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Optimization of macrostructure in aluminium foams

Tan, Serdar
Pure aluminium and aluminium-5wt % TiO2 aluminium foams were produced by powder metallurgy technique with the use of TiH2 as foaming agent. Two sizes of TiH2 were used: 20æm and 3æm. It has been confirmed that high level of compaction is the primary requirement in foaming. It was shown that hot swaging could be used as a method of compaction for foaming as it leads to values close to full density. Pure aluminium foamed at 675°C and 725°C leads to a volume expansion between 90-180 %. A model was developed for pure aluminium to explain the pore initiation and the resultant pore size. The model predicts a critical particle size for TiH2 below which bubbles could not form. The size appears to be in the neighborhood of 30æm for 675°C and 6æm for 725°C and is temperature dependent. Equilibrium pore size appears to be a function of TiH2 particle size and not affected significantly by the temperature of foaming. It has also been shown that depth effect, i.e. hydrostatic pressure of liquid metal, is unimportant in foaming process and can be neglected. According to the model, to produce pores of fine sizes, two requirements must be met: use of fine foaming agent and the use of high foaming temperature. Al-5 wt % TiO2 was foamed at 750°C and 800°C, i.e. at temperatures that yield viscosities similar to pure aluminium. The structure of foamed metal and level of foaming, 120-160%, was similar to pure aluminium. Unlike pure aluminium, internal reactions are dominant feature of TiO2 stabilized systems. Solid content of the system increases as a result of internal reactions between Al-Ti and Al- TiO2. When this change occurs, however, is not known. It is possible that the viscosity of the system may be four times of its original value.