Effects Of Laser Beam Temporal/spatial Morphology On The Microstructure And Properties Of 316L Stainless Steel By Directed Energy Depositio

Laser-based directed energy deposition(L-DED)technology has significant advantages in deposition efficiency,forming size,high-performance parts repair and gradient material preparation.The deposition process involves multi-parameter action,multi-scale space-time evolution and multi-physical field coupling,the research of this technology focuses on the multi-physical field coupling of laser-powder-molten pool,microstructure solidification mechanism and control,deposition process and mechanical properties of parts.As the heat source,the temporal/spatial profile has a significant influence on the heat and flow transport process of molten pool,which determines the characteristics of solidification structure,and finally determines the macro-mechanical properties of parts.Comparing with other kind of alloys,316 L stainless steel is widely used in aerospace,military and marine industries because of its excellent mechanical properties,corrosion resistance and machinability,in addition,the mechanical properties of the material can be further improved by combining the L-DED technology.In view of the fact that the current research focused on the influences of the single-domain(temporal or spatial profile)of laser beam on the microstructure and properties,the temporal and spatial profiles of laser beam were taken as the control object,and the solidification structure,phase structure and mechanical properties were taken as the characterization object,the effects and mechanism of single-profile and multi-profile synergy on the microstructure and properties of 316 L stainless steel fabricated by L-DED are analyzed.Firstly,the evolution of the spatial profile of the laser beam in the transmission direction is measured by the beam quality detection system and fitted by the characteristics parameters identification method,and the single-track,single-layer(STSL)basic deposition experiments were carried out,the regression models for deposition height and width and laser power,scanning speed,powder feeding rate were established,and the single-track multi-layer(STML)z-increment model was verified.Based on the technique of laser beam spatial profile control,STSL and STML deposition experiments on spatial profile,including super-Gaussian profile(SGP)and Gaussian profile(GP)were carried out.Compared with GP,microstructure under the SGP has finer grain and fewer columnar grains,resulting the increase of microhardness due to the fine grain strengthening effect.Subsequently,4 × 4 single-factor deposition experiments on temporal profile were carried out with frequency F and duty cycle Dc of pulsed laser beam as control objects,the data of deposition height,penetration depth and dilution rate of molten pool morphology prepared by STSL specimen,individual grain size and area of microstructure prepared by STML specimens were acquired by image recognition software,the equivalent diameter of equiaxed grain and the ratio of columnar to equiaxed grains were calculated by MATLAB.Compared with continuous wave laser,the equivalent diameter of F80Dc60 is reduced by 27.4%,and the microhardness value is increased by 20 HV.Finally,by combining weight assignment and score calculation method,three optimal temporal profile including F20Dc90 /F80Dc60 /F40Dc90 were selected,the multi-track and multi-layer deposition experiments of temporal profile and spatial profile,which were F20Dc90,F80Dc60,continuous wave profile and GP and SGP,were carried out to explore the synergetic of spatial and temporal profile,the changes of microstructure,phase structure,microhardness and tensile properties including yield strength,tensile strength,elongation rate were analyzed.Compared with the continuous wave laser,the pulse temporal profile can refine the grains,produce more ferrite phases and increase the microhardness.Compared with typical continuous GP,the yield strength,tensile strength and elongation rate under optimal profile SGPF80Dc60 are increased by 12.8%,22.1% and 10.5%,respectively,the mechanical properties of parts can be further improved by temporal and spatial profile synergy.