Study On The Key Factors Affecting The Crack Of Power-mixed Near-dry EDM Strengthening Layer

As a new industrial material in the 21st century,titanium and titanium alloys are widely used in various fields such as aero-engine manufacturing,steam turbine manufacturing,and medical equipment manufacturing.However,with the rapid development of industrial technology,its requirements for high temperature resistance,corrosion resistance,hardness performance,processing performance,and service life are getting higher and higher.The use of traditional titanium alloy matrix materials is limited,and it needs to be surface strengthened.However,crack initiation occurs inevitably in various surface strengthening layers,which reduces the service life of the material.At present,domestic and foreign scholars'research on surface strengthening technology mainly focuses on how to improve the performance of the strengthening layer,and rarely studies the factors affecting the crack initiation of the strengthening layer.Therefore,studying the factors affecting the crack initiation of the strengthening layer is of great significance for improving the service life of the material and expanding the application range of the material.Based on theoretical analysis,finite element simulation and experimental research,this paper used power-mixed near-dry EDM surface strengthening technology to strengthen TC4under different conditions,such as peak current,pulse width,pulse gap,electrode material and polarity,powder material and concentration.Investigating the crack initiation of titanium alloy surface strengthening layer.The process parameters that have a serious impact on the crack initiation of the strengthening layer are obtained.The main research contents are as follows:?1?In order to study the influence of temperature field and stress field distribution on crack initiation of strengthening layer,a finite element model is established,Simulation study on temperature field and stress field distribution in different strengthening parameters during discharge process.The results show that the peak current and pulse width increase gradually,and the temperature field and stress field influence range of the discharge process increase gradually.The radial temperature gradient difference and stress value of the workpiece surface are larger than the axial direction.The surface of the strengthening layer is more susceptible to cracking than the inside,and the pulse width has a greater influence on the crack initiation than the peak current.When I_p=6.6 A is strengthened,the stress value is 337 MPa minimum.When t_on=80 us,the stress is 341 MPa minimum and the number of crack initiation may be the smallest.?2?Performing the surface strengthening experiment of mixed powder electric spark,and analyzing the microstructure and crack initiation distribution of the surface and section of the strengthening layer.The results show that peak current,pulse width,electrode material and polarity,and powder materials are the key factors affecting the crack initiation and distribution of the strengthening layer.The microcracks on the surface of the strengthening layer are mostly short cracks with a dispersive distribution.The average length is 8?m²4?m,and there is no crack branch near the main crack.Among them,the positive electrode of graphite and the negative polarity of copper have the most cracks in the strengthening layer,which are distributed in a network.There are 3⁵ branches around the main crack,90%of the crack nodes are three branches,the average length is 54.426?m,49.122?m,which is much larger than other parameters.Under the conditions of various processing parameters,there is no obvious crack initiation in the section of the strengthening layer,and there is no crack directly into the TC4matrix and through the strengthening layer.The peak current is 6.6⁸.2 A,the pulse width is80¹00?s,the pulse gap is 80¹00?s,the hardness of B₄C powder strengthening layer is the highest under the graphite negative processing condition,and the surface crack initiation distribution of the strengthening layer is less,and the finite element The simulation prediction results are basically the same.?3?A composite strengthening experimental study was carried out,and a process combination with less influence on the crack initiation of the strengthening layer was obtained.The results show that the crack initiation distribution of graphite negative polarity?copper positive strengthening layer is the most serious.The Al powder positive polarity?negative strengthening layer has the lowest cracking parameters and is the optimal composite strengthening process combination.