Friction Stir Welding And Solid-state Defect Repairing Of 7150-T77511 Aluminum Alloy

Friction stir welding as solid-state connection technologies can avoid the defects caused by fusion welding.It has great advantages for connecting lightweight materials such as aluminum alloys.This paper uses non-weld-thinning friction stir welding,solid-state filling repair and self-supporting friction stir welding to solve problems of weld thinning phenomenon,keyhole and supporting,a separate welding tool was designed and optimized to achieve defect-free and high-strength connection of 3 mm 7150-T77511 aluminum alloy.The joint forming,microstructure,mechanical properties and material flow behavior were studied,providing technical support for the friction stir welding defect-free highperformance connection of 7150-T77511 aluminum alloy.Through new design of shoulder welder with built-in small shaft,the “single pass welding,two forming action” method achieves no-thinning and high-strength joints.At welding rotating speed of 800 rpm,welding inclination angle of 1.5°,and welding speed of 100-400 mm/min,there is no defect in the thinned friction stir welding joints.The entire joint is divided into four zones: shoulder affected zone,welding nugget zone,thermo-mechanically affected zone and heat affected zone.Within a certain range,with the increase of welding speed,joint width become narrow,grain refinement occurs,tensile strength of the joints increases,up to 491.8 MPa,reaching 85% of the strength of the base metal.The fracture mode is a brittleductile fracture mode.The microhardness of the joints shows a "W" distribution,and the higher welding temperature on the advancing side causes the strengthening phases to precipitate,and the lowest microhardness value appears on the heat affected zone of the retreating side.The characteristic of the material itself results in a wide range of microhardness affected area,and the increase of the welding speed can improve the overall microhardness of the joint.By designing the consumable welding tool,after the keyhole or the defect is regularly treated,the solid-phase repair is performed in a way of “making the weak strong” to realize the repair of the keyhole or the defect.The size of the regular holes and filler rods is optimized to achieve equal strength repair of the filled joints with a maximum tensile strength of 445.9 MPa,equal to 90.7% of weld strength.Fill repair joints are divided into six zones: filling zone,mixed zone,new welding nugget zone,welding nugget zone,thermo-mechanically affected zone,and heat affected zone.The original filler rod material remains in the repair joint formed filling zone.The filler rod material was mixed with the material of regular hole and formed mixed zone.After the fracture,the filler rod acted as a stirring pin and forms new welding nugget zone during the friction stir process.With the filler rod size close to slightly larger than the regular pore size,the tensile strength of the joint increases.The hardness of mixed zone is higher than that of new welding nugget zone and welding nugget zone due to the fine equiaxed grains of mixed zone.thermo-mechanically affected zone,and heat affected zone experienced the secondary welding thermal cycle,the softening is severe and become the weakest area of repairing joints.By designing a separate welding tool with a supporting stirring pin,the supporting achieves a back supporting function and achieves a non-cushion platesupporting friction stir welding connection.Self-supporting friction stir welding can adapt to different sheet thickness without back supporting after optimize the self-supporting friction stir welding split type welding equipment shape and size.The optimized design of the size avoids the outer area of the shaft shoulder,eliminates the weak connection and incomplete penetration of the conventional friction stir welding.The method has a narrow range of rotational speed,and increasing the welding speed with a certain rotational speed at 550 rpm can reduce or even eliminate internal hole defects,and realizes non-defect friction stir welding connection at a welding speed of 500 mm/min.Due to the structural similarity and good heat dissipation conditions,the grain size of upper welding nugget zone and lower welding nugget zone affected by the shoulder and the lower support is basically the same,but smaller compared to welding nugget zone,the grain morphology of each zone is similar to conventional friction stir welding.Through the non-weld-thinning friction stir welding,keyhole or defect solidstate filling repair and self-supporting friction stir welding,high-strength and nothinning connection of the material was realized,the keyhole was repaired and nosupport welding was realized.Through the optimal design of the welding equipment and the analysis of the correlation between the organization and performance,the performance optimization control is achieved,and a no-weld-thinning high-strength welding without defect was obtained.