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Microstructure and mechanical properties of 308L stainless steel fabricated by laminar plasma additive manufacturing
Materials Science & Engineering A 770 (2020) 138523
Contents lists available at ScienceDirect
Materials Science & Engineering A journal homepage: http://www.elsevier.com/locate/msea
Microstructure and mechanical properties of 308L stainless steel fabricated by laminar plasma additive manufacturing Miao Li, Tao Lu, Jianwen Dai, Xiaojian Jia, Xuhan Gu, Ting Dai * School of Materials Science and Engineering, Jiangsu Key Laboratory of Advanced Metallic Materials, Southeast University, Nanjing, 211189, China
A R T I C L E I N F O
A B S T R A C T
Keywords: Additive manufacturing Laminar plasma 308L Microstructure Mechanical properties
In the present work, a novel direct energy deposition method for metal additive manufacturing is developed employing laminar plasma as the heat source. With a combination of modified process parameters, a highperformance 308L stainless steel component with four hollow straight walls is prepared. The behavior of phase formation, microstructure, density and mechanical properties of the samples with different heights were investigated. Transformation from columnar to equiaxed dendrites can be observed as the height of wall in creases from the substrate to about 30 mm. In the top region of the deposited wall, the equiaxed dendrites are dominant. The average density of the sample reaches 98.3%. Anisotropic property is observed in the bottom and middle regions, while the top region is isotropic.
1. Introduction Metal additive manufacturing is a very attractive new manufacturing process with broad application prospects in the field of aerospace, automobile and petrochemical industry, etc [1–5]. It can be classified into Powder Bed Fusion (PBF) and Direct Energy Deposition (DED) with the benefits of flexible manufacturing, rapid prototyping and high utilization rate of raw materials [1,6,7]. PBF is the most frequently investigated technique with advantages of high feature resolution and lower surface roughness [8,9]. However, PBF is often constrained in terms of the component size that can be built [10], which means that it is unfeasible to manufacture large parts due to slow deposition rate and considerable waste of raw materials. Compared with Powder Bed Fusion, Direct Energy Deposition can realize rapid production of large-scale and complex components with higher deposition rate [7,11]. In addition, utilization of raw materials like wires or powders is higher by Direct Energy Deposition and therefore, it is well-suited for production of components in large scale [12]. In the aspect of heat source of DED, laser, electron beam and plasma are commonly used according to the requirements of materials, cost, and service environment. Plasma additive manufacturing is
mostly used in the aspect of DED due to higher energy density and without the need for vacuum environment [11,13]. In recent years, researchers investigate the shape-forming capability, microstructural evolution and mechanical properties with Wire Arc Additive Manufacturing (WAAM) based on Gas tungsten arc welding (GTAW), Gas metal arc welding (GMAW), Cold metal transfer (CMT) and Plasma arc welding (PAW) [3,4,11–17]. However, these kinds of plasma mentioned above often exist in the state of turbulent form. Compared with turbulent plasma, laminar plasma has better controllability and higher energy density because of special design of the generator [18], which has been successfully used in plasma powder preparation [18], welding [19], thermal spraying [20] and remelting or cladding of metallic surfaces [21,22]. To the best knowledge of the authors, only few papers reported the possibility of additive manufacturing through laminar plasma [23]. Wang et al. focused on the reliability of forming quality based on copper alloy and investigated the effects of processing parameters on the macro-properties of parts. However, the micro structure and mechanical properties were not completely analyzed which are very important for industrial applications. In addition, the microstructure and mechanical properties are closely related to the locations of samples and printing parameters [17]. In this work, wire of 308L stainless steel was used as raw materials
* Corresponding author. E-mail address: [email protected] (T. Dai). https://doi.org/10.1016/j.msea.2019.138523 Received 27 June 2019; Received in revised form 5 October 2019; Accepted 6 October 2019 Available online 7 October 2019 0921-5093/© 2019 Elsevier B.V. All rights reserved.
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Materials Science & Engineering A 770 (2020) 138523
Table 1 Chemical compositions of wire (308L) and substrate (304) (wt.%). Grade
Ni
Cr
Mn
Si
P
S
C
Cu
Fe
308L 304
9.54 7.98
20.71 17.57
2.15 1.17
0.82 0.47
0.018 0.044
0.008 0.004
0.017 0.055
0.013 0.098
Bal. Bal.
