Modelling ceramic mould deformation in the investment casting process

Swain, Luke James (2017). Modelling ceramic mould deformation in the investment casting process. University of Birmingham. Eng.D.

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Abstract

Core and shell deformation during the firing stage of the investment casting process was investigated. Relevant thermo-mechanical and thermal properties were characterised for a commercial core and shell system. A finite-element model was created using the experimentally obtained data. Four major mechanisms were shown to contribute to ceramic mould deformation; general thermal expansion, sintering, phase transformations and creep. The shell system experienced little deformation form phase transformation, the core system exhibited minor deformation from sintering. The sintering behaviour of shell material was observed to be highly dependent on the temperature-time profile of the firing cycle, with lower heating rates resulting in more sintering. The crystallisation of amorphous silica to β-cristobalite in core materials was also seen to follow this trend. A rule of mixtures approach showed that inter-layer interaction in shell materials was minimal. An Avrami-based kinetic modelling approach was applied to pressed silica pellets, showing very good prediction of similar heating rates, but an interaction effect between crystallisation and sintering behaviour needs to be established to predict a wider range of heating cycles. A master sintering curve approach was adapted to incorporate crystallisation to predict the displacement of pellets mimicking core materials. Due to silica crystallisation inhibiting sintering, lower heating rates produced less displacement. In order to validate the finite-element model, an experimental test was developed capturing the deformation of a shell test bar in a three-point bend rig at high temperature using photography. Images were then analysed in ImageJ® to measure the bending of the shell material, this was validated using a transducer attached to a three-point bend knife. Comparison of the finite-element model with the validation test showing good agreement. The creep exponents which were the main proponent of shell bending were optimized to be used in future models.

Type of Work:

Thesis (Doctorates > Eng.D.)

Award Type:

Doctorates > Eng.D.

Supervisor(s):