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Ni joints: Effect of beam irradiation position on mechanical and microstructural properties
j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 7 ( 2 0 0 8 ) 112–117
journal homepage: www.elsevier.com/locate/jmatprotec
Electron beam welding of Zr-based BMG/Ni joints: Effect of beam irradiation position on mechanical and microstructural properties Jonghyun Kim ∗ , Y. Kawamura Department of Material Science, Kumamoto University, Kumamoto 860-8555, Japan
a r t i c l e
i n f o
a b s t r a c t
Article history:
The electron beam welding of 3 mm thick Zr41 Be23 Ti14 Cu12 Ni10 bulk metallic glass (BMG)
Received 12 May 2007
plate to Ni metal was investigated. The BMG was welded to Ni with an electron acceleration
Received in revised form
voltage of 60 kV, a beam current of 13.5 mA, welding speed of 45 mm/s and dw = 0.2 mm. The
5 November 2007
flexural strength of the welded joint (415 MPa) was higher than the yield strength of the Ni
Accepted 14 December 2007
(345 MPa). The electron beam irradiation position had a strong influence on the joint properties. When the electron beam was too far from the crystalline metal, no joining was achieved, and when the electron beam was too close to the crystalline metal it led to crystallization
Keywords:
in the weld.
Bulk metallic glasses
© 2007 Elsevier B.V. All rights reserved.
Welding Interface Ni metal
1.
Introduction
Bulk metallic glasses (BMGs) have excellent strength, hardness and superior corrosion resistance because the atoms in them are randomly placed like those in a liquid. With slow cooling rates, some BMGs can be fabricated directly from the melt into bulk form with a thickness of several tenths of a millimeter (Peker and Johnson, 1993; Inoue et al., 1996; Inoue, 2000; Shin et al., 2004). In spite of such superior characteristics, BMGs have been used in only a few industrial applications such as electrical products and sporting goods, because homogeneous glassy BMGs are not thick enough for structural applications. Recently, this problem was solved by the welding of Zr-based and Cu-based BMGs, which have a relatively low glass forming ability (GFA) (Shoji et al., 2003; Kim et al., 2006, 2007). Moreover, BMGs have been successfully welded to crystalline metals such as Zr and Ti (Kawamura et al., 2001; Kawamura, 2004;
∗
Corresponding author. Tel.: +81 96 342 3705; fax: +81 96 342 3710. E-mail address: [email protected] (J. Kim). 0924-0136/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jmatprotec.2007.12.090
Kim and Kawamura, 2007). In order to extend the engineering applications of BMG materials, welding techniques to join dissimilar materials such as BMGs and commercial crystalline alloys should be developed. The welding of dissimilar metals has been an object of investigation for many years; its growing importance is justified by its technical and economic potential. The functional use of the specific properties of each material in dissimilar material combinations provides flexible design possibilities for products. The weldability of dissimilar metals (BMG/crystalline metal) is determined by the phase transformation of the weld metal in the heat affected zone (HAZ). Reactions in the weld metal and HAZ often result in the formation of inter-metallic phases (crystallization), the majority of which are hard and brittle and are thus detrimental to the mechanical properties and ductility of the joint. Therefore, the weld (weld metal and HAZ) should make use of controlled
j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 7 ( 2 0 0 8 ) 112–117
crystallization to obtain a sound junction between the BMG and the crystalline metal. The welding of BMG to Ni metal was attempted by electron beam welding, which creates a deep and narrow weld, a minimal HAZ and a high cooling rate because the input energy is focused on a highly localized area (on the scale of millimeters) (Powers, 1988). In this present work is focused on the dissimilar welding of electron beam welding of Zr41 Be23 Ti14 Cu12 Ni10 BMG to crystalline Ni metal and the effect of the horizontal focal point position (electron beam irradiation position: EBIP (dw)) on the phase composition.
2.
