Correlation between arc mode, microstructure, and mechanical properties during wire arc additive manufacturing of 316L stainless steel

Materials Science & Engineering A 751 (2019) 183–190

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Materials Science & Engineering A journal homepage: www.elsevier.com/locate/msea

Correlation between arc mode, microstructure, and mechanical properties during wire arc additive manufacturing of 316L stainless steel

T

⁎

Leilei Wanga, , Jiaxiang Xuea, Qiang Wangb a b

School of Mechanical and Automotive Engineering, South China University of Technology, Guangzhou 510641, China Hubei Dayun Automobile Co., Ltd., Shiyan 442500, China

A R T I C LE I N FO

A B S T R A C T

Keywords: Wire arc additive manufacturing Stainless steel Quality assessment Microstructural characterization Mechanical properties

Wire arc additive manufacturing (WAAM) features advantages such as low cost and high disposition rate, and thus WAAM is a feasible additive manufacturing process. Although some characteristics of WAAM have been documented in the literature, the process stability, structural integrity, component morphology, microstructure, and mechanical properties during WAAM under different arc modes are not comprehensively demonstrated and understood. Here, we performed WAAM experiments with 316L stainless steel under different arc modes and a constant deposition rate, and then we discussed the mechanism and impact of the arc mode on the manufacturing process stability, structural integrity, microstructures, and mechanical properties. The results indicate that the SpeedPulse and SpeedArc additive manufacturing processes are relatively stable, significantly efficient, and structurally sound. Although the deposition rate and scanning speed of SpeedPulse WAAM and SpeedArc WAAM are the same, SpeedArc WAAM has a lower heat input and a higher cooling rate. Therefore, SpeedArc WAAM produces a finer solidification structure than SpeedPulse WAAM. The ultimate tensile strengths of the SpeedPulse and SpeedArc additive manufactured specimens along the horizontal direction are greater than 540 MPa and slightly greater than previously reported results. Due to the lower heat input and finer solidification structure, a component produced by SpeedArc WAAM has greater tensile strength and hardness than a component produced by SpeedPulse WAAM.

1. Introduction Additive manufacturing transforms the fabrication of a structurally complicated three-dimensional (3D) component into stepwise additions of thin material layers guided by a digital model [1,2]. Additive manufacturing enables the fabrication of structurally complex components without using a mold, which significantly improves production efficiency and manufacturing flexibility [3,4]. Laser beams, electron beams, and electric arcs are commonly used heat sources during additive manufacturing of metallic components [5–7]. When laser beams or electron beams are selected as the heat source, the energy control is accurate, and the component shape is relatively precise [8,9]. Therefore, investigations regarding additive manufacturing based on laser and electron beams are thorough [10–12]. Direct energy deposition and power bed fusion are the general additive manufacturing processes used when laser beams or electron beams are selected as the heat source [13,14]. However, both of these processes adopt metal powder as the feedstock material, and thus the production efficiency is limited and the production cost is high, which restricts the applications of laser beams

⁎

and electron beams in the manufacture of large-scale metallic components. Wire arc additive manufacturing (WAAM) adopts an arc as the heat source and metal wire as the feedstock material. Generally, the primary cost of the metal wire is approximately 10% of the same weight of metal powder. During the WAAM process, the metal wire is heated, melted, and then transferred to the melt pool and then solidifies at the melt pool boundary and forms digitally designed components [15,16]. WAAM is a kind of droplet-based additive manufacturing process, which is very promising for the direct fabrication of complex thin-walled parts [17,18]. WAAM features a high deposition rate, which is suitable for fabricating large-scale components [19,20]. Furthermore, WAAM features advantages such as low cost and low wastage rate, and thus WAAM is an advantageous alternative additive manufacturing process to other methods based on laser and electron beams [21,22]. The dominant factor that affects the component morphology, microstructure, and mechanical properties during WAAM is the heat input. However, differences in heat input exist when different arc modes are adopted even if the wire feeding rate is held constant.

Corresponding author. E-mail address: [email protected] (L. Wang).

https://doi.org/10.1016/j.msea.2019.02.078 Received 14 February 2019; Received in revised form 20 February 2019; Accepted 22 February 2019 Available online 23 February 2019 0921-5093/ © 2019 Elsevier B.V. All rights reserved.

