Effect of process variables on foaming characteristics of Al-based foams processed through P/M route

摘要

Gas atomized aluminum based powders (ASTM -72mesh to -325mesh) were mixed with 1 weight percent TiH_2 powder (average size approx 10 mu m ) and vacuum hot pressed to 99.8 percent relative density. Foaming of hot pressed samples was carried out by heating them to viscous state in graphite crucibles. Foam samples with density of 0.2 to 0.6g/cc were prepared. Foaming was not affected by change in aluminium powder size or composition. Foaming was successfully carried out using induction heating. However, heating of hot pressed compacts in a resistance furnace failed to produce foam. Significant agglomeration of fine hydride powder was observed in hot pressed compacts leading to localized increase in pore size in the final foam. Agglomeration of TiH_2 powder could be minimized by milling of Al+TiH_2 powder mix for an hour in an attritor prior to pressing. Use of coarse (approx 65 mu m) TiH_2 powder as foaming agent resulted in aluminium foams with high density (lg/cc). Low density foams could not be obtained when coarse TiH., powder was used. Reducing the hydride particle size from 10 to 2.5 mu m did not have a significant effect on foaming behaviour. However, samples containing 1.5 mu m TiH_2 powder could not be pressed to optimum density and failed to foam satisfactorily. A Low power input during induction heating is necessary to ensure foaming occurs throughout the sample and there are no dense portions left after foaming. Control of L/D ratio of the final foam may be necessary to control size and shape of pores in the foamed samples.