Cobalt Base Research Articles

Subsurface changes of a MoM hip implant below different contact zones

Metal-on-metal hip arthroplasties undergo distinct release of toxic metal particles and ions. Thus, it is necessary to minimize this. In order to evaluate the wear behaviour of metal-on-metal hip replacements it is essential to understand the micro-structural changes in the sub-surface region. Previous studies revealed that cobalt chromium metal-on-metal implants are able to alter their mechanical behaviour by adjusting the microstructure to load. The reason for this is the so-called mechanical mixing. This means that a nano-crystal layer is formed by rotating clusters of atoms that incorporate denatured proteins from the interfacial medium. This is followed by a layer of rhombic shaped nano-crystals in between sheared ε -martensite lathes, twins, and stacking faults. Although the primary wear zone has been well characterized, the sub-surface structure of the stripe wear and the non-contact zone of the hip ball have yet to be analysed. For this study a 28-mm cobalt base alloy femoral head and acetabular cup were analysed. The implant was simulator tested for 5 million cycles with the application of micro-separation resulting in a clearly visible stripe wear appearance. The TEM micrograph of the primary wear zone of the ball confirmed the presence of a sub-surface layer of nano-crystals. The thickness of this layer was approximately 200 nm and the average grain diameter ranged from 35 to 40 nm. Within the stripe wear zone the micrographs also revealed a nano-crystal layer but with a thickness of only 50 nm and an average grain diameter from 15 to 20 nm. The carbon and oxygen content was highest closest to the surface which proves the occurrence of mechanical mixing. The non-contact zone of the ball was analysed as well. When compared to the primary wear zone a nano-crystal layer with similar thickness but with an average grain diameter smaller than 15 nm was observed.

Nickel base superalloy GH4049 prepared by powder metallurgy

The process of preparing nickel base superalloy GH4049 from atomized prealloyed powder is studied. The results show that the compacts with relative density over 99% can be made by sintering at 1350 °C for 2 h in vacuum. Sintering in vacuum is better than that in argon atmosphere. The sintering way is supersolidus liquid phase sintering. The tensile strength and yield strength and ductility are about 1113 MPa and 760 MPa and 13% respectively. They are near the wrought superalloy GH4049. There are two kinds of γ′ phase precipitated in the grains. The rectangular γ′ phase is about 200 nm and the global γ′ phase is about 40 nm. The average grain size is below 80 μm. Its property of the compression stress relaxation at 850 °C is better than wrought cobalt base superalloy GH605 (L-605). The results lay the foundation of near-net-shape forming for the powder nickel superalloy GH4049.

Thermodynamic analysis of gaseous phase equilibrium compositions in producing iron-and cobalt-based alloys by diffusion saturation from point sources. II. Calculation of the thermodynamic characteristics of chemical processes in cobalt-based mixtures with different content of chromium

The paper presents a thermodynamic analysis of chemical equilibrium in Co-Cr-Ni-Mo alloys with different chromium content (10, 20, and 30%) produced by point-source diffusion saturation under 0.1 MPa at 500–1500 K. The elements of the alloys are most likely transported by lower chloride molecules participating in cyclic reactions. The limiting concentration of chromium above which no changes occur in either gaseous or condensed phase is 20%. It is experimentally established that chromium rapidly diffuses into the cobalt base, accompanied by the formation of homogeneous alloy powders, under the actual nonequilibrium conditions for the production of cobalt alloy powders containing more than 20% of chromium.

High‐frequency Impedance and Sensitivity of Micro‐fluxgate Sensors Fabricated with Cobalt Base Amorphous Films

AbstractMicro‐fluxgate sensors 2 mm long, 1.5 mm wide were fabricated with CoZrNb amorphous films. Their high‐frequency input/output impedance was measured and evaluated to investigate whether the sensor output and/or sensitivity could be estimated by the complex impedance, especially the reactance. The output reactance changed from 11.1 to 6.1 ohm at 8 MHz by applying the external magnetic field of 10.5 Oe, whereas the input impedance changed from 12.3 to 10.1 ohm. The parasitic capacitance was driven from the measured reactance and resonance frequency. The inductance and inductive reactance could be evaluated with the parasitic capacitance and measured reactance. The tendency of output voltage dependent on frequency is similar to that of inductive reactance. The sensitivity of the fabricated sensor was 17.6 mV/VOe at 8 MHz. Copyright © 2008 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc.

