Emerging Trends in Hybrid Additive and Subtractive Manufacturing

Abstract

The great capability of additive manufacturing to produce parts with complex, even impossible to achieve, geometries through five-or-more-axis machining or other conventional processes opened a promising future decades ago. For its part, mature subtractive manufacturing presents problems of material waste, especially relevant in the case of superalloys used in fields such as aerospace. From the need to overcome the limitations of both and take advantage of their capabilities, the new paradigm of hybrid additive and subtractive manufacturing was born, which today is defined as a hybrid flow of subprocesses that interact with the part in the same machine. This paper presents a review of the emerging trends in additive–subtractive manufacturing over the last five years. This review has been carried out by applying an adaptation of the PRISMA (Preferred Reporting Items for Systematic reviews and Meta-Analyses) methodology to the field of manufacturing engineering. Specifically, open access papers published in English between 2020 and 2024, collected in prestigious journals (classified as Q1 and Q2 within the ranking of their respective categories according to the Journal Citation Report), and peer-reviewed conference proceedings of recognized prestige have been selected. From the analysis of the selected articles, it is concluded that hybrid additive and subtractive manufacturing is especially focused on the aerospace field, using titanium and nickel alloys, combining processes among which DED (directed energy deposition) and milling stand out.

1. Introduction

It is now generally accepted by the research community that the technical origin of additive and subtractive manufacturing lies in the search for synergy generated by the development of a hybrid flow that alternates several additive and subtractive manufacturing subprocesses, in order to reduce the limitations and take advantage of the strengths of both types of manufacturing [1,2,3,4,5], but also to generate capabilities arising from the new paradigm [6].

Although the maturity of additive manufacturing is much lower, given its shorter evolution time, since its first applications it has demonstrated its capacity to produce parts of complex geometry with different types of materials, including metallic ones [7]. Today there are many research groups and works presented in publications and conferences of the highest level on additive manufacturing, and even applied to sectors as cutting-edge and demanding as aerospace or on materials such as superalloys used in these sectors.

For its part, research on the various subtractive manufacturing processes is much more abundant. Its considerable maturity, with a much longer evolutionary process, means that its capabilities and characteristics are much better understood: geometric and dimensional precision, surface quality, and integrity are some of its strengths. However, in recent decades, research has proliferated on its application to new materials and alloys, process planning, and measurement and control systems.

As for the hybrid concept, Zhu et al. [8] already established an important conceptual requirement, such that the subprocesses must interact with each other and affect the workpiece directly to produce effects different from those that would occur with the technologies involved applied independently. GonzalezBarrio et al. [9] start from the premise that hybrid manufacturing is produced from the same machine and is also moving toward reaching a single CAD/CAE/CAM (computer-aided design/computer-aided engineering/computer-aided manufacturing) environment at the software tools level. In turn, while in the early years of the concept the debate on this point was greater [10] and some authors contextualized it in the field of production systems, as a mere combination of different manufacturing technologies along a manufacturing or supply chain, currently researchers focus their work almost exclusively on hybrid flows carried out on the same machine [11]. It is worth noting that at practical levels, given the complexity of the process and the high cost that hybrid machines still have, research work is often forced to propose a theoretical process on a single machine and, due to these limitations, to deconstruct it and execute it in parts on different machines. In this evolution of the concept and research work, we finally reached the point where, with few exceptions in recent scientific literature, it was established that the hybrid flow must be produced on a single machine, and that the result must be not only different but superior to that obtained using, for example, a hybrid manufacturing chain developed on different machines or even facilities [6].

Thus, with these foundations in place, lines of work are beginning to emerge, both in industry and in university and research institutions, focusing on different aspects of hybrid additive and subtractive manufacturing. The need, for example, to reduce material waste is particularly relevant in industries such as aerospace, where complex, high-value parts manufactured using subtractive manufacturing with high-cost superalloys are abundant [12]. The concept of hybrid additive and subtractive manufacturing is also generally related to the additive production of near net shape parts [3], whereby the subtractive subprocesses work in a finishing machining context. In addition, other concepts frequently appear linked to hybrid additive and subtractive manufacturing. These include the development of digital twins, the study of hybrid trajectories with five or more axes, and the development of software tools that truly integrate all stages of the process [6].

Regarding the evolution from a historical perspective, polymeric parts, although not structural, were already being additively manufactured in real-life applications by Bell Helicopter and Boeing starting in 1990. Since then, other major companies in the sector, such as Airbus, Lockheed Martin, GE Aviation, Northrop Grumman, and Honeywell Aerospace, have incorporated parts produced using additive manufacturing. Currently, even the ESA (European Space Agency) and NASA (National Aeronautics and Space Administration), and newer companies, including Space X and Relativity Space, use additive manufacturing. Combustion chambers, fuel tanks, injectors, air ducts, brackets, and clips are some of the parts and components manufactured. Melissa Orme, vice president of additive manufacturing at Airbus, identified 70,000 pieces that would be flying in aircraft and satellites by 2022 [13]. In 2015, Boeing filed a patent application for 300 parts produced using additive manufacturing [14]. The first titanium part was additively manufactured by Airbus in 2017, authorized by the EASA (European Union Aviation Safety Agency), and integrated into the A350 XWB. However, the need for high precision and high geometric, dimensional, and surface quality of functional parts, enabling, for example, assembly operations, requires the use of machining subprocesses [15].

Multitasking machines, which initially integrated different subtractive processes and over time have come to integrate additive and subtractive processes, have a great impact on the reduction of labor costs, as these are reduced, as well as on machinery costs. The time reduction involved in performing all the subprocesses in the same machine without having to remove the part, along with the reduction in required software tools, also represents a significant cost reduction [16]. Since the 1980s, machine tool manufacturers have been presenting their new proposals, as shown in Figure 1 [16].

In recent years, powerful companies such as Mazak and DMG Mori, and other growing companies such as Ibarmia, have developed hybrid additive and subtractive manufacturing machines that alternate between DMD (direct metal deposition) or LMD (laser metal deposition) and machining, especially milling [17]. Indeed, as shown in Figure 2, it was especially from 2014 onwards that a race began among the main machine tool manufacturers to launch their additive and subtractive hybrid manufacturing proposals [6]:

Hybrid additive and subtractive manufacturing, while offering great potential, must address challenges to continue advancing, especially in the interaction phases of the additive and subtractive subprocesses. For this reason, studies are emerging on the effect of machining with lubricants on the subsequent addition of material using LMD. Some studies point to dry machining, while others propose solutions such as the introduction of cleaning phases [17]. One of the most promising fields of application for hybrid additive and subtractive manufacturing is the manufacture of stamping dies [18] and injection molds [2]. The geometric freedom provided by LMD manufacturing, combined with the dimensional, geometric, and surface quality enabled by milling, allows for the manufacture of conformal cooling channels, which allow for faster, more homogeneous, and more efficient cooling. Studies show that this hybrid process also allows achieving the machining characteristics of these parts [18]. Moreover, additive and subtractive hybrid manufacturing machines are beginning to be configured for a wider variety of hybrid processes. Those that integrate PBF (Powder Bed Fusion), which is more oriented toward the production of complete parts, often combine it with milling. Meanwhile, DED (directed energy deposition), which still tends to focus more on coating or repair, is combined with a greater number of machining processes, such as milling, turning, or grinding [19].

Now that hybrid additive–subtractive machines are a reality, and their potential to reduce manufacturing times and waste of expensive materials seems clear, in addition to producing more complex, flexible, and better-featured parts, this poses a challenge for design and manufacturing engineers and operators. The level of knowledge they must acquire not only of each process, additive or subtractive, but of both processes combined, of the synergies that emerge, and, ultimately, of the new paradigm that constitutes hybrid additive–subtractive manufacturing must increase significantly. Predicting geometry, residual stresses, and characteristics such as hardness, porosity, and microstructure, as well as improving monitoring and inspection systems, are also challenges that must be addressed for the use of hybrid machines to become widespread in the manufacturing industry [19].

Significant research efforts are underway to evaluate the interaction of additive and subtractive processes. Ostra et al. [20] analyzed the effect of LMD on the milling process of Inconel 718 parts and concluded that the results in terms of microstructure and mechanical properties are comparable to those of machining the same forged material.

In an attempt to develop a unique CAD/CAE/CAM environment for hybrid additive and subtractive manufacturing, Gonzalez Barrio et al. [21] propose a comprehensive hybrid manufacturing methodology that covers all stages of the process, including design, additive subprocesses (LMD), subtractive subprocesses (milling), and measurement and control processes between each stage and at the end of the entire sequence. They develop a prediction model for the additively produced geometry as a basis for the application of subsequent machining. The entire study is aimed at the aerospace sector and its specific geometries and materials. Thus, it ends by validating its proposal with the manufacture of a Hastelloy^®X (Haynes International, Inc., Hendersonville (North Carolina), USA) blisk on Inconel^®718. It is concluded that, although the methodology is validated and the potential of hybrid additive and subtractive manufacturing is recognized, further study is still needed to address issues such as the design of five-axis trajectories, understanding the interaction of alternating additive and subtractive subprocesses, or bringing programming environments closer to the end user of the hybrid machine. Therefore, in sectors as demanding as the aeronautical sector, it is essential to achieve highly optimized and efficient processes [22], which allow the transition from conventional machining processes, which are highly mature and well-known, to the promising additive and subtractive manufacturing processes, with their clear focus on energy savings, waste reduction, reduced use of polluting refrigerants, and the circular economy.

