For Friction Stir Welding (FSW) of aluminum alloy thick plates, the main reasons of poor weld formation quality are the low temperature of plastic metal in the weld root and its low flowability. Therefore, the assisted heating method was used to preheat the thick plate bottom in this study, and the effect of assisted preheating temperature on weld formation of FSW of thick plates was investigated. The results show that, with the assisted preheating temperature increasing from 20℃ to 80℃, the weld quality improves firstly and then worsens In particular, the width, height and area of nugget zone in the weld show a bulge trend. However, the area of loose zone shrinks first and then expands. When the assisted preheating temperature reaches 40℃, the produced weld has the best quality and the largest size of the nugget zone with the loose zone disappearing into the weld. However, when the assisted preheating temperature raises to 80℃, the obtained weld is the worst and the area of the loose zone is the largest. The study finds that, the enhancement of the flowability and the weld quality are mainly caused by significantly raising the peak temperature and extending high temperature acting time when an appropriate assisted preheating temperature is applied.. The flow pattern of plastic materials in the nugget zone transfers from flowing upward to high temperature zone on the weld top along the rotating pin surface to transverse flow of extruding surrounding cold metal. Nonetheless, when the assisted preheating temperature is too high, the flow pattern of plastic materials in the nugget zone starts to transform into original mode of flowing upward to high temperature zone on the weld top along the rotating pin surface. Also, this transferring degree increases evidently, causing the reappearance of the defects in the loose zone.
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