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Frictional wear evaluation of WC–Co alloy tool in friction stir spot welding of low carbon steel plates
Int. Journal of Refractory Metals & Hard Materials 27 (2009) 931–936
Contents lists available at ScienceDirect
Int. Journal of Refractory Metals & Hard Materials journal homepage: www.elsevier.com/locate/IJRMHM
Frictional wear evaluation of WC–Co alloy tool in friction stir spot welding of low carbon steel plates D.H. Choi a, C.Y. Lee b, B.W. Ahn a, J.H. Choi a, Y.M. Yeon c, K. Song c, H.S. Park d, Y.J. Kim a, C.D. Yoo b, S.B. Jung a,* a
School of Advanced Materials Science and Engineering, Sungkyunkwan University, 300 Cheoncheon-dong, Jangan-gu, Gyeonggi-do 440-746, Republic of Korea Department of Mechanical Engineering, Korea Advanced Institute of Science and Technology, 335 Gwahangno, Yuseong-gu, Daejeon 305-701, Republic of Korea c Department of Advanced Materials Application, Suwon Science College 9-10, Botong-li, Jeongnam-myeon, Hwasung, Gyeonggi-do 445-742, Republic of Korea d Body Manufacturing Engineering Team, Kia Motors Corp., 781-1, Soha-dong, Kwangmyeong-shi, Gyeonggi-do 423-701, Republic of Korea b
a r t i c l e
i n f o
Article history: Received 1 April 2008 Accepted 1 May 2009
Keywords: Friction stir spot welding Hard metals Tool wear Tensile test Low carbon steel
a b s t r a c t Tool wear is a key issue for the friction stir spot welding (FSSW) of steel plates, especially in the automobile industry. In this study, steel plates were welded 500 using FSSW with WC–Co alloy tools of two different compositions. The effect of the weld number on the joint strength and the tool wear characteristics were analyzed by using a non-contact, 3D measurement system, scanning electron microscopy (SEM) with energy dispersive spectrometry (EDS), and X-ray diffraction (XRD). The experimental results indicated that the tool suffered extreme wear and that the joint strength was affected by the worn tool shape after welding. This tool wear was attributed to the formation of a ternary W–Fe–O compound, oxidative wear of WC and fatigue of the Co binder. Crown Copyright Ó 2009 Published by Elsevier Ltd. All rights reserved.
1. Introduction Friction stir spot welding (FSSW) is a significant new solid-state joining method with potential application in the automobile industry. Compared with electric resistance spot welding (ERSW) that is the most commonly used welding method in the automobile industry, FSSW offers a 90% energy saving and 40% equipment saving due to its minimal equipment requirement [1,2]. Furthermore, FSSW is an environmentally friendly spot welding method due to the absence of any fume or spark. In the FSSW process, a cylindrical rotating tool, with a protruding pin centered on one of the circular faces, plunges at a specific rate into the overlapping sheets to a predetermined depth. It is then retracted at a rapid rate either immediately or after a dwell period. The frictional heat generated softens the workpiece and the rotating pin causes material flow in both the circumferential and axial directions. The forging pressure applied by the tool shoulder results in the formation of an annular, solid-state bond around the pin. The retraction of the pin leaves a characteristic exit hole. In FSSW, the tool materials depend on the workpiece material. In light metals such as Al and Mg alloy, the welding tool is commonly made of tool steel and suffers little or no wear, even after * Corresponding author. Fax: +82 31 290 7371. E-mail address: [email protected] (S.B. Jung).
