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 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.     

Modification of tribology and high-temperature behavior of Ti-48Al-2Cr-2Nb intermetallic alloy by laser cladding

In order to improve the tribology and high-temperature oxidation properties of the Ti-48Al-2Cr-2Nb intermetallic alloy simultaneously, mixed NiCr-Cr₃C₂ precursor powders had been investigated for laser cladding treatment to modify wear and high-temperature oxidation resistance of the material. The alloy samples were pre-placed with NiCr-80, 50 and 20%Cr₃C₂ (wt.%), respectively, and laser treated at the same parameters, i.e., laser output power 2.8 kW, beam scanning speed 2.0 mm/s, beam dimension 1 mm × 18 mm. The treated samples underwent tests of microhardness, wear and high-temperature oxidation. The results showed that laser cladding with different constitution of mixed precursor NiCr-Cr₃C₂ powders improved surface hardness in all cases. Laser cladding with NiCr-50%Cr₃C₂ resulted in the best modification of tribology and high-temperature oxidation behavior. X-ray diffraction (XRD), optical microscope (OM), scanning electron microscopy (SEM) and energy-dispersive spectrometer (EDS) analyses indicated that the formation of reinforced Cr₇C₃, TiC and both continuous and dense Al₂O₃, Cr₂O₃ oxide scales were supposed to be responsible for the modification of the relevant properties. As a result, the present work had laid beneficial surface engineering foundation for TiAl alloy applied as future light weight and high-temperature structural candidate materials.     

Wear properties of compact graphite cast iron with bionic units processed by deep laser cladding WC

By simulating the cuticles of some soil animals, the wear resistance of compact graphite cast iron (CGI) processed by laser remelting gets a conspicuous improvement. In order to get a further anti-wear enhancement of CGI, a new method of deep laser cladding was used to process bionic units. By preplacing grooves then filling with WC powders and laser cladding, the bionic units had a larger dimension in depth and higher microhardness. Fe powder with different proportions from 30% (wt.) to 60% (wt.) was added into WC before laser processing for a good incorporation with CGI substrate. The improved laser cladding units turned out to induce higher wear resistance in comparison with laser remelting ones. The depth of the layer reached up to 1 mm. The results of dry sliding wear tests indicated that the specimen processed by laser cladding has a remarkable improvement than the ones processed by laser remelting. It should be noted that the wear mass loss was essentially dependent on the increase in WC proportion.     

Effects of Ta on microstructure and microhardness of Ni based laser clad coating

Through addition of Tantalum, fine TaC particles were in situ synthesized in a NiCrBSi alloy laser clad composite coating. Microstructure, microhardness and abrasive wear resistance of the composite coating were investigated. The result showed that TaC particles were dispersed in Ni based alloy composite coating, refining the microstructure of the coating after laser cladding. Amount of coarse primary carbides such as M7C3 and eutectic of γ-Ni + M23C6 substantially decreased because the formation of TaC particles suppressed the formation of M7C3 and M23C6. On the one hand, fine TaC particles acted as hard phase, which improved the microhardness of the composite coating; on the other hand, a decrease in amount of the coarse M7C3 and eutectic of γ-Ni + M23C6 reduced the crack susceptibility of the Ni based composite coating. Also, Ta element improved the abrasive wear resistance of the Ni based coating.     

Study on Fe-Al-Si in situ composite coating fabricated by laser cladding

Fe-Al-Si in situ composite coating was fabricated on the surface of ASTM A283Gr.D steel by laser cladding with the preplaced powder. The influence of powder composition, laser power and scanning speed on microstructure, microhardness and wear resistance were investigated in this paper. The results show that Fe-Al-Si in situ composite coating with the good metallurgical bond mainly consists of Fe, SiO₂ and Al₂Fe₃Si₄ intermetallic compound. With the increase of laser power and scanning speed, the grain size of coating gets the minimum value. With the increase of laser power and scanning speed, microhardness and wear resistance both get the peak vaule, and their value are three times and 3.5 times those of substrate, respectively. The optimum parameters are followed as: the ratio of the preplaced composite powder: 8:1:1, laser power: 1600 W and scanning speed: 400 mm/min.     

