Study On The Joining Mechanism And Mechanical Properties Of Friction Stir Spot Welded Joints For Aluminum Alloy

Friction stir spot welding is a newly-developed solid state joining process as an alternative of conventional spot joining process such as resistance spot welding and riveting. To determining the effective process parameters scope, the aluminum alloy sheets 6061-T4 and 7075-T6 with 2mm thickness are welded using fixed friction stir spot welding and refill friction stir spot welding, and the influence of process parameters on joint microstructure,mechanical properties and fracture mode has been investigated.At first, the fixed friction stir spot joint and refill friction stir spot joint of AA6061-T4 has been studied. Four distinct zones exist in two types of joins, and in the stir zone of refill friction stir spot joint, microstructure in the center region are coarser than that in the outer periphery, which induce that the hardness values in the center region of the stir zone are lower than that in the outer periphery. The geometries of hook in two types of joins are different: the heights of the hook in the fixed joints are obviously higher than that in the refill joints. And the profiles of hook tend to outwards from the periphery of the exit hole from inwards towards the periphery of the exit hole with the increasing of rotational speed and dwell time, which has some influence on the joint fracture loads. Within the range of process parameters selected in this experiment, the shear tensile loads and cross-tension tensile loads of the refill joints are much higher than that in the fixed joints. The maximum shear strength of refill joint is higher 34.1% than that of fixed joint based on the shear strength of base material. Different fracture modes are observed: shear fracture for fixed joints and shear fracture, plug fracture and plug-shear fracture for refill joints under shear tensile loading, and nugget debonding for fixed joints and plug fracture (fracture in the upper sheet), plug fracture (fracture in the lower sheet) and nugget debonding for refill joints under cross-tension tensile loading. Experimental results indicate that, friction stir spot welding of AA6061-T4 has a wider process window, the optimum process parameters of fixed joints are 2100rpm of rotational speed and 4s of dwell time, and refill joints is 1500rpm of rotational speed and 4s of process time, which make the shear tensile loads and cross-tension tensile loads both reach the maximum.Secondly, refill friction stir spot welding of AA7075-T6 which has bad weldability has been investigated. The heights of the hook are higher than that of the AA6061-T4 joint. And the geometries of hook and bonding ligament are not as regular and coherent as that of AA6061-T4 joints. Many void defects exist in the root of the stir zone of the joint, which induce some differences in fracture path by contrast to AA6061-T4 joints. When the rotational speed is 1500rpm and process time is 3s and 4s, the shear tensile load and cross-tension tensile load both reach the maximum for AA7075-T6 joint, and at this time the maximum shear strength are equal to the 35.2% of the strength of the base material. So comparing to the AA6061-T4, AA7075-T6 is not as fit as AA6061-T4 to refill friction stir spot welding due to the bad weldability.