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Dissimilar welding of Zr41Be23Ti14Cu12Ni10 bulk metallic glass and stainless steel
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Scripta Materialia 65 (2011) 1033–1036 www.elsevier.com/locate/scriptamat
Dissimilar welding of Zr41Be23Ti14Cu12Ni10 bulk metallic glass and stainless steel Jonghyun Kim⇑ and Y. Kawamura Department of Material Science, Kumamoto University, Kumamoto 860-8555, Japan Received 5 May 2011; revised 18 June 2011; accepted 20 June 2011 Available online 24 June 2011
The electron beam welding of 2 mm thick Zr41Be23Ti14Cu12Ni10 bulk metallic glass (BMG) plate to stainless steel was investigated. The BMG was welded to specially designed stainless steel with an electron beam acceleration voltage of 60 kV, a beam current of 20 mA, a welding speed of 66 mm s 1 and an electron beam irradiation position (dw) of 0.2 mm. The flexural strength of the welded joint (642 MPa) was higher than the yield strength of the stainless steel (478 MPa). Ó 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Bulk metallic glass (BMG); Welding; Interface; Stainless steel
Zr-based bulk metallic glasses (BMGs) are attractive materials for scientific research and engineering applications due to their high strength, high hardness, superior corrosion resistance, wide super-cooled liquid region and good glass-forming ability [1–4]. The maximum thicknesses of some Zr-based BMGs have reached several tenths of millimeters. Nevertheless, homogeneous glassy BMGs are not thick enough for structural applications and additional phase stability would be required during engineering processes. To extend the engineering applications of the BMG material, extensive research efforts have been made to develop welding technologies. Specifically, the welding technology of dissimilar materials, such as BMG materials to crystalline alloys, is an important area that should be developed. The functional use of the specific properties of each material in dissimilar material combinations provides flexible design possibilities for products. Electron beam welding has been in practical use for many years, its main advantage being its ability to produce joints with a deep and narrow weld, a minimal heat-affected zone (HAZ), a high cooling rate and insignificant deformation [5]. With electron beam irradiation, the absolute heat input is much lower than common conventional welding processes, such as gas tungsten arc and gas metal arc welding, but the input energy is focused on a highly localized area of the millimeter scale.
⇑ Corresponding
author. Tel.: +81 96 342 3705; fax: +81 96 342 3710; e-mail: [email protected]
Kawamura et al. [6–8] have reported that the Zr41Be23Ti14Cu12Ni10 BMG can be successfully welded to crystalline Zr, Ti and Ni metal without welding defects or crystallization in the weld. Stainless steel (STS) is used more widely than pure metal, such as Zr, Ti and Ni, in a range of applications in the automotive and marine industries. Unfortunately, the welding of BMG to stainless steel is not successful due to its tendency to crystallize in the weld metal. Therefore, we attempted electron beam welding of BMG to STS metal in an effort to extend the possible industrial applications of BMG. In this paper we report the successful results of electron beam welding of Zr41Be23Ti14Cu12Ni10 bulk metallic glass to crystalline STS metal, the effects of welding specimen shape and the effect of the horizontal focal point position (electron beam irradiation position: EBIP (dw)) on the phase composition. Zr41Be23Ti14Cu12Ni10 BMG (25 mm 15 mm 2 mm) and STS316L (30 mm 15 mm 2 mm; C = 0.02, Si = 0.8, Mn = 1.2, Cr = 16.5, Ni = 13.4, Mo = 2.4 and Fe = Bal. (wt.%)) plates were prepared for electron beam welding, which was carried out in a vacuum of 5 10 4 torr using a maximum power of 9 kW. All the sample surfaces were polished to remove existing oxides using 1000-grit SiC paper. The alloys were electron beam welded using different operating parameters in order to obtain sufficient toughness and strength. The electron acceleration voltage was 60 kV, and a beam current of 20 mA was used. The gun–specimen distance was 300 mm, and the electron beam was focused on the top surface of the samples; in each case, the welding speed was 66 mm s 1. The EBIP (dw) was
1359-6462/$ - see front matter Ó 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.scriptamat.2011.06.032
