Reaction layer formation at the graphite/copper-chromium alloy interface

Sessile drop tests were used to obtain information about copper-chromium alloys that suitably wet graphite. Characterization of graphite/copper-chromium alloy interfaces subjected to elevated temperatures were conducted using scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDS), Auger electron spectroscopy (AES), and X-ray diffraction analyses. These analyses indicate that during sessile drop tests conducted at 1130 °C for 1 hour, copper alloys containing greater than 0.98 at. pct chromium form continuous reaction layers of approximately 10-μm thickness. The reaction layers adhere to the graphite surface. The copper wets the reaction layer to form a contact angle of 60 deg or less. X-ray diffraction results indicate that the reaction layer is chromium carbide. The kinetics of reaction layer formation were modeled in terms of bulk diffusion mechanisms. Reaction layer thickness is controlled initially by the diffusion of Cr out of the Cu alloy and later by the diffusion of C through chromium carbide. This article is based on a presentation made in the symposium “High Performance Copper-Base Materials” as part of the 1991 TMS Annual Meeting, February 17–21, 1991, New Orleans, LA, under the auspices of the TMS Structural Materials Committee.     

Carbon fibers coated with chromium carbide using the liquid metal transfer agent technique

Protective coatings of chromium carbide were applied to PAN type carbon fibers by a liquid metal transfer agent (LMTA) technique using copper and tin as transfer agents, and the effect of the coating on the strength of the fibers was evaluated by performing single fiber tensile tests. The coatings were examined metallographically, by X-ray diffractometry, and by scanning electron microscopy. It was found that a tin transfer medium produced a smoother and more uniform coating than a copper transfer medium. The smallest carbide coating obtained using a copper transfer medium was approximately 1 μm, while coatings as thin as approximately 0.14 μm were obtained using a tin transfer medium. It was found that wetting of the fibers by the tin and copper alloys is associated with the spontaneous formation of a carbide layer of significant thickness, after which the carbide layer was found to grow parabolically with time and with an apparent activation energy of 139 kJ/mol. The strength of the carbon fibers decreased with increasing coating thickness.     

含板状晶WC双模组织结构硬质合金的研究

采用超细WC原料，在WC-20Co合金中加入纳米Y2O3，在1440℃的液相烧结温度下获得了含板状晶WC双模组织结构的合金。通过对比低于共晶温度下热压固结工艺与传统液相烧结工艺制备的WC-20Co=1Y2O3硬质合金的组织结构发现，液相烧结合金中WC板状晶是在液相烧结阶段通过液相重结晶形成的，均匀分散在合金中的Y2O3具有促进WC从粘结相中均匀析出长大、抑制WC沿C轴方向生长的作用，因而使合金中粗大的WC呈现板状晶的形貌。研究结果表明，板状晶强化的双模结构WC-20Co—1Y2O3合金具有较好的综合性能。     

Variation in the structure and properties of sintered alloys under the effect of nanodimensional carbon additives

The influence of nanodimensional carbon additives on the structural-phase state and properties of sintered alloys on copper, iron, and tungsten–cobalt bases is considered. It is shown that the undissolved part of the nanodimensional additive can serve as additional nucleation centers of crystals during liquid-phase sintering and crystallization of powder compositions, which promotes structure refining. It is established that the nanodimensional diamond–graphite additive in alloys able to form solid solutions with carbon or strengthening carbide phases leads to an increase in hardness of sintered composites and an improvement in their tribotechnical properties.     

On the kinetics of carbide precipitation during reaction between graphite and Fe-Ti liquids under microgravity

Experiments on the reaction between graphite and liquid Fe-Ti alloys were performed with a mirror furnace on board an airplane during parabolic flights. Small Fe-Ti alloy samples were melted in contact with graphite and held for some seconds at a temperature of 1550 °C. The samples were melted and solidified during a microgravity period. Carbon and titanium atoms reacted in the melt and titanium carbides were formed. In the experiments, a precipitation zone with faceted titanium carbide crystals dispersed in high carbon Fe-C-Ti alloy matrix was obtained near the graphite/alloy interface. The thicknesses of the carbide precipitation zones were measured and effects of alloy composition on the growth rates of the carbide zones were revealed by experiments and calculations. It was shown that the process was controlled by the diffusion of titanium in the liquid at low titanium concentrations and by diffusion of carbon through the precipitation layer at high titanium concentrations in the melt. Supersaturation of the carbide in front of the reaction interface was predicted from the calculations. The analysis showed that homogeneous nucleation of titanium carbide can readily occur in the alloys. Carbide morphologies were analyzed, and the mechanisms which lead to their formation are discussed.     

