Research On Microstructures And Properties Of 316L/Nb Stainless Steel Matrix Material Fabricated By Selective Laser Melting

316L stainless steel is widely used in chemical,marine and automotive industries and so on for its good corrosion resistance,creep resistance,high-temperature resistance,high toughness,non-magnetic properties.However,the requirements for strength,hardness,and wear resistance of 316L stainless steel are increasing in the aerospace field with the rapid development of aerospace technology in recent years.Such as aero-engine systems and aircraft structural parts(the engine's high-pressure compressor wheel and aviation fuel and hydraulic control system and other parts)require not only higher strength but also need better wear resistance.In recent years,SLM technology provides an effective processing method for manufacturing complex-shaped,high-precision metal matrix composite parts due to the development of high-power lasers.The defects of SLM 316L stainless steel are mainly concentrated in microcracks and pores,and the post-treatment process is the main method to eliminate/repair microcracks and enhance the denseness of SLM parts,whereas the more mature post-treatment process is hot isostatic pressing(HIP)treatment.This work analyzed the microstructure and properties of SLM 316 and 316L/Nb composites,in combination with HIP technology,investigating the interaction between the phases in the laser pool and the strengthening mechanism,wear mechanism and corrosion mechanism.Specifically,the microstructures of SLM specimens were analyzed by OM,SEM,TEM,EBSD and EDS under different SLM parameters and content of Nb,and their room temperature hardness,tensile mechanical properties,wear properties and corrosion properties were also tested to optimize the SLM parameters and determine the optimal Nb additions.Moreover,the effects of HIP(950?,120 MPa,2h)on the microstructures and properties of SLM 316L and 316L/Nb composites specimens were investigated to reveal the microstructure evolution of SLM specimens after HIP treatment and its correlation with property changes.The research is mainly divided into three sections as follows:(1)The results show that the microstructure of SLM 316L tends to be sub-crystal(cellular sub-structure)and a large number of nano twins are observed,which can effectively hinder the dislocation movement.With the addition of 1 wt.%Nb,the region of the equiaxed crystal zone in the remelting zone of the melt pool is reduced,and there is a tendency for the equiaxed crystals to transform into equiaxed dendrites and columnar dendrites.When the addition of Nb increases to 5 wt.%,the microstructures of the SLM specimens is still a single-phase austenite,but part of the unmelted Nb particles can be obviously observed and the laser pool boundary becomes blurred,resulting in the lava-like organization microstructure.The columnar crystals in the laser pool are transformed into finer equiaxed crystals by the thermal effect and the cooling rate,accompanied by some small size of bulk precipitates in the fine equiaxed crystals.The mean grain size of the such composites decreases from 20.9?m in 316L to 14.2?m.(2)The 316L was successfully fabricated at a laser power of 214.2 W,a laser scanning speed of 928.1 mm/s,the scanning pitch of 100?m,the powder layer thickness of 40?m and the laser energy density of 57.7 J/mm~3.The Ultimate Tensile Strength(UTS),Yield Strength(YS),and Elongation(EL)are 702 MPa,627 MPa and 36.0%,respectively.With the addition of 5 wt.%Nb,the 316L/Nb composites were successfully fabricated at a laser power of 370 W,a laser scanning speed of 925 mm/s,the scanning pitch of 100?m,the powder layer thickness of 40?m and the laser energy density of 100 J/mm~3,with UTS up to 990 MPa and YS increasing to 840 MPa while maintaining 16.0%elongation.With the addition of Nb(5 wt.%),the microhardness of the composites increased from 213 HV₁ to 290 HV₁.The strength increase of316L/Nb composites is considered to be the sum of fine grain strengthening,dislocation strengthening,precipitation strengthening,thermal mismatch strengthening,solid solute strengthening and load transfer strengthening.The friction coefficient of SLM 316L,316L-(1%)Nb and 316L-(5%)Nb-H are about 0.70,0.76 and 0.87,respectively.However,the wear rate of the composites was greatly reduced under the actions of abrasive wear mechanism and adhesive wear mechanism.The self-corrosion potentials of SLM 316L,316L-(1%)Nb,316L-(5%)Nb-L and 316L-(5%)Nb-H are-0.26 V,-0.22 V,-0.19 V and-0.17 V,respectively.With the addition of Nb,the corrosion resistance of SLM 316L/Nb is better than that of 316L,and 316L-(5%)Nb-H shows the lowest corrosion rate.Many aggregation areas of Mo are formed and evenly distributed in the composites with the addition of Nb.Mo can make the passive film denser and stabler,further improving the corrosion resistance of such composites.Moreover,lots of the Nb-containing nano-precipitates produce along the grain boundary,significantly improving the intergranular corrosion resistance of such composites.(3)The fish-scale-shaped laser pool disappears in HIP 316L,and the microstructure shows a coarser columnar crystal,containing many fine grain structures within the crystal.After HIP treatment,the porosity of SLM specimens decreased and the density increased to more than 98%.The strength of SLM parts decreased significantly,among which the yield strength decreased especially,the elongation increased to different degrees,and the work-hardening ability of the material was greatly improved.After HIP treatment,the friction coefficient of 316L-(5%)Nb-H did not decrease significantly,but there were large local fluctuations in the friction coefficient.the self-corrosion potential of the HIP samples decreased compared with the SLM samples,indicating that their corrosion resistance decreased overall after HIP treatment.