For this reason, comparative studies of hybrid processes are proposed, combining LMD of complex geometries in titanium, nickel, and steel alloys with conventional milling, SAM (Super Abrasive Machining), and SEDM (Sink Electro Discharge Machining). The conclusion is that while the process that integrates the SAM process is faster and SEDM machining presents the best surface finishes, conventional milling provides the hybrid process with excellent surface quality and much lower costs [23].

In this paper, after addressing the origin and evolution of the concept of hybrid additive and subtractive manufacturing, and briefly presenting the most important milestones in the development of hybrid machines, the incorporation of additive manufacturing-based parts in industry, the integration of additive and subtractive subprocesses into a single hybrid flow in the same machine, and establishing the current context in which research on the subject is being developed, a systematic review based on PRISMA (Preferred Reporting Items for Systematic reviews and Meta-Analyses) of the scientific literature on hybrid additive and subtractive manufacturing is developed. Once the starting point has been established, and the conceptual debates have been overcome, it is considered important to systematize the search and analysis of scientific works, with the intention of reducing the risk of bias and being able to answer a series of essential questions previously defined, as explained in the Section 2. After applying the method, a total of 27 papers published between 1 January 2020, and 31 December 2024 (5 years) were analyzed.

The WoS (Web of Science) search platform was used to develop the work, and the SCI (Science Citation Index Expanded) and CPCI-S (Conference Proceedings Citation Index-Science) databases from the Main Collection were used for searches. The inclusion and exclusion criteria for papers are: open access; published in English; from Q1 or Q2 journals or prestigious conference proceedings (Journal Citation Reports). These decisions and criteria are justified in the methodology chapter.

Several conclusions emerge from the analysis. There is a significant dispersion when classifying the articles by area and country, although less so by year of publication. Unquestionably, the aerospace sector is identified by most papers as a key field of application and development for hybrid additive and subtractive manufacturing. The manufacturing of stamping dies and injection molds with conformal cooling channels is also notable. The most used materials in the various experimental studies and simulations are titanium and nickel alloys. The extension of the studies to real-life industrial cases, the inclusion of new materials, including polymers, a deeper understanding of the technologies involved and their integration and interaction, the development of comprehensive CAD/CAE/CAM systems, and the design of optimized processes are some of the future works proposed by the researchers.

2. Methodology

This work continues the line of research on hybrid additive and subtractive manufacturing initiated by the authors, who from the beginning have emphasized reaching a clear and univocal definition of the concept, after collecting different visions and approaches [6]. In this context, a series of systematic reviews based on PRISMA [24] have been proposed. This methodology, although originated in the field of medical research, has since been adapted and used in different fields of knowledge and academic research due to its usefulness and ability to reduce the risk of bias and because it constitutes a tool that significantly favors the repeatability of studies [25].

The experience of previous research and the novelty of the concept of hybrid additive and subtractive manufacturing allow us to identify the most likely risks of bias as availability bias, due to, among other reasons, the lack of methodological consistency and the dispersion in the indexing of works; lax inclusion bias caused by the need to alleviate the apparent lack of works; and selection bias, due to the ease of accessing works known or developed by similar authors. Rigorous application of the established methodology and defined selection criteria will help minimize the risk of these and other types of bias occurring.

To develop this review of the scientific literature on additive and subtractive hybrid manufacturing and then focus on the analysis of current trends and future expectations raised by researchers, the following methodology [12] has been followed, which is explained briefly and clearly below and summarized in Figure 3.

The essential questions that are intended to be answered during the analysis of the selected articles are first defined: What hybrid processes are proposed? Which additive and subtractive technologies are involved? What materials are used and studied? What industrial or research sectors and areas are identified? What are the main challenges and perspectives? Are future works announced?

In the second step, the search platform is defined; in this case WoS (Web of Science), and within its Core Collection, the databases in which the search was conducted were specified, namely SCI (Science Citation Index Expanded) and CPCI-S (Conference Proceedings Citation Index-Science). The search was performed in the topic field, which includes title, abstract, keyword plus, and author keywords. Next, the inclusion and exclusion criteria are established, including quality criteria: open access papers; in English; Q1, Q2 or conference proceedings of recognized prestige, according to the JCR (Journal Citation Report); published between 1 January 2020 and 31 December 2024 (5 years); including both articles and reviews. In the following step, a series of keywords were defined, with which the corresponding Boolean search equations will subsequently be formulated. For the selection of keywords, an initial list with synonyms was created based on the experience acquired [12], and later, the ChatGPT-4 tool was used to identify, after establishing an appropriate context and protocol, and expand the number of relevant synonyms (ChatGPT-turbo, OpenAI, 2025). Three search levels were established [12] with three Boolean equations, so that a first, more restrictive search was carried out, in which fewer records were expected to be found, and the search was progressively expanded until reaching the third and final one, configuring a last equation that is less strict but equally relevant, allowing for a pertinent review to be developed. The authors’ judgment is essential to decide whether to work with the list generated at one level or another. In any case, the list of each level must be contained within that of the following level (1 within 2; 2 within 3). The three proposed equations were submitted to the syntactic analysis of the ChatGPT4 tool, which confirmed the absence of errors or redundancies (ChatGPT-turbo, OpenAI, 2025). Once the decision is made on which record list is to be selected, based on the total number, in the case that it results in an inappropriate number to carry out a manageable and useful review in this context, the first decile or the first quartile of the most cited papers may be chosen. For this purpose, total citations were considered, not average annual citations. Finally, the definitive selection will be made after a critical and detailed reading of the articles not discarded up to this point. Based on the resulting list, a results analysis and a discussion of findings were carried out, mainly addressing the topics stated in the initially defined questions.

The application of the PRISMA methodology and those based on it promotes transparency and precision in both the review process and the presentation of the findings. Its application allows the authors themselves and other researchers to build on previous work; the key concept is the reproducibility of searches and reviews. For this to occur, it is essential that both the methods and the findings be presented in a clear, transparent, and accessible manner [26]. Special importance must be given to communicating the objectives and questions to be addressed, which must be specific, pertinent, and relevant [27]. The use of a checklist based on the PRISMA checklist was essential in this work, allowing for self-assessment of the review and communication process [28].

The selection of WoS as a search platform is widely supported by the scientific literature. Its impact on global scientific production is undeniable and has continued to grow in recent decades, both in terms of the number of papers published in its associated databases and the citations received by those articles [29]. Today, especially in technological fields, the leadership of WoS-associated databases are evident in high-impact papers [30]. Given the review topic, hybrid additive and subtractive manufacturing, framed within the field of advanced manufacturing engineering, the SCI (Science Citation Index Expanded) and CPCI-S (Conference Proceedings Citation Index-Science) databases of the WoS Main Collection [31] were chosen.

It is established as an eligibility criterion that the works must be open access, since this condition follows the current trend focused on promoting greater openness and dissemination of scientific research and its findings, in order to accelerate research and improve education, as well as usefulness for global society, reducing differences in access to knowledge based on criteria such as wealth or geographical location [32]. In addition, it is accepted that those works published in open access have more citations, and therefore greater impact on global research, than those not published openly [33].

English is becoming a fundamental eligibility criterion, as it has become what some authors call the de facto lingua franca of science and research. In fact, it is prioritized in the criteria of various citation indexes, such as the SCI [34]. This circumstance is leading researchers to strive to publish in English, even if it is not their native language, to increase the likelihood of their work being published and cited [35]. Beyond other considerations, and possible positive and negative aspects of this reality, this is not expected to change in the short term [36].

Since, in addition to addressing the most innovative trends in hybrid additive and subtractive manufacturing, this study aims to provide a clear overview of the origin and evolution of this new manufacturing paradigm, articles, conference proceedings, and reviews [37] were eligible.

As a quality criterion, eligible papers must be indexed in the JCR, an evaluation system associated with the Web of Science. Articles included in its databases are classified into quartiles, Q1 to Q4, according to the impact factor. Papers in Q1 and Q2 categories were eligible, as well as proceedings from prestigious conferences that guaranteed peer review.

Even so, the current trend is not to limit oneself to the impact factor and to broaden the vision, hence open access is imposed as a fundamental criterion in many studies and evaluation processes [38]. In this study, after applying the open access and indexing criteria in the Q1 and Q2 quartiles of the JCR, a descending order (from most to least) of citations received was established, compared to the average annual citations, a criterion widely used in conducting systematic reviews [39]. The decision was made to consider the total number of citations of the works, since the aim was to prioritize their relevance at the time of the review over future projections, a concept supported by the average annual citations. The latter would allow articles with few total citations, and therefore little current relevance, to prevail over works with a greater number of total citations, and therefore greater real influence at present, if these had a lower annual average.

The 5-year period, between 1 January 2020 and 31 December 2024, was established taking into account the novelty of hybrid additive and subtractive manufacturing and with the intention of focusing the study on current trends and future challenges [12].

Figure 4 shows the keywords selected for the configuration of the Boolean equations. All words and expressions in the same column are considered synonyms and were joined using the OR connector. The groups of words corresponding to the different columns were linked using AND connectors. The unshaded words were previously defined by the authors. Those shaded in green were generated using the ChatGPT4 tool (ChatGPT-turbo, OpenAI, 2025). In the first equation, all the keywords were integrated and joined using the corresponding Boolean connectors, except for those highlighted in blue. In the second equation, the group outlined in yellow was deleted and the groups outlined in blue were added. In the third and final equation, the groups outlined in red were deleted.

The application of the methodology can be followed with the help of

Table 1. Search strategy and features of selected literature.

