Mechanism Analysis And Experimental Study Of Ceramic Tool Crack Initiation And In Situ Repair

The high brittleness of ceramic tool material leads to high sensitivity to microdefects and the failure of crack propagation in cutting process is the main reason which leads to the reduction of the service life of ceramic tool.In order to investigate the crack initiation and propagation of ceramic tool during cutting and improve the service life of ceramic tool,the process of crack initiation and propagation of Al₂O₃/Ti N self-healing ceramic tool and its repair performance were studied in this thesis.In order to study the dynamic of crack initiation and propagation during the cutting process of ceramic tool,firstly,the ceramic tool model with local micro-morphology was established by Abaqus simulation software,then the ceramic tool damage model was constructed by combining the cohesive force unit,and finally,the cutting model was established.The effect of cutting speed and porosity on the crack initiation and propagation process of the front tool surface of ceramic tool was studied,and the cutting speed of the tool without porosity and with porosity was determined respectively.The results show that when there is no porosity in the tool model,the crack initiation occurs when the cutting speed is greater than 75 m/min.The crack initiation occurs first on the front tool surface,and the stress concentration occurs at the crack tip,and then the crack propagation occurs under the continuous action of external force.When there are pores in the tool model,the crack initiation occurs when the cutting speed of the tool is greater than 66 m/min.Due to the stress concentration phenomenon at the porosity,micro-cracks will occur at the porosity with the increase of the stress on the tool.The crack growth morphology of the tool is that the main crack at the front tool surface and micro-cracks at the porosity expand at the same time,and then converge.The increasing number of cracks accelerates tool failure.The self-healing ceramic material Al₂O₃/Ti N and Al₂O₃/Ti N/Ti Si₂ with crack repair function were prepared by spark plasma sintering（SPS）by adding Ti N and Ti Si₂ with self-healing function to the Al₂O₃ ceramic material.Firstly,the effects of sintering temperature and holding time on material properties were studied.It was found that when the sintering temperature was 1475℃and the holding time was 6 min,the Al₂O₃/Ti N ceramic materials prepared by SPS had good mechanical properties,and the Vickers hardness,flexural strength and fracture toughness were 18.74 GPa,574.5 MPa and 4.52MPa·m^1/2,respectively.When the sintering temperature is 1300℃and the holding time is 6 min,the mechanical properties of Al₂O₃/Ti N/Ti Si₂ ceramic materials are better.The Vickers hardness,flexural strength and fracture toughness are 16.75 GPa,719.31 MPa and 4.7 MPa·m^1/2,respectively.SEM fracture images show that ceramic materials with good mechanical properties have higher transgranular fracture ratio and higher density.By comparing the optimal sintering temperature and mechanical properties of Al₂O₃/Ti N and Al₂O₃/Ti N/Ti Si₂ ceramic materials,it is found that adding Ti Si₂ reduced the optimal sintering temperature of Al₂O₃/Ti N ceramic material by 175℃,and increased the bending strength by 25.20%.The fracture toughness was increased by 4.42%.In situ repair properties of self-healing ceramic materials were studied at 600,700and 800℃.It is found that the surface of cracks repaired by Al₂O₃/Ti N at 700℃has fewer pores and better crack repair effect.After heat treatment at 800℃,cracks are completely repaired but more pores are generated on the surface.Al₂O₃/Ti N/Ti Si₂ ceramic material has the best surface repair effect at 800℃,cracks are effectively filled,and surface pores are less.This is because Ti Si₂ will form an oxide film that prevents the contact between oxygen and the crack,improving the optimal repair temperature of Al₂O₃/Ti N ceramic materials.The cutting performance of self-repairing ceramic tool was studied.Firstly,Abaqus was used to study the temperature of Al₂O₃/Ti N ceramic cutting tool cutting Ti-6Al-4V at different cutting speeds to determine the distribution law of cutting speed and tool temperature when the tool could reach the repair temperature.The possible position of the repair topography of the ceramic tool in the cutting experiment is determined.The temperature cloud map shows that the repair phenomenon may occur within 0.07mm of the distance between the front cutter face and the cutting edge.The high temperature range of the tool is small,because the thermal conductivity of the tool is low,the heat transfer inside the tool is difficult,so that the heat is limited to the local range of the tool.Comparative cutting experiments of Al₂O₃/Ti N and Al₂O₃/Ti N/Ti Si₂ ceramic tools were carried out.The experimental results show that under the same cutting parameters,the cutting temperature and cutting force of Al₂O₃/Ti N/Ti Si₂ ceramic tool are greater than that of Al₂O₃/Ti N ceramic tool.When the cutting speed is 50 m/min,the front tool face of Al₂O₃/Ti N ceramic tool will produce micro-chipping phenomenon,but the Al₂O₃/Ti N/Ti Si₂ ceramic tool will not produce micro-chipping phenomenon.Therefore,adding Ti Si₂ improves the cutting reliability of Al₂O₃/Ti N based ceramic tool.At the same cutting distance,the front tool wear depth of Al₂O₃/Ti N/Ti Si₂ ceramic tool is shallow,and the rear tool wear is increased.There are long chip scratches on the front tool face of ceramic tool.The main wear forms are adhesive wear,while the rear tool face wear forms are adhesive wear and abrasive wear.Due to the high cutting temperature,Al₂O₃/Ti N and Al₂O₃/Ti N/Ti Si₂ self-healing ceramic cutting tools can be observed near the main cutting edge of the tool tip.