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Article

Experimental Investigations on Microstructure and Mechanical Properties of L-Shaped Structure Fabricated by WAAM Process of NiTi SMA

by
Vatsal Vaghasia
1,
Rakesh Chaudhari
1,
Sakshum Khanna
2,
Jash Modi
1 and
Jay Vora
1,*
1
Department of Mechanical Engineering, School of Technology, Pandit Deendayal Energy University, Gandhinagar 382007, India
2
Relx Pvt. Ltd., Delhi 122002, India
*
Author to whom correspondence should be addressed.
J. Manuf. Mater. Process. 2025, 9(7), 239; https://doi.org/10.3390/jmmp9070239
Submission received: 12 June 2025 / Revised: 10 July 2025 / Accepted: 11 July 2025 / Published: 11 July 2025
(This article belongs to the Special Issue Advanced Welding Processes, Additive Manufacturing and Numerical Models: 2nd Edition)
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Abstract

In the present study, an L-shaped multi-walled structure of NiTi shape memory alloy (SMA) was fabricated by using the wire arc additive manufacturing (WAAM) method on a titanium substrate. The present study aims to investigate the fabricated structure for microstructure, macrostructure, and mechanical properties. The 40 layers of L-shaped structure were successfully fabricated at optimized parameters of wire feed speed at 6 m/min, travel speed at 12 mm/s, and voltage at 20 V. The macrographs demonstrated the continuous bonding among the layers with complete fusion. The microstructure in the area between the two middle layers has exhibited a mixture of columnar grains (both coarse and fine), interspersed with dendritic colonies. The microstructure in the topmost layers has exhibited finer colonial structures in relatively greater numbers. The microhardness (MH) test has shown the average values of 283.2 ± 3.67 HV and 371.1 ± 5.81 HV at the bottom and topmost layers, respectively. A tensile test was conducted for specimens extracted from deposition and build directions, which showed consistent mechanical behavior. For the deposition direction, the average ultimate tensile strength (UTS) and elongation (EL) were obtained as 831 ± 22.91 MPa and 14.32 ± 0.55%, respectively, while the build direction has shown average UTS and EL values of 774 ± 6.56 MPa and 14.16 ± 0.21%, respectively. The elongation exceeding 10% in all samples suggests that the fabricated structure demonstrates properties comparable to those of wrought metal. Fractography of all tensile specimens has shown good ductility and toughness. Lastly, a differential scanning calorimetry test was carried out to assess the retention of shape memory effect for the fabricated structure. The authors believe that the findings of this work will be valuable for various industrial applications.

