Special Issue "Additive Manufacturing in the Industry"

A special issue of Applied Sciences (ISSN 2076-3417). This special issue belongs to the section "Applied Industrial Technologies".

Deadline for manuscript submissions: closed (15 November 2020).

Special Issue Editors

Special Issue Information

Dear Colleagues,

The advent of additive manufacturing (AM) processes applied to the fabrication of structural components has created the need for design methodologies and structural optimization approaches that take into account the specific characteristics of the fabrication process. While AM processes give unprecedented geometrical design freedom, which can result in significant reductions of the components’ weight (e.g., through part count reduction), on the other hand they have implications for the fatigue and fracture strength, because of residual stresses and microstructural features. This is due to stress concentration effects and anisotropy that still need research. Moreover, implications for the component behavior when considering the crash aspects need to be explored.


This Special Issue of Applied Sciences aims at gathering together papers investigating the different features of AM processes with relevance for their structural behavior, particularly, but not exclusively, from the viewpoints of fatigue, fracture and crash behavior. Although the focus of this Special Issue is on AM, articles dealing with other manufacturing processes with related analogies can also be included, in order to establish differences and possible similarities. The submission of papers on numerical simulation, experimental work, or a combination of both is welcome.

Prof. Dr. Roberto Citarella
Dr. Venanzio Giannella
Guest Editors

Manuscript Submission Information

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Published Papers (8 papers)

