Fractional solidification for recycled aluminium alloys

Abstract

Climate change, and the need of recycling, are two interconnected topics that are gaining more and more relevance nowadays. In order to decrease the CO2 emissions and reduce, or contain, the pollution that is having devastating effects on our climate, recycling seems to be a necessity that can not be postpone or diminished anymore. In the effort of reducing pollution, the aluminium world (industry and Academia) is playing an important role. Recycling aluminium, in fact, require only 10% of the energy necessary to extract it form the bauxite ore, leading to a massive energy and cost saving, as well to a reduction on the CO2 emissions equivalent to take off the road 900000 cars for 12 months. Up to now, the only closed loop recycling process is the one for usage beverage cans (UBC): used cans are collected, molten together, and used again to create new cans without the addition of any other material. When it comes to the automotive industry things are a bit more complicated. There are two main ways, adopted by industries, to recycle Al alloys depending on their composition. Wrought alloys are recycled remelting the scrap with primary Al, and cast alloys are obtained remelting wrought alloys and adjusting their composition for the different purposes. Although these two methods meet the requirement so far, they will lead to a nonrecyclable scrap surplus in the near future, consequently, new possibilities are being explored. In this work, a method to recycle aluminium alloys based on fractional solidification is proposed. The technology developed is based on an idea proposed by A.L Lux and M.C Flemings in which a semisolid alloy is isothermally squeezed towards a filter. Lux and Flemings tested their method on Sn-Pb alloys and on a small scale (500 g). The technology developed in this thesis, has been tested first on model aluminium alloys, and once the optimised procedure was established, on real aluminium scrap alloys, and on a bigger scale (up to 3 kg). The effect of several parameters (temperature, squeezing procedure, ultrasound, and grain refinement) on the purification efficiency was investigated and a 60% purification efficiency was achieved. Along with the technology development, a more fundamental study was carried out focusing on the investigation of the semisolid deformation, the liquid migration through the mush and the role of morphology on the permeability of the semisolid alloy. Finally, through the use of a numerical model, the feasibility of the Scheil-Gulliver approach to our case study, was investigated.