Shock consolidation: microstructurally-based analysis and computational modeling

The most important microstructural processes involved in shock consolidation are identified and discussed; the energy dissipated by a shock wave as it traverses a powder is assessed. The basic microstructural phenomena are illustrated for a metal (nickel-based superalloy), an intermetallic compound (rapidly solidified Ti 3Al), and ceramics (silicon carbide). Interparticle melting, vorticity, voids, and particle fracture are observed and the plastic deformation patterns are identified. Various energy dissipation processes are estimated: plastic deformation, interparticle friction, microkinetic energy, and defect generation. An analytical expression is developed for the energy requirement to shock consolidate a powder as a function of strength, size, porosity, and temperature, based on a prescribed interparticle melting layer. This formulation enables the prediction of pressures required to shock consolidate materials; results of calculations for the superalloy and silicon carbide as a function of particle size and porosity are represented. The fracture of ceramic particles under shock compression is discussed. Tensile stresses are generated during compaction that may lead to fracture. It is shown that the activation of flaws occurs at tensile reflected pulses that are a decreasing fraction of the compressive pulse, as the powder strength increases. These analytical results are compared to numerical solutions obtained by modeling the compaction of a discrete set of particles with an Eulerian finite element program. These results confirm the increasing difficulty encountered in shock consolidating harder materials, and point out three possible solutions: (a) reduction of initial particle size; (b) reduction of shock energy; (c) post-shock thermal treatment. Two possible and potentially fruitful approaches are to shock densify (collapse voids with minimum bonding) powders and to apply post-shock thermal treatments, and to shock consolidate nanosized powders. The latter method requires high shock energy and careful minimization of the shock reflections.