hundreds of welds [3–7]. For the welding of light metals, the tool durability therefore has a high reliability. However, it is impossible to weld a workpiece material with a high melting point such as steel and Ti alloy using a tool steel. Therefore, in high melting point materials welding, the welding tool must be made of a hard metal with a superior thermal resistance and wear resistance at temperatures higher than 1000 °C. In the FSSW of a high melting point material, the welding tool is usually made of hard metals such as WC–Co, TiC, and polycrystalline boron nitride [8–10]. However, the hard metal tool was also worn during FSSW. Therefore, the examination of tool wear and durability in FSSW is very important, especially in the automobile industry. However, little research has been conducted on tool wear and durability in FSSW. In this study, steel plates for the automobile industry underwent FSSW 500 with two kinds of hard metal tools. The study object was to clarify the variation of joint strength associated with the tool deformation, the wear mechanism in two kinds of hard metal tools during FSSW. 2. Experimental procedures The base material used for welding in this study was a 0.6 mmthick steel plates sheared from a commercial hot annealed coil, with a chemical compositions of Fe–0.002C–0.01Si–0.62Mn– 0.039P–0.008S (all compositions are wt%), with an ultimate tensile strength of 340 MPa and elongation of 44%. The steel plates of
0263-4368/$ - see front matter Crown Copyright Ó 2009 Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijrmhm.2009.05.002
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Fig. 1. External tool shape: (a) WC tool, (b) cap, and (c) shank.
dimensions 100 30 mm were welded in a lap joint configuration with a 30 30 mm overlapped area. FSSW was performed 500 with two kinds of WC–Co tool. The tool 1 was a commercial super hard alloy with the composition of WC–13 wt% Co, and the tool 2 was a special super hard alloy that 6 wt% Ni and 1.5 wt% Cr3C2 compound were added to the first thing for enhancing a high temperature strength. The tool used in this study had a cylindrical pin with a shoulder, as shown in Fig. 1. The initial tool penetration depth was 0.36 mm and was changed proportionately with tool wear. The tool penetration speed and rotation speed were 15 mm/min and 1600 rpm, respectively. After every 100 welds, the tool shape was measured by the Mitutoyo Quick Vision, non-contact, 3D measurement system, and the lap joints underwent tensile shear test at a crosshead speed of 1 mm min1. Plates of the same thickness were used to minimize the effect of eccentricity in the tensile shear test. The microstructure and chemical compositions of the tools were analyzed with a scanning electron microscope (SEM) equipped with energy dispersive spectrometry (EDS) and the phases were characterized with X-ray diffraction (XRD) at 2h ranges of 20–80° and a scan speed of 2°/min.
Fig. 2. Tensile shear strength according to the number of welds.
3. Results and discussion 3.1. Joint strength variation due to the tool deformation The joint strength of every one-hundredth weld and cross-section images of joints were shown in Fig. 2. The joint strength and shapes were affected by the number of welds and by the changes in the tool shape resulting from the tool wear caused by the increasing number of welds. Until 200 welds, the joint strengths by tool 2 were higher than those by tool 1, and those by tool 1 were conversely higher than those by tool 2 in weld numbers from 200 to 500 welds. However, the joint strength of both tools 1 and 2 was higher than the minimum accepted strength by ERSW currently applied in automobile industry, indicating that FSSW using the WC–Co tool guaranteed the reliability of joint until 500 welds. Fig. 3 shows the external tool shape after every one-hundredth weld in order to analyze the effect of tool shape on the joint strength. Both tools 1 and 2 suffered severe wear. Blue powders, assumed to be tungsten oxide, were observed around the edge of tool 1 but not in tool 2, and this was attributed to the presence of Cr3C2 which prevented oxidation [11]. The tool shape after weld-
Fig. 3. External shape of tools 1 and 2: (a) and (e) before welding, and after (b) and (f) 100, (c) and (g) 300, and (d) and (h) 500 welds, respectively.
