Effect of CuCe alloy addition on the microstructure and mechanical performance of brazed diamonds with NiCr alloy

This study was aimed to reduce unwanted thermal damage to diamond grains during brazing. To achieve this goal, the diamonds were brazed using NiCr alloy with the addition of CuCe alloy (i.e. NiCr composite solder). Surface morphology and interfacial microstructure of the brazed samples using NiCr composite solder was characterized. The thermally induced residual stresses of brazed diamond grains were examined by Raman spectroscopy. The static pressure strength and impact toughness of the brazed diamond grains were examined. Constitutional phases of NiCr composite solder after brazing were detected. The results show that the thermal residual stresses values of the brazed diamond grains using NiCr composite solder, in which 2 wt% and 5 wt% CuCe alloy were used respectively, were decreased by 6.4% and 9.7% respectively, meanwhile, the static pressure strength values increased by 4.9% and 13.4% respectively as well as impact toughness values increased by 4.0% and 9.2% respectively, compared with that of the brazed diamond grains using NiCr alloy. Chromium carbides (Cr₃C₂ and Cr₇C₃) were obtained at the bonding interface between diamonds and composite solder. The constitutional phases containing Ce₂Ni₇, which could be beneficial to reduce the thermal damage, were formed in the solder alloy after brazing.     

Measurements of the melting points,liquidus, and solidus of the Mo,Ta, and MoTa binary alloys using a novel high-speed pyrometric technique

The measurement of high phase-transition temperature is equally challenging and useful for developing high-temperature materials for critical applications. Pyrometer is a suitable option to measure the phase-transition temperature above 2300 K. In this work, Mo, Ta, and their isomorphous binary alloys are selected as the target system for high-speed pyrometer temperature measurements in the arc-melting setup. The present phase transitions (melting point, solidus, and liquidus) measurements indicate that the MoTa is an isomorphous system with a narrow freezing range (i.e., an average value < 35 K). The results from this present work would change the existing MoTa phase diagram. The Gibbs energy modeling for liquid (L) and BCC_A2 (α) phases of the MoTa system are then performed, including the latest experimental results from this work. The applications of the current experimental methodology could be extended to measure the high phase-transition temperatures in superalloys with significant commercial values.     

Densified WCu composite fabricated via laser additive manufacturing

Tungsten-Copper (WCu) composites are promising materials for electrical and thermal applications. However, their fabrication remains limited using conventional techniques, such as powder metallurgy, which are not suitable for manufacturing densified complex WCu components. In this work, laser additive manufacturing (LAM) technology was introduced for fabricating W-25 wt%Cu composites. A densified WCu composite, free of apparent cracks and pores, is successfully synthesized using optimum process parameters of laser power 2000 W and laser scan speed 360 mm/min in an Ar atmosphere with less than 10 ppm O. The densification of WCu composites is owing to the sufficient wetting of solid W particles by the molten liquid Cu and the surface smoothing of the W particles. The good wetting of the solid W particles by the liquid Cu is due to the low oxygen content and the combined effect of increased capillary force and reduced friction force. The W grains are rounded, indicating some solution-reprecipitation of sharp particle edges originating from the increase in solubility from curvature effects.     

Experimental investigation on tool wear characteristics of PVD and CVD coatings during face milling of Ti6242S and Ti-555 titanium alloys

Ti6242S and Ti-555, as two typical titanium alloys, are often used to manufacture high-temperature aeroengine parts and landing gear components, respectively. They have different chemical composition and microstructure, which make them have different mechanical properties, and also affect their machinability. In this paper, face milling experiments were carried out to evaluate the wear performance by using CVD-Ti(C, N) + Al₂O₃ + TiN, PVD-(Ti, Al)N + TiN coated and uncoated tools. The results show that Ti-555 has the worse machinability than that of Ti6242S. When milling Ti6242S, all tools suffered adhesive wear and diffusion wear; the wear of Ti(C, N) + Al₂O₃ + TiN coated tool was more serious than that of other tools due to the blunt cutting edge; (Ti, Al)N + TiN coated tool suffered micro chipping and coating peeling with the minimal wear loss. When milling Ti-555, uncoated tool suffered serious chipping, abrasive wear and adhesive wear; Ti(C, N) + Al₂O₃ + TiN coated tool suffered serious chipping and coating peeling with short tool life; (Ti, Al)N + TiN coated tool suffered coating peeling, adhesive wear and diffusion wear. Overall, (Ti, Al)N + TiN coated tools have the longest tool life and are preferred for face milling of Ti6242S and Ti-555 titanium alloys.     

