Study On Microstructure And Mechanical Properties Of 316L Under Rescanning Process By Directional Energy Deposition

Directed energy deposition(DED)technology is promoting the revolution of manufacturing industry.It has outstanding advantages in the direct forming and repair of large and super large complex structural parts.However,the parts created by DED technology contain defects such as pores and poor fusion.The existence of these defects greatly restricts the development of DED technology,and it takes time and effort to improve the defects through subsequent heat treatment.Therefore,laser rescanning(LR)assisted process has been becoming an effective way to solve such problems.This paper focuses on the effects of different rescanning power on the microstructure and mechanical properties of 316 L stainless steel parts manufactured by LR assisted DED.Single pass deposition experiments and multi-layer multi-pass deposition experiments are carried out respectively.The main research contents and conclusions are as follows:(1)The defect repair ability of LR process is explored.For the single pass deposition process,the defect repair mechanism is explored in combination with numerical simulation and experiment.It is found that the internal defects of single pass deposition are mainly pore defects.The relative relationship between bubble overflow velocity and molten pool solidification velocity determines the formation of pore defects.The bubble overflow velocity is determined by the properties of molten pool liquid metal and bubble gas.Adjusting the process to repair the pores is realized by affecting the solidification of the molten pool;in the single channel deposition experiment,under the action of 1200 W LR process,the repair rate of pore defects reached 85.7%;in the multi-layer and multichannel experiment,the density is used to characterize the abundance of internal defects of the samples.Under the 1000 W LR auxiliary process,the defects are the least,and the density is up to 99.29%.Compared with the original deposited parts,the density is increased by 42.3%.(2)The microstructure evolution of 316 L stainless steel formed by DED and the effect of LR auxiliary process on the microstructure were investigated.It is found that the typical microstructure evolution of plane crystal,cellular crystal,dendritic crystal and equiaxed crystal exists in the single channel of DED forming,while in the multi-layer and multi-channel silence,the microstructure of the sample is mainly the subcrystal network structure composed of cellular and dendritic subcrystals;LR process has two influence mechanisms on microstructure: one is epitaxial growth mechanism,the other is grain refinement mechanism.(3)The effect of LR process on the mechanical properties of DED formed parts was explored.The results show that when the LR power is1000 W,the effect is the best,and the performance of DED formed parts is improved most significantly;LR has a significant effect on the plastic properties.When treated with 1000 W LR,the elongation is 48.2%,which is 46.3% higher than that of the original deposited parts,reaching the level of forgings;in other properties,the tensile strength increased slightly,only about 5%,and the hardness increased by more than 11.7%.