Aluminum Low-Silicon (Hypoeutectic) Alloys

Aluminum casting alloys can be customized to encompass a broad range of mechanical performance through chemical and/or process control. There are four main families based on Al-Si (-Cu), Al-Cu, Al-Mg (-Si) and Al-Zn (-Si) - Mg systems. The vast majority of cast Al components are Al-Si systems due to their excellent castability. Some Al-Cu, Al-Mg and Al-Zn alloys exhibit better properties than Al-Si alloys, but their casting characteristics are generally poor. They are particularly prone to hot tearing. Hypoeutectic Al-Si based alloys are well-known casting alloys possessing a combination of highly desirable properties such as castability, weldability, low coefficient of thermal expansion, good corrosion resistance and machinability. Because of these properties, Al-Si alloys are used in the automotive industry, particularly in the production of engines, crankcases, intake manifolds, cylinder blocks, cylinder heads, pistons, cast oil pans and valve lifters. Mechanical properties are primarily controlled by casting structure. The microstructure evolution of hypoeutectic Al-Si alloys during solidification occurs in two stages: formation of primary dendritic Al phase (α-matrix) and subsequent eutectic transformation (eutectic Si particles within the α-matrix). According to the Al-Si binary diagram, the Al-Si eutectic volume fraction can exceed 50% in commonly used hypoeutectic Al-Si alloys such as AlSi7Mg0.3, AlSi9Cu3, AlSi6Cu4. Typically, the Al-Si eutectic comprises 50-90% of the volume fraction of these alloys. The purpose of the research by T. Szymczak, G. Gumienny, I. Stasiak and T. Pacyniak was to observe the effect of Cr, V and Mo added simultaneously on the crystallization process, microstructure and mechanical properties of hypoeutectic Al-Si alloy in pressure die casting. The research was conducted under the production conditions of Wifama-Prexer Ltd., a Research, Innovation and Application Company in Poland. The study used 226 standard hypoeutectic aluminum alloy with pressure die casting. The chemical composition is given in Table 1. Table 1: Chemical composition of 226 Al-Si alloys tested, by weight %. [3] The test additives were added as AlCr15, AlV10 and AlMo8 master alloys. The tested alloys were cast into a DTA sampler and also cast using pressure die casting. A portion of the Cr, V and Mo additives in the alloy was cast into the DTA sampler within an approximate range of 0.05-0.35%. For pressure die casting, the alloys contained 0.05-0.20% Cr, V and Mo. The crystallization process was examined using derivative thermal analysis (DTA).

The DTA curves of Al-Si alloys containing approximately 0.30% and 0.35% Cr, Mo and V showed an additional thermal effect, probably caused by the peritectic crystallization of intermetallic phases containing the aforementioned additives. These phases have a wall-like morphology and relatively large size. Similar phases also occur in pressure die casting alloys with 0.10% or greater additions of Cr, V and Mo.

The appearance of these phases in pressure die casting Al-Si alloys coincides with a decrease in tensile strength Rm and elongation A. It was demonstrated that pressure die castings made from Al-Si alloys containing the aforementioned additives have higher Rm and A than the "226 alloy". Figure 1 shows the SEM microstructure and three-dimensional morphology of eutectic Si in cast AlSi6Cu4 alloy with and without 1000 and 10,000 ppm Sb. The microstructure shown in Figure 1a reveals fine, yet still plate-like eutectic silicon. In Figures 1a and 1b, it is clear that the dimensions of the fine eutectic Si platelets progressively decreased after Sb addition, and the morphology changed from compact plate-like form to rod or fibrous form. The Si particles in the 10,000 ppm samples coarsen, probably as a result of excessive modification (Figure 1c). [2] In all heat-treated specimens (Figure 2), the morphology of eutectic Si was observed as rounded particles. After deep etching, we observed fragmentation of the fine compact Si plate-like phase and its transformation into fine rod-like phase (Figures 2a, 2b, 2c).

References: 1. [1] Products-Cast alloys and products, The Aluminium Automotive Manual, https://www.european-aluminium.eu/media/1544/aam-products-6-castalloys-and-products.pdf/Accessed 20 March 2019;/; 2. [2] M. Farkašová, E. Tillová, M. Chalupová: Modification of Al-Si-Cu cast alloy, FME Transactions, N°3, 2013, Vol. 41, p.210-215; 3. [3]T. Szymczak, G. Gumienny, I. Stasiak, T. Pacyniak: Hypoeutectic Al-Si Alloy with Cr, V and Mo to Pressure Die Casting, Archives of Foundry Engineering Volume 17, Issue 1, 2017, p.153-156; ISSN 2299-2944

    Dr. Zoran Slović Chief Engineer Key to Metals AG Total Materia