No abstract

The temperature distribution of 6061-T6 aluminum alloy plates under a friction stir butt-welding was investigated by using experiment and numerical simulation. A real-time temperature measuring system was used to measure the temperature change in the welding process. Vickers hardness profiles were made on the cross-section of the weld after welding. A commercial software of FlexPDE, a solver for partial different equations with finite element method, was used to simulate the experimental welding process of this study. Comparison the experimental and numerical results, the temperature cycles calculated by numerical are similar to those measured by experiment. The temperature distribution profile obtained from the numerical simulation is symmetrical to the weld center and has a close correspondence with the hardness configuration and the microstructure of the weld. The region with the temperature over 300 °C is the zone of softening within the boundaries of base material and HAZ. The regions of 350 °C with minimum hardness are located near the boundary of HAZ and TMAZ. The maxima temperature about 500 °C distributes around the upper part of the weld center. However, the region above 400 °C only matches with the upper half of the weld nugget.

In this study, the tensile shear strength and the fracture behavior of friction stir spot welded AZ61 joints in lap-shear configuration were investigated. The heat input was measured in FSSW to help analyze the effect of welding parameters on the strength. The tensile shear failure test was performed in a material testing system. The cross section of the joints and the fracture surface of the failed specimens were analyzed using optical microscopy and scanning electron microscopy. Results show that the weld diameter and the tensile shear load increase with increasing the input heat. The path of the material flow formed during FSSW process would provide a good way for crack propagation. All failed specimens in this study appear the same fracture features and show a circumferential failure mode under tensile shear loading conditions. The failure is initiated from a notch tip in the upper sheet loading side, and then propagates along the interface of the upper and lower sheets, then through the stir zone circumference; finally, a small portion of the lower sheet in the lower sheet loading side is torn off with some part of the stir zone.

The effects of tool geometry on the microstructure and tensile shear strength of friction stir spot-welded A6061-T6 Al alloy sheets were investigated in the present study. Friction stir spot welding (FSSW) was carried out at a tool speed of 2500 rpm, plunging rate of 1 mm/s, and dwell time of 3 s. Four types of tools with the same shoulder shape and size, but different pin profiles (threaded cylindrical, smooth cylindrical, threaded triangular, and smooth triangular) were used to carry out FSSW. The mechanical and metallurgical properties of the FSSW specimens were characterized to evaluate the performance of the different tools. Experimental results show that the pin profile significantly alters the hook geometry, which in turn affects the tensile shear strength of the friction stir spot welds. The welds made using the conventional thread cylindrical tool have the largest elongation and yield the highest tensile strength (4.78 kN). The welds made using the smooth cylindrical tool have the lowest tensile strength. The welds made using the threaded triangular and smooth triangular tools both have a tensile-shear load of about 4 KN; however, the welds made using the threaded triangular tool have a better elongation than those made using the smooth triangular tool.