Development of wear resistant hammer heads for coal crushing application through experimental studies and field trials

Coke is one of the most important raw materials for iron making through blast furnace route. Strength and reactivity of coke plays a huge role in efficiency of the counter-current reactor. In order to attain the desired quality of coke, large lumps of coals are crushed in to smaller size fraction in a crushing mill using rotating hammers. However, the heads of these hammers fail prematurely during service that requires their frequent replacements along with interruption in production. This paper presents failure analysis of a coal crushing hammer head with its detailed metallurgical characterization. Analysis revealed that hammer heads fail in abrasive wear mode due to lean chemistry of the alloy used and lower hardness that manifests as higher abrasion rate. 6Cr1Mo steels were supplied in quenched and tempered conditions to a hardness of around 52 HRC. There was presence of only iron chromium carbides that is not sufficient to withstand an application involving extreme wear.The second part of this paper presents detailed chemistry-microstructure-wear resistance correlation for four different alloys; 6Cr1Mo, 20Cr1Mo, complex refractory alloy carbides and tungsten carbide weld deposited hard-facing. Microstructural analysis and phase identification is carried out using scanning electron microscope coupled with electron probe micro-analyzer, x-ray diffraction and Vickers hardness tester. Dry sand abrasion tests were carried out for the existing 6Cr-1 Mo steel and proposed trial specimens. Study shows that the abrasion resistance is strongly a function of type of carbides and their volume fraction. Niobium carbide and tungsten carbides are much more effective in improving the abrasion resistance of alloys compared to primary chromium carbides. Tungsten carbide hard-facing demonstrated the best abrasion resistance. Based on the study, a new design for hammer head with tungsten carbide hard-facing is developed and field trial revealed an improvement in service life greater than three times compared to existing 6Cr1Mo quenched and tempered hammer heads.     

Effect of heat treatments on the Li2O-Al2O3-SiO2-B2O3-BaO glass-ceramic bond and the glass-ceramic bond cBN grinding tools

Influence of heat treatments on the Li₂O-Al₂O₃-SiO₂-B₂O₃-BaO glass-ceramic bond and the glass-ceramic bond cBN grinding tools had been methodically discussed. The results revealed that the different heat treatments mainly varied the content of LiAlSi₂O₆ and LiAlSi₃O₈ in the glass-ceramic bonds which in turn resulted in the variation of bonds' CTE and affected the performance of glass-ceramic bond cBN grinding tools in bending strength. In addition, results of XPS indicated chemical bonds such as NAl, NSi, and NLi bonds were generated at the interface between the bonds and cBN abrasives during the sintering process, which acted as a vital part in improving the holding power for the bonding of glass-ceramic bonds and cBN abrasives. In this study, glass-ceramic bond cBN grinding tools sintered at 860°C for 120 min presented the highest bending strength (89.71 MPa) with the highest potential for high performance grinding tools among all the samples.     

Low temperature molten salt preparation of molybdenum nanoparticles

Pure molybdenum (Mo) nanoparticles (NPs) were synthesized by a novel molten salt technique using Na₂MoO₄ and Al as starting materials and NaCl, KCl and NaF to form a reaction medium. The effects of salt type, reaction temperature and salt to reactant ratio on the synthesis of Mo NPs were investigated in detail. Phase pure Mo NPs with a crystalline size of about 46 nm were synthesized at as low as 650 °C which was much lower than that required by other conventional reduction methods. Among the salts tested, the NaClKClNaF ternary salt showed the best accelerating effect on the low temperature Mo NPs formation, and the optimal weight ratio of salt to reactant was 1.0:1.0. The “dissolution-precipitation mechanism” played a dominant role in the molten salt synthesis of Mo NPs.     

Microstructure and mechanical properties of iron-containing titanium metal-metal composites

Titanium-based metal-metal composites show promises in many applications. In this work, iron-containing titanium-based composites were fabricated by using powder metallurgy method and rotary swaging process. Titanium-based composites with steel addition were sintered at temperature below TiFe eutectic temperature. Increase of the steel addition to 23% resulted in formation of TiFe intermetallic and density decrease in the composites. Further, the as-sintered composites present relatively good workability so hot rotary swaging of the as-sintered composites can be conducted. Steel particles were diffused into the Ti matrix and the composite transformed to a discontinuous β-Ti fiber-reinforced structure. As a result, mechanical properties (in axial direction) of the swaged composites were significantly improved. The tensile strength and elongation reached 1360 MPa and 9.3%, respectively.     

