Pressure assisted segregation in squeeze cast aluminium alloys
A study has been conducted into Pressure Assisted Macrosegregation during the squeeze casting of an aluminium alloy, LM25, and an aluminium based silicon carbide particulate metal matrix composite. The squeeze casting process is particularly prone to this type of segregation which results in large areas of solute rich material in the final casting. The fundamental causes of the segregation have, until now, not been fully understood. This has restricted the use of squeeze casting as it was believed that the cause was a fundamental manufacturing weakness. An existing squeeze casting facility at Warwick was used to produce cast samples for study. The facility produces simple tensile test bars. Inserts for the die were designed to specifically produce the segregation within the castings. Thermocouples were threaded through the die wall and placed within the casting to enable the recording of the thermal history during solidification. Samples were taken from the castings, mounted, ground and polished for study via an optical microscope. A mechanism for the formation of the segregation is proposed. The primary causes of the segregation are shown to be surface tension, of the molten alloy, and the design of the component. The widespread belief that this form of segregation is a fundamental weakness of the squeeze casting process is therefore shown to be false. A theoretical model to a first approximation of the segregation, using the Young and Laplace equation is given. Using the pressure difference across a thin film the minimum web width, or radius of a corner, for no significant segregation can be found. The model is expanded and presented in a graphical form which is easy to understand and provides the designer with some tools to enable a sound casting to be produced. By sensible design the segregation phenomenon can be significantly reduced and in some cases eliminated completely.