Effect Of Double Glow Plasma Hydrogen-free Carburizing On Mechanical Properties Of TC4 Titanium Alloy

TC4 titanium alloy is widely used in aerospace because of its excellent corrosion resistance and high specific strength.It is mainly used in aircraft engine parts.However,the low surface hardness and poor wear resistance of TC4 titanium alloy have limited its development.Therefore,this project uses a double-layer glow plasma hydrogen-free carburizing process to improve the wear resistance of TC4 titanium alloy while avoiding the hidden danger of hydrogen embrittlement.The friction and wear behavior of TC4 titanium alloy after hydrogen-free carburizing under different load conditions and different process parameters was studied through friction and wear tests.The friction coefficient,wear morphology,and wear amount were analyzed from three aspects of the substrate and the impact of carburized layer friction and wear.In addition,because the hydrogen-free carburizing process will have a great impact on the fatigue properties of titanium alloys,this paper uses fatigue tests to study the fatigue behavior of titanium alloys under different stress levels under cyclic loading conditions through hydrogen-free carburizing,and explores effect of hydrogen carburizing on fatigue crack initiation of TC4 titanium alloy.Based on the previous research,this paper analyzed the effects of various factors on the quality of the carburized layer,and found that the temperature of the carburized layer had the greatest impact on the quality of the carburized layer.Therefore,in this subject,three sets of process parameters are formulated by adjusting the source voltage parameter to change the temperature.SEM and EDS analysis results show that under process 1(980-1000?),the thickness of the carbide layer is about10?m,and the thickness of the diffusion layer is about 10?m.In process 2(890-910?),the thickness of the carbide layer is about 6?m,and the thickness of the diffusion layer is about 5?m.In process 3(800-830?),the thickness of the carbide layer is about 6?m,and the thickness of the diffusion layer is about 3?m.The XRD analysis results show that after double glow plasma hydrogen-free carburizing treatment,a strengthened phase composed of Ti C ceramic phase and CTi_0.42V_1.58 is formed on the surface of TC4 substrate.The microhardness test results show that the hardness of the TC4 matrix is only 285HV_0.2,and the surface hardness of the carburized layer are 1088HV_0.2,989HV_0.2,and 931HV_0.2 after three process parameters without hydrogen carburizing treatment,compared with the TC4 matrix hardness increased by 3-4 times.The friction and wear tests were conducted to investigate the wear resistance of the substrate and process 1 samples under different load conditions.The wear morphology results show that the carburized layer mainly shows the characteristics of slight abrasive wear,while the TC4 matrix shows the coexistence of adhesive wear and abrasive wear.The results of specific wear rate show that when the load is 10N,the specific wear rates of TC4 matrix and carburized sample are288.9×10^-6mm³N^-1m^-1 and 12.4×10^-6mm³N^-1m^-1,respectively,which shows that carburized layer has a better protective effect on the matrix.Comparing the wear resistance of carburized samples under three process parameters,under 20N load,the specific wear rate are 146.4×10^-6mm³N^-1m^-1,286.50×10^-6mm³N^-1m^-1,422.23×10^-6mm³N^-1m^-1,indicating that the relationship between the wear resistance of the carburized layer prepared by the three process parameters is shown as:Process 1>Process 2>Process 3.Comparing the fatigue performance of the substrate and the three process parameters through fatigue tests,the results show that the fatigue limit of the TC4 titanium alloy treated by the three processes has been reduced by 70%,50%and 80%relative to the matrix.This shows that the hydrogen-free carburizing process will reduce the fatigue performance of TC4 titanium alloy.The results of the roughness test show that the hydrogen-free carburizing process will increase the roughness of the TC4 surface and reduce the surface integrity of the sample.The titanium alloy is very sensitive to the surface integrity and will promote crack source nucleation in the area of local stress concentration,which adversely affects fatigue performance.And because of the higher temperature in the carburizing process,the titanium alloy grains are coarse and the network structure appears,which causes fatigue sources to appear in multiple regions and accelerates the fatigue rate.