Hot tearing during solidification is frequently observed while casting alloys with a long freezing range (such as the AZ91D magnesium alloy) into molds with complex geometries and varying section thicknesses. Typically, hot tearing may be reduced or eliminated by manipulating the cooling rate, alloy composition or the mold geometry. These parameters directly influence the grain structure and interdendritic regions during the critical stage of an alloy's solidification when hot tears are prone to nucleate. In this work, the effect of a novel TiBor grain refiner on the hot tearing behavior of the AZ91D magnesium alloy was quantitatively studied. The relationship between the grain refinement level, the alloy's cooling rate and the in-situ force evolution during casting solidification was investigated. The results revealed that for the AZ91D alloy, the force evolution rate and the microstructure were the critical determinants of the hot tearing severity. With the addition of the TiBor grain refiner, the grain size of the AZ91D alloy reduced by ∼70%, and the force and force-rate evolution significantly reduced as well, suggesting a homogenization of the internal stresses experienced by the solidifying alloy's microstructure, leading to a nearly complete elimination of hot tearing.