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Tool geometry effect on the characteristics of dissimilar friction stir spot welded bulk metallic glass to lightweight alloys
Journal of Alloys and Compounds xxx (2013) xxx–xxx
Contents lists available at SciVerse ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Tool geometry effect on the characteristics of dissimilar friction stir spot welded bulk metallic glass to lightweight alloys Hyung-Seop Shin ⇑ Department of Mechanical Design Engineering, Andong National University, Andong, Kyungbuk 760-749, Republic of Korea
a r t i c l e
i n f o
Article history: Available online xxxx Keywords: Metallic glasses Friction stir spot welding Superplastic behavior Dissimilar welding Tool geometry
a b s t r a c t A small-scale joining technique for dissimilar friction stir spot welding (FSSW) of a BMG alloy to lightweight crystalline alloys has been developed. An experimental apparatus which could possible give a precise control of the friction time, the plunge speed and the plunge depth of the tool was used. In this study, the influences of tool geometry with different pin shape on the joining characteristics and on the failure load of specimens after dissimilar FSSW were investigated. As a result, it was found that the tool geometry influenced on the welding performance of the BMG alloy to lightweight crystalline alloys at small tool plunge depth and the extent of BMG particles penetrated onto the lightweight alloy side, especially for the Mg alloy/BMG material combination. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction Bulk metallic glasses (BMGs) have attracted much attention due to its unique properties such as high strength, large elastic limit and superior corrosion-resistance ability [1,2]. Several multi-component BMG alloys have recently been developed in bulk sizes at a lower cooling rate of 100 K/s. They have been tried to be applied as a structural material utilizing their superior mechanical properties. However, their engineering and structural applications are still limited due to size applicability. In order to solve the size limit problem and to extend the application field of BMGs, various joining techniques applicable to BMG alloys have been developed. There are some studies on the friction welding of similar and dissimilar BMG rods [3–5]. It is also necessary to develop a new joining technique applicable to sheet parts with a relatively low energy solid-state welding process. Recently, the studies on the application of friction stir welding (FSW) technique to BMG sheets have carried out [6–12]. In order to achieve a good friction stirring in BMG sheets using the FSW process, the studies of related parameters such as material combination, tool geometry, process management involved in thermal and material flow are important. The effect of tool speed on the similar and friction stir spot welding (FSSW) of BMG was investigated by the author’s group [13]. It was found that the BMG showed its characteristic behavior due to superplastic deformation induced in the supercooled liquid region depending significantly on the tool speed. The tool dimension
⇑ Tel.: +82 54 820 5675; fax: +82 54 820 5167. E-mail address: [email protected]
and geometry is also expected to influence on the joining characteristics of FSW or FSSW alloy sheets. The effects of tool dimension on similar FSW or FSSW of Al or Mg alloy sheets have been investigated [14–16], but no result has been reported yet for the dissimilar FSSW of BMG to lightweight alloys. In this study, the dissimilar friction stir spot welding (FSSW) of a BMG alloy to crystalline lightweight alloys has been tried. The influence of tool geometry having different pin shape on the characteristics of dissimilar FSSW between BMG and lightweight alloys were investigated. Depending on the tool geometry, the measurement of the temperature distribution and the vertical load applied during FSSW were carried out and their effects on the failure load as the weld strength were evaluated. 2. Experimental procedures In this study, a commercially available BMG having a composition of Zr41.5Ti13.8Cu12.5Ni10Be22.5 (Vit-1), and an Al alloy (A5052-H32) and an Mg alloy (AZ31B) as lightweight crystalline alloys were supplied. Thermal properties of the Vit-1 BMG that were measured using a differential scanning calorimetry (DSC) are as follows; the glass transition temperature (Tg) and the crystallization temperature (Tx) are 623 K and 705 K, respectively. Therefore, the supercooled liquid region (DTx = Tx Tg) becomes 82 K. The chemical composition and mechanical properties of lightweight crystalline alloys are shown in Tables 1 and 2, respectively. The BMG side specimen for the dissimilar FSSW was machined to a 1 mm-thick coupon-shape with dimensions of 10 80 mm from a 3 mm-thick cast plate. Al and Mg alloy sheets of 1 mm in thickness were cut into a coupon-shape with dimensions of 15 100 mm. Fig. 1 shows the configuration of a single lap-joint structured specimen. The details of apparatus and processes for FSSW used in this study are described in Ref. [11]. The FSSW process was carried out by plunging the rotating tool onto the single lapped specimen as shown in Fig. 1 which is fixed onto the bed of the smallscale computer numerical controlled (CNC) milling machine using a jig. By using the CNC milling machine, it is easy to control both plunge depth and plunge speed of
0925-8388/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2012.12.031
Please cite this article in press as: H.-S. Shin, J. Alloys Comp. (2013), http://dx.doi.org/10.1016/j.jallcom.2012.12.031
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Table 1 The chemical composition of respective lightweight material.
