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ABSTRACT
Ultrahigh-purity aluminium sheets with excellent electrical and thermal conductivity at ultra-low temperature have uses in heat dissipation components in, for example, superconducting equipment. Recently, here has been a requirement for the development of methods of welding these sheets without degrading their excellent conductivity. This study focused on friction stir welding (FSW), which was conducted for 99.999 wt.% (5 N) Al thin sheets with thickness of 0.8 mm. The FSW tool rotation speed was set to 3000 rpm, and both the welding speed and tool insertion depth were varied. Decreasing welding speed suppressed the occurrence of a kissing bond on an FSW back side and increased the area of the stir zone that had ultrafine grains, although residual strain in all the joints was greater than in tungsten inert gas (TIG)-welded joints which consisted of coarse grains similar to the base metal. Fracture occurred in the base metal in tensile tests of the FSW joints except for those made at the maximum welding speed, in contrast to rupture near the fusion line of the TIG-welded joint due to blowholes. On the other hand, the residual resistivity ratio (RRR) of FSW and TIG-welded joints decreased to almost half of that of the base metal. The heat treatment at 500°C for 3 h increased the RRR of the FSW joints to almost the same value as that of the base metal. This is attributed to a significant reduction of residual strain as well as grain coarsening. In TIG-welded joint, there is an evident decrease in the RRR associated with a decrease in purity. Therefore, FSW in conjunction with post-weld heat treatment would be a better choice to achieve high conductivity at ultra-low temperatures in 5 N-Al thin sheet joints, while maintaining a high joining strength.