1. Introduction

Smart materials and their excellent mechanical characteristics, like super elasticity and high corrosion resistance, enable them to be implemented in various applications like aerospace, biomedical, automotive, pressure vessels, marine, etc. [1,2]. Shape memory alloys (SMAs) regain their original form when heated at a particular temperature. [3,4]. Shape memory alloys came into commercial usage when their unique characteristic of shape memory effect was observed in 1962 in a Ni-Ti alloy [5,6]. Due to fewer technological advancements, the basic knowledge of Nitinol, like its complex properties and crystal structure, was not understood until the 1980s [7,8]. These smart alloys have since been implemented in many engineering disciplines. Most of the applications of nitinol are in the medical field for its excellent biocompatibility and high wear and corrosion resistance [9,10,11]. Other applications include the defence sector and aerospace [12]. New applications in biomedical are orthodontics and endosurgical equipment, like stents and bone implants [13,14]. Nitinol has an almost equal composition of nickel and titanium. Traditional machining of nitinol is challenging owing to their larger strength, the requirement of larger machining force, strain hardening, and higher tool wear rate [15,16,17]. Traditional processes also possess difficulties owing to their poor surface quality, burr formations, lower machining efficiency, and decremented shape memory effect [18,19]. Therefore, additive manufacturing (AM) is the best approach for creating the intricate geometries of nitinol components [20,21,22].
Additive manufacturing (AM) is a technique of building objects by adding material layer-by-layer according to the digital 3D model, unlike the conventional subtractive manufacturing methods, which shape the final product by removing the material [23]. The AM process is suitable for applications where the production volume is less, designs are complex, or frequent updates in the design are required [24,25]. It enables the manufacture of complex parts that cannot be made by traditional manufacturing methods [26,27]. AM uses a variety of materials, including both polymers and metals, to create objects. Among the metal-based AM methods, direct energy deposition (DED) has advantages over other metal-based methods [28,29]. The DED process has certain advantages compared to other methods, like significantly higher deposition rates, complete fusion between successive layers, and it is relatively economical [30,31,32]. The DED process utilizes two types of heat sources to melt the materials, which are a laser-based heat source and an electric arc-based heat source [33]. The electric arc-based DED process is also recognized as a wire arc additive manufacturing (WAAM) process. The WAAM is furthermore classified into three categories that are gas metal arc welding (GMAW)-based, gas tungsten arc welding-based, and plasma arc welding-based methods [34]. The rate of deposition achieved by GMAW is approximately 2–3 times greater than the other arc-based methods [35,36]. Because of these advantages, GMAW-based WAAM is well-suited for integration into automated manufacturing systems [37].
In the GMAW process, an electric arc forms between a consumable wire electrode and the workpiece metal, generating heat. The consumable wire electrode is continuously fed from a spool to the arc zone. To shield the molten weld pool from oxidation and contaminants in the atmosphere, carbon dioxide or argon gas is used. The automated GMAW-based WAAM (GMAWAM) process involves several critical parameters, such as voltage (V), travel speed (TS), wire feed speed (WFS), gas flow rate (GFR), gas mixture ratio (GMR), dwell time, and stand-off distance. Selecting these parameters carefully is essential for achieving good weld quality and the desired bead morphology. Thus, careful planning and process optimization are vital to fine-tune these variables, minimizing issues like porosity, spatter, and incomplete fusion, and ultimately enhancing welding efficiency.
Resnina et al. [38] deposited a five-layer NiTi alloy on a titanium substrate using the GMAWAM method with Ni50.9Ti49.1 wire. During the deposition of the first layer, the remelting of the titanium substrate increased Ti concentration to the extent that resulted in the formation of a dendritic Ti-rich NiTi matrix and Ti2Ni precipitates. Layers 2 to 5 exhibited columnar grains internally and near the surface, equiaxed grains. The Ni concentration in the dendritic matrix increased from 48.5 ± 0.25% (at the first layer) to 50.9 ± 0.25% (fourth layer). Therefore, the B2↔B19′ martensitic transformation start temperature ranged from 73 °C (first layer) to 16 °C (fifth layer). During the tensile testing, the inelastic strain in the third layer was initiated at 25–30 MPa, martensite reorientation in the second layer started at 145 MPa, and stress-induced transformation in the fourth layer started at 160–170 MPa. The shape memory effect on heating confirmed the recoverable mechanism in the WAAM-fabricated NiTi structure. Gaofeng et