Fabrication and characterization of Al, AlMgSi, and AlSi foams

Abstract

Manufacturing of metal foams by using powder compact melting (PCM) method, with the most important considerations like material and method selection, powder blending, cold compaction, hot compaction and hot extrusion and foaming is explained briefly. The effects of Al2O3 particle addition on the heat treatment, cell structure and mechanical properties of AlMgSi foams were studied. Alloy and composite foams were manufactured by powder compact melting (PCM) method. A pre-blended mixture of Al, Mg, Si and Cu representing the wrought AlMgSi alloy (6061) were mixed with heat treated TiH2 and Al2O3 ceramic particles, hot compacted and foamed at temperatures between 750 and 800 °C. The amount of <20μm sized Al2O3 particles in the composite foams were 3, 5 and 10 per cent by volume. The effects of different heat treatments on the microhardness of the foams were investigated. Foams that were fully heat treated had the highest hardness values and they performed best with an increase in collapse strength up to 100 per cent over the untreated samples. It was found that the addition of Al2O3 did not affect the hardenability but the strength and the compression stiffness of the composite foams were increased with 3 and 5 vol. per cent Al2O3 addition. This was attributed to the improved cell structure and decreased drainage when the ceramic amount is not more than 5 per cent. The compression test results were interpreted in terms of the foam’s microstructure and correlations were made relating the unloading modulus and compression strength of the foams with the relative density. It was found that the foams were inhomogeneous and their mechanical properties were close to those expected from open cell foams. An important phenomenon in Al foam production is stabilization of the structure. Especially it is very difficult to obtain highly expanded foams made from pure Al powder. In this study, an improvement in the stabilization was achieved by Al2O3 and B4C ceramic powder addition to the Al-TiH2 mixtures. Compaction of the mixture was achieved by hot extrusion. Extruded dense semi-products (precursors) were foamed at 800 °C and the macro and microstructures of the foams were analyzed. It was found that both of the ceramics increased the number of cells and cell size homogeneity of Al foams. The stabilization seems to be enhanced with the presence of solid constituents but the mechanism acting could not be understood clearly. Most of the ceramic particles were partially wetted and segregation between the metal and gas interface was observed. The wetting of B4C particles by the aluminum matrix was relatively better and this resulted in smoother foam cell walls when compared with the cell walls of pure and Al2O3 added Al foams. Finally, a different matrix alloy, hypereutectic AlSi (AlSi14Cu2.5Mg0.6) was used for foam manufacturing by PCM method. The alloy has some advantages over the Mg, Cu and Zn added alloys like good wear resistance, high mechanical strength without aging heat treatment and dimensional stability. Foams manufactured were heat treated, in order to analyze the changes in the microstructure and the mechanical properties. The results showed that, heat treatment caused spheroidizing of eutectic Si phases and depending on the density, the compression strength of the foams could be increased up to 50 per cent. The deformation of the foam was similar to brittle foams and densification strain was higher than ductile foams which makes it a good candidate for energy absorption applications.