High Velocity Compaction Of 316l Stainless And Iron-based Powders

HVC(high velocity compaction) has the special advantage in forming high-density(7.4~7.8g/cm~3)and large P/M parts (weighing up to 5 Kg), So the continuous breakthrough in its application is expected due to low ejection, low springback and graftibility with other powder metallurgy technology, such as die-wall lubrication and warm compaction and so on. In the paper, 316L stainless powder, Fe-2Cu-1C powder and Fe-C powder were used as raw materials, then the effects of impacting time, die-wall lubrication, double-HVC and double sintering on the method of high velocity compaction and the densification mechanism of the process were studied.As the total impacting energy rose up, the green density of 316L stainless power increased gradually, the green density of compacts made by double impact is higher than that by single impact for 0.02~0.05 g/cm~3. With the increasing of the total impacting energy, the maximal impacting force increased, and it was larger for double impact under the same total impacting energy. But there was no relationship between the impacting energy, the impacting time and the withdraw force. In addition, the shock generates perturbation while the rammer strikes the upper punch. The stress wave appears triangle pulsed waveforms, it presents saw tooth waveforms in the loading stage and the stress wave was smoother in the unloading stage. Green density for HVC depends on the maximal impacting stress and duration, the attenuating of stress wave is the process of densification. Compared with double impact, the response time and the duration prolong for single impact under the identical energy, the first loading time and the first delay time shortens gradually for double impact.Green density and maximal impact force of samples with die-wall lubrication were greater than that without die-wall lubrication for Fe-2Cu-1C under the same compacting velocity, but the withdraw force and springback were lower. Compared to traditional repressing and re-sintering technology, double-HVC and double-sintering is an effective approach to achieve higher densities as cost effectively as possible. For iron-based powder, the best density of pre-compacts and the best pre-sintering temperature were respectively 7.0 g/cm~3 and 780℃。Green densities by the pre-sintering and repressing were higher than that of single impact by 0.5 g/cm~3, the hardness and bending strength of the sintering sample were reached to 32HRC and 839MPa respectively. The similar sintering microstructure exists on the surface of particle of repressing compacts, more and finer pearlites are obtained after double sintering, and the fracture appearular.The densification mechanism of high velocity compaction was preliminarily investigated. Friction among particles and adiabatic shear mainly resulted in temperature rising on the surface of particle during HVC. Due to thermal softening, yield strength decreased gradually, then the compact is beginning to deform secondly, the pore is undergoing plastic collapsing. Meanwhile, the isolated pores were filled by shattering in the partial region, leading to further improvement of density of the compact.