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displaced 0, 0.1, 0.15, 0.2 and 0.3 mm from the interface onto the BMG. The welded samples were observed using scanning electron microscopy (SEM) on a polished cross-section that was etched in a solution of 100 ml H2O, 5 ml H2O2 and 2 ml HF. The glassy phase was investigated by micro-area X-ray diffractometry using Cu Ka radiation. A three-point bending test was carried out to evaluate the toughness and bonding strength of the interface. The three-point bending test was performed on 30 mm 5 mm 1.9 mm welded samples at a crosshead speed of 0.16 mm min 1, using an Instron testing machine. In order to estimate the weld strength, the three-point bending strength of the welded sample was examined. Figure 1 shows the three-point bending strength of welded samples at each EBIP (dw = 0, 0.1, 0.15, 0.2 and 0.3 from the interface onto the BMG) using a beam current of 20 mA and a welding speed of 66 mm s 1. The bending strength of the welded sample at dw = 0 is 109 MPa and increases with increasing EBIP up to dw = 0.2, at which level the bending strength is 448 MPa. However, a further increase in EBIP causes a decrease in bending strength. When the electron beam was irradiated 0.4 mm from interface (dw = 0.4 mm), a gap was observed at the interface because welding was not achieved. Figure 2 shows the morphologies, microstructures and phase evolution of the Zr41Be23Ti14Cu12Ni10 BMG/STS316L interfaces of the dw = 0 and 0.2 mm welded samples. As shown in Figure 2(a) and (b), full penetration was obtained for both beam irradiation conditions; however, the melting region of STS316L varied as the EBIP grew more distant from the interface. This is due to the melting temperature of the STS316L (1661 K), which is much higher than that of the BMG alloy (1030 K), and the peak temperature of the STS316L, which decreased as the EBIP receded from the interface [9]. Therefore, the shape of the BMG alloy changes more than that of the STS316L steel after welding. Figure 2(c) and (d) shows the microstructures at an upper zone of the welded BMG/STS316L joint for each EBIP. The microstructure near the interface is quite different depending on the EBIP used. In both conditions, dw = 0 and 0.2 mm, the BMG region is crystallized.
Bending strength (MPa)
600 500 400 300 200 100 0 0.0
0.1
0.2
0.3
Beam irradiated position from interface (mm) Figure 1. Three-point bending strength of Zr-based BMG/STS316L steel welded samples under various electron beam welding conditions.
Figure 2. Low-magnification SEM micrographs of the polished and etched cross-section interface of BMG and STS316L steel weld for (a) dw = 0 and (b) dw = 0.2 mm; the microstructure of the upper region interface for (c) dw = 0 and (d) dw = 0.2 mm; the microstructure of the lower region interface for (e) dw = 0 and (f) dw = 0.2 mm; and (g and h) micro-area X-ray diffraction in the STS316L, weld metal and HAZ.
However, the crystallized region in the joint welded with dw = 0 mm is thicker than that welded with dw = 0.2 mm. This is because the amount of STS316L steel melting increased with the beam position at the interface. Figure 2(e) and (f) shows the microstructures at lower zones of the welded samples. Overall, the BMG region was crystallized in the joint (both upper and lower regions) when welded with dw = 0 mm; however, for dw = 0.2, a sound interface is present only in the lower region. Moreover, the phases of the HAZ retained an amorphous structure after welding, as shown in Figure 2(g) and (h). Welding with dw = 0 mm results in a weld metal formed by crystallization of both upper and lower regions. However, welding with dw = 0.2 mm results in the crystallization of only the upper region. This is due to a difference in the melting temperatures of the base metals and implies that melting occurred relatively easily in the BMG during the welding process. The melting of STS316L then changes the chemical composition in the weld, which has a significant influence on the critical cooling rate of the amorphous structure. Although the Zr41Be23Ti14Cu12Ni10 BMG alloy has good glass-forming ability over a wide compositional range, it decreases sharply due to the diffusion of elements in the STS316L
J. Kim, Y. Kawamura / Scripta Materialia 65 (2011) 1033–1036
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Figure 3. Schematic diagrams for (a and b) the shape of the melting region for different electron beam irradiation positions and (c) the new STS316L design in order to minimize the melting of the STS316L steel.
