Finite Element Analysis Simulation Of Laser Melting Deposition Co-based Alloy Process

Biomedical cobalt-based alloys have attracted increasing attention from medical implants fields due to its excellent corrosion resistance,wear resistance and good mechanical properties.However,the long processing cycle,high customization cost,and difficulty in preparing complex geometric structures restrict the further development and application of medical implant materials.As a additive manufacturing technology,Laser Melting Deposition(LMD)has the characteristics of flexibility,intelligence and networking which combines the prototyping,rapid manufacturing with CAD.LMD provides new method and ideas for the preparation of medical implant materials and widely used in the aerospace,biomedicine,petrochemical and other fields.Laser Melting Deposition is essentially coupled with a number of complex physical and chemical processes with dynamic rapid melting and solidification phenomenon.It requires the deposited parts with high dimensional accuracy,small residual stress,no defects and excellent mechanical properties.The deposited undergo cyclic heating and cooling resulting in various heat treatment and microstructure which affect its mechanical properties.At present,the microstructure and defect regulation of deposited parts are still difficulties and challenges.It is significantly difficult to monitor these transient processes accurately.Therefore,using numerical simulation method to study the temperature and flow distribution of molten pool in multi-field is necessary.The influence of temperature and flow distribution on microstructure evolution laid a theoretical foundation for the further understand,provide predictive guidance for real-time monitoring and feedback control,support for wider application development.The COMSOL Multiphysics software is used to establish the three-dimensional transient heat-flow coupling model for laser melting deposited.The temperature dependent thermophysical parameters,solid-liquid phase change,buoyancy and surface tension of the liquid metal in the melt pool and the attenuation effect of the powder flow are considered.The influence of the process parameters on the temperature and fluid field can be obtained.The microstructure distribution of specimens is analyzed by temperature gradient and the cooling rate.The results show that the heating rate is 1.498× 104 K/s at 0.1 s.Its temperature field is elliptical.The melt pool takes time(0-0.5 s)to gradually form and grow,and the flow rate of the molten pool rapidly increases.Then,the shape of the molten pool and the flow velocity tends to be stable.The temperature gradient G at the edge of the molten pool is greater than the temperature gradient at the center.The solidification rate at the top of the molten pool is greater than it at the bottom.The decreasing of G/R and increasing of G×R result in the microstructure transition of the specimens from top to bottom of the deposition.The order of microstructure transition is plane grain,columnar grain and equiaxed grain.The result of multiple deposition shows that with the increase of deposition layers number,the maximum temperature increase fluctuated and the thermal energy accumulated.The melt pool size is increases resulting in re-melting multilayer.Due to the natural convection caused by buoyancy and the Marangoni convection caused by surface tension,two inward convection loops is formed at melt pool.In addition,the power increase will lead to the increasing of temperature fluctuations in multi-layer deposition and deepen non-uniform temperature distribution.When the laser power is 500 W-600 W,the maximum temperature gradient is 1.81×106 K/m-2.86×106 K/m.The huge gap between temperature gradient at different power shows that it is very important to choose the appropriate laser power in the deposition process.