al. [39] analyzed mechanical and wear performance, and microstructural characterizations of NiTi structures using cold metal transfer (CMT)-based WAAM with optimized depositing parameters. With the optimized parameters of torch TS 30 cm/min and WFS of 6–7 m/min, the process was improved in terms of wettability between the substrate, refined microstructure, and uniformity in the properties. It was found to have transitioned from coarse columnar grains at the base to an equiaxed microstructure at the top. The mechanical properties were improved correspondingly with the wall height. The microhardness increased from 256 to 278 HV, the critical stress increased from 483 MPa to 643 MPa, and elongation increased from 6.39% to 6.75%. The improved microstructural homogeneity and super elasticity of NiTi alloys result from these enhancements, which demonstrate that the WAAM is a viable method for producing high-performance NiTi components with uniform properties. Ajit et al. [40] experimented to fabricate a Belleville spring using WAAM with Nitinol (NiTi) super-elastic (SE) and shape-memory (SM) wires. They studied and correlated the chemical composition, microstructure, martensitic transformation, and mechanical properties. SE samples had finer microstructure, higher hardness and compressive strength, but lower tensile strength than SM samples. Singh et al. [41] explored the fabrication of thin SMA structures for MEMS applications using a hybrid approach combining WAAM with laser marking pre-treatment. Melt pool instability and residual stresses are challenges to traditional methods of producing thin and customized SMA components. Wang et al. [42] investigated the impact of varying deposition current (80 to 120 A) during in situ alloying WAAM on the mechanical and microstructural properties of Ni-rich NiTi alloys. The B2 grains coarsen, the high-angle grain boundary fraction increases, and the texture intensity decreases as the deposition current increases. At higher currents, Ni4Ti3 precipitates enlarge and partially transform into Ni3Ti, while the dominant B2 phase remains present. The phase transformation temperatures increase with current and tensile strength, and ductility decreases due to changes in grain size, precipitate behavior, and defect formation. The best shape recovery (3.2% recoverable strain, 53.9% recovery ratio) is obtained from cyclic tests at 80 A. Deposition current is identified as a key parameter in the optimization of structural and functional properties of WAAM processed NiTi components. Song et al. [43] fabricated high-strength low-alloy (HSLA) steel components using the GMAW-based WAAM process. They fabricated a 26-layer thin wall structure and analyzed the microstructural and mechanical characteristics. The microstructure is mainly acicular ferrite with dendritic growth along the deposition direction. Results indicate that WAAM can produce HSLA steel parts with desirable mechanical properties, which is an alternative to conventional manufacturing methods. Van Thao et al. [44] experimented with the fabrication of thin-walled SS-308L components to investigate the microstructure and tensile properties. The WAAM process parameters kept were welding current, voltage, and travel speed, which were optimized using RSM to achieve the desired weld bead geometry with minimal heat input. Stable and defect-free walls were deposited with a deposition efficiency of 91% using optimal parameters of current 122 A, voltage 20 V, and TS 368 mm/min. The microstructure was columnar austenite dendrites and residual ferrite with an average microhardness of 163 ± 5.36 HV. The tensile properties showed values of YS, UTS, and EL of 343.67 ± 7.53 MPa, 531.78 ± 4.52 MPa, and 39.58 ± 1.38%, respectively, along the building direction. In the deposition direction, the YS, UTS, and EL are 352.69 ± 8.12 MPa, 552.95 ± 4.96 MPa, and 54.13 ± 1.29%, respectively. The value of these mechanical properties is comparable to those of wrought 308L stainless steel, indicating the suitability of WAAM-fabricated walls for industrial applications.
The studied literature has shown the capability of the WAAM process for the fabrication of NiTi SMA components. However, experimental investigations on the fabrication with consistent quality of the L-shaped structure of NiTi SMA that exhibits both favorable microstructure and mechanical properties have not yet been conducted. The favorable microstructure and mechanical properties for NiTi SMA refer to defect-free layer bonding, refined grain structure, and mechanical properties, such as high tensile strength, good ductility, and retention of shape memory effect. The optimal process parameters were utilized to fabricate an L-shaped structure using NiTi SMA wire on a titanium substrate. The fabricated structure was then investigated for microstructure, macrostructure, and mechanical properties. The differential scanning calorimetry (DSC) test was carried out to assess the retention of the shape memory effect for the fabricated structure. The authors believe that the findings of this work will be valuable for various industrial applications.

