Critical Review of Solid-State Welding for Al Alloys with High Joint Efficiency: Friction Stir Welding (FSW) vs. Linear Friction Welding (LFW)

Abstract

Conventional fusion welding process exhibited difficulties in joining Al alloys at the relatively high processing temperature above the melting temperature due to their high coefficient of thermal expansion and thermal conductivity, which cause pore formation and large distortion during solidification, resulting in significant deterioration of mechanical properties in welds. Friction stir welding (FSW), one of solid-state joining methods, was developed as a special welding technique for Al alloys invented at The Welding Institute (TWI) in 1991. Since FSW can be performed below the melting point of Al alloys, the formation of pores and large distortion are greatly inhibited at the weld regions. However, both grain coarsening and dissolution of precipitates during FSW lead to the material softening, which reduces the joint efficiency. While, at a lower processing temperature, the material flow between the rotating tool and workpieces becomes insufficient, generating welding defects. Linear friction welding (LFW) is another solid-state joining process that is suitable for manufacturing Ti-6Al-4 V alloy blade disks, i.e., blisks in the field of aircraft engines. This is caused by a low thermal conductivity of the Ti alloys, suppressing the formation of softening and heat-affected zones under any processing conditions during LFW. The high-pressure LFW process is expected to be a promising process also for producing high-efficiency Al alloy weldments. However, the knowledges and benefits of LFW process for metallic materials with high thermal conductivity such as Al alloys are not well-known. Therefore, this article summarizes the latest reported results on welding temperature, microstructure evolutions, mechanical properties and joint efficiency of LFW Al alloy joints to better utilize the LFW process. Finally, the conclusions of this article including suggestions for further research and development in the Al alloy joints in LFW are provided.