Criteria	Features
Search Platform	Web of Science
Bibliography sources	SCI—CPCI-S
Type of studies	Scientific journals Q1–Q2 and conference proceedings
Publication period	5 years (from 1 January 2020 to 31 December 2024)
Language	English
Type of publication	Only open access
Search 1	Boolean equation (1)
Search 2	Boolean equation (2)
Search 3	Boolean equation (3) (First Decil/Quartile)
Date of last search	4 February 2025
(1)  TS = ((“hybrid manufacturing” OR “hybrid additive and subtractive manufacturing” OR “hybrid additive subtractive manufacturing” OR “hybrid additive manufacturing” OR “AM/SM” OR “AM-SM” OR “hybrid layered manufacturing” OR “Additive-subtractive” OR “Additive/subtractive” OR “Hybrid CNC-Additive Manufacturing” OR “Hybrid Additive and Machining Manufacturing” OR “Additive and subtractive” OR “Additive and Machining” OR “Additive & Subtractive”) AND (“additive manufacturing” OR “AM” OR “3D printing” OR “3D” OR “Direct Digital Manufacturing” OR “Layered Manufacturing” OR “Additive Layer Manufacturing”) AND (“subtractive manufacturing” OR “machining” OR “SM” OR “CNC” OR “Material Removal Processes” OR “Subtractive Fabrication” OR “Material Removal Manufacturing”) AND (“Sector” OR “Industry” OR “Domain” OR “Field” OR “Area”) AND (“trend” OR “Tendency” OR “Lean”) AND (“Rapid Prototyping” OR “Rapid Tooling” OR “Digital Prototyping”)) (2)  TS = ((“hybrid manufacturing” OR “hybrid additive and subtractive manufacturing” OR “hybrid additive subtractive manufacturing” OR “hybrid additive manufacturing” OR “AM/SM” OR “AM-SM” OR “hybrid layered manufacturing” OR “Additive-subtractive” OR “Additive/subtractive” OR “Hybrid CNC-Additive Manufacturing” OR “Hybrid Additive and Machining Manufacturing” OR “Additive and subtractive” OR “Additive and Machining” OR “Additive & Subtractive”) AND (“additive manufacturing” OR “AM” OR “3D printing” OR “3D” OR “Direct Digital Manufacturing” OR “Layered Manufacturing” OR “Additive Layer Manufacturing” OR “LMD” OR “WAAM” OR “DED” OR “laser cladding” OR “FDM” OR “FFF” OR “SLS” OR “SLM” OR “DMLS” OR “EBM” OR “EBAM” OR “MJ”) AND (“subtractive manufacturing” OR “machining” OR “SM” OR “CNC” OR “Material Removal Processes” OR “Subtractive Fabrication” OR “Material Removal Manufacturing” OR “milling” OR “drilling”) AND (“Sector” OR “Industry” OR “Domain” OR “Field” OR “Area”) AND (“trend” OR “Tendency” OR “Lean”)) (3)  TS = (((“hybrid manufacturing” OR “hybrid additive and subtractive manufacturing” OR “hybrid additive subtractive manufacturing” OR “hybrid additive manufacturing” OR “AM/SM” OR “AM-SM” OR “hybrid layered manufacturing” OR “Additive-subtractive” OR “Additive/subtractive” OR “Hybrid CNC-Additive Manufacturing” OR “Hybrid Additive and Machining Manufacturing” OR “Additive and subtractive” OR “Additive and Machining” OR “Additive & Subtractive”) AND (“additive manufacturing” OR “AM” OR “3D printing” OR “3D” OR “Direct Digital Manufacturing” OR “Layered Manufacturing” OR “Additive Layer Manufacturing” OR “LMD” OR “WAAM” OR “DED” OR “laser cladding” OR “FDM” OR “FFF” OR “SLS” OR “SLM” OR “DMLS” OR “EBM” OR “EBAM” OR “MJ”) AND (“subtractive manufacturing” OR “machining” OR “SM” OR “CNC” OR “Material Removal Processes” OR “Subtractive Fabrication” OR “Material Removal Manufacturing” OR “milling” OR “drilling”)))

. It shows the defined exclusion–inclusion criteria and the three configured equations: first equation (1); second equation (2); third equation (3).

The first search, using equation (1), yielded 0 records. Therefore, it is mandatory to continue with equation (2), whose search yielded 2 records. Beyond the relevance of these works, they did not constitute a sufficient subject for conducting a review, so the search was continued, for which equation (3) was used. As expected, the findings of this search were much more numerous. The search returned 153 works, including research articles, reviews, and conference proceedings, ordered by most cited (total citations). The first quartile was selected to continue the evaluation. A preliminary selection of 38 works was therefore made, to which the exclusion–inclusion criteria were applied. A first detailed reading was then performed, which allowed 10 works to be discarded, as they did not address in any way, either theoretically or practically, a hybrid additive and subtractive manufacturing flow. One was discarded because it was Q3 and therefore did not meet the corresponding quality criterion. This means that the final selection of works consisted of 27 entries, which will be analyzed in a later section. Figure 5 shows the flowchart of the complete selection process.

A brief summary of the records obtained can be seen in

Table 2. Selection data.

Selected Articles	Number	%
Total	27	100%
Q1	18	66.67%
Q2	7	25.93%
Proceeding Papers	2	7.40%

, which indicates the works obtained from Q1 and Q2 publications and conference proceedings.

3. Results and Discussion

Before analyzing and discussing the selected papers, a summary is presented, sorted in descending order by total number of citations, with the intention of highlighting those that are most relevant to the research community at the time of the study.

Table 3. General results.

Ref.	Q	Cit.	Year	Institution	Country	Hybrid Flow
[1]	Q1	128	2021	University of the Basque Country	Spain	LPBF + Peripheral milling (Inconel 718)
[2]	Q1	103	2021	University of Groningen	Netherlands	LPBF + CNC milling (316L stainless steel)
[40]	Q1	57	2020	TECNALIA, (BRTA)	Spain	PAW-WAAM + CNC milling (Ti6Al-4V)
[41]	Q1	54	2020	University of Palermo	Italy	---
[3]	Q1	46	2022	Karabük University	Turkey	---
[42]	Q1	45	2021	Oak Ridge National Laboratory	USA	Laser Hot-Wire Deposition + CNC milling (316L stainless steel)
[43]	Q2	42	2020	Palo Alto Research Center	USA	---
[44]	Q1	38	2021	The University of Texas at Dallas	USA	DED + High-speed end milling (Inconel 625)
[45]	Q1	38	2021	University of Calabria	Italy	DED + CNC turning (IN718 powder)
[4]	Q1	38	2020	University of Applied Sciences Aschaffenburg	Germany	SLM + 3-axis milling (Maraging steel 1.2709)
[46]	Q2	36	2023	Ahsanullah University of Science and Technology	Bangladesh	---
[47]	Q2	34	2020	University of Patras.	Greece	DLM + Milling + Vision Inspection (316L stainless steel)
[48]	Q2	35	2020	Fraunhofer Institute for Material and Beam Technology, Dresden.	Germany	LMD + Cryogenic milling (Ti-6Al-4V powder)
[49]	Q1	32	2021	Shenzhen University	China	DED + Dry milling (PEEK, CF/PEEK)
[50]	PP	32	2020	Technical University of Munich	Germany	WAAM + Robotic spindle machining (AlSi12)
[5]	Q1	29	2022	Oak Ridge National Laboratory	USA	DED + CNC milling (316L stainless steel)
[51]	Q2	28	2021	Hamburg University of Technology	Germany	PBF-LB/M + CNC milling (316L stainless steel)
[52]	Q1	28	2021	Inspire AG, Zurich	Switzerland	Casting + CNC milling + DMD (Inconel 718 powder)
[53]	Q2	26	2022	Palo Alto Research Center	USA	Powder Bed Fusion + Multi-axis CNC machining
[21]	Q1	23	2022	University of the Basque Country	Spain	LMD (Hastelloy®X sobre Inconel®718) + 5-axis milling
[54]	Q1	22	2023	RMIT University, Melbourne	Australia	DED/PBF-LB/EBM + Turning/Milling (Ti-6Al-4V)
[11]	Q2	22	2022	ISEP—Polytechnic of Porto	Portugal	---
[55]	Q1	21	2023	City University of Hong Kong	China	2D/3D printing (PDMS + ZrO2/AlON) + Laser engraving/cutting
[56]	Q1	21	2023	Empa, Swiss Federal Laboratories for Materials Science and Technology	Switzerland	WAAM (ER70S-6 steel) + machining (pyramid profile shaping)
[57]	Q1	20	2021	Harbin University of Science and Technology	China	LMD + Side Milling (316L stainless steel)
[58]	-	19	2020	Tshwane University of Technology,	South Africa	Laser AM + CNC machining (Ti-6Al-4V)
[59]	Q1	17	2021	University of Applied Sciences Aschaffenburg	Germany	SLM + high-speed micromilling (Maraging steel)

Where Q: quality criteria; Cit.: citations; PP (proceedings paper); LPBF (Laser Powder Bed Fusion); SLM (selective laser melting); HSM (high-speed machining); PAW (Plasma Arc Welding); WAAM (wire arc additive manufacturing); CNC (Computer Numerical Control); DIW (Direct Ink Writing); PBF-LB/M (Laser-Based Powder Bed Fusion of Metals); EBM (Electron Beam Melting); PDMS (Polydimethylsiloxane); AM (additive manufacturing).

shows the quality criteria (Q1, Q2, PP), the year of publication, and the country of the first author. The column corresponding to the first author’s institution of affiliation is highlighted, as it can serve as a guide for future selective searches for research on additive and subtractive hybrid manufacturing, using methods other than traditional search platforms and databases. Of particular importance is the last column, which presents in a summarized, formula-like manner the additive and subtractive hybrid workflow addressed by each paper, either individually or as the main flow of the study.