Experimental procedure
A Zr41 Be23 Ti14 Cu12 Ni10 BMG plate (25 mm × 15 mm × 2 mm) and Ni metal (30 mm × 15 mm × 2 mm) were prepared for electron beam welding. All the sample surfaces were polished to remove existing oxides using SiC paper with 1000-grit. The formation of a single glassy structure was confirmed by X-ray diffractometry. The butt-welding was carried out at a pressure of 5 × 10−4 Torr using an electron beam welding system with a maximum power of 9 kW. 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 beam currents of 13.5 and 20 mA were used. The gun-specimen distance was 300 mm, the electron beam was focused on the top surface of the samples and the welding speed was 45 and 66 mm/s. The EBIP (dw) was displaced 0, 0.2 and 0.4 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 H2 O, 5 ml H2 O2 and 2 ml HF. The distribution of chemical elements in the weld metal and the interface was determined by energy dispersive X-ray spec-
113
troscopy (EDS) in an environmental SEM. The glassy phase was investigated by micro-area X-ray diffractometry using Cu K␣ 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 at a cross-head speed of 0.16 mm/min for 30 mm × 5 mm × 1.9 mm welded samples using an Instron testing machine.
3.
Results and discussion
Fig. 1 shows the morphologies and microstructures of the BMG/Ni interface of the welded samples at each EBIP (dw = 0, 0.2 and 0.4 mm from the interface onto the BMG) using a beam current of 20 mA and a welding speed of 66 mm/s. As shown in Fig. 1(a) through (c), full penetration was obtained at all beam irradiation conditions, however, the melting region of the Ni metal varied as the EBIP grew more distant from the interface. This is due to the melting temperature of the Ni metal (1726 K), which is much higher than that of the Zr41 Be23 Ti14 Cu12 Ni10 BMG alloy (1030 K) and the peak temperature of the Ni metal, which decreased as the EBIP receded from the interface (Johnson, 1999). Therefore, the shape of the BMG alloy changes more than that of the Ni metal after welding. Fig. 1(d) and (e) shows the microstructure of the welded BMG/Ni metal joint produced by different EBIPs. The microstructure near the interface is quite different depending on the EBIP used. The interface of the welded sample with dw = 0.2 and 0.4 mm is largely divided to into three regions: a region of BMG, a reaction region containing columnar dendrites and a Ni metal region. The reaction region in the joint welded with dw = 0 mm is thicker than that of the welded sample with dw = 0.2 mm. This is because the amount of Ni metal melting increased with the beam position at the interface. When the electron beam was irradiated 0.4 mm from interface
Fig. 1 – Low-magnification SEM micrographs of the polished and etched cross-section interface of weld: (a) dw = 0, (b) dw = 0.2 and (c) dw = 0.4 mm and microstructures of interface between BMG and Ni metal: (d) dw = 0, (e) dw = 0.2 and (f) dw = 0.4 mm.
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j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 7 ( 2 0 0 8 ) 112–117
(dw = 0.4 mm), a gap was observed at the interface because the welding was not achieved. Fig. 2 shows micro-area X-ray diffraction data for the BMG, weld metal and Ni metal in BMG/Ni welded samples as marked in Fig. 1(b) and (c). For the sample welded with dw = 0.2 mm, Fig. 2(a) shows the formation of a Zr2 Ni phase due to crystallization in the weld metal. However, with the beam 0.4 mm from the interface (dw = 0.4 mm), only broad halo patterns were found in the weld metal. Fig. 3 shows fracture morphologies from the three-point bending test on samples produced by dw = 0, 0.2 and 0.4 mm. The bending strength was 95, 198 and 125 MPa, respectively (Fig. 7). The fracture surface shows brittle, cleavage-like fractures, which dominate the dw = 0 sample (Fig. 3(a)). When the electron beam was directed at dw = 0.4 mm, welding was not achieved over most of the area of the fracture surface. However, when the electron beam was aimed 0.2 mm from the interface (dw = 0.2 mm), the fracture surface shows three regions: brittle fracture (A), ductile fracture (B) and non-welded region (C). In particular, in the ductile fracture region (middle region) vein patterns typical of the BMG were observed. To understand these phenomena, we should consider the shape of the melting area together with the melting temperature of the base metals. Kawamura et al. (2001) reported that Zr41 Be23 Ti14 Cu12 Ni10 BMG was successfully welded to crystalline Zr and Ti metals without crystallization in the weld (Kawamura, 2004; Kim and Kawamura, 2007). As mentioned above, in the electron beam welding of a BMG to Ni metal, the melting appears to have occurred in the BMG region, most likely because the melting temperature of Ni is much higher than that of the Zr-based BMG alloy. The melting temperature of the Ni metal (1726 K) is lower than that of Zr (2125 K) and Ti (1941 K) metals. This different melting temperature of base metal implies that melting occurred relatively easily in the Ni metal during the welding process, changing the chemical composition in the weld. This chemical composition change
Fig. 2 – Micro-area X-ray diffractions in the Ni metal, heat affected zone and weld metal as marked in Fig. 1: (a) dw = 0.2 and (b) dw = 0.4 mm.
