Study On The Effect Of Particle Size Distribution Of Metal Powder On The Relative Density

Due to the different particle size distribution of metal powder,the compacting behavior will be different,which leads to different compacting properties such as relative density that is the key property of powder metallurgy products.In order to solve the problem that the standard of particle size distribution of metal powder leading to the limited increase of density of compacted products is not strict,this paper aims to study the effect of particle size distribution of metal powder on the relative density of compact to promote the performance of powder metallurgical products.In this paper,the law of iron powder densification is studied by means of numerical simulation of three-dimensional particle compaction with discrete element method software PFC and compaction experiment,and the suggestion on the revision of particle size distribution standard for powder compounding is put forward,which improves the relative density of iron powder.The following conclusions are obtained.?1?A three-dimensional particle compaction model was established by using PFC software.The model material parameters were calibrated by comparing the mechanical response of the experiment,and then the model of different particle size distribution was established by using this micro-material parameters.The stress-strain law in the process of pressing is obtained.The increase of stress is from slow to fast.When the axial stress is the same,the axial strain of single particle size powder is smaller than that of mixed powder,and the change trend of radial stress and axial stress is the same,but the value of radial stress is smaller than that of axial stress.Furthermore,the lateral pressure coefficient and Poisson's ratio are obtained.It is found that the distribution of powder particle size has little effect on them.Both of parameters above are lower in the initial stage of compaction because the main direction of compaction is axial.When pressing is basically equal in axial and radial,the radial stress is linearly related to the axial stress.?2?The velocity propagation law of powder particles is consistent with the formation of force chain,at the same level,the force chain is first disturbed near the center of the compact.The particle velocity propagation near the die wall is lag and the force chain formation is slow and imperfect.Uniaxial pressing will lead to a decrease in the number of force chains away from one end of the compression force,especially the number of force chains on the side of the end and the weak connection between powder particles.The experiment shows that the surface of the compact is rougher than that of the end of the compression.The number of force chains of mixed particle size powder is much more than that of single particle size powder.Due to the interaction of a large number of weak force chain and strength chain,the surface relative density of compact consisted of the mixed particle size powder is larger than that of single particle size powder.?3?According to the requirement of the experiment,a set of dies was specially designed,and the compacting experiment was carried out after pre-sieving and compounding the mixed size iron powder.Comparing the experimental results with the numerical simulation,it was found that the relative density of the compact consisted of the single particle size powder was the lowest.The relative density of compact consisted of the mixed particle size increases with the increase of the additional fine powder,but too much fine powder will lead to the lack of strong chain correlation among the powder particles,which leads to decrease the density.By comparing the compressibility of each sample,the suggestion on the revision of particle size distribution standard was put forward.The relationship between the size distribution interval and the grade was clarified by introducing the particle size coefficients?₁,?₂ and?₃.The mass fraction of each particle size range is as follows:the proportion of the base powder whose particle size is?₁×D is 60%,the proportion of the additional medium diameter powder whose particle size is?₂×D is 15%,the proportion of the additional fine powder whose particle size is?₃×D is 25%,and D is the particle size corresponding to the classification number of the powder.The particle size coefficients are 0.7<?₁?1.0,0.3<?₂?0.7 and 0<?₃?0.3,respectively,and the revised standard is suitable for 80 mesh,100 mesh and 150 mesh iron powder,respectively for compounding powder.The density was increased by 0.12%⁰.35%compared with the original standard.?4?In order to study the effect of particle arrangement on density in numerical simulation model,5 kinds of ideal distribution models of powder particles were preliminarily discussed.It was found that the porosity of the ideal distribution model of the particle could be reduced to the range of 19.01%²8.60%by filling the additional group of powder particles,which leads to the number of coordination increased.The highest relative density reached 80.99%.