Process Performance Study Of Laser Gas Nitriding On The Titanium Alloy Substrate

Laser gas nitriding method was taken to improve the wear resistance and anti-cavitation performance of the large steam turbine blades.Under the protection of homemade device,high-purity nitrogen and argon were respectively taken as the reactant gas and protective gas.The nitride layer fabricated under different laser power,scanning velocity,nitriding atmospheres were analyzed through the observation of surface morphologies,microstructure,phase composition and micro hardness.The nitride layer fabricated under the optimum process conditions was uniform,crack-free,and without porosity defects.It has reasonable hardness gradient,wear resistance and anti-cavitation performance.The nitride and surface strengthening mechanism was also has been studied.The results show that in the range of 1200W?1800W,the larger the laser power value is,the denser dendritic volume fraction is.When the power is 1800W,the liquid film is cracked under effect of tensile stresses to form the crystal crack.Longitudinal cracks are formed at the boundary of crystal in the center of nitride layer.The results show that in the range of 8mm/s?14mm/s,the results show that the greater scanning velocities is,the smaller the nitride layer will be,and the denser dendritic volume fraction and the more smooth surface quality.In order to obtain the nitride layer with good comprehensive performance,the scanning velocities should be 10mm/s?12mm/s,while the range of energy density is 3.33kJ/cm2?4.OkJ/cm2.Only the critical power density can be reached,a large number of dendritic crystal will be formed.The nitrogen-to-argon flow ratios(N2:Ar)has great influence on the formation of nitride layer.At an energy density of 3.33kJ/cm2,P=1600W,V=12mm/s,N2:Ar=15L/min:5L/min,the main phases of the surface nitride layer are the hard phase TiN0.9 and small amount of Ti and no A1 and V compounds existed.The nitride layer depth can reach 960?m and the average hardness is 650HV0.3,which is 2.03 times that of the substrate.Reaching or exceeding the critical power density provides the nitriding process with molten pool and certain amount of activated nitrogen atoms.Scanning velocities affect the interaction time between the activated atoms and melt.The decrease of nitrogen-to-argon flow ratios(N2:Ar)will suppress adsorption and migration process of N2 molecule in the molten pool surface,and then lead to the inhibition effect of nitriding process.There is a trend that the anti-cavitation performance of nitride layer increases along with the increasing of hardness value.16 hours cavitation test showed that the anti-cavitation performance of nitride layer was 2.61 times than that of substrate.The basket-weave microstructure was constructed by the dendrite,which can reduce the motion impact of the vacuoles.It has a limitation on the region where cavitation phenomenon occurs.