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.     

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.     

Preparation of Mo nanoparticles through hydrogen reduction of commercial MoO2 with the assistance of molten salt

In the present work, the hydrogen reduction of commercial MoO₂ with the assistance of molten salt has been investigated at 720–850 °C. In the case of no salt addition, the prepared Mo particles maintained the big platelet shape of raw MoO₂ powders. However, after adding a small amount of salt (0.1% NaClKCl binary salt, MgCl₂KCl binary salt, or LiCl mono salt), Mo nanoparticles were prepared successfully. It was found that a small amount of molten salt can help Mo to nucleate dispersedly and its growth via chemical vapor transport (CVT) mechanism can be controlled by reducing temperature. At 750 °C, when the NaClKCl binary salt was used, the obtained Mo nanoparticles had both good dispersion and small average size of about 70 nm. Kinetics analyses indicated that the rate controlling step for the hydrogen reduction of MoO₂ was the interface chemical reaction, and after adding salt, the reaction rate constant decreased dramatically, relative to that without salt. The extracted activation energies for pure MoO₂, MoO₂ with 0.1% NaClKCl binary salt and MoO₂ with 0.1% LiCl mono salt were 76.13 kJ/mol, 117.45 kJ/mol and 97.23 kJ/mol, respectively.     

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.     

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.     

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.     

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.     

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.     

Interfacial characteristics of titanium coated micro-powder diamond abrasive tools fabricated by electroforming-brazing composite process

In this study, abrasive tools were fabricated by an electroforming-brazing composite process. The abrasive tools were prepared using micro-powder diamond (MPD) grits with and without titanium (Ti) coatings. The interface characteristics of the MPD diamond grits and filler alloy were investigated. The results show that the diamond grits are uniformly dispersed on the surface of the steel substrate without visible agglomeration, and the NiCr filler alloy has good wetting toward diamond grits. Compared to the uncoated diamond grits, the abrasive grains with a Ti coating have an ideal grain distribution and brazed joints, and no aggregation of Si element observed at its edge. There is a more stable carbide TiC that formed on its surface in addition to the chromium carbide. Furthermore, the coated diamond grits are not graphitized and show a better residual stress state. In the abrasive tools with Ti-coated diamond, numerous intermetallic compounds (IMCs) are formed in the brazing layer, but the formation of theγ-Ni-based solid solution is relatively slight. The IMCs increase the microhardness of the filler alloy and help improve the wear resistance of the solder layer.     

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.     

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.     

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.     

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.     

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.     

Perspectives of metal-diamond composites additive manufacturing using SLM-SPS and other techniques for increased wear-impact resistance

In this paper, a new route is introduced to fabricate parts with increased wear and impact resistance for use in tunneling and mining applications. A combination of selective laser melting (SLM) and spark plasma sintering (SPS) to 3D print functionally graded lattices (FGL) that are later filled with metal-diamond composites was used. It was demonstrated that a cellular lattice plays an important role in the consolidation of diamond particles. Impact-abrasive laboratory experiments with tribological device, developed in-house, were carried out to characterize the fabricated samples. The results show that the balance of nickel, molybdenum, and chromium significantly affects the performance of the fabricated specimens. The addition of a higher content of MoCr, Ni and coated diamond particles guarantees higher impact-abrasive resistance of the composite. Our experimental results show that the FGL structure allows a more accurate distribution of diamond particles (variable metal/refractory material content) across the structure and our finite element simulations showed increased ductility and impact absorption ability due to a more uniform distribution of stresses throughout the volume.     

Influence of alloy composition and thermal history on carbide precipitation in γ-based TiAl alloys

Carbide precipitation in TiAl alloys with different alloying element additions and thermal history was investigated by transmission electron microscopy and high energy X-ray diffraction. The results reveal that Nb addition in TiAl alloys does not increase the carbon solubility in the γ matrix significantly. With increasing carbon concentration in the alloys, a splitting of the P-type carbides takes place earlier in the course of ageing at 800 °C and the formation of H-type carbides is promoted. For a nearly single γ phase alloy Ti51Al5Nb0.5C, H-type carbides are the thermodynamically stable phase. However, with decreasing Al concentration, P-type carbides become thermodynamically stable at low carbon concentration (below 1%) in Ti45Al and Ti45Al5Nb alloys. It is feasible to achieve high thermodynamical stability of the P-type carbides in TiAl alloys through controlling alloying elements.     

High strength Mo/Ti6Al4V diffusion bonding joints: Interfacial microstructure and mechanical properties

Joining Mo to Ti6Al4V is of great interest for applications in the weaponry and aerospace fields. In this work, a diffusion bonding technique is developed to bond a MoZr alloy and Ti6Al4V using a Cr-Ni-Ti-Si composite interlayer. The formation of the interlayer ensured that crack-free bonds were obtained. The mechanical properties and microstructure of the interface were examined. The results show that as the temperature was raised, the thickness of the diffusion layer increased and Kirkendall voids formed on the Mo alloy side. At the Mo/Cr-Ni-Ti-Si interface, a diffusion layer formed as Cr, Ni, Si and Mo dissolved into Ti to form a substitutional solid solution. As a result, the lattice constant of Ti reduced. For diffusion bonding between Mo and Ti6Al4V, a maximum tensile strength of 369 MPa was attained while a processing temperature of 950 °C was used. Ductile fractures were observed at the Mo/Ti6Al4V joints, primarily in Mo adjacent to the Mo/Cr-Ni-Ti-Si interface. The findings in this study open up new possibilities for the use of Cr-Ni-Ti-Si composite interlayer to bind Mo alloys to Titanium alloys.