Simulation of thermal stress in induction-assisted laser cladding

Aim of this work is the prevention of high stresses and cracks which may occur in wear and corrosion protective coatings and three-dimensional structures generated by laser cladding. This may help to overcome the still valid limitations of this technol. with respect to very hard coating materials and high feed rates. Thermal stresses which form during laser cladding can be reduced by using tailored temp. fields consisting of a flat base and a steep peak obtained by induction and laser heating, resp. Optimization of this so called induction assisted laser cladding technol. requires a thorough understanding of the mutual interaction of phys. phenomena such as heat flow, phase transformations, thermal expansion, and plastic flow, which are responsible for the evolution of stress and distortion. This is best achieved by modeling. On the basis of a semi-anal. description of the laser cladding process a three-dimensional FEM model was developed which enables calcns. of the thermally induced stress and strain for single weld beads as well as for coatings consisting of several beads. Since process optimization by parameter studies using that model requires a lot of calcn. time, addnl. simplified models, which are able to reduce significantly the calcn. time, were derived and checked by comparison with 3D FEM calcns. Using the mentioned tools the influence of various parameters such as feed rate, bead shape, substrate thickness etc. as well as of the size and the position of the inductors has been studied. Particular attention has been paid to the thermal and mech. interaction between overlapping weld beads forming coatings or three-dimensional structures. The induction assisted laser cladding technol. is not only able to reduce stresses but also to decrease the structural damage caused by plastic strain. The latter seems to be the more important effect in the prevention of cracks for material couples which have strongly different coeffs. of thermal expansion and for coating materials with low fracture strain.