Figure 4. Low-magnification SEM micrograph of the polished and etched cross-section interface of the weld for (a) dw = 0.2 with the designed STS316L; microstructures of interface for only the (b) upper and (c) lower regions; and (d) flexural stress–deflection curves of the STS316L steel and welded samples with varying STS316L specimen shape.
steel. Therefore, the shape of the melting region should be optimized. The schematic diagram, Figure 3(a) and (b), shows the shape of the melting regions from welding for dw = 0 and 0.2 mm, respectively. The melting region of the STS316L decreased with increasing EBIP. This melting of the STS316L steel can change the chemical composition of the weld because of mixing of the STS316L steel and Zr-based BMG alloys. The melting region of STS316L for dw = 0 is shown Figure 3(a). The STS316L steel melting occurs in the entire interfacial region. However, when the electron beam traveled 0.2 mm on the BMG side of the interface (dw = 0.2), the interface was divided into two different
regions (crystallized and good welded region). In the crystallized region, it appears that the peak temperature was higher than the melting temperature of STS316L steel. Therefore, in future applications, the melting of STS316L should be controlled during welding of Zrbased BMG to STS316L steel. To minimize the STS316L steel melting in the electron beam welding, the STS316L welding specimen should be designed to minimize melting of STS316L steel (see Fig. 3(c)). Figure 4(a)–(c) shows SEM microstructures of the Zr-based BMG/STS316L steel joints welded at an EBIP of dw = 0.2 mm and using the STS316L specimen design shown in Figure 3(c). There are no defects in the weld and the change in the shape of the STS316L steel after
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welding is less than that for the undesigned STS316L steel specimen. It can be seen that with the Zr-based BMG/STS316L steel both the upper and lower region interfaces of the welding sample are homogeneous and pore free. In the upper region interface, a reaction region with a thickness of about 2 lm was observed. However, the thickness of the reaction region is less than that from using an undesigned specimen. Figure 4(d) shows the flexural stress–deflection curves of the welded sample with various EBIPs, together with data for STS316L steel. The yield flexural stress was measured at 478 MPa for STS316L steel. The maximum flexural stress of the BMG/STS316L sample at dw = 0.2 was measured at 408 MPa. The welded sample fractured at the interface between the BMG and STS316L steel. In the sample using the designed STS316L steel, the maximum flexural stress was measured at 642 MPa, which is higher than the yield strength of the STS316L steel. Overall, the present data indicate that the welded sample has adequate joint strength for use in industrial applications. In conclusion, electron beam welding of Zr41Be23Ti14Cu12Ni10 BMG to STS316L steel was attempted in an effort to extend the industrial applicability of BMGs. The results are summarized as follows:
(1) A 2 mm thick Zr41Be23Ti14Cu12Ni10 BMG plate was successfully welded to STS316L steel using an electron acceleration voltage of 60 kV, a beam current of 20 mA, a welding speed of 66 mm s 1 and an EBIP between dw = 0 and dw = 0.3 mm from the interface to the BMG. A welding strength of 642 MPa, which is higher than the yield strength of STS316L base metal, was achieved using an EBIP of dw = 0.2 mm and an engineered STS316L steel sample design. (2) A reaction region at the interface was unavoidable within the scope of this study. [1] A. Peker, W.L. Johnson, Appl. Phys. Lett. 63 (1993) 2342. [2] A. Inoue, N. Nishiyama, T. Matsuda, Mater. Trans. JIM 37 (1996) 181. [3] A. Inoue, Acta Mater. 48 (2000) 279. [4] S.Y. Shin, J.H. Kim, D.M. Lee, J.K. Lee, H.J. Kim, H.G. Jeong, J.C. Bae, Mater. Sci. Forum. 449–452 (2004) 945. [5] D.E. Powers, in: Proceedings of the Conference on Power Beam Processing, 1988, p. 25. [6] Y. Kawamura, S. Kagao, Y. Ohno, Mater. Trans. JIM 42 (2001) 2649. [7] J. Kim, Y. Kawamura, Scripta Mater. 56 (2007) 709. [8] J. Kim, Y. Kawamura, Mater. Proc. Tech. 207 (2008) 112. [9] W.L. Johnson, MRS Symp. Proc. 554 (1999) 311.
