It is found that the inoculation of LM6 Al–Si alloy with grain-refiner Al5Ti1B will improve the mechanical properties of the castings. The hardness can be improved about 10% and the ultimate tensile strength also achieves a significant improvement of 39%. The optimal addition level of grain-refiner is 0.5% to achieve the highest values of hardness and ultimate tensile strength in the castings. Grain refinement also increases the solidification rates of LM6 sand castings for all sections of different modulus. Faster solidification rate coupled with more heterogeneous nucleation sites will promote the formation of finer equiaxed grain sizes as indicated in Fig. 10. The experimental data concludes that LM6 sand castings can be effectively grain-refined with Al5Ti1B by introducing the master alloy in bar form into the melt just before pouring into the mold. The most optimally potent addition level is 0.5 wt.% to achieve the best mechanical properties of hardness and ultimate tensile strength for LM6 sand castings.