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ing revealed the extreme wear in the area between the pin center and edge, whereas the pin center was less worn than any other region. The non-contact 3D measurement system was used to measure the tool shapes of the new tool and tools 1 and 2 after welding. D in Fig. 4 was the worn-out depth of a tool, and DT1 increased abruptly until 200 while DT2 increased step by step, and the values of D in both tools were constantly retained in 200–500 welds, as shown in Fig. 4c. These results indicated that tool 2 was less worn than tool 1 until 200 welds, but then suffered extreme wear after 200 welds, to a level similar to that of tool 1 wear. After 300 welds, the wear was decreased for both tools, which was attributed to the worn tool shape. During welding, the tool was selfsharpened by wear, so that it easily penetrated into the workpiece with increasing number of welds. The diameter of the pin center averaged 6.4 mm for tool 1 (D0T1 ) and 5.3 mm for tool 2 (D0T2 ), indicating that the contact area between tool 1 and the workpiece was broader than that in tool 2 when the tool was inserted into the workpiece. Therefore, due to its higher joint strength, the wear of tool 2 (DT2) was relatively less than that of tool 1 (DT1) until 200 welds. After 300 welds, however, the tools showed similar wear but the pin center diameter of tool 1 (D0T1 ) was broader than that of tool 2 (D0T2 ), and the joint strength of tool 1 was higher than that of tool 2. It was found that these tool deformations in two tools were well corresponded to the variations of joint strength shown in Fig. 2. 3.2. Wear mechanism of welding tools Measuring of worn tool shape was shown that extreme wear occurred during FSSW and the tool 2 had a superior wear resistance to the tool 1 until 200 welds. These results suggested that wear mechanism was different between tools 1 and 2 until 200 welds. Fig. 5 shows the XRD patterns of both tools; before welding and after 300 welds. Before welding, mainly WC peaks, as well as some tungsten oxide (WxOy) peaks, were observed in the XRD patterns of both tools. It was assumed that these oxides were formed during the sintering process. Cr3C2 peaks, however, were not observed in tool 2, which was attributed to the dissolution of the Cr3C2 particles in the Co binder during the sintering process [11]. In the XRD patterns after 300 welds, WxOy peaks were increased and new peaks revealing a ternary W–Fe–O compound were observed in both tools. In FSSW, the tool temperature exceeds 1000 °C, suggesting that WC particles were easily reacted with oxygen in air to form ternary W–Fe–O compounds as a result of reaction between the tool and workpiece. SEM with EDS analysis of the chemical compositions of both tools before and after welding revealed that the white particles were WC and the black layer around the particles was Co binder. In tool 2, especially, Cr and Ni were observed in the Co binder but not in the WC particles (Fig. 6). As stated above, this phenomenon was related with the dissolution of Cr3C2 particles in the Co binder. After welding, two layers, one black and one white, could be distinguished on the tool surface (Fig. 7). EDS analysis of these layers indicated compositions consisting of W, Fe, C, and O, and W, C, and O, respectively. Considering the XRD analysis results, the black and white layers were determined to be ternary W–Fe–O compound and WxOy, respectively. The WxOy amount of tool 1 was more than that of tool 2, due to the presence of Cr3C2 that prevented the oxidization of WC. However, the WxOy area of tool 2 was increased after 300 welds, it corresponds result of XRD analysis. SEM and XRD analyses of the tool surface suggested three causes explaining the tool wear mechanism during welding. How-
Fig. 4. Measurement of tool shape of tools (a) 1 and (b) 2 and (c) distance from tool surface (DT1, DT2).
ever, it was difficult to determine their proportional contribution because the tool wear was caused by a combination of all three wear mechanisms, rather than only one. In presumed order of importance, the first most important was oxidative wear of WC (Fig. 8) [12–14]. It has been reported that when WC is reacted with oxygen, its volume fraction is expanded by about 300% via an oxidation process that generates CO gas in the solid. The pressure of this gas was greater than the fracture strength of solid,
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Fig. 5. X-ray diffraction (XRD) patterns of the surface of tools (a) 1 and (b) 2 before welding, tools (c) 1 and (d) 2 after 300 welds.
Fig. 6. Microstructure of the tool surface before welding for tools (a) 1 and (b) 2.
thereby leading to cracks in the solid. Furthermore, oxide layers are easily fractured, due to residual tensile stresses associated with different coefficients of thermal expansion between the substrate. The second most important mechanism was the increased brittleness due to the transformation of the Co binder [15,16]. The Co binder phase is continuously exposed to severe conditions due to its contact with the workpiece, which leads to the accumulation of fatigue, as discussed in other studies. After sintering, Co binder is considered to have a ductile, predominantly face-centered cubic (fcc), structure. During use, it undergoes plastic deformation, involving dislocation movement and twin formation. This causes the fcc-structure Co binder to be transformed to the more brittle, hexagonal close-packed structure Co, which consequently changes the WC’s mechanical properties. It has been shown that
WC exposed to press cycling fatigue exhibits reduced fracture toughness, erosion resistance and thermal shock resistance. Furthermore, considering the brittleness of the hcp-structure Co, the Co binder was easily fractured and removed from the tool material during welding, thereby explaining the extreme wear that occurred after 200 welds in tool 2. Generally, Cr3C2 increased the corrosion resistance and inhibited the grain growth in hard metals; in addition, it was dissolved in the Co binder during sintering. According to this result, the Cr3C2-containing tool 2 showed superior wear and thermal resistance to tool 1. However, increasing welds, the Co binder underwent phase transformation due to fatigue, both the Co binder and Cr3C2 were fractured and removed from tool 2, and the overall effect of Cr3C2 was decreased. As a result, tool 2 exhibited similar characteristics to
D.H. Choi et al. / Int. Journal of Refractory Metals & Hard Materials 27 (2009) 931–936
Fig. 7. Microstructure of the tool surface after welding of tools 1 and 2 after (a) and (d) 100, (b) and (e) 200, and (c) and (f) 300 welds, respectively.
