超细硬质合金中晶粒非均匀长大机理

采用市售的粒度为0.8μm的WC粉末和粒度为1.6μm的Co粉制备了WC-10%Co超细硬质合金,通过金相显微镜﹑扫描电子显微镜观察了不同烧结温度下制备的试样WC晶粒形貌,对超细晶粒硬质合金非均匀长大现象及机理进行了研究。结果表明:粉末湿磨后的粗大颗粒在烧结过程中起晶核作用,是引起晶粒非均匀长大的关键因素。固相烧结时,烧结体中细小颗粒受到张力的作用发生旋转,当其取向与邻近的大颗粒取向一致时,形成共格界面,以粗大晶粒为核心以并合的方式非均匀长大;液相烧结时,细小晶粒溶解并优先地在大晶粒的某些低能量晶面如(0001)和(1010)面析出,引起晶粒异常长大。本研究中,当烧结温度达到1 410℃时,WC晶粒可异常长大为接近20μm的粗大晶粒。     

超细硬质合金晶粒生长抑制剂VC、Cr_3C_2作用机理的研究

本文以液相复合-连续还原碳化方法制备的掺杂有VC和Cr3C2抑制剂的纳米复合WC-10Co粉末为原料,采用真空烧结+低压处理的工艺制备超细WC-10Co硬质合金,运用原子力显微镜(AFM)和场发射扫描电镜(FESEM)确定VC和Cr3C2抑制剂在硬质合金中的分布,讨论其抑制晶粒生长的机理。一部分VC、Cr3C2抑制剂吸附在WC晶粒表面形成30nm～50nm的沉淀物,降低WC晶粒的表面能;一部分VC、Cr3C2溶解在Co相中,降低WC在液相中的溶解度;其余VC、Cr3C2沉积在WC晶界,从而有效地抑制WC晶粒的长大。     

纳米硬质合金制备技术的研究

研究了 WC-Co纳米硬质合金的制备技术。采用强化球磨、添加晶粒长大抑制剂和低温加压烧结工艺 ,获得了 WC晶粒度接近 2 0 0 nm的硬质合金。研究了 VC和Cr3 C2 两种抑制剂加入量对合金组织、WC晶粒度和性能的影响以及抑制晶粒长大的机理。研究结果表明 ,添加 VC和 Cr3 C2 晶粒长大抑制剂十分有效的抑制了晶粒的长大 ,合金中的 WC晶粒度随抑制剂加入量的增加而细化。但过多的抑制剂不仅会导致碳化物在 WC/Co晶界上大量析出 ,而且也会增加孔隙度 ,结果增加了合金脆性 ,降低了合金的强度 ,其有害影响 VC比 Cr3 C2 更大。采用加压烧结可消除合金中的孔隙提高合金的强度。     

Effect of VC and NbC additions on microstructure and properties of ultrafine WC-10Co cemented carbides

The nanocomposite WC-Co powders were prepared through planetary ball milling method. Effects of grain growth inhibitor addition and the vacuum sintering parameters on the microstructure and properties of ultrafine WC-10Co cemented carbides were investigated using X-ray diffractometer, scanning electron microscope and mechanical property tester. The results show that VC and NbC additions can refine the WC grains, decrease the volume fraction of Co₃W₃C phase in ultrafine WC-10Co cemented carbides, and increase the hardness and fracture toughness of the base alloys. After sintering for 60 min at 1400 °C, the average grain size and hardness of ultrafine-grained WC-10Co-1VC cemented carbide are 470 nm and HRA 91.5, respectively. The fracture toughness of cemented carbide WC-10Co-1NbC alloy is over 7 MN·m^?3/2.     

微量钴对无金属粘结相WC硬质合金烧结致密化与WC晶粒生长行为的影响

采用Cr3C2、VC掺杂超细WC为原料和无压真空烧结工艺,通过合金微观组织结构的观察与分析,研究了添加质量分数为0.3%的Co对无金属粘结相WC硬质合金烧结致密化与WC晶粒生长行为的影响。结果表明,微量Co的存在加速了WC的烧结致密化过程,与此同时也导致了WC晶粒明显的各向异性非连续晶粒长大。在上述研究基础上,提出了一种无金属粘结相WC硬质合金的低成本制备工艺,探讨了超细硬质合金中WC晶粒生长机制,提出了超细硬质合金的质量改进建议。     

