Effect of the Surface State of Steel on the Microstructure and Mechanical Properties of Dissimilar Metal Lap Joints of A
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AS a solid-state welding technology, friction stir welding (FSW) process[1] can join Al alloys and steels and get higher quality joints than fusion welding technology. It is also possible that good welds can be produced when FSW is used to join dissimilar materials.[2–18] The need of light weight in automotive body construction leads to the increasing use of the lap joining of aluminum alloy and steel in the fabrication of vehicles.[19,20] Therefore, the development of reliable joints between aluminum alloy and steel is required. Elrefaey et al. studied the feasibility of friction stir lap welding (FSLW) a commercially pure aluminum plate to a low-carbon steel plate and investigated the effects of welding parameters on joint strength.[14] It was reported that joint strength depended on the depth of the probe tip into the steel surface. When the probe tip did not reach the steel surface, the joint failed under low applied loads. However, slight penetration of the probe tip to the steel surface significantly increased joint strength. Kimapong and Watanabe reported the results about FSLW of 5083 aluminum and SS400 steel.[15,16] They showed that the joint shear strength decreased with an increasing tool tilt angle and a probe diameter due to the formation of a thick Al5Fe intermetallic compound layer at the joint interface. The weld quality of FSLW of aluminum and steel was difficult to control because of the formation of some intermetallic compounds at the weld interface. However, some recent attempts at joining aluminum and zinc-coated steel using FSLW turned out to be successful.[17,18] The adoption of zinccoated steel evidently improved the friction stir weldability of aluminum and steel and made the full-strength joints possible. Chen et al. investigated the FSLW Y.C. CHEN, Researcher, and K. NAKATA, Professor, are with the Joining and Welding Research Institute, Osaka University, Ibaraki, Osaka 567-0047, Japan. Contact e-mail: armstrong@hit. edu.cn Manuscript submitted October 21, 2007. Article published online May 17, 2008 METALLURGICAL AND MATERIALS TRANSACTIONS A
feasibility of AC4C cast aluminum alloy and low-carbon zinc-coated steel.[17] They reported that welding speeds had a significant effect on the tensile properties and fracture locations of the joints. With appropriate welding parameters, full-strength joints could be obtained and joints fractured at the zinc-coated steel base material side. Elrefaey et al. investigated the weldability of 1100H24 Al alloy and zinc-coated steel using FSLW.[18] They found that the Al/zinc-coated steel joint exhibited considerable fracture load, while Al/steel joints were so weak that they fractured at the same probe depth during preparation of the specimen for metallographic analysis. Current reporting literature distinctly shows that the surface state of steel has a significant effect on the weldability of Al and steel in friction stir lap joints. So far, systematic research about the effect of the surface state of steel on the weldability of Al alloy and steel in fricti
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