Effect of heat treatment on microstructure evolution and mechanical properties of 2219 aluminum alloy joint as fabricated by double-pulsed TIG welding

In the present study, the effects of typical post-welding heat treatments, including direct aging treatment (AT) and solution treatment + aging treatment (STAT), on the strengthening effect of 2219-T6 joint fabricated by double-pulsed variable polarity tungsten inert gas welding were investigated comparatively. The evolution of α-Al grain, eutectic structure and precipitations during different post-welding heat treatments were investigated. Moreover, the influence mechanism of microstructure heterogeneity on the plastic deformation behavior of the joint was discussed. Obtained results show that AT affected slightly α-Al grain and eutectic structure, and introduced low-density coarse θ'-Al₂Cu precipitation into the welding seam. AT has a limited strengthening effect on 2219-T6 aluminum alloy joint and a damage to plasticity. While, STAT lead to coarsening of α-Al grain and decrease in the content and size of eutectic structure. Meanwhile, a high-density fine θ''-Al₃Cu particles with a mean diameter of 22 nm was introduced by STAT into the welding seam. STAT can significantly improve the strength and plasticity of the joint, make the strength coefficient of the joint reach up to 0.84 and the elongation increase to 7.0%. The strengthening and plasticizing effect of the joint caused by STAT result from a more homogeneous strength matching among each region of the joint, due to the significant precipitation strengthening of nano θ'' particles in welding seam. That promoted a more uniform overall plastic deformation and lead to the increase in ultimate tensile strength.