Synthesis of WC-Co composite powders with two-step carbonization and sintering performance study

In this study, WC-Co composite powder was synthesized by two-step carbonization method using W, Co and C as raw materials. X-ray diffraction (XRD) showed that the η phase (Co₆W₆C) was kept at 1100 °C for 1 h under vacuum, and it could be completely carbonized into WC-Co composite powders. The surface morphology of WC-Co composite powders was analyzed by scanning electron microscope (SEM). The effects of η phase and second phase (W phase) on WC morphology and Co phase distribution were investigated. Electron backscattered diffraction (EBSD) was used to analyze WC-10 wt% Co cemented carbide particle distribution. Comparison of transverse rupture strength, hardness and fracture toughness of two kinds of WC-10 wt% Co cemented carbides synthesized by WC-Co composite powders + WC and WC + Co respectively, the cemented carbide of composite powders + WC increases the fracture toughness from 11.4 ± 0.3 MPa·m^1/2 to 12.4 ± 0.3 MPa·m^1/2.     

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.     

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。     

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。     

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.     

Effect of Re addition on the microstructure and mechanical properties of WC-10Co cemented carbides fabricated by chemical coating method

WC-Co composite powder with nano‑cobalt coatings doped with rhenium (Re) varying from 0 to 3 wt% were prepared by a novel sol-gel method. The effects of Re on the microstructure and mechanical properties of WC-10(Co, xRe) cemented carbides fabricated by composite powder were studied by XRD, SEM and TEM. The results indicated that C₄H₆CoO₄·4H₂O with or without NH₄ReO₄ was homogenously distributed on WC powder particles. After reduction in H₂ atmosphere, nano‑cobalt with Re was coated on WC particles with a uniform distribution in composite powder. WC-10(Co, xRe) cemented carbide containing Re exhibited homogeneous grain distribution. Moreover, no stabilization effect for hexagonal close-packed (hcp) cobalt by Re addition was found mainly due to the minor additive amount and the unique preparation method. WC grains were refined and binder phases were strengthened due to the dissolved Re in binder phases. With increasing the content of Re from 0 to 3 wt%, an improved microstructure led to better mechanical properties, the hardness increased from 87.8 ± 0.1 HRA to 89.4 ± 0.2 HRA, transverse rupture strength (TRS) increased from 2810 ± 89 MPa to 3410 ± 113 MPa.     

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。     

Role of Co Content on Densification and Microstructure of WC–Co Cemented Carbides Prepared by Selective Laser Melting

WC–Co cemented carbides, well-known as the conventional tooling materials, have not been successfully produced by one step additive manufacturing processes such as selective laser melting(SLM) yet. The microstructure evolution as well as WC grain growth behavior has rarely been investigated in detail during SLM process. In this study, the WC–Co cemented carbides with different Co contents(12–32 wt%) were prepared by optimized SLM processes for comparative investigation of densification behavior, microstructure characterization and mechanical property. The increase in Co content in feedstock carbide granules can improve the densification behavior during SLM process. The SLM processed WC-12 Co shows larger average WC grain size and higher percentage of coarser WC grains as compared with both WC-20 Co and WC-32 Co. The microstructure characterization, combined with finite element simulation, shows the WC grain growth mechanisms include agglomeration and dissolution-deposition of WC during SLM process and agglomeration of WC is an important mechanism especially for WC–Co cemented carbides with Co content as low as 12 wt%. The comparison between horizontal(perpendicular to the SLM laser beam) and vertical(parallel to the SLM laser beam) cross sections of carbides shows that SLM process introduces a certain degree of microstructure and mechanical behavior anisotropy for WC-12 Co, WC-20 Co, and WC-32 Co.     

工艺条件对WC-12%Co超细硬质合金性能的影响

采用不同粒度的WC粉,加入VC、Cr3C2做抑制剂,制备WC-12%Co超细硬质合金。采用D60-25型钴磁仪测量合金磁饱和,利用排水法测定合金密度,采用三点弯曲法在CMT4504拉伸机上检测合金的抗弯强度,试样抛光后在JEOL-6701F扫描电镜下观察合金的显微组织。研究了不同的WC粉末粒度、球磨时间、烧结工艺对WC-12%Co的超细硬质合金性能的影响。结果表明:过压烧结可明显提高合金抗弯强度、硬度和密度;随着球磨时间的增加,合金硬度不断上升,抗弯强度先增后减;采用0.55μm粒度WC粉制备的合金的硬度明显高于0.70μm粒度WC粉制备的合金。在本次实验中,选用0.55μm的WC粉末原料,混合料球磨85 h,通过过压烧结,可制备出性能优良的WC-12%Co超细硬质合金,硬度HV≥1 800,抗弯强度≥3 400 N/mm2。     