2. Materials and Methodology

2.1. Experimental Plan

In the present study, NiTi SMA wire, having a 1.2 mm diameter, was utilized as a filler material on the titanium substrate plate using the GMAW-based WAAM process. NiTi wire, having 55.8% of Ni, and the remainder of Ti, was utilized to fabricate the L-shaped structure. The substrate plate was selected as pure titanium because its chemical composition is compatible with Nitinol, and this compatibility also makes it easier to remove the deposited structure from the substrate plate. Before the deposition process, the substrate plate was thoroughly cleaned and dried to remove any contamination. The experimental setup consists of an automatic GMAW power source, GMAW torch, and wire feeder, argon shielding gas cylinders, computer controller, and a specialized 3D printer. The experiment was performed using a standard automatic GMAW power source (make: Miller Continuum 350) with a machine build volume of 220 mm × 220 mm × 500 mm. The complete experimental setup is shown in Figure 1. The GMAW torch is mounted on the machine’s X-axis. The GMAW torch can move in the X, Y, and Z axes to deposit the wire material on the substrate. A self-operating wire feeder continuously supplies the filler wire electrode through the torch tip, and it is melted via arc heat. A computer program of 40 layers is fed to the machine controller. The controller, having a computer interface, was used to control nozzle movement and to navigate the torch during the deposition. Argon shielding gas is used to shield the molten pool from the atmospheric gas via the torch itself. The shielding gas is supplied before the start of the program to avoid any interaction between the deposited material and ambient gases.
In our recently reported study, optimized process parameters were obtained by using the heat transfer search (HTS) algorithm [45]. In that recently published study, we systematically investigated the effects of three critical WAAM process parameters, WFS, TS, and V, on bead morphology, specifically bead height (BH) and bead width (BW), during single-layer NiTi wire deposition on a titanium substrate. To optimize these process parameters, Box-Behnken Design (BBD) was used to design an experimental matrix, ANOVA was used for statistical significance, and Heat Transfer Search (HTS) algorithm was used for simultaneous optimization. The aim of optimization was to minimize BW (for better precision and dimensional control), and maximize bead height (to build taller structures with fewer layers). Table 1 shows the optimal process parameters obtained from our past study. In the present study, the set of optimized parameters were reused for the fabrication of a more complex L-shaped 40-layer structure, ensuring a reliable baseline from which to assess geometry-specific effects for microstructure and mechanical characterizations. In addition to previous process conditions, the substrate plate of titanium was pre-heated, and an additional dwell time of 45 s was kept among the successive layers of depositions to minimize deposited layers to prevent the development of excessive residual strains and deformations. The built L-shaped structure was then examined for microstructure, macrostructure, mechanical properties, and fracture surface morphologies.
The study reported by Singh et al. [41] and Wang et al. [42] discussed the difficulties associated while fabricating the complex structure, such as non-uniform thermal distribution, increased risk of thermal distortion at the corner joint, lack of fusion/porosity, and inconsistent bead geometry. To overcome these challenges, we have adopted several specific conditions in our current study for the controlled fabrication of the L-shaped structure. Firstly, the substrate plate of titanium was pre-heated for the reduction in thermal gradient among the deposited structure and base to minimize the residual stresses. To allow adequate cooling, and prevention of distortion, a dwell time of 45 s was kept between the successive layers of the structure. We have used optimized process conditions derived in our past study using the HTS algorithm to ensure consistent bead geometry. For uniform deposition, and thermal distribution, the filler wire was rotated 180 degrees after each layer deposition. These mentioned strategies adopted in the current study have ensured defect-free deposition of the structure with strong metallurgical bonding, even in the complex L-shaped structure.

2.2. Testing and Characterization

Initially, the fabricated specimen shown in Figure 2 was removed from the substrate plate using a wire-cut EDM machine. The L-shaped structure was then cut in half using a heavy-duty hydraulic band saw and divided into two segments. Then both sections were grounded and cleaned thoroughly to remove surface irregularities. The corner joint was then cut from the end of either plate for microstructural and macrostructural examination to assess layer bonding, cracks, and other defects.
The tensile, microhardness, macroscopy, and microscopy samples were prepared using a wire-cut EDM machine. Tensile testing was performed as per ASTM E8M standard [46] using a universal testing machine, while microhardness measurements were conducted in accordance with ASTM E384-17 [47] using a load of 150 gf and 15 s dwell time. These samples are cut as shown in Figure 2. These investigations offer a complete evaluation of the mechanical and microstructural properties of the joint and the fabricated structure. The weld bead profile, bonding integrity, grain morphology, and the presence of any defects were analyzed in macro- and microstructural evaluations using an optical microscope. Macro- and microstructural analysis was performed on the cross-sectional samples extracted from various locations, i.e., top, middle, and bottom of the multi-layered structure, including corner regions. The sample surfaces were polished progressively with abrasive papers of different grit sizes (240, 400, 600, 800, 1000, 1200, and 2000) and then fine polished with a 0.5-micron alumina slurry on a velvet cloth to achieve a mirror finish. The specimens were cleaned, polished, and etched by using the solution of HF + 4HNO3 + 5H2O.
To examine the mechanical properties of the structure, tensile tests were conducted to measure strength and ductility, and microhardness for examination of hardness. The three samples were cut from plate 1 (x-axis) along the deposition direction, and another three samples from plate 2 (y-axis) along the build direction to prepare tensile specimens. According to ASTM E8M standards, entire test samples were prepared by using a WEDM machine [46]. The M100 universal testing machine was used to perform tensile testing at ambient temperature. The cross-head travel speed was kept at 10 mm/min. Microhardness of the multi-layered structure was measured at both the corner joint and the build plate along the built direction by following ASTM E384-17 [47]. The evaluation was conducted at three different positions at the top, middle, and bottom on both the built plate and the corner joint. A Vickers microhardness testing machine (Manufacturer: Metkorp Equipments Pvt. Ltd.) was used with a diamond indenter and a load of 150 gf for a dwell time of 10 s.
The shape memory effect (SME) of built specimens was examined by using a DSC test. A DSC test was used to determine the phase transformation temperatures like austenite start temperature (As), martensite start temperature (Ms), austenite finish temperature (Af), and martensite finish temperature (Mf). These four transformation temperatures were compared for the untreated NiTi wire and the built specimen from WAAM. A DSC setup was employed to investigate the phase transformation behavior of the samples, using a heating/cooling rate of 10 °C/min under a constant nitrogen flow.

