Optimization of Mg Blowing Agent Content for Foaming Aluminum

Abstract

Mg-based blowing agents exhibit the potential to yield aluminum foams with better structure and properties than those achieved by using conventional blowing agents. However, all the studies to date used a high amount of such blowing agents (e.g., 15 wt pct Mg) for foaming aluminum. In this study, we investigate the minimum amount of Mg blowing agent required for foaming. Al-Si13-MgX (X = 2.5–15 wt pct) alloy foams were produced employing the powder metallurgy route where Mg acted as the blowing agent. The macro- and microstructure of the foams were analyzed using X-ray tomography, microscopy, and X-ray diffraction. The foams were subjected to hardness and compression tests to evaluate their mechanical properties. Deformation behavior was studied in situ by monitoring the foam surface during the compression test. For the first time to our knowledge, the present study established that 5 wt pct of Mg is sufficient to achieve foam expansion similar to that achieved by 15 wt pct Mg. Moreover, the structural and mechanical properties of the 5 wt pct Mg-containing foams were much superior to the 15 wt pct Mg-containing foams. However, the highest strength was obtained using 10 wt pct of Mg. Many cracks were observed at the early deformation stages of 10 and 15 wt pct Mg-containing foams. We correlate the Mg content with the structure, properties, and deformation behavior of the foams.

Graphical abstract

Appendices

Appendix A

Calculation of Hydrogen Content Present in the Form of Adsorbed Water on Metal Powder Surface

Both Al and Mg powders adsorb water on their surface. The particle size of Al powder used in this study is 23 μm (D₅₀ value). Based on the data presented in Reference 37, the BET surface area of the Al powder used in this study can be taken as ≈ 0.4 m²/g. The surface area of Mg is 1.89 m²/g, as reported in a previous study.^[16] Therefore, it is safe to assume that the overall surface area of the Al and Mg powders used in this study is not < 0.4 m²/g. If we consider Si powder does not adsorb any water, only 87 wt pct of the powder contains adsorbed water. Therefore, in a 100 g precursor, the effective surface area that adsorbs water is ≈ 0.4 × 87 m². Water molecule density present in a monolayer of water adsorbed on metal powder surface is ≈ 10¹⁹ mol/m².^[58] Therefore, the number of moles of water present in a 100 g precursor is ≈ 10¹⁹ × 0.4 × 87. Since there could be up to ten monolayers of water,^[38] the maximum possible number of water molecules is 10¹⁹ × 0.4 × 87 × 10 or 3.48 × 10²¹. N (N is Avogadro’s number, 6.023 × 10²³) number of water molecules contains 2 g of hydrogen. From this, the amount of hydrogen that could be present in the form of adsorbed water layer in a 100 g precursor was estimated to be (3.48 × 10²¹ × 2)/(6.023 × 10²³) ≈ 0.01 g or 0.01 wt pct.

Appendix B

Derivation of Eq. [2]

According to Onck et al.,^[57]

$$ \frac{{\sigma _{{\text{p}}} }}{{\sigma _{{{\text{bulk}}}} }} = \frac{{\left( {\alpha - \frac{1}{2}} \right)^{2} }}{{\alpha ^{2} }} $$

(B.1)

where

$$ \sigma _{{{\text{bulk}}}} = \frac{{2\sigma _{y} }}{3}\left( {\frac{t}{l}} \right)^{2} $$

(B.2)

and

$$ \alpha = W/d $$

(B.3)

Substituting Eqs. [B.2] and [B.3] in [B.1],

$$ \frac{{\sigma _{{\text{p}}} }}{{\sigma _{{\text{y}}} }} = \frac{2}{3}\left( {\frac{t}{l}} \right)^{2} \frac{{\left( {W/d - 0.5} \right)^{2} }}{{\left( {W/d} \right)^{2} }} $$

(B.4)