Fig. 1. (a) Schematic diagram of laminar plasma additive manufacturing. (b) Hollow cubic sample (50 mm height).
laminar plasma generator was used as heat source and the wire fed into the plasma center by means of a bypass wire feeder, which is similar to Gas Tungsten Arc Welding process. In order to promote the formation of laminar plasma and achieve a better protection of the molten pool, argon gas with high purity (99.999%) was used not only to act as a plasma forming gas but also to prevent the formation of oxides at elevated temperature. Table 2 lists the processing parameters used in the experiment which were chosen after some preliminary trials. Fig. 1b presents the hollow cubic sample prepared in this work. The dimension of each wall of cube is of 60 mm � 10 mm � 50 mm. Wire electrical discharge machining was employed to cut the sample in different shapes (as indicated in Fig. 1b) for the characterization and testing. To be specific, in region A, three small block specimens cut from different heights were used for density testing and XRD analysis. In re gion B, tensile specimens were cut along different directions and from different positions (bottom, middle, top), which is clearly shown in Fig. 2. Region C is a 50 mm high sample cut along the cross-section perpendicular to the deposition direction, which is used for metallo graphic observation.
Table 2 Processing parameters used for fabricating samples. Current (A)
Travel speed (mm/s)
Wire feed speed (mm/s)
Argon flow rate (L/min)
Layer height (mm)
60
2.4
20
5
1–2
and hollow cubic samples with height of 50 mm were successfully built using laminar plasma jet together with a bypass wire feeder. Phase composition, microstructural evolution and mechanical prop erties were analyzed and mechanical properties including density and tensile properties were investigated. The results show that typical transformation of columnar dendrites to equiaxed dendrites was observed and anisotropic properties occurred in the bottom and middle area of the sample, while isotropic properties emerged in the top area. 2. Experimental procedure 2.1. Preparation process In this study, the feedstock was AISI 308L wire with a dimension of Φ1.2 mm. AISI 304 plate with a dimension of 150 mm � 150 mm � 20 mm was used as the substrate. The composi tions in weight percentage of AISI 308L and AISI 304 are shown in Table 1, which were measured by Optical Emission Spectrometer (MaXx LMF15, China). The substrates were polished and cleaned with acetone before the manufacturing process in order to guarantee the bonding strength. The laminar plasma equipment was jointly developed by Southeast University and Zhenhuo Technology Co., Ltd. Fig. 1a indicates the schematic diagram of manufacturing process for the equipment. The
2.2. Microstructure and property characterization Phase compositions of the samples were analyzed by X-ray diffractometer (XRD, Bruker D8 DISCOVER) with a Cu-Ka radiation. After mechanical grinding and polishing, the samples were etched with chemical reagent (50 ml HCL, 10 ml HNO3, 100 ml H2O). The EBSD samples were further polished and then etched electrolytically in oxalic acid etching solution (28 V for 60s, 5 ml HCLO4 and 95 ml H20). Then microstructures of samples were observed by optical microscopy (OM) and scanning electron microscopy equipped with a backscatter electron (BSE) detector (SEM, Sirion 200 FEI). The
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Materials Science & Engineering A 770 (2020) 138523
Fig. 2. Schematic diagram of tensile samples of three different areas and size of tensile samples.
