The AZ91D magnesium alloy is a popular casting alloy used for diverse automotive applications, despite its high susceptibility to hot tearing during casting solidification. In the metalcasting industry, hot tearing is manipulated via cooling rate, alloy composition or mold design optimization. In this work, the effect of grain refinement on hot tearing was quantitatively studied and the relationship between the alloy’s cooling rate and in situ force evolution during casting solidification was related to the severity of hot tears. The results suggest that the load evolution rate and microstructure were critical determinants of the hot tear severity for both unrefined and grain-refined alloys. The grain refiners were seen to significantly reduce the overall force and force-rate evolution, which contributed to the elimination of hot tearing in the AZ91D alloy under standard casting conditions.