Optimization of Post Spray Heat Treatments for Cold Sprayed Ni Base Superalloy IN738LC
Kazuhiro OGAWA and Satoshi ONCHI
Abstract: In this study, the effects of heat treatment were clarified, and then the optimum heat treatment condition was investigated for the cold sprayed Ni base superalloy IN738LC coatings. In order to characterize the microstructure of heat treated cold spray coatings, SEM and TEM observation, EBSD analyses were carried out. And also, in order to evaluate mechanical properties, small punch (SP) tests and four-point bending tests were conducted. And as for an as-sprayed IN738LC coating, heat treatment (1121/2h + 843/24h) was applied. Prior to heat treatment, the as-sprayed coating had nano-order crystalline in the vicinity of the interface between coating and substrate. However, after heat treatment, the nano-order grains changed for micron-order grains. This grain growth can induce the improvement of the material properties such as ductibility and adhesion strength etc., and release of residual stress. And then, optimum heat treatment condition was investigated. At first, effects of heat treatment were considered. It was thought that the changes of configuration of f precipitation phases and crystal grain were important for characteristics of heat treated coatings. As a result of heat treatments, it is clear that the crystal grain of as-sprayed coating was mainly changed by solution heat treatment, and f phase precipitated in solution heat treatment and grew in aging treatment. Therefore, the discussion about optimal heat treatment condition was focused on the conditions of solution heat treatment. The coatings were heat-treated at higher temperature or for longer time compared with standard heat treatment of virgin IN738LC. It was clear that solution treatment at higher temperature was especially effective for miniaturization and homogenization of f phases, and growth of crystal grains. In the coating strength tests and adhesion strength tests, the specimen with higher temperature heat-treatment demonstrated better results than that of the specimen with standard heat treatment. Key Words:Cold spray, Ni base superalloy, Heat treatment, Coating, fphase, Small punch test