Optimization of Initial WC Grain-Size Distribution in WC-6Ni Cemented Carbides

The initial WC grain-size distribution, as one of the most important factors influencing the properties of alloys, has been focused on for a long time in some fields other than cemented carbides. In the present work, five groups of WC-6Ni cemented carbides were prepared and the effect of initial WC grain-size distribution on the microstructure and properties of the hard alloys was studied. The results indicate that initial WC grain-size distribution has significant influence on the microstructure and properties of WC-6Ni cemented carbides, except density. When the mass ratio of coarse-WC and fine-WC is 7:3, WC-6Ni cemented carbide samples with initial WC grain-size distribution of WC-1(4.8 μm) and WC-4(0.7 μm) show a relatively good combination properties, while the cemented carbide samples with WC-5(0.4 μm) as the fine-WC component exhibit higher values on some properties, mainly hardness and immersion corrosion resistance in acid and alkali.     

烧结过程中WC晶粒形貌的变化

采用粉末冶金工艺烧结了纯WC,制备了WC-10%Co硬质合金,利用JSM5600LV扫描电镜观察不同烧结温度下制备的试样WC颗粒/晶粒形貌。结果表明:纯WC进行烧结时,烧结过程中在表面张力的作用下,系统向能量最低趋势发展,WC晶粒保持球形或类球形;在有钴的存在下,界面张力状态发生了改变,为了达到系统的稳定状态,WC/Co接触界面向平直化发展,烧结温度达到共晶温度之后,在溶解-析出机理的作用下,WC晶粒有选择性长大,形状更加规则化,最终导致WC/Co界面平直。     

Effects of fine WC particle size on the microstructure and mechanical properties of WC-8Co cemented carbides with dual-scale and dual-morphology WC grains

Dual-scale and dual-morphology WC grained WC-8Co cemented carbides comprising triangular or hexagonal fine WC grains and plate-like coarse WC grains were synthesized by vacuum sintering using Co, flaky graphite, WC, and coarse W as the starting materials. The effects of fine WC particle sizes on microstructure, relative densities, and mechanical properties of the dual-scale and dual-morphology WC grained cemented carbides were investigated. The results revealed that the growth of plate-like coarse WC grains was further promoted with the decrease in the particle size of the added fine WC; hence, their aspect ratio increased. In addition, added fine WC led to the separation of plate-like coarse WC grains so as to break their oriented arrangement and prevent their face contact; hence, plate-like coarse WC grains were completely covered by the Co binder phase. Moreover, the addition of smaller particle size of fine WC contributed to more uniform Co binder phase. When 0.4-μm WC powders was added, the aspect ratio of plate-like coarse WC grains was greater than that of plate-like WC grained cemented carbides without the addition of fine WC. The dual-scale and dual-morphology WC grained cemented carbides by adding 0.4-μm fine WC exhibited good comprehensive mechanical properties, with a transverse rupture strength of 3645 MPa, a Rockwell hardness of 91.5 HRA, and a fracture toughness of 12.3 MPa∙m^1/2.     

Microstructure and mechanical properties of 90WC-8Ni-2Mo2C cemented carbide developed by conventional powder metallurgy

In this paper, the microstructure and mechanical properties of a WC-Ni based cemented carbide with the addition of 2 wt% Mo₂C, processed by conventional powder metallurgy, was investigated. With the addition of only Mo₂C in the WC-Ni alloy system, the wettability between the WC and Ni binder phase was improved, which was confirmed by the increased density, hardness, fracture toughness and flexure strength of the cemented carbide obtained, which is superior than those observed in WC-10Ni cemented carbides and similar to those observed in WC-Co and WC-Ni-TiC-Mo₂C cemented carbides. Microstructural examinations of the developed cemented carbide 90WC-8Ni-2Mo₂C indicated that there was no excessive grain growth of the WC particles during sintering, confirming that Mo₂C is a grain growth inhibitor as effective as other carbides such as VC, TiC, Cr₂O₃, showing that the addition of only Mo₂C is able to improve the overall mechanical properties of the WC-Ni alloy system without sacrificing the toughness.     

碳化钨粗细颗粒搭配对WC—10%Co合金力学性能与组织结构的影响

本实验系统地研究了碳化钨粗颗细粒搭配对WC—10%合金的力学性能与组织结构的影响.优质碳化钨粉,通过空气分级的方法获得三种不同粒度级别的碳化钨.不同粒度级别的碳化钨,以适当的比例搭配和钴粉混合.获得具有“双重晶粒结构”的硬质合金,在凿岩试验中取得了良好的结果.     

