Evolution Process And Microstructure Of Powder TC4 Titanium Alloy During Hot Deformation And Heat Treatment Prepared By Spark Plasma Sintering

In this study,the TC4 titanium alloy is taken as the research object.Through the experimental research on different process parameters under hot compression experimental conditions,the microstructure transformation of the TC4 titanium alloy after spark plasma sintering is carried out by hot deformation and solution-aging treatment.The purpose is to explore the microstructure evolution process of TC4 titanium alloy and accurately describe its law under the condition of reducing the deformation resistance of TC4 titanium alloy in the forging process.The specific work includes:using the Gleeble-1500D thermal simulation test machine,single-pass and multi-pass high-temperature hot compression experiments were carried out by TC4 titanium alloy prepared by spark plasma sintering.The characteristics of the stress-strain curve and the effect of hot deformation parameters on the microstructure are analyzed under different deformation temperatures（850¹050°C）and different strain rates(0.001⁵s^-1).And the influence of temperature and time on mechanical properties and microstructure is analyzed by adjusting the parameters of different solution treatment and aging treatment.The results show that the relative density of TC4 titanium alloy before the hot deformation is 90.2%;after single-pass of hot deformation,the density of TC4 titanium alloy rises to 98%;and the density of TC4 titanium alloy can reach 99.93%after multi-pass.At high temperature（>950°C）,the flow stress is less sensitive to temperature.At the same temperature,the increasingof strain rate promotes the refinement of dynamic recrystallization grains.The microstructure shows that for multi-pass hot deformation,the change in density is the major factor to the flow stress under one-pass hot deformation.For the hot deformation of two-pass and three-pass,with the increase of the amount of deformation and the decrease of the deformation temperature,the lamellarαphase gradually increases,and the acicularαphase gradually decreases.During the development of the microstructure of the TC4 titanium alloy in solution treatment,the coarsening of the lamellarαphase is the major influencing factor.When the temperature of hot deformation is close to the phase transition point and above,it is difficult for the transition from the lathαphase to the equiaxedαphase as decreasingof the temperature during the solution treatment.When the temperature ofhot deformation is below the phase transition point,with the decrease of the solution treatment temperature,the increase ofαphase is mainly reflected in the equiaxedαphase.At this time,the lamellarαphase exists and coarsens.In the aging treatmentprocess,it was found that it is relatively difficult to convert the equiaxedαphase to the lamellarαphase by heating,so it cannot be the main method of the lamellarαphase.However,when the hot deformation temperature is above the phase transition point,as the decreasing of temperature during the aging treatment,the TC4 titanium alloy tends to increase the equiaxedαphase and coarsen the lamellarαphase.The efficiency of this trend is much higher than thatunder the phase transition point during hot deformation by increasing the solution treatment temperature.Therefore,the transition from the lamellarαphase to the equiaxedαphase is efficient.The increase of the equiaxedαphase is greatly affected by the time in the aging treatment,and thetri-modal microstructure is gradually formed after 12h to 20h.When the solution treatment temperature is above the phase transition point,the coarsening of the lamellarαphase is themainly characteristic.However,at this time,it is difficult for the lamellarαphase after coarsening to be transformed into the equiaxedαphase during the aging treatment.Therefore,the key to the formation of the tri-modal microstructureis to controll the increase of the initialαphase and the coarsening of the lamellarαphase.At the same time,the change in microstructure is also reflected in the change in hardness.When the TC4 titanium alloy is in the basket-weavestructure and the widmanstatten structure,the hardness is the highest,which can reach about 438HV;the hardness of the equiaxedαphase is the lowest,which can reach about 348HV;the hardness of the tri-modal microstructure is about 373HV.