Study On Stationary Shoulder Friction Stir Welding Process Of 7B52 Laminated Armored Aluminum Alloy

When traditional fusion welding is used to weld aluminum alloys,defects such as airhole and cracks are prone to occur in the weld,and large residual stresses and welding distortions are produced.Friction stir welding is a solid-phase connection technology that can overcome the typical defects caused by aluminum alloy welding.Therefore,research on the stationary shoulder friction stir welding of 7B52 laminated aluminum alloy is of great significance for its promotion and application in the military field.In this paper,by developing the stationary shoulder friction stir welding(SSFSW)tool,12 mm and 21 mm thick 7B52 laminated aluminum alloy non-defective SFSSW butt joints were successfully obtained.By analyzing the joint morphology,microstructure and mechanical properties,the pros and cons of the process parameters were evaluated.And through solid solution + aging treatment to further improve the strength of the weld.Finally,the tensile and impact fracture mechanisms of the base material and joint are analyzed.The hardness curve of 12 mm thick single-sided welded joints from the welding core zone(WNZ)to the thermomechanical affected zone(TMAZ)to the heat affected zone(HAZ)is basically a "W"-shaped distribution,and the lowest hardness value appears in the HAZ.The second layer has more serious damage to the bonding layer,and the overall hardness of WNZ fluctuates greatly.The maximum tensile strength of the joint is 369 MPa,obtained at 900 rpm& 150 mm / min.The highest impact toughness is 26.8 J / cm2,which is also obtained under the parameters of 900 rpm & 50 mm / min,which is 84.8% higher than the base metal.The mechanical properties of the second weld of the 21 mm thick double-sided welding joint are better than the first weld.The stratified tensile test showed that the strength of the fourth layer joint was the highest,which was 428 MPa.The best heat treatment process for 7B52 aluminum alloy SFSSW single-sided welded joint is 480 ? heat preservation 80 min + 120 ? artificial aging 24 h.Under this process condition,the hardness distribution of the weld seam is uniform,and basically returns to the base metal level.The tensile strength of the joint is 438 MPa,which is 92.2% of the tensile strength of the base metal,which is 18.7% higher than the tensile strength of the welded joint without solution treatment.After solution treatment and aging treatment,the HAZ structure of the weld basically remains unchanged;the long-axis grains of TMAZ have grown,and the TMAZ and WNZ mutation interface on the advancing side is also blurred due to the grain growth;WNZ The original fine grains had grown significantly,and the degree of growth was superfluous to TMAZ.The tensile crack of the base metal originates at the interface between different layers,and the tensile crack of the joint originates at the boundary of TMAZ and WNZ on the advancing side.Because WNZ forms fine equiaxed crystals during the welding process,and the pure aluminum material between the layers is fully stirred into the weld,the impact toughness of the joint is higher than the base metal.