Two important factors affecting hot tearing - semi-solid constitutive behaviour and grain percolation - have been simulated through the use of microstructure models based on granular structures. The semi-solid model geometry is based on a modified Voronoi tessellation, and includes rounded corners to approximate an equiaxed-globular grain structure with liquid surrounding the grains. The percolation model combines solidification and thermodynamic aspects to predict the gradual transition within the mushy zone from a continuous liquid to a coherent solid network, while the constitutive behaviour model uses experimentally-derived data to describe the behaviour of the solid grains. By performing a series of model runs over ranges of grain size and fraction solid, the simulations have revealed an important link between grain size, semi-solid yield stress, strain localisation, and grain coalescence. Furthermore, the models provide insight on the relative importance of each mechanism on hot tear formation, and show promise for improving quantitative hot tearing predictions.