Mechanism Analysis Of Selective Laser Melting And Metallurgy Process Based On Base Element Powder Of Titanium And Boron

Titanium(Ti)has excellent bio-compatibility and corrosion resistance,but the medium strength and hardness of Ti hinder its further utilization,and Ti is insensitive to heat treatment and has high reaction activity.Therefore,it is a very promising technology by using modern additive manufacturing technology,selective laser melting(SLM),to fabricate Ti components with ultra-fine grains and complex structures.This way,not only improves its mechanical strength but also maintains other excellent unique metal properties.However,there is a complex thermal physical phenomenon in the SLM process,especially complex phase change heat transfer and metal fluid flow in the molten pool.They have a great influence on the coagulation and temperature gradient in the molten pool,and then strongly affect the grains size and the growth direction,and the surface morphology of the molten pool channel.Therefore,a large number of research work needs to be used to study the thermal behavior during SLM.For the demand for the excellent performance of titanium alloys,the researchers synthesize diboride titanium(TiB₂)ultra-high temperature refractory ceramic materials by composite pathways.In all B-based alloys of Ti,TiB₂ is the most stable in several titanium boron compounds,which is also the hardest interstitial boride with a hexagonal structure,where boron atoms fill the trigonal prisms formed by the titanium atoms.It has excellent performance in hardness,mechanical strength,wear resistance,and corrosion resistance,but due to its poor manufacturability,its application is still limited.It is a promising idea to use pure elements Ti and B to in-situ synthesis of TiB₂ with selective laser melting and to broaden its application by enabling its 3D free forming.For this reason,the concept of Selective Laser Alloying(SLA)is proposed.However,the in-situ reaction of Ti and B will release a large number of reaction heat.On the one hand,this part of the reaction exotherm can promote the melting of the powder,thereby reducing the energy input of the laser,but at the same time,the existence of this part of the energy makes the heat field more complicated.Once the energy input to the powder bed system is too much,it is easy to cause excessive melting of the powder,which will cause various manufacturing defects.On the contrary,if the input energy is insufficient,it is easy to cause insufficient melting of the powder,so this part requires a quantitative analysis.In addition,the SLA process also involves multi-component diffusion and complex heat and mass transfer in the molten pool.The above problems need to be verified by establishing an SLA model and implementing experiments,which has become a huge challenge restricting the application of TiB₂ ceramic materials in 3D molding.In summary,this article combines the major needs in the engineering field and the current research status at home and abroad and aims at the scientific problems in the SLM process of single-component pure Ti powder and the SLA process of dual-components Ti and B.Three research contents are proposed:First,a mesoscopic simulation of a random powder bed model based on the discrete element method(DEM)was established to study the thermal behavior of single element pure Ti powder during selective laser melting(SLM).By realizing the CFD-DEM coupling,the characteristics of the molten pool under the interaction of laser and powder,and the influence of laser power on the thermal behavior,fluid dynamics and surface topography evolution of the molten pool are studied.The model has been verified by experiments,and finally obtained the laws related to the thermal field,such as the maximum temperature,the temperature change rate,and the lifetime of the molten pool in the case of SLM multi-tracks melting.In addition,the influence of different laser power on SLM characteristics is also studied.Later,in the context of Ti alloy,the idea of using pure elements Ti and B powder in situ synthesis ultra-high temperature refractory ceramic material-TiB₂ is proposed,and it is desirable to broaden its industrial applications by enabling the 3D forming of TiB₂.This method is called Selective laser alloying(SLA).In the SLA process,in order to study the coupling effect of the laser melting powder and the chemical reaction between the powder,numerical simulations were carried out using COMSOL Multiphysics 5.4.Current work is the first successful numerical study of melting and reactions in the SLA process,providing detail related to the molten pool.The free solid-liquid interface evolution,component diffusion,chemical reaction rate,and the effects of evaporation latent heat were discussed by considering the generation of TiB₂ in the calculation.It is found and explained that the component diffusion model,the significant influence of the latent heat of evaporation on the temperature field in the calculation domain,and the concentration distribution of TiB₂ strongly depend on the chemical reaction rate and diffusion coefficient.Finally,the quantitative analysis model has been established to study two energy sources in the SLA process.By considering the concentration distribution of Ti,B and TiB₂,the chemical reaction rate and the effect of laser power on the evolution of the free solid-liquid interface are discussed.The laser energy consumed in the SLA process and the energy released by chemical reaction were theoretically derived.The results show that the heat released from the exothermic reactions between Ti and B is an important energy source,which can reduce laser energy input and help to improve the manufacturing efficiency of the SLA process.In addition,the chemical reaction rates have been found to have a significant effect on the concentration distribution of Ti,B and TiB₂.Moreover,current work also explains that the volume of the molten pool,the lifetime of the molten pool,and the heat source released by the chemical reaction strongly depend on the chemical reaction rate and laser power.