Influence Of Welding Process Parameters On The Joint Formation In Friction Stir Spot Welding Of Aluminum

Aluminum alloy is an ideal material for the fabrication of components in aviation~astronautics, automobiles and ships, because they process high specific strength, specific modulus, fracture toughness, fatigue strength and corrosion-resistance. At present, the overlapping joint of aluminum alloy mainly adopts riveting, brazing and resistance welding. Riveting has complicated manufacturing process, poor watertightness and increase the weight of structure. When brazing, the workpiece produce large distortion, and is prone to produce lack of penetration and corrosion. The resistance welding requires higher power, waste much electric power and produce severe electrode wear.In this paper, the influence of the welding parameters on the joint formation and mechanical properties of friction stir spot welded LY12 A1 alloy were investigated. When the tool penetration depth is fixed, the influence of tool rotation speed, welding time, the shape of pin and tool diameter on the formation and mechanical properties of joint was analyzed. A 3-D finite element model was built to simulate the temperature distribution and flow behavior of plastic metals around the pin.The result shown that, When tool rotation speed is high or welding time is long, a good joint is achieved, the surface shaping of the joint is smooth, and without burr. With the decreasing of tool rotation speed and welding time, the surface shaping became worse, and the diameter of weld nugget is reduced, the deformation width of interface increased in the first, then reduced, the length of faying line in cross section gradually increased. Base on the microstructure character of the joint, it can be divided into Plasticity Zone, Dynamically Quiescent Layer, Heat Affected Zone, Notch Tip Zone and Base Metal. Plasticity zone and dynamically quiescent layer became fine equiaxed recrystallisation under the thermo-mechanical effect, grain in the heat affected zone are grow because of the effect of friction heat. From base metal to plasticity zone, the profile of microhardness exhibite a trend of high-low-sight higher that is the microhardness of plasticity zone is high, but slightly lower than base metal's, the microhardness of dynamically quiescent layer lower than plasticity zone's. the minimum value of microhardness is in the heat affected zone. In the range of experiment parameters, the tensile shear strength of spot increased with the increasing of tool rotation speed and diameter of shoulder. But with the increasing of welding time, the strength increased in the first, and then reduced. For different shape of pin, the force to the plastic metals in the welding is different, resulting in different tensile shear strength, the joint with columnar-pin and counter-clockwise screw thread has the maximum strength, the joint with cone-pin and non-whorl has the minimum strength. When the tool rotation speed was 2500rpm, welding time was 12s, tool diameter was 16mm, the pin was columnar-pin and counter-clockwise screw, the maximum strength was up to 9.24kN per spot. The joint with shear zone and without shear zone were two typical fracture mode of joint. When the energy input is high, the fracture of joint has the shear zone, and the shear tensile strength is high; on the contrary, when the energy input is low, the fracture is without shear zone, and the shear tensile strength is low.Flow behavior of plastic metal indicated that, the plastic metal in the top is mainly influenced by the tool shoulder. Under the shoulder friction and the shear force between the shoulder and material, plastic metal flowed around the rotating pin, the trend of flow decreased with the increasing of distance to the shoulder. Under the effect of pin screw, the plastic metal moved in a spiral flow around the rotating pin and a helical vertical rotational flow was created. When the plastic metal flowed to the bottom of joint, the plastic metal moved upwards and outwards under the block of the backplane at the bottom and the extrusion force of un-plastic metal, when the metal arrived to the top, the plastic metal moved to the bottom under the effect of shoulder.Numerical simulation results indicated that, the plastic metal which around the pin has the highest temperature, the temperature distribution is identical in general around the pin. From pin to base metal, the temperature gradually decreased along radial, from top to bottom, the high temperature region continually decreased. Plastic metal in the shoulder has the same flow direction with the rotating pin, the plastic metal which close to the rotating pin has the highest flow velocity. From pin to the edge of shoulder, the flow velocity of plastic metal gradually decreased, the plastic metal in the top of joint moved from upper to the bottom along the surface of rotating pin. The simulation results is almost identical to the experimental results.