Improvement And Verification Of Yield Model During Sintered Powder Forging Process

Powder forging technology is a sort of new forming technology which combines powder metallurgy with metal material precision forging techniques. This technology can be applied to making parts in high request, high precision, excellent fatigue strength and good tenacity. Because of its low cost, high efficiency and short cycle, lots of attention is paid to this technology by many researchers. In the process of powder upsetting, a variety of factors may restrict the final processes of densification and homogenization. The existing yield models cannot describe the process of powder plastic forming well because of its complex forming mechanism. So the more precise powder forging yield model should be investigated to obtain the better explanation for densification behavior of sintered powder and to offer the theoretical basis for computer simulation of the powder forming process.This paper is mainly about the yield behavior during metal powder forging process based on the method of the theoretical derivation, experiment testing study and Finite Element analysis.To begin with, based on plasticity mechanics of powder sinter porous material, powder body is regarded as compressible continuum, mechanical behavior of plastic forming in powder upsetting forming process was studied. A new expression of plastic Poisson’s ratio and a new yielding model were established via taking the influence of the material parameter of powder and initial relative density of preform body into consideration.Then, on the aspect of experiments, iron powder, copper powder and aluminum powder compaction and upsetting experiments were conducted. The data obtained from experiments was used to draw diagrams of axial strain and relative density, plastic Poisson’s ratio and relative density, the geometry hardening coefficient of powder sintered material and relative density, the yielding spheroidicity surface in different relative density under the yield locus of 3,/- / plane respectively. Compared with other theoretical models, the theoretical value calculated by yield model improved in the paper is more consistent with the experimental value and its accuracy is higher.Finally, by means of simulating iron, copper and aluminum powder upsetting experiments, through the finite element simulation software MSC.MARC, the influence of different process conditions(friction coefficient, height-diameter ratio, the initial relative density of powder body, mold loading speed) on the deformation law of powder compacts is explored. The result shows that energy gradient of plastic deformation increases unsteadily as the values of friction coefficient, loading velocity or unsuitable height-diameter ratio, density gradient of powder increases. The effect of densification is not ideal or uniform. Initial relative density of powder body powder compact has a little effect on evenness of powder compacts, but the increase of the initial relative density can reduce the porosity in body powder and evenness of powder compacts in upsetting. Also, under the same distribution and the maximum density is showed close to top and bottom die. Those spots have the most concentrated stress and strain. The lowest density and the worst densification effect are showed around the drum-shaped spot. Damage cracks appears in the two areas easily and this conclusion is already verified by experiments.