Recovery of rhenium from tungsten‑rhenium wire by alkali fusion in KOH-K2CO3 binary molten salt

Tungsten‑rhenium wire is used in thermocouple and lamp filament manufacturing due to its good thermal sensitivity and high temperature plasticity, and many waste wires are generated in processing and after use. This work focuses on the efficient recovery of high value rhenium from tungsten‑rhenium wire waste with a mass composition represented by W95Re5. The main steps for recovery include alkali fusion, recrystallization, hydrogen reducing and washing. First, WRe wire was decomposed by KOH-K₂CO₃ molten salt to produce potassium perrhenate, where the decomposition ratios of W and Re reached 99.36% and 99.80% using a mass ratio of salt to wire of 3:1, m(KOH) of 80% (m representing the mass fraction of KOH in binary salt), a temperature of 800 °C and a reaction time of 60 min. Then, the decomposed product was leached by water, and from the resulting lixivium high purity KReO₄ crystals were obtained by segregation, which had a perfect rhombic dipyramid morphology and average size of 73.26 μm. Last, the material was reduced to Re powder at 350 °C with a H₂ flow rate of 10 L/min. Re powder, with a purity of higher than 99.5% and fine grain size of 19.37 μm, was obtained after washing with acid and water. This method provided a potential economic process for the recovery of waste WRe wire.     

Hydrogenation and hybridization in hard W-C:H coatings prepared by hybrid PVD-PECVD method with methane and acetylene

The investigations the influence of methane and acetylene additions in the hybrid PVD-PECVD W-C:H coatings driven by DCMS and HiPIMS on plasma polymerization processes, carbon matrix content, its hybridization, hydrogenation and hardness revealed numerous differences related to hydrocarbon gas type and sputtering method. Close correlations between the hydrogen to carbon concentration ratio in the hydrocarbon precursor and contents of carbon phase, its hydrogenation and hybridization were established. The increase of each of these parameters had detrimental effects on hardness due to the consumption of σ bonds for CH at the expense of cross-linking among CC chains. HiPIMS and acetylene were found to provide the highest hardness (~20 GPa) in W-C:H coatings within the hybrid PVD-PECVD regime, most probably due to the best combination of hybridization, hydrogenation and cross-linking of the carbon phase. A qualitative model of growth mechanism of W-C:H coatings during hybrid PVD-PECVD with different hydrocarbon precursors and sputtering methods was proposed.     

Influence of processing parameters on the microstructure and tensile property of 85 W-15Ni produced by laser direct deposition

The plate-like shape 85 W-15Ni parts were produced by laser direct deposition technology with different processing parameters (laser power and scanning speed). The influence of processing parameters and their corresponding laser energy density on the microstructural characterization, phase composition and tensile property of 85 W-15Ni samples was investigated. The results show that the relative density of samples increased with the laser energy density and the densification trend started to slow as the laser energy density reached 380–400 J/mm³, though the highest density value was obtained with laser energy of 425 J/mm³. With the increase of laser energy density, more disorder and fine W dendrites existed at the bonding region between deposition layers and more WW grain boundaries formed at the central region of the layer. The 85 W-15Ni samples produced with different processing parameters consisted of W and γ-Ni phase. To improve the tensile property, it is necessary to increase the laser energy density to obtain denser structure and reduce the residual pores or gaps. However, the excessive laser energy density resulted in the formation of more WW grain boundaries that were detrimental to the tensile property. The best tensile properties were obtained at the laser energy density of 395 J/mm³.     

W-Cu composites reinforced by copper coated graphene prepared using infiltration sintering and spark plasma sintering: A comparative study

In order to solve problems of significant interfacial reactions and agglomeration in graphene reinforced WCu composites, powders of copper coated graphene (Cu@Gr) were pre-mechanically mixed with tungsten and copper powders, and then graphene doped WCu composites were sintered using two different methods, e.g., spark plasma sintering (SPS) and infiltration sintering. Microstructural analysis showed that the doped Cu@Gr powder can effectively inhibit the interfacial reaction between graphene and tungsten, prevent the segregation of graphene, and evenly distribute the copper in the binder phase. When the mixed concentration of Cu@Gr was 0.45 wt%, uniform distributions of W phase and Cu phase were obtained in the composite, and the mechanical properties and conductivity of this composite achieved their best results. When the doping content was further increased to 0.8%, WC phase was found in all alloys, thus resulting in poor mechanical and physical properties. Comparing the microstructures produced using these two methods, the composites prepared using the infiltration sintering method showed network distribution of copper phase and segregation of copper, whereas the composites prepared using the SPS method showed network skeleton phase of tungsten. Although the SPS process was performed in a much shorter time, the mechanical properties of the composites sintered using the SPS process did not show much differences with those sintered using the infiltration sintering method.     

