Cobalt-based Alloys Research Articles

Growth temperature-controlled Gilbert damping and anisotropies in PLD grown epitaxial Co2FeSi Heusler alloy thin film

The utilization of spin rather than charge as a functional aspect in the electronic devices will result in an energy-efficient, thermally stable, and fast operation. Cobalt-based Heusler alloy Co2FeSi (CFS) offers excellent possibilities due to its high curie temperature, large spin polarization, and low Gilbert damping (α). Small damping makes the alloy fascinating for spin transfer torque magnetoresistive random-access memory (STT-MRAM) devices where a low α causes a low value of switching current. The present investigation explored the effect of deposition temperature on the chemical, structural and magnetic properties of the pulsed laser deposition grown single-crystal CFS film of thickness 50 nm on a lattice-matched (001) MgO substrate. The XRD data suggest that Co site atomic ordering and overall film crystallinity improved with temperature, resulting in a minimal value of α obtained from ferromagnetic resonance (FMR) experiment in the film grown at 600°C. The azimuthal angular variation (φ) of FMR field data gives a fourfold anisotropy pattern that confirms the single-crystal CFS film epitaxially grown on the MgO substrate. Two in-plane anisotropies, namely cubic (four-fold) and uniaxial, were obtained in all the samples. This investigation is crucial in the fabrication of fast spintronics based memory and processing devices.

Optimization of material and process parameters for plasma transferred arc hard facing for high temperature applications

The authors in this research paper have investigated the process of hardfacing, which is a well-established manufacturing process for increasing the durability of industrial components, which are subjected to extreme wear, erosion, abrasion. The objective of this study is to establish a scientific methodology to select the most suitable combinations of materials as base-deposition materials for hardfacing, specifically focusing on Nickel-based and Cobalt-based materials. In order to make an informed decision, multi-criteria decision-making method known as “Technique for Order of Preference by Similarity to Ideal Solution (TOPSIS)” is used. This technique permits to deal with wide range of important factors, such as the materials' high-temperature resistance, wear resistance, and corrosion resistance, among others. The paper discusses a step-by-step approach to selecting the best alternative material for hardfacing by considering eight different criteria and assigning preferences to each of the available materials. The results of this methodology suggested that Stellite-6 based cobalt-based alloy is best suitable for AISI 304 grade austenitic stainless steel materials used for high temperature applications.In addition, the paper also studies the effects of various welding parameters on the hardfacing process. This includes parameters such as welding current, welding speed, powder feed rate and the type of shielding gas used. The significance of each parameter is also studied, and the results of these investigations are discussed in detail. Overall, this research paper provides a comprehensive overview of the hardfacing process and the materials and techniques that are best suited for it, making it valuable resource for professionals and researchers in the field.

Impact of operating parameters on electric discharge machining of cobalt-based alloys

The goal of this investigation is to examine how operating parameters (voltage, peak current, pulse-on & pulse-off times) affect the MRR of satellite-6 material that has undergone EDM processing. To carry out the trials, an L9 orthogonal array (OA) for the 4 machining variables at 3 levels each was chosen. Three successive trials were performed in the studies in random order. The best levels for maximising the responses were identified after performing an analysis of variance. The important factors in MRR are current & pulse-on-time. The effects of pulse-off-time & voltage on MRR are negligible. The ANOVA's results also show that discharge current contributes significantly to overall performance with a contribution of (67.87%). The contribution % age for “pulse-on-time, pulse-off-time, voltage & pulse-on-time” are lower at 25.45%, 2.27% & 4.8% respectively.

Coaxial laser cladding of cobalt-base alloy StelliteTM6 on gray cast iron/investigations on friction, wear versus commercial brake pad, and corrosion characteristics

Environmental legislation and the electrification of vehicles place increased requirements on brake disks in terms of wear and corrosion resistance. Departing from a preliminary study, the present investigation examined the friction and wear behavior as well as the corrosion properties of Stellite™ 6 coatings on gray cast iron, which were deposited by laser cladding. The friction and wear experiments were conducted on a pin-on-disk tribometer at contact pressures of 1.0 and 2.0 MPa as well as rotation speeds of 0.2, 0.4, 0.6, 0.8, and 2.0 m/s. The pins are manufactured from an automotive semi-metallic brake pad. The friction behavior of gray cast iron and Stellite™ 6 coatings is similar under changing test conditions. The tribological behavior is strongly influenced by the microstructural and mechanical properties of the coatings and the brake pad material. A third body layer of contact patches is formed on Stellite™ 6, which mainly consists of brake pad components. The transformation of the Co-matrix (fcc → hcp), subsurface fatigue, and tribo-oxidation are the main wear mechanism for Stellite™ 6. The electrochemical characteristics were analyzed by potentiodynamic polarization, and the corrosion rate was determined with Tafel plots in 3.5% NaCl solution, respectively. Compared to gray cast iron, Stellite™ 6 coatings have a pronounced corrosion resistance due to the formation of a protective passive layer of Co and Cr oxides. This corrosion resistance is mainly influenced by the microstructure. The corrosion rate of the coatings increases in parallel with the Fe dilution.