Table 4. Journals and conferences with selected papers.

Journal/Congress	Reference
30th international conference on flexible automation and intelligent manufacturing (FAIM)	[50]
8th international conference on through-life engineering services (TESCONF)	[58]
Additive manufacturing	[5]
Advanced materials	[55]
Computer-aided design	[43,53]
Engineering structures	[56]
Fatigue & fracture of engineering materials & structures	[51]
International journal of advanced manufacturing technology	[47,48]
International journal of heat and mass transfer	[2]
International journal of machine tools & manufacture	[1]
International journal of mechanical sciences	[44]
Journal of manufacturing processes	[41]
Journal of materials processing technology	[42,52]
Journal of materials research and technology (JMR&T)	[3]
Materials	[4,40,54,57,59]
Mechanical systems and signal processing	[21]
Metals	[11]
Micromachines	[46]
Polymers	[49]
Wear	[45]

lists the journals or conferences in which the selected articles have been published. This table has a double interest. It shows the significant dispersion of the works, as most of the top-tier publications have only one work among the selected ones. An increasing number of journals are open to the publication of specific works on the still novel hybrid additive and subtractive manufacturing. Furthermore, it allows researchers to identify those with the greatest number of articles on the topic of study, in order to focus their future publications and selective searches of scientific literature.

3.1. Articles Classification

Following the lead of previous work [12], the selected papers were pre-classified in this order according to the research areas assigned by the WoS, the year of publication, and the country of the first authors. All three criteria are relevant and useful for both the search process and the subsequent analysis.

3.1.1. Research Areas

A typical feature of emerging technologies can be seen in the multiple areas from which the topic of study is approached. It is expected that some of these will consolidate while others will eventually become residual. The most common areas are actually transversal and can encompass a wide variety of research topics. Thus, Engineering and Materials Science appear in 70.4% and 48.1% of the reviewed papers. They are followed by generic areas such as Chemistry and Physics, with a similar analysis. Again, a wide dispersion is observed in the remaining areas; these are more specific areas, which are, however, mentioned in greater or lesser depth in many of the articles analyzed: Digital Twin, Feedback Control, Intelligent Manufacturing, and Smart Systems, among others. In fact, they are revealed to be crucial for the future development of hybrid additive and subtractive manufacturing. Polymer Science also appears as a minority area, with only one article (3.7%) among those analyzed. However, it represents an interesting contribution, given that it opens the door to research with new materials, distinguishing the most common ones, which are those related to nickel and titanium superalloys. Figure 6 shows the distribution described.

3.1.2. Year of Publication

The distribution of the selected articles by year of publication is more homogeneous than it might appear at first glance. It should be noted that the ranking of the preselected works, in order to study the first quartile of the most cited, was based on total citations, with the intention of studying the articles based on their current dissemination and impact rather than their projections. This means that in the established interval, 2020 to 2024, articles from 2020 and 2021 have received a greater number of citations. This indicates, on the one hand, that the process of research consolidation, as occurs in other fields, and of dissemination of works, despite being open access, is not as rapid as might be desired. It can also be concluded that there have been no qualitative leaps, with extraordinary contributions, that would have caused a publication from 2024, for example, to receive an unusual number of citations. The distribution can be seen in Figure 7.

3.1.3. Countries

The distribution of the articles analyzed by country is consistent with previous research [12]. First place is held by Germany and the United States, countries with a long tradition and research capacity. China, as a rising power, competes technologically with the United States and Europe and occupies third place. Spain’s position is particularly noteworthy, with the same number of articles as China. In Spain, it is worth highlighting the important tradition that exists in the machine tool sector. This has led, in recent decades, to an intensification of the technological developments carried out in companies in the sector in the manufacture of hybrid, additive, and subtractive machine tools and, as a consequence, to an increase in the research carried out on the subject in technology centers and universities all over the country [12]. Figure 8 shows the distribution by country.

3.2. Articles Analysis

Following the criteria established in previous research [12], a brief analysis is made of the keywords in the articles analyzed, considering them indicative of research trends and the interests of the authors themselves, who selected them. Although up to 107 different keywords were identified, in the 27 articles analyzed, the greatest concentration occurs around 15 of them. The remaining 92 appear in a single article.

Table 5. Keywords present in several articles.

Hybrid manufacturing	[1,3,11,40,42,43,48,50,52,53,54,56]
Additive manufacturing	[2,3,11,21,40,41,42,44,45,47,54]
Machining	[41,45,54]
Selective laser melting	[4,51,59]
Accessibility analysis	[43,53]
Design for manufacturing	[43]
Digital twin	[46,50]
Directed energy deposition	[42,44]
Hybrid additive manufacturing	[4,59]
Laser metal deposition	[21,48]
Multi-axis machining	[43,53]
Robot	[50,58]
Subtractive manufacturing	[3,42]
Tool wear	[45,54]
Topology optimization	[43,53]

shows those keywords that appear in multiple articles. They appear in descending order. Hybrid manufacturing stands out, present in 44.4% of the works, and additive manufacturing in 40.7%. These percentages seem to point to the gaining importance of hybrid additive and subtractive manufacturing, which is beginning to be truly understood and studied as a concept distinct from additive manufacturing, surpassing the idea of mere finishing machining of additively manufactured parts. Even so, researchers understand that they must contextualize it within the framework of additive manufacturing so that the papers can be correctly indexed and more easily located by researchers. The generic term ‘Machining’ and the more specific ‘Selective Laser Melting’ appear in three of the twenty-seven papers (11.1%), while the remaining terms appear in only two of the twenty-seven, representing 7.4%.

Table 6. Unique keywords.

316L stainless steel	Cutting force	Inconel	Process digitalization
4D printing	Data analytics	Industry 4.0	Residual stress
5-axis machining	Decision support chart	Injection molding	Sensors
Actuators	Design of experiments	Intelligent manufacturing	Shape memory ceramics
Additive materials	Direct metal deposition	Iterative stress reconstruction	Smart system
Additive–subtractive hybrid manufacturing	Distortion	Laser hot-wire deposition	Stress and deformation
Additive–subtractive manufacturing	Dry milling	Laser melting deposition	Structural design
Anisotropy	Electron microscopy	LMD clad sensorization	Superalloys
Application Programming Interface (API)	Energy demand	LPBF	Support structures
Artificial intelligence	Fatigue life extension	Manufacturing processes	Surface characterization
Ball end milling	Fatigue repair	Maraging steel	Surface integrity
CAD/CAM	FDM	Mean stress effects	Surface roughness
CF/PEEK	Feedback control	Metal 3D-printing	Surface treatment
CNC machining	Grain boundary density	Microcontroller	Sustainable manufacturing
Computer-Aided Manufacturing	Heat treatment	Milling	Taguchi method
Configuration space	Heterogeneous precursors	Modelling	Taylor factor
Conformal cooling channels	High-speed machining	Monitoring System	Tensile properties
Construction rules	High-speed milling	Paw	Thin-walled parts
Control systems	Hybrid additive–subtractive manufacturing	Peek	Titanium alloy
Cost	Hybrid additive/subtractive manufacturing	Polymer-derived ceramics	Toolpath planning
Crack arrest	Hybrid directed energy deposition	Porosity	WAAM
Cryogenic machining	Hybrid manufacturing system	Post-processing processes	Wire and laser additive manufacturing (WLAM)
Crystallographic effect	Hybrid process chain	Process chain	Wire arc additive manufacturing

shows, alphabetically, the remaining keywords, which appear in only one article. It is striking that the most specific terms on additive and subtractive hybrid manufacturing appear in only one article. This is justified by the lack of terminological consistency and the low standardization that still affects additive and subtractive hybrid manufacturing. Even so, the complete list is presented because it may be of interest to researchers when setting up search equations for their own systematic reviews or for selective searches to support their experimental articles.

3.2.1. Processes and Materials

Although some of the reviewed articles [1,2,3,4,5] justify the emergence and usefulness of hybrid additive and subtractive manufacturing, literally, with the need to reduce the limitations of additive manufacturing, such as the lack of surface quality, and subtractive, including the waste of high-cost material, and the desire to take advantage of its strengths, the ability to generate complex geometries in the case of additive manufacturing and the great geometric, dimensional, and surface quality in the case of subtractive manufacturing, this idea underlies practically all of the selected works, assuming it in a natural and unquestionable way.

Perez et al. [1] developed an experimental investigation to relate LPBF (Laser Powder Bed Fusion) parameters with cutting forces and milling anisotropy of Incolel 718 parts, in the context of a hybrid additive–subtractive manufacturing flow. After highlighting the capacity of hybrid manufacturing to achieve complex geometries, they emphasize the need for the participation of subtractive manufacturing to provide the parts with the required surface, dimensional, and geometric quality. The rapid heating and cooling cycles of the parts manufactured by LPBF give rise to columnar grain structures, which consequently produce crystalline textures and anisotropy. This makes both the mechanical properties and the cutting forces depend on the spatial directions. Through a study based on a Taylor oblique cutting model, the microstructures of the parts manufactured by LPBF, the layer thickness and the cutting parameters are related. The authors make a relevant contribution by limiting machining parameters to a finishing context, given that the objective of additive manufacturing is to obtain near-net-shape parts.

Research shows that although the influence of the crystalline structure on cutting forces is reduced when working with low VED (Volumetric Energy Density), this increases the impact of grain shape. Under these conditions, the increase in the density of the boundary grain requires lower cutting forces for a tool axis parallel to the main grain axis. On the contrary, they are higher for planes of the shear bands transverse to the columnar grains [1].

