【作者】 吴晓东；

【导师】 杨冠军； 葛鹏；

【作者基本信息】 西安建筑科技大学 ， 材料加工工程， 2009， 硕士

【摘要】 Ti-5Al-5Mo-5V-1Cr-1Fe(BT22)是前苏联1974年研制成功的近β型钛合金。该合金具有深度淬透性和明显的时效强化效应,因此可制成大型锻件、模锻件和高强承力件。在进行新的工艺探索时,大锻件由于尺寸大,对不同尺寸的大规格锻件进行等尺寸实物试验是不经济的,而实验常规试样试验结果会与实际生产过程相差很大,并且具有一定的局限性。基于上述考虑,本文通过实测和ANSYS有限元模拟两种手段研究了Φ170mm×250mm大尺寸锻棒固溶后降温和时效升温过程中内部温度场的变化情况,OM、SEM、TEM及X射线衍射用于对固溶后降温和时效升温过程合金显微组织、断口表面及相进行分析。维氏硬度和室温拉伸用于机械性能的分析。研究结果表明,Φ170mm×250mm锻棒热处理降温过程中其内外温差可达50℃左右,采用ANSYS有限元分析软件能准确的反映工件热处理中的热过程。通过模拟得到降温过程的温度值与实测值相对误差在2%～5%左右。由于相变的原因875℃固溶空冷曲线有一段近似平台,这一平台把875℃固溶后空冷降温过程分三阶段。Φ170mm×50mm锻棒875℃固溶降温过程中发现从心部到边部析出α相逐渐增多,并且随着温度的降低析出的α相越细密。合金在810℃固溶后空冷降温过程中α相的增多不明显但是尺寸宽化明显。875℃和810℃降温硬度曲线表现出硬度先升高再趋于平缓的特点。实验用小尺寸(Φ12mm×70mm)试样经875℃固溶30分钟后为全β组织,550℃时效过程中α相析出并长大,试样硬度逐渐升高,室温拉伸显示断裂为脆性沿晶断裂。Φ170mm×250mm锻棒经875℃固溶2小时后空冷,发现其组织中已经有α相析出,时效过程中初生α相变化不大,主要是次生α相的生成和长大,时效过程中硬度变化曲线平缓在360～400范围内波动。经550℃时效6小时后锻棒三个部位强度基本一致,但是塑性边部明显不如心部和1/2R。Φ12mm×70mm试样经810℃固溶30分钟后为针状初生α相和β相的两相组织,550℃时效24分钟,析出次生α相,硬度也迅速提高,时效65分钟硬度达到峰值,550℃时效6小时硬度无明显升高,拉伸数据显示断裂为韧性断裂。Φ170mm×250mm锻棒经810℃固溶90分钟后空冷,组织为“叉形”结构的α相和β相组成的两相组织。550℃时效24分钟,晶界处先析出了次生α相,550℃时效2小时后硬度达到峰值,随后时效硬度无明显升高趋势。拉伸数据显示边部的强度高于心部,但塑性又不如心部。更多还原

【Abstract】 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.更多还原