Fig. 8. Oxidation of the tool edge of the tool 1 after 100 welds.
Fig. 9. Crack in tool 2 surface in the tool center formed after 200 welds.
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those of tool 1 after 300 welds. The third most important mechanism was the formation of ternary W–Fe–O compounds. In FSSW, most of the observed material transfer was from the workpiece to the tool and ternary W–Fe–O compounds were produced on the tool surface by the reaction between the tool and material. After the welding, the tool was rapidly air cooled, suggesting that cracks were created on the compound surface by thermal expansion and shrinkage, and that tool wear was increased by the spreading of such cracks (Fig. 9). In addition, these ternary W– Fe–O compound layers may be removed from tool surface, this was likely to happen after every welding when tool had fresh surface under high temperature. Consequently, it was considered that synthetic reasons by an oxidation of tungsten and a formation of W–Fe–O ternary compound resulted in the severe wear in the tool 1, and in a case of tool 2, a formation of W–Fe–O ternary compound was mainly attributed to the wear until 200 welds, and an additional contribution of tungsten oxides due to the fatigue of Co binder accelerated the wear after 200 welds. 4. Conclusion Steel plates were subjected to FSSW with two tools of different chemical composition. The effect of the number of welds and tool deformation on the mechanical property of the joint and the tool wear during FSSW were analyzed. The joint strength of the weld was affected by the number of welds, which was attributed to the effect of tool wear on the tool shape. Examination of the tool shape after welding revealed that extreme wear occurred between the pin center and the edge. As the tool tip was self-sharpened during welding, the tool could possibly be initially shaped with a steady-state, worn tool geometry in order to reduce the tool wear rate and produce a more durable tool. Three potential mechanisms to explain the tool wear were obtained by the XRD and SEM analysis results for the worn surfaces, which were an oxidative wear of WC, and a fatigue of the Co binder and a formation of a ternary W–Fe–O compound.
Acknowledgment This work was supported by the WCU program of Korea Science and Engineering Foundation funded by the Korean Government (MOEHRD) (Grant No. R32-2008-000-10124-0). References [1] Sakano R, Murakami K, Yamashita K, Hyoe T, Fujimoto M, Inuzuka et al. In: Proceedings of the third international symposium of friction stir welding, 2001. [2] Iwashita T. US Patent 6601751 B2, 5 August 2003. [3] Badarinarayan H, Yang Q, Zhu S. Effect of tool geometry on static strength of friction stir spot-welded aluminum alloy. Int J Mach Tool Manu 2009;49:142–8. [4] Wang DA, Lee SC. Microstructures and failure mechanisms of friction stir spot welds of aluminum 6061–T6 sheets. J Mater Proc Technol 2007;186:291–7. [5] Mitlin D, Radmilovic V, Pan T, Chen J, Feng Z, Santella ML. Structure–properties relations in spot friction welded (also known as friction stir spot welded) 6111 aluminum. Mater Sci Eng A 2006;441:79–96. [6] Lathabai S, Painter MJ, Cantin GMD, Tyagi VK. Friction spot joining of an extruded Al–Mg–Si alloy. Scripta Mater 2006;55:899–902. [7] Adams DE. Tool penetration during friction stir spot welding of Al and Mg alloys. J Mater Sci 2005;40:6473–81. [8] Santella ML, Grant GJ, Feng Z. Friction-stir spot-welding of high-strength steel. Progr. Rep. Automotive Lightweight. Mater. 