Experimental study on the apparent viscosity of aluminum melt foam

The rheological behavior of the aluminum melt foam dictates the final porous structure of solidified material. To date, experimental investigations into the flow properties of aluminum melt foams remain limited. In this study, a customized rotational viscometer was developed to quantitatively assess the apparent viscosity of aluminum melt foam. Melt foams with various gas volume fractions were prepared via the direct foaming method and subjected to rate-controlled testing across different rotational speeds. The results demonstrate a non-monotonic relationship between apparent viscosity and gas volume fraction, characterized by an initial increase followed by a subsequent decrease, peaking at a volume fraction of ∼70% under low rotation speeds. At lower gas fractions, bubble jamming increases the resistance to bubble rearrangement; however, beyond a critical threshold, progressive film thinning induces rupture under shear loading, leading to a significant drop in viscosity. Furthermore, in our experimental conditions, the melt foam exhibits pronounced shear-thinning behavior, where the mechanical response transitions from a solid-like to a liquid-like state beyond a shear rate of ∼2 s−1. Morphology analysis of the pore structure reveals that high-rate shearing locally refines pores and increases the liquid fraction. This mechanism mitigates the resistance to bubble rearrangement, thereby yielding macroscopic fluid-like behavior. These findings provide fundamental insights into the rheology and structural evolution of aluminum melt foams under shear flow, proposing a viable strategy for tailoring the gradient pore structure of aluminum foams.