机械合金化制备纳米晶Ti-6Al-4V及其注射成形工艺研究

【作者】 王家惠；

【导师】 史庆南；

【作者基本信息】 昆明理工大学 ， 材料学， 2014， 博士

【摘要】 钛及其合金具有低密度,高强度,良好的耐蚀性和力学性能优异等特点,广泛的应用在航空航天、造船、化工、冶金、医疗等领域。其中Ti-6A1-4V(TC4)是使用最广泛的钛合金之一,应用率占钛合金总产量的50%以上,占全部钛合金加工件的95%,是世界各国钛合金应用中的主导。然而,钛的生产成本太高,钛的提取、熔炼、加工十分困难,从而限制了钛及其合金的应用范围。自20世纪90年代起,世界各国学者相继展开了钛及钛合金的注射成形研究,该技术应用到钛及其合金成形上,能够极大的降低成本,提高其利用率,生产范围很广的高性能、复杂形状的零件。机械合金化作为一种细化颗粒与材料显微组织结构的有效方法,日益受到国际材料学界的重视。本文应用机械合金化方法制备纳米晶Ti-6A1-4V合金粉末,并对其注射成形技术展开相关的研究,从而期望发挥注射成形的优势,扩大注射成形钛及钛合金零部件在民用领域的应用。开展的工作具体如下：(1)采用机械合金化方法制备纳米晶Ti-6A1-4V合金粉末1)本研究以HDH Ti粉(粒度<(200目,纯度>99.4%)及铝钒合金粉(粒度<200目,纯度>99%)为原料,按90%HDH Ti粉+10%铝钒合金粉(质量分数)进行配料,采用机械合金化方法制备Ti-6A1-4V合金粉末。借助粒度分析、X-ray衍射、扫描电镜、透射电镜等分析测试手段观察在球磨过程中粉末粒度变化、物相组成及微观形貌的变化情况。分析结果表明：采用机械合金化可以制备纳米晶Ti-6A1-4V合金粉,其反应机理以扩散为主,且该固态反应是缺陷能和碰撞能共同作用的结果。Ti-Al-V混合粉末的组织结构随球磨时间延长发生了明显的改变,部分V固溶于Ti中形成置换固溶体Ti(V)；球磨40小时后都能获得纳米晶,球磨60小时的粉末为纳米晶和多晶的混合物,晶粒尺寸小于60nm；60小时后晶粒尺寸变化缓慢。球磨过程中没有中间相生成。结合面元素扫描和能谱分析,球磨后Ti、Al、V的原子比近似为90：6：4,与Ti-6A1-4V元素成分一致,也即球磨后获得Ti-6A1-4V合金粉末。2)经机械球磨后颗粒形状和尺寸与球磨工艺参数,如球磨机转速、球料比、球磨时间等密切相关。随着球磨时间的延长,粉末颗粒平均粒径呈不断减小的趋势,且在30h到60h之间,粉末粒度尺寸降幅最大,球磨60h后,球磨并不能改变粉末的粒度,70h后容易出现团聚；大的球料比有利于粉末细化和形成固溶体；增加球磨转速,球磨能量大,能使粉末快速细化,促进合金化的进程。综合以上分析,本实验的球磨工艺参数为：球料比20：1、球磨转速330r/min、球磨时间60h。3)分析了球磨过程中的球磨工艺条件对磨球运动状况,尤其是磨球的碰撞行为与粉末的变形的影响,结果表明：磨球运动速率vb、碰撞频率f随球磨转速的增大而增大。装料量mp一定时,增大球料比Rbp,磨球运动速率vb将降低,平均自由程S减小,碰撞频率f增大；Rbp和mp一定时,对于同种材质的磨球,采用大球时的碰撞频率低于小球；大球的平均自由程大,增大球磨罐体积可以增大S。在斜碰时粉末的最大真应变εmax随转速Ω的增大而增大,随碰撞角度θ的增大而减小；剪应变γyx,则随转速Ω、碰撞角度θ的增大而增大。这些理论基础对深入研究机械合金化过程的微观机理具有重要的理论意义,以及对正确选择合理的球磨工艺具有重要的指导意义。4)粉末碰撞过程中的温升未超过100℃,因此在球磨过程中不会有中间相生成,这与测试分析结果相一致。但由于产生了大量的缺陷,使得在较低温度下就能够得到纳米晶Ti-6A1-4V。(2)纳米晶Ti-6A1-4V注射成形技术的研究1)从流变学基本理论出发,分析了纳米晶Ti-6A1-4V合金粉末喂料的流变学行为,深入讨论了剪切速率、温度、机械球磨时间、粉末装载量对剪切粘度的影响,实验结果表明：①随着剪切速率增加、温度升高、球磨时间增加,粘度降低；提高粉末装载量,喂料的粘度增加。本实验喂料的非牛顿指数n值均小于1,且随着表观剪切速率的增加,各喂料n值均减小；在一定剪切速率下,温度升高,n值增加；相同温度和剪切速率下,粉末装载量增加,n值下降；球磨时间延长,,n值上升。②通过实验方法和对经验模型的计算获得机械球磨60h的纳米晶Ti-6A1-4V粉末喂料的临界粉末装载量φmax为69vo1.%。③喂料的粘流活化能Ea和A值都随剪切速率的增加而改变,不同球磨时间粉末制备喂料的Ea值与剪切速率均满足指数关系；球磨60h粉末制备喂料的A与剪切速率满足指数关系,球磨20h和40h粉末喂料的A与剪切速率呈线性关系。本文从Arrhenius方程出发,推导出了一组半经验性本构方程,将温度、剪切速率有机地联系在一起,该方程对注射成形生产有指导意义。2)应用最小二乘法对Cross-Arrhenius四参数粘度模型进行拟合,拟合出经机械球磨60h的纳米晶Ti-6Al-4V合金粉末喂料在140℃.150℃.160℃下的零剪切粘度η0(T)分别为13464.97Pa·s、8080.82Pa·s、6030.5Pa·s,在此基础上应用由“时温等效原理”发展而来的(η/η0～η0γ)主曲线生成方法获得经机械球磨60h的纳米晶Ti-6Al-4V合金粉末喂料的流动主曲线。借助该主曲线,对于进一步研究MIM喂料的其它特性以及生产实践大有益处。3)提出应用DOE方法,通过试验设计来实现注射工艺参数的优化。在此基础上开展了大量的注射实验,应用MiniTab软件对试验结果进行分析。首先应用部分析因实验设计(Fractional Factorial Design)进行因子筛选,选出对实验指标影响显著的单因素和因素间的交互作用。分析结果显示：注射压力、保压压力、注射温度、注射速度、保压压力×注射温度交互作用和注射压力×注射温度交互作用对于注射坯密度影响显著。然后利用Taguchi试验设计方法设计了L27(313)试验矩阵,分析了以上筛选出的四个显著因子和两个交互作用对注射坯密度的影响,优选出工艺参数：熔体温度150℃、注射压力100bar、保压压力90bar、注射速度62%。4)研究了纳米晶Ti-6Al-4V注射坯的脱脂行为和机理,制定了其脱脂工艺路线为溶剂脱脂加后续热脱脂,溶剂脱脂工艺参数为：正庚烷,60℃脱脂4小时；热脱脂工艺参数为：最高脱脂温度为500℃,升温速率为1.℃/min,气体流量为200ml/min,具体研究工作如下：①分析溶剂脱脂过程中脱脂温度、脱脂时间、试样形状、溶剂种类对脱脂率的影响发现：脱脂率随脱脂时间的延长而持续增大；脱脂温度越高,脱脂速率越高,但10个小时之后,温度对脱脂速率的影响不显著；本实验样品溶剂脱脂过程受扩散控制,由扩散控制数学模型计算出正庚烷的活化能为40.39KJ·mol-1·K-1,大于二氯甲烷的活化能(38.41KJ·mol-1·K-1)；在相同温度下,正庚烷的扩散系数高于二氯甲烷的,因此注射坯在正庚烷中的脱脂速率大于在二氯甲烷中的；②应用Ricardo V B.Olivira脱脂数学模型描述脱脂率与尺寸因子As/V、脱脂时间、脱脂温度之间的关系。经计算,随着样品尺寸因子As/V的增加,在相同时间内的脱脂率增加；随着As/V的增加,溶解活化能Q值减小,恒定As/V的试样,时间对溶解活化能的影响不明显。③借助于热重分析及Ti-6Al-4V合金的特性制定了热脱脂工艺曲线,讨论了热脱脂最高脱脂温度和保护气氛气体流量对热脱脂率及脱脂坯残余C、O含量的影响,从而确定合理的最高热脱脂温度：500℃；气体流量：200ml/min。5)通过对纳米晶H-6A1-4V烧结过程的研究获得：本课题制备的烧结坯均为等轴a相和晶间少量β转变组织组成的等轴组织,且随着烧结温度提高和烧结时间的延长,a相的含量增加；提高烧结温度和延长保温时间,纳米晶Ti-6A1-4V合金致密化程度提高,孔隙率下降,烧结坯的硬度增加；本课题在1200℃下保温3小时的烧结制品的致密度达到97.92%,抗拉强度为783MPa,延伸率6.04%。更多还原

