Enhancing ductile and adhesive performance of cold-sprayed Ti6Al4V coatings by multi-step vacuum heat treatment

Abstract

Developing cold spray additive repair for aerospace applications remains a significant challenge due to the lack of ductility and adhesion of hard metal deposits, such as titanium alloys. This study compares the effect of multi-step heat treatment in vacuum and hydrogen atmospheres on cold-sprayed Ti6Al4V coatings on CP-Ti using nitrogen and helium carrier gas. All the as-sprayed coatings were delaminated at strains of 1–3% during three-point bending tests, with detached fragments struck on the surroundings. The nitrogen-sprayed Ti6Al4V sample after multi-step vacuum heat treatment exhibited ductile deformation without fracture or delamination, with an ultimate flexural strength of 887 MPa at a strain of 11.1%. The superior ductility was contributed by the fully equiaxed microstructure in the coating, with α grains < 10 μm and submicron-thin β laths, and adhesion was improved by 40 μm-thick interfacial diffusion bonding. However, the effectiveness of multi-step heat treatment was limited by the as-sprayed quality. The porous helium-sprayed coating, due to nozzle clogging, underwent the same heat treatment but broke into powder and debris upon bending load. Hydrogen charging could enhance interfacial diffusion bonding, leading to the formation of pentagonal α2 grains. This study was also the first to capture the morphology of twinned-pentagonal α2 grains in Ti6Al4V particles, as observed through the SEM fractography of helium-sprayed Ti6Al4V coatings that underwent a multi-step heat treatment in hydrogen. The thermo-hydrogen refinement of microstructure was applicable to Ti6Al4V but not to CP-Ti, as it triggered excess grain growth and severe embrittlement in CP-Ti substrates. This study outlined a strategy to enhance ductility and adhesion of cold-sprayed Ti6Al4V coatings and provided valuable insights into structural cold spray repair for sustainable aviation applications.