Effect of gas temperature on the interfacial bonding of cold-spray additive-manufactured Ti6Al4V

Abstract

The mechanism of the bonding between the particles deposited in cold spraying (CS) is still under debate. Particularly for Ti and its alloys, bonding often occurs without evident jetting at the edge, which is difficult to explain using the traditional bonding theory. To date, a detailed microstructural understanding of the particles has been lacking. Therefore, in this work, the deposition behavior of cold-spray additive-manufactured Ti6Al4V (Ti64) was investigated as a function of gas temperature, and the microstructure of the surface particle interface was analyzed in depth. The results showed that with increasing gas temperatures, particle deformation and bonding between the surface particles and the previously deposited particles improved; accordingly, the porosities of the deposits decreased. Microstructural analysis revealed that abundant fine grains and amorphous phases formed at the interface between the surface particle and the subsurface particles, and that oxygen is likely to concentrate in this area. Therefore, a new bonding mechanism involving nano-recrystallized grains and amorphous structures is proposed. Moreover, the numerical simulation results showed that the maximum interfacial temperature is close to the melting point of Ti64, which plays a significant role in the nanocrystalline- and amorphous-phase formation. In addition, several nanosized cracks were observed at the interface at 700 °C, and the sample interface split at 800 °C. These results, combined with the evolution of the elastic recovery energy and velocity of the surface particle with impact time, lead us to speculate that the continuous fluctuating elastic recovery process may cause microcracks and interfacial tears.