3. Results and discussion
Chemical composition (%) AZ31B A5052-H32
Mg Bal. Al Bal.
Mg 2.80
Al 3.1 Fe 0.4
Cr 0.35
Zn 0.9 Si 0.25
Cu 0.10
Mn 0.4 Mn 0.10
Zn 0.10
Table 2 Mechanical properties of respective lightweight material.
AZ31B A5052-H32
Tensile strength (MPa)
Yield strength (MPa)
Elongation (%)
260 320
200 250
14 10
Fig. 1. Configuration of lap-joint specimen for dissimilar FSSW process.
Fig. 2. Tool geometry adopted; (a) round pin, (b) triangular pin.
the tool. The tool is composed of a cylindrical shoulder part and pin part with diameters of 6.0 and 2.5 mm, respectively, as shown in Fig. 2. To investigate the tool geometry effect on the characteristic behavior during FSSW, two different shapes of pins were used; a round one and a triangular one. The angle of chamfer within the shoulder end surface is 3° and the length of the pin part is 1.1 mm. A plunge speed of 6 mm/min and various plunge depths of 1.8, 1.9 and 2.0 mm were adopted in this study. The tool rotation speed was fixed to 2,700 rpm. During the FSSW process, the temperature distribution around the shoulder of the tool at the surface of the stir zone on the upper-positioned specimen was measured by using an infrared imager (FLIR-ThermaCam SC-2000). The vertical load applied to the specimens was simultaneously measured using a loadcell installed under the jig. The failure load of the specimen after FSSW was evaluated by a tensile-shear test. The SEM observation of the fracture surface and cross section has been done to evaluate the weldability of BMG to lightweight crystalline alloys depending on the tool geometry adopted.
Fig. 3 shows the fracture surface and cross-sectional views of specimens after FSSW using tools having round and triangular shaped pins for respective dissimilar material combinations; wherein (a) is for Al alloy/BMG combination and (b) for Mg alloy/BMG. In these cases, the BMG alloy was located at the lower position of the lap-joint specimen configuration shown in Fig. 1. It could be seen that as the plunge depth of tool increases, the failure mode of specimens after FSSW was changed from the shear fracture at the interface of lap-joint to the pull-out fracture of the upper-positioned alloy material around the indented shoulder part. But in the case of Al alloy/BMG specimens, instead of the shear fracture at interface, they showed that the tensile fracture of the lower-positioned BMG alloy occurred near the stir zone due to the pre-existed defects like voids. The transition of failure mode was related to the balance between the area of weld zone at the interface which was subjected to shear force and the circumferential area surrounding the weld zone from the shoulder edge to the interface which resisted to the pull-out fracture depending on the plunge depth adopted. As the plunge depth increased, the distance from the shoulder edge to the interface decreased; it is 0.3 mm for the plunge depth of 1.8 mm, but it reduces to 0.1 mm for the 2.0 mm case. The cross-sectional views of weld parts represent that good joining were achieved for dissimilar FSSW for both material combinations and the lower-positioned BMG alloy was stirred penetrating into the upper lightweight alloy side which results to a higher failure load [11]. Fig. 4 shows the enlarged views of penetrated BMG particles into the stir zone of upper-positioned Mg alloy side in the case of Mg alloy/BMG combination. From the cross-sectional view of the stirred part, it could be found that in the case of the triangular pin, the lower-positioned BMG alloy was well stirred penetrating into the upper lightweight alloy side which resulted in a higher failure load at larger plunge depth. During dissimilar FSSW process of BMG to lightweight alloys, the maximum temperature around the friction-stirred part and the vertical load applied to the specimen were measured and was shown in Fig. 5(a) and (b), respectively; wherein (a) represents the Al alloy/BMG combination and (b) the Mg alloy one/BMG. As a whole, the behaviors of the vertical load applied with friction time during FSSW were different depending on the pin geometry adopted. By using the round pin tool, the vertical load increased with friction time but it suddenly dropped in magnitude when the pin contacted to the lower-positioned BMG sheet, because the temperature at the stirred zone has reached its supercooled liquid state wherein the BMG starts to show a superplastic deformation. On the other hand, in the case of the triangular pin tool, in the early stage it showed a flat and lower vertical load due to the cutting effect of the sharp pin although that generated much more chips compared to the previous round pin case. The behavior of temperature measured around the stir part on the surface of the upper-positioned specimen during the FSSW process was similar to that of the vertical load. The temperature did not increase in the early stage and it was held constant at around 400 K due to the dominant cutting effect of sharp edge of the triangular pin rather than the friction stirring effect as above mentioned, but it showed a rapid increase with plunging after the shoulder part of the tool made contact with the specimen surface. On the other hand, in the case of Mg alloy/BMG combination of Fig. 5(b), the influence of tool geometry on the behaviors of the vertical load applied and the temperature during friction stirring of the pin part appeared significantly different as compared with the case of Al alloy/BMG combination shown in Fig. 5(a). This was due to the fact that the upper-positioned Mg alloy produced
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3
Fig. 3. Appearances of fracture surface and cross-sectional views at stirred part after FSSW of (a) Al alloy (A5052)/BMG (Vit-1) combination, (b) Mg alloy (AZ31B)/BMG (Vit-1) combination.
