Reduction of thermally induced distortion in laser cladding

Laser cladding is a modern technology for the precise generation of wear and corrosion protective coatings and the computer-assisted build-up of three-dimensional structures. Thereby the powdery coating material is transported by a carrier gas through a nozzle to the melt pool generated by the laser beam. The result is a strong joint between substrate and coating. After solidification the initially liquid coating contracts during cooling, thereby causing stress and distortion which depend on the shape and size of the body as well as on the process parameters and material properties, especially on the coefficients of thermal expansion of the materials involved. Very large distortions can occur if thin sheets are coated. Thus distortion may seriously restrict the application of laser cladding. Two possibilities to reduce thermally induced distortion will be discussed in this paper: 1. External mechanical load by suitable clamping which reduces the final total strain at the expense of enhanced stresses and/or plastic strain. 2. Application of additional heat sources which cause counteracting inelastic strain, e. g. by local heating using electromagnetic induction. A simple heuristic model as well as more accurate 2D and 3D transient FEM calculations are used for the simulation of the evolution of stresses and bending distortion in planar plates during laser cladding. Such calculations facilitate studies of the influence of pre-straining by clamping and of the use of additional heat sources on the resulting stress and strain states, which have the goal to optimize the cladding process with respect to distortion, thereby controlling the danger of cracking and delamination. Using this theoretical basis the possibilities to minimize distortion in laser cladding are studied for two examples. The results are compared with experimental observations