Use of Ti in hard metal alloys – Part I: structural and microstructural analysis

Nanosized WC, and binders Co, Ti and Ti‐Co, are used to process hardmetals. Titanium (Ti) was proposed to reduce and even replace the Co in these composites, verifying the effectiveness of the new binders. Samples of nanosized WC with 10 wt% Co, 9 wt% WC 1 wt% Ti – Co, WC‐10 wt% Ti were cold compacted at 200 MPa and sintered at 1500°C during 1 hour under vacuum of 10^–2 mbar for the processing of hardmetal were performed. The structural characterization by X‐ray diffraction and microstructure by scanning electron microscopy (SEM) and EDS microanalysis of the sintered material. We observed the presence of the W₂C phase in the sintered samples, and Co₃W phases in the samples with Co content and a good distribution of binder phase, leading to formation of small “pool” of Co and Ti and small porosity and well distributed. It was proved that using Ti as binder phase, the neta phase formation was avoided.     

Rapid sintering of ultra-fine WC-10 wt% Co by high-frequency induction heating

High-frequency induction-heated sintering (HFIHS) is utilized to consolidate ultra-fine grain WC-10 wt% Co. Densification to near theoretical density in a relatively short time can be accomplished with insignificant change in grain size. WC-10 wt.% Co with a relative density of up to 99.5% was produced within 1 min with the simultaneous application of 60 MPa pressure. The average grain size of the densified material was about 260 nm and the mean free path in the cemented carbide was about 11 nm. The sintered material had fracture toughness and hardness values of 13 MPa^.m^1/2 and 1886 kg/mm², respectively. The hardness is comparable to literature values but the fracture toughness is about two times higher. These results are interpreted in terms of current effects on sintering and mass transport. Higher heating rates result in higher density with smaller WC grain size, and higher current-induced solubility of WC in Co is proposed as an explanation for the high fracture toughness.     

Effect of TiN addition on sintering behaviour and mechanical properties of WC-10Co hard metals containing Mo2C and Ni

The aim of the present work was to substitute a portion of WC in WC-10 wt% Co hard metal by TiN by modification of the binder phase, and to produce an equivalent grade of hard metal. Sintering studies were carried out in both H₂ and H₂-N₂ (5050) mixture. Introduction of TiN into WC-10Co hard metal resulted in a high sintered porosity, and as a consequence the mechanical properties deteriorated. Partial substitution of cobalt in WC-8.7TiN-12Co by nickel further increased the sintered porosity and led to a non-uniform microstructure. Incorporation of Mo₂C along with cobalt-nickel binder promoted a fine-grained structure, which resulted in better sintered properties than those of WC-8.7TiN-6Co-6Ni hard metal. However, hot isostatic pressing (HIP) treatment of the liquid-phase sintered alloys was effective in eliminating the large pores and thus greatly enhanced the mechanical properties. HIPed hard metal of WC-8.7TiNS-12Co composition showed properties almost equivalent to those of WC-10Co hard metal.     

The Effect of Variable Binder Content and Sintering Temperature on the Mechanical Properties of WC–Co–VC/Cr3C2 Nanocomposites

WC powders with an average crystallite size of 10 nm were successfully prepared by ball milling of micron-sized tungsten carbide powders. Grain growth inhibitors (VC and Cr₃C₂) with concentrations of 0.6 wt% each were added to nanocomposites of WC–9Co and WC–12Co, in both as-received and milled WC. Powder mixtures were then consolidated using spark plasma sintering technique at 1200 and 1300 °C for 10 min under high vacuum and pressure of 50 MPa. The influence of WC crystallite size, Co content, and sintering temperature over microstructure and mechanical properties of the resulting composites were studied through XRD and FESEM. Densification and attained grain sizes of the sintered products were measured by Archimedes principle and Scherrer procedure, respectively. Moreover, microhardness (H_v30) and fracture toughness were measured and compared for each composition to comparatively assess the individual effect. It was observed that the addition of VC and Cr₃C₂ resulted in decreased densification of the synthesized composites. These grain growth inhibitors were found to limit grain sizes to 131 nm with an average hardness of 1592 H_v30 and fracture toughness of 9.23 Mpam^1/2.     

Sintering behaviour and mechanical properties of Cr3C2–NiCr cermets

Cr₃C₂–NiCr cermets are used as metal cutting tools due to their relatively high hardness and low sintering temperatures. In this study, a powder mixture consisting of 75 wt% Cr₃C₂–25 wt% NiCr was sintered at four different temperatures and characterized for its microstructure and mechanical properties. The highest relative density obtained was 97% when sintered at 1350 °C. As the relative density increased, elastic modulus, transverse rupture strength, fracture toughness and hardness of the samples reached to a maximum of 314 GPa, 810 MPa, 10·4 MPa·m^1/2 and 11·3 GPa, respectively. However, sintering at 1400 °C caused further grain growth and pore coalescence which resulted in decreasing density and degradation of all mechanical properties. Fracture surface investigation showed that the main failure mechanism was the intergranular fracture of ceramic phase accompanied by the ductile fracture of the metal phase which deformed plastically during crack propagation and enhanced the fracture toughness.     

