Microstructure evolution of Fe-based WC composite coating prepared by laser induction hybrid rapid cladding

Fe + 50 wt.% WC composite coating was prepared by laser induction hybrid rapid cladding (LIHRC) on steel substrate. The phase and microstructure of the composite coating were investigated by X-ray diffraction (XRD), environmental scanning electron microscope (ESEM) and energy dispersive spectrum (EDS). The results showed that WC particles were dissolved almost completely to precipitate the coarse herringbone M₆C eutectic carbides and the fine dendritic M₆C carbides, and that the partially dissolved WC particles with an alloyed reaction layer were occasionally observed in the whole coating. The phases of the composite coating were composed of supersaturated solid solution α-Fe, retained austenite, Fe₃C, W₂C, M₆C and M₇C₃. The microstructure evolution in the composite coating was represented by the transformation of three parts such as Fe-based metallic matrix, dispersed carbides and incompletely dissolved WC particles. The microhardness of Fe-based WC composite coating was three times much higher than that of the substrate, but was relatively lower than that of Ni-based WC composite coating by LIHRC.     

Microstructure and properties of high chrome steel roller after laser surface melting

Laser surface melting of high chrome steels was achieved by a 5 kW continuous wave CO₂ laser. The microstructure of the laser surface-melted steels was investigated by scanning electron microscopy, transmission electron microscopy and X-ray diffractometry, and the hardness profiles were determined by a Vickers hardness tester. The corrosion behavior in 3.5% NaCl solution was studied by electrochemical corrosion equipment. The large carbides of high chrome steels are completely dissolved and ultrafine dendrites of austenite with submicroscopic M₂₃C₆ carbides precipitation are formed in the melted zone. The austenite in the melted zone has a high tempering stability. The corrosion resistance of the laser surface-melted steels is significantly improved due to the dissolution of carbides and the increase of the alloying elements in the solid solution as well as the large amount of austenite.     

Microstructure and corrosion properties of thick WC composite coating formed by plasma cladding

The thick Ni-coated WC coatings, in a matrix of Nickel-based alloys, were prepared on AISI 1045 steel using plasma cladding equipment. A pre-placed layer of uniform mixture, with different weight fractions of Ni-coated WC powder and Nickel-based alloy powder, on the steel substrate was melted at the high temperature of the plasma jet. The coating composition, microstructure and microhardness were analyzed by X-ray diffraction (XRD), scanning electron microscopy (SEM), energy-dispersive spectrometer (EDS) and microhardness testing. The experimental results show that the metallurgical bond was formed between the coating and substrate. The XRD results show that the coatings contain γ-Ni, carbides (such as M₂₃C₆ and M₇C₃) and boride (such as Fe₂B, Fe₃B phases). SEM shows that all the coatings are crack-free with lower porosity (<1%). It is found that the microhardness and the electrochemical behavior of the coatings are depended on the content of Ni-coated WC powder. The corrosion mechanism for the coatings may be due to the microgalvance corrosion between the phases in the cladding coatings.     

Microstructure and corrosion properties of diode laser melted friction stir weld of aluminum alloy 2024 T351

Friction stir welding is a promising solid state joining process for high strength aluminum alloys. Though friction stir welding eliminates the problems of fusion welding as it is performed below melting temperature (T_m), it creates severe plastic deformation. Friction stir welds of some aluminum alloys exhibit relatively poor corrosion resistance. This research enhanced the corrosion properties of such welds through diode laser surface melting.A friction stir weld of aluminum alloy 2024 T351 was laser melted using a 1 kW diode laser. The melt-depth and microstructure were investigated using optical and scanning electron microscopy. The melt zone exhibited epitaxially grown columnar grains. At the interface between the melted and the un-melted zone, a thick planar boundary was observed. Energy dispersive spectroscopy analyzed the redistribution of elemental composition. The corrosion properties of the laser melted and native welds were studied in aqueous 0.5 M sodium chloride solution using open circuit potential and cyclic potentiodynamic polarization. The results show noticeable increase in the pit nucleation resistance (390 mV) after the laser surface treatment. The repassivation potential was nobler to the corrosion potential after the laser treatment, which confirmed that the resistance to pit growth was improved.     

