Study On Material Characteristic And Failure Of K403Die Used For Isothermal Forging Of Titanium Alloys

Large-scale complex integral components of titanium alloy, satisfying theequipment requirements of lightweight, high performance, high reliability and highefficacy, are widely applied in aeronautics and astronautics. Isothermal forging bylocal loading provides a new way to resolve the forming problems of thesecomponents such as the insufficient forming capacity and an integrated regulation offorming and performance. However, during the isothermal local loading forgingtitanium alloy, the dies are subjected to thermal exposure at a high temperature for along time, the microstructures and properties of the die material may change;Meanwhile, the dies bear large and inhomogeneous forming load, this will result inlocal deformation and even cracking of the dies, which seriously restrict theimplementation of forming processes and forming quality. Thus, to predict thefailure of the dies during the isothermal local loading forging of the large-scaleintegral components of titanium alloy has been the key problem needed to be solvedfor enhancing the reliability and service life of the dies. In this thesis, by usingtheoretical analysis and experiment combined with finite element simulation, thebehaviors of high temperature thermal exposure, high temperature deformation andfracture failure of Ni-based superalloy used for die material are revealed, then thestress analysis, failure prediction of the dies and the optimal mold structural designin the local loading forming process of the large-scale integral components oftitanium alloy are systematically investigated. The main contents and achievementsare as follows:The influences of the thermal exposure temperature and time on themicrostructures evolution and properties of the K403Ni-based superalloy werestudied by using the thermal exposure experiments and the tensile tests at roomtemperature. It is found that the microstructure of K403alloy still presented a typicaldendritic structure after thermal exposure in a temperature range from800℃to950℃for50to200hours. The MC carbides partially decomposed, the strengtheningphase of γ’ grew up together. The coarsening of γ’ became more remarkable as thermal exposure temperature and time increasing; A few of the harmful TCP brittlephases (σ) precipitated after thermal exposure at850℃and900℃, andas thermalexposure time increasing, the precipitation of acicular σ phase increased; The offsetyielding stress and the tensile strength decreased after thermal exposure due to theaggregation and coarsening of strengthening phase γ’ and the precipitation of theacicular σ phase.The effects of the deformation temperature, the strain rate and the stress states onhigh temperature deformation behaviors and microstructures of K403alloy werestudied by thermal compression and tensile simulating tests. The fracture mechanismof K403alloy at high temperatures was revealed, and the high temperature fracturemodel of K403alloy was established. The results showed that with the increase ofthe deformation temperatures, or the decrease of strain rate and stress triaxiality,during K403alloy tensile deformation at high temperature, the peak stress and thebreaking stress of the superalloy decreased, while the reduction of area and thebreaking strain increased. The tensile facture morphology mainly exhibits brittlequasi-cleavage at the temperatures below900℃, while with the increase of thedeformation temperature, the fracture mechanism changed from quasi-cleavagefracture to inter-granular fracture.For the large-scale complex integral bulkhead components and two typicalrib-web components, which reflect the high rib and thin web structuralcharacteristics of the large-scale complex integral components of titanium alloy, thedeformation behaviors and the die stress distributions were revealed. The dangerouszones of the dies were determined, and the effects of the die geometry parameters onthe die stress distributions were discussed. The results showed that the dangerouszones of the die during the isothermal local loading forging are outside round cornerof the cavity in the bottom die used to obtain the outside rib of the ledge, where thestress concentration is prone to occur; Increasing the round corner radius of cavitydie and draft angle can reduce the stress of the die to some extent; While increasingthe wall thickness of the die can significantly reduce the stress of the die, though itseffect weakens when exceeding certain thickness.Based on the established high temperature fracture model of K403alloy and thedeveloped finite element model, the die failure was predicted during the isothermallocal loading forging of large-scale complex integral bulkhead components of titanium alloy. In order to reduce the mould stress and the risk of cracking, the diestructure was optimized by increasing the wall thickness of die and using prestressedcomposite bottom die. It is proved that the prestressed composite bottom die is moreeffective.