Boundary element stress analysis for bi-metallic dies in pressure diecasting

The rate of heat extraction during the pressure die-casting process is central to both the quality and the cost of the finished castings. Energy-extraction rates in traditional steel dies are principally dictated by the cooling arrangement but also by the resistance path offered by the die. It has recently being shown that modification to cooling channel shapes and position can enhance sub-cooled nucleate boiling and substantially increase energy-extraction rates. Reducing the thermal resistance of the coolant boundary layer means that a significant proportion of the thermal resistance path becomes attributable to the die steel. Replacing the steel with copper can greatly improve heat extraction rates. This paper is concerned with the development of an efficient structural model for the analysis of bi-metallic copper-alloy steel dies. A cyclic boundary element stress model has been developed so that stress levels can be predicted and examined in both the copper and steel parts of the die. The stress model is based on the 3D thermoelastic boundary element method and produces stresses and deformation due to both mechanical and thermal loads on the system. The collocation based boundary element method is used for the prediction of transient stress fields over a thermally stabilised casting cycle. A novel approach called the simplex method is used to evaluate domain integrals that appear in the boundary integral formulation due to the presence of a transiently varying thermal field. This method involves meshing the domain with tetrahedral elements and then applying recursive radial integration to these tetrahedral elements. Repetitive application of the recursive scheme effectively reduces the initial volume integral to line integrals. Excellent accuracy is obtained with this method. In order to validate the model predicted strain fields are compared with strain gauge measurements obtained on a purpose built rig designed to be representative of the casting process and on a production die. © 2005 Elsevier B.V. All rights reserved.