Effect Of Thermomechanical Processing On Microstructure Evolution And Mechanical Properties Of TC4-DT Alloy

The effect of thermomechanical processing (TMP) on microstructures and properties was systemtically investigated for TC4-DT alloy by means of optical microscope observation, mechanical property test, scanning electric microscope fractography analysis and transmission electric microscope observation. The structure heredity and evolution of TC4-DT alloy were discussed. The relastionship among different kinds of microstructures and properties was analysed.It was found that the higher the forging temperature, the fewer the content of equiaxed a phase is, however, the size of equiaxed a changed little. The cooling method after forging only changed the size and orientation distribution of lamellar a phase. Under the result of concurrent effects of various mechanisms, including lamellar termination migration, thermal diffusion, local shear, subgrain boundary sliptting,dynamic recovery, recrystallization and reduction of interfacial free energy, lamellar a gradually becomed short and was spheroidized finally.Optimal matching between strength and ductility in TC4-DT alloy was reached after forging at α+β filed and heat treatment with960℃/1h WQ+540℃/6h AC. After conventional annealing, the tensile properties were determined by the microstucture obtained from forging processing. Through the solution and aging treatment, the mechanical properties depended on the different step of TMP. When the solution temperature was in a+(3filed, the forging and heat treatment determined the tensile properties. With temperature rising to near-β field, they were controled by the distribution and size of secondary a phase, but strength had little relation to forgiong process. The tensile strength and ductility were restricted to β grain size after solution treat at980℃following aging at540℃. The fatigue strength of TC4-DT alloy decreased with the increasing of forging temperature. The fatigue crack growth of TC4-DT alloy after forging at a+(3field subsequent to heat treatment at980℃/1h WQ+540℃/6h AC was slower than that after forging at β field and heat treament at960℃/1h WQ+540℃/6h AC. The higher attack toughnes could be achieved after forging at β field. But when deformed below the transus temperature, attack toughnes after forging at near-β field slightly lower than that after forging at α+β field.