Fig. 4. Magnified view of stir zones obtained by dissimilar FSSW of Mg alloy (AZ31B)/BMG (Vit-1) combination using (a) round pin tool and (b) triangular pin tool.
more chips at the early stage in the case of triangular pin due to its difficulty to be plastically deformed, as compared with the Al alloy/
BMG combination case. Thus, in the case of the triangular pin, it showed much lower temperature at the stir zone and no rapid drop
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H.-S. Shin / Journal of Alloys and Compounds xxx (2013) xxx–xxx
Fig. 5. Histories of the maximum temperature around the friction stirred part and the vertical load applied to specimen during dissimilar FSSW of (a) Al alloy (A5052)/BMG (Vit-1) combination, (b) Mg alloy (AZ31B)/BMG (Vit-1) combination.
of the vertical load at around 12 s of friction time which was observed in the round pin case. In order to evaluate the weld strength of the lap jointed specimen after dissimilar FSSW, the tensile-shear tests were carried out and the representative load–displacement curves obtained are shown in Fig. 6(a) and (b). They showed different behaviors after the peak load depending on the failure mode as can be seen in Fig. 3. When the shear fracture occurred at the interface of the weld zone, the load rapidly dropped at peak value, however, when the pull-out fracture around the weld zone occurred, the load decreased gradually with displacement after reaching the peak value. The Al alloy/BMG combination case of (a) showed a higher peak value in failure load and a larger displacement to failure for both pin shapes when compared with the Mg alloy/BMG case of (b). The failure load was determined by the maximum value on the load–displacement curves and plotted against the plunge depth in Fig. 7. As a whole, the dissimilar FSSW of lightweight alloys/BMG combinations produced a higher failure load as compared with the cases for similar FSSW of Al alloy/Al alloy and Mg alloy/Mg alloy combinations [16]. In the case of the round pin tool, little influence of plunge depth on the maximum failure load existed for both material combinations. In the case of the triangular pin tool, however, the maximum failure load was low at a small plunge depth and increased with increasing plunge depth and approached to the value of the round pin case. As a result, the effect of tool geometry on the maximum failure load existed when the plunge depth
was less than 2.0 mm, showing a lower failure load for triangular pin tool due to its dominant cutting effect in this plunge range. But at the plunge depth of 2.0 mm where the tool shoulder was completely in contact with the surface by applying the vertical load to the specimen, the influence of pin geometry did not exist. This means that the effect of the tool geometry appeared on the shear fracture-dominant cases. In the case of the triangular pin tool, the stirring of BMG particles became less and producing less penetration of BMG particles into the lightweight alloy. In order to clarify the weld characteristics for the dissimilar FSSW of Mg alloy/BMG combination, the cross-section of the stir zone was observed and the mixing state was analyzed [13]. The phase constitution of the BMG/lightweight alloy interface was detected by with a micro-beam X-ray diffractometer (Rigaku, D/MAX RAPID-S). The X-ray diffraction (XRD) patterns at corresponding spots around the stirred zone for the dissimilar FSSW of Mg alloy/BMG combination is shown in Fig. 8. The cross-sectional view showed that the lower-positioned BMG alloy was stirred penetrating into the upper Mg alloy side, substantially resulting in a higher maximum fracture load as shown in Fig 4 [13]. At spot No. 2 in the stir zone, the overlap of respective pattern corresponding to the upper and lower-positioned materials can be seen, which represents that a good mixing occurred between dissimilar materials adopted during stirring. At the heat-affected zone (HAZ) in the BMG side of spot No. 3, no occurrence of crystallization was detected from the XRD patterns. The influence of tool geometry on
Please cite this article in press as: H.-S. Shin, J. Alloys Comp. (2013), http://dx.doi.org/10.1016/j.jallcom.2012.12.031
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H.-S. Shin / Journal of Alloys and Compounds xxx (2013) xxx–xxx
(a) 3.0
3.0 Pin length: 1.1 mm Round A5052/BMG(Vit-1) RPM 2,700 Plunge speed 6 mm/min Plunge depth 1.8 Plunge depth 1.9 Plunge depth 2.0
1.5
1.0
0.5
0.5
0.5
1.0
1.5
2.0
2.5
0.0 0.0
3.0
1.0
1.5
2.0
Displacement, mm
(a-1) Round pin
(a-2) Triangle pin
2.5
3.0
2.5
2.5 Pin dpeth 1.1 mm - Round Mg alloy (AZ31B)/BMG (Vit-1) Plunge speed 6 mm/min Plunge depth 1.8 Plunge depth 1.9 Plunge depth 2.0
1.5
Pin length 1.1 mm - triangle Mg alloy (AZ31B)/BMG (Vit-1) RPM 2,700 Plunge speed 6 mm/min Plunge depth 1.8 Plunge depth 1.9 Plunge depth 2.0
2.0
1.0
1.5
1.0
0.5
0.5
0.0 0.0
0.5
Displacement, mm
2.0
Load, kN
1.5
1.0
0.0 0.0
(b)
2.0
Load, kN
Load, kN
2.0
Pin length 1.1 mm - Triangle A5052/BMG(Vit-1) RPM 2,700 Plunge speed 6 mm/min Plunge depth 1.8 Plunge depth 1.9 Plunge depth 2.0
2.5
Load, kN
2.5
0.5
1.0
1.5
2.0
2.5
3.0
0.0 0.0
0.5
1.0
1.5
2.0
2.5
3.0
Displacement, mm
Displacement, mm
(b -1) Round pin
(b-2) Triangle pin
Fig. 6. Representative load–displacement curves obtained after tensile-shear tests for dissimilar FSSW of (a) Al alloy (A5052)/BMG (Vit-1) combination, (b) Mg alloy (AZ31B)/ BMG (Vit-1) combination.