Fig. 3 – Fracture surfaces of the welded samples after the bending test where the dw (a) 0, (b) 0.2 and (c) 0.4 mm and showing (d) the vein patterns typical characteristics of BMG in the middle region as marked in (b) B.
j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 7 ( 2 0 0 8 ) 112–117
115
Fig. 4 – Schematic diagrams for shape of melting region according to different electron beam irradiation position.
has a significant influence on the critical cooling rate of the amorphous structure. The schematic diagram (Fig. 4) shows the shape of the melting regions from the electron beam welding of BMG to Ni metal with different electron beam irradiation positions. The higher thermal conductivity and melting temperature of Ni tends to rapidly dissipate heat away from the weld leading to difficulties in reaching the melting temperature. The melting region of the Ni metal decreased with increasing distance of the electron beam irradiation position from the interface. This melting of the Ni metal can change the chemical composition of the weld because of mixing of the BMG and Ni metal. All of the mechanical and thermal properties and the GFA of a BMG are determined by its chemical composition. The crystallization of the weld metal depends on whether the cooling rate is lower than its critical cooling rate. Any change in the chemical composition from the optimum original chemistry has a significant influence on the critical cooling rate. When the electron beam traveled 0.2 mm on the BMG side of the interface (dw = 0.2), the interface was divided into three different regions (crystallized, welded and non-welded regions). In the non-welded region, it appears that the peak temperature was not high enough to make the molten BMG alloy wet the Ni metal. The electron beam welding of BMG to Ni metal seems to show a stronger dependence on the dw than the welding of BMG to Zr and Ti metal because the melting temperature of Ni is lower than that of these metals. Therefore, melting occurred easily for the Ni metal during the welding, which changed the chemical composition of the weld material. The size of dw seems to have strongly affected joint properties. This shows that if the electron beam is too far from the crystalline metal, no joining is achieved, and if the electron beam is too close to the crystalline metal it leads to crystallization in the weld material. Therefore, the shape of the melting region should be controlled in the welding of BMG to Ni metal. For electron beam welding, the shape of the melting region was influenced by the welding parameters (especially, beam current and welding speed). When the beam current increased, a deeper and narrower weld was achieved.
To increase the temperature around the interface and minimize the Ni melting region, electron beam welding with a beam current of 13.5 mA and welding speed of 45 mm/s was used. Fig. 5 shows the SEM microstructure of the BMG/Ni metal joint welded with a beam current of 13.5 mA, a welding speed of 45 mm/s and dw = 0.2 mm. It can be seen that the BMG/Ni metal interface of the welding sample was homogeneous and pore free. A reaction region with a thickness of about 2 m was observed. However, the thickness of the reaction region was less than that from a beam current of 20 mA and a welding speed of 66.6 mm/s. Fig. 6 shows the concentrations of Zr, Ni, Ti and Cu across the weld material, HAZ and base metal under different welding conditions. The sample welded with a beam current of 20 mA, a welding speed of 66.6 mm/s and dw = 0 shows three different regions. This indicates that the chemical composition of the weld material changed after welding. However, in the samples welded with a beam current of 13.5 mA, a weld-
Fig. 5 – Microstructure near the interface zone of BMG/Ni electron beam welded joint with a beam current of 13.5 mA, welding speed of 45 mm/s and dw = 0.2 mm.
