| 316L stainless steel is used to produce high-performance complex parts because of its excellent comprehensive performance.Laser additive manufacturing technology is high efficiency,flexible,can prepare good microstructure,which can be used to manufacture complex 316 L stainless steel parts.However,the laser additive manufacturing deposition process is accompanied by extremely complex physical,chemical and mechanical behavior,and the forming quality is affected by various process parameters.Among them,the spatial laser beam profiles directly affect the heat transport behavior of the molten pool,which in turn affects the microstructure and properties of the deposited parts.Therefore,understanding the influence mechanism of the spatial laser beam profiles on the thermal transport behavior and microstructure in the deposition process of additive manufacturing is the theoretical basis and prerequisite for the performance control of the formed parts.In this paper,the effect of spatial laser beam profiles on the thermal transport and microstructure of coaxial powder feeding additive manufacturing is investigated for 316 L stainless steel.Firstly,the spatial power density distribution data of the focused laser beam in the coaxial powder feeding additive manufacturing experiment are collected by using the beam quality diagnosis platform.Based on the superGaussian model,the characteristic parameters of the spatial laser beam profiles are identified by fitting method,and the super-Gaussian beam model is reconstructed.Gaussian and elliptical Gaussian beam models are established based on the equivalent laser power density.A coaxial powder beam model was established,and a powder feeding operation measurement experiment for coaxial powder additive manufacturing was conducted to obtain the powder feeding operation parameters in the model.Secondly,a three-dimensional heat transport model of laser additive manufacturing with coaxial powder feeding was established.The effects of four spatial laser beam profiles,including Gaussian(GP),super-Gaussian(SGP),longitudinal elliptical Gaussian(LEGP)and transverse elliptical Gaussian(TEGP),on the morphology characteristics of the molten pool,heat transport and fluid flow within the molten pool were investigated.The results show that the morphology of the molten pool is determined by the spatial profile of the laser beam.the molten pool under LEGP is the gentlest,while the molten pool under TEGP is the steepest.The maximum temperature in the molten pool increases successively under SGP,TEGP,GP and LEGP.The temperature distribution in the liquid phase region under GP and SGP is significantly different,but it is approximately the same in the solid phase region.The molten pools exhibit an outward annular flow pattern under all four spatial laser beam profiles with fluid flows mainly driven by Marangoni shear stress.Heat transfer within the molten pool is dominated by Marangoni convection and heat conduction.The velocity distribution on the trajectory line along the laser scanning direction on the surface of the molten pool is ’ M ’-shaped bimodal distribution.Average fluid velocities within the molten pool decrease successively according to the following order: LEGP,GP,TEGP and SGP.Finally,the solidification model of the solidification interface is established.The influence of the four laser beams on the solidification characteristic parameters of the solidification interface is calculated and analyzed,and the microstructure under four laser beams is predicted.The results show that the characteristic parameters of the spatial profile are highly dependent on the depth of the molten pool.The temperature gradient and profile parameters increase with the depth of the molten pool,and the solidification rate and cooling rate decrease with the increase of the depth of that.The solidification rate decreases successively under GP,SGP and TEGP,and the solidification rate under LEGP is the smallest.When the pool depth is less than0.2mm,the cooling rate increases successively under GP,SGP and TEGP,and decreases when the pool depth is greater than 0.2mm.The crystal morphology of the four spatial laser beam profiles is mainly columnar crystals.Under GP,SGP and TEGP,the dendrite arms spacing at the rear of the molten pool increases successively,and the bottom of the molten pool decreases successively. |
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