Study On Gas-molten Pool Coupling And Regulation Mechanism Of Laser Nitriding Of Titanium Alloys

Titanium alloys have excellent specific strength,corrosion resistance,performance at high temperature,and biocompatibility,and they are widely used in aerospace,energy and power,marine engineering,petrochemical,defense,and biomedical industries.However,the poor surface hardness and abrasion resistance of titanium alloys have limited their wider application.Poor surface properties have become the core issue that restricts the wider application of titanium alloys.For instance,1000 mm super-long titanium alloy steam turbine blades are made of Ti-6Al-4V.Moreover,according to the design requirements,the blade surface microhardness must be higher than 500 HV;however,the surface microhardness of Ti-6A1-4V is only 250 HV,and thus,cannot meet the design requirements.Laser in-situ nitriding of titanium alloy surfaces can effectively improve their surface properties.At present,researchers all over the world study laser in-situ nitriding of titanium alloy surfaces primarily by focusing on parameter-related aspects.Moreover,the results are fragmented,lacking effective evaluation and correlation between process and performance through measurable and controllable physical characteristics and rigorous theoretical models.Further,a theoretical system that reveals the intrinsic connection between process factors and performance and quality control has yet to be established.Due to the poor understanding of the mechanism of titanium alloy nitriding process and the complex and interactive effects of process control elements,the reproducibility of process parameters is poor,and the research and test results of different literature are often contradictory and difficult to verify.In the nitriding process of blade parts with spatial shapes,the poor repeatability and non-uniformity of the process are prominent due to the influence of laser gas coupling and gas and molten pool coupling behavior on the transport and distribution of nitrogen elements.To address the above-mentioned problems,this research focused on the following aspects.（1）This study systematically examined the influence of process parameters on the surface formation of fiber laser nitrided layer and tested the mechanical properties of this layer to optimize the surface formation of nitrided layer and verify the feasibility of laser nitriding process.According to the design specifications,the surface roughness of the turbine blades must be strictly controlled.However,the nitriding layer is hard and thin,and its surface formation cannot be achieved by secondary mechanical processing.Therefore,the effect of process parameters on the surface formation of the nitrided layer was systematically investigated in this study.Univariate analysis was used in this study,that is,other parameters were unchanged,and different laser power,laser scanning speed,nozzle distance,N₂ flow rate,and defocusing amount were used to conduct nitridation experiments and the surface formation of the nitrided layer was measured The experimental results showed that the laser power mainly affected the oxidation behavior of the nitrided layer surface,and excessively high laser power led to the oxidation of the nitrided layer surface.The laser scanning speed primarily affects the surface roughness of the nitrided layer,with increased surface roughness observed under both excessively high and low laser scanning speeds.Nozzle distance significantly affects the nitrided layer surface formation,which must be strictly controlled to be in the range of 3–5 mm;else,the nitrided layer would be highly oxidized.N₂ flow rate has little effect on the nitrided surface formation,and the ideal nitrided layer formation can be obtained under both high and low N₂ flow rates.Defocus amount mainly affects the surface roughness of titanium alloys.Excessively high defocus amount can result in a low power density of titanium alloy surface,and subsequently increase the surface roughness of the nitrided layer.In addition,to clarify the mechanical properties of fiber laser nitrided layer,its microhardness and abrasion resistance were tested.The microhardness of the surface of the fiber laser nitrided layer exhibited a gradient distribution,and the microhardness of the upper surface was the highest,that is,four times that of the base.The abrasion resistance of the nitrided layer was increased by 57.1%compared with that of the Ti-6Al-4V base.The microhardness and abrasion resistance were able to meet the design requirements of titanium alloy turbine blades.（2）Focusing on the mechanism of nitrogen transition to the molten pool,an in-depth investigation of the laser-N₂ coupling behavior was conducted.To explore the effect of the fiber laser on N₂,the spatial conductivity was tested,spatial specific wavelength radiation photographs were taken,and spatial emission spectra were collected.The results of these three experiments showed that the spatial N₂ was not ionized during the fiber laser in-situ nitriding of titanium alloys.Theoretical calculation was performed to determine the fiber laser power density required to ionize the spatial N₂,and the result showed that the fiber laser power density required to ionize spatial N₂ was higher than1830 MW/cm².However,the maximum laser power density output was only 0.17 MW/cm².Therefore,the fiber laser was unable to ionize the spatial N₂.Notably,N₂ cannot be ionized,and there is no shielding effect of plasma on the laser energy;hence,during the fiber laser in-situ nitriding of titanium alloys,the laser did not couple with N₂,and N₂ transitioned to the molten pool surface in the molecular form.The effect of spatial un-ionized N₂ on the nitrided layer was further investigated,and it was found that the nitrided layer had lower nitrogen content,fewer defects,higher oxygen content,distinct black traces on the surface,and lack of smooth area on the surface under spatial un-ionized N₂.The surfaces of nitrided layers under both ionized and un-ionized spatial N₂ shared similarities in that there were nitrogen-rich peaks and titanium-rich troughs on the sides and Ti N dendrites in the middle of the nitrided layer.