Grain refinement mechanism of pure aluminum by inoculation with Al–B master alloys

There has long been a discrepancy in the literature regarding the grain refining potency of Al–B master alloy on pure aluminum. This has made the excellent performance of Al–B master alloy in refining Al–Si foundry alloys rather incredible. Although Al–B master alloy was recently ascertained to be grain refining efficient on commercial purity aluminum (CPAl, 99.7% Al) in our recent work, the reasons why previous studies have shown controversial results are still not fully understood. In this paper we investigated the immanent mechanism underlying this paradox. The key influencing parameters studied were: (i) the detrimental effect of stirring action to the preparation of a reliable master alloy; (ii) the size distribution of the AlB2 particles in the master alloy; (iii) the trace elements like Fe and Si in aluminum; (iv) the cooling condition during solidification. A criterion based on solidification fundamentals was developed to link these parameters to the formation of new equiaxed grains in solidification. The experimental findings strongly supported this criterion to explain the grain refining mechanism of Al–B master alloy on pure aluminum. These results indicate that the grain refinement by inoculation with Al–B master alloy is actually a complicated and multi-factor controlled event rather than a simple judgement of “yes or no”. The present work also provides an insight for designing powerful refiners for a given kind of aluminum or its alloy.

► Al–B master alloy indeed has refining potency on pure aluminum. ► We clarify the grain refinement mechanism of Al–B master alloy on pure aluminum. ► A criterion was developed to show why previous studies have shown discrepancies. ► Stirring was proved to be detrimental to Al–B master alloys.