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7A52 superhard aluminum is a kind of material commonly applied in military vehicle, the paper againsts the practice problem that fatigue strength and wear-resisting of TANDEM MIG welded joint reduce seriously for these defects like hot cracking, serious blow hole and so on, so it is studied that surface nanocrystallization measurment of aluminum alloy welded joint, in order to search a effective path to increase cynthesis mechanical property of welded joint by the technology of the high-energy shot peening surface nanocrystallization.The best welding parameters were firmed by orthogonal measurment and the mechanical property of welded joint after artificial aging such as surface hardness, wear-resisting property, tensile-strength, impact ductility, fatigue strength, residual stress were measured and studied; the surface nanocrystallization technology parameters were preliminary fixed for 7A52 superhard aluminum welded joint and measured surface hardness and wear-resisting property of welded joint, searched the influence regular pattern of surface nanocrystallization to mechanical property of welded joint, disclosured surface nanocrystallization mechanics of welded joint of 7A52 superhard aluminum alloy on theoty. The mainly results and conclusion as follows:The optimized welding parameters and artificial aging parameters technology are: hunting frequency 40 times/min, welding current 40 A, welding voltage 16～24 V, aging temperature 120℃, aging time 20 h. High-energy shot peening time parameter optimization: 20～30 min, other technology parameters of surface nanocrystallization are vibration prequency 50 Hz, ejection distance 33 mm, bullet loading 500 g.The microstructure of base metal before surface nanocrystallization isα(Al)+ T(Mg3Zn3Al2), the microstructure of the welded seam isα(Al)+β(MgZn2)+T(Mg3Zn3Al2), the microstructure of heat-affected zone isα(Al)+T(Mg3Zn3Al2). The grain size of welded seam is 49.5μm, the grain size of heat-affected zone is 98μm. The impact ductility of welded joint is 21.3 J·cm-2; tensile-strength of welded joint is 269.2 MPa; wear rate of welded joint is 0.0187%, wear rate of base metal is 0.0174%; the hardness near welded seam center is lowest, the hardness gradual increase far from seam center and heat-affected zone is stricture that is about 20mm; the fatigue strength of welded joint is 40MPa when the fatigue life is 3×106; the longitudinal residual stress(σx) is major at welded seam, the numeric ofσx is lower than yield strength 219 MPa of welded joint.The grain is equiax crystal after surface nanocrystallization, it appears CuMnZn and AlMn phases in base metal, welded seam and heat-affected zone appears CuMnZn phase, and all appear second-phase SiMnFe in base metal, welded seam and heat-affected zone. The reasons of appearance different phases among base metal , welded seam and heat-affected zone are different chemical constitution proportion of base metal and welding wire and part of metallic elements are melting loss when welded. The average grain size of welded seam surface is 24.7～34.4nm, which is 0.05% of welded seam grain and the average grain size of heat-affected zone surface is 26.6～31.5 nm, which is 0.03% of heat-affected zone grain before surface nanocrystallization, the average grain size of base metal surface is 25.8～34.7nm, the nanometer layer is about 20μm. The deforming layer is about 36.5～159μm, which is gradual increase as the cloudburst time prolong.The hardness number of welded joint increases obviously and change gently and equality after surface nanocrystallization, the hardness of welded seam, heat-affected zone and base metal increasing amplitude is 2.52 double, 1.79 double and 1.11 double, the hardness of welded seam change most, the hardness distribution of original as welded hasn’t changed. The wear rate of welded joint is 0.0056% after surface nanocrystallization, which is 29.9% wear rate of welded joint before surface nanocrystallization. The wear rate of base metal is 0.0049% after surface nanocrystallization, which is 28.2% wear rate of welded joint before surface nanocrystallization.
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