Effect Of Heating Treatment On Microstructure And Mechanical Properties Of BT22(TC18) Titanium Alloy

Ti-5Al-5Mo-5V-1Cr-1Fe(BT22) is a nearβ-type titanium alloy,which has been successfully developed by Soviet Union in 1974.This Alloy can be applied in heavy forgings,die forging and high strength force bearing parts,due to its high hardenability,microstructure inhomogeneity and effects of aging strengthen.In new process research, practicality experiment of heavy forgings is impossible,and the experiment results of small specimens in lab(Φ12×70mm) are different from actual productions.Based on the above considerations,the inner temperature field ofΦ170mm×250mm forging during cooling after solution and heating in aging was studied, according to measured and simulated by ANSYS finite element.OM,SEM,TEM observation and X-ray diffraction were performed to characterize the microstructures, fracture surfaces and phase of this alloy.Vickers hardness and room temperature tensile were performed to characterize the mechanical.The results revealed as following:Difference of internal and external temperature of this forging can reached about 50℃during the cooling process.The thermal process during procedure of heat treatment can be reflected accurately by ANSYS finite element software.The relative error between simulated and measured temperature value during cooling process is about 2%～5%.After 875℃solution,the air cooling curve have an approximate platform,and this cooling process was divided into three stages.From center to edge,theαphase was gradually increased during cooling ofΦ170mm×250mm forging bar after solution at 875℃,and theαphase was fine and dense with the decreased temperature.After solution at 810℃,the increased number of a phase was not obvious but the broaden ofαphase was obviously.The characteristic of samples hardness curve during 875℃and 810℃solution cooling process was raised firstly and then kept stable.After solution 30 minutes at 875℃the microstructure ofΦ12mm×70mm isβ phase,aging at 550℃theαphase was precipitation and growth,the hardness of specimens was gradually increased.After aging 6 hours at 550℃,the experimental date of room temperature tensile was only tensile strength.Reference for fracture scanning, the result showed that the fracture was brittle intergranular fracture.During air cooling after solution at 875℃2 hours,αphase of forging has already precipitated,the primaryαchanged little,but the secondaryαprecipitated and grew up during aging process.The curve of hardness fluctuated gently in HV360～400.the strength of three parts of forging were basically identical,but the plastic of edge was poorer than core and 1/2R.The microstructure ofΦ12mm×70mm speciments were two phase structures after 810℃solution.Aging at 550℃after 24 minutes,secondaryαwere precipitated and hardness was improved;After 65 minutes,hardness reached the peak;After 6hours, hardness was no significantly increased and the fracture was ductile fracture.After solution at 810℃,90 minutes,the microstructure of forging bar were two phase structures,fork typeαandβphase.Aging at 550℃after 24 minutes secondary were precipitated along grain boundary;after 2 hours,the hardness reached the peak. After solution and aging,the date of tensile revealed that the strength and tensile of three parts of forging were uniform.