Determination of residual stresses in processes with multiple thermal loads

Abstract

Abstract Numerical models are used to identify correlations between internal material loads and material modifications. Such correlations are called Process Signatures, which are based on the idea that different processes inducing equal internal material loads produce the same material modifications. Here, machining processes involving repeated thermal loads at high frequency as they occur in laser ablation or electric discharge machining are considered. As an example for the material modifications, the resulting residual stress distribution is determined as a function of the temperature. First, the temperature field in the work piece is computed for about 1000 consecutive heat pulses generated by the laser or electric discharges. An in-house finite-volume method is used to solve the time dependent enthalpy equation on a hierarchical Cartesian grid, which allows a high degree of parallelization such that high-performance computing hardware can be used. The resulting temperature fields and the heating rates for several load numbers are used to determine the residual stress state in the material. This is done in a second step by finding an appropriate model for the thermal loads, which provides a boundary condition for a finite element simulation of the surface near modification process. In this step, phase transformation during heating (austenite formation) and cooling (martensitic formation) are taken into account. Finally, the displacements and residual stresses are calculated by the phase and temperature distributions determined in the previous step. The results are compared with measurements and enable the determination of a Process Signature component describing the relation between repeated heat loads and the resulting residual stresses in the surface layer of the work piece.