Structural Transformation and Mechanical Properties of Cold Sprayed Nickel Coatings after Annealing
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1151-SS01-06
Structural Transformation and Mechanical Properties of Cold Sprayed Nickel Coatings after Annealing Yu Zou1, Ahmad Rezaeian1, Jerzy A. Szpunar1, Eric Irissou2, Stephen Yue1 1 Department of Mining and Materials Engineering, McGill University, Montreal, QC, Canada 2 National Research Council Canada (NRC) – Industrial Materials Institute (IMI), Boucherville, QC, Canada ABSTRACT Cold spray is a relatively new coating technology in which coatings can be produced by powdered particles under large plastic deformation without significant heating. In this paper, nickel coatings were fabricated by cold spray process followed by heat treatment in inert gas. Structural transformation from as-sprayed to annealed coatings was investigated by Electron Backscattering Diffraction (EBSD) in a FEG-SEM. The results show that after cold spraying sub-micron grains and subgrains with high crystal strain appear in the particle bond interface, but not shown in the center of particles. Microstructure was transformed to be uniform and stresses were released after annealing in 400°C for one hour. And ductility and formability were significantly improved due to recovery and recrystallization. Continuous recrystallization after large strain deformation could occur after cold spraying followed by annealing. INTRODUCTION Cold spray is a technique used to build up layers by spraying metal powders onto a substrate at relatively low temperatures [1-3], without thermal problems associated with well known thermal spray technologies. However, metal powders are heavily work hardened after spraying [2, 4], which leads to very brittle material with high hardness and low ductility. Moreover, the microstructure of as-sprayed powder particles is non-uniform due to different strains and strain rates between the particle interface and particle centre [2]. This inhomogeneous microstructure would leave more mechanical residual stresses in the particle bonding interface and reduce mechanical properties in its application.
Figure 1. Schematic diagram of the cold spray system Post processing heat treatment is an obvious approach to homogenize the deformed microstructure and release the residual stresses. Some publications [6-8] show substructure was
removed through recovery and recrystallization by grain boundary migration and the ductility was significantly improved by annealing. Although some researchers [9, 10] discussed about the recrystalization mechanism of the bulk metals after large strain deformation, the recrystallization mechanism of the metal powder particles under high strain deformation is still not understood well. In this paper, EBSD was first systematically used to characterize the microstructure of cold sprayed and annealed coatings and larger areas were investigated than TEM method. Structural transformation and mechanical properties after annealing will be explained. EXPERIMENT Nickel coatings were fabricated by the cold spray system of Kinetic 3000 CGT Technologies, figure 1, using commercially available gas atomized nickel powd
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