Effect Of Boundary Conditions On Microstructure And Properties Of Friction Stir Welding Joints Of Aluminum Alloy

Currently, most researches on friction stir welding are about the influence of weldingspeed, rotation speed and the amount of pressure on the welding joint’s properties, whilethere are few researches on boundary conditions, such as stir tool material, shoulder size,pin shape, backing plates and so on. There is a lack of systematic research on boundaryconditions, for most researches on boundary conditions only involve one or two factors. Aclear understanding of the influence of boundary conditions on welding joint’s propertieshas a non-negligible significance for improving the quality of friction stir welded joints.This research selects2219aluminum alloy as experiment material, and designsexperiments to study the influence of process parameters on the microstructure andproperties of the welding joints and get the optimum process parameters, then welds theworkpieces under different boundary conditions using the optimum process parameters tosystematically study the influence of stir tool material, shoulder size, pin shape and backingplates on joint’s microstructure and properties. The experiment results are as follows.(1) We can get good quality welding joint with smooth surface and dense interiorunder proper process parameters. When the welding speed is100mm/min and the stir tool’srotation speed is1200r/min, the welded joint shows the best performance.(2) When using different shaped stir tool, the surfaces are all relatively smooth, butthere appears varying degrees of burrs on one side of the joint.. When using a cylindricalfriction stir tool and a screw thread friction stir tool, the joints’ intensity factor is more than90%, and there is no big difference in intensity factor when using those2stir tools. Whenusing a cone shape friction stir tool, the intensity factor is significantly lower because of thedefect in joints.(3) When using high-speed steel stir tool and stainless stir tool, the surfaces of jointsare both smooth, and there are no internal defects. The joints both have a high tensilestrength and the intensity factor is up to90%of the base mental. When using CuW80alloystir tool, there is slight burrs on the surface of joint, and there is severe tunnel-typed defectsat the bottom of NZ. The tensile strength is relatively low and the intensity factor is only81.81%.(4) When using different shoulder size stir tools, the surfaces of joints are both smooth.When the shoulder size is Ф9mm, there is much burrs and there is severe tunnel-typed internal defects. When the shoulder size is Ф12mm and Ф15mm, there is little burrs andthere is no internal defects. The tensile strength of joints has a varying degrees of increasewith the increase of shoulder size.(5) Different backing plates have a big difference on all the surface, microstructureand mechanical properties of joints. We should choose backing plate with properconductivity according to the actual situation in order to get high quality welding joints.