Casting die design for optimised solidification

Abstract

A simulation based die design process for Iow pressure die casting has been developed, resulting in reduced design Iead times and increased plant productivity and casting quality. The process enables modification to the tooling at the design level before the prototype is developed. Traditionally, foundries have been engaged in experimental trials to produce quality castings. The die design technique described in this work shifts the iterative process from a casting trial and error approach in the foundry to computer simulation, using solidification analyses coupled with numerical optimisation algorithms. The process consists of the following steps: -- match a computational model to existing measured temperature histories during each cast -- modify thermophysical characteristics of the tooling to control the solidification profiles and decrease the solidification time of the casting (since these are the controlling factors of the process for the casting quality and production capacity, respectively) -- mimic the improvements of the computational model in the tooling design to provide directions in determining ideal locations for chills, cooling circuits and insulation -- fine tune cooling circuit parameters for optimum cooling. The viability of the process is illustrated in two independent applications. The full methodology is implemented in an existing manufacturing process at Ford Alloy Wheel Plant in New Zealand. A two dimensional finite element model of the DN-IOl GL wheel is tuned to reflect process conditions by modifying discrete points of temperature dependent interfacial heat transfer coefficients between the wheel and die. The thermal conductivity and thermal capacity at various locations in the die are then adjusted to reduce the solidification time. A constraint is applied to propagate unidirectional solidification throughout the wheel to avoid compromising the quality of each casting. The altered thermal properties in the new model are compared against the original die material for directions in the tooling design. Regions requiring cooling circuits are subsequently analysed to determine optimum activation periods. Implementation of the modifications to the tooling design, suggested by the improved model, has yielded an 80% increase in productivity and produced a 15% reduction in design Iead time.