Synthesis of WC hard materials using coated powders

Nickel and cobalt were used as binder materials for tungsten carbide powders (WC) hard materials. Ni and Co binder were added individually to the WC powder by two different methods namely, mechanical mixing and chemical electroless coating. In this study WC powders of grain sizes 0.3–1.0 μm were electroless coated with either nickel or cobalt. The loading of either Ni or Co coating was 13 wt.%. The electroless-coating method conditions of both Ni and Co on WC powders are described. The coated powders were cold compacted and sintered in vacuum at different sintering temperatures. For comparison, identical materials compositions were prepared by mixing the powders constituents mechanically, compacted and sintered under the same conditions.The prepared powders and sintered materials were investigated using X-ray diffraction (XRD) and scanning electron microscope (SEM). The results revealed that coated WC materials have smaller values of porosity and more homogeneous microstructure while other properties, such as transverse rupture strength, and hardness exhibit greater values than those produced using mixing elemental powders. It is possible to outline the benefits of coated powder approach in the following: high homogeneity and better distribution of binder materials within WC hard materials, higher density and good interfacial bonding, capability of using fine powders, and possibility of using small alloying and/or reinforcement additions in a more uniform manner.     

超音速火焰喷涂WC-17Co涂层微观结构与性能研究

采用超音速火焰喷涂技术喷涂了两种不同WC颗粒尺寸的WC-17Co粉末.对制备的两种涂层的硬度、孔隙率、断裂韧性、结合强度和电化学腐蚀行为进行了测试.结果表明:具有亚微米结构WC颗粒的粉末制备的涂层,在硬度、孔隙率、断裂韧性方面具有一定优势,而含有大颗粒WC相的粉末制备的涂层在结合强度、腐蚀行为方面优势明显,这说明WC颗...     

硬质相粒度对金属陶瓷断裂韧性的影响

用压痕法测定了具有不同粒度硬质相的Ti(C,N)基和WC基金属陶瓷的断裂韧性,结果发现,Ti(C,N)基金属陶瓷的断裂韧性随硬质相粒度的增大而减小,而WC基金属陶瓷的断裂韧性随硬质相粒度的增大而增大。产生上述现象的主要原因与硬质相的晶体结构有关:在室温条件下,面心立方结构的Ti(C,N)晶体中可能存在｛110｝<11 0>和｛111｝<11 0>两个滑移族(含18个独立滑移系);裂纹从一个Ti(C,N)颗粒扩展至另一个Ti(C,N)颗粒时很容易形成取向有利。当Ti(C,N)颗粒较粗时,极易发生穿晶断裂,并且裂纹连续穿晶扩展时亦不会发生显著的偏转或分叉,Ti(C,N)呈现较强的脆性断裂特征。而密排六方结构的WC晶体只有｛101 0｝<112 3>一个滑移族(含4个独立的滑移系);由于取向不利,裂纹难以连续穿晶扩展,且随WC粒度的增大,其对裂纹的偏转和分叉作用增强,从而导致断裂面表面积增大而增韧。     

烧结温度对WZV材料力学性能和显微结构的影响

为了开发一种新型刀具材料,以WC、ZrO2和VC为原料,利用热压烧结工艺,分别在1500、1550、1600℃和1650℃烧结温度下制备了4种相同成分的WC/ZrO2/VC(WZV)复合材料.分析了烧结温度与刀具材料相对密度、硬度、抗弯强度和断裂韧性之间的关系,研究了烧结温度对刀具材料力学性能和显微结构的影响,确定了该材料合理的烧结温度为1550℃.试验结果表明,ZrO2质量分数为10%的WZV复合粉末经过48 h的高能球磨,在1550℃、30 MPa的热压烧结条件下,可获得相对密度为99.2%,维氏硬度为17.6 GPa,抗弯强度为786 MPa,断裂韧性为11.51 MPa.m1/2的优异性能.此外,通过对材料显微结构和断裂方式的分析,发现烧结温度对材料的断裂方式具有重要影响.     

