Study on Low Axial Load Friction Stir Lap Joining of 6061-T6 and Zinc-Coated Steel
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INTRODUCTION
ALUMINUM alloy and galvanized steel are increasingly being used as the main structural materials in the automotive industry.[1,2] Because of the significantly different properties between Al alloy and steel, the formation of defects in the joint is inevitable when fusion welding is applied. The main defects are holes and cracks, which reduce the performance and strength of the joint.[3] Although braze welding technology can reduce the formation of these defects, its low efficiency and difficult accessibility make its use controversial in certain types of joint structures. Alternatively, friction stir welding (FSW) is expected to reduce the formation of hole and cracks.[4–7] As a solid-state welding technology, FSW can be performed without the use of a filler material or extra protective gas and is regarded as an environmentally friendly welding technology.[8] In the friction stir lap welding of aluminum alloy and steel, many factors affect the strength properties of the joint, such as the geometry and size of the pin, plunge depth of the shoulder, welding speed, and rotation speed. Elrefaey et al. studied the friction stir lap joining of aluminum plate (2 mm thick) and low-carbon steel plate (1.2 mm thick).[9] In their study, the tool rotation speed was lower than 100 rpm. They observed that a joint with better strength properties could be obtained
SHUJIN CHEN, JIAQI ZHANG, DI WANG, JIANXIN WANG, and RUIFENG LI are with the School of Material Science and Engineering, Jiangsu University of Science and Technology, Zhenjiang 212003, P.R. China. Contact e-mail: [email protected] Manuscript submitted September 22, 2018.
METALLURGICAL AND MATERIALS TRANSACTIONS A
when the pin penetrated the steel plate rather than when the pin did not reach the steel. However, their method may weaken the repeatability of the process because the pin becomes quickly worn out. During the Al/Fe friction stir lap welding (FSLW) process, the Al-Fe intermetallic compounds (IMCs) at the interface play a significant role in the lap joint. Das et al. discussed the parameter optimization in Al6061/ HIF GA steel friction stir lap welds. They showed that IMCs formed for both a higher input parameter and lower energy input parameter. In addition, all the IMCs formed were thermodynamically possible, and both the intermetallics formed above 400 °C.[10] Kimapong et al. reported that the shear strength decreased because of the formation of a thick FeAl3 IMC layer, which formed in the joint between 5083 aluminum and SS400 steel.[11,12] Chen et al. examined the effect of the steel surface state on the microstructure and mechanical properties of dissimilar metal lap joints. Among the three types of surface state, the zinc coating appeared to contribute to the tight combination of the two plates. In their study, Fe2Al5 and Fe4Al13 were observed in the IMC layer.[13,14] Chen et al. combined a 3-mm-thick AC4C aluminum alloy plate and 0.8-mm-thick zinc-coated steel sheet using FSLW. They observed that with the increasing rotation speed, the thickness of th
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