Experiment And Numerical Simulation On Temperature And Stress Fields During Selective Laser Melting Of Thermoelectric SnTe

Thermoelectric(TE)conversion technology is expected to solve the energy and environment crises by directly achieve conversion between heat energy and electric energy.However,the traditional fabrication of TE modules is subtractive manufacturing or other material manufacturing,consisted by the bulk material preparation,particle cutting,spraying,electroplating,welding,cleaning,packaging and other processes.The high demand for equipment and large cost of time and raw materials have restricted the large-scale commercial application of TE technology.Therefore,it is urgent to develop a new technology for the preparation of TE devices.As an advanced additive manufacturing(AM)technology,the selective laser melting(SLM)technique could be used to rapidly fabricate three-dimensional objects with complex shapes in the layer-by-layer fashion by selectively melting thin layers of loosely packed powder beds from 3D Computer Assisted Design(CAD)data.Hence,it has been widely applied in the manufacturing of traditional structural materials and components.Recently,attempts have been made to employ SLM to produce thermoelectric materials and devices.But SLM is a complex non-equilibrium thermodynamic process,which includes phase transformations and mass and heat transfer in a short-period of time.The process includes great temperature gradients,rapid cooling rates,and local residual stresses near the molten pool as the laser spot is being rastered.All of the above likely contribute to the formation of defects,such as pores,cracks,and warpage that significantly weaken the mechanical performance of the final product.In this paper,a 3D non-linear transient finite element model,based on sequentially coupled thermo-mechanical field analysis,was set up using the ANSYS program to predict the temperature and stress distribution during the SLM processing of a medium temperature SnTe thermoelectric material.The main research and conclusions are as follows:The moving surface heat source with the Gaussian distribution and various boundary conditions was modeled by a user-defined subroutine written in the ANSYS parametric design language(APDL).The temperature-dependent physical properties of SnTe,the powder-to-solid transition,and the latent heat of melting and vaporization were considered.By simulating the SLM processing of a single SnTe layer,the influence of different processing parameters on the molten pool dimensions,peak temperature,liquid lifetime and cooling rate were analyzed.The optimized parameters were obtained at sanning speed v=300 mm/s,laser power P=10 W,hatch spacing h=40?m and layer thickness d=25?m,resulting in the stable molten pool and uniform temperature field.Varing scanning strategies have little effect on the width,depth and vaporization phenomenon of the molten pool.However,the zoning zigzag scanning strategy can greatly increase the pool length,decrease the temperature gradient and the cooling rate.Based on the simulated temperature field results,keeping the original finite element model and using thermal-mechanical coupling method,combined with the"birth and death”technique to simulate the real powder forming process and the unstress state within the molten pool.The transient thermal stress evolution and the residual stress when the part cooled down to room temperature during SLM process of SnTe were analysed.The results showed that the transient thermal stress performed periodically with the laser spot moved.The residual stress mainly manifested as tensile stress in the scanning direction,and concentrated in the middle of the first track and the junction between powder layer and the substrate;In forming window,the residual stress elevated with the increasing scanning speed and laser power.When v=300 mm/s and P=10 W,stress field distributed uniform and the maximum value keep stable at?_x=670 MPa;The larger hatch spacing h could contributed to the higher residual stress,resulting in the uncontrollable surface forming quality;The influence of layer thickness d on the stress field was in contrary to h;Moreover,shortenning of the single track and reasonable partition of the scanning region could effectively release the residual stress and inhibit the occurrence of crack and warpage.SnTe powder was prepared by self propagating high-temperature synthesis(SHS)method,and the corresponding single track and forming surface was prepared by the SLM experiment under different laser scanning speed.The results showed that:In the forming window,the experimental width and depth of SnTe single track was consistent with the numerical ones of molten pool;The main defects exchanged from pores into balls as the scanning speed increased from 100 mm/s to 400 mm/s,proving the calculated value of maximum temperature and liquid lifetime;Meanwhile,the microstructure of the forming surface was gradually refined with the elevating cooling rate;The residual stress?_x and?_y measured by X-ray diffraction fit highly with the calculated ones.This work has laid a significant foundation for rapid manufacturing of bulk SnTe thermoelectric materials using the selective laser melting process and will serve as a guide for laser processing of other thermoelectrics and perhaps even complete thermoelectric modules.