Study on the sliding bearing of hot dip galvanized sinking roll

As a key component in the hot-dip galvanizing production line, the sink roller ends of the sliding bearing is affected by the high-temperature zinc liquid corrosion wear, which leads to frequent stop for maintenance. The galvanizing industry is looking for more suitable material for bearing to prolong the service life. To improve the corrosion and wear resistance of the sliding bearing, the laser cladding method was used to prepare the cobalt-based and iron-based coatings on the surface of the superalloy. In this paper, the high-temperature zinc liquid corrosion wear test was carried out on the alumina ceramic bushing and three kinds of metal sleeves (316L stainless steel, cobalt-based alloy, and iron-based alloy). By comparing and analyzing the corrosion and wear conditions of the three metal sleeves after the test, the main failure mechanisms were comprehensively analyzed.

Tailoring the intrinsic magneto-electronic, mechanical, thermo-physical and thermoelectric response of cobalt-based Heusler alloys: an ab initio insight.

We conducted a comprehensive analysis of the fundamental properties of CoHfSi and CoHfGe half-Heusler alloys using density functional theory simulations implemented in Wien2k. To begin, structural optimization revealed that both alloys effectively adopt a cubic C1b structure, with Y1 as the dominant ferromagnetic phase. Electronic properties were computed using various approximation schemes, including the Generalized Gradient Approximation and the modified Becke-Johnson potential. The examination of electronic band structures and their accompanying density of states using the modified Becke-Johnson functional approach unveiled their half-metallic nature. In this context, the spin-up channel exhibited semiconductor behaviour, while the spin-down channel displayed metallic characteristics. Additionally, the spin-splitting observed in their resulting band structures contributed to a net magnetism within their lattice structure, making them promising candidates for spintronic applications. We also scrutinized Seebeck coefficients, electrical conductivity, thermal conductivity, and power factor to gain a better understanding of their thermoelectric properties.

Effect of heat-treatment on corrosion performance of Ni and Ni-Cr-based alloys

Metal dusting is described as a severe form of corrosion attack in which iron, steels, nickel- and cobalt-based alloys disintegrate into metal or metal carbide particles in a coke deposit when exposed to strongly carburising gases (carbon activity ac>1) at elevated temperatures (300-800°C). Despite intense research efforts, which led to much progress in mitigating metal dusting, particularly in the oil and gas industry, the complete prevention still remains an issue. Current study focuses on measuring corrosion resistance of heat-treated austenitic Ni, Ni-Cr-based complex alloy compositions containing iron and copper as additional alloying elements.

For high tech industry to find physical characteristics of thin clad layer of Stellite-6 (Cobalt Base Alloys) deposited by laser cladding

For high tech industry to find physical characteristics of thin clad layer of Stellite-6 (Cobalt Base Alloys) deposited by laser cladding

Laser-Based Additive Manufacturing of Magnesium Alloys for Bone Tissue Engineering Applications: From Chemistry to Clinic

Metallic biomedical implants are made from materials such as stainless steel, titanium, magnesium, and cobalt-based alloys. As a degradable biometal, magnesium (Mg) and its alloys are becoming more popular for applications in bone tissue engineering. Mg-based alloys have been found to be biocompatible, bioabsorbable, and bioactive, allowing them to be used as orthopedic implants with a low Young’s modulus. Computer-aided design can be used to design scaffolds with intricate porous structures based on patient-specific anatomical data. These models can be materialized rapidly and with reasonably acceptable dimensional accuracy by additive manufacturing (AM) techniques. It is known that lasers are the most widely investigated energy source for AM’ed Mg, as they offer some distinct advantages over other forms of energy. Recent studies have focused on developing biodegradable Mg scaffolds by using laser-based AM techniques. In this paper, we aim to review the recent progress of laser-based AM for Mg alloys and survey challenges in the research and future development of AM’ed Mg scaffolds for clinical applications.

