【作者】 汪敏；

【导师】 周建新； 南海；

【作者基本信息】 华中科技大学 ， 材料加工工程， 2018， 博士

【摘要】 航空航天类高性能零件结构复杂服役环境苛刻,其超高的耐热性、轻量化及高可靠性需求对以铸锻焊为代表的传统成形方法提出了严峻挑战。而粉末热等静压技术通过耦合粉末冶金与现代模具制造技术可一次整体近净成形出综合力学性能与锻件相当的复杂零件,在航空航天类高性能复杂零件制造方面极具潜力。但目前对粉末热等静压过程的控形控性机理并没有很成熟的研究。粉末热等静压成形的本质是离散态的颗粒在高温高压耦合作用下转变成致密化的连续体。该过程在宏观上表现为包套塑性变形驱使内部粉末致密化,在细观上表现为内部粉末的运动变形,而在微观层面表现为粉末颗粒扩散连接等造成的微观组织演变。这就导致了任何单一尺度的研究手段都无法全面解释其成形成性过程。经典的连续介质的弹塑性有限元方法能够预测宏观整体变形及致密化程度,却忽视了成形过程中的颗粒特性且模拟精度由模型准确性决定,而离散有限元方法能从细观层面分析热等静压过程中颗粒的运动及变形规律,但无法反应组织演变过程,元胞自动机方法通过对微观组织转变的物理本质进行数学建模,可实现微观组织演变过程模拟。因此,本文拟通过数值模拟与实验研究相结合的方法,在宏观变形上采用弹塑性有限元方法、细观尺度采用离散有限元方法、微观层面上采用元胞自动机等手段来分析Ti6Al4V合金粉末热等静压成形过程中形状控制、致密化预测以及组织演变机理等问题。具体内容如下。设计了粉末热等静压过程多尺度研究实验。在宏观控形上,采用圆柱形包套的中断实验制备相应的合金试样,进行单轴实验与Gleeble热压缩实验修正宏观有限元模型,并采用立方体零件验证模型;在细观尺度上,探究粉末热等静压过程中因颗粒受热变形导致的组织形成以及物相转变,同时研究在颗粒致密化不同阶段成形试样所对应的拉伸性能;在微观尺度上主要是研究粉末热等静压过程微观组织的演变机理,采用数学方法对应力应变曲线进行处理修正KM位错密度模型,设计相应的SEM实验观察研究α+β两相区α片层弯曲/折断现象。建立了粉末热等静压过程多尺度数学模型。在宏观数值模型上,修正了Ti6Al4V粉末热等静压成形过程屈服准则及流变应力模型,实现了对粉末热等静压过程宏观变形、致密度分布的预测,典型位置致密度模拟误差在2%以内。通过单轴实验获得了不同致密度下粉末合金试样的应变规律以及屈服强度,基于数据拟合建立了适用于Ti6Al4V合金粉末热等静压过程的Shima修正屈服准则,采用经典的两段式Arrhenius模型建立了反应Ti6Al4V合金热加工过程中高温流变应力模型。在细观尺度上,将热等静压过程简化为高温作用下的粉末压制过程,构建三种不同初始堆积方式的粉末颗粒来研究高温压制条件下颗粒的运动变形情况。在微观尺度上,以经典的位错密度演化KM模型为基础,基于实验数据构建了Ti6Al4V合金热等静压过程加工硬化与动态软化参数的数学模型。在宏观整体变形研究上,首先采用典型的立方体试样验证了所建立的宏观有限元模型的准确性。建立了反应涵盖塑性变形、蠕变以及扩散等致密化机制的两段式数值模型,基于此构建了反应温度、压力以及不同作用时间下粉末体致密度的热等静压图,实现工艺参数快速选取;分析了热等静压过程中“包套-型芯-温度-压力-初始密度”对控形控性效果的影响,结果表明:包套对等静压力的屏蔽效果与壁厚及材料的屈服强度成正比;刚度大的石墨型芯控形效果更好,但局部区域致密度较低;较长的保温平台可以获得更高的致密度,但是容易导致晶粒粗大,微裂纹增多;初始密度分布不均将导致包套非均匀变形而对内部粉末产生剪切作用使颗粒破碎。在细观颗粒运动变形研究上,发现了等静压力逐层传递压缩粉末边界变形的规律,揭示了因边界塑性应变能累积及颗粒表面污染物耦合导致的再结晶原因,阐明了网格状粉末热等静压微观组织形成的物理过程。结合离散有限元与热等静压中断实验,从颗粒尺度研究了热等静压过程中组织及性能动态演变过程。结果表明:初始的Ti6Al4V合金粉末主要由针状α’相和弥散分布的β相组成,在热等静压过程中α’相以两种途径转变为α相和β相;在热等静压过程中,颗粒尺寸大小影响颗粒变形规律,小尺寸颗粒变形更加均匀,且颗粒的塑性变形及塑性应变能大部分累积在粉末颗粒的边界处;对颗粒表面采用XPS分析可知,Ti6Al4V粉末表面覆盖着一层主要由TiO₂、Al₂O₃组成的污染物薄膜,薄膜厚度小于10nm。颗粒表面氧化物的存在可以为异质形核提供形核基底,同时颗粒边界处累计塑性应变能的释放能进一步促进动态再结晶的形成,反应在最终组织上为典型的网格状结构,网格大小接近粉末颗粒尺寸,网格内部为板条状结构,网格边界以等轴结构区分,整体组织为等轴α相、板条α相和β相组成。在工艺为930℃/120MPa时,粉末体已经接近理论致密度,延长保温时间,晶粒有所长大,但由于颗粒之间扩散更加充分,所制备试样综合力学性能已经达到同质材料锻件水平。在微观组织演变上,建立了粉末热等静压初始组织形成及动态再结晶演变的数值模型,揭示了热等静压参数差异影响位错密度演化诱发的再结晶及α片层结构弯曲/折断导致的软化规律。结合微观组织演变的物理规律建立了初始晶粒生成的热力学以及晶界驱动机制,实现了初始晶粒长大模拟;以位错密度为变量建立了动态再结晶晶粒形核、长大、转变的数值模型,实现了Ti6Al4V合金粉末热等静压动态再结晶过程模拟。模拟结果表明:影响动态再结晶的主要因素有温度、应变量与应变速率。其中应变速率与应变量通过改变形核与长大时间影响再结晶晶粒的形态,合理的调控变形量与应变速率能够达到细化晶粒的效果;升高热等静压温度可以降低形核所需要的孕育期时间,能加快晶粒形核,但是过高的温度又容易使晶粒粗大。对于Ti6Al4V这种具有片层结构微观组织,在热成形过程中片层结构的弯曲折断亦会引发软化效果。在相对低的应变速率中,层状结构比高应变率区域的结构更粗大。随着应变率的增加,初始片层结构屈曲/折断的数量增加,初始层状结构更加倾向于被拉伸而不是球化。因此,在较高的应变速率下,软化程度取决于层状结构的弯曲折断情况。更多还原

【Abstract】 High-performance aerospace components are serving in the harsh environments and their high heat-resistance,lightweight,high-reliability,and high-load requirements pose a serious challenge to manufacturing materials and forming methods.The traditional manufacturing methods,like casting,forging and welding,have obvious shortcomings although they are technically mature.It is difficult for them to meet the current requirements for the production of complex parts and components of aerospace.The powder hot isostatic pressing technology can form an overall near-net shape of complex parts with comprehensive mechanical properties by coupling powder metallurgy with mold manufacturing technology,which is of great potential for aerospace high performance complex parts manufacturing.However,the technology of powder hot isostatic pressing is immature worldwide,the essence of powder hot isostatic pressing is that the particles in the discrete state transform into a densified continuum under the action of pressure.This process shows macroscopically that the plastic deformation of the capsule drives the densification of the internal powder.Microscopically,it is characterized by the movement of internal powders,and at the microscopic level,it is the microstructure evolution caused by the powder particle diffusion connection.No single-scale research method can fully explain its formation process.The classical continuous