2005:336–9. [9] Fena Z, Santella ML, David SA, Steel RJ, Packer SM, Pan T, et al. Friction-stir spot welding of advanced high-strength steel: a feasibility study. SAE Trans 2005;114:592–8. [10] Hovanski Y, Santellab ML, Granta GJ. Friction stir spot welding of hot-stamped boron steel. Scripta Mater 2007;57:873–6. [11] Zackrisson J, Jansson B, Uphadyaya GS, Andr´en HO. WC–Co based cemented carbides with large Cr3C2 additions. Int J Refract Met Hard Mater 1998;16:417–22. [12] Lofaj F, Kaganovsklii Y. Kinetics of WC–Co oxidation accompanied by swelling. J Mater Sci 1995;30:1811–7. [13] Lee GG, Kwon HS, Ha G. Oxidation Behavior of WC–Co hard metal. J Kor Powder Metall Inst 2004;11:111–7. [14] Casas B, Ramis X, Anglada M, Salla JM, Llanes L. Oxidation-induced strength degradation of WC–Co hard metals. Int J Refract Met Hard Mater 2001;19:303–9. [15] Vasel CH, Krawitz AD, Drake EF, Kenik EA. Binder deformation in WC–(Co, Ni) cemented carbide composites. Metall Mater Trans A 1985;16:2309–17. [16] Kindermann P, Schlund P, Sockel HG, Herr M, Heinrich W, Gorting K, et al. High-temperature fatigue of cemented carbides under cyclic loads. Int J Refract Met Hard Mater 1999;17:55–68.
Contents lists available at ScienceDirect
Int. Journal of Refractory Metals & Hard Materials journal homepage: www.elsevier.com/locate/IJRMHM
Frictional wear evaluation of WC–Co alloy tool in friction stir spot welding of low carbon steel plates D.H. Choi a, C.Y. Lee b, B.W. Ahn a, J.H. Choi a, Y.M. Yeon c, K. Song c, H.S. Park d, Y.J. Kim a, C.D. Yoo b, S.B. Jung a,* a
School of Advanced Materials Science and Engineering, Sungkyunkwan University, 300 Cheoncheon-dong, Jangan-gu, Gyeonggi-do 440-746, Republic of Korea Department of Mechanical Engineering, Korea Advanced Institute of Science and Technology, 335 Gwahangno, Yuseong-gu, Daejeon 305-701, Republic of Korea c Department of Advanced Materials Application, Suwon Science College 9-10, Botong-li, Jeongnam-myeon, Hwasung, Gyeonggi-do 445-742, Republic of Korea d Body Manufacturing Engineering Team, Kia Motors Corp., 781-1, Soha-dong, Kwangmyeong-shi, Gyeonggi-do 423-701, Republic of Korea b
a r t i c l e
i n f o
Article history: Received 1 April 2008 Accepted 1 May 2009
Keywords: Friction stir spot welding Hard metals Tool wear Tensile test Low carbon steel
a b s t r a c t Tool wear is a key issue for the friction stir spot welding (FSSW) of steel plates, especially in the automobile industry. In this study, steel plates were welded 500 using FSSW with WC–Co alloy tools of two different compositions. The effect of the weld number on the joint strength and the tool wear characteristics were analyzed by using a non-contact, 3D measurement system, scanning electron microscopy (SEM) with energy dispersive spectrometry (EDS), and X-ray diffraction (XRD). The experimental results indicated that the tool suffered extreme wear and that the joint strength was affected by the worn tool shape after welding. This tool wear was attributed to the formation of a ternary W–Fe–O compound, oxidative wear of WC and fatigue of the Co binder. Crown Copyright Ó 2009 Published by Elsevier Ltd. All rights reserved.