3. Results and discussion Fig. 3 shows the XRD patterns obtained from different locations of the fabricated specimens. It is obvious that only γ- (111), γ- (200) and γ(220) peaks can be detected in three samples, which means the fabri cated specimen is composed of a single austenite. Meanwhile, the γ(200) peak is dominant in the bottom sample and its intensity gradually decreases with the increase of overlapped layers (in the middle and top samples, respectively). At the same time, γ- (220) peak appears only in the middle and top samples. This phenomenon indicates a strong crys tallographic orientation of grains in the bottom section, while a poly crystalline trend occurs with the increase of deposition, which is consistent with Krishnan’s results [24]. The vertical-section microstructures of the deposited wall from bottom to top are shown in Fig. 4 and Fig. 5, respectively. In Fig. 4, it is clear that the columnar dendritic morphology is dominant in the bot tom half of the deposited wall. Particularly, the columnar dendrites in the near-substrate part (about 1–2 layers with 5 mm height as shown in Fig. 4e–f) shows a radial growth direction, since the substrate acts as the heterogeneous nucleation of the first-layer fusion and the poly crystalline surface leads to the multi-growth directions and orienta tions of the dendrites. As the deposition layer increases to about 8 mm, the columnar dendrites (with the width of 202.5 � 50.1 μm) grow outward in a direction perpendicular to the substrate as shown in Fig. 4c–d. The orientations of grains are approximately same which is
Fig. 3. XRD patterns (Cu-Kα) of samples at different height.
density of the samples was determined by Archimedes’ method. The tensile properties of samples were tested by the electronic universal testing machine (CMT5105, SANS, China) at a strain rate of 1.0 � 10 3 s 1.
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Materials Science & Engineering A 770 (2020) 138523
Fig. 4. Optical and EBSD images of different height: (a) 8–30 mm; (b) 8–30 mm; (c) 5–8 mm; (d) 5–8 mm; (e) 0–5 mm; (f) 0–5 mm.
consistent with the XRD results since the growth direction is parallel to the temperature gradient caused by the chilling effect of the substrate. However, at the middle of the wall, the columnar dendrites can not maintain continuous with the occurrence of the fine grains located at the deposited layer boundary (Fig. 4a–b, the average grain size of 718.2 � 188.7 μm). In Fig. 5, microstructural evolution from columnar dendrites to equiaxed dendrites can be observed in the top half of the deposited wall (with average grain size of 601.4 � 196.4 μm). To be specific, the transformation from columnar dendrites to equiaxed dendrites is detected for the first time when the deposition height reaches to about 30 mm (Fig. 5e and f). As clearly shown in Fig. 5c and d, columnar dendrites grow at the boundary of each layer, while equiaxed dendritic morphology appears in the inner part of each layer because the tem perature gradient is lower than that at the boundary of each layer. Therefore, the transformation from columnar dendrites to equiaxed dendrites occurs with the decrease of the ratio of the temperature gradient to the solidification rate when solidification maintains stable. In addition, a spatial periodicity of the microstructural evolution mentioned above is visible until the end of printing due to the periodic increase of deposited layers, which is presented in Fig. 5a and b. This phenomenon can also be confirmed by the XRD results that the preferred orientations of grains are weakened with the appearance of equiaxed dendrites.
Table 3 lists the density of samples located at different part. Basi cally, the densities of samples in different height positions are identical with a value of about 98%, which means the density is independent on the height of the deposited wall. In addition, the parts manufactured by laminar plasma technology possess a stable quality without obvious defects such as porosity, cracks, and slag inclusions, compared with the SLMed components of which the relative density is sensitive to laser powers and processing technology, ranging from 85% to 99.5% [8,25, 26]. Fig. 6a shows the tensile curves of the vertical and horizontal samples in different positions of the deposited wall, respectively. The detailed comparisons of the yield strength, ultimate tensile strength and elongations are presented in Fig. 6b-d, respectively. It can be seen that the tensile properties of samples located at the bottom and middle regions are anisotropic but the ones at the top region are isotropic. Specifically, for the samples at the bottom and middle re gion, the yield strength and ultimate tensile strength in the horizontal direction are superior to that in the vertical direction, while the elongation is on the contrary. The fact that the sample in different regions has different tensile properties is closely related to the microstructure mentioned above. Columnar dendrites possess aniso tropic properties, while equiaxed dendrites show isotropic properties since there is no preferred orientation of the grains. A detailed anal ysis of anisotropic properties related to the texture and Schmid factor
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Materials Science & Engineering A 770 (2020) 138523
Fig. 5. Optical and EBSD images of different height: (a) 46–50 mm; (b) 46–50 mm; (c) 34–46 mm; (d) 34–46 mm; (e) 30–34 mm; (f) 30–34 mm.
parameters, a high-performance 308L stainless steel component is prepared. (2) The sample manufactured by laminar plasma shows special microstructure that columnar dendrites can be observed in the bottom and middle regions of the sample. However, the trans formation of columnar dendrites to equiaxed dendrites can be observed when the height of overlapped layers reaches 30 mm. In the top region of the sample, each layer is mainly composed of equiaxed dendrites with less columnar crystals at the layer boundary of the sample. (3) The density of the sample is about 98.3% on average without obvious defects. The tensile test shows that samples have obvious anisotropic properties in mechanical properties in the bottom and middle regions, while the top region is isotropic.