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Research

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Open AccessFeature PaperArticle
Free Vibration Analysis of a Thermally Loaded Porous Functionally Graded Rotor–Bearing System
Appl. Sci. 2020, 10(22), 8197; https://doi.org/10.3390/app10228197 - 19 Nov 2020
Abstract
The present work deals with natural and whirl frequency analysis of a porous functionally graded (FG) rotor–bearing system using the finite element method (FEM). Stiffness, mass and gyroscopic matrices are derived for porous and non-porous FG shafts by developing a novel two-noded porous [...] Read more.
The present work deals with natural and whirl frequency analysis of a porous functionally graded (FG) rotor–bearing system using the finite element method (FEM). Stiffness, mass and gyroscopic matrices are derived for porous and non-porous FG shafts by developing a novel two-noded porous FG shaft element using Timoshenko beam theory (TBT), considering the effects of translational inertia, rotatory inertia, gyroscopic moments and shear deformation. A functionally graded shaft whose inner core is comprised of stainless steel (SS) and an outer layer made of ceramic (ZrO2) is considered. The effects of porosity on the volume fractions and the material properties are modelled using a porosity index. The non-linear temperature distribution (NLTD) method based on the Fourier law of heat conduction is used for the temperature distribution in the radial direction. The natural and whirl frequencies of the porous and non-porous FG rotor systems have been computed for different power law indices, volume fractions of porosity and thermal gradients to investigate the influence of porosity on fundamental frequencies. It has been found that the power law index, volume fraction of porosity and thermal gradient have a significant influence on the natural and whirl frequencies of the FG rotor–bearing system. Full article
(This article belongs to the Special Issue Additive Manufacturing in the Industry)
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Open AccessArticle
Fatigue Cracking of Additively Manufactured Materials—Process and Material Perspectives
Appl. Sci. 2020, 10(16), 5556; https://doi.org/10.3390/app10165556 - 11 Aug 2020
Abstract
Strong efforts are made internationally to optimize the process control of laser additive manufacturing processes. For this purpose, advanced detectors and monitoring software are being developed to control the quality of production. However, commercial suppliers of metal powders and part manufacturers are essentially [...] Read more.
Strong efforts are made internationally to optimize the process control of laser additive manufacturing processes. For this purpose, advanced detectors and monitoring software are being developed to control the quality of production. However, commercial suppliers of metal powders and part manufacturers are essentially focused on well-established materials. This article demonstrates the potential of optimized process control. Furthermore, we outline the development of a new high temperature structural steel, tailored to best utilize the advantages of additive manufacturing techniques. In this context, the impact of production-induced porosity on fatigue strength of austenitic 316L is presented. Additionally, we discuss the first conceptual results of a novel ferritic steel, named HiperFer (High Performance Ferrite), which was designed for increased fatigue strength. This ferritic, Laves phase-strengthened, stainless steel could be used for a wide range of structural components in power and (petro)chemical engineering at maximum temperatures ranging from about 580 to 650 °C. This material benefits from in situ heat treatment and counteracts process-related defects by “reactive” crack obstruction mechanisms, hampering both crack initiation and crack propagation. In this way, increased fatigue resistance and safety can be achieved. Full article
(This article belongs to the Special Issue Additive Manufacturing in the Industry)
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Open AccessArticle
An Innovative Approach on Directed Energy Deposition Optimization: A Study of the Process Environment’s Influence on the Quality of Ti-6Al-4V Samples
Appl. Sci. 2020, 10(12), 4212; https://doi.org/10.3390/app10124212 - 19 Jun 2020
Abstract
Blown powder additive manufacturing technologies are not restricted to the use of a process chamber. This feature allows to build larger components with respect to conventional powder bed processes. This peculiarity is mostly promising for manufacturing large components or repairing/rebuilding parts of large [...] Read more.
Blown powder additive manufacturing technologies are not restricted to the use of a process chamber. This feature allows to build larger components with respect to conventional powder bed processes. This peculiarity is mostly promising for manufacturing large components or repairing/rebuilding parts of large systems. The main downside of using an open environment, even if a protective shielding gas system is adopted, is the lack of control of process atmosphere. This is particularly critical for titanium alloys which are very sensitive to oxygen/nitrogen pick-up; they have a detrimental effect on ductility, by causing embrittlement and possibly leading to the formation of cracks. It is then important to address how environmental factors, such as process atmosphere and platform temperature, impact not only on the processability but also on the final component properties, both from a compositional and mechanical point of view. The correlations between these environmental factors and microstructure, interstitials content, grain size, and hardness were investigated. Moreover, the Hall–Petch equation was then adopted to additive manufacturing microstructures, characterized by a columnar grain morphology, and used to further investigate the relationship intercurring between grains and hardness and how different microstructures might influence this correlation. Full article
(This article belongs to the Special Issue Additive Manufacturing in the Industry)
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Open AccessArticle
Probabilistic Analysis of Fatigue Behavior of Single Lap Riveted Joints
Appl. Sci. 2020, 10(10), 3379; https://doi.org/10.3390/app10103379 - 13 May 2020
Cited by 3
Abstract
This research deals with the fatigue behavior of 200 small single lap multiple-riveted joint specimens, widely used for aeronautic structures. The tests were performed with three different levels of stress with stress ratio R = 0.05; three levels were set: 90 MPa, 120 [...] Read more.
This research deals with the fatigue behavior of 200 small single lap multiple-riveted joint specimens, widely used for aeronautic structures. The tests were performed with three different levels of stress with stress ratio R = 0.05; three levels were set: 90 MPa, 120 MPa and 160 MPa. The fatigue life and critical crack size for all tested specimens were analyzed. According to the results’ analysis, two types of fracture, through-hole and in proximity of the hole, were observed, depending on the level of stress: the higher the applied stress, the more through-hole cracking. Indeed, under the fatigue load with a stress level of 90 MPa, less than 30% of specimens showed cracks propagating through the hole, while, at the stress level of 120 MPa, the percentage reaches 36.3%. At the stress level of 160 MPa, 100% of specimens failed through the hole. Moreover, aimed to use experimental data for probabilistic methods, a statistical analysis was performed according to the Anderson–Darling test. This method allowed the analysis of the datasets, in terms of both fatigue life and critical crack size, providing information about the best distribution function able to fit experimental results. Full article
(This article belongs to the Special Issue Additive Manufacturing in the Industry)
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Open AccessArticle
A Comparative Study of the CMT+P Process on 316L Stainless Steel Additive Manufacturing
Appl. Sci. 2020, 10(9), 3284; https://doi.org/10.3390/app10093284 - 08 May 2020
Cited by 1
Abstract
Adopting the cold metal transfer plus pulse (CMT + P) process, 316L stainless steel wire was treated with a single channel multi-layer deposition experiment under different linear energy. The microstructures of different regions on the deposited samples were observed by optical microscope and [...] Read more.
Adopting the cold metal transfer plus pulse (CMT + P) process, 316L stainless steel wire was treated with a single channel multi-layer deposition experiment under different linear energy. The microstructures of different regions on the deposited samples were observed by optical microscope and scanning electron microscope, and the element distribution in the structure was analyzed by energy dispersive spectrometer. The mechanical properties and microhardness were measured by tensile test method and microhardness tester, respectively, and the anisotropy of tensile strength in horizontal and vertical directions were calculated. Finally, the fracture morphology of the tensile samples were observed by SEM. Experiment results showed that when the difference between the actual and the optimal wire feeding speed matching the specific welding speed was too large, this led to an unstable deposition process as well as flow and collapse of weld bead metal, thus seriously deteriorating the appearance of the deposition samples. The results from metallographic micrograph showed that rapid heat dissipation of the substrate caused small grains to generate in the bottom region of deposition samples, then gradually grew up to coarse dendrites along the building direction in the middle and top region caused by the continuous heat accumulation during deposition. Tensile test results showed that with the increase of linear energy, the horizontal and vertical tensile strength of the as-deposited samples decreased. In addition, the higher linear energy would deteriorate the microstructure of as-deposited parts, including significantly increasing the tendency of oxidation and material stripping. The microhardness values of the bottom, middle and top regions of the samples fluctuated along the centerline of the cross-section, and the values showed a trend of decreasing first and then rising along the building direction. Meanwhile, the yield strength and tensile strength of each specimen showed obvious anisotropy due to unique grain growth morphology. On the whole, the results from this study prove that CMT+P process is a feasible MIG welding additive manufacturing method for 316L stainless steel. Full article
(This article belongs to the Special Issue Additive Manufacturing in the Industry)
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Open AccessArticle
Influences of Material Variations of Functionally Graded Pipe on the Bree Diagram
Appl. Sci. 2020, 10(8), 2936; https://doi.org/10.3390/app10082936 - 23 Apr 2020
Cited by 1
Abstract
The present research is concerned with the elastic–plastic responses of functionally graded material (FGM) pipe, undergoing two types of loading conditions. For the first case, the FGM is subjected to sustained internal pressure combined with a cyclic bending moment whereas, in the second [...] Read more.
The present research is concerned with the elastic–plastic responses of functionally graded material (FGM) pipe, undergoing two types of loading conditions. For the first case, the FGM is subjected to sustained internal pressure combined with a cyclic bending moment whereas, in the second case, sustained internal pressure is applied simultaneously with a cyclic through-thickness temperature gradient. The properties of the studied FGM are considered to be variable through shell thickness according to a power-law function. Two different designs of the FGM pipe are adopted in the present research, where the inner surface in one case and the outer surface in the other are made from pure 1026 carbon steel. The constitutive relations are developed based on the Chaboche nonlinear kinematic hardening model, classical normality rule and von Mises yield function. The backward Euler alongside the return mapping algorithm (RMA) is employed to perform the numerical simulation. The results of the proposed integration procedure were implemented in ABAQUS using a UMAT user subroutine and validated by a comparison between experiments and finite element (FE) simulation. Various cyclic responses of the two prescribed models of FGM pipe for the two considered loading conditions are classified and brought together in one diagram known as Bree’s diagram. Full article
(This article belongs to the Special Issue Additive Manufacturing in the Industry)
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Open AccessArticle
Effect of Heat Input on Formability, Microstructure, and Properties of Al–7Si–0.6Mg Alloys Deposited by CMT-WAAM Process
Appl. Sci. 2020, 10(1), 70; https://doi.org/10.3390/app10010070 - 20 Dec 2019
Cited by 4
Abstract
In order to improve the forming efficiency of Al–7Si–0.6Mg fabricated by wire and arc additive manufacturing process (WAAM), wire with a diameter of 1.6 mm was selected as the raw material. The effect of heat input on the formability, microstructure, and properties of [...] Read more.
In order to improve the forming efficiency of Al–7Si–0.6Mg fabricated by wire and arc additive manufacturing process (WAAM), wire with a diameter of 1.6 mm was selected as the raw material. The effect of heat input on the formability, microstructure, and properties of the WAAM alloy was investigated, and the forming model was established. The WAAM alloys were characterized by electronic universal testing, scanning electron microscopy, energy spectrum analysis, and metallographic microscopy. The results show that Al–7Si–0.6Mg alloy has a large processing window under the cold metal transfer (CMT) process, and it can be well formed with a large range of heat input. The secondary dendrite arm spacing and Fe-phase in the as-deposited alloy gradually increase with an increase in heat input, and slight overburning occurs in the heat affected zone at higher heat inputs. After solid solution and aging treatment (T6 heat treatment), the size of α-Al grain and eutectic silicon grain increases with the increase of heat input. Little anisotropy in the mechanical properties is observed except at higher heat inputs. The tensile strength is 354.5 MPa ± 7.5 MPa, yield strength is 310 MPa ± 5.5 MPa, and elongation is 6.3 ± 0.7%. Full article
(This article belongs to the Special Issue Additive Manufacturing in the Industry)
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Review

