Residual stresses refer to stresses remaining in a structure in the absence of external mechanical or thermal loads. Such stresses are induced where a stress gradient exists in a body and the stresses are larger than the elastic limit. Upon removal of external forces and to maintain the permanent deformation caused by the stress raiser, a self-balanced system of internal forces (residual stresses) remains in the structure. Therefore, residual stresses are usually local (next to the location of the stress concentration) and bounded by the large elastic region. Residual stresses are caused by most manufacturing processes such as welding, machining, and forming; or are intentionally produced by processes like peening and autofrettage.
Cold spray coating uses high particle velocity and impact energy to enable material coating on a substrate at a relatively low temperature. A by-product of cold spray coating is the formation of residual stress due to the peening effect of the particles' collision with the substrate. The high particle impact velocity causes high local stresses which lead to plastic deformation in the substrate in the proximity of the particle-substrate interface. The figure below depicts such plastic zone formation schematically. Due to the localized effect of the collision, the area undergoing plastic deformation is surrounded by a large elastic domain, resulting in the formation of local residual stresses. The dent created from particle impact causes tension at the surface, generating compressive residual stress at and near the surface upon unloading (i.e., adhesion of particle to surface). Such stresses are beneficial for the fatigue life of the coated part. Due to the self-equilibrated nature of residual stress (i.e., zero net internal forces), associated with the compressive residual stress at and near the surface, there is tensile residual stress through the depth to balance off the residual force. A schematic pattern of distribution of residual stress due to collision of a single particle with the substrate is shown. Quantification of residual stresses through experimental measurement and finite element modelling is of interest in this research.
