Casting and anodising of Al alloys- Alloy design, manufacturing process and material properties

Abstract: Casting and semi-solid metal (SSM) casting are widely used to manufacture near-net-shape components of Al-Si alloys in the automotive and electronic industries. In such applications, casting components need to meet the combined requirements of good mechanical properties and corrosion and wear resistance. Hence, a good understanding of the relevant aspects such as material design, manufacturing and surface treatments have a significant impact on the final performance of the component. The objective of this thesis is to understand the interaction among manufacturing and surface treatments and how their combination impacts the microstructure and final properties. The results will accordingly highlight the potential for improving the mechanical and anodising properties of rheocast components.The influence of the most relevant alloying elements has been investigated in this study. It is found that Si and Fe have a significant influence on anodising. During anodising, Si particles are oxidised at a much lower rate than Al phase and embedded in the oxide layer. Due to the presence of Si particles and their morphology, residual metallic Al phase and cracks are introduced in the oxide layer. A reduced number of residual metallic Al phase, as well as defects, can be obtained by changing the Si particle morphology to disconnected fibrous by Sr modification. On the contrary, Fe-rich intermetallics could be partly dissolved during anodising, leaving vacancies or voids as defects in the oxide layer. So, it was proved that by modifying Si particles and removing Fe-rich intermetallics from the surface, the defects in the oxide layer are reduced, and better corrosion protection is achieved.The SSM process increases the microstructural inhomogeneity such as transverse macrosegregation and longitudinal macrosegregation in the cast component. The results show that the presence of surface liquid segregation (SLS) layer by transverse macrosegregation does not have a significant impact on the corrosion resistance and hardness of the oxide layer of as-cast surfaces compared to liquid casting. The longitudinal macrosegregation influences the corrosion protection provided by the anodised layer but does not affect the hardness of the rheocast component before or after anodising. In this study, it is also found that, during the casting of Al-Si alloys, the surface of the component can be enriched in Fe-rich intermetallics due to the SLS or interaction with the die material. Despite this affects only the very superficial thickness, it has a big impact on the corrosion resistance and hardness of the oxide layer.This study has revealed that the high value of the oxide layer thickness, as well as the hydrothermal sealing, is not a guarantee for improving the corrosion resistance of the oxide layer. An increase of the oxide layer thickness by increasing applied voltage or anodising time decreases both the corrosion resistance and hardness of the oxide layer. Moreover, the hydrothermal sealing after anodising significantly decreases the corrosion protection provided by the anodised layer in Al-Si alloys due to cracks formation.This study has observed that the casting defects such as oxide film, cold shots and the solute-rich layer which are related to the casting process dominate the fatigue behaviours of the SSM cast components. The fractographic examination indicates that the oxide film, cold shots and solute-rich layer act as crack initiation points during fatigue testing. Therefore, it was found that, in these conditions, the anodising does not have an evident impact on fatigue properties, despite the anodising process adds a brittle anodised layer on the surface.