Scripta Materialia 65 (2011) 1033–1036 www.elsevier.com/locate/scriptamat
Dissimilar welding of Zr41Be23Ti14Cu12Ni10 bulk metallic glass and stainless steel Jonghyun Kim⇑ and Y. Kawamura Department of Material Science, Kumamoto University, Kumamoto 860-8555, Japan Received 5 May 2011; revised 18 June 2011; accepted 20 June 2011 Available online 24 June 2011
The electron beam welding of 2 mm thick Zr41Be23Ti14Cu12Ni10 bulk metallic glass (BMG) plate to stainless steel was investigated. The BMG was welded to specially designed stainless steel with an electron beam acceleration voltage of 60 kV, a beam current of 20 mA, a welding speed of 66 mm s 1 and an electron beam irradiation position (dw) of 0.2 mm. The flexural strength of the welded joint (642 MPa) was higher than the yield strength of the stainless steel (478 MPa). Ó 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Bulk metallic glass (BMG); Welding; Interface; Stainless steel
Zr-based bulk metallic glasses (BMGs) are attractive materials for scientific research and engineering applications due to their high strength, high hardness, superior corrosion resistance, wide super-cooled liquid region and good glass-forming ability [1–4]. The maximum thicknesses of some Zr-based BMGs have reached several tenths of millimeters. Nevertheless, homogeneous glassy BMGs are not thick enough for structural applications and additional phase stability would be required during engineering processes. To extend the engineering applications of the BMG material, extensive research efforts have been made to develop welding technologies. Specifically, the welding technology of dissimilar materials, such as BMG materials to crystalline alloys, is an important area that should be developed. The functional use of the specific properties of each material in dissimilar material combinations provides flexible design possibilities for products. Electron beam welding has been in practical use for many years, its main advantage being its ability to produce joints with a deep and narrow weld, a minimal heat-affected zone (HAZ), a high cooling rate and insignificant deformation [5]. With electron beam irradiation, the absolute heat input is much lower than common conventional welding processes, such as gas tungsten arc and gas metal arc welding, but the input energy is focused on a highly localized area of the millimeter scale.
⇑ Corresponding
author. Tel.: +81 96 342 3705; fax: +81 96 342 3710; e-mail: [email protected]
Kawamura et al. [6–8] have reported that the Zr41Be23Ti14Cu12Ni10 BMG can be successfully welded to crystalline Zr, Ti and Ni metal without welding defects or crystallization in the weld. Stainless steel (STS) is used more widely than pure metal, such as Zr, Ti and Ni, in a range of applications in the automotive and marine industries. Unfortunately, the welding of BMG to stainless steel is not successful due to its tendency to crystallize in the weld metal. Therefore, we attempted electron beam welding of BMG to STS metal in an effort to extend the possible industrial applications of BMG. In this paper we report the successful results of electron beam welding of Zr41Be23Ti14Cu12Ni10 bulk metallic glass to crystalline STS metal, the effects of welding specimen shape and the effect of the horizontal focal point position (electron beam irradiation position: EBIP (dw)) on the phase composition. Zr41Be23Ti14Cu12Ni10 BMG (25 mm 15 mm 2 mm) and STS316L (30 mm 15 mm 2 mm; C = 0.02, Si = 0.8, Mn = 1.2, Cr = 16.5, Ni = 13.4, Mo = 2.4 and Fe = Bal. (wt.%)) plates were prepared for electron beam welding, which was carried out in a vacuum of 5 10 4 torr using a maximum power of 9 kW. All the sample surfaces were polished to remove existing oxides using 1000-grit SiC paper. The alloys were electron beam welded using different operating parameters in order to obtain sufficient toughness and strength. The electron acceleration voltage was 60 kV, and a beam current of 20 mA was used. The gun–specimen distance was 300 mm, and the electron beam was focused on the top surface of the samples; in each case, the welding speed was 66 mm s 1. The EBIP (dw) was
1359-6462/$ - see front matter Ó 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.scriptamat.2011.06.032