A new route for preparing Mo-10wt.%Cu composite compacts

In this work, high pure Mo-10 wt% Cu composite powders with a ultrafine particle size are prepared by a two-step reduction process (first carbothermal reduction and then hydrogen reduction). Ammonium heptamolybdate and copper nitrate trihydrate powders are used as molybdenum and copper source, respectively. The mixtures of raw materials are calcined at 400 °C firstly to form the composite oxides which are then reacted with insufficient carbon black at 1050 °C for 2 h. The as-prepared powders are further reduced by hydrogen at 750 °C for 2 h to obtain the ultrafine Mo-10 wt% Cu composite powders. The experimental results show that the residual carbon of the Mo-10 wt% Cu powders can be decreased to 0.015 wt%, and the composite powders have an average particle size of 200 nm. The sintering behavior of ultrafine MoCu powders and the properties (vickers hardness and thermal conductivity) of samples after sintering are investigated. High sintering temperature is beneficial to increase the density of the compact. At 1200 °C, the relative density of the MoCu compacts is 98.8%. The vickers hardness and thermal conductivity of the Mo-10 wt% Cu composites sintered at 1200 °C for 3 h are 233 HV and 130 (W/m·k), respectively.     

Manufacture of a new kind diamond grinding wheel with Al-based bonding agent

A new kind diamond grinding wheel with Al-based bonding agent was prepared in this paper. The influence of sintering temperature to the relative density (R.D.), hardness and service life of diamond grinding wheels with AlSnTi, AlSnTiNiCo, AlSnTiNi and AlSnNiCo bonding agent was studied. The microstructure of different bonding agent sintered at different temperature was observed. The service life of the Al-based grinding wheels was compared with Cu-based or resin-based ones. The results showed that the AlSnTiNiCo is the best composition system in this research. The best sintering temperature is 300 °C. The sample has a high relative density after sintered at 300 °C. The retention of Al-based bonding agent to diamond grit is strong. The service life of this Al-based diamond grinding wheel is about three times as long as that of resin-bonded grinding wheel.     

Effect of Mo content on microstructure and Wear resistance of Mo-Fe-B claddings

Mo-Fe-B alloy powder blocks were designed and prepared to produce claddings with Mo₂FeB₂ phase on Q235 steel using an argon arc welding process. The effects of Mo content on microstructure, phase chemistry, microhardness and wear resistance of claddings were systematically studied. In addition, the formation conditions of hard phase were theoretically analyzed by thermodynamic calculations. The results showed that the claddings consisted of Mo₂FeB₂, Cr₂B₃, MoB and FeCr. Additionally, calculated results revealed that the Mo₂FeB₂ can stably exist in the high- temperature liquid phase and has good thermodynamic stability. Moreover, the maximum microhardness value of claddings was about 1052 HV_0.5.The claddings with 30% and 25% Mo contents had better wear resistance.     

Nitridation of Ti6Al4V alloy under ultrasonic impact treatment in liquid nitrogen

X-ray photoelectron spectroscopy and scanning electron microscopy with energy dispersive X-ray microanalysis were employed to study the effect of mechanical treatment on the surface chemical state, composition and morphology of commercial Ti6Al4V alloy. It has been demonstrated that ultrasonic impact treatment of Ti6Al4V in a liquid nitrogen (LN2) environment results in substantial grain refinement and formation of nitrides and oxynitrides of all the alloy components. The mechanochemical synthesis of the nitrides and oxynitrides from LN2 was found to efficiently occur not only within the impacted area but outside it as well. The layer with a high content of incorporated nitrogen and average atomic ratio N/Ti = 0.94 was estimated to be at least ～2 μm thick. As a result of the combined effect of cryogenic deformation, nanoscale grain refinement and nitriding, a 3-fold increase in the microhardness of Ti6Al4V alloy is obtained. The impact treatment in LN2 was observed to be accompanied by material transfer from the hardened steel pin to the alloy followed by incorporation of the pin components into Ti6Al4V and formation of the FeN, FeC and TiC bonds.     

