Al-Si Foundry Alloys Research Articles

Anomalous microsegregation in Al–Si foundry alloys

Measurements of Si and Mg concentrations across dendrites in a commercial Al–Si–7Mg alloy have shown a higher Si concentration in dendrite centers decreasing towards the dendrite boundaries, which is opposite to what would be expected from the phase diagram.

Grain refinement mechanisms of hypoeutectic AlSi alloys

The general concept of the grain refinement mechanism in hypoeutectic AlSi alloys has been adopted from that in wrought alloys without consideration of the influence of the silicon content. In the present investigation the commercial grain refinement practice in AlSi foundry alloys has been experimentally simulated by introducing synthetic TiB2 crystals directly into AlSi melts. To explore the possible use of a new generation of AlTiC master alloys in these alloys, the behaviour of synthetic TiC has also been studied. Experimental findings indicate that in the presence of Si, TiC crystals are highly unstable in aluminium melts. TiB2 crystals alone do not nucleate the α-phase, whereas in the presence of dissolved Ti a ternary AlTiSi interfacial layer is formed on the TiB2 which subsequently nucleates the α-Al, via a peritectic reaction. However, due to a drastic decrease in the peritectic temperature, the efficiency of grain refinement is greatly reduced. A theoretical analysis indicates that when boron is added in excess of TiB2 stoichiometry, or simply as AlB2, grain refinement occurs by a eutectic reaction.

Effect of grain refinement on the fluidity of two commercial Al-Si foundry alloys

The effect of grain refinement on the fluidity of AlSi7Mg and AlSi11Mg has been investigated by spiral tests. Two different types of grain refiners have been evaluated. An AlTi5Bl master alloy was added to different Ti contents. Since the commercial alloys had a high initial content of titanium, model alloys were made to investigate the fluidity at low grain refiner additions. Commercial alloys grain refined only by boron additions have also been investigated. The results from the fluidity measurements have been verified by measuring the dendrite coherency point of the different cast alloys. Although different, the two methods show similar trends. The spirals from each fraction grain refiner cast were subsequently investigated metallographically at the tip of the spirals and at a reference point a distance behind, but no obvious difference in structure was observed. For both alloys, an increase in fluidity is observed as the content of grain refiner increases above 0.12 pct Ti, while the fluidity is impaired with increased grain refinement below 0.12 pct Ti. The alloys grain refined with ~0.015 pct B show the highest fraction solid at dendrite coherency, the smallest grain size, and the best fluidity.

The effect of na and Sr modification on surface tension and volumetric shrinkage of A356 alloy and their influence on porosity formation

Modification of the eutectic Si in Al-Si foundry alloys by adding strontium or sodium is, unfortunately, accompanied by an increase of porosity in the casting. In an attempt to understand the nature of this problem, this study used a sessile-drop method to investigate the effect of Sr and Na on surface tension and volumetric shrinkage, two probable causes of porosity occur-rence. The addition of 0.01 wt pct Sr and 0.005 wt pct Na to A356 alloy decreases the surface tension of the liquid by about 19 and 10 pct, respectively, and may increase the volume shrink-age by about 12 pct. These changes to surface tension and volumetric shrinkage promote the early formation of the pores during solidification and give the availability of a longer period of growth prior to complete solidification, resulting in a larger pore size. The effect of surface tension on the pores is more significant than volumetric shrinkage. Although the predicted pore diameter increases with lower surface tension or higher volumetric shrinkage, these two effects alone do not seem able to completely account for the observed increase in porosity that is associated with modification.

Magnesium Effects on the Microstructure of Unmodified and Modified Al-Si Alloys

Small amounts of magnesium are often used to improve the mechanical properties of Al-Si foundry alloys. The effect of this magnesium on the eutectic microstructure has been investigated at magnesiu...