Cold Spray of Metallic Composites: From Dissimilar Bonding and Microstructure Evolution Towards Production of Gradient NiTi Intermetallics

Owing to low-temperature deposition conditions and high deposition rate, cold spray offers unique advantages in the manufacturing of metal composites. However, coating composition deviation from feedstock powders can affect the phase evolution of intermetallic forming coatings during post heat treatment. This work aims at optimizing the coating composition/structure of cold-sprayed blended Ni and Ti powders for the production of NiTi intermetallic coatings/parts after heat treatment.

Numerical modelling and experimental analysis of single particle impact and coatings of Ni and Ti pairs are performed. Microstructure studies reveal that either twinning or combination of twinning-microband form at Ni splat of Ni-Ti coatings. Different extent of recrystallization is found at Ni/Ni and Ni/Ti interfaces of Ni splat boundaries while Ti splats have very fine recrystallized grains at all impact interfaces.

Microstructural investigation of composite coatings after annealing show that the composition of coatings and spraying parameters have a critical role in the final intermetallic phase composition. Intermetallic growth mechanisms during heat treatments show the importance of the impact sequence. Thermal etching analysis of composite NiTi coatings show that NiTi phase forms with a gradient microstructure, attributed to the grain refinement of cold sprayed samples at the splat boundaries and intermetallic nucleation mechanism.

Keywords: Composite Ni-Ti coating, physically blended powder, sequence of dissimilar material impact, asymmetrical bonding, FEM modelling, grain refinement, twinning and microbands, heat treatment, and gradient microstructure forming.