Novel cobalt base superalloy and its high-temperature flow behavior

A novel cobalt base superalloy containing titanium and aluminum was investigated through metallography, tensile test, and high-temperature isothermal compression deformation. The results show that proper content of titanium and aluminum with can improve the strength and ductility of the cobalt base superalloy. The Co 3(Ti, Al) compound with L12 structure precipitates when the novel superalloy is aged at 800°C for 20 h. The ɛ phase with hcp structure also precipitates when the superalloy is deformed by 28% and then aged at 650°C for 4 h. The ɛ phase can exist at 800°C. The superalloy has excellent high-temperature mechanical properties. Its maximum flow stress at 850°C is in the range of 360-475 MPa when the strain rate is between 0.0021 and 2.1 s −1. The flow stress of the superalloy during high-temperature deformation can be described by the Zener-Hollomon parameter with a hot deformation activation energy of 397 kJ·mol −1.

Cost and performance evaluation of different surface treated dies for hot forging process

In this work, the effect of various surface treatment methods used to improve the hot forging die life were investigated in the forging department of a plant located in Turkey. For this aim, the surface treatments such as one layer of aluminum titanium nitride (AlTiN) surface coating, a special multilayer coating called TOKTEK (trademarks of the Teknoplasma, Industry and Trade Corporation), nitriding, and the weld overlay coating of the contact surface of the hot forging dies were studied under the same conditions. Then, the results were compared with the as received (uncoated) die. Performance tests, made on a real production line under continuous standard working parameters, showed that all of these methods improved die life in various amounts but the best results were obtained by weld overlay of the die contact surfaces with cobalt base welding electrodes.

Spark plasma sintering cobalt base superalloy strengthened by Y–Cr–O compound through high-energy milling

Powder cobalt base superalloy prepared by high-energy balling and spark plasma sintering are studied through metallography and compression test. The experimental results shows that the powder size and crystalline grain decrease and distortions of crystal lattice increases with milling time. These trends slow down after milling for 8 h. Addition of 1.5% methanol can lessen powders adherence and agglomeration. The relative density of the sintered compact is 99.2% by spark plasma sintering at 1100 °C and 40 MPa. Its microstructure is fine with average grain size below 5 μm. Its compression strength and elongation are 2506 MPa and 2.5%. Annealing heat treatment at 1300 °C can improve its elongation but decrease its strength. They are 23.0% and 1982 MPa, respectively. The added nano-particles Y 2O 3 are changed into Y–Cr–O compounds to strengthen the cobalt base superalloy.

A DTA Study on HVOF Thermally Sprayed WC–M Coatings

Five types of tungsten carbide based powders with different chemical compositions (WC-12Co, WC-17Co, WC-10Ni, WC-10Co-4Cr and WC- 20Cr-7Ni) were deposited onto ST37 mild steel substrate using high velocity oxy fuel (HVOF) spray technique. The feedstock powders and sprayed coatings were studied by using X-ray diffraction (XRD), and differential thermal analyzing (DTA). The results were shown during HVOF thermal spraying, WC-M powders become partially melted before being sprayed on the surface of the substrate with supersonic speed. In these types of coatings, the crystallographic structures are normally non equilibrium, because the cooling rates of the deposited splats are very high due to the cold substrate acting as a thermal sink. These partially melted powders are then rapidly solidified to an amorphous phase. XRD analysis showed that the amorphous phase was existed in all of the as sprayed coatings. The amorphous phase in WC-12Co, WC-17Co and WC-10Ni coatings was transformed to crystalline phases by heat treatment at high temperature. Heat treatment of these coatings at high temperature also resulted in partially dissolution of WC particles and formation of new crystalline phases. In cobalt base coatings, the new phases were eta carbide phases like Co6W6C and Co3W3C but in WC-10Ni coating a NiW intermetallic phase was formed. Heat treatment of WC-10Co-4Cr and WC-20Cr-7Ni coatings did not change the amorphous phases in these coatings. Differential thermal analysis of cobalt containing coatings revealed an exothermic reaction at approximately 880°C. This exothermic reaction may be related to the transformation of the amorphous phase to eta phases. On the contrary, DTA analysis of feedstock powders of these coatings showed an endothermic reaction at approximately 1000°C. DTA analyses of nickel containing cermets also showed similar results. Differential thermal analysis of chromium containing cermets did not show any noticeable exothermic or endothermic reactions.

