Pre-heat treatment of gas atomized Al 2024 powder and its effects on the properties of cold spray coatings

• Online Access: http://hdl.handle.net/2047/D20328709

Cold spray (CS) is a solid-state powder consolidation process used to produce coatings of varying thicknesses and to build free standing parts. Micron-sized particles are accelerated at supersonic speeds toward a substrate and experience mechanical and metallurgical bonding upon impact. CS has a wide range of applications, both reparative and structural. Commercially available CS feedstock includes gas-atomized powders which possess rapidly solidified dendritic microstructures. Al 2024, Al 6061, and Al 7075 are three such alloys, widely accepted for fabrication and repair of aeronautical components for their desirable strength-to-weight ratios. However, CS deposits often lack in ductility and fracture toughness which is unfavorable for load bearing components. At present, there is a need to develop new procedures and standards for high-temperature powder processing, which will increase the reproducibility of CS coatings and allow for manipulation of properties in the as-deposited state. Existing literature on pre-heat-treating Al 2024 powders to improve ductility of as-sprayed parts is limited. In this work, a study was conducted to investigate the effects of pre-heat-treating Al 2024 powder on the properties of cold spray deposits. Three unique powder samples were studied: (i) as-received, no heat treatment, (ii) solid solution heat-treated (495 °C, 1 hr), and (iii) annealed (415 °C, 2.5 hrs). Particles were analyzed for changes in size, microstructure, and micro-hardness. Each powder sample was then cold sprayed on to an Al 2024-T351 substrate using Helium as the processing gas. The deposits were tested to determine micro-hardness and tensile properties in the as-sprayed condition. Scanning electron microscopy revealed a dendritic/cellular microstructure within the as-received powder, which was eliminated with both solution treatment and annealing. Particle hardness decreased by 33.4 % after annealing, but showed no significant change after solution treatment, implying that full solution may not have been achieved. Particles agglomerated during both high temperature heat-treatments due to sintering effects, resulting in a measurable increase in size distributions. Solution heat-treated powder produced a decrease in as-sprayed ductility and no significant change in ultimate tensile strength (UTS), while annealed powder produced a decrease in as-spayed UTS and no significant change in ductility. Elastic modulus remained constant across all CS deposits.