Grain refinement of pure aluminium and Al–7Si with Al–3B master alloy

Abstract

The potential of AlB₂ particles in the grain refinement of aluminium alloys with and without Si was investigated in an effort to develop potent grain refiners for Al–Si based foundry alloys. The Al–Si alloys are grain refined with the Al–3B alloy, whereas pure aluminium is not. The cast grain size in the former decreases with increasing Si content. The addition of 200 ppm B into the Al–7Si alloy, the basis of most commercial foundry alloys, produces remarkably fine grains with no fading effect. The Al–B system relies on a eutectic reaction L→α (Al)+AlB₂ at 659·7°C to offer grain refinement, which occurs predominantly via heterogeneous nucleation of α (Al) grains on AlB₂ particles thus made available. The grain refinement starts at a critical Si content enough to depress the liquidus temperature of the alloy sufficiently below the eutectic reaction temperature. The grain refining efficiency improves with increasing Si content owing to a decreasing liquidus temperature, which in turn facilitates the formation of an increasing number of solid nuclei. Alloys with insufficient level of Si to displace the liquidus temperature of the alloy below the eutectic reaction temperature cannot take advantage of this reaction and thus enjoy grain refinement. This is clearly why pure aluminium with a liquidus point above the eutectic reaction temperature is not grain refined with the Al–3B alloy at all.