Research On Powder Bed Spreading Process Based On Selective Laser Melting

Selective laser melting has attracted the attention of scientific researchers because of its high degree of freedom,low cost,and fast speed.As a direct manufacturing method that uses high-power laser to melt the metal powder layer by layer,the powder spreading process is critical to the quality of the part building.In the process of powder spreading,the characteristics of the metal powder and the spreading parameters can affect the quality of the laid powder layer after spreading,which can affect the quality of the formed parts directly or indirectly.In this paper,a discrete element model considering powder melting and solidification shrinkage during multi-layer powder spreading is proposed,which is used to analyze the particles movement to reveal the relationship between microscopic motion law of the particle system and the macroscopic behavior of the powder layer during powder spreading.It guides the quality control process of the built parts and provides a theoretical basis at the micro level for the selective laser melting process of titanium alloys.The main content of this work includes the following aspects: Firstly,the geometric size,shape,friction coefficient,etc.of the microscopic powder particles were measured or calibrated using experimental testing and model calibration methods.Secondly,the discrete powder method was used to establish the top powder supply model and the initial powder model was generated.The powder spreading process was simulated and analyzed.The relationship between the powder spreading plane,powder layer thickness and powder flow,defect formation,and powder spreading quality was analyzed.The powder base morphology,particle kinetic energy and pressure and other indicators were used to analyze the particles movement law.Simultaneously,the finite volume method was used to simulate and analyze the energy effect and melting depth of the laser melting process of powder bed with different layer thickness.Finally,the experiments of single melt tracks and tensile sample were carried out to analyze the relationship between the quality of powder spreading and the quality of melt tracks and the mechanical properties of the built parts.It is concluded that the thickness of the powder layer has a great influence on the density and uniformity of the powder layer.The density increases with the thickness of the layer and eventually stabilizes.At the same time,the rough base surface can also effectively improve the low layer thickness parameters,which can reduce the accumulation defects of the powder layer by affecting the movement of the layer of particles in direct contact with it;insufficient powder spreading at low layer thickness and incompletely melted particles at high layer thickness can result in single melt tracks discontinuous,moderate powder layer thickness(90?m-120?m)can built a stable,uniform and continuous melt tracks;the particle's translational kinetic energy,rotational kinetic energy and pressure are the key internal properties that affect the packing density;the difference in remelting times are the main causes of different performance of parts with various layers heights,and the regional difference in parts performance mainly comes from that the particle pressure in the edge area is less than the center area in the powder coating process.