Semi-solid alloys are deformed in a wide range of pressurised casting processes; an improved understanding and modelling capability are required to minimise defect formation and optimise productivity. This thesis combines thin-sample in-situ X-ray radiography of semisolid Al-Cu alloy deformation with 2D coupled lattice Boltzmann method – discrete element method (LBM-DEM) simulations. Mechanisms of strain heterogeneity and localisation are identified during semi-solid deformation in globular Al-Cu alloys with various combinations of initial solid fraction and strain rates. The calibrated set of LBM-DEM simulations is then used to obtain information that is not available in X-ray imaging to extract deeper insights into the semi-solid deformation behaviours observed in the experiments.
It is found that the local contraction and dilation of the percolating grain assembly are highly influenced by the initial solid fraction. When deforming a low solid fraction alloy, macroscopic contraction due to grains being pushed together increases the local liquid pressure and expels liquid from the sample surface. In contrast, deforming high solid fraction alloys leads to macroscopic shear-induced dilation by grains pushing each other apart and surface menisci are sucked-in due to the decrease in interstitial liquid pressure.
It is also shown that the macroscopic behaviour of semi-solid alloy deformation undergoes a range of rheological transitions with increasing solid fraction. First from a suspension to a percolating solid network and, later, from net dilation to shear cracking. These transitions are investigated with LBM-DEM simulations, and the transition to shear cracking is shown to be related to the local decrease in interstitial liquid pressure caused by shear-induced dilation.
The verified coupled LBM-DEM simulation is shown to exhibit a load:deformation response consistent with the critical state framework of soil mechanics, indicating that this approach can be useful for modelling thermomechanics in casting processes.Open Acces
