Optimization of the laser path and power in the surface heat treatment of diesel engine exhaust values

Abstract

In this article, a model for the process of CO₂ laser treatment of diesel engine exhaust valves is presented. The treatment consists on the remelting of a nickel based alloy which has been previously plasma sprayed on the valve seat. Such process can be well described by means of a transient heat transfer equation, even if some numerical method must be applied when both temperature dependent thermophysical properties and realistic geometry description of the workpiece are taken into account. For this work the Finite Element Method, within the framework of ANSYS®, was chosen, to solve that equation and this was done in the framework of ANSYS®. The use of the process parameters previously assessed during a the theoretical and experimental study of box shaped workpieces requires a fine tuning based on simulation in order to be sure that the treatment of the valve seat gets comparable results to that of the flat workpieces. Among the important conditions to be satisfied stands the need for the temperature map to be parallel to the valve seat surface and so have the same treated depth. Besides, as the beam describes a circular path, coming back to the starting position, the initial temperature at that point will be higher than for rest of the workpiece. Therefore the beam power has to be optimized in order to get adapted to this new situation. Phase transition is included in the model by using the right thermophysical properties and a correct estimation of the effective absorption coefficients in both phases. This work shows the advantages of the process modelling to support the optimization of laser surface treatment of complex geometry workpieces using the model validation performed in simpler workpieces.