Development of a Bioactive Surface on a Co-Cr-Mo Alloy during Investment Casting or Heat Treatment

Two different techniques were used to promote a bioactive surface on a cobalt base alloy: i) the cobalt alloy melt was cast into wollastonite-coated cavities of an investment mold, or ii) wollasonite-encapsulated as-cast samples were heat treated at 1220°C for 1 h, this is the typical treatment performed to this alloy for improving its mechanical behavior. In vitro bioactivity was assessed by immersing samples in a simulated body fluid for 21 days. Potentially bioactive layers were obtained in both of the cases. A thicker apatite layer was formed on the samples obtained by investment casting. However, since the heat treatment needs to be performed, the heat treatment method is also a promising technique for promoting the bone-bonding ability of this Co alloy.

Tribological Behavior of Laser-Cladding Ni60 and Co-Cr-W Coatings at Elevated Temperature

Nickel base Ni60 and cobalt base Co-Cr-W coatings were prepared on substrate of refractory alloy steel 4Cr14NiW2Mo by laser-cladding technology. A 2 kW transverse-flowing CO2 laser device was used for cladding alloy powder. Another Co-Cr-W coating was formed by bead welding. Under room temperature, 250 oC.and 420 oC, reciprocate sliding wear tests with a ball-on-flat contact for three coatings have been carried out. For the laser cladding coating, a good metallurgical bonding with the substrate was obtained. Different tribological behaviors of three coatings were observed. For the two Co-Cr-W coatings, the friction coefficients decreased with the increase of test temperature, and larger spot-like stripping pit and wider ploughing trenches in the wear scars were presented. The wear mechanism was the combination of abrasive wear, delamination and oxidation wear. For the Ni60 laser cladding layer, the friction coefficient hardly changed with the elevated temperature, and the wear mechanism mainly performed as abrasive wear due to its high oxidation resistance. The wear tests showed that the Ni60 clad coating had provided excellent wear resistance than Co-Cr-W layers under high temperature reciprocating sliding wear condition.

A heat treatment method for obtaining a bioactive cobalt base alloy

In the aim to promote bioactivity on a cobalt base alloy (ASTM F-75), as-cast substrates were heat treated in contact with bioactive material. Polished as-cast samples were encapsulated in a powder mixture of hydroxyapatite–bioactive glass (HA–BG). The heat treatment of the encapsulated metallic substrates was performed at 1220 °C for 1 h. For the in vitro bioactivity assessment, heat treated samples were immersed in simulated body fluid (SBF) for 21 days. In order to evaluate the effect of the heat treatment on the possible transformation of the HA–BG mixture a ceramic specimen was also prepared. The metallic surfaces, before and after immersion in SBF, were analyzed by Scanning Electron Microscopy (SEM), Energy Dispersive X-Ray analysis (EDX) and X-Ray Diffraction (XRD). Small ceramic agglomerates, composed mainly of Ca, P, O, Si and Na, were observed on the metallic surface before immersion in SBF. After 21 days, a continuous bone-like apatite layer was formed. This may indicate that the heat treated material is potentially useful for bone replacement and tissue regeneration under highly loaded conditions. The HA–BG mixture transformed partially to β-tricalcium phosphate (β-TCP) after heat treatment.

The influence of volumetric dilution on the strain induced γ → ɛ martensitic transformation in GTAW processed Co–Cr–Mo alloy

Mechanical properties of cobalt base alloys are greatly affected by strain induced martensitic transformation (SIT). The present study aims to investigate the influence of volumetric dilution on the SIT and work-hardening in GTAW processed Co–Cr–Mo alloy (Stellite 21) during wear test. The worn surfaces were studied by XRD, SEM and microhardness tester. The results showed that the development of SIT—ɛ phase was strongly influenced by iron content, whereas the work-hardening was less sensitive to dilution. The volume fraction of ɛ phase was increased by decrease in volumetric dilution.

