Research of Mechanical Mechanism in Uniaxial Pressing Process of Powder Metallurgy
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摘要: 粉末冶金是一项节能、节材、高效、近净成形的先进制造技术，粉末冶金压制过程中的力学理论是决定压制毛坯的致密性和均匀性，并对最终形成产品的质量有决定性影响的关键问题。本文利用离散单元法建立了粉末冶金单轴压制的离散元数值模型，研究了粉末颗粒单轴压制过程中的力链演变规律、应力变化规律及配位数、滑动分数等力学参数的变化规律。研究结果表明，随着上模的逐步下压，粉末颗粒间的细观力链的强度逐渐增强，压制后期细观力链也呈现出明显的沿y轴走向的方向性特性；x、y向应力在压制初期较小，压制后期粉末颗粒系统的x、y向应力大小随着上模的逐步下压基本上呈线性增长的变化趋势；配位数在压制初期较小，在压制后期则随着上模的逐步下压而逐渐增大，滑动分数在压制初期和后期较小，在压制中期则较大。Abstract: Powder metallurgy was an advanced manufacturing technology in saving energy, saving material, high efficiency and near-net forming, while the mechanical theory of powder metallurgy in the pressing process was the key problem that determined the compactness and uniformity of the pressed blank and has a decisive influence on the quality of the final product. A numerical analytical model for uniaxial pressing of powder metallurgy was established via discrete element method (DEM). Using the present model, the force chain evolution law, the stress variation law, as well as the variation of coordination number and the variation of sliding fraction were studied. The research results showed that the strength of microscopic force chain among powder granules gradually increased with the descending of upper die. In addition, the meso-force chains in the later pressing stage also showed obviously directional characteristics along the y-axis. The stress in x and y direction was smaller in the initial pressing stage, moreover, the stress in x and y direction increased linearly with the descending of upper die in the later pressing stage. The coordination number was smaller in the initial pressing stage, moreover, the coordination number increased with the descending of upper die in the later pressing stage. The sliding fraction was smaller in the initial and later pressing stage, moreover, the sliding fraction was larger at the middle pressing stage.
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Key words:
- powder metallurgy /
- force chain evolution /
- coordination number /
- sliding fraction /
- stress variation
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表 1 模型参数
参数名称 数值 上模切向刚度ksp 200 GPa 墙切向刚度ksd 200 GPa 上模摩擦因数fp 0.25 颗粒密度ρ 7 850 kg/m3 颗粒泊松比ν 0.26 颗粒数量N 1 200 上模法向刚度knp 200 GPa 墙法向刚度knd 200 GPa 墙摩擦因数fd 0.25 颗粒弹性剪切模量G 206 GPa 颗粒摩擦因数fg 0.25 下压速度u 0.1 m/s 
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