复合添加Cr_3C_2/NbC对超细晶WC-10%Co硬质合金组织和性能的影响

采用真空烧结方式制备超细晶WC-10%Co硬质合金,研究了复合添加晶粒长大抑制剂Cr3C2/NbC对合金组织及性能的影响。结果表明:在1400℃烧结条件下,添加剂的组成为0.4%Cr3C2+0.3%NbC时(文中含量均为质量分数),合金具有最好的综合力学性能,抗弯强度可达1951MPa、硬度1615HV30;WC平均晶粒尺寸为490nm。经SEM观察并通过EDS发现,抑制剂Cr3C2/NbC固溶到粘结相Co中,改变界面能,限制WC晶粒的长大;同时,也会降低WC在粘结相Co中的溶解度,从而起到细化晶粒的作用。     

Synthesis of Cr-doped APT in the evaporation and crystallization process and its effect on properties of WC-Co cemented carbide alloy

WC grain size has significant effect on WC-Co cemented carbide alloy properties. In order to inhibit WC grain growth during sintering process, grain growth-inhibitor Cr₃C₂ is usually added to tungsten carbide powder in advance through mechanical milling. While, homogeneous distribution of Cr₃C₂ in the tungsten carbide powder is difficult to achieve and result in abnormal growth of WC grains. For this purpose of growth-inhibitor uniform distribution, (CH₃COO)₃Cr is added into ammonium tungstate solution during evaporation and crystallization process to prepare Cr-doped APT powder, which can be used as precursor for ultrafine-grained WC-Co cemented carbide alloy preparation. Compared with conventional APT powder, the Cr-doped APT has smaller particle size and bulk density, moreover, chromium is evenly distributed within it. The Cr-doped APT is then used to produce Cr-doped tungsten powder, which also has smaller particle size than that of conventional tungsten powder. Cr-doped tungsten powder is subsequently prepared into tungsten carbide powder and WC-Co cemented carbide alloy through carbonization and sintering process, respectively. Compared with conventional WC-Co cemented carbide alloy, the obtained WC-Co cemented carbide alloy has smaller mean WC grain size (0.36 μm), and more uniform microstructure. Furthermore, the phenomenon of WC grain abnormal growth during sintering process is not observed, because the grain growth-inhibitor Cr₃C₂ is well dispersed in tungsten carbide and cobalt composite powder. Results show that the obtained WC-Co cemented carbide alloy presents better mechanical properties (HRA, bending strength, coercive force) than those of conventional WC-Co cemented carbide alloy. Accordingly, the novel addition of (CH₃COO)₃Cr during the evaporation and crystallization process is the key factor of ultrafine-grained WC-Co cemented carbide alloy production.     

Tungsten carbide platelet-containing cemented carbide with yttrium containing dispersed phase

A fine and platelet tungsten carbide patterned structure with fine yttrium containing dispersed phase was observed in liquid phase sintered WC-20% Co-1%Y2O3 cemented carbide with ultrafine tungsten carbide and nano yttrium oxide as starting materials. By comparing the microstructures of the alloy prepared by hot-press at the temperature below the eutectic melting temperature and by conventional liquid phase sintering, it is shown that hexagonal and truncated trigonal plate-like WC grains are formed through the mechanism of dissolution-precipitation （recrystallization） at the stage of liquid phase sintering. Yttrium in the addition form of oxide exhibits good ability in inhibiting the discontinuous or inhomogeneous WC grain growth in the alloy at the stage of solid phase sintering.     

Effect of powder particle size on gradient formation and grain growth in ultrafine crystalline gradient cemented carbide

Sinter-HIP combining vacuum and pressure in one-step sintering process has been applied to prepare ultrafine crystalline cemented carbide with a surface gradient layer enriched in binder. The effect of powder particle size on gradient formation and grain growth has been examined. The results show that the gradient layer thickness increases with decreasing WC and Ti(C,N) powder particle size. The number of abnormal WC grains increases with decreasing WC powder particle size. The formational mechanism of the gradient cemented carbide with ultrafine grains is discussed through analyzing the decomposability of nanoscale Ti(C,N), atomic diffusion and grain growth during one-step Sinter-HIP process.     

