Friction Stir Welding of AZ31B-H24 Magnesium Alloy in Lap Configuration

Abstract

Friction stir welding (FSW) being an enabling solid-state joining technology can be suitably applied for the assembly of lightweight magnesium alloys. In this study, AZ31B-H24 Mg alloy sheets with a thickness of 2 mm were friction stir welded in lap configuration using two tool rotational rates of 1000 and 1500 rpm and two welding speeds of 10 and 20 mm/s. The joint quality was characterized in terms of the residual stresses, welding defects, microstructure, and texture. The mechanical properties including hardness, room and elevated temperature tensile and fatigue properties were also evaluated and correlated to the structure and defects. It was observed that the hardness decreased from the base metal (BM) to the stir zone (SZ) across the heat-affected zone (HAZ) and thermomechanically-affected zone (TMAZ). The lowest value of hardness appeared in the SZ. With increasing tool rotational rate or decreasing welding speed, the average hardness in the SZ decreased owing to increasing grain sizes, and a Hall-Petch-type relationship was established. The shear tensile behavior of the lap joints was evaluated at low (-40°C), room (25°C), and elevated (180°C) temperatures. The failure load was highest in the lower heat input condition that was obtained at a tool rotational rate of 1000 rpm and a welding speed of 20 mm/s at all the test temperatures, due to a smaller hooking height, larger effective sheet thickness, and lower tensile residual stress, as compared with other two welding conditions that were obtained at a higher tool rotational rate or lower welding speed. The lap joints usually fractured on the advancing side of the top sheet near the interface between the TMAZ and the SZ. Elevated temperature testing of the weld assembled at a tool rotational rate of 1000 rpm and a welding speed of 20 mm/s led to the failure along the sheet interface in a shear fracture mode due to the high integrity of the joint that exhibited large plastic deformation and increased total energy absorption. Fatigue fracture of the lap welds always occurred at the interface between the SZ and TMAZ on the advancing side where a larger hooking defect was present (in comparison with the retreating side). The welding parameters had a significant influence on the hook height and the subsequent fatigue life. A relatively “cold” weld, conducted at a rotational rate of 1000 rpm and welding speed of 20 mm/s, gave rise to almost complete elimination of the hooking defect, thus considerably (over two orders of magnitude) improving the fatigue life. Fatigue crack propagation was basically characterized by the formation of fatigue striations concomitantly with secondary cracks.