Friction stir welding of AISI 1080 steel using liquid CO2 for enhanced toughness and ductility

In this study, FSW of AISI 1080 high carbon (0.85 wt-%) steel was carried out at a temperature below its A(C1) under two sets of conditions, one entailing natural cooling and the other rapid cooling with liquid CO2. The joint structures and mechanical properties obtained under both sets of conditions were investigated, whereupon it was found that defect free joints can be successfully fabricated under either. The resulting joint microstructures mainly consisted of grain refined ferrite with some globular cementite. Higher (harder) joint microhardness profiles were obtained under liquid CO2 cooling below A(C1) than under natural cooling below A(C1). In tensile testing, all joint fractures were observed to occur in the base metal. Also, stir zone tensile strength and toughness were found to be higher in liquid CO2 cooled samples (820 MPa and 76.7 mm N respectively) than in naturally cooled samples. Note that in both cases, the joints were formed at temperatures below A(C1).