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j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 7 ( 2 0 0 8 ) 112–117
Fig. 7 – Flexural stress–deflection curves of the Ni metal and welded sample with various e-beam welding conditions (a) with a beam current of 20 mA, welding speed of 66.6 mm/s and dw = 0, (b) 20 mA, 66.6 mm/s and dw = 0.2, (c) 20 mA, 66.6 mm/s and dw = 0.4 and (d) 13.5 mA, 45 mm/s and dw = 0.2.
4.
Conclusion
We have examined the conditions (electron beam irradiation position; EBIP) for welding Zr41 Be23 Ti14 Cu12 Ni10 BMG to Ni metal with electron beam welding. The results are summarized as follows.
Fig. 6 – Qualitative line profiles across the joint (a) with a beam current of 20 mA, welding speed of 66.6 mm/s and dw = 0 and (b) with a beam current of 13.5 mA, welding speed of 45 mm/s and dw = 0.2 mm.
(1) The electron beam welding of Zr41 Be23 Ti14 Cu12 Ni10 BMG to Ni metal was achieved using an electron acceleration voltage of 60 kV, a beam current of 13.5 mA, a welding speed of 45 mm/s and dw = 0.2 mm. The flexural strength of the welded joint (415 MPa) was higher than the yield strength of the Ni metal (345 MPa). (2) The dw has a strong influence on joint microstructure and mechanical properties. When the electron beam was too far from the crystalline metal, no joining was achieved, and when the electron beam was too close to crystalline metal it leads to crystallization in the weld material.
references ing speed of 45 mm/s and dw = 0.2 mm, there was no change in the concentration distribution of elements in the weld material. In order to estimate weld strength, we examined the threepoint bending strength of the Ni metal and the welded sample. Fig. 7 shows the flexural stress–deflection curves of the welded sample with various e-beam welding conditions together with data for Ni. The yield flexural stress was measured at 345 MPa for Ni metal. For the welded sample with a beam current of 13 mA, welding speed of 45 mm/s and dw = 0.2, the maximum flexural stress was 415 MPa. The welded sample fractured on the interface between the BMG and the Ni metal. However, the flexural strength of 415 MPa is higher than the yield strength of the Ni metal. This present data indicate that the welded sample had adequate joint strength for use in industrial applications.
Inoue, A., 2000. Stabilization of metallic supercooled liquid and bulk amorphous alloys. Acta Mater. 48, 279–306. Inoue, A., Nishiyama, N., Matsuda, T., 1996. Preparation of bulk glassy Pd40 Ni10 Cu30 P20 alloy of 40 mm in diameter by water quenching. Mater. Trans. JIM 37, 181–184. Johnson, W.L., 1999. Bulk glass-forming metallic alloys: science and technology. MRS Symp. Proc. 554, 311–339. Kawamura, Y., 2004. Liquid phase and supercooled liquid phase welding of bulk metallic glasses. Mater. Sci. Eng. A 375–377, 112–119. Kawamura, Y., Kagao, S., Ohno, Y., 2001. Electron beam welding of Zr-based bulk metallic glass to crystalline Zr metal. Mater. Trans. JIM 42, 2649–2651. Kim, J., Kawamura, Y., 2007. Electron beam welding of the dissimilar Zr-based bulk metallic glass and Ti metal. Scr. Mater. 56, 709–712.