3. Results and Discussion

3.1. Fabrication of L-Structure

A multi-layer L-shaped structure with 40 layers was successfully fabricated as shown in Figure 3 using the optimized process parametric setting of voltage of 20 V, TS of 12 mm/s, and WFS of 6 m/min using the GMAW-based WAAM, achieving a total wall height of 100 mm. A dwell time of 45 s was maintained between two successive layers to ensure sufficient cooling, reduce residual stresses, and prevent deformation. To improve dimensional accuracy, the filler wire was rotated 180 degrees during each layer deposition. Identical layer-on-layer deposition can be seen for the wall structure. In between the layer-on-layer deposition, seamless fusion was observed. The deposited L-shaped structure exhibits strong metallurgical bonding without any sign of disbonding. Although minor lumps of molten Nitinol wire were observed along the edges, they were completely removed during the post-processing. The experimental results validate that these optimized parameters successfully produced an L-structure made of NiTi SMA material.

3.2. Macrostructure and Microstructure

Macro- and microstructural analysis was performed on the cross-sectional samples extracted from various locations, i.e., top, middle, and bottom of the multi-layered structure of the corner region. The sample surfaces were polished progressively with abrasive papers of different grit sizes (240, 400, 600, 800, 1000, 1200, and 2000) and then fine polished with a 0.5-micron alumina slurry on a velvet cloth to achieve a mirror finish. The etchant solution of HF + 4HNO3 + 5H2O was used for the macro- and microstructural specimens. Figure 4 shows the macrostructure of the corner joint from the bottom to the top regions. The macrographs clearly show the absence of pores and cracks on the NiTi deposited structure joint. Also, the bonding among the layers was observed to be continuous with complete fusion. This shows that the parametric settings, along with other manufacturing conditions, adopted for the fabrication of the L-shaped structure were adequate and suitable.
Microstructure images were observed at two prominent locations where a subsequent change in microphases was expected. Figure 5a shows the microstructure of the area between the two middle layers, and Figure 5b shows the microimage of the topmost layer (i.e., last layer deposited). As shown in Figure 5a, the region between two successive layers, referred to as the interface region, along with the bottom of the upper layer and the top of the lower layer, is highlighted. The microstructure in this area exhibits a mixture of columnar grains (both coarse and fine) interspersed with dendritic colonies. These colonies appear in isolated patches, commonly referred to as “islands,” and resemble martensitic colonies. However, due to repeated thermal cycles during the layer-by-layer metal deposition process, these regions experience extended solidification times. As a result, the supersaturated carbon in austenite (initially forming martensite) gradually transforms into bainite. In the interface region, additional heat input from the subsequent layer further alters the structure, leading to the formation of coarse, rounded features characteristic of that zone [48,49]. Figure 5b shows the microstructure of the top surface of the final deposited layer. It can be observed that finer colonial structures are present in relatively greater numbers. This is because the top layer solidifies in a comparatively cooler environment, with no subsequent layers deposited afterward [50,51]. However, at the time of deposition, the underlying layer was still warm, resulting in moderate cooling rates. This thermal condition leads to the formation of a mixed microstructure consisting of fine columnar grains (as indicated by arrows) along with coarser regions, though the finer colonies appear slightly more abundant. Similar microhardness trends have been reported in previous studies by Zeng et al. [52], and Lin et al. [53]. The similar trend can be confirmed while observing the hardness values, which increase while measuring from bottom to top owing to an increase in the finer microconstituents yielding higher hardness.