【Abstract】 Titanium and its alloys have received considerable attention recently in aerospace, navigation, automotive, biological engineering, sports goods and other fields. The advantages of these materials include low density, high specific strength, excellent corrosion resistance, and excellent mechanical properties. Among those, Ti-6Al-4V(TC4)has been the most highly used titanium alloy, its utility ratio being more than50%of the total titanium alloy production, accounting for95%of the total titanium alloy machined part, and it is the world leading in the applications of titanium alloys. However, its shortcomings such as the high production cost, difficulty in extraction, melting and machining, limit the application of titanium and its alloys. Since the1990s, scholars around the world have launched researches on metal injection molding of titanium and its alloys. The application of this technology to titanium and its alloy forming can greatly reduce the cost, improve the utilization of a wide range of production of the high performance and complicated parts. Mechanical alloying has increasingly brought to the attention of the international academic materials for an effective method of refined grain and material microstructure structure. In this paper, mechanical alloying is adopted to prepare nanocrystalline Ti-6A1-4V alloy powder; and then systematic study is done on powder injection molding process of nanocrystalline Ti-6A1-4V alloy, thus can take advantage of MIM and can expand the application of titanium and titanium alloy components in civilian areas. The main steps carried out are as follows:(1) Mechanical alloying is adopted to prepare nanocrystalline Ti-6A1-4V alloy powder1) The initial material used in the experiments are HDH titanium powder (particle size<74μm, purity>99.5%) and Al-V powder (particle size<74μm, purity>99%).Mixtures of90wt%Ti and10wt.%Al-V alloy powder as starting powders have been milled in a QX-2planetary ball mill. With laser scattering particle analyzer, X-ray diffraction, scanning electron microscopy (SEM), transmission electron microscopy (TEM) and other analytical testing methods, this paper observes changes in the particle size distributions of the powders, phase composition and microstructure changes in the milling process. The results show that: mechanical alloying can prepare nanocrystalline Ti-6A1-4V alloy powder, the reaction mechanism dominated by diffusion, and the solid-state reaction is the result of defects in energy and interaction of the collision energy. Organizational structure of Ti-Al-V powder mixture changed obviously with increasing milling time, and substitution solid solution Ti(V)formed in the process. After milling for40h can obtain nanocrystalline, nanocrystalline and polycrystalline mixtures obtained for60hour. The grain sizes are less than60nm and little change in grain size after milling for60h. There is no intermediate phase generated in milling process. After milling, the atomic ratio of Ti, Al, V is near to90:6:4, which is consistent with the Ti-6A1-4V elemental composition with element surface scanning and energy spectrum analysis.2) The particle shape and size of mechanically milled powder change in close response to milling process parameters, such as angular velocity of milling, ball-to-powder ratio and milling time. With the increase of the milling time, the average particle diameter of the powder particles has a continuously decreasing trend, with the largest decline in30h to60h; after being milled for60h, milling does not change the powder particle size; and particles begin to reunite after70h. Larger ball-to-powder ratio would favor powder refinement and the formation of the solid solution. Increasing the angular velocity of milling would increase milling energy and refining speed as well as promote the process of alloying. Through the above experimental research and theoretical analysis, this study selects reasonable parameters in mechanical alloying process: ball-to-powder