Effect of Welding Parameters on Microstructure and Mechanical Properties of Friction Stir Lap Welds of an Ultrahigh Strength Steel

In this study, effect of welding parameters on microstructureMicrostructure and mechanical propertiesMechanical properties of an ultrahigh strength steelUltrahigh strength steel (UHSS) friction stir lap welded with Co-based alloy toolTools was examined. Friction stir lap weldingFriction stir lap welding of the UHSS was successfully done at various welding parameters. A decrease in rotational speed decreased fraction of martensite and hardness in the stir zone. Tensile shear load of the weld increased with decreasing the rotational speed. In the welds produced at the high rotational speed, crack was initiated from the tip of the lapped interface and then propagated into the stir zone having high fraction of martensite, possibly resulting from the stress concentration. On the other hand, the welds produced at the low rotational speed failed in the softest region of the heat-affected zone. This study showed that the reduced brittleness of the stir zone arising from the low rotational speed was an effective strategy for the superior mechanical propertiesMechanical properties of the lap welds of the UHSS.