Carlos H Caceres

The ductility of casting alloys is usually low and it is thus important to simultaneously assess the effect of changes to the microstructure and heat treatment on both ductility and strength of the material. The use for this purpose of the quality index charts* is common in the casting industry with regard to the Al-Si-Mg casting alloys A356 and A357. An analytical method of generating quality index charts for any alloy system is presented. Applications of the method are illustrated with case studies involving Al-Si-Mg, Mg-Al-Zn, and Al-Si-Cu-Mg casting alloys. The analytically determined charts indicate the limits to microstructural improvement available for each material. The possibility of using the charts to optimize the relation between mechanical performance, chemical composition, solidification conditions, and temper is discussed.

The cracking of Si particles during plastic deformation has been studied for different microstructures produced by varying the solidification rate and length of solution treatment. The number of cracked particles increases with the applied strain. The larger and longer particles are more prone to cracking. In coarser structures particle cracking occurs at low strains, while in finer structures the progression

The microstructure and tensile behavior of two Al-7 pct Si-Mg casting alloys, with magnesium contents of 0.4 and 0.7 pct, have been studied. Different microstructures were produced by varying the solidification rate and by modification with strontium. An extraction technique was used to determine the maximum size of the eutectic silicon flakes and particles. The eutectic Si particles in the unmodified alloys and, to a lesser extent, in the Sr-modified alloys are larger in the alloys with higher Mg content. Large Fe-rich π-phase (Al9FeMg3Si5) particles are formed in the 0.7 pct Mg alloys together with some smaller β-phase (Al5FeSi) plates; in contrast, only β-phase plates are observed in the 0.4 pct Mg alloys. The yield stress increases with the Mg content, although, at 0.7 pct Mg, it is less than expected, possibly because some of the Mg is lost to π-phase intermetallics. The tensile ductility is less in the higher Mg alloys, especially in the Sr-modified alloys, compared with the lower Mg alloys. The loss of ductility of the unmodified alloy seems to be caused by the larger Si particles, while the presence of large π-phase intermetallic particles accounts for the loss in ductility of the Sr-modified alloy.

By changing the solidification rate, chemical modification and length of solution treatment we show that the ductility of the Al7Si0.4Mg casting alloy depends on the dendrite cell size and the size and shape of the silicon particles. For the strontium-modified alloy the ductility has a minimum at intermediate cell sizes, the fracture mode being transgranular for the larger cell sizes

Samples of commercial Al–7Si–0.4/0.7Mg–T6 casting alloys deformed to fracture were anodised to reveal the grain boundaries and the proportion of transgranular/intergranular fracture was determined. The number and area fraction of Si particles cracked on cell and grain boundaries was also determined. Samples of unmodified and Sr-modified alloy, with magnesium contents of 0.4 and 0.7% and dendrite arm spacings (DAS) between

The Bauschinger effect has been studied in an Al-7% Si-0.4% Mg casting alloy for a range of dendrite cell sizes and aspect ratios of the Si particles. The internal stresses increase linearly with the imposed pre-strain for strains up to 0.007, gradually saturating thereafter. For a given value of the cell size the internal stresses reach a higher saturation value

The strain dependence of particle cracking in aluminum alloys A356/357 in the T6 temper has been studied in a range of microstructures produced by varying solidification rate and Mg content, and by chemical (Sr) modification of the eutectic silicon. The damage accumulates linearly with the applied strain for all microstructures, but the rate depends on the secondary dendrite arm spacing and modification state. Large and elongated eutectic silicon particles in the unmodified alloys and large π-phase (Al9FeMg3Si5) particles in alloy A357 show the greatest tendency to cracking. In alloy A356, cracking of eutectic silicon particles dominates the accumulation of damage while cracking of Fe-rich particles is relatively unimportant. However, in alloy A357, especially with Sr modification, cracking of the large π-phase intermetallics accounts for the majority of damage at low and intermediate strains but becomes comparable with silicon particle cracking at large strains. Fracture occurs when the volume fraction of cracked particles (eutectic silicon and Fe-rich intermetallics combined) approximates 45 pct of the total particle volume fraction or when the number fraction of cracked particles is about 20 pct. The results are discussed in terms of Weibull statistics and existing models for dispersion hardening.