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Kim, J., Lee, D., Shin, S., Lee, C., 2006. Phase evolution in Cu54 Ni6 Zr22 Ti18 bulk metallic glass Nd:YAG laser weld. Mater. Sci. Eng. A 434, 194–201. Kim, J., Lee, C., Lee, D.M., Sun, J.H., Shin, S.Y., Bae, J.C., 2007. Pulsed Nd:YAG laser welding of Cu54 Ni6 Zr22 Ti18 bulk metallic glass. Mater. Sci. Eng. A 449–451, 872–875. Peker, A., Johnson, W.L., 1993. A highly processable metallic glass: Zr41.2 Ti13.8 Cu12.5 Ni10.0 Be22.5 . Appl. Phys. Lett. 63, 2342–2344. Powers, D.E., 1988. Electron beam welding–an over view. In: Metzbower, E.A., Hauser, D. (Eds.), Proceedings of the
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Conference on Power Beam Processing. ASM International, Ohio, pp. 25–33. Shin, S.Y., Kim, J.H., Lee, D.M., Lee, J.K., Kim, H.J., Jeong, H.G., Bae, J.C., 2004. New Cu-based bulk metallic glasses with high strength of 2000 MPa. Mater. Sci. Forum 449–452, 945–948. Shoji, T., Kawamura, Y., Ohno, Y., 2003. Joining of Zr41 Be23 Ti14 Cu12 Ni10 bulk metallic glasses by friction welding method. Mater. Trans. JIM 44, 1809–1816.
journal homepage: www.elsevier.com/locate/jmatprotec
Electron beam welding of Zr-based BMG/Ni joints: Effect of beam irradiation position on mechanical and microstructural properties Jonghyun Kim ∗ , Y. Kawamura Department of Material Science, Kumamoto University, Kumamoto 860-8555, Japan
a r t i c l e
i n f o
a b s t r a c t
Article history:
The electron beam welding of 3 mm thick Zr41 Be23 Ti14 Cu12 Ni10 bulk metallic glass (BMG)
Received 12 May 2007
plate to Ni metal was investigated. The BMG was welded to Ni with an electron acceleration
Received in revised form
voltage of 60 kV, a beam current of 13.5 mA, welding speed of 45 mm/s and dw = 0.2 mm. The
5 November 2007
flexural strength of the welded joint (415 MPa) was higher than the yield strength of the Ni
Accepted 14 December 2007
(345 MPa). The electron beam irradiation position had a strong influence on the joint properties. When the electron beam was too far from the crystalline metal, no joining was achieved, and when the electron beam was too close to the crystalline metal it led to crystallization
Keywords:
in the weld.
Bulk metallic glasses
© 2007 Elsevier B.V. All rights reserved.
Welding Interface Ni metal
1.
Introduction
Bulk metallic glasses (BMGs) have excellent strength, hardness and superior corrosion resistance because the atoms in them are randomly placed like those in a liquid. With slow cooling rates, some BMGs can be fabricated directly from the melt into bulk form with a thickness of several tenths of a millimeter (Peker and Johnson, 1993; Inoue et al., 1996; Inoue, 2000; Shin et al., 2004). In spite of such superior characteristics, BMGs have been used in only a few industrial applications such as electrical products and sporting goods, because homogeneous glassy BMGs are not thick enough for structural applications. Recently, this problem was solved by the welding of Zr-based and Cu-based BMGs, which have a relatively low glass forming ability (GFA) (Shoji et al., 2003; Kim et al., 2006, 2007). Moreover, BMGs have been successfully welded to crystalline metals such as Zr and Ti (Kawamura et al., 2001; Kawamura, 2004;
∗
Corresponding author. Tel.: +81 96 342 3705; fax: +81 96 342 3710. E-mail address: [email protected] (J. Kim). 0924-0136/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jmatprotec.2007.12.090
Kim and Kawamura, 2007). In order to extend the engineering applications of BMG materials, welding techniques to join dissimilar materials such as BMGs and commercial crystalline alloys should be developed. The welding of dissimilar metals has been an object of investigation for many years; its growing importance is justified by its technical and economic potential. The functional use of the specific properties of each material in dissimilar material combinations provides flexible design possibilities for products. The weldability of dissimilar metals (BMG/crystalline metal) is determined by the phase transformation of the weld metal in the heat affected zone (HAZ). Reactions in the weld metal and HAZ often result in the formation of inter-metallic phases (crystallization), the majority of which are hard and brittle and are thus detrimental to the mechanical properties and ductility of the joint. Therefore, the weld (weld metal and HAZ) should make use of controlled
j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 7 ( 2 0 0 8 ) 112–117
crystallization to obtain a sound junction between the BMG and the crystalline metal. The welding of BMG to Ni metal was attempted by electron beam welding, which creates a deep and narrow weld, a minimal HAZ and a high cooling rate because the input energy is focused on a highly localized area (on the scale of millimeters) (Powers, 1988). In this present work is focused on the dissimilar welding of electron beam welding of Zr41 Be23 Ti14 Cu12 Ni10 BMG to crystalline Ni metal and the effect of the horizontal focal point position (electron beam irradiation position: EBIP (dw)) on the phase composition.