Table 3 Density of samples of different height. Region
Bottom
Middle
Top
Density (%)
98.15
98.47
98.27
can be found in the supplementary material. The anisotropic proper ties are also discovered in other additive manufacturing techniques, which are the results of complex thermal effects of additive manufacturing [27–31]. In addition, the required ultimate strength, yield strength and elongation of 308L stainless in the annealed or rolled state are 515 MPa, 205 MPa and 40%, respectively, which means the laminar plasma technology developed in this work shows a promising application in the fabrication of complex components with high performance.
Data availability The raw/processed data required to reproduce these findings cannot be shared at this time as the data also forms part of an ongoing study.
4. Conclusions (1) A novel direct energy deposition method for metal additive manufacturing is developed employing laminar plasma as the heat source. With a combination of modified process
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Materials Science & Engineering A 770 (2020) 138523
Fig. 6. Tensile properties of the printed sample at different region: (a) curves of engineering stress vs. engineering strain; (b) ultimate strength; (c) yield strength; (d) Elongation.
Declaration of competing interest [7]
No potential conflict of interest was reported by the authors.
[8]
Acknowledgements
[9]
This work was supported by the National Key R&D Program of China (2018YFB1106100).
[10] [11]
Appendix A. Supplementary data
[12]
Supplementary data to this article can be found online at https://doi. org/10.1016/j.msea.2019.138523.
[13]
References
[14]
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components – process, structure and properties, Prog. Mater. Sci. 92 (2018) 112–224. S.W. Williams, F. Martina, A.C. Addison, J. Ding, G. Pardal, P. Colegrove, Wire þ Arc Additive manufacturing, Mater. Sci. Technol. 32 (7) (2016) 641–647. Z. Sun, X. Tan, S.B. Tor, W.Y. Yeong, Selective laser melting of stainless steel 316L with low porosity and high build rates, Mater. Des. 104 (2016) 197–204. S.-F. Wen, X.-T. Ji, Y. Zhou, C.-J. Han, Q.-S. Wei, Y.-S. Shi, Corrosion behavior of the S136 mold steel fabricated by selective laser melting, Chin. J. Mech. Eng. 31 (1) (2018). J.L. Bartlett, X. Li, An overview of residual stresses in metal powder bed fusion, Addit. Manuf. 27 (2019) 131–149. Q. Wu, Z. Ma, G. Chen, C. Liu, D. Ma, S. Ma, Obtaining fine microstructure and unsupported overhangs by low heat input pulse arc additive manufacturing, J. Manuf. Process. 27 (2017) 198–206. J. Gordon, J. Hochhalter, C. Haden, D.G. Harlow, Enhancement in fatigue performance of metastable austenitic stainless steel through directed energy deposition additive manufacturing, Mater. Des. 168 (2019). F. Martina, J. Mehnen, S.W. Williams, P. Colegrove, F. Wang, Investigation of the benefits of plasma deposition for the additive layer manufacture of Ti–6Al–4V, J. Mater. Process. Technol. 212 (6) (2012) 1377–1386. J. Ge, T. Ma, Y. Chen, T. Jin, H. Fu, R. Xiao, Y. Lei, J. Lin, Wire-arc additive manufacturing H13 part: 3D pore distribution, microstructural evolution, and mechanical performances, J. Alloy. Comp. 783 (2019) 145–155. S. Jhavar, N.K. Jain, C.P. Paul, Development of micro-plasma transferred arc (μ-PTA) wire deposition process for additive layer manufacturing applications, J. Mater. Process. Technol. 