(a)
3.0
the XRD patterns did not appear, it however appeared on the extent of BMG particle penetrated into the upper-positioned lightweight alloy side of the lap-structure.
Max. failure load, kN
2.5 2.0
4. Summary
1.5 Al alloy(A5052)/BMG(Vit-1) RPM 2,700 Pin length 1.1 mm Plunge speed 6 mm/min Round pin Triangle pin
1.0 0.5 0.0 1.8
1.9
2.0
Plunge depth, mm
(b)
3.0 Mg alloy(AZ31B)/BMG(Vit-1) RPM 2,700 Pin length 1.1 mm Plunge speed 6 mm/min Round pin Triangle pin
Max. failure load, kN
2.5 2.0 1.5 1.0
The influences of tool geometry on the characteristics of dissimilar friction stir spot welding (FSSW) between bulk metallic glass (BMG) and lightweight alloy sheets of Al or Mg alloy were investigated. The tool geometry influenced on characteristics of the vertically applied load and the temperature at the stir zone during dissimilar FSSW of BMG to lightweight alloys. In the case of the triangular pin tool, its sharp edge produced much more chips due to the dominant cutting effect rather than the friction stirring effect, which resulted to the lowering of the vertical load and the temperature at the early stage of friction stirring as compared with the round pin case. The effect of tool geometry on the failure load only existed at smaller tool plunge depth and appeared on the extent of BMG particles penetrated into the upper lightweight alloy side. This was significant in the case of the Mg alloy/BMG combination as compared with the Al alloy/BMG case. Acknowledgements
0.5 0.0 1.8
1.9
2.0
Plunge depth, mm Fig. 7. The maximum failure load against the plunge depth; (a) Al alloy (A5052)/ BMG (Vit-1) combination, (b) Mg alloy (AZ31B)/BMG (Vit-1) combination.
This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Educations, Science and Technology (20100024639). The author would like to thank Mr. Woo-Hyuk Kwon for his assistance during the FSSW experiment.
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Fig. 8. XRD patterns at corresponding spots on the cross-sectional view around stirred zone for dissimilar FSSW of Mg alloy/BMG combination.
References [1] W.H. Wang, C. Dong, C.H. Shek, Bulk metallic glasses, Mater. Sci. Eng. R 44 (2004) 45–89. [2] W.L. Johnson, Bulk amorphous metal – an emerging engineering material, JOM 54 (2002) 40–43. [3] Y. Kawamura, Liquid phase and supercooled liquid phase welding of bulk metallic glasses, Mater. Sci. Eng. A 375–377 (2004) 112–119. [4] H.S. Shin, Y.J. Jeong, H.Y. Choi, H. Kato, A. Inoue, Friction welding of Zr55Al10Ni5Cu30 bulk metallic glasses, Mater. Trans. 46 (2005) 2768–2772. [5] H.S. Shin, J.S. Park, Y.C. Jung, J.H. Ahn, Y. Yokoyama, A. Inoue, Similar and dissimilar friction welding of Zr–Cu–Al bulk glassy alloys, J. Alloys Comp. 483 (2009) 182–185. [6] D. Wang, B.L. Xiao, Z.Y. Ma, H.F. Zhang, Friction stir welding of Zr55Cu30Al10Ni5 bulk metallic glass to Al–Zn–Mg–Cu alloy, Scr. Mater. 60 (2009) 112–115. [7] J. Kobata, Y. Takigawa, S.W. Chung, H. Tsuda, H. Kimura, K. Higashi, Nanoscale amorphous ‘‘band-like’’ structure induced by friction stir processing in Zr55Cu30Al10Ni5 bulk metallic glass, Mater. Lett. 61 (2007) 3771–3773. [8] Y.S. Ji, H. Fujii, M. Maeda, N. Nakata, H. Kimura, A. Inoue, K. Nogi, Friction stir welding of Zr55Cu30Ni5Al10 bulk metallic glass, Mater. Trans. 50 (2009) 1300–1303. [9] Y. Sun, Y.S. Ji, H. Fujii, N. Nakata, K. Nogi, Microstructure and mechanical properties of friction stir welded joint of Zr55Cu30Al10Ni5 bulk metallic glass with pure copper, Mater. Sci. Eng. A 527 (2010) 3427–3432.