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displaced 0, 0.1, 0.15, 0.2 and 0.3 mm from the interface onto the BMG. The welded samples were observed using scanning electron microscopy (SEM) on a polished cross-section that was etched in a solution of 100 ml H2O, 5 ml H2O2 and 2 ml HF. The glassy phase was investigated by micro-area X-ray diffractometry using Cu Ka radiation. A three-point bending test was carried out to evaluate the toughness and bonding strength of the interface. The three-point bending test was performed on 30 mm 5 mm 1.9 mm welded samples at a crosshead speed of 0.16 mm min 1, using an Instron testing machine. In order to estimate the weld strength, the three-point bending strength of the welded sample was examined. Figure 1 shows the three-point bending strength of welded samples at each EBIP (dw = 0, 0.1, 0.15, 0.2 and 0.3 from the interface onto the BMG) using a beam current of 20 mA and a welding speed of 66 mm s 1. The bending strength of the welded sample at dw = 0 is 109 MPa and increases with increasing EBIP up to dw = 0.2, at which level the bending strength is 448 MPa. However, a further increase in EBIP causes a decrease in bending strength. When the electron beam was irradiated 0.4 mm from interface (dw = 0.4 mm), a gap was observed at the interface because welding was not achieved. Figure 2 shows the morphologies, microstructures and phase evolution of the Zr41Be23Ti14Cu12Ni10 BMG/STS316L interfaces of the dw = 0 and 0.2 mm welded samples. As shown in Figure 2(a) and (b), full penetration was obtained for both beam irradiation conditions; however, the melting region of STS316L varied as the EBIP grew more distant from the interface. This is due to the melting temperature of the STS316L (1661 K), which is much higher than that of the BMG alloy (1030 K), and the peak temperature of the STS316L, which decreased as the EBIP receded from the interface [9]. Therefore, the shape of the BMG alloy changes more than that of the STS316L steel after welding. Figure 2(c) and (d) shows the microstructures at an upper zone of the welded BMG/STS316L joint for each EBIP. The microstructure near the interface is quite different depending on the EBIP used. In both conditions, dw = 0 and 0.2 mm, the BMG region is crystallized.
Bending strength (MPa)
600 500 400 300 200 100 0 0.0
0.1
0.2
0.3
Beam irradiated position from interface (mm) Figure 1. Three-point bending strength of Zr-based BMG/STS316L steel welded samples under various electron beam welding conditions.
Figure 2. Low-magnification SEM micrographs of the polished and etched cross-section interface of BMG and STS316L steel weld for (a) dw = 0 and (b) dw = 0.2 mm; the microstructure of the upper region interface for (c) dw = 0 and (d) dw = 0.2 mm; the microstructure of the lower region interface for (e) dw = 0 and (f) dw = 0.2 mm; and (g and h) micro-area X-ray diffraction in the STS316L, weld metal and HAZ.
However, the crystallized region in the joint welded with dw = 0 mm is thicker than that welded with dw = 0.2 mm. This is because the amount of STS316L steel melting increased with the beam position at the interface. Figure 2(e) and (f) shows the microstructures at lower zones of the welded samples. Overall, the BMG region was crystallized in the joint (both upper and lower regions) when welded with dw = 0 mm; however, for dw = 0.2, a sound interface is present only in the lower region. Moreover, the phases of the HAZ retained an amorphous structure after welding, as shown in Figure 2(g) and (h). Welding with dw = 0 mm results in a weld metal formed by crystallization of both upper and lower regions. However, welding with dw = 0.2 mm results in the crystallization of only the upper region. This is due to a difference in the melting temperatures of the base metals and implies that melting occurred relatively easily in the BMG during the welding process. The melting of STS316L then changes the chemical composition in the weld, which has a significant influence on the critical cooling rate of the amorphous structure. Although the Zr41Be23Ti14Cu12Ni10 BMG alloy has good glass-forming ability over a wide compositional range, it decreases sharply due to the diffusion of elements in the STS316L
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Figure 3. Schematic diagrams for (a and b) the shape of the melting region for different electron beam irradiation positions and (c) the new STS316L design in order to minimize the melting of the STS316L steel.
