The Meshfree Numerical Simulation And Experimental Research On The Process Of Selective Laser Melting

Selective Laser Melting(SLM)is a kind of additive manufacturing processes.Compared with the traditional manufacturing processes,it has many advantages in many aspects,such as no mold,high formation accuracy,less pollution and ease to realize automation,so it can be applied for formation of the complicate components with a certain strength.Therefore,the SLM process will have a wide range of application prospects in the engineering field.However,there are a high temperature and non-linear metallurgical process of physical and chemical to generate in the SLM.Moreover,it is affected by many factors in the scanning process,and the plenty of complex physical phenomena will be generated during the SLM,such as the status of fluid-solid coupling,the absorption and conduction of laser energy,and the morphology formation of molten pool and the evolution of single track.Obviously,the quality of the parts are affected by the these factors,and there some difficulties to achieve the simulation and analysis of the melt pool morphology and single bead when the the grid method is used in the SLM.In this dissertation,the application of smooth particle hydrodynamics(SPH)meshfree method is studied to simulation the SLM process.Meanwhile,the related experiments are conducted to validate the results of numerical simulation,and the rules of forming process are also studied in the experiments..1)The theory and computational processes of SPH method are systematically studied,and a numerical model of SPH for the SLM process is established by using the SPH method.In the modeling process,the SPH governing equations are deduce for the SLM process when the influences of the convection source term and the enthalpy source term are taken into account.Meanwhile,in order to keep the interaction of particles of different interfaces more ordered in the simulation,the XSPH technique is introduced when the position vector of each particle is updated.In addition,to prevent the liquid particles from penetrating the solid region,a repulsive force model is applied to the fluid particles near the boundary when they are tending to penetrate each other,and a local search method of particle is proposed to improve the computational efficiency based on the linked-list search algorithm.2)The numerical model of surface tension model for the SLM process is established by using the SPH method,and the physical model contains two major components,one is the capillary force and the other,Marangoni force,and they both are related to the temperature.The results show that the SPH method can be used to simulate the heat conduction problem,and it can be also used to simulate the movement and deformation of the fluid respectively,which will afford the foundation for the SPH method to simulate and research the SLM process.3)The influence of surface tension on the morphology evolution of molten pool is studied during the SLM process.In the simulation,it is worth noting that there is a variation on the thermo-physical properties of powder material with the influence of temperature,such as the density,thermal conductivity and the viscosity,and the SPH program is developed by the Fortran language,which is used to simulate the two-dimensional SLM process.Meanwhile,the evolution of the surface morphology of the molten pool is studied and analyzed under the different laser process parameters.The results show that the longitudinal surface morphology of molten pool will present a series of curves distribution due to the influence of surface tension,and the longitudinal morphology of a molten pool looks like that of a wave in the simulation.Moreover,the experiments are conducted to validate the correctness of the simulation results,it is found that the simulation results of the longitudinal morphology of molten pool are agreed well with the actual situations,so the effectiveness of the SPH is proved by the calculation of SLM process.In addition,the deformation distributions of the longitudinal morphology of molten pool will increase with increasing laser power or decreasing scanning speed.4)The temperature distribution and the evolution of the solidified bead in SLM are simulated and studied respectively by using the SPH method during the three-dimensional SLM process.In the simulation,a three-dimensional transient mathematical model for the SLM process is firstly established.Afterwards,based on the powder system of 304 L stainless steel,the distributions of temperature and solidified bead are simulated and analyzed at various processing parameters by using the SPH method.The results show that the solidified bead forms as the powder material melts,and its shape changes from initially planar to a semicylindrical bead because of the influence of the surface tension in the scanning process,Meanwhile,it is found that there is a discontinuous temperature distribution between the unmelted powder layer and the substrate material.Both the temperature distribution and the solidified bead deformation increase in the SLM process with the increase of the laser power and the porosity of the powder particle,but they both are apt to decrease with the increase of the scanning speed and the thickness of the powder layer.Experiments into the deformation distribution of the solidified bead and the dimensions of the molten pool are carried out under the same processing parameters,and the results are largely consistent with these from the simulation.Therefore,it indicates that the simulation of solidified bead in single-pass selective laser melting is completed by using the SPH method5)The process parameters of SLM for forming metal component are studied in the experiments.Firstly,the experiments for the single bead and single layer are conducted respectively,the influences of different parameters are studied and analyzed on the formation quality when the powder system of 304 L stainless steel is used in the SLM.Afterwards,the experiment study on the formation of solid block is conducted based on the single-layer melting.The results show that when the other process parameters are held constant in the experiments,the laser power is 250 W,the scanning speed is 15 cm/s,the thickness of metallic powder is 60 μm and the overlapping rate is 60% respectively,and the S-shaped scanning strategy is used in the experiments,the solid block with the better quality will be obtained after the scanning process.Finally,the metallic powder of 304 L stainless steel is applied to fabricate the thin-walled parts such as flower shape and mesh structure by the equipment of SLM.