Calphad coupled microstructure modelling of the az91e mg alloy gravity die casting

Résumé

In this work, a Calphad coupled quantitative equiaxed grain growth modelling of the AZ91E Mg alloy gravity die casting is presented. For that purpose, both macro- and microscales are considered. The microstructure grain growth model is fully coupled to a commercial computational thermodynamics software, and the resulting code is implemented in a commercial casting simulation software that analyses the macroscale heat transfer. In this way, the microstructure calculations are introduced in industrial casting environments. The microstructure modelling takes into account the nucleation event and the equiaxed globular and dendritic grain growth analysis, including the impingement of grains. The solute diffusion in liquid is considering in the solidification analysis. Additional insight regarding castings is gained with the microstructure modelling, obtaining as result the final average grain growth, secondary dendrite arm spacing, and phase distribution and their composition. More detailed temperature-time curves are also obtained, depicting the solidification sequence with precision. Moreover, the effect of the cooling rate on those results is also discussed. The computational thermodynamics provide the calculation with multiphase and multicomponent systems. Additionally, the experimental calorimetry techniques to gather the latent heat amount and release pattern are avoided, since the heat release is obtained as a result of the calculations. The usual assumption of considering proportional the released latent heat and the solid fraction evolution is disclaimed by the thermodynamic calculations. The AZ91E Mg alloy is the main working material in this thesis, and therefore, the thermophysical properties of the alloy are characterised. The thermophysical properties are crucial parameters to simulate casting processes. In particular, the thermal expansion, density, specific heat, thermal diffusivity and thermal conductivity of the alloy are measured and calculated, including the solid, liquid and solid-liquid phase temperature ranges. Furthermore, the influence of the aluminium content in the latter thermophysical properties is analysed by measuring the AZ31 and AZ61 Mg alloys. Experimental castings are also performed to obtain a four step rectangular geometry. The thermal history of the middle point of each step has been recorded. The micrographic inspection of the measurement positions is carried out and the average grain size and grain density is measured. The measured grain density values are used in the microstructure modelling to describe the nucleation. Finally, the simulation results are validated by comparing to actual casting experiments. Specifically, the measured cooling curves and average grain sizes are compared to calculated values, obtaining good agreement of results.