LIQUID-FLOW DENSIFICATION IN THE TUNGSTEN CARBIDE–COPPER SYSTEM

Abstract

The fluid-flow stage of densification in two-phase sintering, with minimum contribution from intersolubility effects and change of particle shape, has been studied by selecting the insoluble tungsten carbide-copper system. Density determinations, photography of specimen shrinkage, and microscopic examination were carried out over a range of copper contents, with two carbide particle sizes, for sintering in a hydrogen atmosphere.

When the copper melts it flows into regions of high carbide density to form carbide/copper colonies. If these occupy a minor proportion of the compact, densification is limited and determined by the larger, “rigid” carbide part of the compact, but if the colonies predominate there is massive shrinkage on fluid flow. Overall densification subsequent to fluid flow is unaffected by the presence of the copper. The copper may, however, be redistributed within the compact as hydrogen in pores near the surface diffuses out and the pores shrink, drawing copper from central regions to form a dense skin.

As densification proceeds the carbide particles form a rigid inter-connected framework. On cooling, the copper contracts more than the solid framework so that, even if at high temperatures the compact is fully dense, shrinkage porosity results on solidification.

The structure after the fluid-flow stage is highly dependent on the initial processing. Mixing produces agglomerates of copper that, on melting, flow into the surrounding carbide matrix leaving behind large voids. Ball-milling, in contrast, yields a more uniform green structure and hence a more uniform compact after flow of the copper.     

Structure of the surface layer of a hard-metal cold-upsetting tool

Conclusions In the structure of the WC and Co phases making up VK20K hard metal the presence was discovered, in the as-sintered condition, of dislocations and twins, which may be attributed to the generation, during the postsintering cooling of the alloy, of large thermal stresses as a result of the difference in the coefficients of thermal expansion of its tungsten carbide grains and metallic cobalt interlayers. Operation increased the number of dislocations and twins and brought about fragmentation of twins in the Co and WC phases, which is evidence that their wear was preceded by appreciable plastic deformation. In the structure of VK20K alloy in the as-sintered condition and in its surface layer after wear the carbide W₂C was found on the surface of the carbide WC between the WC grains and Co-phase interlayers, and it is therefore reasonable to assume the existence under such conditions of an extremely thin transition layer of virtually pure tungsten.Translated from Poroshkovaya Metallurgiya, No. 12(216), pp. 83–88, December, 1980.     

Toward prediction of microstructural evolution during laser surface alloying

The stability of tungsten carbide particles in iron-rich and nickel-rich liquid during the laser surface alloying (LSA) process was investigated. Kinetic calculations indicate a rapid dissolution of tungsten carbide particles in iron-rich liquid, as compared with the dissolution rate in nickel-rich liquid. Optical microscopy indicated a heterogeneous microstructure around the tungsten particles that is in agreement with concentration gradients predicted by kinetic calculations. The work demonstrates the applicability of computational thermodynamics and kinetic models for the LSA process.     

稀土-Cr-V联合掺杂对WC-Co合金中液相出现温度的影响

以采用相同的WC、Co原料与相同的湿磨工艺制备的WC-11Co混合料为参比对象,研究了RE(混合稀土)-Cr-V联合掺杂与La2O3-Cr-V联合掺杂对WC-11Co合金中液相出现温度的影响。合金中液相出现温度采用差示扫描量热法(DSC)与热重法(TG)确定。结果表明,无论以RE-Co预合金形式,还是以稀土氧化物形式加入稀土,稀土-Cr-V联合掺杂的WC-Co合金中液相集中出现温度(共晶温度)仅受Cr-V的影响。Cr-V联合掺杂可使WC-Co合金中液相集中出现温度由1 374.8℃降低至1 328.1～1 328.9℃。尽管RE-Co合金体系在较低的温度下就可出现液相,由于RE与Co之间只形成金属间化合物,不形成固溶体,RE-Co预合金掺杂不会降低合金中液相集中出现温度。     

Reactions during infiltration of

The infiltration sequence of graphite fibers with liquid aluminum alloyed with titanium was studied. The titanium concentration was chosen such that a severe reaction occurred between the fibers and the melt. Aluminum carbide and titanium carbide, as well as an aluminide phase were formed. The phenomenon occurring during the infiltration sequence was explained with the aid of the ternary-phase diagram Al-Ti-C. The effect of the reaction on the infiltration height is discussed.     

盐酸法处理硬质合金粉双回收Co和WC新工艺研究

设计了用盐酸法处理废旧硬质合金粉,双回收高纯度金属钴和碳化钨的工艺,并采用液相还原法一步制备金属钴,是一种工艺简单、能耗少、成本低、污染小的新工艺。研究考察了盐酸浓度、反应时间、温度对钴浸出量的影响,并对回收的金属钴和碳化钨进行了XRD、SEM等表征,结果表明得到的金属钴为粒径0．2～0．55μm的球形微粒,碳化钨为质地疏松易于球磨的颗粒,且两种回收物的纯度较高。     

不同电极材料对电火花成型加工特性的实验研究

电火花成型加工过程中,不同的电极材料对成型工件的表面质量及加工时间均有影响,本文在相同的电参数与非电参数下分别以石墨、钨铜、紫铜3种工具电极材料来加工不锈钢工件并对实验的结果进行分析.实验结果发现:钨铜电极成型的工件表面质量最好,可以达到0.841um的高精度水平,满足航空航天精密零件的加工要求,同时加工时间为12.6...     