Feng et al. [2] conducted a review of the scientific literature published on the manufacturing of molds with conformal cooling (CC), integrating L-PBF and CNC milling in a hybrid flow. The objective of CC is to achieve faster and more homogeneous cooling than that obtained with conventional channels.

Throughout their systematic review, they focus on the additive manufacturing of multimaterial molds, such as 316L stainless steel/H13 hot work steel or steel/Cu combinations, after which they conclude that monomaterial additive manufacturing presents higher quality. They also conclude that the great capacity of hybrid additive and subtractive manufacturing to achieve higher standards in the production of parts with complex geometry, with high surface, geometric, and dimensional quality, becomes especially relevant and applicable in the mold manufacturing industry with conformal cooling (CC), since with traditional processes it would be impossible to provide the channels with the characteristics and qualities obtained through the integration and alternation of additive and subtractive subprocesses within the same manufacturing flow [2].

Veiga et al. [40] conduct a practical investigation to study the quality of hybrid additive and subtractive manufacturing (PAW-WAAM and milling) of Ti6Al-4V titanium parts. The study is focused on PAW (Plasma Arc Welding), a type of WAAM (wire arc additive manufacturing) technology, and its feasibility is validated in combination with CNC milling. In fact, it is concluded from the study that the wall manufactured in their experiments meets aeronautical standards and does not present significant variations due to the position and orientation of the samples analyzed. Neither does the torque change significantly when performing up-milling or down-milling, although up-milling does improve surface quality. Torque is not affected by cutting depth, but it is affected by speed. Surface quality also improves when the tool approaches the clamping system.

The work of Ingarao et al. [41] presents comparative models of costs and accumulated energy demand of subtractive and hybrid additive–subtractive processes. These models consider the complete life cycle, including the operations that follow the hybrid manufacturing phase, such as final surface machining or the removal of support structures. The research proposes models based on hybrid additive and subtractive manufacturing of Ti-6Al-4V parts through Electron Beam Melting and turning. The use phase is also considered. In the model, variables include the shape, the possibilities for part lightening, and the usage time. To compare the subtractive and hybrid processes, the premise is that both can meet the geometric requirements, which indicates the eminently economic focus of the study. Different approaches are alternated in the study: identical part; weight reduction thanks to additive manufacturing; focus on improving usage conditions. Although the study concludes that the hybrid process is the most advantageous from the point of view of energy demand, it does not go so far as to affirm the same from an environmental or economic perspective, as these are highly conditioned by the geometry or recyclability of the materials.

Korkmaz et al. [3] conduct a technical review of hybrid manufacturing processes of metallic parts present in the scientific literature. Within this study, hybrid additive and subtractive manufacturing is addressed, identifying pairs of materials and processes such as 6511 Steel through SLM–milling (selective laser melting–milling), 316L Steel through DED–milling, ER2319 aluminum alloy through Laser Cold Metal Transfer, or 316 Stainless Steels through directed energy deposition—Process Laser Remelting Process.

Feldhausen et al. [42] conduct an experimental study to analyze the mechanical properties and microstructure of 316L stainless steel parts manufactured through hybrid additive and subtractive manufacturing (DED and milling). For the fabrication of the designed hexagonal parts, a five-axis hybrid machine, Mazak VC500A/5x AM, was used. Both porosity and part distortion during the process were studied. The study led to the conclusion that hybrid manufacturing, in which the machine process is not interrupted, reduces total cycle time by 68%. It also reduced the average relative porosity by 83% and the relative elongation at failure by 71%, compared to the additively manufactured part. Hybridization does not affect the crystalline structure. With these results, the future of hybrid additive and subtractive manufacturing, although it must still mature, is promising.

The study by Mirzendehdel et al. [43] proposes a topology optimization (TO) framework with the objective of automating the design of complex parts manufacturable on five-axis machine tools. For this purpose, multi-axis machining constraints are established, with the consequent reduction of time and material in trial-and-error processes, which are inevitable due to the mismatch between design and manufacturing models. Although the study focuses on the machining process, the author directs it toward hybrid manufacturing, first by including the concept “hybrid manufacturing” among its keywords, and then, in the conclusions, by pointing out as future work the extension of the proposed method to hybrid additive and subtractive manufacturing. Although briefly, it clearly defines what the hybrid process should be, alternating additive and subtractive subprocesses. To extend the method, the defined concept of inaccessibility measure field (IMF) must be applied to the additive subprocess, both in terms of accessibility and in cases of over-deposition or under-cut. Clearly, the study is focused on a context of five-axis CNC manufacturing, in which hybrid flows are included.

Sunny et al. [44] address in an experimental study the influence of the residual stress (RS) produced by additive manufacturing on the stresses and distortion induced by the subsequent machining of these parts. To carry out the study, the DED manufacturing of thin walls is simulated by finite element software, followed by a high-speed milling phase. The material considered in the simulations is Inconel 625. In the simulations, one hypothesis is used in which the residual stresses generated by additive manufacturing are disregarded, and another hypothesis in which the stresses produced by the DED process are incorporated. The work confirms the importance of the residual stresses generated by the rapid heating and cooling cycles of DED on the stresses and distortion produced by subsequent milling. It also concludes that the design of the machining strategy is crucial in the effect caused by high-speed machining of DED parts.

Careri et al. [45] carry out an investigation on the wear processes of turning tools for parts made of Ni alloys, specifically IN718, manufactured by DED. In the experiments, additive and subtractive subprocesses are also combined with heat treatments to evaluate their effect. Adhesive and abrasive wear are identified as the main types of tool wear. In the case of the as-deposited material, the wear is due to the low surface quality, although no tool failures occur. When heat treatment is applied to the DED part, although the machinability is similar to that of the forged material, high-speed turning causes rapid tool failure, and surface quality depends to a greater extent on the cutting parameters.

Working with a hybrid machine (Matsuura LUMEX Avance-25, Fukui, Japan), Wüst et al. [4] manufacture maraging steel 1.2709 parts in a hybrid way, using SLM and five-axis milling. Subsequently, they carry out a statistical optimization study of surface roughness. For this purpose, they rely on the Taguchi method. The study was carried out distinguishing between contour surfaces and hatch, or filling surfaces, and varying the laser power, the scanning speed, and hatch distance. Energy input and energy density were also considered. In milling, variations of feed per tooth, cutting speed, and radial depth were considered. It is concluded that the application of the analysis method and the selection of the most favorable combination of parameters allow for the reduction of surface roughness to 9.0 μm (40% compared to the values obtained with the manufacturer’s recommendations) and the roughness of contour surfaces to 7.5 μm (37.5%), which confirms the usefulness of the Taguchi method [4].

In the study by Stavropoulos et al. [47], hybrid additive and subtractive manufacturing is addressed from the perspective of a hybrid process chain. This is done by considering the repair of a gas turbine blade, manufactured in 316L stainless steel. The research develops a hybrid process planning methodology aimed at improving the life-to-value ratio. The method is based on simulation and experimentation. Although it is concluded that the results in terms of surface roughness are similar, it is emphasized that the selected process chain does affect different KPIs (Key Performance Indicators), such as tool wear or total manufacturing time.

Moritz et al. [48] address in their study hybrid additive and subtractive manufacturing, LMD and cryogenic milling. They justify it on the basis that the latter leaves surfaces clean of coolants that could affect the subsequent additive process within the hybrid process. The experimental part was carried out with cryogenic CO₂ milling of Ti-6Al-4V parts. The influence on surface integrity and tool wear was evaluated. The study alternated hybrid and subtractive sequences, AM–SM–AM (additive manufacturing–subtractive manufacturing–additive manufacturing). The additive subprocess was carried out with an ABB IRB 4400 multi-axis robot and the subtractive one with an ABB IRB 6660 robot, integrated into the same station.

Guo et al. [49] seek with their study the reduction of the staircase effect and the improvement of surface quality in parts manufactured through hybrid additive and subtractive manufacturing, FDM (Fused Deposition Modeling) and dry milling. They use polymeric materials, PEEK (Polyetheretherketone) and CF/PEEK (Carbon Fiber Reinforced Polyetheretherketone), for their work, in which they evaluate the relationship between raster angle and layer thickness and spindle speed, depth of cut, and feed rate per tooth. Dry milling is used, considering the surface requirements of molds and medical implants. The study demonstrates that the dry milling subprocess can substantially improve the surface quality of the parts.

Reisch et al. [50] configured a robotic WAAM system, with the possibility of adaptation to hybrid additive and subtractive manufacturing of large-sized parts in AlSi12. They emphasize the monitoring system to ensure compliance with quality requirements and aim for the industrialization of the system. The research covers the entire process, from design and process planning to WAAM deposition and precision milling. Everything is configured and modeled beforehand with Siemens NX. It is confirmed that the best results are obtained with a tilt angle of 0° and a lead angle of 0°, −5°, and −15°. A digital twin of the manufactured part will be the basis for future adaptations of the process.

The review by Feldhausen et al. [5] on hybrid additive and subtractive manufacturing concludes that the complexity of the parts, as well as of the manufacturing strategies and toolpaths, is an inherent part of the process. For this reason, the crucial importance of CAM software and the CAM strategies developed to date by different industries is analyzed and highlighted for a successful application. However, the quality of the result is linked not only to CAM but also to the associated processes, mentioning DED and milling, the materials, applied heat treatments, intricate geometries, or the level of the machine tools used.