3.3. Mechanical Characterizations

3.3.1. Microhardness

The mechanical characterization was investigated with MH testing for the L-shaped structure in the corner joint in all of the regions from the bottom to the top sections. A Vickers microhardness testing machine (Manufacturer: Metkorp Equipments Pvt. Ltd.) was used with a diamond indenter and a load of 150 gf for a dwell time of 10 s. A total of 10 sections were identified to measure the MH values from bottom to top, as shown in Figure 6. Three readings were taken at each location for more accurate and reliable results. The MH value of 283.2 ± 3.67 HV and 371.1 ± 5.81 HV was obtained at the bottom-most layer and top-most layers, respectively, for the built structure. Figure 7 shows the variations in MH values for all ten readings taken across the built structure. The obtained findings have demonstrated that the bottom section has shown lower MH values compared to their subsequent top sections. This is due to it being the first layer deposited directly onto the cold substrate plate, along with the influence of the resulting HAZ [54,55]. Moreover, the values of MH were determined by the results of the microstructure described in segment 3.2. The deposited top layers exhibited finer grain structure as compared to the coarse grains obtained in the bottom layers. Similar microhardness trends have been reported in previous studies [56,57]. However, the overall MH values have clearly shown a consistent and identical summary for the whole built wall, depicting the overall mechanical stability.

3.3.2. Tensile Test

To examine the mechanical properties of the structure, tensile tests were also conducted to measure the strength and ductility of the specimens. The three samples were cut from the X-axis plate along the deposition direction, and another three samples from the Y-axis plate along the build direction to prepare tensile specimens. According to ASTM E8M standards, entire test samples were prepared by using a WEDM machine. The M100 universal testing machine was used to perform tensile testing at ambient temperature. The cross-head travel speed was kept at 10 mm/min. The results of the tensile test obtained from the X-axis plate along the deposition direction and the Y-axis plate along the build direction have been depicted in Table 2. For the deposition direction, the average UTS and EL were obtained as 831 ± 22.91 MPa and 14.32 ± 0.55%, respectively, while the build direction has shown average UTS and EL values of 774 ± 6.56 MPa and 14.16 ± 0.21%, respectively. A graphical representation of these findings is shown in Figure 8. The Hall–Petch relationship confirms the deviation in the obtained findings of UTS and EL [42,58]. The elongation exceeding 10% in all samples suggests that the fabricated structure demonstrates properties comparable to those of wrought metal [53]. Overall, all the specimens from deposition as well as build direction have shown superior mechanical performance, which confirms the adequacy of the GMAWAM method of NiTi SMA for various applications. However, tensile test values attained in horizontal specimens have shown good tensile strength as compared to the vertical tensile test specimens. For specimens in the horizontal direction, the limited number of layers emphasizes the significant influence of grain boundaries on the tensile behavior of the material. Because grain boundaries are high-energy areas, cracks require more energy to propagate through them. Therefore, the tensile test specimens in the horizontal direction have shown higher values than vertical direction. Additionally, UTS in deposition direction was observed to be uniform with negligible deviation among them. The top portion specimen exhibited, to some extent, higher tensile strength owing to the recurring thermal contact, which has lowered the cooling rate [59]. Overall, all of the specimens from deposition as well as build direction have shown superior mechanical performance, which confirms the adequacy of the GMAWAM method of NiTi SMA for various applications. Similar tensile properties were reported in previous studies by Wu et al. [60], and Zhang et al. [61].
Figure 9 and Figure 10 show the fractography of one of the tensile test specimens from horizontal and vertical directions, respectively. As illustrated in Figure 9 and Figure 10, both specimens experienced failure within the gauge section following elastic deformation. A large number of evenly spaced dimples observed on the fracture surface suggests that the material possesses significant toughness [62]. The dense arrangement of dimples also indicates good ductility. Similar fracture features were observed in samples taken from both the top and middle regions. Similar fracture characteristics were also observed in the remaining tensile test specimens.

