SIMULATION OF SOLIDIFICATION OF CERAMIC SHELL INVESTMENT CASTINGS BY USING FEM

Abstract

Ceramic shell investment casting is one of the important casting processes. Numerical simulation of solidification is useful in getting high quality castings and minimising product cost and scrap. In this work, three-dimensional numerical simulations of wax and A-356 (an alloy of aluminium with 6% Si and 0.7 % Mg) solidification in aluminium die and ceramic shell mould respectively, using the finite element technique and the ANSYS software program, were made. Dimensional changes between a pattern die and its wax pattern occur as a result of complex phenomena such as thermal expansion—contraction and hot deformation (elastic, plastic, and creep). The wax pattern dimensions are determined by thermophysical and thermomechanical properties of wax. The viscoelastic model is assumed to simulate the wax solidification and its shrinkage. The thermophysical properties of A-356 were considered temperature dependent, while for material of ceramic shell mould these properties were considered constant. Owing to the temperature dependent thermophysical properties of A-356, this type of problem is of nonlinear characteristic. The latent heat is generated during phase change between liquidus and solidus temperatures and was incorporated in analysis by using enthalpy method. The air gap formed between the casting and the mold at interface affects the heat transfer rate. The gap formation during solidification at interface is very critical to be simulated. In the present work, gap formation is not considered i.e. the interfacial heat transfer coefficient is taken constant. From the first analysis the net contraction is calculated in each dimension of stepped wax pattern. In second simulation the temperature distribution was plotted to understand the cooling pattern and to find out the location of probable hot spot.