Characteristics of cast aluminium-silicon alloys : microstructures and mechanical properties

Sammanfattning: Applications of aluminium alloys are spreading in many industrial fields due to the excellent combination of castability, mechanical performance and lightness assured by such material. This project aims to establish a methodology in bringing the foundry process closer to the casting designer, and thereby leading to a more reliable and more optimised design. Improvement in the degree of integration between processing, metallurgical and mechanical properties of cast aluminium alloys will lead to a shorter lead-time from the very first design attempt, and sounder components, both of which strengthen the competitiveness of the material and the foundry industry.The microstructural features and mechanical properties of cast aluminium alloys are sensitive to composition, melt treatment, the casting process and formation of defects during mould filling, solidification and post-solidification treatment. The microstructural characteristics and the mechanical properties of A1-Si based alloys have been investigated by studies on various different commercial cast components and through systematically designed casting experiments with alloys containing various Si, Mg, Cu, Fe and Mn concentrations solidified under a wide range of cooling conditions. In this work, a gradient solidification thechnique has been employed, providing samples with a low content of oxide films and few porosity defects and with a well-fed and homogenous microstructure. In such samples the alloy's mechanical properties are determined by the microstructure constituents and not by any defects. The influence of the casting process on the microstructural formations and tensile properties has been thoroughly elucidated. Furthermore, relationships between microstructure characteristics and mechanical properties are assessed and discussed. The results indicate that Mg and Cu, due to the formation of A12Cu and Mg2Si have a significant role on the alloy strength, especially after thermal treatment. Significant advances have been mead in understanding the relationship between Fe and Mn levels and their contribution to the tensile strength. While Fe and its compounds, particularly the A15FeSi-needles, adversely influence the ultimate tensile strength and ductility, Mn has been found to not fully neutralize the deleterious character of iron, and especially the influence it has on ductility.Algorithms to predict the micorstructural parameters such as porosity, grain size, the volume fraction of different phases, especially A12Cu and Mg2Si, and their influence on the tensile strength, have been developed. The relationships developed between process, microstructure and mechanical properties have been implemented in commercial simulation software.