Effect of Dwell Time on Joint Interface Microstructure and Strength of Dissimilar Friction Stir Spot-Welded Al-5083 and
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INTRODUCTION
JOINING of aluminum (Al) to steel alloys by conventional fusion welding processes is confronted by some difficulties such as the difference in their melting points as well as the formation of thick and brittle intermetallic (IM) phases at the joint interface.[1,2] Solid-state welding processes are more applicable for Al/steel joining since thinner IM layers are formed in comparison with the fusion welding techniques.[3–5] The tool rotational speed and dwell time are the most important parameters influencing the friction stir spot welding (FSSW) joint quality. The effects of such parameters on the mechanical properties of friction stir spot-welded Al joints have been explored by several researchers in recent years.[6–11] In the present study, the main goal is to use FSSW to weld dissimilar Al/steel sheets using a new process in which the tool does not enter lower steel sheet. In fact, this process implements the heat originating from stirring of only the softer base material (here Al) to weld Al-5083/St-12 alloy sheets with no mechanical ESKANDAR FEREIDUNI, MOJTABA MOVAHEDI, and AMIR HOSSEIN KOKABI are with the Department of Materials Science and Engineering, Sharif University of Technology, Tehran, Iran. Contact e-mail: [email protected] HOSSEIN NAJAFI is with the Institute of Applied Laser, Photonics and Surface Technologies (ALPS), Bern University of Applied Sciences, Burgdorf 3400, Switzerland. Manuscript submitted April 6, 2015. METALLURGICAL AND MATERIALS TRANSACTIONS A
mixing. Similar processes have been previously applied by several researchers[12–15] for the friction stir lap welding (FSLW) of dissimilar Al/steel sheets. Chen and Nakata[13] studied the influence of steel surface state on microstructure and mechanical properties of dissimilar FSLWed Al/steel joints in which the penetration of pin tip into the steel sheet was avoided. Chen et al.[14] observed that when the penetration depth increased and the pin tip just touched the steel surface, a completely continuous IM layer formed at the joint interface which was responsible for improved joint strength. In another study, Girard et al.[15] used a new technique for the friction stir welding (FSW) of dissimilar metals such as Al/steel and Al/Cu in butt joint configuration which was called friction stir diffusion bonding. Simar et al.[16] invented a method for the FSLW of two sheets with different melting temperatures in which the temperature at the interface of the sheets reached to a value higher than the melting temperature of the bottom sheet with lower melting temperature, and the sheets were joined. Among the works on the FSSW technique, few studies have been done to join dissimilar Al/steel sheets using this process. The results presented by Sun et al.[17] and Chen et al.[18] showed that the high weld strength was attributed to the IMC free interface. However, Bozzi et al.[19] stated that the presence of an IM layer at the interface is necessary to improve the weld strength. Chen et al.[18] joined 6111-T4 Al alloy to DC04 steel sheets with a
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