Hot Extrusion Enhanced Homogenization of Microstructure in a Spray Deposition Aluminum Alloy

Abstract

Homogeneous heat treatment serves the significant roles in eliminating the segregation and tuning the microstructure of alloy ingot. It usually cost tremendous power and time to achieve a homogeneous microstructure for aluminum alloys. In this article, the hot extrusion was directly introduced on the spray deposited aluminum alloy 7055 ingot before performing heat treatment to explore the newly feasible homogeneous routine. Equiaxed grains without dendrites or PPB were obtained in our current parameters of spray deposition, which allowed the as-deposited alloy to be deformed without being subjected to pre-homogeneous heat treatment. Significant amount of stored energy was produced during hot extrusion at 420 • C with area reduction ratio of 6.25, which effectively promoted the homogeneity of microstructure and reduced significantly the heat treatment time. A newly feasible short routine, heat treatment at 450 • C /6 h + 470 • C/1 h following the hot extrusion, proved capable of obtaining a homogeneous microstructure for the spray deposited aluminum alloy 7055. macro segregation could be improved by long-time homogenization heat treatment after casting, but it took tremendous power and time to achieve a uniform distribution of elements. It was well accepted that the diffusion of atoms would be accelerated under the combined effects of heat and stress [5]. However, the cast billet cannot be directly deformed due to the existence of macro segregation and casting defects, i.e., voids, concentration of elements, which would lead to the occurrence of cracks if not controlled properly. Thus, the as-cast aluminum alloy 7055 was usually subjected to homogeneous heat treatment at 46-47 • C for 24 h before deformation [6].Fortunately, spray deposition, a rapid solidification process, was proposed in the 1960s and was firstly applied in fabrication of super-high strength aluminum alloy in the 1990s [7,8]. With the assistance of spray deposition method, the aluminum alloy with high alloying could be produced with segregation being controlled at the micro-scale level. In this situation, the as-deposited alloy could be deformed under controllable parameters without being subjected to pre-homogenization heat treatment [9]. Recently, significant efforts have been made to study the hot deformation behaviors [10][11][12][13][14][15], quenching sensitivity [16][17][18][19], corrosion resistance [20-23] and weldability [24-29] of 7055. However, few reports were concerned with the improvement of homogenization in microstructure with less energy or short processes. In this article, hot extrusion was introduced to directly extrude the billet after spray deposition, wherein the microstructure evolution and corresponding mechanism during hot deformation and extrusion were uncovered. The subsequent heat treatment was employed to evaluate the effects of parameters on microstructure evolution of the hot-extruded alloy. Significant amount of stored energy was produced during hot extrusion at 420 • C with the area reduction ratio of 6.25, whic...