Abstract
For AlSi7 foams, microstructure modification by variation of solidification rates and addition of Sr, B and TiB2/TiAl3 was investigated and its transfer to powder metallurgical metal foaming processes demonstrated. Microstructural characterization focused on grain size and morphology of the eutectic phase. Cooling rates during solidification were linked to secondary dendrite arm spacing, establishing a microstructure-based measure of solidification rates. Effects of refining and modification treatments were compared and their influence on foam expansion evaluated. Studies on foams focused on comparison of micro- and pore structure using metallographic techniques as well as computed tomography in combination with image analysis. Reference samples without additives and untreated as well as annealed TiH2 as foaming agent allowed evaluation of pore and microstructure impact on mechanical performance. Evaluation of expansion and pore structure revealed detrimental effects of Sr and B additions, limiting the evaluation of mechanical performance to the TiB2 samples. These, as well as the two reference series samples, were subjected to quasi-static compression testing. Stress-strain curves were gained and density-dependent expressions of ultimate compressive strength, plateau strength and tangent modulus derived. Weibull evaluation of density-normalized mechanical properties revealed a significant influence of grain size on the Weibull modulus at densities below 0.4 g/cm3.
Highlights
IntroductionMetallic foams based on the powder compact melting or Fraunhofer process have reached sufficient levels of maturity to allow series production, e.g., for applications in the transport, machine tool and even the building industry [1,2,3,4]
The diagram confirms that refining and modifying additions can reduce grain size significantly even at the highest cooling rate that could be realized by water quenching
Though in principle successfully applicable, the results obtained show that grain refinement of the type employed here cannot lead to grain sizes that match or undercut the dimensions of typical cell walls
Summary
Metallic foams based on the powder compact melting or Fraunhofer process have reached sufficient levels of maturity to allow series production, e.g., for applications in the transport, machine tool and even the building industry [1,2,3,4]. The underlying process is based on hot compaction of a mixture of matrix metal, usually aluminum or an aluminum alloy, and foaming agent powders, usually. The resulting precursor material is heated above its melting point and expands to yield a liquid foam, which is stabilized by solidification. The method was patented in 1990 by Baumeister et al and has since been described in several publications [5,6].
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