1. Introduction Friction stir spot welding (FSSW) is a significant new solid-state joining method with potential application in the automobile industry. Compared with electric resistance spot welding (ERSW) that is the most commonly used welding method in the automobile industry, FSSW offers a 90% energy saving and 40% equipment saving due to its minimal equipment requirement [1,2]. Furthermore, FSSW is an environmentally friendly spot welding method due to the absence of any fume or spark. In the FSSW process, a cylindrical rotating tool, with a protruding pin centered on one of the circular faces, plunges at a specific rate into the overlapping sheets to a predetermined depth. It is then retracted at a rapid rate either immediately or after a dwell period. The frictional heat generated softens the workpiece and the rotating pin causes material flow in both the circumferential and axial directions. The forging pressure applied by the tool shoulder results in the formation of an annular, solid-state bond around the pin. The retraction of the pin leaves a characteristic exit hole. In FSSW, the tool materials depend on the workpiece material. In light metals such as Al and Mg alloy, the welding tool is commonly made of tool steel and suffers little or no wear, even after * Corresponding author. Fax: +82 31 290 7371. E-mail address: [email protected] (S.B. Jung).
hundreds of welds [3–7]. For the welding of light metals, the tool durability therefore has a high reliability. However, it is impossible to weld a workpiece material with a high melting point such as steel and Ti alloy using a tool steel. Therefore, in high melting point materials welding, the welding tool must be made of a hard metal with a superior thermal resistance and wear resistance at temperatures higher than 1000 °C. In the FSSW of a high melting point material, the welding tool is usually made of hard metals such as WC–Co, TiC, and polycrystalline boron nitride [8–10]. However, the hard metal tool was also worn during FSSW. Therefore, the examination of tool wear and durability in FSSW is very important, especially in the automobile industry. However, little research has been conducted on tool wear and durability in FSSW. In this study, steel plates for the automobile industry underwent FSSW 500 with two kinds of hard metal tools. The study object was to clarify the variation of joint strength associated with the tool deformation, the wear mechanism in two kinds of hard metal tools during FSSW. 2. Experimental procedures The base material used for welding in this study was a 0.6 mmthick steel plates sheared from a commercial hot annealed coil, with a chemical compositions of Fe–0.002C–0.01Si–0.62Mn– 0.039P–0.008S (all compositions are wt%), with an ultimate tensile strength of 340 MPa and elongation of 44%. The steel plates of
0263-4368/$ - see front matter Crown Copyright Ó 2009 Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijrmhm.2009.05.002
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Fig. 1. External tool shape: (a) WC tool, (b) cap, and (c) shank.
dimensions 100 30 mm were welded in a lap joint configuration with a 30 30 mm overlapped area. FSSW was performed 500 with two kinds of WC–Co tool. The tool 1 was a commercial super hard alloy with the composition of WC–13 wt% Co, and the tool 2 was a special super hard alloy that 6 wt% Ni and 1.5 wt% Cr3C2 compound were added to the first thing for enhancing a high temperature strength. The tool used in this study had a cylindrical pin with a shoulder, as shown in Fig. 1. The initial tool penetration depth was 0.36 mm and was changed proportionately with tool wear. The tool penetration speed and rotation speed were 15 mm/min and 1600 rpm, respectively. After every 100 welds, the tool shape was measured by the Mitutoyo Quick Vision, non-contact, 3D measurement system, and the lap joints underwent tensile shear test at a crosshead speed of 1 mm min1. Plates of the same thickness were used to minimize the effect of eccentricity in the tensile shear test. The microstructure and chemical compositions of the tools were analyzed with a scanning electron microscope (SEM) equipped with energy dispersive spectrometry (EDS) and the phases were characterized with X-ray diffraction (XRD) at 2h ranges of 20–80° and a scan speed of 2°/min.
Fig. 2. Tensile shear strength according to the number of welds.
3. Results and discussion 3.1. Joint strength variation due to the tool deformation The joint strength of every one-hundredth weld and cross-section images of joints were shown in Fig. 2. The joint strength and shapes were affected by the number of welds and by the changes in the tool shape resulting from the tool wear caused by the increasing number of welds. Until 200 welds, the joint strengths by tool 2 were higher than those by tool 1, and those by tool 1 were conversely higher than those by tool 2 in weld numbers from 200 to 500 welds. However, the joint strength of both tools 1 and 2 was higher than the minimum accepted strength by ERSW currently applied in automobile industry, indicating that FSSW using the WC–Co tool guaranteed the reliability of joint until 500 welds. Fig. 3 shows the external tool shape after every one-hundredth weld in order to analyze the effect of tool shape on the joint strength. Both tools 1 and 2 suffered severe wear. Blue powders, assumed to be tungsten oxide, were observed around the edge of tool 1 but not in tool 2, and this was attributed to the presence of Cr3C2 which prevented oxidation [11]. The tool shape after weld-
Fig. 3. External shape of tools 1 and 2: (a) and (e) before welding, and after (b) and (f) 100, (c) and (g) 300, and (d) and (h) 500 welds, respectively.
