Optimization of Porosity in Cast Aluminium Foams

Abstract

Closed-cell ultra-light aluminium foams are often used as structural materials in aerospace, railways and automobile sectors as they provide good strength to weight ratio with high impact strength and corrosion resistance. Melt -based metal foams can be casted using indirect foaming techniques, wherein gas bubbles are created inside the melt which has been pre-treated in a suitable way. The properties of these metal foams are directly affected by nature of porosity, which is difficult to measure and control experimentally. Therefore, theoretical solutions have to be developed to control this porous structure. In the present work, a aluminium foam, prepared using foaming method using blowing agents with porosity ~86%, is used, where the effect of melt viscosity and solidification time on gas bubble size and its rising velocity inside melt have been theoretically studied. The calculations showed that rising velocity of hydrogen gas bubbles, aluminium melt viscosity and solidification time are responsible for motion of bubbles in the aluminium melt. Control of this movement of bubbles can result in optimum porosity in the solidified foam. A relationship has been established for viscosity enhancement due to the addition of additives like Ca, Al2O3, SiC, TiB2, SiO2, BN and their influence on bubble size and its rising velocity during solidification of this foam. Overall, the study suggested amount of additives that are required to be added in a given volume of aluminium melt, to increase melt viscosity and to maximize bubble entrapment. The results also showed that a cooling time of ~8 s leads to efficient bubble entrapment with uniform pore structure and hence properties of aluminium foam.