Relationship Between Fatigue Properties And Process Parameters Of Friction Stir Welding Joints For Aluminum Alloy

Friction stir welding(FSW)is a solid-state joining process which based on the friction welding.Owing to its uniqueness and superiority,this new welding technique has displayed great dominance in welding aluminum and aluminum alloy since it has been created.So far,the research and development of FSW is still in its initial stage in China,the fatigue behavior research of friction stir welding joints has just begun. In-depth studying the effects of various parameters on fatigue properties of friction stir welding joints will provide an important basis for the promotion and application of this technique,establishment rational and effective fatigue assessment norms.In this paper,the effects of process parameters on microstructure and fatigue properties for aluminum alloy 7050-T7451,7075-T651,6082-T6 and 2024-0 FSW butt joints have been investigated.The results show that it can form a joint with a good look and without internal defects when the rotation speedωat 400～800rpm and the welding speedυat 40～100mm/min for aerospace aluminum alloys 7050-T7451 and 7075-T651.The welding nugget area,grain size,heat-affected zone area and grain deformation in TMAZ have increased with the rate ofω/υincreasing.The low cycle fatigue performance is better when the welding speed at 40 mm/min than that at other welding speed for the FSW joints of aluminum 7050-T7451.The low cycle fatigue performance is outstanding and the hardness of welding nugget zone are higher than that in the base metal when parameters at 600/60 and 800/80(r/mm)for 7075-T651 joints.In high stress strain condition,the circulatory hardening appears in the low cycle fatigue life of the welded joint when parameters at 800/80(r/mm).On the basis of good weld surface,the high heat input of FSW parameters can improve joint fatigue performance for the 7000 series aluminum alloy.At 1x10~6 cycle,the fatigue strengths of the joints when parameters at 600/60 and 800/80(r/mm)are about 80MPa and which is about 91MPa when parameters at 400/40(r/mm)for the aluminum alloy 7075 - T651.There is nonlinear dependence between high cycle fatigue strength and tensile strength.The friction stir weld ability of 6082-T6 is excellent,the comprehensive performance is better in normsl than that in other norms,and the material suitable for welding in low heat input parameters.when welding speed is 50 mm / min,the fatigue strength is little influenced by the rotation speed.When the rotational speed is 500rpm,the cycle times longer than 2x10~5,fatigue strengthen is higher than other joints. The weld zone has been strengthened,and tensile strength and micro-hardness of FSW joints are higher than the base metal for 2024-O aluminum alloy.At 1x10~6 cycle, the fatigue limiting are 82.8MPa,68.9MPa and 66.1MPa when the ration speed at 500r/mim,700 r/mim and 900 r/mim respectively.When the parameter at 500/50r/mm the joints have better overall performance.With the progressive promotion of friction stir welding technology,the request of FSW fatigue design specifications become more and more urgent.Because of a wide range of parameters for aluminum alloy FSW,the research of fatigue properties should be more extensive and in-depth.