Herstellung HVOF gespritzter,feinststrukturierter Cermetschichten unter Verwendung von feinen WC‐12Co Pulvern

Manufacturing of HVOF sprayed, finest structured cermet coatings using fine WC‐12Co powders The continuous increase in productivity and performance of modern sheet metal forming processes combined with the employment of novel, high strength materials cause high wear on tool systems. Coating technologies like thermal spraying provide a high potential to functionalize and to protect the surface of forming tools. However, it has to be ensured that the high shape and dimensional accuracy of the tool contour is preserved after the application of a wear protective coating. This aim cannot be achieved using currently applied, thermally sprayed coating systems with conventional, coarse grained microstructure. To solve this problem, novel finest structured coatings have been developed in this study by thermal spraying of fine WC‐12Co powders using the HVOF technique. For this purpose the influence of varying HVOF combustion gas compositions on the spray process as well as on the corresponding coating properties has been investigated. Next to a high surface quality the focus was placed on achieving coatings with high hardness and corresponding high wear resistance, low porosity as well as a good adhesive strength on the substrate material.     

Metal fibre reinforced hydroxy-apatite ceramics

The reinforcement of hydroxy-apatite ceramics with metal fibres is discussed. Hastelloy X and FeCralloy fibres were dispersed in hydroxy-apatite powder slurry. The fibre-powder slurries were dried and sieved over a wide aperture sieve. The resulting granules were used for die pressing. Volume fractions used were 10, 20 and 30%. The compacts obtained in this way were isostatically repressed at 4 k bar. These compacts showed considerable strength and toughness. Hot-pressing of the compacts was done at about 1000'C and a pressure between 0.2 and 1.0 k bar for 15 min. The resulting materials were characterized by fractography and strength, fracture toughness, Young's modulus and hardness measurements. Both strength and fracture toughness increased while Young's modulus and hardness decreased with increasing volume fraction of fibres. The strength and fracture toughness of composites containing 20 vol % metal fibres showed an increase of the strength and fracture toughness by a factor of about 2 and 6, respectively, as compared with the strength of about 100 M Pa and a toughness of 1 .0 MPa m^1/2 for the sintered, pure matrix materials. The results obtained are also promising for other metal fibre-ceramic matrix composites.     

Experimental Investigation and Optimization of Machining Characteristics in Ultrasonic Machining of WC–Co Composite Using GRA Method

WC–Co composite material is highly demanded in manufacturing industries, because of its unique properties such as excellent hardness with toughness, higher mechanical strength, and good dimensional stability. The present investigation is aimed at studying the impact of different experimental conditions (by varying cobalt content, thickness of work piece, tool profile, tool material, abrasive grit size, and power rating) on responses of interest (material removal rate and tool wear rate) in ultrasonic drilling of WC–Co composite material. The experiments have been planned by using Taguchi's L-36 orthogonal array and grey relation analysis has been applied for optimization of multiple responses. Analysis of variance is also employed to find the significant factors. Significant effects are observed for process variables such as cobalt content, abrasive grain size, and power level. Tools with higher hardness delivered better machining performance.     

低压烧结温度对一步法制备超细晶WC-Co基硬质合金组织及性能的影响

采用等离子球磨技术制得W-C-10Co-0.9VC-0.3Cr_3C_2纳米复合粉体,并利用单向模压成型法将其压制成生坯,再经低压烧结一步法制备成硬质合金。研究表明,等离子球磨3h所获得的复合粉体呈片层状形貌,并且成分分布均匀。在1 380℃及1 400℃烧结时,由于等离子球磨的特殊作用,VC、Cr_3C_2对WC晶粒长大抑制作用突显。1 380℃烧结制备的硬质合金,致密度为99.2%,WC平均晶粒尺寸为250nm,硬度和横向断裂强度分别为92.3HRA和2 443 MPa,具有最佳的WC晶粒尺寸与致密度配合,以及最佳的综合力学性能。     

Mechanical properties of reaction sintered TiN ceramics with B4C addition

Reaction sintering of TiN with B₄C addition was developed to densify the composite without the application of external pressure. The process utilizes high affinity of B for Ti which leads to the formation of extremely fine highly active TiB₂. The addition of 6–8 wt% B₄C is sufficient to increase the sintered density to over 96% theoretical density, fracture toughness to 3.5 MPa·m^1/2, flexural strength to 415 MPa, and hardness to 14 GPa. The major toughening mechanism was identified to be the crack deflection caused by the presence of hard and tough TiB₂ particles. The large improvements in mechanical properties make this in situ produced composite viable material for applications requiring higher level of reliability.     

