In this research, large gas-atomized copper powder was selected as the feedstock. Some powder was annealed in a vacuum circumstance to avoid to the greatest extent the effect of grain boundaries on the high velocity impact behavior of particles during cold spraying. Some powder was oxidized in a resistance furnace to clarify the effect of surface oxide films. Both the annealed and the oxidized Cu powders were deposited by cold spraying with respect to the single impacts and coating deposition under the same gas condition. In addition, the rebounded copper particles were collected for morphology analysis compared to the adhered particles. The results show that the average size of the rebounded particles is apparently increased compared to the starting powder because of the rebound of the larger particles and the intensive plastic deformation of particles. For the deposited particles, obvious plastic deformation causes the higher hardness of the coatings. The last but not the least finding in this study is the rebounded particles also experienced large deformation and possible shear instability at the impact interfaces.

In this study, the impact melting phenomenon at the interfaces of particles deposited by cold spraying and its effect on coating microstructure were examined. Different powders with various thermal and mechanical properties were selected as feedstock. They are respectively Al2319, Ti, Ti-6Al-4V, Ni and MCrAlY powders. The results showed that most of the sprayed materials possibly experienced local melting at the contact interfaces of particles under certain gas conditions. Low melting point and reaction with the atmosphere are the two main factors contributing to the impact fusion during cold spraying. The results indicated that the local melting would benefit the formation of the metallurgical bonding between the deposited particles and enhance the coating adhesion.

In this paper, the microstructure and microhardness of the Al-12Si coating produced by cold spraying with a relatively large powder were investigated. It is found that a thick, dense and well bonded Al-12Si coating could be produced by cold spraying with a relatively large powder through the control of spray conditions. The critical velocity for large Al-12Si particles was about 500 m/s under the spray conditions in this study. The as-deposited coating had the same phase as the Al- 12Si powder. The localized interface melting occurred resulting from both the adiabatic shearing upon impact and the thermal effect of hot gas. The precipitation of fine Si particles occurred in α-Al matrix in the coating deposited at an elevated gas temperature because of the thermal effect of hot gas during coating deposition. The dispersed Si particles in the coating improved the coating microhardness.