多孔烧结材料锻造镦粗成形致密的特性研究

粉末锻造是一种新型高性能的精密塑性成形技术，但其成形时的特殊性导致目前缺乏完善的、普遍适应的粉末锻造成形理论。通过对还原铁粉烧结多孔体预成形坯锻造镦粗成形致密实验，研究了不同初始相对密度、不同高径比、不同摩擦条件对预成形坯锻造镦粗成形、致密的影响规律。得到了粉末锻造镦粗的断裂极限准则，绘制出了断裂极限应变迹线，并着重分析了初始相对密度、高径比和摩擦因子对镦粗断裂极限的影响，确定了镦粗成形极限工艺参数曲线。实验研究结果为粉末锻造预成形坯与模具的设计、工艺参数的优化奠定了理论基础，并提供了有价值的实验数据。     

不同初始条件下粉末锻造镦粗成形致密过程的数值模拟

运用有限元软件MSC.MARC对多孔体粉末烧结材料锻造镦粗成形致密过程进行了数值模拟,研究了不同初始条件(相对密度、高径比、摩擦因子)对粉末锻造成形致密规律及致密机制的影响,并对其结果进行了分析,为预成形坯和模具的优化设计提供理论依据.     

粉末多孔体烧结材料闭式模锻塑性成形致密研究

通过对粉末冶金预成彤烧结坯闭式模锻塑性成形实验,研究了不同初始相对密度、不同高径比、不同摩擦等条件对成形致密的影响规律,探讨了粉末锻造镦粗、复压两阶段对成形致密的不同作用,同时对实验结果进行分析与比较.结果表明:闭式模锻成形时延长镦粗阶段,有利于提高致密化效果,但镦粗并不能使预成形坯完全致密;复压比镦粗阶段致密困难,但致密化速率却明显提高,能至近全致密.     

粉末烧结材料塑性变形过程的有限元分析

根据多孔材料的塑性理论，推导了分析多孔材料塑性变形和致密化过程的有限元公式。编制了有限元分析程序。通过对粉末烧结元柱体镦粗过程的有限元分析，说明了多孔体镦粗过程的变形特性和致密化过程。证实了有限元公式和有限元程序的正确性。     

钼粉末多孔体烧结材料的制备及镦粗致密实验研究

运用粉末冶金成形技术对高纯钼粉材料进行制备、烧结,对钼粉未烧结体进行镦粗成形,研究了在不同初始条件(相对密度、高径比、摩擦条件)下钼粉末烧结体成形致密规律和变形特性,为钼传统的制备技术的改良、预成形坯和模具的优化设计提供大量实验数据.     

烧结材料塑性变形与致密

用实验确定了烧结材料的等效屈服强度,并以此为基础建立了烧结材料的后继屈服条件。用烧结铜进行了单向压缩、平面变形和闭式镦粗试验,分析了预制坯变形抗力与致密的关系。提出了烧结材料塑性加工中预制坯的密度设计原则,并指出剪切塑性变形有利于致密。     

粉末盘预制坯两次镦粗成形工艺的有限元模拟与分析

采用商用有限元软件，对FGH96合金盘坯的两次镦粗成形工艺进行了有限元数值模拟。通过对丕同圆弧半径模具镦粗成形过程的模拟，分析研究了模具圆弧半径对预制坯相对直径比以及镦粗后盘坯变形均匀程度的影响规律，同时对镦粗过程中等效应变速率的变化规律进行了分析，得到了合理的模具圆弧半径的取值范围，从而为粉末盘件预制坯的工艺制定提供了一种新的选择。     

Ti6Al4V钛合金粉末烧结体镦粗过程致密化规律研究

利用Deform-3D软件对Ti6Al4V粉末烧结体镦粗变形过程进行了热力耦合模拟,分析了不同变形参数下粉末烧结体镦粗变形的致密化行为及规律。结果表明:粉末烧结体在镦粗过程中,各区域密度、应力、应变分布不均匀;致密过程中密度分布与应变分布一一对应,且密度应变与体积应变成线性关系;致密过程受变形程度、变形温度、初始相对密度及变形速度等因素的影响。     

烧结温度对Ni16Cr9Al多孔材料性能的影响

以粒度小于25 μm的Ni16Cr9Al预合金粉末为原料,采用模压成形、真空烧结的方法制备了Ni16Cr9Al多孔材料,研究了烧结温度对Ni16Cr9Al多孔材料性能的影响。结果表明：Ni16Cr9Al粉末压坯在烧结过程中由于烧结颈的形成、长大,体积发生收缩,随着温度的升高,烧结体的孔隙度和孔径减小,强度提高,1130 ℃具有良好的三维孔隙结构,高于1150 ℃,孔隙减少,材料逐渐致密     

