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

Hierarchical Morphology and Formation Mechanism of Collision Surface of Al/Steel Dissimilar Lap Joints via Electromagnetic Pulse Welding

“…The rapid expansion induced by such an interfacial depressurization promotes the radial growth of each micropore to form a spherical pore, and the solidification shrinkage also continuously contributes to the coalescence of pores, resulting in the formation of a regionalized porous structure, as schematically shown in Table 3. In addition, a similar thin melting membrane porous structure was also reported in [20], which could be an amorphous structure without grain boundaries generated under high strain, local heating, and rapid solidification. Further studies would be to explore how the porous structures are formed via vacuum electromagnetic pulse welding, where the interference of air could be precluded.…”

“…The further magnified view of zone D in Figure 4h is shown in Figure 4i, displaying a distinct T-shaped crack. In the process of EMPW, a peak temperature up to 2750 • C could be reached [18] at a rapid heating rate of~10 9 • C/s [19], followed by an ultra-fast cooling rate of~10 10 • C/s [20]. Such an instant local melting followed by swift solidification would result in large residual stresses causing microcracks and even thin amorphous layers.…”

“…As the energy dissipates and the collision angle increases, the tremendous quantity of heat arising from severe plastic deformation, Joule heating, collision energy dissipation, and supersonic gaseous compression [20,31,32] give rise to the local melting in the weldingaffected zone or layer IV, accompanied by porous structures and splashed metal debris, as shown in Figure 5. In this stage, the surface tension of melt, dissolved or trapped air, instant high temperature, and pressure provoked by a high-velocity impact (being as high as 1000 m/s [23]) would affect the formation of porous structure and the further interfacial bonding.…”

“…In this stage, the surface tension of melt, dissolved or trapped air, instant high temperature, and pressure provoked by a high-velocity impact (being as high as 1000 m/s [23]) would affect the formation of porous structure and the further interfacial bonding. Qu et al [33] reported that the heterogeneous porous structure could be interpreted as a result of dissolved air in the molten metal; thereby, the trapped air nucleates to form micropores at the high velocity and pressure, then gradually grows into the characteristic porous structure with the rapid cooling (~10 10 • C/s [20]) of the molten mass. Rice and Tracey established a void growth model, where the rate of growth depends on the hydrostatic stress, von Mises norm of the Cauchy stress tensor (the equivalent stress), and equivalent plastic strain [34,35].…”

“…First, the cavitation phenomenon happens to maintain the conservation of the total volume in the material subjected to an instantaneous local melting due to an ultra-high heating rate of 10 9 • C/s [19], followed by the rupture of melt due to a significantly large inherent surface tension, leading to the nucleation of micropores. Then, the interface undergoes a sudden decrease of temperature at a rate of as high as~10 10 • C/s, accompanied by an ultra-high depressurization rate in the order of~10 11 MPa/s [20]. The rapid expansion induced by such an interfacial depressurization promotes the radial growth of each micropore to form a spherical pore, and the solidification shrinkage also continuously contributes to the coalescence of pores, resulting in the formation of a regionalized porous structure, as schematically shown in Table 3.…”