Laser induction hybrid rapid cladding of WC particles reinforced NiCrBSi composite coatings

In order to investigate the microstructure characteristics and properties of Ni-based WC composite coatings containing a relatively large amount of WC particles by laser induction hybrid rapid cladding (LIHRC) and compare to the individual laser cladding without preheating, Ni60A + 35 wt.% WC composite coatings are deposited on A3 steel plates by LIHRC and the individual laser cladding without preheating. The composite coating produced by the individual laser cladding without preheating exhibits many cracks and pores, while the smooth composite coating without cracks and pores is obtained by LIHRC. Moreover, the cast WC particles take on the similar dissolution characteristics in Ni60A + 35 wt.% WC composite coatings by LIHRC and the individual laser cladding without preheating. Namely, the completely dissolved WC particles interact with Ni-based alloy solvent to precipitate the blocky and herringbone carbides, while the partially dissolved WC particles still preserve the primary lamellar eutectic structure. A few WC particles are split at the interface of WC and W₂C, and then interact with Ni-based alloy solvent to precipitate the lamellar carbides. Compared with the individual laser cladding without preheating, LIHRC has the relatively lower temperature gradient and the relatively higher laser scanning speed. Therefore, LIHRC can produce the crack-free composite coating with relatively higher microhardness and relatively more homogeneous distribution of WC particles and is successfully applied to strengthen the corrugated roller, showing that LIHRC process has a higher efficiency and good cladding quality.     

Microstructure evolution during surface alloying of ductile iron and austempered ductile iron by electron beam melting

Alloying and microstructural modification of surfaces by electron beam has become popular to tailor the surface properties of materials. Surface modification of as-received ductile iron, Ni-plated ductile iron and Ni-plated austempered ductile iron was carried out by electron beam melting to improve the surface properties. Martensitic structure evolved in the heat affected zone and ledeburite structure was produced in the molten zone of the ductile iron. Microhardness of the melted specimens enhanced considerably as compared to the as-received samples. However the microhardness of melted Ni-plated samples is lower than that of the unplated specimens. X-ray diffraction clearly revealed the formation of an austenite and Fe₃C phases in the electron beam molten zone. The broadening of peaks suggests refinement of the microstructure as well as internal stresses generated during electron beam melting.     

Microstructure and corrosion behavior of austenitic stainless steel treated with laser

Surface modification of AISI316 stainless steel by laser melting was investigated experimentally using 2 and 4 kW laser power emitted from a continuous wave CO₂ laser at different specimen scanning speeds ranged from 300 to 1500 mm/min. Also, an investigation is reported of the introduction of carbon into the same material by means of laser surface alloying, which involves pre-coating the specimen surfaces with graphite powder followed by laser melting. The aim of these treatments is to enhance corrosion resistance by the rapid solidification associated with laser melting and also to increase surface hardness without affecting the bulk properties by increasing the carbon concentration near the surface. Different metallurgical techniques such as optical microscopy, scanning electron microscopy (SEM), and X-ray diffraction (XRD) were used to characterize the microstructure of the treated zone. The microstructures of the laser melted zones exhibited a dendritic morphology with a very fine scale with a slight increase in hardness from 200 to 230 Hv. However, the laser alloyed samples with carbon showed microstructure consisting of γ dendrite surrounded by a network of eutectic structures (γ+carbide). A significant increase in hardness from 200 to 500 Hv is obtained. Corrosion resistance was improved after laser melting, especially in the samples processed at high laser power (4 kW). There was shift in I_corr and E_corr toward more noble values and a lower passive current density than that of the untreated materials. These improvements in corrosion resistance were attributed to the fine and homogeneous dendritic structure, which was found throughout the melted zones. The corrosion resistance of the carburized sample was lower than the laser melted sample.     

Study on the solidification microstructure in AZ91D Mg alloy after laser surface melting

Laser surface melting (LSM) is known to enhance the wear and corrosion resistance of Mg alloys, but its effect on microstructural evolution of Mg alloys is not well understood. An effort has been made to study the effect of rapid solidification following LSM on the microstructural evolution of AZ91D Mg alloy. The results of X-ray diffractometry, scanning electron microscopy, transmission electron microscopy and energy-dispersive X-ray spectroscopy indicated that the solidification microstructure in the laser-melted zone was mainly cellular/dendrite structure of primarily α-Mg phase and continuous network of β-Mg₁₇Al₁₂ phase. Numerical prediction of the laser-melted zone suggested that cooling rates increased strongly from the bottom to the top surface in the irradiated regions. An attempt has been made to correlate dendrite cell sizes of the solidification microstructure with the cooling rates in the laser-treated AZ91D Mg alloy.     