As Braun et al. [51] recognize the great potential of metal additive manufacturing, in this case PBF-LB/M (Laser-Based Powder Bed Fusion of Metals), they investigated its combination with subtractive manufacturing, integrated in a hybrid manufacturing flow, to overcome the porosity of the additively produced material and improve its surface roughness, factors that reduce its corrosion and fatigue resistance. The study is specifically applied to the effect on the fatigue resistance of AISI 316L produced through hybrid additive and subtractive manufacturing. It also compares it with the same parameters of wrought 316L. The study confirms that the as-built material has low fatigue strength, while the machining of the additively manufactured part significantly increases its fatigue strength (up to 87%). Since most of the major defects are found in the 250 μm closest to the surface, this allows for improvement in surface quality and integrity.

Among the routes analyzed by Braun et al. [51] are the following hybrid routes: PBF-LB/M + CNC Milling; PBF-LB/M + HIP (Hot Isostatic Pressing) + CNC Milling; PBF-LB/M + Annealing + CNC Milling. These are compared with conventional routes such as: Wrought (316L) + Heat Treatment + CNC Milling; PBF-LB/M (as-built); PBF-LB/M + Annealing; PBF-LB/M + HIP.

Soffel et al. [52] conducted an experimental study in which they analyze various manufacturing sequences, combining and alternating casting, milling, and additive manufacturing DMD, to evaluate the interface strength and the resulting mechanical properties. Among the studied flows, the following fit hybrid additive and subtractive manufacturing: Casting + CNC milling + DMD; Casting + heat treatment + CNC milling + DMD. The authors conclude that the hybrid sequences are suitable for the manufacturing and repair of Inconel 718 parts. When comparing them, it is observed that the first, without heat treatment, presents a smoother transition, a better interface strength, and less thermal affectation. In contrast, the route with heat treatment before machining presents worse mechanical and interface behavior due to the precipitation of undesired phases (such as δ) and heterogeneous microstructures.

The work of Mirzendehdel et al. [53] is oriented toward hybrid additive and subtractive manufacturing with materials that require the use of sacrificial support structures. For this reason, they propose a systematic methodology of topology optimization (TO) that allows removal while overcoming accessibility problems. Their approach is validated through the simulation of the manufacturing of several parts, in Ti-6Al-4V, from the aerospace, robotics, and automotive sectors: GE bracket, +Z (Powder Bed Fusion + 3-axis CNC machining); quad-copter (Powder Bed Fusion + 14-orientation CNC milling); upright example (Powder Bed Fusion + Multi-tool CNC milling with vise fixture). The authors conclude that the approach ensures the successful removal of the support structures from the design phase. To achieve this, they apply the idea of the ‘inaccessibility measure field’ (IMF) to the near net shape parts produced in the additive phase of hybrid additive and subtractive manufacturing.

González Barrio et al. [21] developed a hybrid additive and subtractive manufacturing methodology for high added-value parts, within a single CAM environment. In the work, an API (Application Programming Interface) is developed for the additive manufacturing module, with options for Planar LMD and three-axis and five-axis LMD. A methodology for predicting the shape manufactured by LMD is also developed, for the subsequent milling process. To validate the methodology, a blisk made of Hastelloy^®X over Inconel^®718 is selected for manufacturing. Work is carried out in the NX™ environment from Siemens. Milling is then applied to provide the geometric, dimensional, and surface quality required in the aerospace industry context. Finally, deviations from the designed part are studied. After a hybrid additive and subtractive manufacturing process, which requires continuous five-axis work due to the complexity of the selected geometry, it is concluded that the deviation does not exceed 10% compared to the predicted geometry.

Zhang et al. [54] developed a state of the art on the machining of Ti-6Al-4V parts manufactured additively, which led them to directly address various hybrid additive and subtractive manufacturing processes (DED/PBF-LB / EBM + Turning/Milling). Porosity, residual stresses, surface defects, or anisotropy resulting from the additive subprocesses, and cutting forces, heating during the process, and hardness, resulting from the machining process, are some of the aspects studied.

Sebbe et al. [11] conducted an extensive review of the scientific literature on hybrid additive and subtractive manufacturing oriented toward the production of complex parts. They present a comprehensive summary of the different hybrid flows addressed in the reviewed articles. This allows the authors to approach the process solidly from a technological point of view, considering challenges and opportunities. Beyond overcoming the weaknesses of the independent additive and subtractive subprocesses through their combination, they highlight the versatility of hybrid manufacturing, its ability to reduce the waste of high added-value materials, such as aluminum and titanium alloys used in the aerospace industry, and the possibility of performing the entire process in a single machine. In their work, they identify the issue addressed by many authors concerning the residual stresses caused by the additive subprocess, with the resulting problems of distortion or requirements for heat treatments.

The work of Liu et al. [55] develops a novel approach compared to most studies on hybrid additive and subtractive manufacturing. In the hybrid process, the additive manufacturing of ceramic precursors is combined—that is, polymeric materials loaded with ceramic materials—by Blade Coating (2D printing) or DIW (Direct Ink Writing) (3D additive manufacturing), followed by the application of the subtractive subprocess, laser cutting, and ending with the application of the thermal process, pyrolysis, which converts it into ceramic material. This last step is what leads the authors to call it 4D-additive–subtractive manufacturing. In their experimental development, they work with PDMS (Polydimethylsiloxane) materials with ceramic particles of ZrO₂, AlON, or Al₂O₃.

In the study by Ghafoori et al. [56], ER70S-6 wire is used to repair S355J2+N steel parts by additive manufacturing WAAM, given the metallurgical compatibility of both materials. The repaired specimens present central cracks caused for the experimental approach. After the material addition process, a subtractive subprocess, CNC milling (pyramid shaping), is applied. Finally, they were subjected to HCF (High-Cycle Fatigue) loading to evaluate the results and draw conclusions. It is demonstrated that the hybrid process reduces the stresses in the crack area and reduces premature detachment. Pyramid shaping milling helps to reduce stresses. The process as a whole limits the tendency of the crack to propagate.

Wu et al. [57] analyzed the effect of the machining subprocess, within hybrid additive and subtractive manufacturing, on the stresses generated by the additive manufacturing of 316L stainless steel and, therefore, on the resulting deformation of the part. The experimental approach was carried out using a three-axis LMD machine (Beijing Beiyi Machine Tool Co., Ltd., Beijing, China). The hybrid process was also simulated, considering temperature and stresses, using Finite Element Method through ANSYS Mechanical APDL19.2. Thanks to the simulation, it is confirmed that the direction has a significant influence on the stress distribution in the additively manufactured thin walls. Their milling significantly alters both the values and the distribution of the residual stresses. The milling force changes tensile stress into compressive stress, and its extended gradient distribution transforms into regional distribution. Finally, the result of the experimental work confirms the conclusions of the simulation.

Oyesola et al. [58] developed a theoretical cost model of hybrid additive and subtractive manufacturing, as a tool for the MRO (Maintenance, Repair, and Overhaul) sector, especially focused on the aerospace sector. The study is centered on the manufacturing of parts. Their intention is to integrate in the model the benefit of cost reduction. In the work, analytical techniques are applied to help the aerospace sector increase its competitiveness. Although the process is not addressed from a technical point of view, it is described clearly and precisely: additive and subtractive subprocesses are alternated in the same machine, where the part can remain fixed, to achieve free and complex geometries with the high quality required by the sector. The difficulty lies in identifying the costs of the different phases of the hybrid process, understanding its complexity. This will allow the cost–benefit balance to be adjusted, optimizing the different variables. The authors apply their model to the case of an aerospace bracket manufactured in Titanium 6Al-4V.

The work of Sommer et al. [59] is novel, as it designs a series of rules for a hybrid additive and subtractive manufacturing process, SLM and high-speed micromilling. The intention of the set of rules developed is to serve as guidance during the design phase. To define them, different elements recognizable in any manufacturable geometry are analyzed: cylinders, overhangs, chamfers, radii, holes, etc., both from the additive point of view and from the hybrid additive and subtractive perspective. The designed rules are applied in the manufacturing of different parts in Maraging steel 1.2709 on the hybrid machine Lumex Avance-25 (Matsuura Machinery GmbH, Wiesbaden, Germany), using SLM and high-speed micromilling. The injection mold with conformal cooling stands out, as it perfectly synthesizes the concept of hybrid additive and subtractive manufacturing. The authors consider the advantages of applying the established rules in hybrid manufacturing to be demonstrated. Figure 9 shows an example of the application of hybrid additive and subtractive manufacturing in the production of molds with conformal cooling (CC) channels.

Table 7. Additive and subtractive subprocesses in five most cited articles.

Ref.	Q	Cit.	Year	AM Subprocess	SM Subprocess	Material
[1]	Q1	128	2021	LPBF	Milling	Inconel 718
[2]	Q1	103	2021	LPBF	CNC milling	316L stainless steel
[40]	Q1	57	2020	PAW-WAAM	Milling	Ti6Al-4V
[42]	Q1	45	2021	Laser Hot-Wire Deposition	Milling	316L stainless Steel
[44]	Q1	38	2021	DED	High-speed milling	Inconel 625

provides a summary of the five most cited articles that address a specific hybrid manufacturing flow, breaking down the main additive and subtractive subprocesses and indicating the work material. As it can be seen, although the additive subprocesses are varied, it is clear that the subtractive subprocess generally integrated into the different hybrid flows is milling. In turn, the materials column clearly synthesizes the materials on which hybrid additive and subtractive manufacturing is focused.

