A new nature of microporous architecture with hierarchical porosity and membrane template via high strain rate collision

This paper presents the formation of an unusual porous structure at Al/Al interface joined by magnetic pulse welding. The porous structure consists of a hierarchical microporous architecture with pore size of less than 2 mu m that represents more than 80% over the whole area, in which 38% of them are sub-micron size pores. It also exhibits ultra-thin wall, sufficiently thin enough to behave as an electron-transparent material with a wall thickness of 50 nm. The formation of this porous structure is attributed to a cavitation process of a molten material in three stages including, (1) nucleation, (2) growth and coalescence and (3) solidification. Further analysis of this cavitation process using a dedicated numerical simulation reveals that the nucleation and growth of pores may arise from vaporization governed by a rapid isothermal and isochoric expansion of liquid metal due to the high rate of depressurization. This result explains the potential mechanism of coalescence for the creation of the open porosity. The difference between cavitation onset and highly developed porous structure is attributed to the difference in depressurization during various welding conditions. Welding performed with high intense collision enables to have large impact pressure, therefore those welding conditions provide higher depressurization gap and severe fluid rupture due to significantly large inherent surface tension of the fluid. A fast solidification at a high cooling rate of 10 degrees C/ns enables to freeze the micropores and resulting with the solidified porous architecture.