Investigation on microstructure of aluminum alloys produced by a novel recycling additive manufacturing technique using sheets as feeding material

During recent years Additive Manufacturing techniques of metals has raised particular attention in scientific areas and industrial sectors thanks to their ability to produce complex, integrated parts and no need for necessary post-production steps like in traditional methods. Recycling for AM industry can play an important role in terms of economic and environmental sustainability. Indeed, most of wastes produced by several industries are in sheet form and developing techniques to transform these sheets directly into valuable products can be a promising path for the future of AM industry. Recycling by Laser for Additive Manufacturing (CYCLAM), was developed by the team of Prof. Alexander Kaplan at the Luleå University of Technology in year 2017. This technique was designed using waste steel sheets as feeding material instead of powder or wire, as typical in AM industry. Due to the high thermal conductivity and tendency to form oxides, Aluminum is considered as a challenging material to be processed by CYCLAM technique. Throughout this thesis, several set-ups and configurations of the CYCLAM process were investigated and tested using sheets of three different Al alloys. Specifically, the AA2024, AA6082, and AA7021 Al alloys were produced by CYCLAM and in-depth microstructural investigations were carried out. AA2024 and AA6082 alloys were produced with vertical configuration, whereas the AA7021 alloy was produced by horizontal configuration. Single tracks were produced and no defects were detected in the AA2024 alloy. A columnar grain structure with equiaxed grains on the top-side of single tracks and a cellular substructure within grains were identified. Al2Cu, Al2CuMg and Mg2Si phases were predicted by performing numerical simulations of Scheil curves, and then detected on as-built samples by EDS and XRD analyses. The aging response of the alloy starting from as-built and solution treated conditions was investigated and the hardness peak of 141 HV was achieved after 16 hours (T6 condition). A similar study was carried out on the AA 7021 and AA6082 alloys. Nevertheless, some cracks along grain boundaries and close to the substrate were observed within their microstructure.

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