Effect of Dwell Time on Friction Stir Spot Welded Dual Phase Steel

Abstract

Friction stir spot welding (FSSW) is a relatively new welding technique which has already been successfully commercially applied on automotive aluminum alloys which, in turn, lead to significant energy savings and infrastructure cost-reductions. Due to the success of FSSW on Al alloys FSSW has generated tremendous interest in the automotive industry as an alternative joining process to resistance sport welding for steel body panels. Currently there is limited data on the mechanical properties and microstructure of friction stir spot welded steel thus the current aim was to investigate how these parameters chamged as a function of some critical process variables. In this instance spot welding of 2 mm thick high strength dual phase steel (DP800) was undertaken using a 3.5mm pin length tungsten rhenium tool with a tool rotation speed of 1000 rpm and plunge rate of 100 mm/min. Different dwell times of 0, 0.5, 1, 2 and 4 seconds were then applied prior to tool retraction to study the effect of dwell time on the microstructure and mechanical properties of the spot welds. Investigation of the weld spot joints revealed three distinct microstructural zones; a) thermo-mechanically affected zone (TMAZ), b) heat affected zone (HAZ), and c) parent material zone. The TMAZ shows an acicular martensite microstructure while the HAZ shows a ferrite/martensite structure in a very fine scale when compared to parent material zone. The Parent material is a typical dual phase structure which is ferrite matrix with a dispersion of martensite. Therefore, the TMAZ was harder while HAZ was softer when compared with parent material zone. The tensile strength obtained from tensile tests and the bonding ligament which has been reported to be measures of strength were also found to increase with dwell time.