超细硬质合金烧结过程中WC扩散-溶解-析出特征与机理研究

以08型WC与Co制成WC-10%Co试样条,脱蜡后分别于1 200﹑1 250﹑1 300﹑1 350℃进行烧结并分别保温1 h和5 h。采用扫描电镜观察烧结过程中WC晶粒形貌的变化,采用差热分析仪研究WC-10%Co的共晶温度。研究结果表明:WC-10%Co在低于共晶温度的烧结过程中发生了WC晶粒形貌由等轴形向棱边平直的多边形变化,并随着温度升高,晶粒尺寸增大。这表明在固相烧结时超细WC与钴相存在溶解析出过程,改变WC晶粒形貌并使晶粒长大。     

烧结温度和保温时间对微波制备WC-8Co硬质合金显微组织的影响(英文)

采用2.45GHz高功率多模腔微波炉制备WC8Co硬质合金,对压坯的收缩率和合金的显微组织进行研究。结果表明:液相温度出现在1300°C附近;在烧结温度1450°C下保温5min能获得几乎全致密的合金试样。微波烧结法制备的合金晶粒要比真空烧结制备的合金晶粒尺寸细小且分布更均匀。另外,WC晶粒的尺寸和分布主要取决于烧结温度;保温时间对合金晶粒的影响很小,无论在1450°C下保温多长时间WC平均晶粒的尺寸始终保持在2.7μm。     

放电等离子烧结纳米硬质合金的研究

采用放电等离子烧结 (SPS)和普通真空烧结两种烧结工艺烧结 92WC - 8Co纳米硬质合金。放电等离子烧结 ,在 115 0℃的烧结温度、4.5kN压力下保温 5min ,烧结体就完全致密 ,其合金中的WC晶粒度小于 2 0 0nm ,硬度可达到 94.2HRA。真空烧结达到完全致密 ,烧结温度需 140 0℃ ,保温时间 30min ,WC晶粒度为 (30 0 40 0 )nm ,硬度最高为 93HRA。结果表明 :放电等离子烧结硬质合金的温度显著降低 ,烧结时间大大缩短 ,有效地抑制了WC晶粒的长大。SPS还显著降低微孔等缺陷 ,制品性能也大大提高。     

Experimental and simulation studies on grain growth in TiC and WC-based cermets during liquid phase sintering

The grain growth behaviors of TiC and WC particles in TiC-Ni, TiC-Mo₂C-Ni, WC-Co and WC-VC-Co alloys during liquid phase sintering were investigated for different Ni or Co contents and compared with the results of Monte Carlo simulations. In the experimental study, TiC-Ni and WC-Co alloys had a maximum grain size at a certain liquid volume fraction, while the grain size in TiC-Mo₂C-Ni and WC-VC-Co alloys increased monotonically with an increasing liquid volume fraction. These results mean that the grain growth of these alloys cannot be explained by the conventional mechanisms for Ostwald ripening, namely diffusion or reaction controlled processes. Monte Carlo simulations with different energy relationships between solidliquid interfaces predicted the effect of the liquid volume fraction on grain size similar to the experimental results. The contiguous boundaries between solid (carbide) particles appear to influence the grain growth behavior in TiC- and WC-based alloys during liquid phase sintering.     

Carbide grain growth in cemented carbides sintered with alternative binders

Cemented carbides are composites made of a hard refractory ceramic phase and a ductile binder, most commonly WC and Co, respectively. Since the use of cobalt in the hard metal industry is questioned by the new European regulation on chemicals, extensive research has been done to develop new grades based on a Co-Ni-Fe binder. With similar mechanical, physical properties and affinity to C and W, nickel and iron are the best candidates for an efficient binder in cemented carbides. As mechanical properties are strongly dependent on the materials microstructure, and especially on the WC grain size, understanding the effect of the binder on the final microstructure is crucial.In this work, the carbide grain growth behaviour of WC-M alloys (M = Co, Ni, Fe) with different carbon contents is discussed from qualitative and quantitative microstructural analyses. Whereas grain growth is more or less inhibited in WC-Fe alloys, increasing carbon content promotes grain growth in WC-Co and WC-Ni alloys, with a slight abnormal grain growth in case of Ni binder. Different mechanisms for grain growth are discussed, in relation with the observed morphology of WC grains after sintering.     

两步碳化制备纳米WC粉末及其合金性能

研究了两步碳化工艺对氢还原/碳化制备的纳米WC粉末及其WC-Co合金性能的影响。结果表明,WC粉末的晶粒聚集和异常粗大颗粒主要是由于碳化初期钨颗粒因烧结合并增粗,而钨粉碳化不完全主要是由于碳化后期的温度偏低,利用先低温碳化后高温碳化的两步碳化工艺不仅能够有效抑制纳米颗粒烧结合并增粗,而且可以使钨粉充分碳化,得到颗粒细小、均匀,W2C含量极少的WC粉末;采用1120℃碳化加1180℃碳化的两步碳化工艺制备出的138 nm的WC粉末,W2C含量少于0.5%(质量分数),以其为原料制备的WC-Co烧结体显微组织结构均匀,为超细晶硬质合金,综合性能优良,洛氏硬度HRA高达93.7,抗弯强度高达4380 MPa。     

