Effect Of Shoulder Geometry On Plastic Metal Flow, Microstruture And Properties Of Friction Stir Welding

In the paper, five tools with different shoulder geometries were used to friction stir welding 2A12-T4 aluminum alloy coupling with aluminum foils as a marker material. The flow behavior of plastic metal was observed according to the distribution of tracing materials in the horizontal, longitudinal and transverse direction, and the three-dimensional model of plastic flow in the weld was established. The effect of plastic flow on microstructure and mechanical properties was analyzed, which can provide an experimental basis of the formation for friction stir welding.The results show that, the force suffering from the shoulder metal changes, and which results in different flow patterns of plastic metal and morphologies of the cross section. Increasing the forging pressure of the shoulder can increase the frictional force between plastic metals, and which is conducive to improve plastic flow. Moreover, more plastic metals migrate towards to the bottom of the weld, which increases the nugget size, weld compactness and decreases loose defect in the weld.The flow velocity of plastic metal increases with increasing small groove channel in the shoulder, and the sucking-extruding effect become stronger. The shoulder drives more plastic metal moving to the stirred zone of the pin(SZ) and the bottom of the weld, leading to a bigger nugget width. When the cavity of the channel in the shoulder is excess, and excessive plastic metals squeezed into the cavity in the shoulder. With rising the temperature, the stickiness of the plastic metal decreases, and which impedes metal flow and causes the welding defect including loose structure and hole in the weld.The lateral groove channel can enhance the force of the shoulder for the plastic metal moving toward the horizontal direction, so more plastic metals migrate downward in the weld and the nugget width increases. When a tool with a convex cone and spiral groove shoulder is used, it is advantageous to drive quickly plastic metal moving to the bottom of the weld with a biggest flow velocity, and the weld width is the largest and the height of onion rings is the smallest.The average value of ultimate tensile strength of the joints produced by the tool with a lateral large groove channel shoulder is 278.95MPa(64.3% of base material). The main reason is that the shoulder can drive plastic metal flowing down with a radial pattern, and smaller extrusion force for plastic metal at the bottom can not make the Al-cladding penetrate into the weld. The Al-cladding moving upward into the weld is the highest when a tool with a convex cone with spiral grooved shoulder is used. The Al-cladding between the weld nugget and thermo-mechanically affected zone can weaken the mechanical properties of the joint, where the microcracks initiate and expand easily, and which causes a lowest strength of 196.04 MPa.