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ing revealed the extreme wear in the area between the pin center and edge, whereas the pin center was less worn than any other region. The non-contact 3D measurement system was used to measure the tool shapes of the new tool and tools 1 and 2 after welding. D in Fig. 4 was the worn-out depth of a tool, and DT1 increased abruptly until 200 while DT2 increased step by step, and the values of D in both tools were constantly retained in 200–500 welds, as shown in Fig. 4c. These results indicated that tool 2 was less worn than tool 1 until 200 welds, but then suffered extreme wear after 200 welds, to a level similar to that of tool 1 wear. After 300 welds, the wear was decreased for both tools, which was attributed to the worn tool shape. During welding, the tool was selfsharpened by wear, so that it easily penetrated into the workpiece with increasing number of welds. The diameter of the pin center averaged 6.4 mm for tool 1 (D0T1 ) and 5.3 mm for tool 2 (D0T2 ), indicating that the contact area between tool 1 and the workpiece was broader than that in tool 2 when the tool was inserted into the workpiece. Therefore, due to its higher joint strength, the wear of tool 2 (DT2) was relatively less than that of tool 1 (DT1) until 200 welds. After 300 welds, however, the tools showed similar wear but the pin center diameter of tool 1 (D0T1 ) was broader than that of tool 2 (D0T2 ), and the joint strength of tool 1 was higher than that of tool 2. It was found that these tool deformations in two tools were well corresponded to the variations of joint strength shown in Fig. 2. 3.2. Wear mechanism of welding tools Measuring of worn tool shape was shown that extreme wear occurred during FSSW and the tool 2 had a superior wear resistance to the tool 1 until 200 welds. These results suggested that wear mechanism was different between tools 1 and 2 until 200 welds. Fig. 5 shows the XRD patterns of both tools; before welding and after 300 welds. Before welding, mainly WC peaks, as well as some tungsten oxide (WxOy) peaks, were observed in the XRD patterns of both tools. It was assumed that these oxides were formed during the sintering process. Cr3C2 peaks, however, were not observed in tool 2, which was attributed to the dissolution of the Cr3C2 particles in the Co binder during the sintering process [11]. In the XRD patterns after 300 welds, WxOy peaks were increased and new peaks revealing a ternary W–Fe–O compound were observed in both tools. In FSSW, the tool temperature exceeds 1000 °C, suggesting that WC particles were easily reacted with oxygen in air to form ternary W–Fe–O compounds as a result of reaction between the tool and workpiece. SEM with EDS analysis of the chemical compositions of both tools before and after welding revealed that the white particles were WC and the black layer around the particles was Co binder. In tool 2, especially, Cr and Ni were observed in the Co binder but not in the WC particles (Fig. 6). As stated above, this phenomenon was related with the dissolution of Cr3C2 particles in the Co binder. After welding, two layers, one black and one white, could be distinguished on the tool surface (Fig. 7). EDS analysis of these layers indicated compositions consisting of W, Fe, C, and O, and W, C, and O, respectively. Considering the XRD analysis results, the black and white layers were determined to be ternary W–Fe–O compound and WxOy, respectively. The WxOy amount of tool 1 was more than that of tool 2, due to the presence of Cr3C2 that prevented the oxidization of WC. However, the WxOy area of tool 2 was increased after 300 welds, it corresponds result of XRD analysis. SEM and XRD analyses of the tool surface suggested three causes explaining the tool wear mechanism during welding. How-
Fig. 4. Measurement of tool shape of tools (a) 1 and (b) 2 and (c) distance from tool surface (DT1, DT2).
ever, it was difficult to determine their proportional contribution because the tool wear was caused by a combination of all three wear mechanisms, rather than only one. In presumed order of importance, the first most important was oxidative wear of WC (Fig. 8) [12–14]. It has been reported that when WC is reacted with oxygen, its volume fraction is expanded by about 300% via an oxidation process that generates CO gas in the solid. The pressure of this gas was greater than the fracture strength of solid,
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Fig. 5. X-ray diffraction (XRD) patterns of the surface of tools (a) 1 and (b) 2 before welding, tools (c) 1 and (d) 2 after 300 welds.
