Effect of post-weld heat treatments on the microstructure and mechanical properties of underwater friction stir welded joints of 7003-T4/6060-T4 aluminium alloys

To further improve the mechanical properties of underwater friction stir welded joints of 7003/6060 aluminium alloys, post-weld heat treatment (PWHT) was carried out. The results show that the combination of water-cooling and PWHT is an efficient method to greatly improve the strength of the dissimilar joints. Through solution heat treatment followed by artificial aging (STA), the largest ultimate tensile strength (UTS) of the dissimilar joint can reach 254.0 MPa, which is equal to the UTS of 6060-T6, and the elongation of the joint is 12.7%. The improvement in strength is attributed to reprecipitation of the high density of the beta" phase. For direct artificial aging (AA) after welding, the largest UTS (218.9 MPa) is obtained when aged at 175 degrees C for 8 h. Although the pre-existing beta ' phase grows coarse to some extent, GP zones and the beta" phase are reprecipitated during AA, which leads to a great increase in joint strength compared with the as-welded samples. However, the elongation severely deteriorates to 4.4% due to strain localization caused by the decreasing width of the soft region. Abnormal grain growth (AGG) takes place in the stir zone (SZ) of all joints subjected to STA, while equiaxed grains with limited growth are retained in joints subjected to AA. Compared with the distribution of precipitation phases, the grain size plays a minor role in the mechanical properties of the dissimilar joints.