Multi Scale Numerical Simulation Of Warm Compaction Of Iron Powder

With the rapid development of powder metallurgy(PM)technology,the PM products have been widely applied in the many areas such as mechanics,automobiles,aeronautics etc.In the PM production process,powder compaction can create significant effects of the final property of the compact.In traditional cold die compaction process,the temperature effects are not considered and the performance of the compact is not high.While the powder warm compaction can obtain higher relative density and strength in the compact with low compaction pressure through the utilization of temperature,which can increase the production rate and improve the quality of the product.In this thesis,the commercialized MSC.Marc finite element method(FEM)software was used to model the two-dimensional(2D)warm compaction of iron powder from both the macro and micro scales.Firstly,the simulation on the warm compaction of iron powder in the die was carried out by using continuum mechanics based FEM method,where the iron powder in the die is regarded as the continuum.The relationship between the relative density of the compact and the compaction pressure was obtained,and the effects of the temperature on the relative density of the compact were analyzed.Meanwhile,the powder flow behavior,stress and strain,residue elasticity as well as the densification mechanism during compaction were studied.Secondly,the coupled discrete element method(DEM)and FEM were conducted from particulate scale to simulate the warm compaction of iron powder.The study was mainly focused on the effects of operating parameters such as compaction pressure and temperature etc.on the compaction densification.And the macro and various micro properties of the obtained compact were characterized and analyzed.Meanwhile,the compaction densification mechanism was identified.The results show that:(1)During the warm compaction of iron powder,when the temperature is being fixed,the relative density of the compact increases with the compaction pressure.The larger the temperature,the higher the relative density with the same compaction pressure,the more uniform the relative density distribution.While,the temperature should not be too large,because extra high temperature can destroy the structure of the lubricant,which has been proved in the physical experiments.Therefore,in current simulation,the temperature range of 20-100℃ was chosen for study.(2)In the simulation on the warm compaction of iron powder from continuous scale,the axial displacement of the powder changes with the height.Due to the effect of die wall friction,the axial displacement of the iron powder even at the same height is also different,it is lagged at the region close to the die wall.Meanwhile,the variation of the radial displacement indicates that the iron powder moves from dense packing area to loose packing area during compaction,and compared with the axial displacement,the magnitude of the radial displacement is much less.(3)In the simulation of warm compaction of the iron powder from particulate scale,when the temperature is fixed,the contacts between iron particles change from point to line contact with the increase of the compaction pressure.And with the point contact,the force network formed by normal forces is simple,where the iron particle underneath the force chain formed by "arching" or "bridging" structure of local particle packing is shielded without deformation.With the increase of the compaction pressure,the point contacts between iron particles become line contact;in this case,the force chain structure in the compact becomes more complicated.The destruction of the old force chain formed by "arching" or "bridging" structure makes the plastic deformation of the underneath iron particle through the suffered action forces from its neighboring particles,which completes the adjacent void filling and realizes the densification.(4)During warm compaction,with the increase of the temperature,the relative density of the compact increases accordingly,while the increasing rate gradually decreases.When the temperature is between 80-90℃ and compaction pressure is between 700-800MPa,the warm compaction effects are optimal,which is in good agreement with the physical experiments.