Coarsening of the Nanolamellar Structure in a Soft Magnetic Alloy

Crystallization from a cobalt base Co65Si15B 14Fe4Ni2 amorphous magnetic alloy yielded an interesting highly modulated nanolamellar structure with lamellar spacing of about 5 nm. Coarsening of this structure was investigated using X-ray diffraction (XRD), transmission electron microscopy (TEM), convergent beam electron diffraction (CBED), and X-ray electron dispersive spectrum (XEDS) techniques and was found to initiate at grain boundaries, which provided nucleation sites for equilibrium phases. Rod-shaped Co2B precipitates were observed along the nanolamellae; such precipitates also played a role in coarsening. A coarse lamellar structure subsequently developed which eventually broke up into equiaxed grains. A metastable Co3Si phase was also observed to form by an allotropic transformation. The coarsening of the nanolamellar structure was found to occur largely by discontinuous coarsening processes. The crystallographic relationship between the equilibrium and metastable phases was determined

Role of welding process energy on the microstructural variations in a cobalt base superalloy hardfacing

Cobalt base superalloy hardfacings have been deposited by manual arc welding. Such deposits of total thickness 15 to 17 mm are realised in four successive layers on a steel substrate. The aim of this study is to analyse the role of the welding energy on hardfacings microstructural variations. Microstructure of these hardfacings is formed of a mixture of eutectic precipitates between primary dendrites of a cobalt-rich fcc phase. The high resulting dilution with the substrate induced by such manual arc welding process generates a chemical gradient in the deposit thickness able to involve various carbides volume fraction: the eutectic precipitates volume fraction varies in the reverse order of dilution with no significant difference versus welding energy. On the other hand, the fact of producing hardfacing in successive layers causes a secondary precipitation. In other terms, this kind of deposit involving high temperatures for a sufficient long time makes the dendritic Cr and Co-rich zones precipitate. Thus, fine cuboids particles of Cr 23C 6 carbides are formed in the dendrites. In accordance with higher welding energy used, the higher temperatures reached generate the Cr 23C 6 carbides size increasing.

Mechanical and tribological properties of newly developed Tribaloy alloys

Outstanding combination of mechanical, wear and corrosion performance has been achieved in Laves intermetallic materials, termed Tribaloy alloys. In these two-phase alloys the solid solution provides high mechanical strength and fracture toughness while the Laves intermetallic phase offers excellent wear resistance. However, conventional Tribaloy alloys usually have low tensile strength and fracture toughness compared with ductile materials due to the large volume fraction of Laves phase, which has limited their application in many cases. The present research is aimed at developing advanced Tribaloy alloys with increasing ductility. Two new cobalt base alloys were developed in this research. The specimens were fabricated with a centrifugal casting technique. The material characterization was performed using the differential scanning calorimetry (DSC), scanning electron microscope (SEM), indentation and ball-on-disc tribological techniques.

Room and high temperature wear behaviors of nickel and cobalt base weld overlay coatings on hot forging dies

The lifetime of dies used for production of forged parts is variable and determined by wear rate, plastic deformation, thermal and mechanical fatigue. These tools are subjected to extreme temperature at high unit pressures for short durations and must withstand multiple cycles while maintaining dimensional stability. This paper is an investigation into the use of nickel and cobalt base superalloys as wear resistant hardfacing materials on H11 tool steel. Three weld overlay alloys including Inconel 625, Stellite 6 and Stellite 21 were deposited on H11 steel substrates using tungsten inert gas welding (TIG) process. Wear tests were carried out using a pin-on-disk wear tester at room temperature and 550 °C. Microhardness of the weld overlays was obtained and the worn surfaces were examined by scanning electron microscopy (SEM). The results of the laboratory tests were then compared with the results obtained from field studies. The results showed a much lower wear at high temperature with the weld overlays due to formation of smooth compacted oxide layers on the wear surfaces. Inconel 625 showed the best wear behavior among the weld overlays at high temperature.

