Grain refinement of LM6 Al-Si alloy sand castings to enhance mechanical properties

Al-Ti-B master alloys are well accepted in Al-Si alloys casting industry as grain-refiners to enhance the castings quality. Grain refinement is achieved in aluminium alloys by inoculating the melt in the furnace with specific amount of selected grain-refiner. The grain-refiners come in different chemical compositions and there is no guideline to add the optimal level of them into the melt for sound castings. This research work investigates the effect of Al5Ti1B on the mechanical properties of LM6 Al-Si alloy sand casting. The typical LM6 aluminium alloy contains 10-13 wt.% of silicon and thus inherently solidifies with coarse grain sizes. The mechanical properties ascertained are hardness and ultimate tensile strength (UTS). The grain macrostructures of the castings are studied by optical and scanning electron microscopes. The experimental work is performed on a sand casting of different modulus, which inherently induces different cooling rates to enable a simple correlation between cooling rate and grain refinement level. The addition level of Al5Ti1B into the melt ranges from 0 wt.% to 1 wt.% with the increment of 0.25 wt.%. The experimental results show that the mechanical properties of LM6 sand casting can be optimally improved by grain refinement of 0.5 wt.% AL5Ti1B. Further increase of grain-refiner quantity does not provide any more significant improvement. The data shows that solidification rate is directly proportional to the addition level of grain-refiners but inversely proportional to the casting modulus. In the original sand casting of LM6, section of lower modulus with higher solidification rate has better mechanical properties. When the optimal level of grain-refiner is added, more uniform mechanical properties are achieved throughout the casting irrespective of section modulus.