Impact of friction stir welding on the mechanical and microstructural properties of austempered ductile iron and mild steel joints

Abstract

This study investigates the dissimilar welding of Austempered Ductile Iron (ADI) with mild steel by using friction stir welding (FSW). Both materials are widely utilized in many fields owing to the exceptional properties and relatively competitive costs. However, joining two materials with different chemical and mechanical properties presents significant challenges. After many modifications in FSW parameters, a successful butt-joint was achieved by fixing the ADI in the retreating side (RS) and mild steel in the advancing side (AS), using rotational speed 800 RPM and travelling speed 50 mm/min. These parameters have resulted a well-formed stir zone (SZ) and thermo-mechanical affected zone (TMAZ) without grooves or cracks. The microstructure of the weld regions exhibited deformation of graphite nodules into strips and mainly martensitic structure at SZ, but no carbide phases or cracks were observed in the SZ. The analysis of the as-welded samples showed that the maximum microhardness values are 590 HV and 406 HV at the SZ and TMAZ respectively, and the tensile strength is 350 MPa when fractured at AS-TMAZ. Post-weld tempering at 370°C for 1 hour has resulted a formation of tempered martensite at the SZ, and an obvious reduction in the microhardness values to 395 HV at SZ and 350 HV at TMAZ. The tensile strength has slightly improved and fractured at the mild steel base metal. These results demonstrate a successful joining of ADI and mild steel using FSW.