INFLUENCE OF ROUGH SURFACE OF DEPOSITED AREA ON QUALITY OF POWDER SPREADING DURING SELECTIVE LASER MELTING: DISCRETE ELEMENT SIMULATIONS
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摘要: 选区激光熔化中, 铺粉质量会极大地影响产品的最终质量. 然而, 成形区粗糙表面对铺粉质量影响的研究较少. 因此, 本文以成形区粗糙表面作为新的铺粉基板, 通过离散元法, 研究铺粉过程中成形区的表面形貌和工艺参数对铺粉质量的影响, 并分析铺粉过程中金属粉末在成形区粗糙表面的颗粒动力学和颗粒沉积机制. 结果表明, 将激光扫描方向与铺粉方向旋转一定角度可有效提高粉末层质量, 增加铺粉层厚可减小成形区粗糙表面对铺粉质量的影响. 减小搭接率可提高成形区对颗粒的滞留能力, 从而使更多的颗粒沉积在成形区, 提高粉床填充密度, 但是粉末颗粒会与成形区的粗糙表面碰撞, 产生颗粒迸溅现象. 此外, 铺粉过程中, 由于成形区粗糙度的增大, 成形区粗糙表面上的粉堆产生的强力链、力拱数量多于表面光滑的成形区. 在滚轮作用下, 力拱断裂导致颗粒重新排列, 形成致密的粉末层. 在成形区边界处, 力拱的产生会最终导致边界处的粉末层出现空斑缺陷. 本研究有助于通过优化工艺提高粉床质量.Abstract: The final quality of the printed products is greatly affected by the powder spreading in selective laser melting (SLM). However, little attention is paid on the influence of rough surface of deposited area on the quality of the powder spreading. Therefore, a rough deposited area is modeled as the new substrate for investigating the effect of surface morphology of the deposited substrate and processing parameters on the quality of powder bed during spreading using the discrete element method (DEM). The particle dynamic behavior and powder bed formation mechanism of metal powder on the surface of the deposited area during powder spreading are analyzed. The numerical results show that the quality of powder layer can be effectively improved by rotating the powder spreading direction to a certain angle along the laser scanning direction of the bottom layer, and the influence of the surface of deposited area can be significantly reduced by increasing the powder bed thickness. Reducing the hatch overlap rate can improve the retention capacity of particles in the deposited area, so that more particles are deposited in the deposited area, thereby increasing the packing density of the powder bed. But the powder collides with the rough surface of the deposited area causing more particles to splash. During spreading, the number of strong chains produced by the powder pile on the rough deposited area is more than that on the smooth deposited area due to the increase in roughness of the deposited area. Meanwhile, under the action of the roller, the force arch is destroyed and the particles are rearranged to form a dense powder layer. At the boundary of the deposited area, the generation of force arch will eventually lead to the appearance of vacancy defects in the powder layer at the boundary. The present study is helpful to improve the quality of powder bed during the powder spreading.
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Key words:
- selective laser melting /
- powder spreading /
- discrete element method /
- rough surface /
- powder bed
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图 17 成形区的边界处, (a)强力链导致的空洞, (b)未出现空洞, (c)-(d)分别为(e)-(f)的剖面图, (e)-(f)为俯视角度下的粉末层形貌图(续)
Figure 17. The boundary of the formed region. (a) Cavities caused by the strong force chains, (b) cavity-free, (c)-(d) are the cross-sections of (e)-(f), respectively. (e)-(f) are the top view of the powder layer morphology (continued)
表 1 铺粉模型参数
Table 1. Powder spreading parameters
Parameter Symbol Value length of build platform L/mm 3 width of build platform W/mm 1 roller diameter Dr/mm 5 spreading velocity V/(mm·s−1) 50 angular velocity ω/(rad·s−1) 2π gap height H/μm 40 ~ 70 track height h/μm 40 powder density ρ316L/(kg·m−3) 7.8 × 103 Young's modulus E/MPa 2.2 × 103 Poisson’s ratio ξ 0.3 restitution coefficient e 0.9 static friction coefficient μs 0.6 rolling friction coefficient μr 0.01 surface energy density γ/(mJ·m−2) 0.097[20] power diameter D/μm 10-40 -
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