激光选区熔化铺粉过程的数值模拟及粉层表征

NUMERICAL SIMULATIONS OF POWDER SPREADING PROCESS IN SELECTIVE LASER MELTING AND POWDER LAYER CHARACTERIZATION

激光选区熔化作为一种先进的金属增材制造技术,可用于快速制备力学性能优良且形状复杂的金属零件.激光选区熔化成形性能优异零件的基础条件之一是金属粉层颗粒紧密均匀排布,而粉层颗粒的排布情况受颗粒流动性、铺粉速率和粉层厚度等因素的影响.本文构建了用于获取颗粒性质、分析铺粉工艺对粉层质量的影响和高效模拟多层铺粉的离散元计算模型.通过对比金属钨颗粒粉堆休止角和致密度的模拟与实验结果,得到了金属钨颗粒的滚动摩擦系数和表面能密度,阐释了滚动摩擦系数和表面能密度对金属钨颗粒流动性的影响机制,完善了铺粉离散元模拟需要的物性参数.使用单层铺粉离散元模型研究了铺粉速率和粉层厚度对粉层质量的影响,通过对粉层密度、配位数分布、粉层表面粗糙度和粉层均匀性的表征,确定了使粉层颗粒紧密均匀排布的铺粉工艺窗口.构建了高效多层铺粉离散元新模型,该模型能够准确刻画已熔化粉体的粗糙表面,识别完全未熔化颗粒,并使用可移动球颗粒表示完全未熔化金属颗粒,消除了完全未熔化颗粒不可移动的非物理限制,显著提高了多层铺粉离散元模拟的效率与拟真程度.

As an advanced metal additive manufacturing technology,selective laser melting(SLM)is capable of fabricating metal components rapidly with excellent mechanical properties and complex geometries.One of key foundations for producing high-performance components using SLM is a relatively uniform distribution of metal powder particles in compact powder layers,which is affected by powder flowability,powder spreading speed,and layer thickness.Here,the discrete element modeling for obtaining particle properties,analyzing effects of spreading processes on powder layer quantity and efficient simulations on the multi-layer spreading process are performed.The rolling friction coefficient and surface energy density of tungsten particles are obtained by comparing experimental and numerical results of repose angle and relative density of the tungsten powder pile.Effects of rolling friction coefficient and surface energy density on particle flowability are revealed,and all physical parameters of tungsten particles required in the discrete element modeling are obtained.Effects of powder layer thickness and spreading speed on the layer quality are quantitatively examined based on the discrete element modeling of single-layer spreading process.Through evaluation on packing density,coordination number distribution,layer surface roughness and layer uniformity,the spreading process window is determined for a powder layer consisting of closely and uniformly distributed metal powder particles.By identifying completely unfused metal powder particles in the melted powder bed,a new discrete element model capable of efficiently simulating the multi-layer spreading process in a physical way is established.In the new discrete element model,rough surfaces of fused metal parts are characterized accurately,and completely unfused metal powder particles are represented by movable spherical particles which enables the elimination of unphysical limitation of the movement of completely unfused metal powder particles.Having these advantages,the new discrete element model significantly improves the efficiency and fidelity of simulations on the multi-layer spreading process.