Abstract
In a recent work we reported that the combined addition of Nb and B has higher grain refinement effectiveness than the addition of these individual chemical elements. In this work a commercial Al–12Si–0.6Fe–0.5Mn alloy is employed to fully characterise the refining potency of Nb–B inoculation. It is found that the addition of Nb powder and KBF4 flux to Al–Si melt introduces potent nuclei with low lattice mismatch with the primary α-Al dendrites (i.e. low undercooling) and, thus, promote noteworthy grain refinement through heterogeneous nucleation. Nb–B inoculation induces a significant increment of the volumetric number of grains, decreases the undercooling needed for solidification and makes the grain size of the Nb–B inoculated material less sensitive to the cooling rate employed to solidify the material.
Highlights
The grain refinement of Al and its alloys is a common practise because materials characterised by fine equiaxed a-Al structure show improved mechanical properties and better formability [1]
TiB2 particles coated with a layer of Al3Ti constitute the heterogeneous nucleation substrates from which a-Al grains can grow whilst Al3Ti intermetallics dissolve in the melt to form a-Al (Al(l) + Al3Ti(s) ? a-Al(s) peritectic reaction) during solidification [5,6,7]
It is worth mentioning that the lattice mismatch between Al and Al3Nb is quite small (f = 4.2%) as in the case of Al3Ti intermetallics [20].The refining potency and effectiveness of Nb–B inoculation was tested on Al–12Si–0.6Fe–0.5Mn
Summary
The grain refinement of Al and its alloys is a common practise because materials characterised by fine equiaxed a-Al structure show improved mechanical properties (strength and toughness) and better formability [1]. The presence of free Ti in the melt is paramount for the refinement of Al because Ti has the highest growth restriction factor on Al [8] This aspect, presence of TiB2, Al3Ti and solute Ti, are responsible for the high efficiency of commercial Al–Ti–B master alloys compared to Al–Ti master alloys. Due to its electronic structure, Ti is characterised by very high reactivity (with both interstitial and substitutional alloying elements) and, reacts with Si to form titanium silicides which depletes the Ti content in the melt, reducing the efficiency of grain refinement.
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