The Influence of Alloy Composition and Liquid Phase on Foaming of Al–Si–Mg Alloys

Abstract

The foaming behaviour of aluminium alloys processed by the powder compaction technique depends crucially on the exact alloy composition. The AlSi8Mg4 alloy has been in use for a decade now, and it has been claimed that this composition lies in an “island of good foaming”. We investigated the reasons for this by systematically studying alloys around this composition by varying the Mg and Si content by a few percent. We applied in situ X-ray 2D and 3D imaging experiments combined with a quantitative nucleation number and expansion analysis, X-ray tomography of solid foams to assess the pore structure and pore size distribution, and in situ diffraction experiments to quantify the melt fraction at any moment. We found a correlation between melt fraction and expansion height and verified that the “island of good foaming” actually exists, and foams outside a preferred range for the liquid fraction—just above TS and between 40–60%—show a poorer expansion performance than the reference alloy AlSi8Mg4. A very slight increase of Si and decrease of Mg content might further improve this foam.

Controllable Design of Structural and Mechanical Behaviors of Al–Si Foams by Powder Metallurgy Foaming

“…It is evident that the formation of all Al foams made by PMF method at cellular level is followed by four stages: bubble nucleation, growth, merger, and collapse. [ 40–43 ] Likewise, the Al foams’ expansion process can be divided into four corresponding parts: 1) initial expansion stage, 2) rapid expansion stage, 3) stable expansion stage, and 4) decline stage (Figure 2a). With the increasing Si contents, the duration time to reach maximum expansion rate is significantly delayed (Figure 2b,c).…”

“…Moreover, the possibility of film rupture decreases and finally delays the beginning of bubbles merging. [ 40 ] Among these four kinds of samples, AlSi9 has the maximum expansion rate of 463.6%. However, when Si contents exceed a certain threshold of 12 wt%, the maximum expansion rate declines because the formation of cellular films is impeded by high flowability of melt and the expansion rate is limited.…”

Temperature dependence of film rupture and internal structural stability in liquid aluminium alloy foams

“…For the AlSi8Mg4 alloy with a eutectic temperature of 558 °C, which is markedly below the lowest holding temperature of 580 °C, this effect is not observed. It also differs from AlSi9 in that a larger amount of liquid phase is present at low holding temperatures [57].…”

Cellular Metals: Fabrication, Properties and Applications

“…Seven contributions focus on the advances in production and manufacturing technologies for aluminum alloy foams and the understanding of the foaming behavior and the fundamental relationships between their structure and their properties, with some even exploring nano-sized reinforcement elements. García-Moreno et al [1] investigate the influence of the alloy composition and the role of the liquid phase and its influence on the gas nucleation and foaming of Al-Si-Mg alloys prepared by the powder metallurgy method using in-situ X-ray radioscopy, X-ray tomography, and X-ray tomoscopy. Lehmhus et al [2] analyze the effects of the solidification rates and addition of Sr, B, and TiB2/TiAl3 on the foaming behavior and microstructural, morphological, and mechanical properties of aluminum foams fabricated by the powder metallurgy method.…”