Influence of Zr on Al-Ti-B-Based Grain Refiners in AlSiMgCuZr Alloy

TL;DR

This paper explores how zirconium affects grain refinement in aluminum alloys and finds ways to mitigate its negative impact through optimized casting practices.

Contribution

The study reveals that optimized Al-5Ti-1B addition and reduced holding time can counteract Zr poisoning in grain refinement of AlSiMgCuZr alloys.

Findings

Reducing holding time and increasing Al-5Ti-1B addition improves grain refinement efficiency in the presence of Zr.

The finest grain structure (150–170 μm) and most homogeneous hardness were achieved with continuous feeding of the grain refiner at 80 ppm B.

Optimized casting practices can mitigate Zr poisoning effects in AlSiMgCuZr alloys.

Abstract

One of the most effective methods of improving the properties of aluminium alloys is grain refining using Al-Ti-B master alloys. In contrast, zirconium is a key alloying element, used mainly in 2xxx and 7xxx series aluminium alloys, where it contributes to dispersion enhancement and reduces the rate of dynamic recrystallisation. However, even trace amounts of zirconium—just a few hundredths of ppm—significantly reduce the performance of Al-Ti-B grain refiners, a phenomenon known as ‘Zr poisoning’. This study investigates the impact of holding time and the level of Al-5Ti-1B addition on the microstructure and properties of an AlMgSi(Cu) alloy containing 0.15 wt.% Zr, cast as 7-inch DC billets. The structure and phase distribution were characterised using optical microscopy (OM), scanning electron microscopy (SEM) with energy-dispersive spectroscopy (EDS), and transmission electron microscopy (TEM). Grain size and morphology were evaluated through macrostructure analysis (etched cross-sections and polarised light microscopy), while chemical and elemental distributions were analysed via SEM-EDS and STEM-EDS mapping. Additionally, Brinell hardness measurements were conducted across the billet diameter to assess the correlation between grain size and mechanical properties. The results show that reducing holding time and increasing the Al-5Ti-1B addition improves grain refinement efficiency despite the presence of Zr. The finest grain structure (150–170 μm) and most homogeneous hardness distribution were achieved when the grain refiner was continuously fed during casting at 80 ppm B. These findings are supported by the literature and contribute to a deeper understanding of the Zr poisoning effect and its mitigation through optimized casting practice.

Figures