Synthesis of tungsten carbide nanoparticles in WO3 pyrolysis in a plasma arc

Synthesis of tungsten carbides is investigated in the process of WO₃ pyrolysis in a dc electric arc during dispersion of a composite electrode (graphite-WO₃) in a helium medium at 25 Torr. The synthesized material is analyzed by high-resolution transmission electron microscopy. The interplanar spacings of nanocrystallites are measured directly by using both TEM images and electron diffractometry. The size distribution functions of nanoparticles containing tungsten are measured. It is shown that W, WC, W₂C, and WO₃ nanoparticles can be obtained in the conditions of the present experiments. The content of these compounds in the synthesized material was quantitatively assessed. It is shown that the W₂C content forms the dominant portion of nanoparticles in the synthesized material.     

Microstructure of nodular cast iron after excimer laser surface processing

Nodular iron of martensitic structure was treated by means of a XeCl laser prototype. The energy density varied from 0.3 to 5 J/cm² and the number of shots from 4 to 40. Conversion electron Mössbauer spectroscopy, conversion X-ray Mössbauer spectroscopy and grazing incidence X-ray diffraction were used to characterize the irradiated surface. Some Rutherford backscattering spectrometry measurements were performed to control surface oxidation and carbon distribution. It is shown that after irradiation austenite formed in a rather deep heat affected zone (10–20 m) compared to the thickness of the melted zone ( 1 m). The austenite amount as well as its carbon content increase with energy density and number of shots up to a threshold of carbide formation. Beyond the threshold Fe₂C, Fe₃C and Fe₅C₂ formed only in the melted zone. The carbon content as a function of depth is constant in the melted layer, then decreases quickly from the melted layer-heat affected zone interface down its initial value. The carbon content is shown to govern the evolution of phases content in the melted layer depending on the laser treatment conditions.     

Influences of laser remelting on microstructure of nanostructured Al2O3-13 wt.% TiO2 coatings fabricated by plasma spraying

The effects of laser remelting on microstructure of nanostructured Al₂O₃-13 wt.% TiO₂ ceramic coatings prepared by plasma spraying with agglomerated powders were studied. The microstructure of the feedstock, as-sprayed and laser-remelted coatings were investigated by using scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS) and X-ray diffractometry (XRD). The results indicate that the plasma-sprayed ceramic coating consists of both fully melted regions and partially melted regions. The totally ceramic coating, especially the fully melted regions, has a typical plasma-sprayed lamellar-like structure as the conventional coating, and has some pores. According to the difference of microstructures, the partially melted regions can be divided into liquid-phase sintered regions (a three-dimensional net or skeleton-like structure: Al₂O₃-rich submicron particles embedded in the TiO₂-rich matrix) and solid-phase sintered regions (remained nanoparticles). The lamellar defect of the as-sprayed coating is erased, and the compactness of the coating is improved significantly after laser remelting. The laser-remelted region composed of fine equiaxed grains, which are different from the conventional column-like crystals along the direction of the heat current. Due to the rapid solidification of laser remelting process, there are still some nanoparticles in the remelted region because of an insufficient time for grains growth.     

Microstructure and microhardness analysis of the hexagonal oxides formed on the surface of the AISI 304 stainless steel after Nd:YAG pulsed laser surface melting

The laser surface melting (LSM) technique was adopted to modify the surface layer microstructure of the AISI 304 stainless steel in this paper. The results showed that the hexagonal morphologies have been successfully fabricated on the surface after LSM. These hexagons had side lengths of about 0.5-1 μm and were characterized by X-ray diffraction (XRD), scanning electron microscope (SEM), field emission scanning electron microscope (FESEM) and high resolution transmission electron microscope (HRTEM). It was proved by the XRD that the stainless steel surface mainly consisted of γ-Fe, Cr₂O₃, Fe₂O₃ and some manganese oxides. The FESEM micrographs showed that the hexagonal oxides were regular hexagons in geometry. The HRTEM micrographs also indicated the presence of the hexagons on the surface of the stainless steel. The spacing values were calculated from the HRTEM micrograph and the SAED pattern, and the hexagonal oxide phases determined by these spacing values were consistent with those verified by the XRD. After LSM, the microhardness of the stainless steel was significantly improved.     