大型铝镁合金法兰盘镦冲成形超声探伤缺陷分析及控制

针对大型5083铝镁合金法兰盘自由锻镦粗与冲孔成形过程中近内孔区域超声探伤出现的质量问题，建立了镦粗后饼坯冲孔有限元分析模型，模拟获得了冲孔过程法兰坯近内孔区域材料的流动规律，分析得出，法兰坯近内孔区域超声探伤质量缺陷为反面压平过程中近内孔区域材料流动存在径向分流所致。基于上述分析，提出对现行的正向冲孔-反面压平-连皮冲除工艺流程中的反面压平工序进行调整，即在反面压平工序中留存正向冲孔的冲子以约束近内孔区域材料的内流。进一步模拟表明，改进后的冲孔工艺能够显著改善近内孔区域材料的流动状态，规避了导致近内孔区域质量缺陷的径向分流不利因素。生产实际表明，调整后的工艺方案使得大型法兰盘镦冲成形过程中近内孔区域缺陷问题得到了有效解决。     

Effect of compact structure on phase transformation kinetics from anatase phase to rutile phase and microstructure evolution during sintering of ultrafine titania powder compacts

The effect of compact structure both on phase transition kinetics and on the densification during sintering of n-TiO₂ compacts has been investigated. The compact structures varied from loose powder pack to very high density using different compaction pressures. The compact structure is verified to significantly affect phase transition kinetics and densification during sintering. The onset temperature of anatase -> rutile phase transition decreased with the increase of the compact density. The highest density compact of 86.9% showed phase transition at the lowest temperature and no detrimental effect on densification. The compact structure, i.e., the coordination number of particles in the compacts, is considered to significantly affect both the phase transition kinetics and the subsequent densification during the sintering of n-TiO₂ compacts.     

The effect of particle size on the densification kinetics of tungsten powder during spark plasma sintering

The influence of particle size on the densification kinetics of tungsten powder during spark plasma sintering was investigated. The densification rate of tungsten powder in the intermediate sintering stage decrease with increasing particle size, resulting in a delay in the sintering stages of coarse powder. The isothermal densification kinetic behaviors of tungsten powder show that the densification of tungsten powder can be divided into two kinetic stages: a low-stress exponent segment (n = 1.5) and a high-stress exponent segment (n = 3 or 4). With increasing of particle size, n increases from 3 to 4, and the activation energy decreases from 304 to 254 kJ/mol for the high-stress exponent segment. This is because the densification mechanism has a tendency to change from diffusion creep to dislocation creep or dislocation glide as the particle size increases. The evolution of the activation energy exactly matches the transformation of the deformation mechanism, indicating that the densification activation energy does not reflect a barrier to densification, but rather a barrier to deformation with different deformation mechanisms.     

齿轮粉末锻造的变形力分析及其过程数值模拟

应用粉末烧结材料广义塑性理论,分析和计算了齿轮粉末锻造成形镦粗、复压的变形力和密度,运用MARC有限元数值模拟了粉末锻造齿轮的成形过程,讨论了数值模拟和理论计算结果。     

Micro-FAST成形过程中电流大小对颗粒变形的影响

创新性地采用多物理场活化烧结微成形技术(Micro-FAST)制备了316L不锈钢和纯Cu微型齿轮。探讨了电流大小对316L不锈钢和纯Cu粉末体系在烧结过程中颗粒变形的影响。结果表明,在电场、力场和温度场的耦合作用下,升温过程是粉末体系实现致密化的主要过程。由于电塑性效应的影响,粉末体系在致密化过程中表现出的显著特征之一是颗粒的塑性变形。电流越大,颗粒的变形量越大,粉末体系的轴向尺寸减小量越多,最终微型烧结体的相对密度也越大。     

Effects of processing parameters in P/M steel forging on part properties: A review part II forging of sintered compact

In the last decade, powder metallurgy (P/M) technology has made marked advances in competitive manufacturing. P/M offers design opportunities that are not possible with other methods, as well as significant cost savings. The processing parameters, material characteristics, individual stages of parts production, deformation and densification mechanics and tooling, and preform design influence the properties of the P/M part and related economics. Therefore, a review of the various parameters involved in the different stages of P/M steel forging in net-shape manufacturing and their implications on resulting properties of the P/M parts is presented in a three-part review. Part I discussed the issues of powder preparation, compaction, and sintering in the stages of preparing a sintered compact. This review (Part II) identifies key parameters in forging the sintered compact that influence the properties of the powder forged part. Part III reviews currently available analysis methods for studying the powder forging process.     

