Efficacy of Borides in Grain Refining Al-Si Alloys

Leandro Bolzoni,Nadendla Hari Babu

doi:10.1007/s11661-018-5017-1

Leandro Bolzoni, Nadendla Hari Babu

Open Access

https://doi.org/10.1007/s11661-018-5017-1

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Abstract

The grain refining efficacy of titanium, aluminum, and niobium borides, as well as niobium aluminides, introduced via commercial and lab made master alloys on Al-Si alloys was investigated. Significant grain refinement is achieved via the introduction of these heterogeneous nuclei regardless of their chemistry, stoichiometry of the master alloy, and addition rate. However, the grain refinement is affected by variable such as contact time, as the inoculating particle may sediment or be poisoned, and cooling rate. Specifically, a faster cooling rate generally leads to finer grains due to the lower time for grain growth. In the case of borides, the chemical inoculation efficiency is greatly affected by their thermodynamic stability in molten Al-Si alloys. Conversely, the grain refining potency of properitectic Al3Nb remains unaffected. The underlying grain refining mechanism was finally investigated using current models based on the growth restriction factor Q to simultaneously consider the effect of nucleant potency and alloy chemistry. Among Ti-, Al-, and Nb-based borides with similar particle size and distribution, the latter are the most efficient to grain refine Al-Si alloys.

Highlights

THE control of nucleation is a fundamental aspect in many scientific disciplines such as chemical engineering, atmospheric science, plants biology, and food industry as well as engineering materials like superconductors and metals
Excess Ti contributes to the refinement of the cast structure due to its high growth restriction factor in LEANDRO BOLZONI is with the Waikato Centre for Advanced Materials (WaiCAM), The University of Waikato, Private Bag 3105, Hamilton 3240, New Zealand
By means of a comparative study, in this work we address the following unresolved issues about the grain refinement of commercial Al-Si alloys: (i) effect of stoichiometry and addition rate of Al-Ti-B master alloys, (ii) effect of the addition and addition rate of Al-B master alloys, (iii) effect of addition of Al-Nb master alloy, (iv) effect of the use of different borides (i.e., TiB2, AlB2, and NbB2), and (v) effect of the processing conditions such as contact time and cooling rate

Summary

Introduction

THE control of nucleation is a fundamental aspect in many scientific disciplines such as chemical engineering, atmospheric science, plants biology, and food industry as well as engineering materials like superconductors and metals. In the case of engineering alloys, inoculation of the molten metal leading to finer grains is highly beneficial as the fluidity of the alloy increases, and the number of defects and micro- and macro-porosity decreases.[1] The facilitated casting process is economically important because the quality of the casting is improved and the rejection rate reduced.[2] Chemical inoculation of Al alloys is a standard procedure and is normally done by adding Al-Ti-B master alloys, called grain refiners. Excess Ti contributes to the refinement of the cast structure due to its high growth restriction factor in LEANDRO BOLZONI is with the Waikato Centre for Advanced Materials (WaiCAM), The University of Waikato, Private Bag 3105, Hamilton 3240, New Zealand. Contact e-mail: leandro.bolzoni@ gmail.com NADENDLA HARI BABU is with the BCAST, Brunel University London, Kingston Lane, Uxbridge UB8 3PH, UK

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