Effect of Relative Humidity on Pressure-Induced Foaming (PIF) of Aluminium-Based Precursors

摘要

Pressure induced foaming (PIF) is a novel technique which provides the possibility of foaming precursors prepared from powders without any addition of blowing agent. Along the manufacturing route the powders are exposed to relative ambient humidity (rH) during storage and compaction. Interaction during storage was studied using mass spectrometric (MS) analysis and heavy water (D_2O) as a tracer on Al, Si, and Al_(12)Mg_(17) powders. It was found that after exposure to 75 percent rH for 21 days at room temperature the water and hydrogen content in the powders is increased. A mixture of as-received powders was prepared to yield an AlSiMg alloy. The mixture was then used to hot-press compacts under three different atmospheres: air, vacuum and 75 percent rH. MS analysis of the three kinds of compacts demonstrated that ambient humidity also interacts during compaction. The most relevant effect is that a humid atmosphere during compaction increases the hydrogen amount available for foaming. PIF experiments on the different compacts are consistent with the MS results. Compacts hot-pressed under vacuum yielded an area expansion factor of 1.6 and the most uniform pore structure. Compacts exposed to 75 percent rH during compaction produced an area expansion factor of 3.5 and the least uniform pore structure. For those compacts, big pores were formed as the result of cracking during early stages.