Effects of processing parameters on structure of Ni-based WC composite coatings during laser induction hybrid rapid cladding

The relationships between the processing parameters (i.e. laser specific energy, powder density, preheated temperature of substrate and types of substrate) and the structure characteristics of Ni-based WC composite coatings during laser induction hybrid rapid cladding (LIHRC) were investigated systematically. The results show that laser specific energy, cladding height and contact angle have a linear relation with powder density, as can provide the predictions of laser processing parameters according to the geometrical characteristics of a single laser track (i.e. cladding height, cladding width). Moreover, dilution of composite coating increases with the increasing of laser specific energy and the preheated temperature of substrate, while reduces with the increasing of powder density. The types of substrate also have an important effect on dilution of composite coating, as has a strong dependence on the thermophysical properties of substrate (i.e. melting point, resistivity and permeability).     

Study on fast linear scanning for a new laser scanner

In the field of lidar system design, there is a need for laser scanners that offer fast linear scanning, are small size and have small a rotational inertia moment. Currently, laser scanners do not meet the above needs. A new laser scanner based on two amplified piezoelectric actuators is designed in this paper. The laser scanner has small size, high mechanical resonance frequencies and a small rotational inertia moment. The size of the mirror is 20 mm×15 mm. To achieve fast linear scanning performance, an open-loop controller is designed to compensate the hysteresis behavior and to restrain oscillations that are caused by the mechanical resonances of the scanner's mechanical structure. By comparing measured scanning waveforms, nonlinearities and scan line images between the uncontrolled and controlled scanner, it was found that the scanning linearity of linear scanning was improved The open-loop controlled laser scanner realizes linear scanning at 250 Hz with optical scan angle of ±12 mrad.     

Phase composition and tribological properties of Ti-Al coatings produced on pure Ti by laser cladding

Ti-Al coatings with ∼14.7, 18.1, 25.2 and 29.7 at.% Al contents were fabricated on pure Ti substrate by laser cladding. The laser cladding Ti-Al coatings were analyzed with X-ray diffraction (XRD), scanning electron microscope (SEM) and X-ray energy dispersive spectroscopy (EDS). It was found that with the increase of Al content, the diffraction peaks shifted gradually to higher 2θ values. The laser cladding Ti-Al coatings with 14.7 and 18.1 at.% Al were composed of α-Ti and α₂-Ti₃Al phases, while those with 25.2 and 29.7 at.% Al were composed of α₂-Ti₃Al phase. With the increase of Al content, the cross-sectional hardness increased, while the fracture toughness decreased. For the laser cladding Ti-Al coatings, when the Al content was ≤18.1 at.%, the wear mechanism was adhesive wear and abrasive wear; while when the Al content ≥25.2 at.%, the wear mechanism was adhesive wear, abrasive wear and microfracture. With the increase of Al content, the wear rate of laser cladding Ti-Al coatings decreased under 1 N normal load, while the wear rate firstly decreased and then increased under a normal load of 3 N. Due to its optimized combination of high hardness and high fracture toughness, the laser cladding Ti-Al coating with 18.1 at.% Al showed the best anti-wear properties at higher normal load.     

Combined laser/sol-gel synthesis of calcium silicate coating on Ti-6Al-4V substrates for improved cell integration

New studies have shown that tricalcium silicate powder is a bioactive material and can encourage bone-implant integration. This paper reports the synthesis of Ca₂SiO₄ coating on Ti-6Al-4V samples by laser irradiation under submerged conditions. The results of using a 160-1500 LDL 1.5 kW diode laser (rectangular spot = 2.5 mm × 3.5 mm, λ = 808 and 940 nm with equal intensities) is reported. A number of experiments were carried out varying laser parameters, such as scanning speed and laser power. Coatings are evaluated in terms of microstructure, elemental composition (XRD), SEM and wettability. The in vitro biocompatibility of the samples is investigated by monitoring 2T3 osteoblast cell growth on the samples.     

Crystal growth,structural, linear and nonlinear optical studies of 4-methyl-4′-N’-methylstilbazolium tosylate single crystals

Organic 4-methyl-4′-N’-methylstilbazolium tosylate, a new derivative of the stilbazolium tosylate family compound was synthesized by condensation method. The optical quality single crystals with dimension 5 mm × 4 mm × 2 mm were grown by slow evaporation technique at 40 °C. The crystal system and lattice parameters were found from single crystal X-ray diffraction studies. The optical transmittance, cut-off wavelength (402 nm) and band gap energy (3.09 eV) were estimated by UV–visible studies. The surface laser damage threshold study was carried out for MMST crystal using Nd:YAG laser. The third-order nonlinear optical susceptibility (χ³) for MMST crystal was estimated by employing Z-scan technique using 1064 nm laser.     

