Effect of tool shape on interfacial microstructure of friction stir spot welded aluminum/steel lap joint

摘要

Lap joining of an aluminum plate or an aluminum alloy plate and a low carbon steel plate was performed using friction stir spot welding. Two kinds of the welding tools were used in the present study. One type consisted of a probe and a shoulder. Another type had a flat bottom without the probe. The welding tool was inserted from the Al plate-side which was overlapped on the steel plate and the tip of the tool was held above the Al/steel interface. The holding time was controlled in the range from 3 s to 60 s. Microstructure of the welding interface was examined by using a scanning electron microscope and a transmission electron microscope. Lap joining was achieved with either tool type. Grain size was refined in Al close to the welding interface, however it was not changed in steel. In the joint welded by using a probe-free tool, an intermetallic compound (IMC) layer was produced at the welding interface. The thickness of the IMC layer was about 1 urn at the central position of the welded area. The thickness decreased with increasing distance from the central position of the welded area, i.e., decreased toward outside of the welded area. TEM observation revealed that the IMC layer was AI_(13)Fe_4 phase. In the joint welded by using the tool with a probe, the welded area increased with increasing holding time. The IMC layer was produced along the welding interface. Thickness of the IMC layer also increased with increasing holding time. Precise backscattered electron image observation revealed two different contrasts in the IMC layer. Energy dispersive X-ray analysis revealed that Al_(13)Fe_4 phase was produced at the Al side and AI_5Fe_2 phase was produced at the steel side. Thickness of the AI_(13)Fe_4 phase was almost constant at any holding time. On the other hand, thickness of the AI_5Fe_2 phase increased with increasing holding time. Such a characteristic growth manner of the IMC is considered to be due to increment of heat input induced by stirring of Al and friction heat generated between the Al and the rotating welding tool.