Friction stir welding of ultrahigh-purity aluminum thin sheets never to lower high conductivity at ultra-low temperature

Ultrahigh-purity aluminum sheets with excellent electrical and thermal conductivity at ultra-low temperature have uses of a heat dissipation component, e.g. for a superconducting equipment. Recently, development of welding methods for these sheets without degradation of the excellent conductivity has been required. This study focused on friction stir welding (FSW), which was conducted for 99.999wt.% (5N) Al thin sheets with thickness of 0.8mm. The FSW tool rotation speed was set to 3000rpm, and both the welding speed and tool insertion depth were changed. Decreasing welding speed suppressed the occurrence of a kissing bond on an FSW back side and increased the stir zone area having ultrafine grains, although residual strains in all the joints were more than tungsten inert gas (TIG)-welded joints which consisted of coarse grains similar to the base metal. Base metal fracture occurred in tensile tests of the FSW joints except that at the maximum welding speed, in contrast to rapture near the fusion line of the TIG-welded joint due to blowholes. On the other hand, 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 3h increased RRR of the FSW joints to almost the same value as that of the base metal. This is attributed to significant reduction of residual strain as well as grain growth. Decreasing RRR of the TIG-welded joint seemed to be attributed to decrease in purity. In order to obtain high conductivity at ultra-low temperature in the 5N-Al thin sheet joints keeping higher joining strength, the FSW in conjunction with post weld heat treatment would be a better choice. © 2020 Japan Welding Society. All rights reserved.