金属粉末の衝撃成形における衝撃波伝播解析

Abstract

For the analysis of shock compaction of NiTi alloy powder by using a powder gun, the Hugoniot parameters, duration time of shock pressure, and the positions where a plane shock wave collides with an excessive radial shock wave due to the difference in shock impedance between the powder sample and the copper capsule were calculated on the basis of Rankine-Hugoniot equations, the Mie-Grüneisen equation of state and some assumptions. For the flyer velocities of 0.65 to 1.7 km s−1, the Hugoniot parameters were estimated as a function of the flyer velocity and the formulas on the shock duration time in the powder were derived from the propagation behavior of a plane shock wave. The shock duration time calculated numerically by the formulas depends on the depth from the sample head, and the thickness and the flyer velocity. The maximum value of the shock duration time for the present assembly is limited to about 2μs by the rarefaction wave from the capsule side. The positions where two shock waves collide are generally given by three lines on the cross-section of the sample, which approach the sample side with increasing flyer velocity. The region influenced directly by two shock waves, where the shock energy is deposited in excess, corresponded to the large molten band near the circumference of the recovered sample. The melted mass fraction as a thermal effect of the shock compaction was estimated by the shock energy in order to compare with that in the central region of the recovered sample which is compressed up to solid density by a plane shock wave. The generation of cracks and voids in the melt part between powder particles was discussed by considering the shock duration time and the solidification time of the melt part.The microstructure changes of the recovered samples are good explainable in terms of present numerical results, and the present method is taken to be effective for the analysis of shock compaction process.