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Abstract

Although hardmetal has been manufactured, by the powder metallurgy route for well over 55 years, the characterization of the starting powders, basically tungsten carbide and cobalt, still commands the attention of powder metallurgists. Three fine cobalt powders and two 1·2 μm (FSSS) tungsten carbide powders, freely available, have been both chemically and physically characterized. Various techniques of measuring physical characteristics, such as FSSS, BET gas adsorption, pore volume and area, sedimentation, SEM powder, and SEM polished and etched sections of copper impregnated powders, have been studied. The techniques have been discussed in relation to their advantages and disadvantages. The measurement by FSSS and BET gas adsorption give fairly reproducible results, as does the sedimentation technique. However, by reference to the two SEM techniques it has been shown that the very important characteristic of particle shape and, more importantly for tungsten carbide powder, particle agglomeration (due to the manufacturing route) is not fully revealed. The two SEM techniques are indispensable for defining shape and agglomeration characteristics. It is suggested that by using the SEM copper impregnated powder method and a semi-automatic image analysing system the true crystal (grain) size distribution of tungsten carbide powders can be evaluated. The chemical purity of cobalt and tungsten carbide powders has significantly increased in recent years. Some preliminary results from milled tungsten carbide powders are discussed. PM/0393     

WC粉末粒度与形貌对硬质合金中WC晶粒度、晶粒形貌与合金性能的影响

探讨了粗颗粒与特粗颗粒两种粒度级别以及平面化表面与球化表面两种形貌特征的WC原料对WC-Co硬质合金中WC晶粒度、晶粒形貌以及合金性能的影响。结果表明,分别采用费氏粒度为11.4￣13.4μm,与22.0￣28.3μm两种粒度级别的WC粉末为原料制备合金,尽管两种合金硬度之间存在明显差别,但是两种合金的晶粒度相差很小,在4.0￣4.3μm之间,同属一种粒度级别。WC原料的原始形貌对合金中WC晶粒形貌与合金性能影响很小,碱金属掺杂原料制备的合金中WC晶粒结晶完整性相对较差。因此,高纯原料是制备高性能硬质合金的基础。     

A Feasibility Study on Manufacturing of Functionally Graded Structures Using SiC Ceramic Powder in a Steel‐Based Matrix by Gas Tungsten Arc Welding (GTAW)

The paper presents a feasibility study on application of the gas tungsten arc welding (GTAW) process to obtain metal composite functional coating for advanced tribological application. Silicon carbide (SiC) particles in the form of powder was added to a weld pool in autogenous mode as well as with an additional filler wire. Powder feeding was carried out at different angles and with varying separation distance from the welding torch. The metallurgical characterization of the cladded structure was carried out using optical as well as scanning electron microscopy (SEM). Energy dispersive X‐ray spectroscopy (EDS) was performed to analyze the composition of the deposited weld metal. It has been observed that due to low SiC density it was difficult for particles to penetrate the weld pool. Also the added SiC was found to be dissociated into Si and carbon (C) and the large amount of dissolved C in the weld pool resulted in formation of graphite phases.     

熔融制样-X射线荧光光谱法测定钨铁合金中硅锰磷铜钨

钨铁合金是冶炼高速钢等钨合金钢的加入剂,目前采用化学湿法分析钨、硅、锰、磷和铜的含量,实验流程很长,操作相对繁琐。实验采用碳酸锂和过氧化钠作氧化剂预氧化样品、熔融方法制样,建立了X射线荧光光谱法(XRF)测定钨铁合金中硅、锰、磷、铜、钨5 种元素含量的检测方法。绘制校准曲线时,采用钨铁标准样品中加入三氧化钨纯物质及硅、锰、磷、铜标准溶液的方法扩展了校准曲线线性范围。实验表明,以四硼酸锂为熔剂,溴化铵为脱模剂,试样与熔剂比例为1∶30,在1150℃熔融炉中熔融10min,制得表面光滑、无气孔、无结晶的均匀玻璃片。实验方法用于测定钨铁合金样品中硅、锰、磷、铜、钨,测定结果的相对标准偏差(RSD,n=8)为0.20%~5.2%;按照实验方法测定3个钨铁合金样品中硅、锰、磷、铜、钨,并与相应的国家标准方法进行比对,结果相一致。     

AN ELECTRON-MICROSCOPE AND X-RAY INVESTIGATION OF THE MILLING OF TUNGSTEN CARBIDE/COBALT MIXTURES

Abstract

It is well known that some interaction takes place between the two components when mixtures of cobalt and tungsten carbide are milled. To gain a further insight into this phenomenon, the milling process has been studied by means of the electron microscope and by the BET and X-ray methods.

