Greater emphasis on improved fuel efficiency has led to increased efforts to reduce the weight of automobiles. The use of aluminium alloys in the automotive industry has thus increased because of their relatively low density and good mechanical properties. Hypoeutectic aluminium-silicon alloys are the most commonly used aluminium-based foundry alloys.
The drive for reduced component weight is accompanied by a drive for improved economies of production. This has led to an increase in the use of recycled alloys. Recycled alloys are inherently less pure than primary alloys. The effect of this reduced purity on the casting properties of hypoeutectic Al-Si foundry alloys forms the subject of this thesis.
Iron is the most common impurity element in aluminium alloys. In high concentrations, it is known to reduce both the castability and the mechanical properties of aluminium-silicon foundry alloys. Manganese is commonly referred to as a neutraliser of iron because it has a beneficial effect on the mechanical properties of iron-containing aluminium-silicon alloys. The effect of manganese on the castability of these same alloys is not well defined.
An experimental program has been undertaken to investigate the effect of iron and manganese on the castability of aluminium-silicon foundry alloys. A diverse matrix of compositions, encompassing 5-9wt%Si, 0-3wt%Cu, 0-1wt%Fe, and 0-0.5wt%Mn was investigated. The amount of porosity measured in a plate casting was used as an indicator of the castability of alloys. Thermal analysis and interrupted solidification experiments were also performed to characterise the evolution of the microstructure during solidification. Three-dimensional representations of specific microstructural features were developed by coupling serial sectioning with a virtual reconstruction software package.
Porosity levels in the plate castings were observed to increase with increasing iron concentration in Al-9wt%Si-0.5wt%Mg alloys with 0, 1, and 3wt%Cu. In the copper containing Al-5wt%Si-0.5wt%Mg alloys, a minimum in porosity at an iron concentration of 0.4wt% was observed. In the copper-free Al-5wt%Si-0.5wt%Mg alloys, porosity was observed to increase with increasing iron concentration.
The effect of manganese on the porosity levels in Al-9%Si-0.5%Mg alloy castings was investigated using the same plate casting design. The effect of manganese was found to be dependent on the iron and copper concentrations of the alloy.
The number of Al-Si eutectic nucleation events was found to decrease for increasing iron concentration in Al-9%Si alloys across a range copper concentrations. Thus, the size of the Al-Si eutectic grains increases with increased iron concentration. Manganese additions were found to partially reverse this effect. It is proposed that mushy zone permeability is dependent on the Al-Si eutectic grain size.
Models have been developed to describe the effect of iron, manganese and copper on porosity levels in hypoeutectic Al-Si foundry alloys. Porosity levels in Al-9%Si- 0.5%Mg alloy castings may be explained by considering the effects of changes in the Al-Si eutectic grain size and the iron- and copper-bearing intermetallic particles and morphology. These models have been used to generate guidelines to predict possible porosity levels based on the alloy composition.