Study On Fabrication Of Al Foams By Powder Metallurgy

Aluminum foams is a kind of metal material with pores included. It can be used as engineering material as well as functional material. In this dissertation, the powder metallurgy process for making Al foams was investigated and the dynamics of foaming was discussed.The effects of compaction and sintering process on the foamability of Al were investigated. Aluminum will not foam with compaction force below 400 MPa and sintering time less than 40 minutes.The effect of foaming process parameters on the porosity of Al foams was investigated. The porosity rises with increasing foaming temperature, prolonging foaming time and increasing concentration of foaming agent. The porosity increases from 56 % to 85 % when temperature increases from 680 ℃ to 730 ℃. The porosity increases from 68 % to 83 % when foaming time ranges from 8 minutes to 20 minutes. The porosity increases from 58 % to 83 % when the concentration of foam agent increases from 0.6 wt% to 3 wt%.The effect of foaming process parameters on the structure of Al foams was investigated. The foam structure becomes uneven with increasing temperature. The bubbles grow up, coarsen and rupture with prolonging foaming time. More bubbles coarsen and rupture with increasing concentration of foaming agent.The most optimum process for making aluminum foams by powder metallurgy was concluded to be:, firstly, the powder mixture is compacted at 400 MPa; secondly, the precursor is sintered at 460 ℃ for 40 minutes and is compacted again at 150 MPa; lastly, the precursor was foamed at 720 ℃ for 10 minutes.The thermodynamics and dynamics of sintering process have been analyzed. The use of finer Al powder can accelerate sintering. The addition of Mg also can facilitate sintering, for Mg reacts with oxide film on the surface of Al powder with formation of MgAl2O4 , which breaks the oxide film.The dynamics of foaming for Al foam was discussed. The foam evolution process includes nucleation, expansion and rupture of bubbles. The dimensions of active pores for bubble nucleation range from 0.000087 to 2.1 milimeter. The expansion velocity of bubbles is determined by the diffusion velocity of hydrogen through boundary layer in the molten aluminum. The stability of bubbles is mainly governed by the properties of liquid-gas interface. The rupture of bubbles will be retarded with less surface tension, higher viscosity of molten liquid and stronger surface adsorption of bubbles. Among these factors, the effect of surface adsorption on the stability of bubbles is considered to be most important.