激光选区熔化铜合金多道成形的熔池行为和缺陷机理

Melt Pool Behaviour and Defect Mechanism in Multi-track Formation of Selective Laser Melting Copper Alloys

目的针对激光选区熔化成形铜合金零件中孔隙缺陷的质量问题,研究不同扫描间距下多道熔池的动力学行为,以及不规则未熔合孔隙缺陷的生成机制。方法构建多物理场的高保真数值模型,采用离散元法搭建粉末床,结合两相流法追踪金属相自由表面,并通过射线追踪算法研究多道成形时的全局吸收率,深入分析扫描间距对熔池演变以及未熔合孔隙缺陷的影响。结果前一熔道的扫描对后一熔道有预热效果,因此第2熔道的宽度和吸收率总是大于第1道。随着扫描间距的增加,热积累效应减弱,第2道熔池的宽度和深度随之减小。扫描间距小会增加熔道间的重叠部分,不利于提升零件的构建速率,但无限地增加扫描间距会导致未熔化孔隙缺陷。通过对比70、100、130μm扫描间距下的成形熔道发现,70μm熔道的搭接率接近50%,100μm能形成良好的搭接,且没有缺陷生产,而130μm熔道有明显的孔隙缺陷及未完全熔化的粉末。结论扫描间距的变化会造成液态熔体流动、传热和传质的改变,进而改变激光选区熔化铜合金多道成形的熔池动力学行为与吸收率,最终影响成形件的质量。选用合适的扫描间距可以有效地避免未熔合缺陷,并保证相邻熔道间有良好的搭接率。

In order to solve the quality problem of pore defects in selective laser melting copper alloy parts,the work aims to investigate the kinetic behavior of the multi-track melt pool and the irregular un-fusion pore defect formation mechanism at hatch spacing.During the selective laser melting,the interaction between the material and the laser takes short time and small space scale and is presented throughout the process of heating,melting,flowing and solidification of the powder.The melt pool evolution mechanism can not be observed by experimental means.In order to gain a deep insight into the melt pool dynamics,a high-fidelity numerical model with multi-physics fields is proposed.To simulate the complex process of metal powder melting and even evaporation,multiple phenomena have been considered,such as thermo-capillary forces,Marangoni stress,recoil pressure,temperature-dependent convective and radiative heat flux,and laser-powder interaction.The random packing of the metal powder laid on the substrate is modeled by the commercial DEM software EDEM.The metal phase free surface was traced by the volume of fluid method.The global absorptivity at multi-track forming was also investigated by ray-tracing algorithm.Base on the model,the effect of hatch spacing on melt pool evolution and un-fusion pore defects was analyzed in depth.The results showed that the scan of the previous track had a preheating effect on the latter track,so the width and absorptivity of the second track were always greater than those of the first track.As the scan spacing increased,the heat accumulation effect decreased,so the width and depth of the second melt pool decreased slightly.The global absorptivity curve for each single track could be divided into three processes of increasing stage,decreasing stage,stable stage.The global absorptivity curve of the second track did not vary much with the increasing hatch spacing.A small hatch spacing would increase the overlap between fusion tracks and generate good connection of adjacent tracks without gaps,which was not conducive to improving the building rate of the part.However,an infinite increase in hatch spacing would decrease the overlap zone and even non-connection in the adjacent tracks led to un-fusion pore defects.By comparing the formed tracks at 70μm,100μm and 130μm hatch spacing,it was found that the overlap rate at 70μm was close to 50%,100μm could form a good overlapping and no defects were observed,while at 130μm track,obvious pore defects and incompletely melted powder were observed.The different hatch spacing causes changes in liquid melt flow,heat and mass transfer,which in turn changes the melt pool kinetic behavior and absorptivity for multi-track formation of selective laser melting copper alloys,ultimately affecting the quality and mechanical properties of the formed part.This indicates that proper hatch spacing can effectively avoid un-fusion pore defects and ensure a good overlapping between adjacent fusion tracks.The results of this study provide a reference for understanding the melt pool dynamics during selective laser melting of copper alloys and the control of internal pore defects.