[10] K. Fujiwara, S. Fukumoto, Y. Yokoyama, M. Nishijima, A. Yamamoto, Weldability of Zr50Cu30Al10Ni10 bulk glassy alloy by small-scale resistance spot welding, Mater. Sci. Eng. A 498 (2008) 302–307. [11] H.S. Shin, Y.C. Jung, Characteristics of friction stir spot welding of Zr-based bulk metallic glass sheets, J. Alloys Comp. 504 (2009) S279–S284. [12] H.S. Shin, Y.C. Jung, Characteristics of dissimilar friction stir spot welding of bulk metallic glass to lightweight crystalline metals, Intermetallics 18 (2010) 2000–2004. [13] H.S. Shin, Y.C. Jung, J.K. Lee, H.J. Kim, Influence of tool speeds on dissimilar friction stir spot welding characteristics of bulk metallic glass/Mg alloy, Met. Mater. Int. 18 (2012) 685–689. [14] H. Badarinarayan, Q. Yang, S. Zhu, Effect of tool geometry on static strength of friction stir spot-welded aluminum alloy, Int. J. Mach. Tools Manuf. 49 (2009) 142–148. [15] Y. Tozaki, Y. Uematsu, K. Tokaji, Effect of tool geometry on microstructure and static strength in friction stir spot welded aluminum alloys, Int. J. Mach. Tools Manuf. 47 (2007) 2230–2236. [16] H.S. Shin, Y.C. Jung, K. Choi, Effects of tool speed on joining characteristics during friction stir spot welding of Mg-alloy (AZ31B) sheet, J. Kor. Weld. Join. Soc. 29 (2011) 532–538.
Please cite this article in press as: H.-S. Shin, J. Alloys Comp. (2013), http://dx.doi.org/10.1016/j.jallcom.2012.12.031
Contents lists available at SciVerse ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Tool geometry effect on the characteristics of dissimilar friction stir spot welded bulk metallic glass to lightweight alloys Hyung-Seop Shin ⇑ Department of Mechanical Design Engineering, Andong National University, Andong, Kyungbuk 760-749, Republic of Korea
a r t i c l e
i n f o
Article history: Available online xxxx Keywords: Metallic glasses Friction stir spot welding Superplastic behavior Dissimilar welding Tool geometry
a b s t r a c t A small-scale joining technique for dissimilar friction stir spot welding (FSSW) of a BMG alloy to lightweight crystalline alloys has been developed. An experimental apparatus which could possible give a precise control of the friction time, the plunge speed and the plunge depth of the tool was used. In this study, the influences of tool geometry with different pin shape on the joining characteristics and on the failure load of specimens after dissimilar FSSW were investigated. As a result, it was found that the tool geometry influenced on the welding performance of the BMG alloy to lightweight crystalline alloys at small tool plunge depth and the extent of BMG particles penetrated onto the lightweight alloy side, especially for the Mg alloy/BMG material combination. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction Bulk metallic glasses (BMGs) have attracted much attention due to its unique properties such as high strength, large elastic limit and superior corrosion-resistance ability [1,2]. Several multi-component BMG alloys have recently been developed in bulk sizes at a lower cooling rate of 100 K/s. They have been tried to be applied as a structural material utilizing their superior mechanical properties. However, their engineering and structural applications are still limited due to size applicability. In order to solve the size limit problem and to extend the application field of BMGs, various joining techniques applicable to BMG alloys have been developed. There are some studies on the friction welding of similar and dissimilar BMG rods [3–5]. It is also necessary to develop a new joining technique applicable to sheet parts with a relatively low energy solid-state welding process. Recently, the studies on the application of friction stir welding (FSW) technique to BMG sheets have carried out [6–12]. In order to achieve a good friction stirring in BMG sheets using the FSW process, the studies of related parameters such as material combination, tool geometry, process management involved in thermal and material flow are important. The effect of tool speed on the similar and friction stir spot welding (FSSW) of BMG was investigated by the author’s group [13]. It was found that the BMG showed its characteristic behavior due to superplastic deformation induced in the supercooled liquid region depending significantly on the tool speed. The tool dimension
