Casting of aluminium alloys

Casting is one of the main routes for producing aluminium-alloy parts. Good castability for foundry alloys includes a relatively high fluidity, low melting point, short casting cycles, relatively low tendency for hot cracking, good as-cast surface finish and chemical stability. In hypoeutectic alloys, it is common to refine the size of the aluminium crystals by grain refinement for applications requiring good mechanical properties. In Al-Si alloys, a treatment called eutectic modification is often used to change the eutectic silicon from coarse platelike into fine fibrous morphology, which is less harmful to mechanical performance. Main shape casting processes include high-pressure die casting, low-pressure die casting, permanent mould casting and sand casting. Each process has its own strategy for filling the alloy into the mould/ die and feeding the solidification shrinkage

The Influence of Intensification Pressure on the Gate Microstructure of AlSi3MgMn High Pressure Die Castings

This article focuses on the influence of intensification pressure (I.P.) on the feeding through the gate during high pressure die casting (HPDC). Two values of intensification pressure, the lowest and highest possible for the HPDC machine used, were applied to cast AlSi3MgMn tensile-bar specimens. The castings produced with higher I.P. contained a lower total fraction of porosity, as expected. Microstructural characterisation of the gate region showed markedly different features in and adjacent to the gate at the two levels of I.P. used. The microstructures indicate a change in feeding mechanism with increasing I.P. At high I.P. shear band-like features exist through the gate, suggesting that strain localisation in the gate is involved in the feeding of solidification shrinkage during the I.P. stage. At low I.P. such shear bands were not observed in the gates and feeding was less effective, resulting in a higher level of porosity in the HPDC parts

Microstructure formation in high pressure die casting

In order to optimise the high pressure die casting (HPDC) process, more understanding of microstructure and defect formation is essential. This article gives an overview of the key microstructural features and the mechanisms of microstructure formation in this process. The incavity solidified grain size in HPDC can be as low as 10 μm, but externally solidified crystals (ESCs) as large as 200 μm are often also present. The eutectic microstructure is very fine with an interlamellar spacing around 500 nm. Bands of positive macrosegregation and sometimes with cracks/porosity, so-called defect bands, are also often observed. Intensification pressure (IP) is one of the major factors governing the porosity level in the casting. At high IP, defect bands form in the gate region and appear to be assisting the feeding during the intensification stage

Formation of the surface layer in hypoeutectic Al-alloy high-pressure die castings

A layer of distinctive microstructure known as the surface layer or the skin is often observed near the casting surface of high-pressure die cast (HPDC) parts. With its different microstructure, the surface layer could influence mechanical performance, corrosion properties and also pressure tightness of the whole cast component. This research aims to develop a better understanding of surface layer formation, which is essential to control the microstructure and therefore properties of HPDC components. In this study, microstructural characterization has been performed on HPDC specimens cast from an HPDC-specific Al alloy for structural applications, AlMg5Si2Mn. Most regions in the samples investigated contain ∼10-30 μm globular-rosette primary α-Al grains and [Al + MgSi] eutectic, while very dendritic primary α-Al grains are also present in the surface layer. The surface layer was observed in the region where the alloy did not directly impinge on the die surface during die filling (the so-called indirect impingement zone). In the region between the surface layer and inner regions (the so-called surface-layer edge), the only primary grain population is the very dendritic grains also observed in the surface layer. The surface layer formation is related to shearing at the interface between two parts of material containing different solid fractions (f) which can occur either during the die-filling or during the intensification stage

An overview of defect bands in high pressure die castings

Bands of positive macrosegregation, porosity and/or cracks commonly follow the outer contour of high pressure die cast (HPDC) components. This paper overviews defect bands in hot- and cold-chamber HPDC components manufactured using Mg, Al and Zn-based alloys. A hot chamber HPDC AM50 magnesium steering wheel is used as a case study, and defect band formation is discussed based on recent research on strain localization in partially solid alloys