A novel process for integrated forming and reaction synthesis of NiAl alloy curved shells

Abstract

Abstract NiAl alloy is an important lightweight and heatproof structural material used for the service temperature up to 1000 °C. However, there are huge challenges in fabricating NiAl alloy sheets and forming thin-walled components. In order to solve this problem, a novel process of integrated forming and reaction synthesis using laminated Ni/Al foils as the initial blank was developed for fabricating NiAl alloy curved shells. The laminated Ni/Al foils were first formed into a curved shell at room temperature, then the shell was in-situ heat-treated under a proper pressure until the pure Ni and Al were transformed into intermetallic NiAl by reaction synthesis. The microstructure, defects such as voids and mechanical properties of the shell were investigated in order to evaluate the feasibility of the integrated process. The influence of loading pressure for fabricating the shell on the void size was studied quantitatively. A theoretical model was proposed to predict the thickness distribution of the shell. It was shown that the shell can be successfully fabricated by the forming and reaction synthesis integrated process. Thinning occurred in the whole region of the shell with the minimum and maximum thickness strain locating at ±80° and 0°, respectively. The voids in the shell mainly exist in the original Al foil regions and original Ni foil regions. The voids in the central region of the NiAl3 diffusion layers were inherited to the central region of fine-grained layers. The diffusion of Ni layers at high temperatures led to the appearance of linearly distributed voids in the central region of coarse-grained layers. Loading pressure facilitated microplastic deformation, leading to a maximum contact area between the mating surfaces, which further promoted the interdiffusion. Increasing the pressure enhanced the voids shrinkage while decreasing the size of voids. The difference in the displacement increment of each region prevented the edge region of the shell from achieving full densification, resulting in the uneven distribution of density and mechanical properties.