⇑ Tel.: +82 54 820 5675; fax: +82 54 820 5167. E-mail address: [email protected]
and geometry is also expected to influence on the joining characteristics of FSW or FSSW alloy sheets. The effects of tool dimension on similar FSW or FSSW of Al or Mg alloy sheets have been investigated [14–16], but no result has been reported yet for the dissimilar FSSW of BMG to lightweight alloys. In this study, the dissimilar friction stir spot welding (FSSW) of a BMG alloy to crystalline lightweight alloys has been tried. The influence of tool geometry having different pin shape on the characteristics of dissimilar FSSW between BMG and lightweight alloys were investigated. Depending on the tool geometry, the measurement of the temperature distribution and the vertical load applied during FSSW were carried out and their effects on the failure load as the weld strength were evaluated. 2. Experimental procedures In this study, a commercially available BMG having a composition of Zr41.5Ti13.8Cu12.5Ni10Be22.5 (Vit-1), and an Al alloy (A5052-H32) and an Mg alloy (AZ31B) as lightweight crystalline alloys were supplied. Thermal properties of the Vit-1 BMG that were measured using a differential scanning calorimetry (DSC) are as follows; the glass transition temperature (Tg) and the crystallization temperature (Tx) are 623 K and 705 K, respectively. Therefore, the supercooled liquid region (DTx = Tx Tg) becomes 82 K. The chemical composition and mechanical properties of lightweight crystalline alloys are shown in Tables 1 and 2, respectively. The BMG side specimen for the dissimilar FSSW was machined to a 1 mm-thick coupon-shape with dimensions of 10 80 mm from a 3 mm-thick cast plate. Al and Mg alloy sheets of 1 mm in thickness were cut into a coupon-shape with dimensions of 15 100 mm. Fig. 1 shows the configuration of a single lap-joint structured specimen. The details of apparatus and processes for FSSW used in this study are described in Ref. [11]. The FSSW process was carried out by plunging the rotating tool onto the single lapped specimen as shown in Fig. 1 which is fixed onto the bed of the smallscale computer numerical controlled (CNC) milling machine using a jig. By using the CNC milling machine, it is easy to control both plunge depth and plunge speed of
0925-8388/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2012.12.031
Please cite this article in press as: H.-S. Shin, J. Alloys Comp. (2013), http://dx.doi.org/10.1016/j.jallcom.2012.12.031
2
H.-S. Shin / Journal of Alloys and Compounds xxx (2013) xxx–xxx
Table 1 The chemical composition of respective lightweight material.
3. Results and discussion
Chemical composition (%) AZ31B A5052-H32
Mg Bal. Al Bal.
Mg 2.80
Al 3.1 Fe 0.4
Cr 0.35
Zn 0.9 Si 0.25
Cu 0.10
Mn 0.4 Mn 0.10
Zn 0.10
Table 2 Mechanical properties of respective lightweight material.
AZ31B A5052-H32
Tensile strength (MPa)
Yield strength (MPa)
Elongation (%)
260 320
200 250
14 10
Fig. 1. Configuration of lap-joint specimen for dissimilar FSSW process.
Fig. 2. Tool geometry adopted; (a) round pin, (b) triangular pin.
the tool. The tool is composed of a cylindrical shoulder part and pin part with diameters of 6.0 and 2.5 mm, respectively, as shown in Fig. 2. To investigate the tool geometry effect on the characteristic behavior during FSSW, two different shapes of pins were used; a round one and a triangular one. The angle of chamfer within the shoulder end surface is 3° and the length of the pin part is 1.1 mm. A plunge speed of 6 mm/min and various plunge depths of 1.8, 1.9 and 2.0 mm were adopted in this study. The tool rotation speed was fixed to 2,700 rpm. During the FSSW process, the temperature distribution around the shoulder of the tool at the surface of the stir zone on the upper-positioned specimen was measured by using an infrared imager (FLIR-ThermaCam SC-2000). The vertical load applied to the specimens was simultaneously measured using a loadcell installed under the jig. The failure load of the specimen after FSSW was evaluated by a tensile-shear test. The SEM observation of the fracture surface and cross section has been done to evaluate the weldability of BMG to lightweight crystalline alloys depending on the tool geometry adopted.