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Open AccessReview
Wire Arc Additive Manufacturing of Stainless Steels: A Review
Appl. Sci. 2020, 10(5), 1563; https://doi.org/10.3390/app10051563 - 25 Feb 2020
Cited by 6
Abstract
Wire arc additive manufacturing (WAAM) has been considered as a promising technology for the production of large metallic structures with high deposition rates and low cost. Stainless steels are widely applied due to good mechanical properties and excellent corrosion resistance. This paper reviews [...] Read more.
Wire arc additive manufacturing (WAAM) has been considered as a promising technology for the production of large metallic structures with high deposition rates and low cost. Stainless steels are widely applied due to good mechanical properties and excellent corrosion resistance. This paper reviews the current status of stainless steel WAAM, covering the microstructure, mechanical properties, and defects related to different stainless steels and process parameters. Residual stress and distortion of the WAAM manufactured components are discussed. Specific WAAM techniques, material compositions, process parameters, shielding gas composition, post heat treatments, microstructure, and defects can significantly influence the mechanical properties of WAAM stainless steels. To achieve high quality WAAM stainless steel parts, there is still a strong need to further study the underlying physical metallurgy mechanisms of the WAAM process and post heat treatments to optimize the WAAM and heat treatment parameters and thus control the microstructure. WAAM samples often show considerable anisotropy both in microstructure and mechanical properties. The new in-situ rolling + WAAM process is very effective in reducing the anisotropy, which also can reduce the residual stress and distortion. For future industrial applications, fatigue properties, and corrosion behaviors of WAAMed stainless steels need to be deeply studied in the future. Additionally, further efforts should be made to improve the WAAM process to achieve faster deposition rates and better-quality control. Full article
(This article belongs to the Special Issue Additive Manufacturing in the Industry)
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