Influence of remelting on AlSi$_9$Cu$_3$(Fe) alloy properties

The focus of casting production on raw materials as a starting point of all industrial value chains is more intense due to legislative and recently the difficulties faced by casting manufacturers. Critical (CRMs) and strategic raw materials (SRMs) are often indispensable inputs for a wide set of strategic sectors including renewable energy, the digital industry, the space and defence sectors and health sector which are all connected to the metal industry. Aluminium and its alloys plays an important CRMs and SRMs. Recycling has become a very important term for environmental protection as it reduces the carbon footprint of the foundry supply chain. The importance of recycling or the use of secondary or scrap raw materials is demonstrated by the fact that only 5% of greenhouse gases are released in the production process compared to the production of primary aluminium. Standard aluminium alloy AlSi$_9$Cu$_3$(Fe) (EN AC 46000) is widely used in the automotive and transport industry. High mechanical properties such as strength and hardness, as well as elongation and corrosion resistance are the main advantages of AlSi$_9$Cu$_3$(Fe) alloy. The quality of an alloy is mainly influenced by the properties of the raw material, the melting treatment and the casting technology. The significant use of secondary, i.e. recycled raw material and also of CRM, requires special attention to the chemical composition due to possible deterioration caused by repeated remelting, which can lead to a deterioration of mechanical and other performance properties. A prerequisite for good functional properties is the development of the microstructure. In this work, the influence of completely returned material (secondary raw material—scrap) as the only input charge material for the production of AlSi$_9$Cu$_3$(Fe) alloys by remelting on the development of the microstructure due to thermodynamic interactions of elements present was investigated. The presence of wide range of alloying elements AlSi$_9$Cu$_3$(Fe) alloys indicates development α-Al$_{15}$Si$_2$M$_4$ (M=Cr, Fe and Mn), β-Al$_5$FeSi, Al$_2$Cu and even more complex one such as Al$_3$Cu$_2$Mg$_9$Si$_7$ using theoretical modelling. Complex solidification path indicates primary aluminium α$_{Al}$, eutectic phase α$_{Al}$+β$_{Si}$, intermetallic phase on the iron base in Al$_5$FeSi and “Chinese script” morphology, intermetallic phase on the magnesium and copper base such as Al$_2$Cu and complex intermetallics such as Al$_3$Cu$_2$Mg$_9$Si$_7$ phase. Thermodynamic effects of elements interaction during solidification sequence significantly influence on solidification path and manner. Although the investigated samples exhibit high tensile strength and elongation, a slight deterioration of the chemical composition, and therefore in thermodynamic effect, has a significant influence on the development of the microstructure. Despite the deterioration of chemical composition, obtained microstructure was correct and, therefore, justified achieved high mechanical properties. Based on the investigation of the thermodynamic, microstructural and mechanical properties of the secondary AlSi$_9$Cu$_3$(Fe) alloy, the completely return raw material was characterised as a high-quality charge material with good application and recycling potential.