Research On Microstructure And Properties For Welded Joint Of 7A52 Aluminum Alloy Before And After Surface Nanocrystallization

7A52 aluminum alloy of medium strength is a common material in military products. In view of the present situations of the poor composite performance and the low strength in the welded joint, artificial aging and High Energy Shot Peening (HESP) technology combined methods were used in this paper to process the TANDEM MIG welding, and the microstructure and properties before and after Surface Nanocrystallization were researched, the better parameter of Surface Nanocrystallization for welded joints of 7A52 aluminium alloy were obtained.Microstructure and Properties for welded joint of 7A52 aluminum alloy was characterized before and after surface nanocrystallization by means of Optical Microscope, XRD, TEM, Option Microhardness and other test methods, the results show that welded joints of 7A52 aluminum alloy base material has obvious banded; weld was equiaxed, its grain size was about 50μm around before HESP treatment. The average microhardness of surface for parent matel and weld were about 113 HV and 97 HV. Wear rate was 84.2% and 96.8% for parent metal and welded joints. Tensile strength of welded joint was 271.4MPa before Surface Nanocrystallization.Through HESP treatment, with increasing of time for HESP, the grain size of surface for welded joint have been decreased and microstrain have been reduced accordingly. The banded structure in parent metal, heat affected zone have been removed. Microstructure of welded joints were relatively uniform, fine in all regions. The surface hardness of welded joints in all regions have significantly improved. The microhardness of the longitudinal section gradually decreases along with the distance form the surface, while with depth further increasing, the hardness tend to stabilize until to the same with the parent metal.When HESP time was 50 min, the grain size and grain boundary have not seen in the surface of welded joint, the thickness of the parent metal deformation was about 50μm, the thickness of weld deformation was about 70 μm, and the heat-affected zone deformation was about 60μm. XRD tests available. The grain size of surface for parent metal, weld, HAZ were about 31 nm, 35 nm, 41 nm. Obtained by TEM analysis, the grain size of surface for parent metal, weld and heat affected zone were about 20 nm, 28 nm and 32 nm. the grain size of surface for weld and heat affected zone were larger than the grain size of parent metal, and the distribution of nanocrystals were not fine and uniform than parent matel. Also found MgZn2 has precipitated in theα-Al phase; Al4Cu9 has precipitated in theα-Al phase in the weld and HAZ.When HESP time were 50 min, the average microhardness of surface for parent metal was about 270 HV, the average microhardness of surface for weld was about 185 HV. The hardness of surface for parent metal was about 2.5 times increased and weld was about 2 times increased relative to before the HESP. Wear rate of the parent matel was 64.5%, wear rate of welded joint was 69.7% after HESP treatment. Wear resistance of parent matel can increase about 20%, wear resistance of welded joints can increased about 27.1%. Tensile strength of welded joint was 394.1MPa. The specimen tensile strength increased about 45.2% after HESP.Results shown that the overall performance for welded joints of 7A52 aluminum alloy have significantly increased by the artificial aging and HESP treatment. While shot distance as a 37mm, ball made of GCr15, ball diameter forφ8 mm, vibration frequency was 50 Hz, the HESP time was 50 min, the better parameter of Surface Nanocrystallization was obtained.