Influence of La additions on solidification and microstructure in the as-cast state of the aluminium alloy EN AW-7175

Abstract

The high-strength aluminium alloys of the 7xxx series, which belong to the Al–Zn–Mg–Cu system, are known for their exceptional properties and are often used for applications where strength, fatigue resistance, stress corrosion resistance and wear resistance are required. The alloy EN AW-7175 requires an understanding of the precipitation process during solidification and cooling, where different phases are formed depending on the Zn/Mg ratio and cooling rate. Microstructural defects in the as-cast state affect the mechanical properties, prompting the investigation of La additions to refine the microstructure and improve the mechanical properties. In this study, the influence of La additions on the solidification and microstructure of the alloy EN AW-7175 in the as-cast state is investigated. Thermodynamic calculations, DSC and SEM analyses were performed. Samples with La additions (0.05–0.17 mass%) were compared with a reference sample (0 mass% La). La additives have only a minimal effect on the liquidus temperature and show minor differences in the solidus temperature in equilibrium calculations. The solidification interval decreases slightly compared to the reference sample, which is consistent with the Scheil simulations. The DSC results show reduced liquidus and solidus temperatures, while the solidification interval remains largely unchanged by the addition of La. The addition of La alongside Al13Fe4, Mg2Si and the eutectic α(Al) + σ[Mg(Zn, Cu, Al)2] leads to the formation of two new La-based phases: Al20Cr2La and LaSi2. Al20Cr2La modifies the Al45Cr7 phase and solidifies first, while LaSi2 modifies Mg2Si. As the La content increases, the Mg2Si content decreases until it completely disappears at a La content of more than 0.1 mass%. On the contrary, according to the literature, the grain size increases somewhat with a higher La content.