214 (5) (2014) 1102–1110. O. Yilmaz, A.A. Ugla, Microstructure characterization of SS308LSi components manufactured by GTAW-based additive manufacturing: shaped metal deposition using pulsed current arc, Int. J. Adv. Manuf. Technol. 89 (1–4) (2016) 13–25. J. Ge, J. Lin, Y. Lei, H. Fu, Location-related thermal history, microstructure, and mechanical properties of arc additively manufactured 2Cr13 steel using cold metal transfer welding, Mater. Sci. Eng. A 715 (2018) 144–153. O.P. Solonenko, A.V. Smirnov, Advanced oxide powders processing based on cascade plasma, J. Phys. Conf. Ser. 550 (2014). H. Hamatani, F. Watanabe, N. Mizuhashi, S. Takeuchi, Y. Hirota, S. Matsubayashi, K. Tsukakoshi, Y. Hasegawa, T. Asano, T. Motoyoshi, T. Miura, K. Tanaka, K. Yamamoto, T. Nose, O.P. Solonenko, A.V. Smirnov, Asme, Development of
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laminar plasma shielded hf-erw process - advanced welding process of HF-ERW, vol. 3, 2013. S.-H. Liu, C.-X. Li, H.-Y. Zhang, S.-L. Zhang, L. Li, P. Xu, G.-J. Yang, C.-J. Li, A novel structure of YSZ coatings by atmospheric laminar plasma spraying technology, Scr. Mater. 153 (2018) 73–76. W. Ma, Q. Fei, W. Pan, C. Wu, Investigation of laminar plasma remelting/cladding processing, Appl. Surf. Sci. 252 (10) (2006) 3541–3546. W.X. Pan, X. Meng, G. Li, Q.X. Fei, C.K. Wu, Feasibility of laminar plasma-jet hardening of cast iron surface, Surf. Coat. Technol. 197 (2–3) (2005) 345–350. J. Wang, K. Yang, Y. Zhang, H. Li, X. Zhu, Rapid manufacturing of hollow copper cylinder with laminar plasma torch, Zhenkong Kexue yu Jishu Xuebao/J. Vac. Sci. Technol. 36 (3) (2016) 5–8. T.E. Abioye, A. Medrano-Tellez, P.K. Farayibi, P.K. Oke, Laser metal deposition of multi-track walls of 308LSi stainless steel, Mater. Manuf. Process. 32 (14) (2017) 1660–1666. J. Wang, W.J. Wu, W. Jing, X. Tan, G.J. Bi, S.B. Tor, K.F. Leong, C.K. Chua, E. Liu, Improvement of densification and microstructure of ASTM A131 EH36 steel samples additively manufactured via selective laser melting with varying laser scanning speed and hatch spacing, Mater. Sci. Eng. A 746 (2019) 300–313.
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Contents lists available at ScienceDirect
Materials Science & Engineering A journal homepage: http://www.elsevier.com/locate/msea
Microstructure and mechanical properties of 308L stainless steel fabricated by laminar plasma additive manufacturing Miao Li, Tao Lu, Jianwen Dai, Xiaojian Jia, Xuhan Gu, Ting Dai * School of Materials Science and Engineering, Jiangsu Key Laboratory of Advanced Metallic Materials, Southeast University, Nanjing, 211189, China
A R T I C L E I N F O
A B S T R A C T
Keywords: Additive manufacturing Laminar plasma 308L Microstructure Mechanical properties
In the present work, a novel direct energy deposition method for metal additive manufacturing is developed employing laminar plasma as the heat source. With a combination of modified process parameters, a highperformance 308L stainless steel component with four hollow straight walls is prepared. The behavior of phase formation, microstructure, density and mechanical properties of the samples with different heights were investigated. Transformation from columnar to equiaxed dendrites can be observed as the height of wall in creases from the substrate to about 30 mm. In the top region of the deposited wall, the equiaxed dendrites are dominant. The average density of the sample reaches 98.3%. Anisotropic property is observed in the bottom and middle regions, while the top region is isotropic.
