Evolution of the Structure of Local Regions of Fused Metal in Explosion-Welded Nickel-Aluminum Composites Under Heat Treatment

Abstract

The methods of electron, optical, and atomic force microscopy are used to study the structure, morphology and phase composition of local regions of fused metal in an explosion-welded nickel-aluminum composite. It is shown that the diffusion zone formed due to the heat treatment repeats the contour of the fuse in the first stage and then "absorbs" it upon duration of the hold thus leveling the phase composition. A Ni 2 Al 3 aluminide layer forms on the side of nickel and a NiAl 3 layer forms on the side of aluminum. INTRODUCTIONProduction of bimetallic and multilayer compounds of the aluminum-nickel system with the help of a process combining explosion welding, rolling and heat treatment requires knowledge of the laws of formation of chemical microscopic inhomogeneity in all stages of the production process [1 -3].There are no published data on the effect of local regions of fused metal formed on explosion-welded layer boundaries in nickel-aluminum compounds under welding modes superior to the optimum one on the formation of structure and properties of continuous intermetallic layers forming a layered intermetallic nickel-aluminum composite. Investigation of this effect has become the aim of the present study. METHODS OF STUDYThe initial materials were sheets of nickel of grade NP 2 and aluminum of grade AD 1 with a thickness of 2.5 and 5.0, respectively. Bimetallic composites were formed by explosion welding by an angular scheme, i.e., we varied the process gap from 0.5 to 15 mm, which changed the rate of collision from 354 to 810 m/sec and the fraction of the kinetic energy spent on plastic deformation of the surface layers of the metals (W 2 ) from 0.85 to 2.30 MJ/m 2 .The heat treatment was conducted in an air atmosphere in a SNOL 8.2/1100 furnace. The metallographic analysis was performed with the help of light (Olympus BX61 modular motorized microscope with digital DP-12 camera for imaging) and scanning (Versa 3D scanning electron microscope) microscopy. The area of the fused zones was measured while processing the digital images with the "AnalySIS" software. The microscopic x-ray spectrum analysis was performed with the help of a Versa 3D electron microscope equipped with an EDAX Trident XM 4 energy dispersive spectrometer. The microhardness of the structural components was measured using a PMT-3M device by the method of reconstructed indentation of a diamond tetrahedral pyramid with apex angle 136°(GOST 9450-76) at a load of 0.2 -1 N. The homogeneity of the structure of the fused local zones was determined with the help of a Solver Pro atomic force microscope. RESULTS AND DISCUSSIONIt is known that the degree of fusion in explosion-welded joints is determined by the level of plastic deformation of the surface layers of the metals and hence by the fraction of the kinetic energy W 2 spent on its implementation [4]. By the data of the metallographic study the fused zones have a round shape and a not etching structure if the boundary of the joint has a poorly developed wave profile (1.00 < W 2 < 1.85 MJ/m 2 ) ...