Achieving a Unique Double Hook Morphology in FSSW of Aluminium-Steel through Sheet Placement and Pin Length Variation

Abstract

Abstract In friction stir spot welding (FSSW) of dissimilar metals, the softer material having a lower melting point is generally placed on the top to maximise the tool life. Therefore, in Al-steel FSSW, aluminium is usually positioned on the top. However, with the development of hard and durable tool metals like tungsten carbide and PCBN, the placement of steel sheets on the top is also possible without compromising the tool's life. This study focuses on the Al-steel FSSW joint formation in various configurations by placing aluminium and steel sheets on top alternatively, along with variation in pin length. The joints fabricated in different configurations were compared in respect of joint morphology and strength. The placement of steel sheets on the top resulted in the formation of a unique morphological feature named "double hook", which enhanced the mechanical interlocking between the two sheets on joining. The double hook feature was unheard of because of researchers' reservations against placing the hard metal sheet on the top. The variation in pin length also contributed to the variation in the size of hooks and joint strength. The FSSW joints were investigated by considering metallography, thermal cycle, mechanical properties and fracture analysis. It was observed that the positioning of steel on the top, along with a moderate pin length penetrating up to half the thickness of the bottom plate, resulted in the joints with optimal interlocking and maximum joint strength.