ratio is20:1, the milling speed is330r/min, and milling time is60h.3) This paper studies the effect of the parameters of mechanical alloying process on the ball moving situation, especially impact behavior of milling ball and the deformation of the powder. The results show that velocity of milling ball vb and impact frequency f increase in proportion to angular velocity of milling. When loading capacity mp is decided, the velocity of milling ball vb reduces, average free path S decreases, and impact frequency f increases with the ball-to-powder ratio Rbp increasing. When Rbp and mp are decided, for the same material milling balls, impact frequency using a large balls is lower than that of small balls, and increasing jar mill volume can increase S because average free path of large balls is large. The maximum true strain εmax in the oblique collision increases with milling speed Ω increasing, while decreases with the impact angle Ω increasing; the shear strain γyx increases with the increasing of the milling speed Ω and the impact angle θ. These theoretical study has important theoretical significance on further study of the microscopic mechanism of mechanical alloying process, and have important guiding significance on selecting reasonable parameters in milling process.4) Results showed that temperature rise is not high in the collision, so there is no intermediate phase generated in milling process, which is consistent with the results of test and analysis. However, nanocrystalline Ti-6A1-4V can be obtained at lower temperatures for generation of a large number of defects.(2) Research on MIM of nanocrystalline Ti-6Al-4V1) Starting from the basic theory of rheology, the paper analyzes the rheological behaviors of Ti-6A1-4V feedstocks, and deeply discusses the influence of the shear rate, temperature, mechanical alloying time,powder loading on shear viscosity. The experimental results show that:①The viscosity of feedstocks reduces with the increasing of the shear rate, temperature, and milling time; the viscosity of feedstocks increases with the improving of powder loading; The n value of the all feedstocks in this experiment are smaller than1, and the value of n decreases with the increasing of shear rate. Under a certain shear rate, n value increases with temperature raised; under the same temperature and shear rate, the value of n decreases with powder loading increasing, the value of n increases with the milling time prolonged.②By the density components experiment and mathematical calculations of empirical models, the critical powder loading of Ti-6A1-4V feedstocks is69vol%.③The flow activation energyEa and A value change with the increasing of shear rate, Ea value of the feedstocks fabricated from different milling time meet the index relationship between shear rate; A value of the feedstock fabricated from milling for60h satisfies index relationship between shear rate, but A value of the feedstocks fabricated from milling for20h or40h has a linear relation with the shear rate. From the Arrhenius equation, a set of empirical constitutive equations are established, two important factors, namely, temperature and shear rate are contained in these equations. The equation as a guidance to the injection molding production.2) The fitting empirical parameters by using the least square method are extracted. For Ti-6A1-4V feedstocks fabricated by the powder milling for60h, zero shear viscosity η0(T)value is13464.97Pa·s,8080.82Pa·s and6030.5Pa·s at140℃,150℃and160℃, respectively. The master curve (η/η0～η0γ) developed from time-temperature superposition is used to obtain a master curve for Ti-6A1-4V feedstocks. With the master curve, it be beneficial to