Fig. 3 shows the fracture surface and cross-sectional views of specimens after FSSW using tools having round and triangular shaped pins for respective dissimilar material combinations; wherein (a) is for Al alloy/BMG combination and (b) for Mg alloy/BMG. In these cases, the BMG alloy was located at the lower position of the lap-joint specimen configuration shown in Fig. 1. It could be seen that as the plunge depth of tool increases, the failure mode of specimens after FSSW was changed from the shear fracture at the interface of lap-joint to the pull-out fracture of the upper-positioned alloy material around the indented shoulder part. But in the case of Al alloy/BMG specimens, instead of the shear fracture at interface, they showed that the tensile fracture of the lower-positioned BMG alloy occurred near the stir zone due to the pre-existed defects like voids. The transition of failure mode was related to the balance between the area of weld zone at the interface which was subjected to shear force and the circumferential area surrounding the weld zone from the shoulder edge to the interface which resisted to the pull-out fracture depending on the plunge depth adopted. As the plunge depth increased, the distance from the shoulder edge to the interface decreased; it is 0.3 mm for the plunge depth of 1.8 mm, but it reduces to 0.1 mm for the 2.0 mm case. The cross-sectional views of weld parts represent that good joining were achieved for dissimilar FSSW for both material combinations and the lower-positioned BMG alloy was stirred penetrating into the upper lightweight alloy side which results to a higher failure load [11]. Fig. 4 shows the enlarged views of penetrated BMG particles into the stir zone of upper-positioned Mg alloy side in the case of Mg alloy/BMG combination. From the cross-sectional view of the stirred part, it could be found that in the case of the triangular pin, the lower-positioned BMG alloy was well stirred penetrating into the upper lightweight alloy side which resulted in a higher failure load at larger plunge depth. During dissimilar FSSW process of BMG to lightweight alloys, the maximum temperature around the friction-stirred part and the vertical load applied to the specimen were measured and was shown in Fig. 5(a) and (b), respectively; wherein (a) represents the Al alloy/BMG combination and (b) the Mg alloy one/BMG. As a whole, the behaviors of the vertical load applied with friction time during FSSW were different depending on the pin geometry adopted. By using the round pin tool, the vertical load increased with friction time but it suddenly dropped in magnitude when the pin contacted to the lower-positioned BMG sheet, because the temperature at the stirred zone has reached its supercooled liquid state wherein the BMG starts to show a superplastic deformation. On the other hand, in the case of the triangular pin tool, in the early stage it showed a flat and lower vertical load due to the cutting effect of the sharp pin although that generated much more chips compared to the previous round pin case. The behavior of temperature measured around the stir part on the surface of the upper-positioned specimen during the FSSW process was similar to that of the vertical load. The temperature did not increase in the early stage and it was held constant at around 400 K due to the dominant cutting effect of sharp edge of the triangular pin rather than the friction stirring effect as above mentioned, but it showed a rapid increase with plunging after the shoulder part of the tool made contact with the specimen surface. On the other hand, in the case of Mg alloy/BMG combination of Fig. 5(b), the influence of tool geometry on the behaviors of the vertical load applied and the temperature during friction stirring of the pin part appeared significantly different as compared with the case of Al alloy/BMG combination shown in Fig. 5(a). This was due to the fact that the upper-positioned Mg alloy produced
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Fig. 3. Appearances of fracture surface and cross-sectional views at stirred part after FSSW of (a) Al alloy (A5052)/BMG (Vit-1) combination, (b) Mg alloy (AZ31B)/BMG (Vit-1) combination.
Fig. 4. Magnified view of stir zones obtained by dissimilar FSSW of Mg alloy (AZ31B)/BMG (Vit-1) combination using (a) round pin tool and (b) triangular pin tool.
more chips at the early stage in the case of triangular pin due to its difficulty to be plastically deformed, as compared with the Al alloy/
BMG combination case. Thus, in the case of the triangular pin, it showed much lower temperature at the stir zone and no rapid drop
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Fig. 5. Histories of the maximum temperature around the friction stirred part and the vertical load applied to specimen during dissimilar FSSW of (a) Al alloy (A5052)/BMG (Vit-1) combination, (b) Mg alloy (AZ31B)/BMG (Vit-1) combination.
of the vertical load at around 12 s of friction time which was observed in the round pin case. In order to evaluate the weld strength of the lap jointed specimen after dissimilar FSSW, the tensile-shear tests were carried out and the representative load–displacement curves obtained are shown in Fig. 6(a) and (b). They showed different behaviors after the peak load depending on the failure mode as can be seen in Fig. 3. When the shear fracture occurred at the interface of the weld zone, the load rapidly dropped at peak value, however, when the pull-out fracture around the weld zone occurred, the load decreased gradually with displacement after reaching the peak value. The Al alloy/BMG combination case of (a) showed a higher peak value in failure load and a larger displacement to failure for both pin shapes when compared with the Mg alloy/BMG case of (b). The failure load was determined by the maximum value on the load–displacement curves and plotted against the plunge depth in Fig. 7. As a whole, the dissimilar FSSW of lightweight alloys/BMG combinations produced a higher failure load as compared with the cases for similar FSSW of Al alloy/Al alloy and Mg alloy/Mg alloy combinations [16]. In the case of the round pin tool, little influence of plunge depth on the maximum failure load existed for both material combinations. In the case of the triangular pin tool, however, the maximum failure load was low at a small plunge depth and increased with increasing plunge depth and approached to the value of the round pin case. As a result, the effect of tool geometry on the maximum failure load existed when the plunge depth