2.
Experimental procedure
A Zr41 Be23 Ti14 Cu12 Ni10 BMG plate (25 mm × 15 mm × 2 mm) and Ni metal (30 mm × 15 mm × 2 mm) were prepared for electron beam welding. All the sample surfaces were polished to remove existing oxides using SiC paper with 1000-grit. The formation of a single glassy structure was confirmed by X-ray diffractometry. The butt-welding was carried out at a pressure of 5 × 10−4 Torr using an electron beam welding system with a maximum power of 9 kW. 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 beam currents of 13.5 and 20 mA were used. The gun-specimen distance was 300 mm, the electron beam was focused on the top surface of the samples and the welding speed was 45 and 66 mm/s. The EBIP (dw) was displaced 0, 0.2 and 0.4 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 H2 O, 5 ml H2 O2 and 2 ml HF. The distribution of chemical elements in the weld metal and the interface was determined by energy dispersive X-ray spec-
113
troscopy (EDS) in an environmental SEM. The glassy phase was investigated by micro-area X-ray diffractometry using Cu K␣ 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 at a cross-head speed of 0.16 mm/min for 30 mm × 5 mm × 1.9 mm welded samples using an Instron testing machine.
3.
Results and discussion
Fig. 1 shows the morphologies and microstructures of the BMG/Ni interface of the welded samples at each EBIP (dw = 0, 0.2 and 0.4 mm from the interface onto the BMG) using a beam current of 20 mA and a welding speed of 66 mm/s. As shown in Fig. 1(a) through (c), full penetration was obtained at all beam irradiation conditions, however, the melting region of the Ni metal varied as the EBIP grew more distant from the interface. This is due to the melting temperature of the Ni metal (1726 K), which is much higher than that of the Zr41 Be23 Ti14 Cu12 Ni10 BMG alloy (1030 K) and the peak temperature of the Ni metal, which decreased as the EBIP receded from the interface (Johnson, 1999). Therefore, the shape of the BMG alloy changes more than that of the Ni metal after welding. Fig. 1(d) and (e) shows the microstructure of the welded BMG/Ni metal joint produced by different EBIPs. The microstructure near the interface is quite different depending on the EBIP used. The interface of the welded sample with dw = 0.2 and 0.4 mm is largely divided to into three regions: a region of BMG, a reaction region containing columnar dendrites and a Ni metal region. The reaction region in the joint welded with dw = 0 mm is thicker than that of the welded sample with dw = 0.2 mm. This is because the amount of Ni metal melting increased with the beam position at the interface. When the electron beam was irradiated 0.4 mm from interface
Fig. 1 – Low-magnification SEM micrographs of the polished and etched cross-section interface of weld: (a) dw = 0, (b) dw = 0.2 and (c) dw = 0.4 mm and microstructures of interface between BMG and Ni metal: (d) dw = 0, (e) dw = 0.2 and (f) dw = 0.4 mm.