Correlative atomic scale characterisation of secondary carbides in M50 bearing steel

Correlative atom probe tomography (APT) and transmission electron microscopy (TEM) are used to characterise the microstructure and chemistry of carbide precipitation in M50 bearing steel. This is a prerequisite in establishing relationships between the microstructure and hydrogen embrittlement (HE) resistance. Secondary carbides are the focus of this study, as they play a major role in improving HE-resistance. Secondary carbides are observed in APT, with compositions close to M₄C_3, M₂C and M₃C. Correlative TEM measured orientation relationships between the martensite matrix and carbides, enabling the confirmation of M₃C cementite precipitates in the corresponding APT reconstruction. Additionally, other precipitates observed in TEM were correlated to the M₂C carbides in APT data. The M₄C₃ carbides are found to have a significantly lower volume fraction than the M₂C carbides.     

CaO–MgO–SiO2 glass ceramics: Transferred arc plasma (TAP) synthesis and microstructural characterization

In this paper, synthesis of CaO–MgO–SiO₂ glass ceramic using transferred arc plasma (TAP) processing method is illustrated. Homogeneous mixture of 51.6% SiO₂, 35.6% CaO and 12.8% MgO prepared by dry mixing in a ball mill was kept in the anode well (which is the melting bed) of the 10 kW transferred arc plasma torch. It was melted in plasma at an operating power of 5 kW (by varying the processing time for 3, 5 and 8 min). The melt was cooled to solidify by applying forced air on it. The resulting samples were characterized for microstructure and phase composition. The phases were identified by scanning electron microscopy (SEM), using the back-scattered electron (BSE) image mode and X-ray diffraction (XRD) and energy dispersive X-ray analysis (EDX). The microstructure was examined using optical microscopy (OM) and scanning electron microscopy. The micro-hardness, density and porosity measurements for the synthesized samples were carried out. Differential thermal analysis (DTA) was performed to study the thermal evolution. The results show the formation of diopside phase in the transferred arc plasma melted CaO–MgO–SiO₂ glass ceramic system achieved with in a quite considerable short time of plasma processing. The method indicated that TAP technique could be a promising, time saving and one-step manufacturing process for the production of functional bulk glass ceramics.     

Characterization of ultra-fast deposited polycrystalline graphite by a CO2 laser-assisted combustion-flame method

High deposition rate, 750 μm/min, crystalline graphite was deposited on WC substrates by a CO₂ laser-assisted combustion-flame method at laser powers between 300 and 800 W. The structures, which were identified as pillars, were characterized by various methods. The pillars were cylindrical in shape and grew to a size of approximately 3 mm in length and in a few minutes. The laser power did not affect the overall length of the pillar, but caused changes in the physical shape. X-ray and electron diffraction results revealed the pillars to be crystalline graphite regardless of the laser power. Investigation of the pillars by scanning electron microscopy showed two distinct microstructural areas: an inner core of dense material surrounded by an outer shell of lamellar-like material. The core/shell microstructure was unaffected by the level of CO₂ laser power.     

Dry sliding wear behavior of TIG welding clad WC composite coatings

In this study, melted tungsten carbide powders on the surface of AISI 4340 steel was applied by using tungsten inert gas (TIG) method. It was observed that it has been solidified in different microstructures depending on the production parameters. As a result of microstructure examinations, in the surface modified layers an eutectic and dendrite solidification was observed together with WC, W₂C phases. In the layer produced, the hardness values varied between 950 and 1200 HV. The minimum mass loss was observed in the sample, which was treated in 1.209 mm/s production rate, 0.5 g/s powder feed rate and 13.9 kJ/cm heat input.     