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spray transfer mode. SpeedArc WAAM is distinguished from the conventional GMAW process by a short and particularly forceful arc in the spray arc range and therefore features exceptional directional stability and high energy density. The primary metal transfer mode during SpeedArc WAAM is the short-circuited transfer mode. Argon with a 99.99% purity was used as the shielding gas. The length of one single layer is 150 mm. The scanning path was scanned back and forth, which means that the scanning direction in the current layer is opposite to that in the previous layer. The standstill time during the transition to the next adjacent layer is 20 s. The process parameters are shown in Table 2.

Another particular phenomenon found in WAAM is the transfer of liquid droplets across the arc from the wire electrode to the melt pool [23,24]. The liquid droplet temperature is higher than the solidus temperature, and part of heat is transferred to the melt pool by the liquid droplet [25]. However, differences in liquid droplet transfer exist when different metal transfer modes are adopted even if the wire feeding rate is held constant. Therefore, what is essential but seldom reported in the literature is to reveal what and how the metal transfer mode affects the component morphology, microstructure, and mechanical properties during WAAM. Cong et al. [26] systematically investigated the effects of different metal transfer modes during the cold metal transfer (CMT) process on the porosity characteristics of additively manufactured Al-6.3%Cu alloys, and their results indicated that heat input is one of the critical factors that enables the CMT pulse advanced (CMT-PADV) process to control the porosity rate. Luo et al. [27] conducted WAAM on aluminum alloys using pulsed arcs and nonpulsed arcs, and their results indicated that pulsed arcs can achieve higher droplet transfer frequencies and that the size of the droplet in the pulsed arc approach is smaller than that in the nonpulsed arc approach. Although some characteristics of WAAM under various arc modes have been documented in the literature, the component morphology, integrity, microstructure, and mechanical properties are not comprehensively demonstrated or understood. Stainless steel 316L is a kind of austenitic stainless steel that is widely used in marine and offshore equipment, automobiles, and nuclear reactors due to its outstanding corrosion resistance, high strength, high ductility, and relatively low cost [28,29]. Here, we established an experimental platform and then carried out WAAM with 316L stainless steel under different arc modes but at the same deposition rate. Arc current and arc voltage data were collected during the additive manufacturing process to evaluate the manufacturing process stability. X-ray CT tests were conducted on tensile samples of the component to evaluate the structural integrity of the manufactured component. The microstructures at different locations in the components were characterized to reveal the relationship between the arc modes and microstructures. Tensile tests were conducted on the different locations of the components, and the fracture morphologies were analyzed to reveal the relationships between the metal transfer modes and mechanical properties.

2.2. Microstructural characterization After deposition, two samples were extracted near the middle of the component using a wire-electrode cutting machine to reveal the transverse cross-section. The metallography sample locations are shown in Fig. 2(a), "A" and "B" represent the metallography sample location of the bottom layer and upper layer, respectively. Then, the metallography samples were cleaned, mounted and ground with a series of silicon carbide papers up to 2000 grit. Metallographic polishing was conducted with a series of 3 mm and 1 mm polycrystalline diamond suspensions followed by a final polishing with 0.05 mm colloidal silica to obtain a mirror finish. The 316L samples were etched by immersion for approximately 1 min using a hydrochloric acid: nitric acid: glycerol solution mixed at 20:10:20 to reveal the grain boundaries and subgrain structure. Metallography was performed using a Leica DMI3000M optical microscope. 2.3. Mechanical property tests After deposition, horizontal-direction tensile specimens were cut from the additive manufactured components; the tensile specimen locations are shown in Fig. 2(b). Then, the tensile samples were ground to a thickness of 2 mm with water cooling. An AG-IC 50 kN universal testing machine was used for the tensile tests; the displacement rate was set at 2 mm/min, and the average value of four tensile tests was calculated to guarantee accuracy. A Nova Nano SEM 430 was used to inspect the microstructures and fracture morphologies of the components; the instrument was operated at a voltage of 20 kV. Microhardness measurements were conducted using a Shimadzu HMV-2T microhardness tester equipped with an optical microscope. A 500 g load with a dwell time of 10 s was applied for all indentations. Hardness traces were recorded at 2 mm intervals along the vertical direction from the bottom layer to the top layer of the component.