（3）In this study,the transport mechanism of nitrogen in the molten pool was used to investigate the coupling behavior of nitrogen and the molten pool.To study the nitriding molten pool convective process,the nitriding layer characteristics were analyzed offline,and it was found that the nitrided layer contained irregular cross-sectional morphology,the nitrogen content and microhardness were in a gradient distribution along the depth,and the microstructure of the nitrided layer consisted of continuous Ti N layer on the top,equiaxed crystal layer in the middle,and Acicular structure at the bottom layer.It can be inferred from the cross-sectional characteristics of the nitrided layer that the nitriding molten pool had unique convective process.The convective process on the surface of the molten pool was photographed online,and it was found that the convective behavior could be divided into large and small convections,showing intermittent,localized,and random characteristics,which confirmed the previous speculation.The flow process of the nitriding molten pool was numerically simulated.The simulation results showed that due to the formation of Ti N in the molten pool,the melting point of the molten pool and the viscosity increased,resulting in its unique convective process.The transport behavior of nitrogen was also numerically simulated.The simulation results indicated that molten pool convection was the main factor promoting the transport of nitrogen,and diffusion had very weak effect.Because of the intermittent and localized nature of the nitriding molten pool convections,nitrogen transport was not thoroughly carried out,and nitrogen in the molten pool and nitrided layer was in a gradient distribution along the depth.Overall,the effect of nitrogen was reflected in the influence of nitride on the thermophysical properties of the molten pool,which in turn affected its convective behavior.The effect of the molten pool on nitrogen was exhibited by the dominant role the molten pool convection played in nitrogen transport.As the convection was intermittent and localized,nitrogen transport was not sufficient,and the nitrogen in the nitrided layer was in a gradient distribution along the depth.The surface of the nitrided layer had high nitrogen content,low plasticity,and toughness.Moreover,it was prone to cracks.Nitrogen exhibited a strong coupling effect with the molten pool and directly impacted the composition,structure,properties,and defects of the nitrided layer.（4）In this study,Ar was used to dilute N₂ to control the quality of the nitrided layer and the mechanism of the regulation was investigated in detail.N₂ dilution by Ar was used to regulate the flow process and nitrogen transport in the nitriding molten pool.The flow behavior of the nitriding molten pool surface was photographed online under different N₂ and Ar flow ratios.The results indicated that the dilution of N₂ by Ar changed the convective behavior of the molten pool from intermittent convection to continuous convection,from small local convection to large local convection,and from slow convection to fast convection.Therefore,the use of Ar to dilute N₂ can effectively regulate the convective process of the nitriding molten pool.Numerical simulations of the molten pool flow behavior in diluted N₂environment showed that the convection was continuous with a larger convection area and faster convection speed due to the gradually generated Ti N phase on the surface of the molten pool and the decrease in its melting point.Moreover,the temperature of the melt pool surface was always higher than the melting point.The cross-sectional characteristics of the nitrided layer were examined.It was discovered that the cross-sectional morphology of the nitrided layer changed from irregular to a regular semi-ellipsoidal shape,the nitrogen content and microhardness of the nitrided layer were more uniformly distributed,the size of the equiaxed crystals decreased and became sparser,the range of the acicular structure expanded,and the cracking of the nitrided layer reduced.In conclusion,the use of Ar for the dilution of N₂ can delay the generation of Ti N phase in the molten pool,which affected its thermophysical properties,and further regulated the flow behavior and the nitrogen transport mechanism in the molten pool.It led to a more uniform nitrogen distribution in the nitrided layer and reduced the nitrogen content of the surface,which effectively improved the plasticity and toughness of the top layer and avoided the formation of cracks and defects on the surface of the nitrided layer.