J.R. Griffiths

Damage by the cracking of silicon particles in an Al-7Si-0.4Mg casting alloy

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 of damage is more gradual. Between 3 and 10% of the particles crack prior to fracture. The stresses in the particles can be calculated using current models of dispersion hardening and the particle cracking can be described by Weibull statistics. Assuming that fracture occurs when a critical level of damage is attained, the ductility of the alloy can be expressed as a function of the dendrite cell size and the average size and aspect ratio of the cracked Si particles.

The deformation and fracture behaviour of an AlSiMg casting alloy

Abstract 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 and intergranular for the finer cell sizes. For the unmodified alloy, the ductility also has a minimum at intermediate cell sizes and the fracture mode is, again, transgranular at large cell sizes and intergranular at fine cell sizes. The ductility of the large cell size unmodified materials is low, being dominated by the large elongated silicon particles. If the unmodified alloy is solution treated for shorter times it is more brittle because the silicon particles are more elongated.

The influence of microstructure on the Bauschinger effect in an Al-Si-Mg casting alloy

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 when the Si particles have a larger aspect ratio. At constant aspect ratio of the Si particles, smaller cell sizes result in higher internal stresses at large strains. Current models for dispersion hardening can be used to calculate the stresses in the particles. The calculations are in very good agreement with the experimental results.

Oxide films, pores and the fatigue lives of cast aluminum alloys

In the absence of gross defects such as cold shuts, the fatigue properties of castings are largely determined by the sizes of microstructural defects, particularly pores and oxide films. In contrast, the effects of grain size, second-phase particles, and nonmetallic inclusions are insignificant. The authors review the fatigue properties of castings made by gravity die casting, sand casting, lost-foam casting, squeeze casting, and semisolid casting, and compare A356/357 alloys with 319-type alloys. The application of fracture mechanics enables the properties to be rationalized in terms of the defects that are characteristic of each casting process, noting both the sizes and types of defect. The differences in the properties of castings are entirely attributed to their different defect populations. No single process is inherently superior. For defects of the same size (in terms of projected area normal to the loading direction), oxide films are less detrimental to fatigue life than pores. Areas of current controversy are highlighted and suggestions for further work are made.