Comparative analysis of weld strength of nickel-chromium and cobalt-chromium base metal alloys when submitted to tungsten inert gas welding.

Long span base metal alloy fixed prosthesis and partial dental prosthesis frameworks most often require welding for better fit and adaptation. The type of metal alloys used and its effect on the weld strength of the prosthesis have to be investigated. The aim of the study is to comparatively analyze the weld strength of nickel-chromium (Ni-Cr) and cobalt-chromium (Co-Cr) base metal alloys when subjected to tungsten inert gas (TIG) welding. 10 nickel- and cobalt-based specimens were obtained by casting of castable resin patterns of dimension 25 mm × 3 mm × 1 mm. Welding of the samples was carried out using the Lampert PuK 2 welding equipment, and a three-point bend test using a universal testing machine was carried out to evaluate the weld strength of the specimens. Statistical analysis was carried out using the SPSS software version 23. Higher weld strength was observed in the Co-Cr alloy (mean = 898 MPa) than in the Ni-Cr alloy (mean = 690 MPa), when submitted to TIG welding (P < 0.05). TIG welding could be a better alternative for joining base metal alloys for long span edentulous cases, and cobalt-based alloys could be better alternatives to nickel-based alloys that usually tend to cause hypersensitivity reactions.

Temperature dependent sliding wear behaviour of Stellite 6 alloy

The present work investigates the frictional and sliding wear characteristics of cobalt-based alloy, Stellite 6 at room temperature (RT), 150 °C, and 300 °C. Wear tests were performed using a ball on plate reciprocating tribometer with a tungsten carbide ball as a counter surface. Scanning electron microscopy (SEM) and Raman spectroscopy techniques were used to investigate worn surface morphology and chemical characteristics, respectively. The wear test results clearly indicate that the specific wear rate increased with increasing temperature. Abrasion, oxidation, and delamination wear mechanisms were observed as predominant at higher temperatures. The formation of CoO and Co3O4 phases were evident from the Raman spectroscopy analysis, which is responsible for the reduction of friction, particularly on the samples tested at elevated temperatures. Formation and removal of the tribo-oxide layers at higher temperatures are responsible for increased wear rate as compared to the sample tested at room temperature. The results demonstrate a strong influence of temperature on the wear and friction characteristics of the alloy.

Thermal monitoring for directed energy deposition of stainless steel, bronze, and cobalt-based alloy

Laser cladding and Directed Energy Deposition are two related processes that allow the deposition of specific surface coatings and the production of additively manufactured parts. In both processes, the selection of optimised parameters results in the deposition of high-density material with low dilution. However, the thermal and geometrical conditions constantly change during the process and the parameters need to be continually adapted in order to avoid defects or poor properties. In this context, the development of closed-loop monitoring systems is crucial in order to widen the field of possible applications towards more complexity, with a more stable process and higher materials properties. In this research, the possibility of thermal monitoring with middle-wave and long-wave infra-red cameras is investigated for Directed Energy Deposition of 316L, Stellite 21 and CuSn10. The melt pool length and the cooling rate are extracted from thermal imaging while the laser power was varied, and these results are compared to the materials properties of the deposited tracks. The main results show that an increase of melt pool length results in a decrease of porosity and an increase of dilution, which induces a change of hardness. The melt pool length can be regulated by adjusting the laser power in order to keep both the porosity and the dilution within acceptable values.

Data-driven pitting evolution prediction for corrosion-resistant alloys by time-series analysis

Corrosion initiation and propagation are a time-series problem, evolving continuously with corrosion time, and future pitting behavior depends closely on the past. Predicting localized corrosion for corrosion-resistant alloys remains a great challenge, as macroscopic experiments and microscopic theoretical simulations cannot couple internal and external factors to describe the pitting evolution from a time dimension. In this work, a data-driven method based on time-series analysis was explored. Taking cobalt-based alloys and duplex stainless steels as the case scenario, a corrosion propagation model was built to predict the free corrosion potential (Ecorr) using a long short-term memory neural network (LSTM) based on 150 days of immersion testing in saline solution. Compared to traditional machine learning methods, the time-series analysis method was more consistent with the evolution of ground truth in the Ecorr prediction of the subsequent 70 days’ immersion, illustrating that time-series dependency of pitting propagation could be captured and utilized.