2. Materials and methods 2.1. Material fabrication A commercial 316L stainless steel plate with dimensions of 250 × 100 × 5 mm3 was selected as the substrate, and 316L stainless steel wire with a diameter of 1.2 mm was used to deposit the components. Table 1 shows the chemical compositions of the stainless steel plate and wire [30]. Arc additive manufacturing experiments with two different arc modes were performed by using a Lorch power source integrated with a 6-axis FANUC robot. SpeedPulse and SpeedArc modes are the two kinds of representative arc modes proposed by Lorch Schweißtechnik GmbH. The actual current and voltage waveform during WAAM were collected and are shown in Fig. 1. The SpeedPulse mode involves a modified I-I-I-controlled, nonshort-circuiting pulse welding process that operates at a constant frequency and combines the characteristics of a classic pulse arc with those of a classic spray arc to achieve high process reliability. The primary metal transfer mode during SpeedPulse WAAM is a projected

3. Results and discussions 3.1. Process stability and component quality The distribution map of the arc current and arc voltage of the SpeedPulse and SpeedArc additive manufacturing processes are shown in Fig. 3. During SpeedPulse additive manufacturing, short-circuits do not occur because the arc voltage is always greater than 15 V and the arc does not extinguish because the arc current is always greater than 40 A. The arc current and arc voltage repeats well in every cycle during SpeedPulse additive manufacturing, which suggests that the droplet transfer and process stability of SpeedPulse additive manufacturing are favorable. SpeedArc additive manufacturing also features exceptional directional stability because the arc current remains almost constant at 140 A. Therefore, both SpeedPulse and SpeedArc additive manufacturing are stable additive manufacturing processes. Fig. 4 shows a multilayer component produced by SpeedArc additive manufacturing, and the corresponding process parameters are shown in case 2 of Table 2. The height of the component is relatively

Table 1 Chemical compositions of 316L substrate and filler wire. Element

Cr

Ni

Mo

Mn

Si

C

P

S

N

Fe

wt%

17.09

10.61

2.38

1.17

0.59

0.013

0.011

0.011

0.09

Bal.

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Fig. 1. Arc current and voltage waveform of SpeedPulse and SpeedArc modes.

Defects such as macroporosity and cracking are not found on the presented surfaces. The SpeedPulse additive manufactured component has a larger average width than the SpeedArc additive manufactured component when the wire feeding rate is the same. The overall height of the SpeedPulse and SpeedArc additive manufactured components are 37 mm and 44 mm, respectively. Consequently, the layer thickness of the SpeedPulse and SpeedArc additive manufactured components are 1.23 mm and 1.47 mm, respectively. During SpeedPulse additive manufacturing, the peak current reaches as high as 220 A, and the metal transfer mode is projected spray transfer. The filler wire is rapidly melted, and a droplet is generated in this short time interval when the peak current is applied. The overheated droplet is accelerated by the electromagnetic force and then impinges into the melt pool at a high speed [32]. Therefore, heat is introduced to the interior zone of the melt pool, and the SpeedPulse additive manufactured component features a smaller layer thickness and a larger layer width. X-ray CT tests were conducted on every four tensile samples for both of the additive manufacturing processes. Fig. 6 demonstrates typical Xray CT photographs of the tensile specimens from the SpeedPulse and SpeedArc additive manufactured components. Macroporosity and cracking in the X-ray CT photograph appear white in tensile specimens. However, no white zones are observed in the X-ray CT photographs, which indicates that defects such as macroporosity and cracking do not exist in the tensile samples. Therefore, SpeedPulse and SpeedArc additive manufacturing are structurally sound additive manufacturing processes.

