Study On Numerical Simulation Of SLM Forming Process Based On SPH Method

Metal selective laser melting(SLM)process is an important 3D printing technology,and its forming is a complex nonlinear process involving multi-scale and multi-physical fields.It is difficult to observe the specific physical phenomena of SLM forming process directly by experimental means,so it is urgent to develop reliable numerical simulation methods to reveal the mechanism of thermal-fluid-solid coupling in SLM forming process.The existing grid numerical simulation methods have some limitations: Lagrangian grid method is mainly used for thermal stress simulation,but it is difficult to consider the flow of molten pool.Euler grid method is mainly used to simulate the flow of molten pool,but it is difficult to track the moving interface.Meshless methods such as smoothed particle hydrodynamics(SPH)can naturally track free surfaces and moving interfaces,and have special advantages in manufacturing simulation.In this paper,a high precision SPH method for simulating SLM forming process is proposed,which is expected to provide an effective way for manufacturing process and process simulation of increasing material in China.The specific work and research contents are as follows:(1)Develop high-accuracy,stable and conserved SPH numerical model.Introduce the basic theory and method of SPH method,and the discrete form of governing equation is obtained based on integral approximation and particle approximation.Study the conservative high-precision SPH method,and through the symmetrization method,the conserved kernel gradient correction(KGC)technique is developed to solve the non-conservation problem of the traditional KGC technology.Study the homogenization technology of SPH particles,and the stability and accuracy of SPH method are improved by introducing particle shifting technology(PST)and X-SPH technology.(2)Develop a high precision SPH method to simulate the SLM forming process.Introduce the continuous surface force(CSF)model,the smooth curvature method is used to improve the stability of the CSF model when simulating the violent flow in a small molten pool,and a method suitable for transforming the surface force of single-phase flow into volume force is developed to improve the calculation accuracy of the CSF model.An improved KGC-SPH method for simulating molten pool flow is established by introducing Gaussian heat source model and material melting constitutive model.Numerical examples are given to verify the accuracy and effectiveness of the improved KGC-SPH method in simulating SLM forming process.Finally,based on the improved high-precision SPH method,study the effects of process parameters such as laser power and scanning speed on the forming quality of SLM.It is found that under the same simulation conditions,with the increase of laser power,the porosity decreases,but the metal spatter increases and the surface quality decreases.As the scanning speed increases,the metal spatter decreases and the surface quality gets better,but the pores increase.(3)Develop a parallel 3D SPH method based on GPU to simulate SLM forming process.Study the parallel computing framework of 3D high-precision SPH method,and develop the parallel code and program of SPH method based on CUDA.Compare the 3D CSF model with the surface tension model of surface reconstruction and select the stable surface tension model.The 3D SPH method is verified by numerical examples.The SLM forming process is analyzed based on numerical simulation,and study the influence of process parameters such as laser spacing on the forming quality of multi-channel SLM.It is found that under the same simulation conditions,the metal spatter decreases but the surface quality decreases with the increase of laser spacing.