Critical Assessment of Two-Dimensional Methods for the Microstructural Characterization of Cemented Carbides

Cemented carbides, or hard metals, are ceramic–metal composites usually consisting of tungsten carbide particles bound by a cobalt-based alloy. They are the backbone materials for the tooling industry, as a direct consequence of the outstanding range of property combinations, depending on their effective microstructural assemblage, i.e., the physical dimensions and relative content of their constitutive phases. Hence, reliable microstructural characterization becomes key for hard metal grade selection and quality control. This work aimed to assess the practical two-dimensional characterization methods for the most important one- and two-phase properties of cemented carbides, i.e., the carbide grain size, phase fraction, carbide contiguity, and binder mean free path. Three different methods—point, line, and area analysis—were implemented to characterize four microstructurally distinct grades. The images were acquired by optical and scanning electron microscopy, with the latter through both secondary and backscattered electrons. Results were critically discussed by comparing the obtained values of properties and the different characterization methodology. Inspection technique combinations were finally ranked based on accuracy, accessibility, and operability considerations. The line method was used to analyze all the properties, the area method, for the one-phase properties, and the point method, for only the phase fraction. It was found that the combination of optical microscopy and the line analysis method was suitable for a direct inspection and rapid estimation for carbides above fine grain size. The most precise results were achieved using line analysis of the images obtained by the backscattered electrons of the scanning electron microscope.

Laser metal deposition characterization study of metal additive manufacturing repair of rail steel specimens

ABSTRACT Research on rail steel repair with Stellite 6 powder using a laser metal deposition process was investigated and reported in this study. Rail failures due to excessive wear and other rail head defects can increase the risk of operation and reliability problems in service life. This study examines the deposited material compatibility of a cobalt-based alloy, Stellite 6 with the head-hardened R350HT grade rail steel substrate. Materials characterization work includes metallurgical, wear, and shear properties testing and analysis. Microstructural analysis and hardness measurements of the cross-section from raster scan pattern deposition strategy of Stellite 6 on rail steel substrate surface showed minimal dilution and high hardness of the deposited layer with no presence of defects. Experimental testing of rail wear was conducted using a laboratory ball-on-disc tribometer setup and the wear coefficients for Stellite 6 and rail steel were derived from the wear volume analysis using Archard’s wear model. The material interface bond strength was evaluated from shear testing and analysis to measure the shear interface strength of the metallurgical bond between deposited material on a rail steel substrate.

Cavitation-Erosion behavior of laser cladded Low-Carbon Cobalt-Based alloys on 17-4PH stainless steel

Stellite 728 (C14) with low-carbon and high-molybdenum was used for cavitation resistance application in this research. The specimens were fabricated on the surface of a martensitic precipitation hardening stainless steel called 17-4PH by laser cladding technique. The cavitation erosion behavior in 3.5 wt% NaCl solution is investigated using a 20 kHz vibratory cavitation erosion test machine. The electrochemical corrosion test was also explored to reveal the mechanism of the synergistic effect of cavitation and electrochemical corrosion. The experimental results of the Stellite 728 were compared with those of well-known cavitation-resistant Stellite 6 and wrought Stellite 6B. The surface morphology and microstructure evolution after cavitation were analyzed by means of metallographic microscope (OM), scanning electron microscope (SEM), energy dispersive spectroscopy (EDS), X-ray diffraction (XRD) and 3D optical surface morphometry to explore the cavitation erosion mechanism of the specimens. The experimental results show that the Co-based alloys have better cavitation erosion resistance than 17-4PH, and laser cladded Stellite 728 has better cavitation resistance in 3.5 wt% NaCl solution than laser cladded Stellite 6 and wrought Stellite 6B, owing to the formation of large amounts of Mo-rich intermetallic compound in the alloy.

Feasibility Analysis of Machining Cobalt-Chromium Alloy (Stellite-6) Using TiN Coated Binary Inserts.