The annealing and re-oxidation of oxidized CuNi alloys

A study has been made of the effects of an intermediate, isothermal annealing treatment in argon on the oxidation kinetics in dry oxygen of Cu10%Ni and Cu24%Ni at 800°C and Cu80%Ni at 1000°C using a semi-automatic microbalance. Changes in scale morphology and composition have been investigated using various techniques.Oxidation of Cu10%Ni and Cu24%Ni produces external scales consisting of a thick, inner Cu₂O layer and a thin, outer CuO layer, together with nickel-rich internal oxide in the adjacent alloy. Oxidation of Cu80%Ni yields an external scale consisting of a thick, inner NiO layer and a thin, outer CuO layer, together with a few nickel-rich internal oxide particles in the adjacent alloy. During annealing, the outer CuO layers on the three alloys dissociate to give Cu₂O and oxygen. With Cu80%Ni only, further dissociation to give copper metal takes place after long annealing times. In all cases, the annealing treatment leads to a reduction in the cation vacancy gradient across the scale and a reduction in the total vacancy level in the scale due to the reduction in oxygen activity at the oxide-gas interface.In Cu10%Ni and Cu24%Ni, internal oxide formation during annealing stimulates dissociation of the Cu₂O scale near the oxide-alloy interface to give copper metal and oxygen. Similarly, internal oxide formation stimulates dissociation of the NiO layer on Cu-80%Ni, although to a lesser extent than for the copper-rich alloys.During re-oxidation, the kinetics are partly determined by the extent of scale thinning during the prior annealing treatment, giving more rapid weight gains than expected from those recorded during the initial oxidation period. Nevertheless, particularly for Cu-80%Ni, where the scale thinning is relatively small, the reduction in the cation vacancy gradient across the scale and the reduction in the total vacancy level in the scale do result in a reduced oxidation rate compared with the rate recorded during the initial oxidation period.     

Study on the formation of core–rim structure in Ti(CN)-based cermets

Ti(CN)-based cermets were synthesized from Ti(CN)WCMo₂CTaCNiCo composite powders by vacuum-low pressure sintering. The phase evolution and the formation of core–rim structure in Ti(CN)-based cermets were systemically investigated during difference reaction stages at 950–1450 °C. The results show that the secondary carbides such as Mo₂C and TaC are begun to dissolve at 950 °C, finished at 1150 °C, and the solution temperature of WC phase is range from 1150 to 1300 °C, which are result in increase of the cermets lattice constant. At the same time, the inner rim is also formed, and Ti(CN)-based cermets are composed of (Ti, W, Mo, Ta)(CN) and Ni/Co solid solution phase. While at 1350 °C, it was found that the outer rim began to precipitate from the liquid phase with the metal binder. With increase of sintering temperature, mechanical properties of cermets improved obviously were related intimately to the increase of outer rim thickness.     

Effect of boron on the thermodynamic stability of amorphous polymer-derived Si(B)CN ceramics

The reason for the higher thermal persistence of amorphous polymer-derived SiBCN ceramics (T ～ 1700–2000 °C) compared to SiCN ones (T ～ 1500 °C) has been a matter of debate for more than a decade. Despite recent experimental results which indicate a major kinetic effect of boron on the thermal persistence of the ceramics, no experimental investigation of the thermodynamic stability of the materials has been reported. In this work, we present measured energetics of a series of the amorphous ceramics with various boron contents (0–8.3 at.%) using high-temperature oxidative drop-solution calorimetry. Through measurement of the drop-solution enthalpies in molten sodium molybdate at 811 °C, the formation enthalpies of the amorphous ceramics from crystalline components (SiC, BN, Si₃N₄, C) at 25 °C were obtained and found to be between ?1.4 and ?26.6 kJ g-atom^?1. The determined enthalpy data plus the estimated positive entropy of formation values point to the thermodynamic stability of the amorphous ceramics relative to the crystalline phases, but such stabilization diminishes with increasing boron content. In contrast, the higher boron content increases the temperature of Si₃N₄ crystallization despite less favorable energetics for the amorphous phase, implying more favorable energetics for crystallization. Thus the so-called “stability” of SiBCN ceramics in terms of persistence against Si₃N₄ crystallization appears to be controlled by kinetics rather than by thermodynamic stability.