Figure 4. Low-magnification SEM micrograph of the polished and etched cross-section interface of the weld for (a) dw = 0.2 with the designed STS316L; microstructures of interface for only the (b) upper and (c) lower regions; and (d) flexural stress–deflection curves of the STS316L steel and welded samples with varying STS316L specimen shape.
steel. Therefore, the shape of the melting region should be optimized. The schematic diagram, Figure 3(a) and (b), shows the shape of the melting regions from welding for dw = 0 and 0.2 mm, respectively. The melting region of the STS316L decreased with increasing EBIP. This melting of the STS316L steel can change the chemical composition of the weld because of mixing of the STS316L steel and Zr-based BMG alloys. The melting region of STS316L for dw = 0 is shown Figure 3(a). The STS316L steel melting occurs in the entire interfacial region. However, when the electron beam traveled 0.2 mm on the BMG side of the interface (dw = 0.2), the interface was divided into two different
regions (crystallized and good welded region). In the crystallized region, it appears that the peak temperature was higher than the melting temperature of STS316L steel. Therefore, in future applications, the melting of STS316L should be controlled during welding of Zrbased BMG to STS316L steel. To minimize the STS316L steel melting in the electron beam welding, the STS316L welding specimen should be designed to minimize melting of STS316L steel (see Fig. 3(c)). Figure 4(a)–(c) shows SEM microstructures of the Zr-based BMG/STS316L steel joints welded at an EBIP of dw = 0.2 mm and using the STS316L specimen design shown in Figure 3(c). There are no defects in the weld and the change in the shape of the STS316L steel after
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welding is less than that for the undesigned STS316L steel specimen. It can be seen that with the Zr-based BMG/STS316L steel both the upper and lower region interfaces of the welding sample are homogeneous and pore free. In the upper region interface, a reaction region with a thickness of about 2 lm was observed. However, the thickness of the reaction region is less than that from using an undesigned specimen. Figure 4(d) shows the flexural stress–deflection curves of the welded sample with various EBIPs, together with data for STS316L steel. The yield flexural stress was measured at 478 MPa for STS316L steel. The maximum flexural stress of the BMG/STS316L sample at dw = 0.2 was measured at 408 MPa. The welded sample fractured at the interface between the BMG and STS316L steel. In the sample using the designed STS316L steel, the maximum flexural stress was measured at 642 MPa, which is higher than the yield strength of the STS316L steel. Overall, the present data indicate that the welded sample has adequate joint strength for use in industrial applications. In conclusion, electron beam welding of Zr41Be23Ti14Cu12Ni10 BMG to STS316L steel was attempted in an effort to extend the industrial applicability of BMGs. The results are summarized as follows:
(1) A 2 mm thick Zr41Be23Ti14Cu12Ni10 BMG plate was successfully welded to STS316L steel using an electron acceleration voltage of 60 kV, a beam current of 20 mA, a welding speed of 66 mm s 1 and an EBIP between dw = 0 and dw = 0.3 mm from the interface to the BMG. A welding strength of 642 MPa, which is higher than the yield strength of STS316L base metal, was achieved using an EBIP of dw = 0.2 mm and an engineered STS316L steel sample design. (2) A reaction region at the interface was unavoidable within the scope of this study. [1] A. Peker, W.L. Johnson, Appl. Phys. Lett. 63 (1993) 2342. [2] A. Inoue, N. Nishiyama, T. Matsuda, Mater. Trans. JIM 37 (1996) 181. [3] A. Inoue, Acta Mater. 48 (2000) 279. [4] S.Y. Shin, J.H. Kim, D.M. Lee, J.K. Lee, H.J. Kim, H.G. Jeong, J.C. Bae, Mater. Sci. Forum. 449–452 (2004) 945. [5] D.E. Powers, in: Proceedings of the Conference on Power Beam Processing, 1988, p. 25. [6] Y. Kawamura, S. Kagao, Y. Ohno, Mater. Trans. JIM 42 (2001) 2649. [7] J. Kim, Y. Kawamura, Scripta Mater. 56 (2007) 709. [8] J. Kim, Y. Kawamura, Mater. Proc. Tech. 207 (2008) 112. [9] W.L. Johnson, MRS Symp. Proc. 554 (1999) 311.