Microstructural characterization and microscopy analysis of laser cladding Stellite12 and tungsten carbide

Stellite12 cobalt base alloys with different WC content were deposited on SK3-carbon tool steel by laser cladding. The behavior of WC particulates, including the dissolution, distribution, and microstructures of WC–Co–Cr–C composite coatings with rapidly solidifying were investigated using scanning electron microscopy (SEM), electron probe microanalysis (EPMA) and X-ray diffraction (XRD). Several significantly different solidified microstructures were characterized by dendrites, interdendritic eutectics, faceted dendrites (third phase) and the retaining of WC particles in the laser clads of Stellite12 + WC, under different laser energy densities. When WC was completely melted and fully dissolved into the Stellite12 melt pool, the basic solidification, characterized by the matrix and faceted dendrites (third phase) in various shapes, and the compositional evolution of the clad layers remained nearly identical, no matter the added WC was increased to 10%, 20% or 40% (wt.%). The faceted dendrites (third phase) contained the majority of W as well as some Cr, Co while more Cr and Co were located in the matrix. The X-ray diffraction analyses indicated the existence of σ-Co, M 23C 6, M 6C and M 7C 3 (M = W, Cr, Co) in the Stellite12 alloys with different WC contents when deposited on substrates by laser cladding.

Experimental observation of tool wear, cutting forces and chip morphology in face milling of cobalt based super-alloy with physical vapour deposition coated and uncoated tool

Cobalt base superalloys are used extensively in applications requiring good wear, corrosion and heat resistance. The main goal of this study is to examine the effect of machining conditions (cutting speed, feed rate and depth of cut) on tool wear, chip morphology and cutting forces in symmetric face milling of cobalt base superalloy with physical vapour deposition coated and uncoated inserts. with the aim of achieving to achieve this goal, 90 milling experiments were carried out with different cutting speeds, feed rate and depth of cut under dry cutting conditions. The settings of machining parameters were determined by using general full factorial design method. Chip morphology, cutting forces and tool wear were compared by using PVD coated and uncoated hard metal inserts which are obtained dependent on feed rate, cutting speed and cutting depth. The cutting forces increase as the feed rate and depth of the cut increases, but cutting speeds’ effect on cutting forces has not been observed for symmetric face milling.

Laser cladding for wear‐resistant cobalt base alloy and tungsten carbide composite coatings

Stellite12 cobalt base alloys with different WC contents were deposited on SK3‐carbon tool steel by laser cladding. The behavior of WC particulates, including dissolution and distribution, and the microstructure of WC‐Co‐Cr‐C composite coatings with rapid solidification were investigated. Several significantly different solidified microstructures were characterized by dendrites, eutectics, faceted dendrites and the retained WC particles in the laser cladding WC + Stellite12 coatings, under different laser energy densities. When WC was melted and dissolved into the Stellite12 melt pool, the basic structure of solidification, characterized by the matrix and faceted dendrites in various shapes, and the contents of the faceted dendrites remained nearly identical. The faceted dendrites contained the majority of W as well as some Cr, Co while more Cr and Co were located in the matrix. The X‐ray diffraction analyses indicated the existence of σ‐Co, M23C6, M6C and M7C3 (M = W, Cr, Co) in the Stellite12 with different WC contents when deposited on substrates by laser cladding. The faceted dendrites provided the coatings with excellent resistance during dry sliding wear test. A higher content of WC gave higher volume fractions of faceted dendrites that imparted excellent wear resistance to the coating.

High frequency permeameter with semi-rigid pick-up coil

In this study, we propose the application of semi-rigid cable loop as a single turn shielded loop pick-up coil for the high frequency permeameter. Since the semi-rigid cable pick-up coil has simple structure, it is very easy to make the pick-up coil with bending and conventional soldering. The permeability of cobalt base amorphous ribbon was investigated using the developed permeameter for demonstrating its performance. The permeability of the amorphous ribbon was driven from the S-parameters measured using a network analyzer and permameter having the semi-rigid pick-up coil.