Study On Key Factors Affecting Crack Initiation And Propagation In Electrospark Strengthened Layer

With the high-quality development of industrial technology,the surface properties of TC4 titanium alloy parts are required to be higher and higher.Due to the poor wear resistance and low hardness of TC4 titanium alloy,the application scope of TC4 titanium alloy is limited.Powder mixed quasi dry EDM surface strengthening technology can improve the surface properties,but the cracks in the strengthening layer affect the surface properties.Therefore,it is of great significance to study the key factors of crack initiation and propagation for improving the life of strengthening layer in special environment.Based on the theory and experiment,the surface strengthening and high temperature resistance of TC4 titanium alloy were tested under different peak current,pulse width and powder composition.The specific research contents are as follows:(1)The strengthening mechanism of powder mixed quasi dry EDM surface strengthening technology and the formation mechanism of strengthening layer crack were studied.The process of EDM discharge,the formation process of strengthening layer and the conditions of crack formation were analyzed.(2)Based on ABAQUS finite element simulation software,the stress field of TC4 titanium alloy strengthened by powder mixed quasi dry EDM was simulated under different peak current,pulse width and B4 C,Al and Ni powder as strengthening medium,and the stress field of strengthened layer under 650 ? high temperature test condition was simulated.The results show that when the peak current is 8.2A,pulse width is 100 ? s and Al powder is used as strengthening medium particles,the stress value of strengthening layer is the smallest,about 3.763×108pa,and the number of crack initiation and propagation length may be the smallest.(3)The powder mixed quasi dry EDM strengthening test of TC4 titanium alloy and the high temperature resistance test of strengthening layer at 650 ? were carried out respectively,the effects of peak current,pulse width and powder types(B4C,Al and Ni powders)on the microstructure and crack initiation distribution of the modified strengthening layer were studied,as well as the microstructure,crack initiation and propagation of the strengthened layer under high temperature test conditions.It is found that when the peak current is 5.3-6.6A,the average crack length is the smallest,which is 8.634-9.893?m;when the Al powder is strengthened,the average crack length is the smallest,which is 16.562?m.Under the high temperature test conditions,the increase of crack initiation is the largest when the peak current is 6.6A;when B4 C powder is strengthened,the increase of crack initiation is the least;under the conditions ofpeak current of 6.6-8.2A,pulse width of 90-100?s and B4 C powder mixing,the strengthening layer has the best effect of resisting crack initiation and crack propagation at high temperature,and the hardness of strengthening layer is the best The best performance.(4)B4C,Al and Ni powders were mixed according to the same mass ratio as strengthening powder.The microstructure,crack initiation and propagation of strengthening layer under different mixed powder conditions were analyzed.It is found that when B4 C and Al powder are mixed,the number of crack initiation on the strengthened layer decreases,and the average crack length is 19.037?m,the strengthening layer has better comprehensive properties and fewer cracks.