1. Introduction Metal additive manufacturing is a very attractive new manufacturing process with broad application prospects in the field of aerospace, automobile and petrochemical industry, etc [1–5]. It can be classified into Powder Bed Fusion (PBF) and Direct Energy Deposition (DED) with the benefits of flexible manufacturing, rapid prototyping and high utilization rate of raw materials [1,6,7]. PBF is the most frequently investigated technique with advantages of high feature resolution and lower surface roughness [8,9]. However, PBF is often constrained in terms of the component size that can be built [10], which means that it is unfeasible to manufacture large parts due to slow deposition rate and considerable waste of raw materials. Compared with Powder Bed Fusion, Direct Energy Deposition can realize rapid production of large-scale and complex components with higher deposition rate [7,11]. In addition, utilization of raw materials like wires or powders is higher by Direct Energy Deposition and therefore, it is well-suited for production of components in large scale [12]. In the aspect of heat source of DED, laser, electron beam and plasma are commonly used according to the requirements of materials, cost, and service environment. Plasma additive manufacturing is
mostly used in the aspect of DED due to higher energy density and without the need for vacuum environment [11,13]. In recent years, researchers investigate the shape-forming capability, microstructural evolution and mechanical properties with Wire Arc Additive Manufacturing (WAAM) based on Gas tungsten arc welding (GTAW), Gas metal arc welding (GMAW), Cold metal transfer (CMT) and Plasma arc welding (PAW) [3,4,11–17]. However, these kinds of plasma mentioned above often exist in the state of turbulent form. Compared with turbulent plasma, laminar plasma has better controllability and higher energy density because of special design of the generator [18], which has been successfully used in plasma powder preparation [18], welding [19], thermal spraying [20] and remelting or cladding of metallic surfaces [21,22]. To the best knowledge of the authors, only few papers reported the possibility of additive manufacturing through laminar plasma [23]. Wang et al. focused on the reliability of forming quality based on copper alloy and investigated the effects of processing parameters on the macro-properties of parts. However, the micro structure and mechanical properties were not completely analyzed which are very important for industrial applications. In addition, the microstructure and mechanical properties are closely related to the locations of samples and printing parameters [17]. In this work, wire of 308L stainless steel was used as raw materials
* Corresponding author. E-mail address: [email protected] (T. Dai). https://doi.org/10.1016/j.msea.2019.138523 Received 27 June 2019; Received in revised form 5 October 2019; Accepted 6 October 2019 Available online 7 October 2019 0921-5093/© 2019 Elsevier B.V. All rights reserved.
M. Li et al.
Materials Science & Engineering A 770 (2020) 138523
Table 1 Chemical compositions of wire (308L) and substrate (304) (wt.%). Grade
Ni
Cr
Mn
Si
P
S
C
Cu
Fe
308L 304
9.54 7.98
20.71 17.57
2.15 1.17
0.82 0.47
0.018 0.044
0.008 0.004
0.017 0.055
0.013 0.098
Bal. Bal.
Fig. 1. (a) Schematic diagram of laminar plasma additive manufacturing. (b) Hollow cubic sample (50 mm height).
laminar plasma generator was used as heat source and the wire fed into the plasma center by means of a bypass wire feeder, which is similar to Gas Tungsten Arc Welding process. In order to promote the formation of laminar plasma and achieve a better protection of the molten pool, argon gas with high purity (99.999%) was used not only to act as a plasma forming gas but also to prevent the formation of oxides at elevated temperature. Table 2 lists the processing parameters used in the experiment which were chosen after some preliminary trials. Fig. 1b presents the hollow cubic sample prepared in this work. The dimension of each wall of cube is of 60 mm � 10 mm � 50 mm. Wire electrical discharge machining was employed to cut the sample in different shapes (as indicated in Fig. 1b) for the characterization and testing. To be specific, in region A, three small block specimens cut from different heights were used for density testing and XRD analysis. In re gion B, tensile specimens were cut along different directions and from different positions (bottom, middle, top), which is clearly shown in Fig. 2. Region C is a 50 mm high sample cut along the cross-section perpendicular to the deposition direction, which is used for metallo graphic observation.
