Investigation On Densification Mechanism And High Velocity Compaction Of Warm316L Stainless Steel Powders

HVC（high velocity compaction）has the special advantage in forming high density andlarge P/M parts, and it has wide application areas due to high density, low withdraw force andlow springback etc. In order to develop high density powder metallurgy forming technology,a new concept combining high velocity compaction and warm compaction called warm highvelocity compaction (WHVC) was presented. A new warm high velocity compaction formingequipment which adopts gravitational potential energy was designed and developed. Afterbeing improved, this equipment reached the experimental requirements. Dr. Guan designednew warm high velocity compaction forming apparatus which adopts mechanical springsinstead of hydraulic cylinder as hammer driver. In this paper, by means of these two devices,316L stainless steel powders were regarded as raw material. Then the effects of the impactvelocity, temperature, hammer mass, H/D, powder mass and particle size on the green density,the green strength, the sintered density and the tensile strength of sintered compacts werediscussed and the densification mechanism of the process was studied. The main results areas follows:As the impact velocity, temperature and hammer mass rise up, the green and sintereddensities, the green strength and tensile strength of samples increase gradually. When impactvelocity increases from2m/s to18m/s, the compact density increases from5.13g/cm~3to7.61g/cm~3at room temperature, and when impact at160℃, with the same velocity, compactdensity increases from5.43g/cm~3to7.76g/cm~3. The influence of temperature on the density isless than the impact velocity. When impact velocity increases from10m/s to18m/s, thestrength of samples compacted at room temperature increases from62.1MPa to89.3MPa,the strength of samples compacted at160℃increases from72.5MPa to94.1MPa. There isno certain relationship between radial springback of green compact and impact velocity. Asthe green density rises up, the increment density decreases. At the same impact velocity,temperature and hammer mass, as the H/D rises up, the green strength, the green and sintereddensities decrease gradually. The green density at top is the highest, then the middle, and thelast for bottom. At the same impact energy, the green and sintered densities decreasegradually as the powder mass rises up. As the impact energy per weight rises up, the green and sintered densities increase. At the same impact velocity, the highest green and sintereddensities of samples can be achieved for the powders of-200mesh, while the second for thepowder of-100mesh and the last for the powders of-325mesh. For the mixture of-200+325mesh and-325mesh, when-325mesh powders proportion of10wt%, the green density andsintered densities are the highest. When impact energy is lower, the green density made bydynamic loading facility is higher than that made by mechanical springs HVC experimentalequipment, but when impact energy is higher, the green density are basically the same.The densification mechanism of (warm) high velocity compaction was preliminarilyinvestigated. The research found that the impact pressure is greater than static pressure in theHVC, and when the impact velocity increasing, the impact pressure is greater, then thesuppression effect is more obvious. In the HVC, powders’ plastic deformation andfragmentation phenomenon can be found, and there have cold welding, that increases thegreen strength. There are a lot of crystal twin in the powder particles. Through the XRD ofgreen samples, we find that the diffraction peak width of crystal face at half height increaseswith impact velocity, which indicates that as the impact velocity rises up, powders’ plasticdeformation increase. In the process of HVC, the temperature effect caused by friction,plastic deformation and heat insulation is beneficial to the densification of powders.