Foaming without blowing agent – Pressure induced foaming (PIF)
Pressure Induced Foaming (PIF) of metals is a novel foaming method based on the Powder Metallurgical (PM) foaming route. PIF, in contrast to the traditional PM route, does not need any extra blowing agent admixed to the metal powders. The precursor can be produced by uni-axial hot-compaction, extrusion, etc. The method consists of heating up the precursor above its melting temperature in a gas-tight furnace under a certain gas overpressure, e.g. Al to 680 °C under 9 bar. After the precursor is melted, foaming is easily induced by controlled release of the overpressure, e.g. to atmospheric pressure. This way the gas contained in the precursor nucleates during melting and expands as consequence of the outer pressure decrease, producing a metallic foam. Different cover gases like air or argon, different alloys like Al99.7 and AlSi7, and different overpressures up to 9 bar were successfully tested. In-situ X-ray radioscopy allowed us to follow the melting, gas nucleation, and foaming phases during the process, and perform quantitative analysis. We found out that adsorbates on the powder surfaces provide the most gas volume to PIF. Several advantages of this new method are a very homogeneous pore size distributions and a very precise and easy adjustment of expansion, density and pore roundness. In addition, it is noted that the pressure reduction rate dp/dt allows controlling the foaming time, from less than one second to several minutes. The PIF method has been patented and seems to be relevant from the application point of view. From the scientific side it helps to understand the role of the blowing agent during foaming due to comparison.