A synthesis of the reviewed studies allows us to conclude that, in order to achieve parts manufactured by hybrid additive and subtractive manufacturing with adequate physical and mechanical properties, it is essential to overcome some challenges, among which the following are worth highlighting: [1,5,11,40,42,44,46,48,49,51,52,54,55,56,57]

• Controlling grain orientation and phase formation so as to improve the microstructure of the material, as this directly influences the mechanical properties.
• Optimizing the process parameters to minimize porosity, as this can reduce the mechanical strength and durability of the parts.
• Controlling process conditions such as cooling rate and material composition to ensure adequate mechanical properties such as tensile strength and hardness.

Addressing these challenges comprehensively is key to achieving high quality parts.

3.2.2. Areas and Sectors

Hybrid manufacturing combines the advantages of additive and subtractive manufacturing. Therefore, it allows, on the one hand, to produce parts with high precision and good surface finishes, taking advantage of the great capacity of machining processes to finish parts with a high level of detail and, on the other hand, to optimize the material by combining both techniques, as waste is reduced by allowing additive manufacturing to generate parts of complex shapes with dimensions very close to those included in the design plans which, subsequently, by means of subtractive manufacturing, will be brought to the final dimensions of the part. Therefore, it finds a direct field of application in sectors such as aeronautics and aerospace, automotive, and medical, where complex parts with high precision are required [1,2,5,11,21,40,41,46,48,52,54,58,59].

In their work on the influence of the anisotropy of near net shape parts manufactured by LPBF on milling parameters, Perez-Ruiz et al. [1] point out the manufacturing of aircraft engine parts and power system components as the natural application of hybrid additive and subtractive manufacturing (LPBF–milling). In this regard, they reinforce their statement by highlighting the capability of the process to ensure the assembly of the manufactured parts in complex systems. The work of Feng et al. [2] clearly identifies a sector of application and development, with great importance in the manufacturing industry in general, by pointing out the use of hybrid additive and subtractive manufacturing (L-PBF and milling) in the manufacturing of conformal cooling channels in molds, as an alternative to manufacturing with straight-drilled cooling channels.

Once again, the aerospace sector is pointed out in the work of Veiga et al. [40] as a current field of application and future development of hybrid additive and subtractive manufacturing. Moreover, it is specified in the production of titanium alloy parts, for which it is confirmed that machines are already available. Ingarao et al. [41] present a different perspective when locating the field of application of hybrid additive and subtractive manufacturing, since they do not link it directly to any industrial or research sector, although they model the manufacturing of Ti-6Al-4V parts, which are of great importance in the aerospace sector. On the contrary, they contextualize it in what could be considered a transversal sector, such as sustainable economy, with a clear environmental vision: cost reduction, energy savings, recyclability, and reduction of material waste. In turn, Korkmaz et al. [3] state that, at the time of their study, it is the company Airbus that uses most of the aircraft parts designed and manufactured through hybrid manufacturing. Once again, the aerospace sector is presented as the field with the greatest potential for the development and application of hybrid additive and subtractive manufacturing. Sunny et al. [44] also point to the aerospace sector, specifically to the manufacturing of monolithic structures, as a field of application for hybrid additive and subtractive manufacturing, and they do so by highlighting the challenge that distortion and residual stresses caused by machining in additively manufactured parts represent for its applications.

The study by Careri et al. [45] is contextualized by the need of the aerospace sector to manufacture and repair high added-value parts with a superior finish to that obtained through their manufacturing by DED, something that is achieved with hybrid additive and subtractive manufacturing. Likewise, Stavropoulos et al. [47] propose the manufacturing of a gas turbine blade, which shifts the focus of the application sector of hybrid additive and subtractive manufacturing from the aerospace field to the energy industry, considered from the point of view of manufacturing and not of generation, although with similar parts. Moritz et al. [48] identify the aerospace and biomedical sectors as potential fields of application for hybrid additive and subtractive manufacturing, both for repair and for the manufacturing of parts with high requirements. Guo et al. [49] point out, while acknowledging the problem of the staircase effect in FDM processes as the basis of hybrid additive and subtractive processes, the mold manufacturing, aerospace, and medical sectors, with high surface quality requirements, for the application of hybrid additive and subtractive manufacturing.

The aerospace industry, with a focus on reducing the waste of high-value materials, is the sector in which Reisch et al. [50] place the application of hybrid additive and subtractive manufacturing, in this case integrating WAAM and milling. Similarly, throughout the entire work by Feldhausen et al., the aerospace sector appears transversally in the analysis of the reviewed articles [5] as the main target of hybrid additive and subtractive manufacturing. Likewise, the manufacturing of conformal channels is mentioned, to allow for faster, more homogeneous, and more efficient cooling of molds, which are fundamental in the general manufacturing industry. On the other hand, since their study focuses on fatigue resistance, Braun et al. [51] mention throughout it different applications in which this factor is relevant, as in the case of the manufacturing of turbine blades. Corrosion resistance, improved by hybrid processes, points toward the offshore industry.

Throughout the work of Soffel et al. [52], sectors such as aerospace and the energy industry are mentioned again, but the automotive industry is also mentioned. In all of them, with the presence of complex high added-value parts, hybrid additive and subtractive manufacturing is indicated. The fact that the experimental part of the research is carried out with Inconel 718 reinforces its orientation toward the aerospace sector. The parts whose hybrid manufacturing is simulated by Mirzendehdel et al. [53] are once again framed within the aerospace and automotive sectors, which reinforces the trend followed by researchers in recent years.

González-Barrio et al. [21] clearly contextualize the application and future development of hybrid additive and subtractive manufacturing in the aerospace field, which they highlight with the part (blisk) and the material (Inconel 718) studied. In their review, Zhang et al. [54] clearly identify the interest shown by the automotive, biomedical, and aerospace sectors in the hybrid additive and subtractive manufacturing of titanium parts, driven by the complexity and high mechanical, geometric, and surface finish requirements of many of their components. Throughout the research of Liu et al. [55], the uses of 4D hybrid additive and subtractive manufacturing are not only projected onto the aerospace sector, with numerous examples and scenarios, or the biomedical sector, but a new field is also proposed: the restoration of cultural heritage. The work of Oyesola et al. [58] on a cost model for hybrid additive and subtractive manufacturing is clearly focused, throughout the entire article, on the aerospace sector, which is highlighted by the case study of the aerospace bracket. However, it should be noted that it is oriented toward the MRO business for aerospace companies, which in itself constitutes a sector of great economic, industrial, and technological importance. Sommer et al. [59] clearly define, and justify their definition based on the capabilities of hybrid additive and subtractive manufacturing to achieve intricate geometries with appropriate surface finishes and surface integrity, the injection mold sector—as fundamental in so many industries—as the main field of application.

Table 8. Sectors and applications. Five most cited articles.

Ref.	Q	Cit.	Year	Sector	Applications
[1]	Q1	128	2021	Aerospace industry	Aircraft engine parts Power system components
[2]	Q1	103	2021	The general manufacturing industry	Molds with CC channels Stamping dies with CC channels
[40]	Q1	57	2020	Aerospace industry	Titanium alloy parts
[44]	Q1	38	2021	Aerospace industry	Manufacturing of monolithic structures
[45]	Q1	38	2021	Aerospace industry	Manufacturing and repair of high added-value parts

presents the five most cited articles among those that clearly identify a sector or area of application and development for hybrid additive and subtractive manufacturing. Some specific applications are also included. A clear predominance of the aerospace sector is observed, with a focus on high added-value parts.

3.2.3. Challenges and Outlook

Parts made by additive manufacturing generally require further processing to achieve the characteristics specified in the design plans. Hybrid manufacturing is very useful as it reduces production time and costs as, in many cases, it avoids the need for post-treatment. Case studies can be found in industries such as aerospace, automotive and medical. With these new hybrid additive–subtractive manufacturing techniques, there is a broad consensus that parts with complex geometries are first produced by additive manufacturing and then the surface finish is improved to meet the specifications of each sector. Moreover, the process can be carried out through several repeated additive–subtractive sequences, so that the improvement in surface quality can reach areas that would be inaccessible once the part is finished [1,2,5,11,21,46,50,51,52,58].

Sommer et al. and Liu et al. provide detailed evidence of how hybrid processes improve surface quality and precision in metallic and ceramic contexts [49,59]; however, other studies support this advantage across different applications. Furthermore, Wu et al. highlight the role of intermediate machining in reducing distortion and enhancing surface uniformity [57], while Ghafoori et al. show that machining WAAM repairs reduces stress concentrators by refining deposited profiles [56]. González-Barrio et al. emphasize the value of adaptive toolpaths in correcting geometric deviations in LMD processes [21], and Ding et al. confirm that finishing improves surface integrity and removes thermal defects [54]. Moreover, Perez-Ruiz et al. [1] point out as the main challenge the development of predictive models for the cutting forces in the milling of parts manufactured by LPBF, given the nonlinearity of this process. The complexity of the interactions between the additive and subtractive subprocesses must be addressed, focusing on grain characteristics and crystallographic structure.

Hybrid manufacturing systems entail substantial initial investment costs, primarily due to the need for specialized equipment and highly skilled operators [21,47]. Nevertheless, opinions are divided regarding their overall economic viability. Oyesola et al. posit that the utilization of hybrid additive and subtractive manufacturing methodologies can yield cost-effectiveness in high-value applications, such as aerospace Maintenance, Repair, and Overhaul (MRO), particularly when evaluated through activity-based costing models that account for lifecycle benefits [58]. Conversely, alternative studies posit that, in the absence of such niche contexts, the elevated per-unit cost and diminished scalability render hybrid manufacturing less viable for small to medium production volumes [21,47]. Furthermore, Feng et al. [2] outline a path of work and evolution by highlighting that one of the main current challenges is the complexity and high cost of hybrid manufacturing machines. The future of the mold industry with conformal cooling channels is promising, but it requires the development of more economically affordable solutions. They also anticipate that the manufacturing of CC channels by hybrid additive and subtractive manufacturing will expand to gas turbine engines, gears, and other systems and parts with special self-cooling requirements.