114
j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 7 ( 2 0 0 8 ) 112–117
(dw = 0.4 mm), a gap was observed at the interface because the welding was not achieved. Fig. 2 shows micro-area X-ray diffraction data for the BMG, weld metal and Ni metal in BMG/Ni welded samples as marked in Fig. 1(b) and (c). For the sample welded with dw = 0.2 mm, Fig. 2(a) shows the formation of a Zr2 Ni phase due to crystallization in the weld metal. However, with the beam 0.4 mm from the interface (dw = 0.4 mm), only broad halo patterns were found in the weld metal. Fig. 3 shows fracture morphologies from the three-point bending test on samples produced by dw = 0, 0.2 and 0.4 mm. The bending strength was 95, 198 and 125 MPa, respectively (Fig. 7). The fracture surface shows brittle, cleavage-like fractures, which dominate the dw = 0 sample (Fig. 3(a)). When the electron beam was directed at dw = 0.4 mm, welding was not achieved over most of the area of the fracture surface. However, when the electron beam was aimed 0.2 mm from the interface (dw = 0.2 mm), the fracture surface shows three regions: brittle fracture (A), ductile fracture (B) and non-welded region (C). In particular, in the ductile fracture region (middle region) vein patterns typical of the BMG were observed. To understand these phenomena, we should consider the shape of the melting area together with the melting temperature of the base metals. Kawamura et al. (2001) reported that Zr41 Be23 Ti14 Cu12 Ni10 BMG was successfully welded to crystalline Zr and Ti metals without crystallization in the weld (Kawamura, 2004; Kim and Kawamura, 2007). As mentioned above, in the electron beam welding of a BMG to Ni metal, the melting appears to have occurred in the BMG region, most likely because the melting temperature of Ni is much higher than that of the Zr-based BMG alloy. The melting temperature of the Ni metal (1726 K) is lower than that of Zr (2125 K) and Ti (1941 K) metals. This different melting temperature of base metal implies that melting occurred relatively easily in the Ni metal during the welding process, changing the chemical composition in the weld. This chemical composition change
Fig. 2 – Micro-area X-ray diffractions in the Ni metal, heat affected zone and weld metal as marked in Fig. 1: (a) dw = 0.2 and (b) dw = 0.4 mm.
Fig. 3 – Fracture surfaces of the welded samples after the bending test where the dw (a) 0, (b) 0.2 and (c) 0.4 mm and showing (d) the vein patterns typical characteristics of BMG in the middle region as marked in (b) B.
j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 7 ( 2 0 0 8 ) 112–117
115
Fig. 4 – Schematic diagrams for shape of melting region according to different electron beam irradiation position.
has a significant influence on the critical cooling rate of the amorphous structure. The schematic diagram (Fig. 4) shows the shape of the melting regions from the electron beam welding of BMG to Ni metal with different electron beam irradiation positions. The higher thermal conductivity and melting temperature of Ni tends to rapidly dissipate heat away from the weld leading to difficulties in reaching the melting temperature. The melting region of the Ni metal decreased with increasing distance of the electron beam irradiation position from the interface. This melting of the Ni metal can change the chemical composition of the weld because of mixing of the BMG and Ni metal. All of the mechanical and thermal properties and the GFA of a BMG are determined by its chemical composition. The crystallization of the weld metal depends on whether the cooling rate is lower than its critical cooling rate. Any change in the chemical composition from the optimum original chemistry has a significant influence on the critical cooling rate. When the electron beam traveled 0.2 mm on the BMG side of the interface (dw = 0.2), the interface was divided into three different regions (crystallized, welded and non-welded regions). In the non-welded region, it appears that the peak temperature was not high enough to make the molten BMG alloy wet the Ni metal. The electron beam welding of BMG to Ni metal seems to show a stronger dependence on the dw than the welding of BMG to Zr and Ti metal because the melting temperature of Ni is lower than that of these metals. Therefore, melting occurred easily for the Ni metal during the welding, which changed the chemical composition of the weld material. The size of dw seems to have strongly affected joint properties. This shows that if the electron beam is too far from the crystalline metal, no joining is achieved, and if the electron beam is too close to the crystalline metal it leads to crystallization in the weld material. Therefore, the shape of the melting region should be controlled in the welding of BMG to Ni metal. For electron beam welding, the shape of the melting region was influenced by the welding parameters (especially, beam current and welding speed). When the beam current increased, a deeper and narrower weld was achieved.