During the milling of cobalt powder the number of stacking faults and the amount of the hexagonal phase both increase. On milling 80:20 tungsten carbide/cobalt mixtures increasing agglomeration of cobalt and fine tungsten carbide particles was found with rise in milling intensity. The larger tungsten carbide particles appeared to have a smooth surface. However, if the cobalt was dissolved in hydrochloric acid, the true surface of the tungsten carbide particles was revealed. This became rougher with increasing milling intensity. From this it can be concluded that, during milling, cobalt settles between the surface irregularities.

The variation in distribution of the cobalt that results from different milling conditions leads to a difference in sintering behaviour. A dilatometric study has been made of this aspect.

The original particle size of the cobalt used for hard-metal mixtures does not affect the properties of the sintered product, if the mixtures are milled very intensively.     

碳纤维表面沉积碳化钨膜研究

通过W(CO)6化学气相沉积(MOCVD)工艺,在碳纤维表面沉积得到了碳化钨膜材料,研究了碳纤维表面沉积碳化钨膜的工艺条件.结果表明:在400℃沉积温度下沉积时间15min,一定浓度的W(CO)6可解离沉积在碳纤维表面形成致密碳化钨膜;膜层厚度0.4μm,物相为WC1-x相,与碳纤维基体结合强度高.DTA分析表明,复合...     

Interface characterization of ceramic fiber-reinforced ti alloy composites manufactured by infrared processing

Interfacial reactions between several ceramic fibers (SCS-0, SCS-6, and carbon fibers) and a liquid titanium-nickel-copper alloy were investigated using electron microscopic analysis. Composite spec-imens were produced using a rapid infrared manufacturing (RIM) process. In SCS-O/Ti alloy com-posites, SiC dissolved in the alloy. The main reaction product was discontinuous agglomerates of titanium carbide which formed from the reaction between dissolved carbon and titanium. Polygonal precipitates of Ti₅Si₃, which are believed to have formed during cooling, were also noticed. Two distinct interface morphologies were observed in these composites: uniform fronts caused by iso-thermal dissolution and scalloped fronts formed as a result of an accelerated dissolution mechanism caused by localized heating. The presence of the accelerated dissolution mechanism suggests that SiC fibers cannot be infiltrated with liquid titanium alloys without applying a coating. In the C/Ti system, carbon fibers reacted with the liquid alloy to form a continuous layer of Ti_xC_1-x. Further growth of this layer occurred by the diffusion of carbon atoms across the reaction product. In SCS-6/Ti alloy composites, free carbon present in the coating formed a discontinuous layer of Ti^C,^, whereas SiC particles dissolved in the alloy. Due to channeled dissolution in the coating, the accel-erated dissolution mechanism was not observed in these composites. As a result, the presence of the carbon-rich coating prevented degradation of the fibers. Although the coating present on SCS-6 fibers moderately retarded reactions in the SiC/Ti alloy composite system during infrared liquid infiltration, it is recommended that the fibers be coated with pure carbon to effectively limit the attack of the fiber by molten titanium. Formaly Postdoctoral Fellow, Department of Materials Science and Engineering, University of Cincinnati     

DENSIFICATION PROCESSES IN THE TUNGSTEN CARBIDE-COBALT SYSTEM

Abstract

Densification of the tungsten carbide-cobalt system has been investigated by determining the effect of the principal sintering variables-composition, temperature and time of sintering, particle size, ball-milling-and by studying the processes that occur. Considerable shrinkage takes place during heating, before the eutectic temperature is attained. A 9% cobalt alloy sintered entirely in the solid state to give comparable density and mechanical properties to those attained by liquid-phase sintering, but the sintering time was increased by a factor of 10. Densification proceeds from nuclei created by ball-milling, which packs the porous cobalt agglomerates with tungsten-carbide particles; if the cobalt particles are only mixed with the tungsten carbide, then on sintering they flow out into the matrix leaving behind voids that do not fill. Densification is characterizedby two features: first, tungsten-carbide particles cement together with cobalt between grains to form clusters and filaments; secondly, the clusters and filaments contract. The solubility of the tungsten carbide in the cobalt is important, since densification occurs far less rapidly when copper is used as the binder phase. Shrinkage can virtually cease before the compact is fully dense, either because voids form as a result of unsatisfactory mixing, or, with a small amount of cobalt, because the periphery of the compact sinters to full density before the interior, preventing further overall densification.