Recycling of aluminum scrap through direct Hot Rolling Process

CARTA, MAURO

2024-02-09

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Abstract

Aluminum, with its unique blend of properties, has risen to become one the most abundant structural metal. In terms of industrial-scale production, it ranks second only to iron and steel. Aluminum can be produced from raw materials, referred to as primary aluminum, or by recycling scraps (secondary aluminum). Recycling offers a more sustainable solution, minimizing the environmental footprint that comes with primary production. The term "Solid State Recycling (SSR)" denotes a group of recycling processes that eliminate the need for melting. When comparing recycling efficiency, energy use, cost, and environmental consequences, SSR stands out as a more fitting choice. It can transform scrap, particularly chips, directly into bulk and semi-finished products with enhanced mechanical properties, avoiding the remelting phase. While various SSR methods exist, a process solely reliant on rolling is missing. To grasp the industrial importance of metal rolling in the industry, consider this: approximately 90% of steel, aluminum, and copper alloys are subjected to a rolling process at least once during their manufacturing. That’s why a solid-state recycling process based on rolling could be crucial in the industrial field. Particularly important would be the development of a process that can be directly applied into existing industrial rolling plant, with little or no alterations. The purpose of this dissertation is to discuss and develop a process with this very purpose in mind. The newly developed process was applied to the major family of wrought aluminum alloy: AA1xxx (Pure Al), AA5xxx (Al-Mg), AA6xxx (Al-Mg-Si) and AA3xxx (Al-Mn). The samples were studied in terms of mechanical properties, microstructure, and corrosion properties. In the last part of this dissertation, LCA analysis will be applied to study the environmental impact of the recycling technique compared to traditional routes based on melting. The thesis highlights several key findings regarding the solid-state recycling of aluminum chips/scraps to obtain high-quality sheets/strips. Firstly, by comparing the microstructures of the bulk material and the recycled-chips, it is clear that all recycled chips have a layered-like microstructure, attributable to the inherent nature of the chips used as base material. SEM analysis revealed that a broken oxide layer aids in maintaining the continuity of the aluminum matrix. In terms of mechanical properties, the study found that chips undergoing heat treatment followed by direct hot rolling exhibited properties very similar to those of the bulk material. Additionally, the application of Accumulative Roll Bonding (ARB) process to the compacted chips, without a prior heat treatment, leads to an enhancement in both mechanical properties and surface quality. However, this route faced challenges such as roller wear and material adherence due to the absence of lubricants. A significant advancement was made by plating the chips with pure aluminum strips. This not only protected the chips during rolling but also enabled the use of lubricants, effectively overcoming the previous challenges. In conclusion, the thesis demonstrates the feasibility and potential of recycling aluminum alloy chips into high-quality strips. This is achieved through optimized rolling processes that successfully address issues related to waste reduction, microstructural integrity, and the enhancement of mechanical properties.