Fig. 6. Microstructure of the tool surface before welding for tools (a) 1 and (b) 2.
thereby leading to cracks in the solid. Furthermore, oxide layers are easily fractured, due to residual tensile stresses associated with different coefficients of thermal expansion between the substrate. The second most important mechanism was the increased brittleness due to the transformation of the Co binder [15,16]. The Co binder phase is continuously exposed to severe conditions due to its contact with the workpiece, which leads to the accumulation of fatigue, as discussed in other studies. After sintering, Co binder is considered to have a ductile, predominantly face-centered cubic (fcc), structure. During use, it undergoes plastic deformation, involving dislocation movement and twin formation. This causes the fcc-structure Co binder to be transformed to the more brittle, hexagonal close-packed structure Co, which consequently changes the WC’s mechanical properties. It has been shown that
WC exposed to press cycling fatigue exhibits reduced fracture toughness, erosion resistance and thermal shock resistance. Furthermore, considering the brittleness of the hcp-structure Co, the Co binder was easily fractured and removed from the tool material during welding, thereby explaining the extreme wear that occurred after 200 welds in tool 2. Generally, Cr3C2 increased the corrosion resistance and inhibited the grain growth in hard metals; in addition, it was dissolved in the Co binder during sintering. According to this result, the Cr3C2-containing tool 2 showed superior wear and thermal resistance to tool 1. However, increasing welds, the Co binder underwent phase transformation due to fatigue, both the Co binder and Cr3C2 were fractured and removed from tool 2, and the overall effect of Cr3C2 was decreased. As a result, tool 2 exhibited similar characteristics to
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Fig. 7. Microstructure of the tool surface after welding of tools 1 and 2 after (a) and (d) 100, (b) and (e) 200, and (c) and (f) 300 welds, respectively.
Fig. 8. Oxidation of the tool edge of the tool 1 after 100 welds.
Fig. 9. Crack in tool 2 surface in the tool center formed after 200 welds.
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those of tool 1 after 300 welds. The third most important mechanism was the formation of ternary W–Fe–O compounds. In FSSW, most of the observed material transfer was from the workpiece to the tool and ternary W–Fe–O compounds were produced on the tool surface by the reaction between the tool and material. After the welding, the tool was rapidly air cooled, suggesting that cracks were created on the compound surface by thermal expansion and shrinkage, and that tool wear was increased by the spreading of such cracks (Fig. 9). In addition, these ternary W– Fe–O compound layers may be removed from tool surface, this was likely to happen after every welding when tool had fresh surface under high temperature. Consequently, it was considered that synthetic reasons by an oxidation of tungsten and a formation of W–Fe–O ternary compound resulted in the severe wear in the tool 1, and in a case of tool 2, a formation of W–Fe–O ternary compound was mainly attributed to the wear until 200 welds, and an additional contribution of tungsten oxides due to the fatigue of Co binder accelerated the wear after 200 welds. 4. Conclusion Steel plates were subjected to FSSW with two tools of different chemical composition. The effect of the number of welds and tool deformation on the mechanical property of the joint and the tool wear during FSSW were analyzed. The joint strength of the weld was affected by the number of welds, which was attributed to the effect of tool wear on the tool shape. Examination of the tool shape after welding revealed that extreme wear occurred between the pin center and the edge. As the tool tip was self-sharpened during welding, the tool could possibly be initially shaped with a steady-state, worn tool geometry in order to reduce the tool wear rate and produce a more durable tool. Three potential mechanisms to explain the tool wear were obtained by the XRD and SEM analysis results for the worn surfaces, which were an oxidative wear of WC, and a fatigue of the Co binder and a formation of a ternary W–Fe–O compound.
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