Thermal Response of Aluminum Engine Block During Thermal Spraying of Bores: Comparison of FEA and Thermocouple Results

Thermally sprayed coatings have used in place of iron bore liners in recent aluminum engine blocks. The coatings are steel-based, and are sprayed on the bore wall in the liquid phase. The thermal response of the block structure determines how rapidly coatings can be applied and thus the investment and floor space required for the operation. It is critical not to overheat the block to prevent dimensional errors, metallurgical damage, and thermal stress cracks. This paper describes an innovative finite element procedure for estimating both the substrate temperature and residual stresses in the coating for the thermal spray process. Thin layers of metal at a specified temperature, corresponding to the layers deposited in successive thermal spray torch passes, are applied to the substrate model, generating a heat flux into the block. The thickness, temperature, and application speed of the layers can be varied to simulate different coating cycles. The temperature field in the block is calculated using a transient thermal analysis with convective cooling on exposed surfaces. The stresses in the coating are computed using a residual stress relaxation method. Computed temperature fields are compared to thermocouple measurements from two V8 blocks sprayed using the Plasma Transferred Wire Arc (PTWA) process the Ford Essex Engine Plant. Measured and computed peak temperatures agree well. The application of the procedure to eliminate a prototype thermal stress crack and to the investigation of torch paths is described.