高速压制成形粉末流动过程的格子Boltzmann方法数值模拟

Numerical simulation of powder flow in high velocity compaction by lattice Boltzmann method

根据高速压制工艺过程的特点,底部和侧面边界采用反弹与反射的混合格式,顶部通过改变分布函数构造能模拟动态压制过程的边界格式。利用粉体的初始密度和压制最大理想密度建立一个动态的松弛函数来代替格子Boltzmann方法的固定松弛时间。对圆柱体及单级台阶的圆柱体模具的高速压制成形过程进行二维数值模拟,直观显示压制过程中粉末的流动以及应力波的形成与传播过程。模拟结果表明:模具形状对应力波的波形有较大影响;压制完成后,圆柱体压坯最大密度差为12 mg/cm3,密度分布均匀且左右对称,压坯顶部和底部密度较中间部分密度稍大,与实验结果基本相似;圆柱体压坯中存在密度较低的横向窄带区域,揭示成形坯有可能形成分层或断裂。

According to the characteristics of high velocity compaction process, the rebound and reflection mixed boundary condition was used on the bottom and sides of the mould. By changing distribution function, a kind of boundary condition which can simulate the dynamic process was proposed. A relaxation function was established according to the powder flow characteristic using initial and maximum density. Two-dimensional numerical simulations were given to cylinder mould and single step cylinder mould. The dynamic evolution process of powders and the stress wave transformation process were presented. The results indicate that the shape of the mould has a great influence on the wave pattern of stress. The density of green compact is uniformly and symmetrically distributed, and the greatest density difference is 12 mg/cm^3. The top and bottom parts have slight higher density than the middle part. These results are consistent with the experimental results. A narrow low density band is found in the green compact which reveals that there may exist density stratification.