Laser Gas Alloying Process And Transmission Characteristics Of TC4 Alloy

Titanium and its alloys have excellent characteristics such as high specific strength,good corrosion resistance,and good biocompatibility.Therefore,titanium and its alloys are widely used in aerospace,medical,and energy engineering fields.For example,the new generation steam turbine blades are made of TC4 titanium alloy,but there will be problems such as low hardness,high friction factor and poor wear resistance.In order to meet the actual application,the surface modification and strengthening treatment of TC4 titanium alloy is needed to improve the hardness and wear resistance.Traditional surface strengthening methods(such as high-temperature nitriding,ion implantation technology,physical vapor deposition,and chemical vapor deposition,etc.)have low production efficiency,affect the performance of the substrate,the combination of the modified layer and the substrate is not strong,and the modified layer A series of problems and shortcomings such as too shallow depth,and these processing methods are not suitable for the treatment of large heterogeneous titanium alloy workpieces.Therefore,in this paper,a new process of laser gas nitridation is used to strengthen the surface of TC4 alloy.Laser gas nitridation has the advantages of chemically and metallurgically combining the nitriding layer with the substrate,which can realize local processing and is suitable for processing large shaped workpieces.For laser gas nitriding of large titanium alloy workpieces such as steam turbine blades,the desired nitrided layer should have characteristics such as high hardness,high wear resistance,large layer depth,and good surface quality.Most domestic and foreign scholars have studied the process optimization of laser gas nitriding and the microstructure and properties of modified layers,and there are few theoretical studies.In this process,the photoionization behavior of the laser heat source and gas interaction,and the flow characteristics of the material in the laser molten pool will affect the formation and distribution of nitrides,and then affect the quality of the nitrided layer.Scholars at home and abroad have less research on such issues.In this context,this paper focuses on determining the photoionization behavior of the laser heat source and gas during the laser gas nitriding process,clarifying the characteristics of nitrogen as the solute and the reaction medium;studying the flow characteristics of the material in the laser molten pool;optimization Process parameters,establish the relationship between process parameters,laser melting pool flow characteristics and the performance of the nitriding layer,so as to provide an effective reference for industrial production.This article mainly researches from the following three aspects:1.To investigate the photoionization behavior of the laser heat source and the gas before the nitrogen enters the laser melting pool.Under the experimental conditions of laser scanning speed of 10 mm / s,laser defocus distance of 5 mm,and total gas flow of 10 L / min,a laser gas nitriding experiment was performed on a TC4 plate with different laser powers as variables.The filter imaging method was used to photograph the photoionization behavior caused by the interaction between the laser heat source and the gas.As a result,no nitrogen plasma was observed.The optical ionization behavior generated by the interaction between the laser heat source and the gas was studied by means of the spectrometer detection method.As a result,no characteristic wavelengths of nitrogen atoms and nitrogen ions were found in the collected spectral lines;combined with theoretical calculation analysis,it was finally determined that the fiber gas laser was used to perform laser gas nitridation of TC4 alloy,and nitrogen did not exist before entering the laser molten pool.Away behavior.2.To study the transmission characteristics of the material in the laser molten pool of the laser gas nitridation process after nitrogen enters the laser molten pool.Using the high-speed camera system to study the surface flow behavior of the laser melt pool in the process,the observation found that the flow direction of the surface of the laser melt pool flows from the middle of the melt pool to the surroundings,and the flow of the melt pool surface is calculated by the marker The speed is about 0.714 m / s.Since the flow inside the molten pool during laser gas nitridation cannot be visually observed and measured,this topic uses an industrial microscope to photograph and observe the cross-sectional morphology of the sample after nitridation,in order to indirectly react the melt The flow behavior of the material in the pool,when the laser scanning speed is 10 mm / s,it is considered that the flow in the laser melt pool is a "two-phase flow" flow mode of the titanium nitride liquid phase and the titanium metal liquid phase at different laser power densities..Due to the different laser power density,the surface area of liquid titanium nitride is different,and the binding force of solid phase titanium nitride is different,and different "two-phase flow" flow patterns have appeared.3.To study the relationship between laser process parameters,different flow patterns of materials in the laser melt pool and the properties of the nitriding layer.A large number of process experiments were carried out,and it was found that different process parameters will cause different flow patterns in the laser melt pool,and the performance of the nitrided layer under each process parameter is compared and analyzed.It is found that the first "two-phase flow" flow pattern The performance obtained below is optimal,and friction and wear experiments were carried out on the samples of the optimal process.It was found that the friction coefficient of TC4 alloy after laser gas nitridation decreased and the wear resistance was significantly improved.