Effect of Cu on the microstructures and properties of WC-6Co cemented carbides fabricated by SPS

The aim of this work is to study the effect of Cu on sintering temperature, densification, microstructure and mechanical properties of WC-6Co cemented carbides fabricated by spark plasma sintering (SPS). Fine grained WC powders with an average size of 1.2 μm, were investigated. Microstructures, hardness, fracture toughness and wear resistance of WC-6(Co/Cu) cemented carbides were measured and observed using SEM, mechanical property test. The results show that the sintering temperature of WC-6Co cemented carbides can be decreased obviously with Cu added; addition of Cu reduced grain size to 0.85 μm, but led to lower density. The adding amount of Cu should be controlled within a certain range, and the samples adding the appropriate proportion of Cu can obtain higher hardness and wear resistance.     

WC-6%Co硬质合金的冲击磨粒磨损性能研究

用MLD-10型动载磨粒磨损试验机研究了不同晶粒度YG6硬质合金的冲击磨粒磨损性能。对试样进行了失重测量,并用FESEM分析磨损试样表面形貌。结果表明,硬质合金的耐磨性能与粘结相自由程、WC晶粒度有关;在本实验条件下,随着WC晶粒度或粘结相自由程增加,合金耐磨性先增加后减小,特定WC晶粒度配比的混晶WC-6%Co合金的抗冲击耐磨性能最好。合金的磨损失效机制也发生了转变:细晶合金主要表现为WC颗粒的剥落;随WC晶粒度增加,合金主要表现为大颗粒WC破碎后脱落。在以SiC为磨料的冲击磨粒磨损试验中,WC-6%Co合金的磨损是以WC颗粒的破碎和碎片脱落为主要的失效机制。     

超细/纳米结构WC-10Co-4Cr涂层的透射电镜表征与性能分析

利用真空原位还原碳化反应合成超细/纳米WC-Co复合粉末,通过添加一定量Cr获得WC-10Co-4Cr复合粉末,经团聚造粒获得喷涂用复合粉末喂料,采用超音速火焰(HVOF)喷涂系统制备出超细/纳米结构的WC-10Co-4Cr涂层。利用X射线衍射仪,扫描电子显微镜和透射电子显微镜对涂层的物相、显微组织结构、元素分布特征等进行了系统表征,并对涂层耐磨性、耐蚀性进行了测试分析。结果表明:基于原位反应合成WC-Co复合粉制备的超细/纳米结构WC-10Co-4Cr涂层具有较好的耐磨性和耐腐蚀性。涂层以WC为主相,含有非晶结构的粘结相Co(Cr),同时存在少量六方晶体结构的W_2C相和非晶复相W_2C+Co(Cr)。对涂层中元素Co和Cr的分布进行了量化分析,得到其从WC晶粒到相界到共晶区再到Co区的变化规律。结合WC-10Co-4Cr复合粉末和超音速火焰喷涂工艺的特点,阐释了Cr在WC-10Co-4Cr涂层分布状态的形成原因,并讨论了对涂层性能的影响。     

粗晶碳化钨粒度对WC-Co合金晶粒度的影响

选用供应态分别为30μm和12μm的二种粗颗粒WC粉末,研究不同方法表征的粉末粒度与合金晶粒度的关系。结果表明:三种粉末粒度测定方法给出的结果都呈现粒度越粗合金的晶粒度也越粗的规律。粗颗粒WC的研磨态粒度与合金的晶粒度相当接近,金相法测得的12μmWC的晶粒分布与所制备的合金的晶粒度的一致性比30μmWC制备的合金要好。粗晶WC研磨态的Fsss粒度可以用于评价粗晶WC晶粒度,也可以预测WC-Co合金的晶粒度。     

High temperature wear resistance and thermal fatigue behavior of Stellite-6/WC coatings produced by laser cladding with Co-coated WC powder

The Stellite-6/WC composite coatings were produced on AISI H13 hot work tool steel by laser cladding with mixture of Co-coated WC (WC-12Co) particles and Stellite-6 powder. The phase composition, microstructural characterization, high temperature wear resistance and thermal fatigue behavior of Stellite-6/WC coatings were investigated and compared with the properties of the coatings produced from mixture of WC particles and Stellite-6 powder. The results showed that using the WC-12Co particles alleviated the decomposition of WC and resulted in the weaker intensity of W₂C, CoC_x and Co₆W₆C peaks in the X-Ray Diffraction (XRD) patterns. Compared with using the WC particles directly as the coating material, using the WC-12Co particles could further improve the wear resistance of coatings according to the relative lower width and depth of wear scars at the same WC content. In addition, fewer fatigue cracks were observed on the surface of coatings made by adding WC-12Co particles under the same thermal fatigue conditions, which indicates that using WC-12Co is beneficial to extend the life of Stellite-6/WC coatings.     

抑制剂对WC-15wt%Co硬质合金性能的影响

研究了抑制剂的不同种类和添加量对WC-15wt%Co硬质合金性能的影响。添加抑制剂可以抑制烧结过程中的晶粒长大及非均匀长大,提高合金的各项性能。在原始WC粉末粒度较粗的合金中添加一定量的VC/Cr3C2,也可以制取WC晶粒度达到亚微米级的合金。