Microstructure Evolution And Vacuum Tribological Properties Of Surface Nitriding Layer On A TiZrAlV Alloy Fabricated By Laser Nitriding

TiZrAlV alloy is considered a potential material for aerospace,marine ships and other industrial fields because of its excellent high specific strength and dimensional stability.However,its poor surface properties such as hardness,wear resistance,and fatigue resistance also lead to surface wear and fatigue failure when it is in service under severe working conditions.In this paper,the Laser Surface Nitriding:LSN technology is used to prepare a nitrided layer with high hardness and high wear resistance on the surface of Ti Zr Al V alloy by changing LSN parameters such as laser beam diameter and scanning speed.X-ray diffractometer（XRD）,scanning electron microscope（SEM）,energy dispersive spectrometer（EDS）,transmission electron microscope（TEM）and electron probe X-ray microanalysis（EPMA）were used to study the evolution of phase composition and microstructure of nitrided layer with LSN parameters and the temperature field distribution of Ti Zr Al V alloy during LSN treatment was simulated by ABAQUS software.Additionally,a microhardness tester determined the samples’hardness distribution and evolution law along the direction from the surface to the inside of the matrix and a friction and wear tester conducted a vacuum rolling load-bearing experiment on the ready samples.The frictional wear behavior of the vacuum was analyzed using a frictional wear tester and the wear mechanism was analyzed.The results of the investigation into the phase composition and microstructure evolution of nitrided layers under various laser parameters indicate that the two primary constituent phases of nitrided samples are theαphase and the nitride phase（Ti（Zr）N and Ti（Zr）N0.3）.The（002）and（101）peaks of theαphase gradually decline as the laser beam diameter or scanning speed lowers,whereas the（200）and（101）peaks of Ti（Zr）N and Ti（Zr）N0.3 progressively increase,indicating a gradual increase in the amount of nitride present and a gradual rise in density.Additionally,observation of the laser beam’s microstructure reveals that as it progressively shrinks from 3 mm to 1 mm（when the spot diameter is 1 mm～397μm）,the average thickness of the nitride layer increases significantly.When the scanning speed gradually decreases from 25 mm/s to 10 mm/s,the nitride layer thickness exhibits a similar evolution trend（when the scanning speed is 10 mm/s～334μm）.This is mainly due to the increase in laser energy density.The results of elemental distribution such as EPMA and EDS confirm that the main nitride is Ti（Zr）x Ny.At the same time,it can be confirmed that the composition distribution of elements such as Zr,Al and V has evolved after laser nitridation.ABAQUS software was used to analyze how laser processing factors affected the temperature field.The analysis shows that the peak temperature of the sample surface deviates from the center of the laser spot to the backside.In the direction of width,the temperature gradient is zero at the center of the molten pool;In the depth direction of the molten pool,the highest gradient exists at the surface,then falls while approaching the bottom.As the laser beam diameter or scanning speed decreases,the surface peak temperature gradually increases（when the laser beam diameter decreases from 3mm to1mm,the maximum surface temperature rises from 1980℃to 3009℃;when the scanning speed decreases from 25 mm/s to 10 mm/s,the maximum surface temperature rises from1648℃to 2175℃）and the molten pool’s depth and breadth gradually grow,causing the temperature gradient to rise in the direction of the molten pool’s depth and width.The evolution of microhardness and friction and wear morphology of surface nitrided layers under different laser parameters were studied,and the results showed that:As the laser beam diameter or laser scanning speed decreases,the hardness of the laser nitrided layer gradually increases(the hardness is～897 HV_0.5 when the laser beam diameter is 1mm,and the hardness is～871 HV_0.5 when the scanning speed is 10 mm/s),while the friction coefficient and wear mass loss of the nitrided layer gradually decrease,(When the laser beam diameter is 1mm,the sample has the lowest COF value,about 0.333,which is 15.98%lower than the original sample.The wear volume and wear rate are the smallest,respectively0.0096 mm³ and 0.85×10^-8 mm³/Nmm).The reason is that laser nitriding can refine surface grains and generate hard nitrides.When the light spot diameter or scanning speed gradually decreases,the dissolved nitrogen content in the nitriding layer during liquefaction increases,increasing the volume fraction of the nitrided phase formed,increasing hardness and friction,and wear resistance.The study confirmed that the wear of the substrate material is abrasive wear and adhesive wear and the wear of the nitrided layer is mainly abrasive wear,manifested by micro-cutting and peeling of hard phases.