Physical Field Modeling And Spatter Behavior In Selective Laser Melting Process

Selective Laser Melting（SLM）forming is an important area of development for new additive manufacturing technologies.SLM is a powder layer-based manufacturing process that can produce complex geometries from different types of materials（e.g.metals,ceramics and composites）.It holds great promise for aerospace,medical and energy storage applications.However,powder fusion technology has been hampered by the unstable forming properties of parts,leading to the build-up of residual stresses and defects such as spatter,porosity,spherification and cracking.Due to the short interaction time between the laser and the powder layer,the dynamic behaviour inside the melt pool or melt trajectory and the defect formation mechanism are difficult to characterise experimentally.This paper therefore shows the whole process of powder layers from melting,melt drop spattering,melt path forming and cooling solidification by developing a high fidelity powder-scale laser melting model for selective laser melting and forming.The main contents and conclusions of this paper include the following.（1）316L stainless steel powder particles as raw materials,the use of discrete unit method based simulation software EDEM to establish a randomly distributed powder layer model corresponding to the actual powder,using Flow-3D software as a platform,loaded on the laser heat source to achieve the laser heat source to act on the metal phase and gas phase interface unit position in the region,in order to achieve the numerical simulation of selective laser melting process.A series of complex evolutionary processes such as thermal melting of the melt pool,droplet spattering,melt flow and solidification of the melt channel during the laser scanning process are obtained by means of numerical simulation.（2）The effect of laser action on melt pool dynamics and defect formation during powder forming is investigated.The spattering behaviour of the molten droplets is an unavoidable source of forming defects in the metal powder layer fusion process.The results show that metal vapour action,in addition to the effects of the metal vapour,is not only the most important aspect of the spattering process.The results show that metal vapour action,in addition to directly causing the depression of the melt pool,is also involved in the melt flow and is an important factor in the induction of droplet spattering.The higher the energy in the melt pool,the greater the instability of the melt flow and the more likely it is that defects such as porosity and spalling will occur during the flow.The high temperature melt flow velocity is between 1 and 6 m/s and the melt droplet spatter velocity is between 1 and 4 m/s.In combination with experimental analysis,the trajectory of the droplet splash and the "secondary explosion" and "spinning ball" behaviour of the droplet splash in the air are traced.The splash droplet splits into smaller sub-droplets by bursting to release excess energy,and the splash velocity of the split droplet is reduced,resulting in the experimentally observed’secondary explosion’.The spherical droplets fly out of the melt pool in a "spin" motion above the pool,confirming the role of the gas flow around the pool in guiding the droplet splash.The simulation statistics are in good agreement with the experimentally recorded results.（3）The variation of melt pool surface morphology and dimensions under different process parameters（powder layer accumulation density,powder particle size,laser power and laser scanning speed）was investigated,and the influence of heat input rate on the spattering behaviour during the forming process was analysed.The results show that metal vapour action and inert gas flow jointly drive the melt pool flow and droplet spattering behaviour,with the adjustment of process parameters,the volume morphology and spattering direction of the spattering droplets change,within the process parameters window of laser power 100 W≤ P ≤ 150 W and laser travel speed 1100 ～ 2000 mm/s,the influence of spattering on the forming plane is relatively small.The influence of spatter on the forming plane is small.（4）Single-pass remelting scanning experiments and multi-pass scanning experiments were carried out using the process parameters set in the model to obtain data on the geometry of the melt pool and the dynamic characteristics of the melt pool melting process under the corresponding process conditions.The mechanism of densification and elimination of residual holes during laser remelting is analysed.The density of SLM manufactured parts can be efficiently increased by laser in-situ remelting.A simulation of a selective laser melting single layer multi-pass process is realised.The results show that as the number of scanned passes increases,the width of the curing track for subsequent laser scan trace fusion becomes significantly larger than the width of the curing track from the initial scanning stage.Single direction scanning tends to lead to uneven edges in the forming area when the number of passes to be melted in the forming plane increases,affecting the accuracy of the forming dimensions,while scanning in the opposite direction allows the dimensions between adjacent fusion passes to complement each other,which can attenuate the effect of melt flow on the forming dimensions.