S. Huo

The effect of trace elements on the sintering of an Al-Zn-Mg-Cu alloy

Trace elements can have a significant effect on the processing and properties of aluminium alloys, including sintered alloys. As little as 0.07 wt% (100 ppm) lead, tin or indium promotes sintering in an Al-Zn-Mg-Cu alloy produced from mixed elemental powders. This is a liquid phase sintering system and thin liquid films form uniformly throughout the alloy in the presence of the trace elements, but liquid pools develop in their absence. Analytical transmission electron microscopy indicates that the trace elements are confined to the interparticle and grain boundary regions. The sintering enhancement is attributed to the segregation of the microalloying addition to the liquid-vapour interface. Because the microalloying elements have a low surface tension, they lower the effective surface tension of the liquid. This reduces the wetting angle and extends the spreading of the liquid through the matrix. An improvement in sintering results

On development of sintered 7xxx series aluminium alloys

AbstractThe sintering characteristics and the tensile properties of the 7000 series Al–Zn–Mg–Cu alloys, fabricated using elemental powders in a conventional press and sinter powder metallurgy process, are examined. Microalloying with 100 ppm of lead or tin enhances the sintering response of these alloys significantly, with a corresponding increase in the tensile strength. The system has aspects of both transient and supersolidus liquid phase sintering. Zinc melts and eutectic liquids form during heating to the sintering temperature but these liquid phases are absorbed by the aluminium on further heating. Sintering above the solidus induces the formation of additional liquid. Because it has aspects of both transient and supersolidus liquid phase sintering, the system is extremely sensitive to process variables, including particle size, sintering temperature, and heating rate, but insensitive to green density. When the alloy composition and the process variables are optimised, tensile strengths in excess of...

Freeform fabrication of aluminum metal-matrix composites

A series of metal-matrix composites were formed by extrusion freeform fabrication of a sinterable aluminum alloy in combination with silicon carbide particles and whiskers, carbon fibers, alumina particles, and hollow flyash cenospheres. Silicon carbide particles were most successful in that the composites retained high density with up to 20 vol% of reinforcement and the strength approximately doubles over the strength of the metal matrix alone. Comparison with simple models suggests that this unexpectedly high degree of reinforcement can be attributed to the concentration of small silicon carbide particles around the larger metal powder. This fabrication method also allows composites to be formed with hollow spheres that cannot be formed by other powder or melt methods.

Effect of particle size ratio and particle clustering on sintering and stiffness of aluminium matrix composite

Abstract A metal matrix composite with a matrix composition of Al–3·8Cu–1Mg–0·75Si–0·5Sn was fabricated by sintering a particulate ceramic reinforcement with elemental metal powders. The sintering characteristics were examined as a function of reinforcement volume fraction, particle size and particle size ratio. At high volume fractions of reinforcement, densification ceased completely and the compacts expanded during sintering. Coarse reinforcement particles, not large particle size ratios, maximised the sintering response. Clustering of the ceramic was studied using Dirichelet tessellations of sintered microstructures. While clustering is evident in the sintered microstructures, it does not correlate to sintered density through the particle size ratio because the ceramic particles appear to rearrange in the presence of a large volume of sintering liquid. The effect of particle size is not due to clustering but maybe due to ceramic particle surface area.

Aluminium powder metallurgy

Conventional press and sinter aluminium powder metallurgy is a well-developed cost-effective process for net-shaped fabrication of complex parts via die compaction and sintering. This chapter provides an overview of the PM process in general and sintering in particular. Key issues in the PM processing of aluminium are considered, including the roles of magnesium and the atmosphere on sintering and how these factors affect densification and the microstructure.

Distortion in a sintered 7000 series aluminium alloy

Abstract The 7000 series aluminium alloys processed using elemental powder mixtures are prone to distortion, which is manifest as hourglassing or waisting in cylindrical specimens. By characterising the density distribution using hardness measurements, it is shown that the green density is not evenly distributed through a part, even though aluminium is relatively soft and readily compacted. Because the density equilibrates during sintering, the non-uniform green density leads to distortion. The cause of this distortion is a result of differential shrinkage, which occurs during sintering as well as on solidification during cooling from sintering. Distortion can be controlled by increasing the compaction pressure, which homogenises the green density and does not affect the tensile properties.