Finite element and experimental investigation on the effects of temperature, strain and strain rate on microstructure and mechanical properties of FSSWed TRIP steel joints

FSSWed TRIP steel joints were investigated at four rotational speed of 900, 1200, 1500 and 1800 rpm. A finite element model was developed to obtain thermal history, strain and strain rate during welding and the results validated by experimental data. The microstructure, microhardness distribution and shear tensile strength were examined. The FSSW process successfully produced high integrity completely defect-free joints at all the proposed welding parameters. As a result of temperature and strain distribution imposed by process, three different zones were determined in welding region: stir zone with recrystallized structure, thermomechanical affected zone with major phase of martensite and heat affected zones. The maximum microhardnees was achieved at stir zone and decreased by getting away from keyhole. Totally the microhardness increased by increasing rotational speed. It was obtained that the strength of joints increased to a maximum value of 9.9 kN in 1500 rpm and then decreases to 8.4 kN at 1800 rpm. By increasing rotational speed, the strain rate increased and causes to recrystallization of the prior austenite grains, but in high rotational speed, the higher temperature causes to grain growth in recrystallized grains. The fracture surfaces of joints showed a dimple pattern ductile fracture in all cases except 1800 rpm that the fracture was less ductile which agrees with lower tensile elongation of it.