Table 2 Process parameters during arc additive manufacturing. Arc mode

SpeedPulse WAAM

SpeedArc WAAM

Mean current I /A Mean voltage U /V Arc power P /W Layer n Scanning speed v1/(mm s−1) Wire feeding rate v2/(m min−1) Gas flow rate Q/(L min−1) Layer thickness δ/(mm)

22.1 135 2984 30 10 4.5 25 1.5

19.5 140 2730 30 10 4.5 25 1.8

3.2. Microstructures Fig. 7 displays an electron backscattered diffraction (EBSD) map of a component produced by SpeedArc WAAM. The EBSD map is vertical to the scanning direction, and the deposition direction is marked. The various colors correspond to the orientation of grains with respect to the crystal lattice, and each color corresponds to a unique combination of Euler angles. The grain growth orientation map along the vertical direction is shown in Fig. 7(a); almost the whole map shows red and yellow colors. The results indicate that most grain orientations are along the 001 direction, which is the same as the vertical direction. The austenite (γ) distribution map is shown in Fig. 7(b), and the results indicate that the primary phase of the WAAM component is austenite. Fig. 8 shows the optical micrographs along the transverse section of a specific layer of the wire arc additive manufactured component. Ferrite (δ), which is shown in gray, distributes within the austenite (γ) matrix, which is shown in white [33]. Fig. 8(b) and (c) show highermagnification micrographs of the top and bottom locations of the specific layer. The grains grow nearly along the vertical direction, which shows good agreement with Fig. 7. Cellular structures were observed adjacent to the fusion line, as shown in Fig. 8(c). Then, the cellular

Fig. 2. (a) Metallography sample locations. (b) Tensile sample dimensions.

equal, which proves that the distortion is acceptable. However, the lateral surface of the component exhibits a slightly corrugated morphology due to layer-by-layer deposition. A smooth surface can be achieved by further machining if necessary. The part is well fabricated, and the width is relatively uniform; rare defects can be found on the surface of the component. The scanning speed and wire feeding rate of the WAAM process are 10 mm/s and 75 mm/s, respectively. The calculated deposition rates of the SpeedPulse and SpeedArc additive manufacturing processes are 2.4 kg/h, which are obviously higher than the deposition rates of typical powder bed fusion-laser, powder bed fusion-electron beam and direct energy deposition-powder feed additive manufacturing approaches [31]. Therefore, SpeedPulse and SpeedArc additive manufacturing are significantly efficient additive manufacturing processes. The cross-sectional morphologies of components fabricated by SpeedPulse and SpeedArc additive manufacturing are shown in Fig. 5. 185

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Fig. 3. Distribution map of the arc current and arc voltage of SpeedPulse and SpeedArc additive manufacturing.

component, the microstructures were obtained from the same layer of the component to make a meaningful comparison. Secondary dendrite arm spacing (SDAS) is an essential index in the evaluation of arc additive manufactured components. The mechanical properties are improved when the SDAS is smaller. The measured SDAS of the components produced by SpeedPulse WAAM and SpeedArc WAAM are shown in Table 3. From the comparison of the SDAS of the bottom layer and SDAS of the top layer, a conclusion can be made that the SDAS of the top layer is larger than that of the bottom layer. The results show good agreement with the previously observed results reported by Hofmeister [34]. The two reasons responsible for this phenomenon are as follows. First, the deposit starts although the previous layer does not cool to room temperature. As a result, the initial temperature of both the component and substrate increases at higher layers. Second, heat can be lost by both the component and substrate at the bottom layer, while less heat can be lost by the substrate at higher layers. In other words, the effective surface area for heat loss decreases at higher layers [35]. When the initial temperature increases and the effective surface area for heat loss decreases at higher layers, the cooling process becomes longer, and the cooling rate becomes lower. Note that the scale of the solidification structure is significantly affected by the cooling rate. The relation between the SDAS (λ2 ) and cooling rate (GR ) is given as follows [36,37]:

Fig. 4. Full appearance of a multilayer component by SpeedArc WAAM.

Fig. 5. Cross-sectional morphology of multilayer components produced by (a) SpeedPulse WAAM and (b) SpeedArc WAAM.

λ2 = 50(GR)−0.4

(1)

where GR is the cooling rate in K/s. Eq. (1) indicates that the SDAS increases as the cooling rate decreases. Therefore, the solidification structure becomes coarser at higher layers because of the lower cooling rate. From the comparison of the SDAS of the SpeedPulse WAAM with the SDAS of the SpeedArc WAAM, a conclusion can be made that the SDAS produced by SpeedArc WAAM is smaller than that produced by SpeedPulse WAAM at both the bottom layer and top layer. Although the deposition rates of SpeedArc WAAM and SpeedPulse WAAM are the same, SpeedArc WAAM has a lower heat input. Therefore, SpeedPulse WAAM has a larger melt pool, wider layer width, and a lower cooling rate. According to Eq. (1), SpeedArc WAAM produces a finer solidification structure in both the bottom layer and higher layer due to the higher cooling rate.

