Feeding and Distribution of Porosity in Cast Al-Si Alloys as Function of Alloy Composition and Modification

Unmodified, Na-modified, and Sr-modified castings of Al-7 pct Si and Al-12.5 pct Si alloys were cast in molds in which it was possible to create different cooling conditions. It is shown how solidification influences the distribution of porosity at the surface and the center of the castings as a function of modification and Si content in sand- and chill-cast samples. Eutectic modification, Si content, and cooling conditions have a great impact on the distribution of porosity. Unmodified and Na-modified castings are more easily fed with porosity tending to congregate near the centerline of the casting, while Sr-modified castings solidify in a mushy manner that creates a more homogeneous distribution of porosity in the casting. The amount of porosity was highest in the Sr-modified alloys, lower in the Na-modified alloys, and lowest in the unmodified alloys. The size of the porosity-free layer and the effectiveness of the feeders were greater in the castings made with the steel chills due to the increased thermal gradients and consequent increase in the directionality of solidification

Study of heterogeneous nucleation of eutectic Si in high-purity Al-Si alloys with Sr addition

The official published version can be accessed from the link below - Copyright @ 2010 The Minerals, Metals & Materials Society and ASM InternationalAl-5 wt pct Si master-alloys with controlled Sr and/or P addition/s were produced using super purity Al 99.99 wt pct and Si 99.999 wt pct materials in an arc melter. The master-alloy was melt-spun resulting in the production of thin ribbons. The Al matrix of the ribbons contained entrained Al-Si eutectic droplets that were subsequently investigated. Differential scanning calorimetry, thermodynamic calculations, and transmission electron microscopy techniques were employed to examine the effect of the Sr and P additions on eutectic undercoolings and nucleation phenomenon. Results indicate that, unlike P, Sr does not promote nucleation. Increasing Sr additions depressed the eutectic nucleation temperature. This may be a result of the formation of a Sr phase that could consume or detrimentally affect potent AlP nucleation sites.This work is financially supported by the Higher Education Commission of Pakistan and managerially supported from the OAD

Segregation band formation in Al-Si die castings

Banded defects are often found in high-pressure die castings. These bands can contain segregation, porosity, and/or tears, and changing casting conditions and alloy are known to change the position and make-up of the bands. Due to the complex, dynamic nature of the high-pressure die-casting (HPDC) process, it is very difficult to study the effect of individual parameters on band formation. In the work presented here, bands of segregation similar to those found in cold-chamber HPDC aluminum alloys were found in laboratory gravity die castings. Samples were cast with a range of fraction solids from 0 to 0.3 and the effect of die temperature and external solid fraction on segregation bands was investigated. The results are considered with reference to the theological properties of the filling semisolid metal and a formation mechanism for bands is proposed by considering flow past a solidifying immobile wall layer

Relationship between tensile and shear strengths of the mushy zone in solidifying aluminum alloys

Strength development in the mushy zone during solidification of three aluminum alloys (Al-4 wt pct Cu, Al-7 wt pct Si-1 wt pct Cu, and Al-7 wt pct Si-4 wt pct Cu) has been measured with two different techniques - horizontal tensile testing and direct shear cell testing. The strength results from the two methods correspond with one another to a much higher degree than suggested by the results presented in the literature. Tensile strength starts to develop at the maximum packing solid fraction, confirmed by the shear strength measurements. The maximum packing fraction represents the point where the internal network structure of the mushy zone is established and ligaments of the network must be broken to rearrange the dendrites. The data indicate a converging trend of the shear and tensile strength at high solid fractions, therefore indicating that the deformation mechanisms are also becoming similar. The results presented in this article suggest that it is possible to develop constitutive equations for the mechanical properties of the mushy zone over the entire solid fraction regime, i.e., from coherency to complete solidification. These equations could be used for the prediction of stress development as well as defect formation

Correlation between hydrogen migration and microstructure in cast Mg alloys

A simple casting process has been applied to produce binary Mg-X (X = Ni, Cu or Al) in situ composites for hydrogen storage. Besides a major single phase of primary Mg, nanostructured eutectic networks of Mg-MgNi, Mg-MgCu and Mg-MgAl were formed during solidification of Mg-Ni, Mg-Cu and Mg-Al alloys, respectively. Each intermetallic phase has different effects on the hydrogen sorption kinetics of the Mg alloys due to their different reactivities with hydrogen. In this study, the relationship between hydrogen migration and microstructure of the cast Mg alloys is investigated. The influence of hydrogenation/dehydrogenation cycling on the formation of porous structures is further discussed

The five feeding mechanisms

This paper summarizes a number of studies on the flow and deformation behavior of solidifying Al-Si alloys and discusses these results with reference to the five feeding mechanisms described by Campbell. Distinct changes in rheological response at the coherency (f) and maximum packing (f) solid fractions are directly correlated with transitions between feeding mechanisms. The effect of alloy and solidification conditions on the development of mush shear strength during solidification is discussed with respect to both burst feeding at stresses exceeding the mush strength and also viscoplastic deformation of the mush at stresses below the mush strength at higher solid fractions

New approaches to understand modification and nucleation mechanisms of hypoeutectic Al-Si Alloys

Recently we have developed methods to enable the application of state-of-the-art analysis techniques for the determination of modification mechanisms, nucleation and growth during solidification in non-facet/facet hypoeutecic alloys. This work aims to use this unique method to develop grain refiners while keeping eutectic modification for use in the casting industry. The application focuses on the development of eutectic grain refiners for modified hypoeutectic aluminium-silicon alloys, which will result in major material property and processing improvements. New experimental approaches are μ-XRF(X-ray fluorescence) in Synchrotron radiation, EBSD (electron back-scattering diffraction) in SEM and FIB (focusing ion beam) in TEM