further studies of MIM feedstocks as well as production practice.3) This paper proposes to optimize of injection process parameters by the application of DOE (Design of Experiment) method. On this basis, a large number of injection experiments are carried out, the results of which are analyzed by MiniTab software. A new design of experiment-fractional factorial design is put forward to study the interaction effect between process parameters. The important interaction and single factors are thus screened. Fractional factorial experiment results show that the injection pressure, holding pressure, injection temperature, injection speed, holding pressure and injection temperature interaction, injection temperature and injection pressure interaction significantly affect the density of injection part. The Taguchi orthogonal experimental design methods are then employed to design L27(313) test matrix and to analyze the influence of four significant factors and two interactions on the density of injection part. The optimized process parameters are:injection temperature150℃, the injection pressure100bar, the holding pressure90bar, and injection speed62%.4) Studied the debinding behavior and mechanism of nanocrystalline Ti-6A1-4V injection parts, and then a two-step debinding process is selected:solvent debinding and thermal debinding to remove the binder. Parameters of solvent debinding are:debinding for4hours in normalheptane at60℃; parameters of thermal debinding are:temperature500℃,heating ratel℃/min,and the gas flow rate200ml/min.①Effects of debinding temperature, debinding time, part geometry, solvent on binder removal are investigated, it is found that debinding rate increases with extension of the debinding time; and the higher the debinding temperature is, the higher the debinding rate is. But after10hours, the effects of temperature on the debinding rate is less clear than before, and the debinding rate at different temperature gradually converge. By study of the solvent debinding kinetic study, solvent debingding process of this experiment sample is found to be predominantly diffusion-controlled. From the mathematical model, the diffusion active energy of normalheptane is calculated as40.39KJ·mol-1·K-1, which is higher than methylene chloride (38.41kJ·mol-1·K-1), The diffusion coefficient of normalheptane is higher than methylene chloride at the same temperature.②Relationships between removed binder quantities, debinding time and temperature are described by Ricardo V. B. Olivira equation. Through calculation, with the surface area/volume ratio As/V increasing, debinding rate at the same time increases and solubilization energy Q decreases.For sample with constant As/V, time has no significant effect on the solubilization energy.③A thermal debinding curve is established by thermal gravimetric analysis(TGA) and Ti-6A1-4V alloy features. The discussion is mainly done on the influence of thermal debinding temperature and the gas flow rate of protective atmosphere on debinding rate and residual carbon content and oxygen in debinding stage, which results in the determination of a reasonable thermal debinding temperature being500℃and the gas flow rate being200ml/min.5) By research on sintering process, the nanocrystalline Ti-6A1-4V was found to have a equiaxed structure, revealing a grains with intergranular β phase. The percentage of a phase in the alloys increased with increasing sintering temperature and times. Theoretical densities and hardness of samples increased, porosity decreased with increasing sintering temperature and times. Theoretical densities, maximum ultimate tensile strength and elongation were obtained 97.92%,783MPa and6.04%at1200℃for3h.更多还原