Effect of tool–workpiece interface temperature control on the weld quality of a bobbin-tool friction-stir-welded aluminum alloy

Temperature control is an effective approach to ensure the weld quality produced by friction stir welding (FSW). In this study, the temperature of the tool–workpiece interface was controlled by a predictive PI (PPI) control method during bobbin-tool friction stir welding (BT-FSW). When the interface temperature was constant, the temperature of the heat affected zone (HAZ) was recorded at different welding speeds. The influence of interface temperature control on the thermal cycle of the HAZ was analyzed. The influence of interface temperature control on the material flow behavior was discussed using a theoretical model. The results of tensile tests showed that the tensile properties were not affected by changes in the tool rotational speed caused by controlling the interface temperature. In addition, regardless of the welding speed, there were optimal welding interface temperatures that caused a weld to have the highest tensile strength or elongation. The weld with the highest tensile strength broke in the HAZ, while the weld with the lowest tensile strength broke on the advancing side in the weld nugget zone because of the tunnel defects in this area. The high interface temperature will lead to these kinds of defects, because the high interface temperature will make the friction coefficient drop sharply, so that the driving effect of the tool on the material will be greatly weakened. The results of tensile tests indicated that interface temperature control can effectively ensure the weld quality.