Practical examples are evident across multiple sectors, in the aerospace sector for the production and repair of engine parts and aircraft structures, in the automotive industry, hybrid processes are applied to the manufacturing of engine components and suspension systems, and, in the medical field, for customized implants and prostheses [1,2,11,21,46]. However, current results are difficult to compare due to the lack of standardized testing and validation protocols. Some authors offer partial solutions, such as FEM-based predictive models validated experimentally [57], or design rules tailored to specific machines [59]. Others call for coordinated international efforts to define hybrid manufacturing-specific standards and benchmark systems [47].

Regarding the challenges of hybrid additive and subtractive manufacturing in the demanding context of the aerospace industry, there is a broad consensus in the literature that hybrid additive and subtractive manufacturing poses significant challenges in terms of process planning and automation. One of the most critical issues is the absence of commercially mature CAM/CAx tools capable of synergistically managing additive and subtractive stages in an integrated and automated workflow. Veiga et al. [40] make clear the importance of further analysis of milling strategies to optimize the process and emphasize that the digitalization of hybrid workflows is still at a critical stage, constrained by the large number of process variables and poor communication between machines and design software. Ingarao et al. [41] argue that beyond current automation limitations, future research should focus on leveraging hybrid manufacturing for geometric complexity and weight reduction through part re-design. They also point out the need to increase studies that combine environmental and economic sustainability and the development of decision-making tools to choose between processes. In turn, with the aim of being able to predict results, both in the production and design phases, to reduce material waste, Korkmaz et al. [3] point out the need to use simulation software. The possibility is also opened for the participation of artificial intelligence in hybrid manufacturing. The study concludes that, in the hybrid context, the difficulty of combining different additive technologies makes it necessary to develop hybrid processes that are independent of the additive manufacturing method. For their part, subtractive processes must evolve to avoid requiring special tools, aiming for greater simplicity and autonomy. Feldhausen et al. [42], based on their study on the properties of 316L stainless steel manufactured by hybrid additive and subtractive manufacturing, state that this will be applicable to the manufacturing of large and complex parts, among which nosecones and marine propulsors are mentioned. Thus, the marine sector appears as a field of application for hybrid additive and subtractive manufacturing. Likewise, Sunny et al. [44] present as future work the application of the simulation methodology used in their study on the effect of residual stress from DED manufacturing on the stresses and distortion generated by subsequent high-speed milling to new hybrid flows involving other additive technologies.

Based on the results of their study, Careri et al. [45] identify as both a challenge and a line of future work the necessity to develop processes that adapt to the needs of manufacturers. In this respect, Stavropoulos et al. [47] agree with the need to develop a complete software solution for the generation of hybrid additive and subtractive processes that would address economic and technical challenges. This solution would establish a solid foundation for the evaluation of hybrid manufacturing in relation to structural integrity and repair standards. On the other hand, Moritz et al. [48] develop a study in which they propose that in the future, further understanding of the parameters of cryogenic cooling and their influence on the hybrid additive and subtractive process will be necessary, in order to optimize it.

Looking ahead, Feldhausen et al. [5] indicate that closed-loop control systems, the increased capacity for surface characterization, and the continuous improvement of the machines involved will be key to the development of hybrid additive and subtractive manufacturing. This will require great research effort in what they call digital manufacturing. In this regard, Braun et al. [51], in their comparison of traditional, additive, and hybrid additive and subtractive manufacturing routes, with a focus on fatigue resistance, propose to further investigate in the future the evaluation methods for local fatigue. For this, they point out several options, such as crack growth studies or stress-based models, which can be combined with models that consider production-related defects. For their topology optimization (TO) model for the manufacturing of parts with sacrificial structures, Mirzendehdel et al. [53] propose as future developments the definition of a general fixturing constraint.

In this respect, González-Barrio et al. [21] highlight the need to deepen the understanding of the hybrid process in all its stages, as well as of each of the technologies and materials involved. They also indicate the need to develop CAD/CAE/CAM environments that cover the entire hybrid additive and subtractive manufacturing process, in this case for parts as complex as an 18-blade blisk, within a single software tool. Zhang et al. [54] point out that the most widely used technologies in hybrid additive and subtractive manufacturing for the production of near net shape parts have been PBF and DED. Regarding research, they conclude that these have also been the most present. They end by noting the lack of studies, and therefore the need to increase them, on WAAM manufacturing in the studied hybrid context. Sebbe et al. [11] point out that the risks of metal powder, with serious safety implications, the training of operators, waste management, and cutting fluids will be factors to study and improve in the future to make hybrid additive and subtractive manufacturing successful.

Table 9. Future works. Five most cited articles.

Ref.	Q	Cit.	Year	Future Works
[1]	Q1	128	2021	Development of predictive models for the cutting forces
[2]	Q1	103	2021	Development of affordable hybrid machines
[40]	Q1	57	2020	Improvement of milling strategies
[41]	Q1	54	2020	Development of process decision-making tools.
[3]	Q1	46	2022	Development of simulation software

lists the five most cited articles that clearly identify future work in the context of hybrid additive and subtractive manufacturing. It is worth noting that all the proposed research directions fall within the CAD/CAE/CAM framework, highlighting its great importance for the development of hybrid manufacturing.

Current research on hybrid manufacturing is heavily focused on metallic alloys, particularly titanium, Inconel, and stainless steels, while applications involving high-performance polymers, ceramics, or composite materials remain scarce [55,59]. However, opinions diverge on the feasibility of extending hybrid additive and substrative manufacturing to non-metallic materials, in their study to relate the parameters of polymer additive manufacturing and dry milling within a hybrid additive and subtractive manufacturing flow Guo et al. [49] propose the possibility of applying the method to new amorphous polymers and high-performance polymers. Liu et al. present encouraging results using hybrid approaches with shape-memory ceramics, demonstrating the potential of additive–subtractive integration in complex ceramic architectures [55]. For Liu et al., in the future the range of materials used in 4D hybrid additive and subtractive manufacturing should be expanded, such as UV/ozone. They also propose increasing research in the production of parts that combine hard and soft materials in the biomedical field. Conversely, other studies, such as that of Wu et al., argue that adapting hybrid additive and subtractive manufacturing to non-metal systems would require significant advances in deposition parameters, thermal processing, and fixturing technologies, which are still in early stages of development [57].

Finally, they highlight the need to improve the mechanical properties of the materials used. In the work of Ghafoori et al. [56], a very interesting and somewhat novel approach is proposed, as they state that single-sided deposition should be investigated—that is, the application of the process from only one side of the part—in order to avoid the accessibility problems that a real case of in situ repair may present.

4. Conclusions

This paper presents a systematic review of the emerging trends in hybrid additive–subtractive manufacturing that have emerged in the last 5 years. After an introductory review of the origin and evolution of hybrid additive and subtractive manufacturing, and establishing the concept of this as a new manufacturing paradigm, in which additive and subtractive manufacturing subprocesses are integrated to overcome limitations, leverage strengths, and generate new synergies, a systematic review was carried out, based on PRISMA (Preferred Reporting Items for Systematic reviews and Meta-Analyses), that allowed the selection of some experimental and review articles collected in specialized journals (classified as Q1 and Q2 within the ranking of their respective categories according to the Journal Citation Report), and peer-reviewed conference proceedings of recognized prestige, published in English, and open access. The selection was made from the SCI (Science Citation Index Expanded) and the CPCI-S (Conference Proceedings Citation Index–Science) databases of the Web of Science (WoS) Core Collection. After selecting the works, classification of the studies was carried out and, through detailed reading, the corresponding analysis and discussion of the findings was presented.

The main conclusions are presented below.

• The concept is generally established, having moved beyond the initial debates, as the lack of integration of CAM/CAx tools for several additive and subtractive manufacturing subprocesses, alternated with each other, interacting among them and within the same machine, to produce a different and superior result compared to what could be obtained through the mere combination of processes along a supply manufacturing chain.
• The most studied manufacturing processes, both from an experimental and theoretical point of view and through simulations, are DED (direct energy deposition) in the case of additive subprocesses and milling for the machining subprocesses integrated into hybrid additive and subtractive manufacturing flows.
• The leading sector for the application and development of hybrid additive and subtractive manufacturing throughout the majority of the reviewed articles is the aerospace sector, followed by the sector of stamping dies and injection molds with conformal cooling channels. The biomedical and automotive sectors are mentioned much less frequently.
• The most commonly used materials in both theoretical and practical or experimental developments are titanium and nickel alloys. To a lesser extent, some studies address stainless steels.
• A lack of studies on standardization and certification processes is confirmed, as well as on approval by various national and international public agencies—for example, in the aerospace sector—of parts for real application.
• From the analysis of the selected works, it is deduced that hybrid additive and subtractive manufacturing must mature, and for this, it is essential to significantly increase knowledge of the additive and subtractive subprocesses involved, both individually and in terms of their interaction and their application to materials such as the superalloys used in the aerospace field.
• Researchers must identify or define, in the near future, hybrid parameters that make sense within the new concept of hybrid additive and subtractive manufacturing, rather than merely combining and studying interactions between traditional parameters of the integrated additive and subtractive processes.
• The study of hybrid parameters is proposed as future work, such as the number of additive–subtractive interactions that occur in the hybrid process, analyzing their effect on the outcome of the manufacturing process.