3.4. DSC Test

The shape memory effect (SME) of the built specimen was evaluated using DSC by comparing its phase transformation temperatures with those of the untreated NiTi SMA wire. A DSC setup was employed to investigate the phase transformation temperature of specimens by using a heating/cooling rate of 10 °C/min under a constant nitrogen flow. A small sample was extracted from the built L-shaped wall structure for the DSC test. The test consists of heating and cooling of the specimens, during which a plot illustrating the relationship between temperature and phase changes is generated. Table 3 shows the transformation temperatures obtained for the untreated NiTi SMA wire and WAAM specimen. Figure 11a,b show the DSC curve obtained for the untreated NiTi SMA wire and WAAM specimen sample, respectively. Transformation temperatures were identified by locating the intersections between the baseline and tangents drawn to each peak. During the cooling phase, exothermic peaks signified the forward transformation from austenite (B2) to martensite (B19′) in both the NiTi wire and WAAM sample. Conversely, endothermic peaks appeared upon heating, corresponding to the reverse transformation from martensite (B19′) back to austenite (B2). The transformation temperatures observed in both the base metal wire and WAAM samples were comparable, suggesting that the shape memory effect remained intact after the fabrication of the L-shaped wall structure using the GMAW-based WAAM process.

4. Conclusions

In the present study, 40 layers of L-shaped multi-walled structure NiTi SMA was fabricated on titanium substrate through GMAWAM method. The present study aimed to investigate the fabricated structure for microstructure, macrostructure, and mechanical properties. From the derived results, key findings were summarized as follows:
  • The 40 layers of L-shaped structure was successfully fabricated at optimized parameters of wire feed speed at 6 m/min, travel speed at 12 mm/s, and voltage at 20 V. Identical layer-on-layer deposition was observed along with seamless fusion.
  • The macrographs demonstrated the continuous bonding among the layers with complete fusion without presence of pores and cracks on the NiTi deposited corner joint.
  • The microstructure in the area between the two middle layers has exhibited a mixture of columnar grains (both coarse and fine) interspersed with dendritic colonies due to extended solidification times, while the microstructure in the top most layers has exhibited finer colonial structures in relatively greater numbers.
  • The microhardness (MH) test has shown the average values of 283.2 ± 3.67 HV, and 371.1 ± 5.81 HV at the bottom- and top-most layers respectively. The bottom section has shown lower MH values due to the fact that initial layers were deposited onto the cold substrate plate, along with the influence of the resulting HAZ. The overall MH values have clearly shown a consistent and identical summary for the whole built wall depicting the overall mechanical stability.
  • The tensile test values attained in deposition direction have shown good tensile strength as compared to the build direction specimens. The average UTS, and EL for deposition direction was obtained as 831 ± 22.91 MPa, and 14.32 ± 0.55% respectively, while the build direction has shown average UTS and EL values of 774 ± 6.56 MPa, and 14.16 ± 0.21% respectively. The elongation exceeding 10% in all samples suggests that the fabricated structure demonstrates properties comparable to those of wrought metal. Fractography of all tensile specimens has shown good ductility and toughness.
  • Lastly, a DSC test was carried out to assess the retention of shape memory effect for the fabricated structure. The transformation temperatures observed in both the base metal wire and WAAM samples were comparable, suggesting that the shape memory effect remained intact after the fabrication of the L-shaped wall structure using a GMAW-based WAAM process.
  • The authors believe that the findings of this work will be valuable for various industrial applications.

Author Contributions

Conceptualization, V.V., R.C., and J.V.; methodology, V.V., R.C., and J.V.; software, J.M., S.K., and R.C.; validation, V.V., R.C., J.M., S.K., and J.V.; formal analysis, V.V.; investigation, V.V., R.C., J.M., S.K., and J.V.; resources, V.V., R.C., and J.V.; data curation, V.V., R.C., and J.V.; writing—original draft preparation, V.V., R.C., and J.V.; writing—review and editing, R.C., and J.V.; visualization, V.V., R.C., J.M., S.K., and J.V.; supervision, R.C., and J.V.; project administration, R.C., and J.V.; funding acquisition, R.C., and J.V.; All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Data Availability Statement

Data presented in this study are available in this article.

Conflicts of Interest

Author Sakshum Khanna was employed from company Relx Pvt. Ltd., other authors declare no conflicts of interest.