Fig. 6. X-ray CT photographs of the tensile specimens produced by (a) SpeedPulse WAAM and (b) SpeedArc WAAM.

structures develop into fine columnar structures far away from the fusion line. Finally, the fine columnar structures develop into coarse columnar structures, and secondary dendrites can be clearly observed farther away from the fusion line, as shown in Fig. 8(b). For the next layer, the grains grow in the same manner. Fig. 9 shows the microstructures from the central transverse sections of the components produced by SpeedPulse and SpeedArc additive manufacturing with different layers. To reveal the effect of the metal transfer mode on the microstructure of the additive manufactured

3.3. Mechanical property Fig. 10 displays the Vickers hardness values of the SpeedPulse and SpeedArc additive manufactured components. The hardness measurement were performed at 2 mm intervals from the bottom layer to the top layer. The Vickers hardness values of the SpeedPulse and SpeedArc 186

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Fig. 7. EBSD map of a component produced by SpeedArc WAAM: (a) grain growth orientation map and (b) austenite (γ) distribution map.

Although the heat input of SpeedArc WAAM is slightly lower than that of SpeedPulse WAAM, the heat dissipation path and heat dissipation effect become worse during the deposition of the top layer. Therefore, the SDAS of SpeedArc WAAM is smaller than the SDAS of SpeedPulse WAAM at the bottom layer. According to the Hall-Petch relationship, the Vickers hardness at the bottom layer of the component produced by SpeedArc WAAM is higher than that of the component produced by SpeedPulse WAAM.

additive manufactured components are greater than 175 HV. The heat input of SpeedArc WAAM is slightly lower than that of SpeedPulse WAAM, and the substrate acts as a good heat sink during the deposition of the bottom layer. Therefore, there is no significant difference in SDAS between the SpeedArc and SpeedPulse WAAM at the bottom layer. Furthermore, no significant difference in Vickers hardness is observed between the SpeedPulse and SpeedArc additive manufactured components in the bottom layer.

Fig. 8. (a) Optical micrographs along the transverse section of a specific layer of a component produced by WAAM. Figures (b) and (c) show higher-magnification micrographs of the top and bottom locations of the specific layer. 187

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Fig. 9. Optical metallurgy graphs at the transverse cross-sections of the wire arc additive manufactured 316L components: (a) bottom layer of the component produced by SpeedPulse WAAM, (b) bottom layer of the component produced by SpeedArc WAAM, (c) top layer of the component produced by SpeedPulse WAAM, and (d) top layer of the component produced by SpeedArc WAAM. Table 3 Measured SDAS values of the components produced by SpeedPulse WAAM and SpeedArc WAAM. Arc mode

SpeedPulse WAAM

SpeedArc WAAM

SDAS of the bottom layer SDAS of the top layer

10.40 ± 0.43 µm 12.02 ± 1.69 µm

9.60 ± 0.92 µm 10.74 ± 0.38 µm

Fig. 11. Stress-strain curves of the SpeedPulse and SpeedArc additive manufactured tensile specimens along the horizontal direction.

Fig. 11 displays the stress-strain curves of the SpeedPulse and SpeedArc additive manufactured tensile specimens along the horizontal direction. The samples were cut from the middle layer of the additive manufactured component. All samples experienced elastic deformation and plastic deformation prior to fracture. It is clear that the ultimate tensile strengths of all samples were greater than 540 MPa. Experimentally measured and previously reported tensile properties of additive manufactured 316L stainless steel are shown in Table 4. The tensile strength and elongation of the SpeedArc additive manufactured component along the horizontal direction is higher than that of the

Fig. 10. Vickers hardness of the SpeedPulse and SpeedArc additive manufactured components.