沉积时间对ZrTiN涂层微观结构及性能的影响

采用多弧离子镀膜法,在YT15及YG8硬质合金基体上沉积ZrTiN硬质涂层,并分析涂层微观形貌、物相组成、涂层的厚度、硬度及结合力等性能参数随沉积时间的变化。结果表明:沉积时间不会影响ZrTiN涂层的晶面指数,涂层中只有单一面心立方结构相形成,且晶面指数始终为(111);涂层的晶粒尺寸约为10nm且随沉积时间变化不大。沉积时间为120min时,涂层的显微硬度和结合力明显下降。     

Synthesis of boron suboxide (B6O) with alkaline earth metal oxide materials with improved properties

The pure boron suboxide (B₆O) powder admixed with 1.08?vol% of MgO, CaO, and CaCO₃ (corresponding to 1.5, 1.4, and 0.7?wt%, respectively) were synthesized by hot-pressing and the mechanical properties were evaluated. Pure B₆O powders were sintered at a temperature of 1900°C, while the B₆O-alkaline earth metal oxide admixed powders were sintered at a temperature of 1850°C. All sintering was done under an applied load of 80?MPa in an argon environment. The microstructures and phase analyses were studied by scanning electron microscopy and X-ray diffraction techniques, respectively. The addition of the alkaline earth metal oxide additives resulted in the formation of boride and borate phases, depending on the oxide used. The mechanical properties of the hot-pressed materials were characterized based on their density, hardness, and fracture toughness. Between 95 and 98% of the theoretical densities were achieved and the result indicate a good combination of hardness (between 31.6 and 32.1?GPa) and fracture toughness (between 6.1 and 6.8?MPa.m^0.5) in the B₆O-alkaline earth metal oxide materials. The introduction of the additive enhances the fracture toughness of the pure B₆O-materials, while the fracture mode observed in the sintered materials is mainly transgranular.     

板状WC晶粒WC-(Co-Ni)硬质合金的组织和性能

采用真空烧结法制备了板状WC晶粒WC-(Co-Ni)硬质合金,通过XRD、SEM、EDS等手段研究了Ni/(Ni+Co)比对硬质合金组织和性能的影响规律。结果表明:随着Ni/(Ni+Co)比的增大,硬质合金显微组织中板状WC晶粒的比例逐渐减少,硬质相颗粒的尺寸逐渐增大且平均长厚比逐渐减小。当Ni/(Ni+Co)比过大时,硬质合金中硬质相颗粒出现了团聚现象,使其力学性能显著降低。当Ni/(Ni+Co)比为0.3和0.5时,WC-(Co-Ni)硬质合金的综合力学性能较高,这与其硬质相颗粒较细和平均长厚比较大有关。当Ni/(Ni+Co)比为0.5时,WC-(5Co+5Ni)硬质合金具有较优的综合力学性能,其抗弯强度、硬度和断裂韧性分别为2 448 MPa、90.0HRA、21.2 MPa·m~(1/2)。     

Characteristics of dissimilar laser-brazed joints of isotropic graphite to WC–Co alloy

The effect of Ti serving as an activator in a eutectic Ag–Cu alloy filler metal in dissimilar laser-brazed joints of isotropic graphite and a WC–Co alloy on the joint strength and the interface structure of the joint is investigated in this study. To evaluate the joint characteristics, the Ti content in the filler metal was increased from 0 to 2.8 mass%. The laser brazing was carried out by irradiating a laser beam selectively on the WC–Co alloy plate in Ar atmosphere. The threshold content of Ti required to join isotropic graphite to WC–Co alloy was 0.4 mass%. The shear strength at the brazed joint increased rapidly with increasing Ti content up to 1.7 mass%, and a higher Ti content was found to be likely to saturate the shear strength to a constant value of about 14 MPa. The isotropic graphite blocks also fractured at this content. The concentration of Ti observed at the interface between isotropic graphite and the filler metal indicates the formation of an intermetallic layer of TiC.     

Three-point bending fatigue behavior of WC–Co cemented carbides

WC–Co cemented carbides with different WC grain sizes and Co binder contents were sintered and fabricated. The three-point bending specimens with a single edge notch were prepared for tests. In the experiments, the mechanical properties of materials were investigated under static and cyclic loads (20 Hz) in air at room temperature. The fatigue behaviors of the materials under the same applied loading conditions are presented and discussed. Optical microscope and scanning electron microscopy were used to investigate the micro-mechanisms of damage during fatigue, and the results were used to correlate with the mechanical fatigue behavior of WC–Co cemented carbides. Experimental results indicated that the fatigue fracture surfaces exhibited more fracture origins and diversification of crack propagation paths than the static strength fracture surfaces. The fatigue fracture typically originates from inhomogeneities or defects such as micropores or aggregates of WC grains near the notch tip. Moreover, due to the diversity and complexity of the fatigue mechanisms, together with the evolution of the crack tip and the ductile deformation zone, the fatigue properties of WC–Co cemented carbides were largely relevant with the combination of transverse rupture strength and fracture toughness, rather than only one of them. Transverse rupture strength dominated the fatigue behavior of carbides with low Co content, whilst the fatigue behavior of carbides with high Co content was determined by fracture toughness.