基于离散元法的固体颗粒介质传力特性研究

Research on Pressure-transfer Characteristics in the Solid Granule Medium Forming Based on the Discrete Element Method

固体颗粒介质成形工艺是采用固体颗粒介质代替刚性凸模(凹模)的作用,对金属板料、管材拉深胀形的先进工艺,在复杂零件精密成形、难加工材料成形、温热成形等方面具有独特优势。为揭示该工艺中固体颗粒介质的传力特性,采用离散元法(Discrete/distinct element method,DEM)数值模拟固体颗粒介质在单轴压缩下的受力过程,从力链角度分析固体颗粒介质在压缩过程中细观结构的变化规律,并以直径1 mm不锈钢球为传力介质,自行设计颗粒介质传力性能试验,数值模拟结果与实测值吻合较好。研究发现颗粒配位数与体积份额呈幂函数关系,侧压系数与压应力亦呈幂函数关系,且当内部力链结构趋于稳定时,侧压系数趋于定值。应用散体力学研究方法推导出固体颗粒介质压力衰减规律,进而得到介质传力极限距离,这对如何准确控制成形中颗粒介质压力分布,提高加工工件的成形性能具有重要意义。

Solid granules medium forming（SGMF） is an advanced forming technology used for deep drawing and bulging of sheet metals and pipes,where the rigid punch（or die） is replaced by solid granules medium,and it has unique advantages in precision forming of complicated parts,hard deformation materials forming,warm forming etc.To reveal the pressure-transfer characteristics of granules medium in SGMF,the stressed process of solid granule medium under uniaxial compression is numerically calculated in discrete/distinct element method（DEM）,besides,the evolution law of micro structure of the solid granule medium in the compression process is analyzed in terms of force chains.To verify the discrete element method,a granules medium force-transfer performance test is designed,in which the stainless steel balls with diameter of 1mm are used as pressure-transfer medium,and the simulation result is in good agreement with the actually measured value.It is also found that the coordination number changes with the solid fraction in power function,so does the lateral pressure coefficient with the compressive stress,and the lateral pressure coefficient tends to be constant when the internal force chains structure keeps stable.In addition,the pressure decay law and limit pressure-transfer distance for solid granules medium are both deduced from mechanics of granular media,which has important significance for accurately controlling the pressure distribution and improving the formability of the workpieces.