Mesoscopic Modelling And Experimental Verification Of Selective Laser Melting Copper Alloy

Copper as well as its alloys is widely used in aerospace,machinery manufacturing,electronics and other industrial field,due to excellent electrical and thermal conductivity,corrosion resistance,etc.With the development of the aerospace industry,it is difficult for traditional manufacturing technologies to meet the manufacturing needs of aerospace high-performance components in terms of lightweight,complexity,structural integration,etc.Selective laser melting(SLM),as an important branch of additive manufacturing technology,has attracted much attention with advantages of high design freedom,short production cycle,excellent performance of formed parts,and net-shaping capability,which brings promising prospect for the manufacturing of high-end aerospace products of copper alloy.However,due to the high laser reflectivity and good thermal conductivity of copper,the copper alloy SLM process presents issues such as narrow process window,low relative density and poor surface quality,which are closely related to the physical phenomena generated in the SLM process.Having indepth insights into the forming mechanisms during SLM copper alloy is of great significance for reducing the forming defects and developing high-quality copper alloy aerospace products.In this study,the SLM process of Cu-Cr-Zr copper alloy is investigated via a combination of numerical modelling and experiments.Specially,several critical issues are systematically studied,including the effects of material properties on the melt-pool thermodynamics,the porosity forming and evolution mechanisms,the transient laser-material energy coupling,and the formation mechanisms of surface defects.The main research contents and conclusions are as follows:(1)Investigation on the basic characteristics of the thermodynamics within the melt pool during SLM copper alloy.Based on the discrete element method and the finite volume method,the randomly distributed metal powder bed model and the SLM thermal fluid dynamic model are established respectively.The laser heat source model was optimized and improved in terms of the heat source loading method and the value of the effective absorptivity.Combined with numerical simulation and experiments,the basic characteristics of the melt-pool thermodynamics during SLM of cooper alloy are analyzed.The results show that the SHS heat source loading method is beneficial to improve the calculation efficiency and convergence.The effective absorptivity measured by in-situ calorimetry can improve the prediction accuracy.As the melt pool changes from conduction mode to keyhole mode,the effective absorptivity increases significantly.Compared with the commonly used material 316 L stainless steel,the melt pool of copper alloy is nearly circular with the tail vanished in the end.Additionally,a conspicuous preheating effect is observed in the SLM process of copper alloy.As less energy input is applied,the preheating effect is weakened,and the ultra-high thermal conductivity leads to the reduction of the melt-pool size and the appearance of balling characteristics.(2)Investigation on the porosity formation and evolution mechanisms during SLM copper alloy.The 3D building process of SLM copper alloy is simulated based on the SLM thermal fluid dynamics model to reveal the underlying physic of the pore formation and evolution.As the scanning speed is high,the melt pool is likely to form a distorted track morphology under the action of surface tension due to the lack of energy input.As the scanning speed is too low,a deep keyhole will be induced by the strong recoil pressure,and the instability of the keyhole wall will lead to the formation of internal pores.When conducting double-track reciprocating scanning,since the temperature of the powder bed at the track beginning is lower than at the end,there is a conspicuous difference in the track morphology at the beginning and end,thus generating porosity defects at the track ends.As the hath space is too large,the gas gap within the unfused particles will remain in the overlap region and form pore defects.When conducting multiple-layer scanning,the distorted morphology of the 1st layer will significantly deteriorate the track quality of the subsequent layer,resulting in poor interlayer bonding and porosity.The keyhole-induced internal pores can not be eliminated by subsequent layer scanning and will gradually accumulate as the number of layers increases.(3)Investigation on the transient laser-material energy coupling mechanisms in SLM copper alloy.A full ray-tracing method is developed to track the multiple laser reflection and absorption behaviors in SLM copper alloy.Besides,a temperaturedependent absorption rules is used in the simulation to correct the unacceptable simulation error caused by the constant complex refractive index which is widely used for the Fresnel absorption.Based on the ray-tracing method and high-fidelity CFD model,the transient laser-material energy coupling mechanisms under three different melt-pool modes are discussed in detail.For the case of continuous track,the global absorptivity generally undergoes three stages: increasing,declining,and keeping stable.With the action of the recoil pressure,the depression region presents the light trapping effect,which promotes the multiple laser reflection and absorption.As the balling effect occurs,the global absorptivity fluctuates intensely which is attributed to the unstable melt pool caused by the poor wettability.More specifically,the vertical reflections on the emerging exposed substrate greatly contribute to the reduction of global absorptivity and lead to the intense oscillation.For the keyhole case,the absorptivity level is much higher than that in the case of the continuous track and balling effect.The instability of the keyhole wall will cause severe oscillation of the global absorptivity and unstable energy coupling,which is not conducive to stable SLM process.(4)The investigation on the effect of the contour scanning sequence on the surface quality in SLM copper alloy.The difference of the powder bed configurations caused by different contour scanning sequence is clarified by theoretical analysis.Through the CFD simulation coupled with the ray-tracing method,the laser reflection and absorption behaviors,melt-pool thermodynamics and defect evolution mechanisms are discussed in detail for different contour scanning cases.For post-contour scanning,the previous consolidated layer in the infill zone provides a flat reflector where the multiple reflections are eliminated.For the pre-contour scanning,the presence of powder particles in the infill zone helps enhance the global absorptivity,resulting in a relatively higher level of laser absorption.The liquid melt is prone to migrate towards the powder zone to attach more partially melted powders when conducting contour scanning.The pre-contour scanning can restrict the melt pool shifting and be beneficial to a better melting behavior that generates a continuous track morphology.Since post-contour scanning weakens the laser absorption,the contour track is more susceptible to the balling effect with more unfused powders attached to the edge.The measured roughness magnitudes in the lateral vertical surfaces of SLM specimens are generally consistent with the numerical analysis,showing that a lower roughness can be achieved by the precontour scan strategy in the range of relatively lower linear energy density.