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Figure 1. The GMAW-based WAAM setup.
Figure 1. The GMAW-based WAAM setup.
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Figure 2. Macrostructure, microstructure, tensile, and MH sample preparation on the fabricated L-structure.
Figure 2. Macrostructure, microstructure, tensile, and MH sample preparation on the fabricated L-structure.
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Figure 3. Fabricated L-structure of 40 layers.
Figure 3. Fabricated L-structure of 40 layers.
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Figure 4. Macroscopy of the top, middle, and bottom zones of the corner joint.
Figure 4. Macroscopy of the top, middle, and bottom zones of the corner joint.
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Figure 5. Microstructure at (a) middle zone layers, and (b) top zone layers.
Figure 5. Microstructure at (a) middle zone layers, and (b) top zone layers.
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Figure 6. Locations of MH measurements across the built wall structure.
Figure 6. Locations of MH measurements across the built wall structure.
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Figure 7. MH measurements from bottom to top portion of the built wall structure.
Figure 7. MH measurements from bottom to top portion of the built wall structure.
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Figure 8. Tensile properties for horizontal and vertical specimens.
Figure 8. Tensile properties for horizontal and vertical specimens.
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Figure 9. Horizontal tensile test after (a) fracture, and (b) SEM fractography.
Figure 9. Horizontal tensile test after (a) fracture, and (b) SEM fractography.
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Figure 10. Vertical tensile test after (a) fracture, and (b) SEM fractography.
Figure 10. Vertical tensile test after (a) fracture, and (b) SEM fractography.
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Figure 11. DSC curve for (a) untreated NiTi wire, (b) WAAM specimen.
Figure 11. DSC curve for (a) untreated NiTi wire, (b) WAAM specimen.
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Table 1. Optimal process conditions.
Table 1. Optimal process conditions.
VariablesOptimal Value
Wire feed speed (m/min)6
Travel speed (mm/s)12
Voltage (V)20
Dwell time (s)45
Argon gas flow (L/min)15
Filler wireNiTi wire with 1.2 mm dia.
Substrate plateTitanium
Table 2. Tensile test results.
Table 2. Tensile test results.
SpecimenUTS, MPaEL, %
Deposition direction
(Horizontal)		H-181114.21
H-282613.83
H-385614.92
Average831 ± 22.9114.32 ± 0.55
Built direction
(Vertical)		V-178114.38
V-276813.97
V-377314.14
Average774 ± 6.5614.16 ± 0.21
Table 3. Phase transformation temperatures.
Table 3. Phase transformation temperatures.
Specimens As (°C)Af (°C)Ms (°C)Mf (°C)
NiTi SMA wire55958545
WAAM sample49887842
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MDPI and ACS Style

Vaghasia, V.; Chaudhari, R.; Khanna, S.; Modi, J.; Vora, J. Experimental Investigations on Microstructure and Mechanical Properties of L-Shaped Structure Fabricated by WAAM Process of NiTi SMA. J. Manuf. Mater. Process. 2025, 9, 239. https://doi.org/10.3390/jmmp9070239

AMA Style

Vaghasia V, Chaudhari R, Khanna S, Modi J, Vora J. Experimental Investigations on Microstructure and Mechanical Properties of L-Shaped Structure Fabricated by WAAM Process of NiTi SMA. Journal of Manufacturing and Materials Processing. 2025; 9(7):239. https://doi.org/10.3390/jmmp9070239

Chicago/Turabian Style

Vaghasia, Vatsal, Rakesh Chaudhari, Sakshum Khanna, Jash Modi, and Jay Vora. 2025. "Experimental Investigations on Microstructure and Mechanical Properties of L-Shaped Structure Fabricated by WAAM Process of NiTi SMA" Journal of Manufacturing and Materials Processing 9, no. 7: 239. https://doi.org/10.3390/jmmp9070239

APA Style

Vaghasia, V., Chaudhari, R., Khanna, S., Modi, J., & Vora, J. (2025). Experimental Investigations on Microstructure and Mechanical Properties of L-Shaped Structure Fabricated by WAAM Process of NiTi SMA. Journal of Manufacturing and Materials Processing, 9(7), 239. https://doi.org/10.3390/jmmp9070239

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