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Table 4 Experimentally measured and previously reported tensile properties of additive manufactured 316L stainless steel. Metal transfer mode

UTS/MPa

YS/MPa

SpeedPulse WAAM SpeedArc WAAM Cold metal transfer-based WAAM [38] Wrought [39]

550 ± 6 553 ± 2 533 ± 23 525–623

418.0 417.9 235 ± 6 255–310

SpeedPulse additive manufactured component. Microstructural refinement is known to have a significant influence on the strengths of metals and alloys. Some literature has described the effect of grain size on yield strength in the recognized Hall-Petch relation [40,41]:

σy = σ0 +

ky d

(2)

where d is the grain size, σy is the yield strength, and σ0 and k y are material constant properties of materials that represent the yield stress of a grain-free material and the strengthening coefficient, respectively. According to the Hall-Petch equation, a finer microstructure indicates a higher tensile strength. Therefore, the SpeedArc additive manufactured component features higher tensile strength due to its finer solidification structure. Another conclusion that can be drawn is that the experimentally measured ultimate tensile strengths of the components produced by SpeedPulse and SpeedArc WAAM are higher than previously reported tensile strength by cold metal transfer-based WAAM. The commercial criterion for the tensile strength of wrought 316L stainless steel is 525–623 MPa. Therefore, the ultimate tensile strengths of the components produced by SpeedPulse and SpeedArc WAAM lie in this range, which indicates that the tensile strengths of the components produced by SpeedPulse and SpeedArc WAAM are up to standard. Fig. 12 shows representative SEM images of the fracture morphologies of the components produced by SpeedPulse WAAM and SpeedArc WAAM. Enormous dimples with a relatively uniform distribution are observed on the fracture surface, which proves that the fracture mode is a ductile fracture and that the as-formed materials feature good toughness [39,42]. However, apparent differences regarding the dimple dimensions and depths can be observed between the components produced by SpeedPulse and SpeedArc WAAM. The dimple dimensions and depths of the component produced by SpeedArc WAAM are slightly greater than those produced by the SpeedPulse WAAM, which indicates

Fig. 13. Energy-dispersive spectroscopy results of the particle elements in the dimples of the components produced by (a) SpeedPulse WAAM and (b) SpeedArc WAAM.

that the component fabricated by SpeedArc WAAM has better ductility. The inference agrees reasonably with the experimental test results, as shown in Table 4. Some particles located at the bottom of the dimples are observed in Fig. 12. Energy-dispersive spectroscopy (EDS) is used to identify the elements of the particles. Fig. 13 shows the EDS results of the particle elements in the dimples of the components produced by (a) SpeedPulse WAAM and (b) SpeedArc WAAM. According to the EDS results, the atomic percentage of oxygen reaches 42.93% for SpeedPulse WAAM and 37.85% for SpeedArc WAAM. The EDS results indicate that the particles in the dimples are oxidizing impurities. Oxidizing impurities commonly act as a brittleness phase and ultimately become the initiation points of cracks. Therefore, the oxidation on the substrate should be removed entirely prior to the deposit, and the manufacturing process should be absolutely protected against oxidative invasion.

Fig. 12. Representative scanning electron microscopy (SEM) images of the fracture morphologies of the components produced by (a) SpeedPulse WAAM and (b) SpeedArc WAAM. 189

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4. Conclusions [15]

1. SpeedPulse and SpeedArc additive manufacturing processes are relatively stable, significantly efficient, and structurally sound. 2. The secondary dendrite arm spacing (SDAS) of the top layer is larger than that of the bottom layer. Although the deposition rates and scanning speeds of SpeedPulse WAAM and SpeedArc WAAM are the same, SpeedArc WAAM has a lower heat input and a higher cooling rate. Therefore, SpeedArc WAAM produces finer solidification structures in both the bottom layer and higher layer. 3. The ultimate tensile strengths of the SpeedPulse and SpeedArc additive manufactured specimens along the horizontal direction are greater than 540 MPa and slightly higher than previously reported results. Enormous dimples with relatively uniform distributions are observed on the fracture surfaces, which proves that the fracture modes are ductile fractures and the as-formed materials feature favorable toughness values. The tensile strengths and hardness values of the components produced by SpeedArc WAAM are higher than those of components produced by SpeedPulse WAAM due to the finer solidification structure provided by SpeedArc WAAM.

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Acknowledgments

[24]

This project is supported by the National Natural Science Foundation of China (Grant no. 51875213).

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Conflicts of interest

[26]

The authors declare that there is no conflict of interest regarding the publication of this paper.

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