was less than 2.0 mm, showing a lower failure load for triangular pin tool due to its dominant cutting effect in this plunge range. But at the plunge depth of 2.0 mm where the tool shoulder was completely in contact with the surface by applying the vertical load to the specimen, the influence of pin geometry did not exist. This means that the effect of the tool geometry appeared on the shear fracture-dominant cases. In the case of the triangular pin tool, the stirring of BMG particles became less and producing less penetration of BMG particles into the lightweight alloy. In order to clarify the weld characteristics for the dissimilar FSSW of Mg alloy/BMG combination, the cross-section of the stir zone was observed and the mixing state was analyzed [13]. The phase constitution of the BMG/lightweight alloy interface was detected by with a micro-beam X-ray diffractometer (Rigaku, D/MAX RAPID-S). The X-ray diffraction (XRD) patterns at corresponding spots around the stirred zone for the dissimilar FSSW of Mg alloy/BMG combination is shown in Fig. 8. The cross-sectional view showed that the lower-positioned BMG alloy was stirred penetrating into the upper Mg alloy side, substantially resulting in a higher maximum fracture load as shown in Fig 4 [13]. At spot No. 2 in the stir zone, the overlap of respective pattern corresponding to the upper and lower-positioned materials can be seen, which represents that a good mixing occurred between dissimilar materials adopted during stirring. At the heat-affected zone (HAZ) in the BMG side of spot No. 3, no occurrence of crystallization was detected from the XRD patterns. The influence of tool geometry on
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(a) 3.0
3.0 Pin length: 1.1 mm Round A5052/BMG(Vit-1) RPM 2,700 Plunge speed 6 mm/min Plunge depth 1.8 Plunge depth 1.9 Plunge depth 2.0
1.5
1.0
0.5
0.5
0.5
1.0
1.5
2.0
2.5
0.0 0.0
3.0
1.0
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2.0
Displacement, mm
(a-1) Round pin
(a-2) Triangle pin
2.5
3.0
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2.5 Pin dpeth 1.1 mm - Round Mg alloy (AZ31B)/BMG (Vit-1) Plunge speed 6 mm/min Plunge depth 1.8 Plunge depth 1.9 Plunge depth 2.0
1.5
Pin length 1.1 mm - triangle Mg alloy (AZ31B)/BMG (Vit-1) RPM 2,700 Plunge speed 6 mm/min Plunge depth 1.8 Plunge depth 1.9 Plunge depth 2.0
2.0
1.0
1.5
1.0
0.5
0.5
0.0 0.0
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Displacement, mm
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Load, kN
1.5
1.0
0.0 0.0
(b)
2.0
Load, kN
Load, kN
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Pin length 1.1 mm - Triangle A5052/BMG(Vit-1) RPM 2,700 Plunge speed 6 mm/min Plunge depth 1.8 Plunge depth 1.9 Plunge depth 2.0
2.5
Load, kN
2.5
0.5
1.0
1.5
2.0
2.5
3.0
0.0 0.0
0.5
1.0
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Displacement, mm
Displacement, mm
(b -1) Round pin
(b-2) Triangle pin
Fig. 6. Representative load–displacement curves obtained after tensile-shear tests for dissimilar FSSW of (a) Al alloy (A5052)/BMG (Vit-1) combination, (b) Mg alloy (AZ31B)/ BMG (Vit-1) combination.
(a)
3.0
the XRD patterns did not appear, it however appeared on the extent of BMG particle penetrated into the upper-positioned lightweight alloy side of the lap-structure.
Max. failure load, kN
2.5 2.0
4. Summary
1.5 Al alloy(A5052)/BMG(Vit-1) RPM 2,700 Pin length 1.1 mm Plunge speed 6 mm/min Round pin Triangle pin
1.0 0.5 0.0 1.8
1.9
2.0
Plunge depth, mm
(b)
3.0 Mg alloy(AZ31B)/BMG(Vit-1) RPM 2,700 Pin length 1.1 mm Plunge speed 6 mm/min Round pin Triangle pin
Max. failure load, kN
2.5 2.0 1.5 1.0
The influences of tool geometry on the characteristics of dissimilar friction stir spot welding (FSSW) between bulk metallic glass (BMG) and lightweight alloy sheets of Al or Mg alloy were investigated. The tool geometry influenced on characteristics of the vertically applied load and the temperature at the stir zone during dissimilar FSSW of BMG to lightweight alloys. In the case of the triangular pin tool, its sharp edge produced much more chips due to the dominant cutting effect rather than the friction stirring effect, which resulted to the lowering of the vertical load and the temperature at the early stage of friction stirring as compared with the round pin case. The effect of tool geometry on the failure load only existed at smaller tool plunge depth and appeared on the extent of BMG particles penetrated into the upper lightweight alloy side. This was significant in the case of the Mg alloy/BMG combination as compared with the Al alloy/BMG case. Acknowledgements
0.5 0.0 1.8
1.9
2.0
Plunge depth, mm Fig. 7. The maximum failure load against the plunge depth; (a) Al alloy (A5052)/ BMG (Vit-1) combination, (b) Mg alloy (AZ31B)/BMG (Vit-1) combination.
This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Educations, Science and Technology (20100024639). The author would like to thank Mr. Woo-Hyuk Kwon for his assistance during the FSSW experiment.
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Fig. 8. XRD patterns at corresponding spots on the cross-sectional view around stirred zone for dissimilar FSSW of Mg alloy/BMG combination.
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Please cite this article in press as: H.-S. Shin, J. Alloys Comp. (2013), http://dx.doi.org/10.1016/j.jallcom.2012.12.031