New aspects on pulsed laser deposition of aligned carbon nanotubes

We have grown carbon nanotubes (CNT) by pulsed laser deposition (PLD) at 1000 °C in Ar atmosphere. A Nd/YAG laser was used for irradiation of a graphite target containing Ni and Co rods. High-resolution scanning electron microscopy (HRSEM) and transmission electron microscopy (TEM) images showed that “closed” carbon nanotubes were grown between clusters of metallic particles, so that the individual nanotubes were arranged in parallel to each other forming a shape of “Rope-Bridge”. The nanotubes structure was analyzed by high-resolution transmission electron microscopy (HRTEM) and their type was found to be of MWNT, containing about five SWNT. Total diameter was 5-20 nm and their length was about 1 μm. High homogeneous distribution carbon nanotubes were grown and different structures were observed such as well-aligned carbon nanotubes, bamboo-like and Y-junction carbon nanotubes.     

Parametric analysis of the selective laser melting process

Selective laser sintering/melting (SLS/SLM) technology is used for manufacturing net-shaped objects from commercial Inox 904L powder with ≤20 μm particle size. Experiments were carried out on PHENIX-PM100 machine equipped with a 50 W cw fiber laser. Powder is layered by a roller over the surface of a 100 mm-diameter build cylinder. Optimal parameters of layer thickness and power input per unit speed for SLM were determined. It was shown that the greater the value of P/V ratio is, the larger is the remelted line (called as “vector”). Influence of the shifting of consecutive single vectors on the process of forming the first layer was studied. Different strategies for forming objects with less than 1 mm-sized inner structures were tested, as, for example, forming a 20 mm × 20 mm × 5 mm box with 140 μm-thick inner compartment walls.     

Beam concentration and homogenization for high power laser diode bar

A novel optical element is presented and applied for beam concentration and homogenization of laser diode (LD) bar. It consists of a tapered SiO₂-rod with twisted surfaces which is designed and optimized by the optical system design software and fabricated by a curved surface grinder and an optical polishing lathe. Results show that a rectangular output beam spot with uniform intensity distributions both at slow and fast axis is obtained at the output facet of the rod and the beam size is only 1.3 mm ∗ 0.8 mm.     

The effect of laser remelting in the formation of tunable nanoporous Mn structures on mild steel substrates

Nanoporous manganese was fabricated by a three-step process involving high power laser cladding of a homogeneous Cu₄₀Mn₆₀ alloy coatings onto a mild steel substrate, laser remelting for tuning the grain size and the composition homogeneity followed by selectively electrochemical de-alloying for removal of Cu element and formation of nanoporous Mn. The microstructure and homogeneity of the precursor Cu₄₀Mn₆₀ alloys have a significant influence on the evolution of nanopores during selectively electrochemical de-alloying. Laser remelting can significantly refine the microstructure. The second dendrite arm spacing decreases with increasing of laser remelting scanning speed. A SDAS of 1.17 μm was obtained at the laser scanning speed of 133 mm/s. When the remelting scanning speed reaches 100 mm/s, a nanoporous structure with average pore size less than 100 nm was achieved under optimized dealloying electrode current density about 2 mA/cm². Nanoporous Mn with nanopore sizes ranging from 80 to 130 nm was fabricated by this method. Surface-enhanced Raman scattering characteristics of the nanoporous materials have been investigated. It is found that smaller nanoporosity leads to significant improvements in surface-enhanced Raman scattering.     

Surface treatment with linearly polarized laser beam at oblique incidence

An effective method for surface heat treatment with 10.6 μm linear polarized laser beam at oblique incidence is reported. A circular focused laser spot on the workpiece surface, simultaneously with 2.2–4 times increasing of the absorption are obtained in the 70–80° range of the incidence angle. The main element of the experimental setup is the astigmatic focusing head which focalize the laser beam into an elliptical spot of ellipticity >3 at normal incidence. At a proper incidence angle (obtained by the focusing head tilting) the focused laser spot on the work piece surface gets a circular form and p-state of polarization is achieved.We performed laser heat treatment (transformation hardening, surface remelting) of the uncoated surface, as well as the alloying and cladding processes by powder injection. An enhancement of the processing efficiency was obtained; in this way the investment and operation costs for surface treatment with CO₂ laser can be significantly reduced. Several technical advantages concerning the pollution of the focusing optical components, powder jet flowing and reflected radiation by the work piece surface are obtained.     

