Effect of heat treatment and extrusion processing on the microstructure and mechanical properties of spray formed 7055 alloy

The effect of extrusion and heat treatment processes on the microstructure and mechanical properties of the spray formed 7055 alloy were systematically studied by three-dimensional X-ray microscope, X-ray diffraction, scanning electron microscopy, transmission electron microscopy, atom probe tomography and tensile testing. It was found that the main second-phases in the as-deposited 7055 alloy include theta (Al2Cu) and Mg(Zn,Al,Cu)(2) phases with similar structure to the eta (MgZn2) phase dissolved with Al and Cu. The Al2Cu particles embedded in Mg(Zn,Al,Cu)(2) phase are widely distributed on the grain boundaries. Compared with the as-cast 7055 alloy, the S (Al2CuMg) phase and T (Al2Mg3Zn3) phases were not detected in the as-deposited alloy due to the change of solidification paths caused by rapid solidification. The microporosity caused by spray forming is always observed in the as-deposited alloy, these micro-pores can be effectively eliminated by the extrusion processing. Meanwhile, compared with extrusion directly after preheating at 420 degrees C/24 h, additional high temperature two-stage homogenization treatment before extrusion can reduce the adverse effect of S (Al2CuMg) phase and promote the precipitation of high density of Al3Zr dispersoids, which significantly increases the recrystallization resistance and improve the mechanical properties of the alloy.

Automated summary

The study systematically investigated the influence of extrusion and heat treatment processes on the microstructure and mechanical properties of spray formed 7055 alloy. It was found that the second phases present in the as-deposited alloy could be effectively eliminated by extrusion processing, leading to improvements in recrystallization resistance and mechanical properties. Additional high temperature two-stage homogenization treatment before extrusion was also shown to promote the precipitation of Al3Zr dispersoids, further enhancing the alloy's mechanical properties.