To increase the temperature around the interface and minimize the Ni melting region, electron beam welding with a beam current of 13.5 mA and welding speed of 45 mm/s was used. Fig. 5 shows the SEM microstructure of the BMG/Ni metal joint welded with a beam current of 13.5 mA, a welding speed of 45 mm/s and dw = 0.2 mm. It can be seen that the BMG/Ni metal interface of the welding sample was homogeneous and pore free. A reaction region with a thickness of about 2 m was observed. However, the thickness of the reaction region was less than that from a beam current of 20 mA and a welding speed of 66.6 mm/s. Fig. 6 shows the concentrations of Zr, Ni, Ti and Cu across the weld material, HAZ and base metal under different welding conditions. The sample welded with a beam current of 20 mA, a welding speed of 66.6 mm/s and dw = 0 shows three different regions. This indicates that the chemical composition of the weld material changed after welding. However, in the samples welded with a beam current of 13.5 mA, a weld-
Fig. 5 – Microstructure near the interface zone of BMG/Ni electron beam welded joint with a beam current of 13.5 mA, welding speed of 45 mm/s and dw = 0.2 mm.
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j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 7 ( 2 0 0 8 ) 112–117
Fig. 7 – Flexural stress–deflection curves of the Ni metal and welded sample with various e-beam welding conditions (a) with a beam current of 20 mA, welding speed of 66.6 mm/s and dw = 0, (b) 20 mA, 66.6 mm/s and dw = 0.2, (c) 20 mA, 66.6 mm/s and dw = 0.4 and (d) 13.5 mA, 45 mm/s and dw = 0.2.
4.
Conclusion
We have examined the conditions (electron beam irradiation position; EBIP) for welding Zr41 Be23 Ti14 Cu12 Ni10 BMG to Ni metal with electron beam welding. The results are summarized as follows.
Fig. 6 – Qualitative line profiles across the joint (a) with a beam current of 20 mA, welding speed of 66.6 mm/s and dw = 0 and (b) with a beam current of 13.5 mA, welding speed of 45 mm/s and dw = 0.2 mm.
(1) The electron beam welding of Zr41 Be23 Ti14 Cu12 Ni10 BMG to Ni metal was achieved using an electron acceleration voltage of 60 kV, a beam current of 13.5 mA, a welding speed of 45 mm/s and dw = 0.2 mm. The flexural strength of the welded joint (415 MPa) was higher than the yield strength of the Ni metal (345 MPa). (2) The dw has a strong influence on joint microstructure and mechanical properties. When the electron beam was too far from the crystalline metal, no joining was achieved, and when the electron beam was too close to crystalline metal it leads to crystallization in the weld material.
references ing speed of 45 mm/s and dw = 0.2 mm, there was no change in the concentration distribution of elements in the weld material. In order to estimate weld strength, we examined the threepoint bending strength of the Ni metal and the welded sample. Fig. 7 shows the flexural stress–deflection curves of the welded sample with various e-beam welding conditions together with data for Ni. The yield flexural stress was measured at 345 MPa for Ni metal. For the welded sample with a beam current of 13 mA, welding speed of 45 mm/s and dw = 0.2, the maximum flexural stress was 415 MPa. The welded sample fractured on the interface between the BMG and the Ni metal. However, the flexural strength of 415 MPa is higher than the yield strength of the Ni metal. This present data indicate that the welded sample had adequate joint strength for use in industrial applications.
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