Additive manufacturing of WC-13%Co by selective electron beam melting: Achievements and challenges

Additive manufacturing is a powerful tool for rapid prototyping and fabricating metal articles having a complicated geometry. This method is known to be used almost solely for the manufacture of articles consisting of pure metals and alloys. In the present work the possibility of obtaining dense carbide articles by a single-step process of additive manufacturing based on selective electron beam melting was evaluated. A new technology for fabricating cemented carbide granules suitable for selective electron beam melting was developed. It includes conventional granulating WC-Co powders followed by solid-state pre-sintering and preliminary screening of the granules. After that their liquid-phase sintering and final screening are carried out to obtain a desired fraction needed for the additive manufacturing process. Results of experiments on selective electron beam melting at different scan rates and current values indicated that it was possible to obtain non-porous carbide articles of complex geometry from WC-Co granules initially containing 13 wt% Co. The selective electron beam melting process led to the evaporation of some liquid Co and very intense local WC grain growth resulting in peculiar microstructures of the cemented carbide articles comprising layers with medium-coarse and abnormally large WC grains. A near-surface layer of the cemented carbide articles obtained by additive manufacturing is characterized by a high roughness comparable with the mean size of the original WC-Co granules.     

Ni3Al添加量对WC-Co硬质合金中WC晶粒形状的影响

采用粉末冶金制备技术,以粗WC粉末、Co粉和WC＋Ni3Al预合金粉末为原料制备出WC-40vol％（Co—Ni,Al）硬质合金。利用扫描电镜和透射电镜研究了不同NbAl含量对WC-40vol％（Co—Ni3Al）硬质合金中WC晶粒形状的影响规律。结果表明：W在Co粘结相中的固溶度接近25．4wt％,而W在Ni,Al粘结相中的固溶度接近9．5wt％,随着NbAl含量的增加,粘结相对W的固溶度减小,合金中的WC晶粒圆钝和细小;WC晶粒表面上出现明显的台阶。相应的,延长烧结时间,WC—Co—Ni3Al硬质合金具有与WC—Co硬质合金相同的WC生长行为,WC-40vol％（Co—Ni3Al）硬质合金中的WC晶粒表面上的台阶处出现明显的刻面。     

Study of solid-state sintered fine-grained cemented carbides

Submicron cemented carbides are most often produced by liquid phase sintering. To retard the grain growth during sintering, these materials are sintered at low temperatures and with addition of grain growth inhibitors, e.g. Cr and V. The common hypothesis is that the sintered material would benefit from a more evenly distributed inhibitor in the WC raw material, in order to control the grain growth during both the solid-state and liquid phase part of the sintering.

Aiming to study the distribution of Cr specifically after solid-state sintering, a Cr-doped WC-powder was mixed with Co and excess of carbon, and subsequently hot-pressed for 1 h and 30 MPa at 1200 °C. This transmission electron microscopy study shows that Cr is concentrated to the surfaces of the WC grains in the sample after mentioned solid-state sintering.     

超粗晶WC-Co硬质合金的研究现状

超粗晶硬质合金是一类性能优异的新型合金,广泛用于采矿、凿岩、轧辊等领域,应用前景广阔。化学包覆法和纳米粉末溶解法是超粗晶WC-Co硬质合金制备的主要方法,介绍了超粗硬质合金原料WC颗粒平均粒度选择的动力学理论,碳含量对超粗合金中WC晶粒生长粗化的影响以及通过引入其它元素强化粘结相以提高合金寿命的探索结果。此外,分析了以合金硬度为常数时,晶粒尺寸与合金性能的关系,指出在保持合金硬度不变时,通过增大WC晶粒尺寸来提高合金的耐磨性和韧性是可行的。最后,展望了超粗晶合金的发展方向。     

超细WC-Co硬质合金及其磨损性能研究

采用低温化学镀方法在超细WC颗粒表面进行金属钴包覆,烧结包覆后的复合粉体制备新型硬质合金NYG（WC-3%Co）.研究了超细WC-Co硬质合金的力学性能、断口形貌和显微结构,在销盘式磨损试验机上进行干滑动磨损实验.结果表明,在硬质合金烧结过程中,沿WC晶界均匀分布的金属钴不仅起粘结剂作用,也起抑制剂作用阻碍晶粒的长大;新型硬质合金的抗弯强度、断裂韧性、硬度和耐磨性能均得到较大提高;在干滑动摩擦条件下,新型WC-Co硬质合金的失效以塑性变形及细小碳化钨相颗粒脱落为特征.