elastic-plastic finite element method can predict the overall macroscopic deformation and densification,but ignore the particle characteristics during the forming process.However,the discrete finite element method can analyze the movement and deformation of particles in the process of hot isostatic pressing from the meso-level,but it can not reflect the process of tissue evolution.The cellular automaton method mathematical modeling of the physical nature of the microstructure transformation can simulate the microstructure evolution process.Therefore,this paper intends to combine numerical simulation and experimental methods to analyze the related issues in hot isostatic pressing of Ti6Al4V alloy powder,we have used the finite elemet method to study the macroscopic deformation,the coupled discrete element method to study the mesoscopic particle movement and deformation,and the cellular automata method to study the micro dynamic recrystallization.Specific researches and results are as follows:Multiscale experiments were designed.As to the macroscopic controllment,the corresponding alloy samples were prepared by the cylindrical capsule interrupt experiments,and the uniaxial and Gleeble thermocompression experiments were used to modify the macro-finite model,and the cubical component was used to verify the established model;At the mesoscale,the formation of the microstructure and the phase transformation of the particles due to thermoplasticity during the hot isostatic pressing of powders were investigated.At the same time,the tensile properties of the formed parts at different stages of hot isostatic pressing were studied.At the microscopic scale,the mechanism of the microstructure evolution during powder hot isostatic pressing was studied,mathematical methods were used to process the softening curve to modify the KM dislocation density model,and a corresponding SEM experiment was designed to study the bending/breaking phenomenon ofα-sheets in theα+βtwo phase region.Multi-scale mathematical models were established.As to the macroscopic numerical model,the strain regularity and yield strength of powder alloy parts at different densities were obtained by uniaxial experiments.The Shima modified yield criterion for Ti6Al4V powder hot isostatic pressing process was established based on data fitting.The classic two-stage Arrhenius model was used to establish a high-temperature flow stress model for the thermal processing of reactive Ti6Al4V alloys.Based on the hypothesis of continuum medium,a hot isostatically-elastic-plastic finite element model of powder was constructed.On the mesoscale,the hot isostatic pressing process was simplified to the powder compaction process under high temperature.Three different initial accumulation methods of powder particles were constructed to study the movement and deformation of the particles under high temperature pressing conditions.On the microscopic scale,based on the classical KM model of dislocation density evolution,the work hardening and dynamic softening parameters of Ti6Al4V alloy hot isostatic pressing process were modified based on experimental data.In the study of overall macroscopic deformation,A typical cubic sample was fabricated by the powder hot isostatic pressing to verify the established macroscopic finite element model.A two-stage numerical model was established to describe the densification mechanism of plastic deformation,creep and diffusion.Based on this,a hot isostatic pressing diagram was established which can reflect effects of temperature and pressure on the densification process at different reaction times.The influence of"capsule-core-temperature-pressure-initial density"on the shape control in the process of hot isostatic pressing has been analyzed.The results show that:The capsule shielding effect on the isostatic pressure is proportional to the wall thickness and the yield strength of the material,the bigger stiffness of the graphite core is better for control,but the