Towards Producing a High-Strength Al7075-Type Aluminum Alloy from a Recycling Scrap Stream: The Role of Melt Cleanliness, Bifilm Defects and Microstructural Evolution

Abstract

Recycling of high-strength aluminum alloys has gained increasing attention due to sustainability demands. However, producing scrap-derived 7xxx alloys remains challenging, as melt contamination, oxide films, and porosity-related defects can significantly deteriorate metallurgical quality. In this study, the influence of melt processing on defect formation, microstructural evolution, phase constitution, and hardness response of Al-Zn-Mg-Cu (7075-type) alloys produced from scrap was systematically investigated. The alloys were prepared under different processing conditions, including untreated scrap, degassing, and combined degassing with Ti and Sr additions. Melt quality was evaluated using the reduced pressure test, followed by quantitative bifilm analysis, porosity characterization, optical microscopy, SEM-EDS analysis, X-ray diffraction (XRD), and hardness measurements. The untreated alloy exhibited the highest bifilm index, pore population, and defect severity, indicating poor melt cleanliness. Degassing significantly reduced bifilm-related defects and suppressed pore formation, while Ti and Sr additions promoted a more refined and modified solidification structure. XRD and SEM-EDS analyses confirmed the presence of typical 7075-type phases dominated by the alpha-Al matrix, with interdendritic enrichment of Zn, Mg, and Cu. Improved melt cleanliness and structural refinement resulted in a slight increase in hardness. The results demonstrate that appropriate melt processing strategies can effectively enhance the metallurgical quality of recycled high-strength aluminum alloys.