Table 2 Processing parameters used for fabricating samples. Current (A)
Travel speed (mm/s)
Wire feed speed (mm/s)
Argon flow rate (L/min)
Layer height (mm)
60
2.4
20
5
1–2
and hollow cubic samples with height of 50 mm were successfully built using laminar plasma jet together with a bypass wire feeder. Phase composition, microstructural evolution and mechanical prop erties were analyzed and mechanical properties including density and tensile properties were investigated. The results show that typical transformation of columnar dendrites to equiaxed dendrites was observed and anisotropic properties occurred in the bottom and middle area of the sample, while isotropic properties emerged in the top area. 2. Experimental procedure 2.1. Preparation process In this study, the feedstock was AISI 308L wire with a dimension of Φ1.2 mm. AISI 304 plate with a dimension of 150 mm � 150 mm � 20 mm was used as the substrate. The composi tions in weight percentage of AISI 308L and AISI 304 are shown in Table 1, which were measured by Optical Emission Spectrometer (MaXx LMF15, China). The substrates were polished and cleaned with acetone before the manufacturing process in order to guarantee the bonding strength. The laminar plasma equipment was jointly developed by Southeast University and Zhenhuo Technology Co., Ltd. Fig. 1a indicates the schematic diagram of manufacturing process for the equipment. The
2.2. Microstructure and property characterization Phase compositions of the samples were analyzed by X-ray diffractometer (XRD, Bruker D8 DISCOVER) with a Cu-Ka radiation. After mechanical grinding and polishing, the samples were etched with chemical reagent (50 ml HCL, 10 ml HNO3, 100 ml H2O). The EBSD samples were further polished and then etched electrolytically in oxalic acid etching solution (28 V for 60s, 5 ml HCLO4 and 95 ml H20). Then microstructures of samples were observed by optical microscopy (OM) and scanning electron microscopy equipped with a backscatter electron (BSE) detector (SEM, Sirion 200 FEI). The
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Materials Science & Engineering A 770 (2020) 138523
Fig. 2. Schematic diagram of tensile samples of three different areas and size of tensile samples.
3. Results and discussion Fig. 3 shows the XRD patterns obtained from different locations of the fabricated specimens. It is obvious that only γ- (111), γ- (200) and γ(220) peaks can be detected in three samples, which means the fabri cated specimen is composed of a single austenite. Meanwhile, the γ(200) peak is dominant in the bottom sample and its intensity gradually decreases with the increase of overlapped layers (in the middle and top samples, respectively). At the same time, γ- (220) peak appears only in the middle and top samples. This phenomenon indicates a strong crys tallographic orientation of grains in the bottom section, while a poly crystalline trend occurs with the increase of deposition, which is consistent with Krishnan’s results [24]. The vertical-section microstructures of the deposited wall from bottom to top are shown in Fig. 4 and Fig. 5, respectively. In Fig. 4, it is clear that the columnar dendritic morphology is dominant in the bot tom half of the deposited wall. Particularly, the columnar dendrites in the near-substrate part (about 1–2 layers with 5 mm height as shown in Fig. 4e–f) shows a radial growth direction, since the substrate acts as the heterogeneous nucleation of the first-layer fusion and the poly crystalline surface leads to the multi-growth directions and orienta tions of the dendrites. As the deposition layer increases to about 8 mm, the columnar dendrites (with the width of 202.5 � 50.1 μm) grow outward in a direction perpendicular to the substrate as shown in Fig. 4c–d. The orientations of grains are approximately same which is
Fig. 3. XRD patterns (Cu-Kα) of samples at different height.
density of the samples was determined by Archimedes’ method. The tensile properties of samples were tested by the electronic universal testing machine (CMT5105, SANS, China) at a strain rate of 1.0 � 10 3 s 1.
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Fig. 4. Optical and EBSD images of different height: (a) 8–30 mm; (b) 8–30 mm; (c) 5–8 mm; (d) 5–8 mm; (e) 0–5 mm; (f) 0–5 mm.
