Numerical and experimental study on the effects of welding environment and input heat on properties of FSSWed TRIP steel

In this paper, the effect of input heat and welding environment on microstructure and mechanical properties of friction stir spot welded TRIP steel sheets were investigated. Six types of joints produced in both air and water environments and under rotational speeds of 900, 1350, and 1800 rpm. Then, the microstructure and mechanical properties of them were studied. The thermal histories, strain, and strain rate distributions of cases obtained by finite element modeling. According to the temperature and strain distribution and microstructural observations, four different zones determined in welding region: stir zone, thermomechanically affected zone, and high and low temperature heat-affected zones. It is obtained at in-air welds by increasing the rotational speed, the strength of joints increase to a maximum value because of higher strain rate and more recrystallization of prior austenite grains. The strength then decreases due to high amount of heat input and growth of recrystallized grains. Thermal history of underwater welds showed lower peak temperature and rapid cooling rate. Also, by increasing rotational speed in underwater joints, the strength and hardness increased because of microstructure refinement. The fracture surfaces of joints showed a dimple pattern ductile fracture in all cases except 1800 rpm in-air joint that the fracture was less ductile which agrees with lower tensile elongation of it.