Grain refinement of AA333 aluminium cast alloy by Al–Ti granulated flux

Abstract

Addition of grain refiner is an option to obtain higher mechanical properties of aluminium cast alloy. Grain refiner will react with molten aluminium and will form nucleant particles that initiate solidification. Therefore, the grain refiner will also be useful to control the solidification processes to reduce shrinkage formation. This study evaluated grain refinement in AA333 aluminium alloys by using Al–Ti granulated flux. Experiments were conducted by adding 0.056 and 0.078 wt.% Ti granulated flux to the molten aluminium during rotary gas bubble floatation and it was subsequently transferred into the Low Pressure Die Casting (LPDC) holding furnace. The pressure in the LPDC holding furnace was initially 256 kPa and no stirring was applied. The melt was injected to cylinder head moulds with cycle time of 180 s. Samples were cut at the thin and thick sections of the cylinder head to analyze the effects of cooling rate on grain refinement. Fading of grain refiner was studied for the period of 4 h. Hardness testing and microstructural observation were conducted. The results showed that addition of Al–Ti granulated flux for 0.056 and 0.078 wt.% Ti, increased the hardness and lowered the dendrite arm spacing (DAS). The refinement is more significant in thin samples, because of the assistance by higher cooling rate. The mechanism of grain refinement of Al–Ti granulated flux in AA333 alloy is a combination of high growth restriction factor by the large amount of solute and nucleation by Al 3Ti particles. Fading of grain refiner was detected by the increase in DAS, the lowering of Ti concentration in the melt and the reduction in hardness and strength. The pressure utilized in LPDC was not adequate to give stirring effect to prevent fading.