3D statistical analysis of a copper powder sintering observed in situ by synchrotron microtomography

Sintering of a copper spherical powder was observed in situ by X-ray microtomography at the European Synchrotron Radiation Facility. Morphological tools have been developed to obtain three-dimensional (3D) quantitative microstructural characteristics of the particle packing. The average microstructural changes (neck size, coordination number, pore size distribution) and the macroscopic densification are in agreement with the predictions of the mean field sintering models, which assume a uniform shrinkage of the powder compact. The present 3D results confirm that the effect of the inherent heterogeneity of a random dense particle packing on the average microstructural changes during sintering is limited.     

高密度纯钨的低温活化烧结工艺及其致密化行为

采用溶胶喷雾干燥-煅烧-氢热还原法制备了BET粒径为0.21μm的超细纯钨粉末,并利用球磨处理进一步活化粉末。研究了超细纯钨粉末形貌及其性能随球磨时间的变化特征,探索了未球磨、球磨5h及球磨10h3种超细纯钨粉末烧结致密工艺,此外还详细研究了纯钨烧结体组织形貌、晶粒尺寸及显微硬度等性能随烧结温度及球磨时间的变化规律。结果表明,球磨处理对超细纯钨粉末的烧结起到了极大的活化作用,由球磨10h粉末制成的压块在1900℃下烧结2h其致密度即可达97.3%,比传统微米级纯钨粉末制成的压块达到相同烧结致密度的温度降低了600℃以上。同时,球磨处理可以大幅降低钨粉的起始烧结温度和再结晶温度,获得组织更加均匀细小、力学性能(硬度)更加优良的钨烧结体。     

Densification and microstructural evolution of a high niobium containing TiAl alloy consolidated by spark plasma sintering

Gas-atomized Ti–45Al–7Nb–0.3W alloy powders were consolidated by the spark plasma sintering (SPS) process. The densification course and the microstructural evolution of the as-atomized powders during SPS were systematically investigated by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD) and electron back-scattered diffraction (EBSD) techniques. As a result of SPS densification, special (α + γ) precipitation zones are formed in the initial stage of sintering, and the residual β phases in the microstructure of the powders are fragmentated. During the following SPS course, α₂/γ lamellar colonies at the edge of the precipitation zone, α₂ and B2 phase as well as dynamic recrystallized γ grains are found to form. For the as-atomized powders sintered at 1000 °C, the densification is preceded by the early rearrangement of the powder particles and the following formation of sintering necks. For the powders sintered at 1200 °C, plastic deformation plays an important role in densification. Local melting and surface bulging between two adjacent particles can also serve as one of the densification mechanisms. In the later stage of sintering, the growth of sintering necks controlled by diffusion and the pore closure would make important contributions to the densification.     

烧结工艺对纳米AgSnO2复合材料致密度的影响

对化学镀法制备的纳米AgSnO2粉末分别采用常压、热压和真空烧结的方法制成AgSnO2复合材料，研究了烧结方法及工艺对致密度的影响。结果表明，采用常压烧结，虽然成型压力、烧结温度和时间对致密度有一定的影响，但复压、复烧后，材料的致密度十分接近。热压能在短时间内获得96％以上的致密度，但复烧时膨胀严重，甚至出现鼓泡。真空烧结对去除吸附物有利，但仍会有少量残留并导致复烧时出现膨胀。在较低的压力下成型并采用分段保温烧结，不但能消除吸附物的影响，复烧后还可以得到94％的致密度。研究表明，纳米粉末表面大量的吸附物以及由此造成的封闭在压坯中的气体是阻碍烧结致密化的根本原因。     

Pressure consolidation of amorphous ZrO2–Al2O3 by plastic deformation of powder particles

Amorphous ZrO₂–Al₂O₃ powders undergo densification at low temperatures (<650°C) and moderate uniaxial pressures (~750 MPa). It is established that large pressure dependent densification and little time dependent densification occur. Viscous sintering is not the dominant densification mechanism. Study of the particle size effect in densification of amorphous ZrO₂–40% Al₂O₃, and comparison with hot pressing of borosilicate glass powder at 500 and 550°C and cold compaction of silver powder, clearly indicate the possibility of compaction of amorphous ZrO₂–Al₂O₃ by plastic deformation. Good agreement was seen between a model for the compaction of ductile metal powders and the observed hot pressing behaviour.