Abstract：7A04 aluminum alloy plate was jointed by submerged friction stir welding(SFSW), and welded joints were treated (Post Weld Heat Treatment, PWHT), and the effect of post weld heat treatment on the microstructure and mechanical properties in SFSW was investigated. The results show that PWHT joints exhibit dispersively distributed fine precipitates phase morphology, are significantly superior than the feature of the small amount of precipitates with dispersed distribution in SFSW joints. Compared with SFSW joints, the mechanical properties of joints are improved significantly by PWHT. The average hardness of the weld joints nugget zone is increased by 39.7HV, and the tensile strength is increased by 67MPa, reaches 96.1% of the base material, strain hardening capacity of the joints is also enhanced, the tensile fracture exhibits mixed fracture feature of microporous polymerization and cleavage.
 ANDREATTA F, TERRYU H, WIT J H W. Corrosion behaviour of different tempers of AA7075 aluminium alloy[J]. Electrochemical Acta, 2004, 49(17):2851-2862.
 MISHRA R S, MA Z Y. Friction stir welding and processing[J]. Materials Science and Engineering R, 2005, 50(1-2):1-78.
 THOMAS W M, NEEDLHAM J C, DAWES C J, et al. Friction stir butt welding[P]. UK Patent:9125978.8, 1991-12-6.
 WANG W, WANG K S, GUO Q. Effect of friction stir processing on microstructure and mechanical properties of cast AZ31 magnesium alloy[J]. Rare Metal Materials and Engineering, 2012, 41(9):1522-1526.
 WANG K S, WU J L, WANG W, et al. Underwater friction stir welding of ultrafine grained 2017 aluminum alloy[J]. Journal of Central South University, 2012, 19(8):2081-2085.
 李继忠,马正斌,董春林,等. 异种铝合金搅拌摩擦焊材料流动行为研究[J]. 材料工程, 2014, (6):1-10. LI J Z, MA Z B, DONG C L, et al. Material flowing behaviors of friction stir welding by dissimilar aluminum alloys[J]. Journal of Materials and Engineering, 2014, (6):1-10.
 FRATINI L, BUFFA G, SHIVPURI R. Mechanical and metallurgical effects of in process cooling during friction stir welding of AA7075-T6 butt joints[J]. Acta Materialia, 2010, 58(6):2056-2067.
 SRIVATSAN T S, VASUDEVAN S, PARK L. The tensile deformation and fracture behavior of friction stir welded aluminum alloy 2024[J]. Materials Science and Engineering:A, 2007, 466(1/2):235-245.
 SHEN Z K, YANG X Q, ZHANG Z, et al. Microstructure and failure mechanisms of refill friction stir spot welded 7075-T6 aluminum alloy joints[J]. Materials and Design, 2013, 44(1):476-486.
 崔俊华,柯黎明,刘文龙,等. 搅拌摩擦焊接全过程热力耦合有限元模型[J]. 材料工程, 2014, (12):11-17. CUI J H, KE L M, LIU W L, et al. Thermo-mechanical coupled finite element model for whole process of friction stir welding[J]. Journal of Materials and Engineering, 2014, (12):11-17.
 李敬勇,周小平,董春林,等. 6082铝合金双轴肩搅拌摩擦焊试板温度场研究[J]. 航空材料学报, 2013, 33(5):36-40. LI J Y, ZHOU X P, DONG C L, et al. Temperature fields in 6082 aluminum alloy samples bobbin-tool friction stir welded[J]. Journal of Aeronautical Materials, 2013, 33(5):36-40.
 BENAVIDES S, LI Y, MURR L E, et al. Low-temperature friction-stir welding of 2024 aluminum[J]. Scripta Materialia, 1999, 41(8):809-815.
 ZHANG H J, LIU H J, YU L. Microstructure and mechanical properties as a function of rotation speed in underwater friction stir welded aluminum alloy joints[J]. Materials and Design, 2011, 32(8-9):4402-4407.
 LIU H J, ZHANG H J, YU L. Effect of welding speed on microstructures and mechanical properties of underwater friction stir welded 2219 aluminum alloy[J]. Materials and Design, 2011, 32(3):1548-1553.
 HU Z L, YUAN S J, WANG X S, et al. Effect of post-weld heat treatment on the microstructure and plastic deformation behavior of friction stir welded 2024[J]. Materials and Design, 2011, 32(1):5055-5060.
 BOONCHOUYTAN W, RATANAWILAI T, MUANGJUNBUREE P. Effect of pre/post heat treatment on the friction stir welded SSM 356 aluminum alloys[J]. Procedia Engineering, 2012, 32(2):1139-1146.
 SHARMA, DWIVEDI D K. Effect of post weld heat treatments on microstructure and mechanical properties of friction stir welded joints of Al-Zn-Mg alloy AA7039[J]. Materials and Design, 2013, 43(2):134-143.
 ZHANG Z M, YU J M, WANG Q, et al. Effects of multiple plastic deformations on microstructure and mechanical properties of 7A04-T6[J]. Rare Metal Materials and Engineering, 2011, 40(Suppl 3):69-72.
 DIERINGA H. Properties of magnesium alloys reinforced with nanoparticles and carbon nanotubes:a review[J]. Journal of Material Science, 2011, 46(2):289-306.