On the role of magnesium and nitrogen in the infiltration of aluminium by aluminium for rapid prototyping applications

Selective laser sintering has been used to fabricate an aluminium alloy powder preform which is subsequently debound and infiltrated with a second aluminium alloy. This represents a new rapid manufacturing system for aluminium that can be used to fabricate large, intricate parts. The base powder is an alloy such as AA6061. The infiltrant is a binary or higher-order eutectic based on either Al-Cu or At-Si. To ensure that infiltration occurs without loss of dimensional precision, it is important that a rigid skeleton forms prior to infiltration. This can be achieved by the partial transformation of the aluminium to aluminium nitride. In order for this to occur throughout the component, magnesium powder must be added to the alumina support powder which surrounds the part in the furnace. The magnesium scavenges the oxygen and thereby creates a microclimate in which aluminium nitride can form. The replacement of the ionocovalent Al2O3 with the covalent AlN on the surface of the aluminium powders also facilitates wetting and thus spontaneous and complete infiltration. (C) 2004 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Eutectic modification of Al-Si alloys with rare earth metals

The effects of different concentrations of individual additions of rare earth metals (La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu) on eutectic modification in Al-10mass%Si has been studied by thermal analysis and optical microscopy. According to the twin-plane re-entrant edge (TPRE) and impurity induced twinning mechanism, rare earth metals with atomic radii of about 1.65 times larger than that of silicon, are possible candidates for eutectic modification. All of the rare earth elements caused a depression of the eutectic growth temperature, but only Eu modified the eutectic silicon to a fibrous morphology. At best, the remaining elements resulted in only a small degree of refinement of the plate-like silicon. The samples were also quenched during the eutectic arrest to examine the eutectic solidification modes. Many of the rare-earth additions significantly altered the eutectic solidification mode from that of the unmodified alloy. It is concluded that the impurity induced twinning model of modification, based on atomic radius alone, is inadequate and other mechanisms are essential for the modification process. Furthermore, modification and the eutectic nucleation and growth modes are controlled independently of each other.

Aluminium phosphide as a eutectic grain nucleus in hypoeutectic Al-Si alloys

Aluminium phosphide (AlP) particles arc often suggested to be the nucleation site for eutectic silicon in Al-Si alloys, since both the crystal structure and lattice parameter of AlP (crystal structure: cubic K(4) over bar m; lattice parameter: 5.421 Angstrom) are close to that of silicon (cubic Fd3m, 5.431 Angstrom), and the melting point is higher than the Al-Si eutectic temperature. However, the crystallographic relationships between AlP particles and the surrounding eutectic silicon are seldom reported due to the difficulty in analysing the AlP particles, which react with water during sample preparation for polishing. in this study, the orientation relationships between AlP and Si are analysed by transmission electron microscopy using focused ion-beam milling for sample preparation to investigate the nucleation mechanism of eutectic silicon on AlP. The results show a clear and direct lattice relationship between centrally located AlP particles and the surrounding silicon in the hypoeutectic Al-Si alloy.

Grain refinement efficiency and mechanism of aluminium carbide in Mg-Al alloys

Detailed microscopic examination using optical and electron microscopes suggests that Al4C3, often observed in the central regions of magnesium grains on polished sections, is a potent substrate for primary Mg. Calculations of the crystallographic relationships between magnesium and Al4C3 further support the experimental observations. (c) 2005 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Influence of the beta phase on the corrosion performance of anodised coatings on magnesium-aluminium alloys

The effect of the beta phase in Mg-Al alloys on the corrosion performance of an anodised coating was studied. It was found that the corrosion resistance of the anodised coating was closely associated with the corrosion performance of the substrate alloy. In particular, Mg alloys with a dual phase microstructure of alpha + beta with intermediate aluminium contents (namely 5%, 10% and 22% Al) after anodisation had the highest corrosion rate and the worst corrosion resistance provide by the anodised coating. The poor performance of an anodised coating was attributed partly to lower corrosion resistance of the substrate alloy and partly to the higher porosity of the anodised coating. (c) 2004 Elsevier Ltd. All rights reserved.

Corrosion of magnesium alloys in commercial engine coolants

A number of magnesium alloys show promise as engine block materials. However, a critical issue for the automotive industry is corrosion of the engine block by the coolant and this could limit the use of magnesium engine blocks. This work assesses the corrosion performance of conventional magnesium alloy AZ91D and a recently developed engine block magnesium alloy AM-SC1 in several commercial coolants. Immersion testing, hydrogen evolution measurement, galvanic current monitoring and the standard ASTM D1384 test were employed to reveal the corrosion performance of the magnesium alloys subjected to the coolants. The results show that the tested commercial coolants are corrosive to the magnesium alloys in terms of general and galvanic corrosion. The two magnesium alloys exhibited slightly different corrosion resistance to the coolants with AZ91D being more corrosion resistant than AM-SC1. The corrosivity varied from coolant to coolant. Generally speaking. an oraganic-acid based long life coolant was less corrosive to the magnesium alloys than a traditional coolant. Among the studied commercial coolants. Toyota long, life coolant appeared to be the most promising one. In addition. it was found that potassium fluoride effectively inhibited corrosion of the magnesium alloys in the studied commercial coolants. Both general and galvanic corrosion rates were significantly decreased by addition of KF, and there were no evident side effects on the other engine block materials, such as copper, solder. brass. steel and aluminium alloys, in terms of their corrosion performance. The ASTM D 1384 test further confirmed these results and suggested that Toyota long life coolant with 1%wt KF addition is a promising coolant for magnesium engine blocks.

The effect of aluminium content on the mechanical properties and microstructure of die cast binary magnesium-aluminium alloys

This paper investigates the relationship between mechanical properties and microstructure in high pressure die cast binary Mg-Al alloys. As-cast test bars produced using high pressure die casting have been tested in tension in order to determine the properties for castings produced using this technique. It has been shown that increasing aluminium levels results in increases in yield strength and a decrease in ductility for these alloys. Higher aluminium levels also result in a decrease in creep rate at 150 degrees C. It has also been shown that an increase in aluminium levels results in an increase in the volume fraction of eutectic Mg17Al12 in the microstructure.

The effect of grain refinement and cooling rate on the hot tearing of wrought aluminium alloys

Using modifications to the Rappaz-Drezet-Gremaud hot tearing model, and using empirical equations developed for grain size and dendrite arm spacing (DAS) on the addition of grain refiner for a range of cooling rates, the effect of grain refinement and cooling rate on hot tearing susceptibility has been analysed. It was found that grain refinement decreased the grain size and made the grain morphology more globular. Therefore refining the grain size of an equiaxed dendritic grain decreased the hot tearing susceptibility. However, when the alloy was grain refined such that globular grain morphologies where obtained, further grain refinement increased the hot tearing susceptibility. Increasing the cooling decreased the grain size and made the grain morphology more dendritic and therefore increased the likelihood of hot tearing. The effect was particularly strong for equiaxed dendritic grain morphologies; hence grain refinement is increasingly important at high cooling rates to obtain more globular grain morphologies to reduce the hot tearing susceptibility.