local density is lower.Longer insulation platforms can achieve higher densification but can easily lead to coarse grains and increased micro cracks.Inhomogeneous initial density distribution will cause non-uniform deformation of the capsule and shearing of the internal powder to break the particles.At the meso level,based on the hot isostatic pressing interruption test,combined with the finite element and discrete element analysis,the dynamic evolution of microstructure and mechanical properties during hot isostatic pressing were studied.Results show that the initial Ti6Al4V alloy powder mainly consists of acicularα’phase and dispersedβphase.During the hot isostatic pressing,theα’phase transforms intoαphase andβphase in two ways.In the process of hot isostatic pressing,the size of the particles affects the powders deformation law,and the deformation of the smaller size particles is more uniform.The plastic deformation of the powder particles mostly concentrates on the boundaries of the powder particles,and the larger plastic strain energy accumulates at the boundaries.XPS analysis of the particle surface shows that the surface of the Ti6Al4V powder is covered with a layer of pollutant film mainly composed of TiO₂ and Al₂O₃ with the surface oxide thickness less than 10 nm.The presence of oxide on the surface of the particle can provide a nucleation substrate for the heterogeneous nucleation,and the release of the cumulative plastic strain energy at the particle boundary can further promote the formation of dynamic recrystallization.The final microstructure is a typical grid-like structure;the size of the grid is close to the size of the powder particles.The interior of the grid is a lath-like structure.The boundaries of the grid are distinguished by the equiaxed structure.The overall structure is composed of the equiaxedα-phase and the lathαandβ.In the condition of 930℃/120MPa,the degree of densification of alloy powder has been close to the theoretical value,with the holding time extension,although the grain will be grown,the diffusion between the particles are more fully.Yield strength of the prepared alloy sample is 922MPa;tensile strength is 1048MPa,13.9%elongation after break,this has reached to the level of forgings.At the micro level,the thermodynamics and grain boundary driving mechanism of initial grain growth have been established in conjunction with the physical laws of microstructure evolution and realized the initial grain growth simulation.A numerical model of dynamic nucleation,growth and transformation of the dynamic recrystallization grains was established using the dislocation density as a variable,and the dynamic simulation of the dynamic recrystallization process of Ti6Al4V alloy powder hot isostatic pressing was realized.The simulation results show that the main factors affecting the dynamic recrystallization are the temperature,the strain and the strain rate during hot isostatic pressing.Among them,strain rate and strain amount affect the morphology of recrystallized grains by changing the nucleation and growth time,and reasonable regulation of deformation and strain rate can achieve the effect of grain refinement;Increasing the hot isostatic pressing temperature can reduce the incubation period required for nucleation and accelerate the nucleation of the crystal grains,however,the high temperature is easy for the grains to grow coarse.For the Ti6Al4V alloy with a lamellar microstructure,the bending fracture of the lamellar structure during high temperature thermoforming also causes the softening effect.The lamellar structure at relatively low strain rates is coarser than the structure at high strain rates.As the strain rate increases,the number of initial lamellar structures buckling/kinking increases and the initial layer structure tends to be stretched rather than spheroidized.Therefore,at higher strain rates,the degree of softening behavior depends on the portion of the curved laminar structure.更多还原