Effect of Al-3Nb-1B Master Alloy on the Grain Refinement of AZ91D Magnesium Alloy

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AS the lightest metal structural materials, magnesium alloys have attracted significant interest in industrial production due to their low density, high specific strength, and good recyclability. Among the broad range of magnesium alloys, Mg-Al-based alloys, e.g., AZ91D magnesium alloy, etc. take a dominating position of consumption, but their low strengths and poor formability still do not meet the needs of some important parts in vehicles and other applications.[1] It is well known that grain refinement of the cast metal products is one of the most effective routes to improve the mechanical properties and formability. For pure magnesium and Al-free magnesium alloys, zirconium is an extremely effective grain refiner. However, the presence of Al in Mg-Al-based alloys poisons the grain refining efficiency of Zirconium.[2] Although many grain refining methods have been developed for Mg-Al-based alloys such as carbon inoculation,[3,4] melt superheating,[2,4] melt vibration or shearing,[5,6] and addition of solute elements,[7–9] to date, there is still lack of a commercially suitable grain refiner for the Mg-Al-based alloys. Recently, Bolzoni et al.[10] reported that pure Nb and KBF4 powders were added simultaneously to Al melt to favor the formation of Nb-based compounds (i.e., NbB2 and Al3Nb). They found that the binary Al-Si cast alloys were efficiently refined because the Nb-based compounds can act as heterogeneous nucleus of a-Al. It is LEI ZHANG, Lecturer, WEI ZHOU and PENGHE HU, Masters, and QUAN ZHOU, Associate Professor, are with Key Laboratory for Microstructural Control of Metallic Materials of Jiangxi Province, Nanchang Hangkong University, Nanchang 330063, P.R. China. Contact e-mail: [email protected] Manuscript submitted on November 12, 2015. Article published online April 5, 2016. METALLURGICAL AND MATERIALS TRANSACTIONS B

well known that TiB2 can be used to grain refine Al alloys, and it can also refine the Mg alloys.[11,12] This is because TiB2 has a same hexagonal structure with a-Mg and the smallest disregistry between TiB2 and a-Mg is only 5.6 pct.[13] Hence, TiB2 can act as the heterogeneous nucleation substrate of a-Mg. As we all know, NbB2 is also hexagonal crystal structure and its lattice constants (a = 0.3102 nm, c = 0.3285 nm) are similar to those of TiB2 (a = 0.3028 nm, c = 0.3228 nm), which may act as the heterogeneous nucleation substrates of a-Mg. In this study, an Al-3Nb-1B master alloy has been prepared by in situ synthesis method from Al melt and Nb + KBF4 mixed powder. The microstructure of Al-3Nb-1B master alloy and its grain refinement effect on AZ91D magnesium alloy were studied. Moreover, the grain refinement mechanism of Al-3Nb-1B master alloy on AZ91D magnesium alloy was discussed.

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EXPERIMENTAL PROCEDURE

The Al-3Nb-1B master alloy was prepared in an electric resistance furnace using high purity Al (99.99 wt pct), Nb powder (99.9 wt pct, particle size