Beam Profile Characteristic Based Investigation Of Heat-fluid Transport Mechanism In Laser Direct Metal Deposition

Laser direct metal deposition(LDMD)is an additive manufacturing technique characterized by multi-disciplinary crossing and multi-technology fusion.This technique has integrated manufacturing ability to control shape and performance,and is easy to fabricate the excellent performance monolithic complex metal parts for meeting functionalized,lightweight and personalized requirements.Due to these unique technological merits,the LDMD has been successfully applied in high-end equipment manufacturing fields such as aerospace,vehicle,mold and biomedical engineering.However,the extreme variability and the high complexity in the deposition process make the microstructure,performance and dimensional accuracy of the parts from LDMD difficult to meet the needs of high-end products.The current research shows that the microstructure,mechanical performance and thermal deformation are closely related to the heat-fluid transport during the deposition process,which means that to find the regulation strategy of the heat-fluid transport is an important way to control shape and performance.The clear understanding of heat-fluid transport mechanism is the vital theoretical basis for the realization of heat-fluid transport regulation.To this end,the present disser tation systematically carries out the research on the heat-flow transport mechanism during the LDMD process based on the beam profile.The main results obtained are summarized as follows:(1)Considering the complex evolution characteristics of the spatia l profile of the focused fiber laser beam in free space and the absence of the accurate beam model,a modeling strategy of the beam profile is proposed based on a beam characteristic parameter identification method,and the beam profile model is then fabri cated.It is found that the super-Gaussian order of super-Gaussian beam(SGB)obeys the Lorentz distribution.And the principle of customizing the Gaussian profile(GP)or super-Gaussian profile(SGP)by adjusting the laser defocusing amount is clarified.(2)A homogeneous transformation based on modeling approach is proposed for a coaxial powder stream(CPS)with multi-powder stream components,aiming at addressing some existing shortcomings in the numerical investigation of the CPS,such as the time-consuming in numerically solving partial differential equation,and the difficulty in parametric modeling for the traditional analytical model based on solid geometry.A data processing method combining dimension reduction and normalization is proposed to properly solve the problem on spatial dimension and physical dimension inconsistency between measurement and simulation data.The influence of the structures of multi-channel coaxial powder feeding head on the CPS is investigated by the sensitivity analysis method,it is further suggested that the radial distance and the injection angle of the nozzle are the key parameters for designing the powder-fed head.(3)In order to understand the influence mechanism of the beam profiles on the energy transport in the laser-powder coupling process,the laser energy attenuation model is established via the Lambert-Beer law,and the powder heat transport model is developed based on the homogeneous transformation theory and lumped parameter method.When the laser spot is much smaller than the CPS spot,the beam profile does not affect the laser power loss rate,but affect the heat transport of the CPS in the laser-powder coupling process.It is confirmed that the heat transport characteristics of the CPS in the laser-powder coupling can be regulated by modulating the proper beam profile.(4)Based on the laws of conservation of mass,momentum and energy,apparent heat capacity method and Arbitrary Lagrangian-Eulerian(ALE)method are employed to establish a transport dynamic model for the molten pool,which includes the heat-fluid transport,the solid/liquid interface evolution and free interface evolution.The transport dynamic characteristics,such as the geometry of molten pool,heat transport and fluid flow,are revealed under the action of the SGP and GP.The results indicate that the beam profiles have an important influence on the size and temperature of the molten pool,the temperature gradient of the solidification interface,the average velocity of the fluid and its steady-state response time.The increase of laser power can induce the oscillation of fluid velocity,and the amplitude of oscillation is positively related to the laser power.An important finding is that the position of the boundary between the two vortices at the front and rear of the molten pool is consistent with that of the peak temperature of the molten pool.On both sides of the boundary,the Marangoni shear stress and vortex momentum along the horizontal direction are reversed,resulting in the ult ra-low flow velocity at the boundary.Dimensionless analysis shows that the SGP is beneficial to refine the microstructure and reduce the thermal deformation of the parts,while the GP has advantages in preheating the CPS and improving the powder utilizati on and the mass deposition rate.