Research On The Arrest Process And Mechanism Of Nickel-based Alloy Cracks By Pulsed Current Treatment

Among the various damages of metal materials,cracks are extremely harmful.The stress concentration at the crack will reduce the bearing capacity and even cause material failure.Pulsed current crack arresting uses pulsed current as energy input,which has the advantages of short crack arresting time and influence on non-cracks zone.In this paper,the pulsed current is used to arrest the prefabricated cracks in the GH4169 nickel-based alloy sample,and the external force is loaded on both ends of the sample to improve the crack arrest effect.The temperature field and current density field simulation,pulsed current crack arrest process,microstructure and mechanical properties evolution of crack area are studied.The ABAQUS finite element analysis software was used to simulate the current and temperature distribution in the process of pulsed current crack arrest,and the appropriate pulsed current parameters were determined.The simulation results show that when the peak current is 1000 A,the current density of the matrix part is 76.89 A/mm²,the current density at the crack tip is 200.20 A/mm²,and the metal melting point is reached within1.64 s.Combine with the simulation results and the actual test conditions,the appropriate frequency and duty cycle of the pulsed current are determine to be 20 Hz and 0.5.On the basis of simulation,the influence of discharging time and external force on crack arrest effect was studied experimentally.When discharging time is too short or the external force is too small,the best crack arrest effect cannot be achieved,but when the time is too long or the external force is too large,the sample deforms and break.When the discharging time is 2 s,the crack tip melts,which is close to the simulation results.When the discharging time is 3.5 s and the external force is 500 N,the prefabricated crack of 2.5mm length in GH4169 nickel-based alloy is repaired.The microstructure evolution before and after crack arrest was compared and observed.After crack arrest,the microstructure of the original crack and its adjacent area of 11.13 mm² change from columnar crystal to fine equiaxed crystal,the average grain size decrease from 929.97μm to 16.59μm,and Laves phase is found at the grain boundary.The maximum texture intensity decreases from 32.61 to 1.97,which reduce the anisotropy.The current density of the matrix part is relatively low,and the Joule heat effect is not obvious,the microstructure of the matrix part does not change before and after the crack arrest,indicating the flow concentration effect of the pulsed current at the crack tip.In order to comprehensively consider the crack arrest effect,the changes of mechanical properties before and after crack arrest were observed.The results show that the average hardness near the crack of the specimen decreases from 446 HV to 211 HV after crack arrest.The reason is that Laves phases consume strengthening elements Mo and Nb at the crack tip after crack arrest and the dislocation density is reduced by electronic wind.The maximum tensile force of the specimen is increased from 1.48 k N to8.60 k N,which do not reach the maximum tensile force that the matrix could withstand.This is due to oxidation on the crack surface before crack arrest.After crack arrest,a continuous distribution oxide band compose of Al₂O₃ and Ti O₂ with a width of 2.37μm appear at the original crack area,which affect the further improvement of tensile properties.In this paper,the pulsed current arresting of prefabricated cracks in nickel-based alloy is realize,which can provide reference for crack arresting of other metal materials.