Effects of annealing temperature on microstructure and mechanical properties of cold sprayed AA7075

Abstract

This paper examines the softening behaviour of cold sprayed 7075 aluminum alloy after isothermal annealing. The as-received powder was deposited by a high-pressure cold spray device using heated nitrogen as propellant gas. To investigate the effects of post-deposition heat treatment, the excised samples were isochronally annealed in the temperature range of 200 to 400 °C. The feedstock powder and the free-standing coatings were initially characterized through various electron microscopy techniques (SEM, EDS, EBSD) and relevant microstructural features were determined. Mechanical properties of the as-sprayed and heat-treated samples were evaluated by quasi-static tensile and microhardness testing. It has been shown that extensive plastic deformation during cold spraying resulted in a dense coating with relatively low internal porosity. Specimens in an as-deposited state possessed a high level of strain hardening and exhibited a brittle rupture associated with intersplat cracking. When subjected to heat treatment, the cold spray deposits showed a general trend of microhardness reduction and progressive sintering of the microstructure with increasing annealing temperature. Furthermore, post-mortem observation revealed a gradual transition in fracture mechanism, manifested by improving material ductility and the occurrence of typical dimple morphology. The aspects responsible for the softening of the cold sprayed 7075 alloys are discussed further.This paper examines the softening behaviour of cold sprayed 7075 aluminum alloy after isothermal annealing. The as-received powder was deposited by a high-pressure cold spray device using heated nitrogen as propellant gas. To investigate the effects of post-deposition heat treatment, the excised samples were isochronally annealed in the temperature range of 200 to 400 °C. The feedstock powder and the free-standing coatings were initially characterized through various electron microscopy techniques (SEM, EDS, EBSD) and relevant microstructural features were determined. Mechanical properties of the as-sprayed and heat-treated samples were evaluated by quasi-static tensile and microhardness testing. It has been shown that extensive plastic deformation during cold spraying resulted in a dense coating with relatively low internal porosity. Specimens in an as-deposited state possessed a high level of strain hardening and exhibited a brittle rupture associated with intersplat cracking. When subjected to heat treatment, the cold spray deposits showed a general trend of microhardness reduction and progressive sintering of the microstructure with increasing annealing temperature. Furthermore, post-mortem observation revealed a gradual transition in fracture mechanism, manifested by improving material ductility and the occurrence of typical dimple morphology. The aspects responsible for the softening of the cold sprayed 7075 alloys are discussed further.