Multi-refiners for sustainable aluminium manufacturing

Aluminium is, in principle, infinitely recyclable and when compared with primary production, recycling can save 95% of the energy and 90% of the greenhouse gas emissions. However, aluminium recycling is still a technologically challenging process due to the accumulation of elemental impurities at each cycle, which downgrades the material properties. Impurity elements, such as Fe and Si, have a strong tendency to segregate during solidification and precipitate into coarse and brittle intermetallic compounds (IMCs), which adopt anisotropic morphologies (e.g. plate-like) and compromise the material performance even at small volume fractions. Removal of these deleterious contaminating elements from end-of-life sources is a huge challenge, as monitoring their level in the scrap during sorting is practically unfeasible. The current industry solution to the downcycling problem is to maintain tightly controlled alloy compositions and dilute recycled scrap with virgin, near-pure alloy, with all attendant logistical, cost and emission penalties.

A radical alternative, is to shift from composition tuning to microstructure tuning, wherein properties are engineered using designed solidification conditions to manipulate impurities into forming benign and finely dispersed IMCs, rather than the ‘naturally’ occurring plate-like detrimental compounds. However, practical applications of this concept are still underexploited because the methodologies to promote these more benign IMC morphologies are not known.

This project will investigate how to manipulate IMC shape and size by controlling their nucleation through inoculation. Inoculation is already widely used in industry to refine the primary aluminium grains, though only in a handful of cases, and not yet applied to secondary phases. The primary goal of the project will be to develop the necessary science and understanding to design new nucleant compounds for the most problematic IMCs present in automotive and aerospace aluminium alloys. The work will involve the development of methods to manipulate the surface chemistry and size distribution of nucleants to enhance their efficiency. Chemical and deposition techniques will be utilised together with electron microscopy for rapid throughput analysis of the nucleant-IMC interfaces, which control the nucleation efficiency. Time-resolved synchrotron X-ray imaging will be employed to investigate nucleant performance under industrial-relevant solidification conditions. Further extension of the project will investigate scalable processes to manufacture nucleant-containing master alloys that can be practically used in a commercial environment.