consistent with the XRD results since the growth direction is parallel to the temperature gradient caused by the chilling effect of the substrate. However, at the middle of the wall, the columnar dendrites can not maintain continuous with the occurrence of the fine grains located at the deposited layer boundary (Fig. 4a–b, the average grain size of 718.2 � 188.7 μm). In Fig. 5, microstructural evolution from columnar dendrites to equiaxed dendrites can be observed in the top half of the deposited wall (with average grain size of 601.4 � 196.4 μm). To be specific, the transformation from columnar dendrites to equiaxed dendrites is detected for the first time when the deposition height reaches to about 30 mm (Fig. 5e and f). As clearly shown in Fig. 5c and d, columnar dendrites grow at the boundary of each layer, while equiaxed dendritic morphology appears in the inner part of each layer because the tem perature gradient is lower than that at the boundary of each layer. Therefore, the transformation from columnar dendrites to equiaxed dendrites occurs with the decrease of the ratio of the temperature gradient to the solidification rate when solidification maintains stable. In addition, a spatial periodicity of the microstructural evolution mentioned above is visible until the end of printing due to the periodic increase of deposited layers, which is presented in Fig. 5a and b. This phenomenon can also be confirmed by the XRD results that the preferred orientations of grains are weakened with the appearance of equiaxed dendrites.
Table 3 lists the density of samples located at different part. Basi cally, the densities of samples in different height positions are identical with a value of about 98%, which means the density is independent on the height of the deposited wall. In addition, the parts manufactured by laminar plasma technology possess a stable quality without obvious defects such as porosity, cracks, and slag inclusions, compared with the SLMed components of which the relative density is sensitive to laser powers and processing technology, ranging from 85% to 99.5% [8,25, 26]. Fig. 6a shows the tensile curves of the vertical and horizontal samples in different positions of the deposited wall, respectively. The detailed comparisons of the yield strength, ultimate tensile strength and elongations are presented in Fig. 6b-d, respectively. It can be seen that the tensile properties of samples located at the bottom and middle regions are anisotropic but the ones at the top region are isotropic. Specifically, for the samples at the bottom and middle re gion, the yield strength and ultimate tensile strength in the horizontal direction are superior to that in the vertical direction, while the elongation is on the contrary. The fact that the sample in different regions has different tensile properties is closely related to the microstructure mentioned above. Columnar dendrites possess aniso tropic properties, while equiaxed dendrites show isotropic properties since there is no preferred orientation of the grains. A detailed anal ysis of anisotropic properties related to the texture and Schmid factor
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Fig. 5. Optical and EBSD images of different height: (a) 46–50 mm; (b) 46–50 mm; (c) 34–46 mm; (d) 34–46 mm; (e) 30–34 mm; (f) 30–34 mm.
parameters, a high-performance 308L stainless steel component is prepared. (2) The sample manufactured by laminar plasma shows special microstructure that columnar dendrites can be observed in the bottom and middle regions of the sample. However, the trans formation of columnar dendrites to equiaxed dendrites can be observed when the height of overlapped layers reaches 30 mm. In the top region of the sample, each layer is mainly composed of equiaxed dendrites with less columnar crystals at the layer boundary of the sample. (3) The density of the sample is about 98.3% on average without obvious defects. The tensile test shows that samples have obvious anisotropic properties in mechanical properties in the bottom and middle regions, while the top region is isotropic.
Table 3 Density of samples of different height. Region
Bottom
Middle
Top
Density (%)
98.15
98.47
98.27
can be found in the supplementary material. The anisotropic proper ties are also discovered in other additive manufacturing techniques, which are the results of complex thermal effects of additive manufacturing [27–31]. In addition, the required ultimate strength, yield strength and elongation of 308L stainless in the annealed or rolled state are 515 MPa, 205 MPa and 40%, respectively, which means the laminar plasma technology developed in this work shows a promising application in the fabrication of complex components with high performance.
Data availability The raw/processed data required to reproduce these findings cannot be shared at this time as the data also forms part of an ongoing study.
4. Conclusions (1) A novel direct energy deposition method for metal additive manufacturing is developed employing laminar plasma as the heat source. With a combination of modified process
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Fig. 6. Tensile properties of the printed sample at different region: (a) curves of engineering stress vs. engineering strain; (b) ultimate strength; (c) yield strength; (d) Elongation.
Declaration of competing interest [7]
No potential conflict of interest was reported by the authors.
[8]
Acknowledgements
[9]
This work was supported by the National Key R&D Program of China (2018YFB1106100).
[10] [11]
Appendix A. Supplementary data
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Supplementary data to this article can be found online at https://doi. org/10.1016/j.msea.2019.138523.
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