Microstructure and wear behavior of γ/Al4C3/TiC/CaF2 composite coating on γ-TiAl intermetallic alloy prepared by Nd:YAG laser cladding

As a further step in obtaining high performance elevated temperature self-lubrication anti-wear composite coatings on TiAl alloy, a novel Ni-P electroless plating method was adopted to encapsulate the as-received CaF₂ in the preparation of precursor NiCr-Cr₃C₂-CaF₂ mixed powders with an aim to decrease its mass loss and increase its compatibility with the metal matrix during a Nd:YAG laser cladding. The microstructure of the coating was examined using X-ray diffraction (XRD), scanning electron microscope (SEM) and energy dispersive spectroscopy (EDS) and the friction and wear behavior of the composite coatings sliding against the hardened 0.45% C steel ring was evaluated using a block-on-ring wear tester at room temperature. It was found that the coating had a unique microstructure consisting of primary dendrites TiC and block Al₄C₃ carbides reinforcement as well as fine isolated spherical CaF₂ solid lubrication particles uniformly dispersed in the NiCrAlTi (γ) matrix. The good friction-reducing and anti-wear abilities of the laser clad composite coating was suggested to the Ni-P electroless plating and the attendant reduction of mass loss of CaF₂ and the increasing of it's wettability with the NiCrAlTi (γ) matrix during the laser cladding process.     

Microscopical study of the formation of adiabatic shear bands in 4340 steel during dynamic loading

In this study, optical microscopy, scanning electron microscopy, transmission electron microscopy, X-ray diffraction and electron probe microanalyser were used to analyse the changes in microstructure of AISI 4340 steel specimens caused by impact at high strain rates and large strains. The structures of the steel prior to dynamic deformation and after dynamic deformation were examined to understand on a microscale level, the mechanism of formation of adiabatic shear bands (ASBs). The study also includes the structural changes that occur during post-deformation annealing processes which may relate to understanding of the mechanism of formation of ASBs. Prior to deformation, the tempered steel specimens consisted of lenticular laths of α-ferrite with precipitated platelet and spherical M₃C carbides. After impact, the structure inside the shear band was characterized by refined and recrystallized grains immersed in dense dislocation structures. In addition, residual carbide particles were observed inside the shear bands due to deformation induced carbide dissolution. Regions away from the shear bands developed ‘knitted’ dislocation walls, evolving gradually into sub-boundaries and highly misoriented grain boundaries at increasing strains, leading to grain refinement of the ferrite. After impact, annealing the shear bands at 350?°C resulted in an increase in hardness regardless of the heat treatment before impact, amount of deformation and the time of annealing. This is because of the occurrence of extensive reprecipitation of dissolved carbides that existed in the steel structure prior to deformation. It is concluded that dynamic recovery/recrystallization, development of dislocation structures and carbide dissolution all contribute simultaneously to the formation of ASBs in quench-hardened steels.     

Solidification microstructure of AZ91D Mg alloy after laser surface melting

The solidification microstructure plays a critical role in determining the surface properties of laser-treated magnesium alloys. The purpose of this paper is to study the solidification microstructures of AZ91D Mg alloy following millisecond- and nanosecond-pulse Nd:YAG laser irradiation. The solidification microstructural evolution of laser-melt AZ91D Mg alloy was investigated using X-ray diffractometry, scanning electron microscopy, energy-dispersive X-ray spectrometer and transmission electron microscopy. Much refined α-Mg phase and β-Mg₁₇Al₁₂ intermetallics were observed in the microstructure after laser surface melting. Periodic and successive structure was observed in the millisecond irradiated surface and the melt depth was more than 100 μm. The solidification microstructure was mainly cellular/dendrite structures together with a large number of β-Mg₁₇Al₁₂ nano-particles. Micron holes were found in the nanosecond irradiated surface and the melt depth was shallow at 50 μm. Millisecond-pulse Nd:YAG laser was found to be more suitable for Mg alloy surface treatment due to sufficient melt depth.     

Phase Composition of Oxidised Layers Grown on Steel Exposed to Liquid Lead at 749 K

The corrosion resistance of AISI 316 LN stainless steel was studied in contact with stagnant, oxygen-saturated liquid lead at 749 K for times up to 1200 h. The reaction products were analysed by means of optical and scanning electron microscopy, electron probe microanalysis, X-ray diffraction analysis and Mössbauer spectroscopy. Thin layers of largely variable thickness formed on the surface, mainly consisting of Fe₃O₄, with small amounts of FeO in the inner regions, Fe₂O₃ and an Fe-Pb-O ternary product in the outermost regions. The alloying elements Cr and Ni diffused to a different extent into Fe₃O₄. A mechanism is proposed to explain formation and growth of the reaction products.