The objective of the study was to check the feasibility of machining Stellite 6, a cobalt–chromium superalloy, using TiN-coated carbide inserts in an end milling operation. The inserts were coated using the magnetron sputtering process. The sputtering power and gas flow rate were considered as the variables during the coating process. The performance of the coated binary carbide insert was checked during the end milling of Stellite 6 (~45 HRC) through an experiment with a Taguchi design. Experimental runs based on an orthogonal array were executed for each insert type to check the feasibility of machining this cobalt-based alloy. Adequate precision and the optimum parametric conditions were determined and are reported in this study. Analysis of variance (ANOVA) with a two-factor interaction model was also undertaken to forecast the key elements influencing surface roughness. Based on the ANOVA model, the depth of the cut, combined with the insert type, was the factor that had the greatest influence on surface roughness, followed by the cutting feed, whereas the cutting velocity had the least significance based on the tests. Moreover, the regression analysis demonstrated that the created model can be used to accurately forecast surface roughness in end milling of Stellite 6 with confidence intervals of 95%.

Coaxial Laser Cladding of Cobalt-Base Alloy Stellite™ 6 on Grey Cast Iron Analysis of the Microstructural and Mechanical Properties Depending on the Laser Power

A high-power diode laser was used to generate single- and multi-bead coatings of Stellite™ 6 by coaxial laser cladding over flat grey cast iron (EN-GJLP-200) as a preliminary study to develop a wear and corrosion resistant coating for brake disks on a cost-effective substrate. In this article, we have focused on a detailed quantitative analysis of the effect of different laser powers (1.5, 2.0, 2.5, 3.0, 3.5, and 4.0 kW) on the bead geometry, dilution, microstructure, and hardness. Coatings dilution or composition depends directly on the laser power as well as bead geometry. The typical microstructure of the coatings comprises a solid solution of α (hcp)- and β (fcc)-Co with a dendritic structure as a metal matrix and an interdendritic lamellar eutectic, which contains predominantly β-Co, chromium carbides Cr7C3 and Cr23C6 as well as blocky tungsten carbide W2C. Coating hardness depends on the chemical composition and microstructure that is modified by the deposition parameters. Low laser power results in high carbide fraction and most refined microstructures, accounting for harder coatings.

High-Energy Computed Tomography as a Prospective Tool for In Situ Monitoring of Mass Transfer Processes inside High-Pressure Reactors-A Case Study on Ammonothermal Bulk Crystal Growth of Nitrides including GaN.

For the fundamental understanding and the technological development of the ammonothermal method for the synthesis and crystal growth of nitrides, an in situ monitoring technique for tracking mass transport of the nitride throughout the entire autoclave volume is desirable. The feasibility of using high-energy computed tomography for this purpose was therefore evaluated using ex situ measurements. Acceleration voltages of 600 kV were estimated to yield suitable transparency in a lab-scale ammonothermal setup for GaN crystal growth designed for up to 300 MPa operating pressure. The total scan duration was estimated to be in the order of 20 to 40 min, which was sufficient given the comparatively slow crystal growth speed in ammonothermal growth. Even shorter scan durations or, alternatively, lower acceleration voltages for improved contrast or reduced X-ray shielding requirements, were estimated to be feasible in the case of ammonoacidic growth, as the lower pressure requirements for this process variant allow for thinned autoclave walls in an adapted setup designed for improved X-ray transparency. Promising nickel-base and cobalt-base alloys for applications in ammonothermal reactors with reduced X-ray absorption in relation to the maximum operating pressure were identified. The applicability for the validation of numerical simulations of the growth process of GaN, in addition to the applicability of the technique to further nitride materials, as well as larger reactors and bulk crystals, were evaluated.

Multi-spectroscopic analysis of high temperature oxides formed on cobalt-chrome-molybdenum alloys

Thanks to their thermal stability, resistance to oxidation and mechanical strength, cobalt-chrome molybdenum alloys are considered an ideal alloy for high temperature applications. The surface oxide layer evolves as a function of time and temperature, changing its chemical structure and increasing its thickness from a few nanometers to various microns. Making use of various diffractographic and spectroscopic techniques, namely X-ray diffraction, X-ray photoelectron spectroscopy, Raman spectroscopy and glow-discharge optical emission spectroscopy, coupled with complementary analysis, this work gives new insights on the chemical bonding, crystallographic structure, thickness and elemental composition of the oxide layers as a function of both time and temperature of oxidation. Results show that the initial nanometric passive layer of Co3O4 evolves into a metastable, sub-micrometric CoCrO4 structure and finally stabilizes into a micrometric Cr2O3 at the highest temperatures. This paper fills a fundamental gap in the understanding of the chemistry and stability of Cobalt-based alloys used for high temperature applications, such as in poppet or exhaust valves, aerospace components or hot gas turbines. Once calibrated, this innovative, complete surface characterization approach can be ideally extended to other metallic alloys.