【作者】 李敏；

【导师】 李春峰；

【作者基本信息】 哈尔滨工业大学 ， 材料加工工程， 2011， 博士

【摘要】 钛及其合金具有优异的力学性能、耐热及耐腐蚀性能好、密度低、良好的生物相容性等综合性能,被广泛应用于航空、化工、生物工程等领域。粉末冶金是制备高性能、低成本钛合金的理想工艺。将钛合金的粉末成形和热氢处理技术相结合,可以降低钛合金粉末成形时的固结温度、缩短成形时间、降低制件的孔隙率、相应地提高制件的力学性能。Ti6Al4V是典型的α+β型的钛合金,具有良好的综合力学性能,是研究和应用最广泛的钛合金。磁脉冲压实使粉末在冲击磁场力作用下高速成形。此方法可以使粉末在加热、真空或保护气氛条件下成形。它能得到完全致密的粉末压坯,广泛应用于黑色金属、有色金属、金属间化合物、陶瓷、复合材料等粉末的压制。本文采用理论分析、数值模拟、工艺试验及微观分析相结合的方法对Ti6Al4V粉末磁脉冲压实的变形行为和机理进行了系统研究。采用工艺试验及微观分析相结合的方法对粉末烧结体的组织结构和性能进行了研究。为了改善其耐磨性,对粉末烧结体进行了热氧化和氧化+氧扩散两种表面处理,并对处理后的试样进行了滑动摩擦试验。利用压电力传感器测量磁脉冲压力,并建立了其与压坯密度的关系通式。研究了磁脉冲压实能量密度与压坯孔隙率的关系。采用松散耦合法对磁脉冲压实过程进行分析。在ANSYS中,将放电回路简化为一个RLC电路。把模拟得到的电流作为边界条件,利用ANSYS/Multiphysics模块建立电流激励的电磁场模型。以电磁场模拟结果为边界条件,在MSC.MARC中,利用powder模块和Shima屈服准则建立粉末压实模型。把模拟得到的速度作为边界条件,在MSC.MARC中,利用Johnson-Cook本构关系建立粉末微观变形模型。模拟结果表明,驱动片主要受轴向磁场力作用,并且其沿驱动片径向分布不均匀,沿驱动片厚度方向呈梯度分布。放电电压、压实温度、摩擦系数对压坯平均相对密度和相对密度的均匀性有明显的影响。放电电压对压实速度有比较大的影响。粉末颗粒的变形和温升主要集中于表面层,并且随放电电压和压实速度的升高而增加。随着磁脉冲压实温度的升高,置氢Ti6Al4V粉末压坯相对密度和硬度不断增加。相同压力下,磁脉冲压实的粉末压坯相对密度比传统静态压实的高12个百分点左右。烧结体真空退火后的相对密度、硬度和抗压强度基本上都是在200℃的压实温度下达到最大值。随着氢含量的增加,烧结体真空退火后的抗压强度先升高后降低。随着放电电压的增加,烧结体真空退火后的相对密度、硬度和抗压强度都不断的升高。烧结体真空退火后显微组织为典型的α+β组织。随着氢含量的增加,等轴的颗粒组织不断增加。试样经热氧化处理后在表面形成一层致密的不导电涂层,且随氧化时间和氧化温度的增加,氧化层逐渐增厚,但氧化层过厚会出现脱落现象。热氧化的产物主要是TiO₂和Al₂O₃,氧化层最外层为一薄层Al₂O₃,次外层为TiO₂。氧化层以内存在厚度为140μm左右的硬化层。试样经氧化+氧扩散处理后氧化层的TiO₂分解,形成低价态的氧化物TiO。硬化层的厚度可以达到210μm左右。经两种表面处理工艺处理后的试样磨损率都比未处理的试样低2个数量级。氧化+氧扩散处理后试样的硬化效果和耐磨性都比热氧化的试样好。更多还原

【Abstract】 Titanium and titanium alloys have been widely used in aviation, biomedical and chemical industries due to their outstanding properties, such as good comprehensive mechanical properties, low specific weight, high specific strength, high temperature strength, excellent anti-corrosion performance and high biocompatibility. Powder metallurgy （PM） is an ideal approach for fabrication of high–performance and low–cost titanium alloys. The combination of PM and thermohydrogen processing （THP） can reduce the temperature, time of consolidation and the porosity of sintered body during the sintering process, as well as improving its mechanical properties. Ti6Al4V, a typicalα+βtitanium alloy, is most widely used and investigated because of its excellent comprehensive mechanical properties. Magnetic pulse compaction （MPC） allows the powder compaction to carry out successfully under conditions of heating, vacuum or protective atmosphere. This method can achieve the automated mass production and fully dense （or near fully dense） powder compact by using high strength impact loading, which has been widely investigated in the compaction of ferrous metals, nonferrous metals, intermetallic compounds, ceramic and composite materials.In this dissertation, the deformation behavior and mechanism of MPC of Ti6Al4V alloy powder are systematically investigated by theoretical analysis, numerical simulation, experiments and microanalysis. The microstructure and properties of sintered bodies are investigated by the combination of experiments and microanalysis. Thermal oxidation （TO） process and TO + oxygen diffusion （OD） process have been investigated to improve the wear-resistance of sintered samples. Then the sliding frictional properties of the treated samples are investigated.The compaction pressure of MPC is measured by an electric pressure sensor, and the general formula of it and the density of compact is established. The correlation of compaction energy density and the porosity of compact are studied. The MPC process is analysed by a loose coupling numerical scheme. The discharge circuit is simplified as a RLC circuit in ANSYS. As the boundary condition of simulated current, the electromagnetic field model of current incentive is established by ANSYS/Multiphysics module. As the boundary condition of electromagnetic field simulation result, the powder compaction analysis model is established by Shima yield criteria and the powder module in MSC.MARC. As the boundary condition of simulated compaction velocity, the micro deformation model of powder is constructed based on the Johnson-Cook relation by using MSC.MARC. The simulation results show that the driver plate is mainly subjected to the axial magnetic force and its distribution is uneven along with the radial direction of the drive plate. The magnetic force along with the driver plate thickness is a gradient distribution. The average relative density of compact and the distribution of relative density are apparently affected by discharge voltage, compaction temperature and friction coefficient. The discharge voltage has a large effect on the compaction velocity. The deformation and temperature rise of powder particles manily occur in the surface layer, and they enhance with increasing discharge voltage and compaction velocity.The relative density and hardness of green body of hydrogenated Ti6Al4V powder compacted by MPC increase as compaction temperature rises. The relative density of compact pressed by MPC is about 12 percent higher than that prepared by static compaction under the same compaction pressure. The relative density, hardness and compressive strength of vacuum annealed samples can be increased by increasing discharge voltage, and they basically reach their maximum values at 200℃. As hydrogen content rises, the compressive strength of vacuum annealed samples first increases and then decreases. The microstructure of vacuum annealed samples consists of primaryαphase andβphase. The number of equiaxed grains increases with the augment of hydrogen content.The coating of samples processed by TO is dense and insulated, and its thickness increases as the temperature and time increase, but the coating may break off when it is too thick. The primary substance of the coating is Al₂O₃ which is on the surface of the coating and TiO₂ inside. On the inner side of the coating, there is a hardened layer with thickness of about 140μm. The TiO₂ of coating of samples treated by TO + OD decomposes to TiO. The thickness of hardened layer reaches approximately 210μm. The wear rate of samples treated by the two methods is about two orders of magnitude lower than that of untreated sample. The TO + OD treatment appears to be more efficient in surface hardening and wear resistance than the TO process.更多还原