Microstructure and phase transformations in laser clad CrxSy/Ni coating on H13 steel

Laser cladding was carried out onto H13 steel with preplaced NiCrBSi+Ni/MoS₂ powders using CO₂ laser under the optimized experimental parameters of laser power 2 kW, scanning velocity 6 mm/s and laser beam diameter 3 mm. An X-ray diffractometer and scanning electron microscope with energy dispersive spectroscopy were applied to analyze the microstructure and phase compositions of the coating. Thermodynamic calculation was performed with Thermo-Calc software on the basis of a commercially available Ni-based Alloys׳ database. The experimental results show that MoS₂ decomposed and S reacted with Cr to form nonstoichiometric Cr_xS_y during the laser cladding process. The coating consists of spherical Cr_xS_y particles, primary γ-Ni dendrite, interdendritic eutectic (γ-Ni+NiMo) and precipitated NiMo. The precipitated NiMo was fine and uniformly distributed in primary γ-Ni dendrite. The calculated results and experimental data indicate that the solidification process in the coating during laser cladding process was liquid→liquid+Cr_xS_y→ liquid+Cr_xS_y+γ-Ni→liquid+Cr_xS_y+γ-Ni+ eutectic (γ-Ni+NiMo). A solid state phase transformation (fine and uniformly distributed NiMo precipitated from γ-Ni) occurred after the solidification process. The calculations agree well with the experimental data and it is helpful to understand the phase transformation and microstructure evolution in the coating.     

Investigation of temperature and stress fields in laser cladded coatings

Temporal and spatial distributions of temperature and strain-stress have been modelled and investigated experimentally for the laser cladding process. The model corresponded to experimental conditions where the multilayer protective coatings were prepared by direct laser cladding of stellite SF6 powder on X10Cr13 chromium steel by means of a 1.2 kW CO₂ laser. For calculations the effect of base preheating, temperature dependent material properties, and also influence of time-break between cladding of the consecutive layers were taken into account. The calculated temperature fields indicated good bonding of the substrate and coating, which was in agreement with the micro-analytical test results. A decrease of the number of microcracks in the coating with an increase of substrate preheating temperature was concluded from stress calculations and confirmed in the experiment. Moreover, an increase of the cracking susceptibility with an increase of the time delay between cladding of the consecutive layers was evidenced by modelling. The best technological results were obtained for the case of single-layer coatings prepared on a preheated substrate and for higher coating thickness required the processing of consecutive layers with a possibly short time delay is advisable due to effective usage of laser beam energy for preheating and lower temperature gradients.     

In situ Video-STM study of the potential-induced (1 × 1) → “hex” transition on Au(1 0 0) electrode surfaces in Cl containing solution

The potential-induced (1 × 1) → “hex” transition on Au(1 0 0) electrodes in 0.01 M Na₂SO₄ + 1 mM HCl was studied by in situ scanning tunneling microscopy at high time resolution (Video-STM). According to these observations the elementary units of the “hex” surface reconstruction, hexagonally-ordered strings in the Au surface layer, are highly dynamic nanoscale objects. Isolated “hex” strings exhibit dynamic fluctuations in structure and position on the millisecond timescale. These fluctuations exceed the mobility of multistring “hex” domains by several orders of magnitude and can be explained by collective dynamic processes within the strings. Furthermore, the observations reveal a novel 1D mass transport mechanism along the strings, details on the nucleation and growth of “hex” strings and complex string restructuring processes, facilitating “hex” domain ripening.     

A study on wear resistance and microcrack of the Ti3Al/TiAl + TiC ceramic layer deposited by laser cladding on Ti-6Al-4V alloy

Laser cladding of the Al + TiC alloy powder on Ti-6Al-4V alloy can form the Ti₃Al/TiAl + TiC ceramic layer. In this study, TiC particle-dispersed Ti₃Al/TiAl matrix ceramic layer on the Ti-6Al-4V alloy by laser cladding has been researched by means of X-ray diffraction, scanning electron microscope, electron probe micro-analyzer, energy dispersive spectrometer. The main difference from the earlier reports is that Ti₃Al/TiAl has been chosen as the matrix of the composite coating. The wear resistance of the Al + 30 wt.% TiC and the Al + 40 wt.% TiC cladding layer was approximately 2 times greater than that of the Ti-6Al-4V substrate due to the reinforcement of the Ti₃Al/TiAl + TiC hard phases. However, when the TiC mass percent was above 40 wt.%, the thermal stress value was greater than the materials yield strength limit in the ceramic layer, the microcrack was present and its wear resistance decreased.