Papers by Keyword: Laser Remelting

Abstract: Pores and weak bonding are the inherently drawbacks for thermally sprayed coating. Laser beam is an attractive approach to remelt thermal spray coating for obtaining fully dense coating with metallurgical bonding with substrate. However, defects of holes or cracks are highly inevitable with unmatching remelting processing parameters. In this work, a thermally sprayed Cr₃C₂-NiCr cermet coating by high velocity oxygen fuel spraying was post-processed by laser remelting with a series of varying beam energy densities from 37.5 J/mm² to 225 J/mm². The defect evolution was investigated by both experimental and numerical simulation methods. Large holes and through-thickness cracks were typical defects observed in the remelt coating by optical microscopy. The experimental results show that remelting-induced defects evolve into three stages with laser energy density. The effect of energy density on remelt structure was further verified with the temperature field by numerical simulation with ABAQUS code. The stress field interpreted the crack formation at periodical formation sites. The results on the defect evolution shed light on obtaining functional coatings for industrial applications.

Abstract: Conventional and nanometer aggregate ZrO₂-7wt.%Y₂O₃ ceramic powders taken as raw materials, plasma spraying and plasma spraying-laser remelting compound technology was used to prepare conventional and nanostructured thermal barrier coatings on the TiAl alloy surface. Effects of powder structure (feedstock) and laser remelting on organizational structure and phase of the coatings were analyzed using scanning electron microscope (SEM) and X-ray diffractometer (XRD). Results indicate that: conventional plasma sprayed ceramic coating presents typical lamellar stacking features; plasma sprayed nanostructured coating consists of fully melted region and partially melted region, presenting a two-phase structure. Under the comprehensive impacts of laser power, energy density, temperature field distribution in the laser action region, ceramic heat conductivity coefficient and coating thickness and other factors, the coating presents obvious lamellar structural features after laser remelting; the upper part is compact columnar crystal remelting region and the lower part is residual plasma spraying region. Due to toughening effect of residual nanoparticles in the remelting region of laser remelted nanostructured coating, grain-boundary strength is high and there are a considerable number of transgranular fractures, but the fractures in the remelting region of laser remelted conventional coating are basically intergranular fractures. Conventional plasma sprayed ceramic coating is mainly of tetragonal phase together with a small quantity of monoclinic phases, but nanometer plasma sprayed ceramic coating only has non-equilibrium tetragonal phases. After laser remelting, both conventional coating and nanometer coating mainly have non-equilibrium tetragonal phases with a small quantity of cubic phases.

Abstract: Plasma spraying technique was used to prepare MCrAlY coating on TiAl alloy surface, the coating was treated through the laser remelting process, and then scanning electron microscope (SEM) and X-ray diffractometer (XRD) were used to analyze surface topography and phase composition before and after hot corrosion behavior of the coating. Results indicate that after laser remelting treatment, lamellar structure of the coating disappears with improved compactness and most defects like pores and inclusion eliminated. Plasma sprayed MCrAlY coating can significantly improve hot corrosion property of TiAl alloy, which can be further improved after laser remelting. MCrAlY coating mainly experiences surface oxidation reaction and internal sulfurization reaction during the hot corrosion in the high-temperature molten salt, which mainly generate corrosion products such as Al₂O₃, Cr₂O₃, NiO, NiCr₂O₄, Ni₃S₂ and CrS.

Abstract: The CoCrAlYTaSi/YSZ thermal barrier coating was prepared on the surface of GH586 by plasma spraying and laser remelting process. The coating prepared by plasma spraying, YSZ have no phase transformation. After remelting, YSZ also have no phase transformation, but the grain of the preferential growth orientation changed. Plasma spraying coating is mainly composed of t and t' phase, all the t phase transform into t' phase and recrystallization after remelting. After remelting, the substrate grain and γ' phase size are obviously grown up, leading to the decrease of high temperature oxidation resistance of substrate. In the process of oxidation, no obvious phase transformation are taken place in the plasma spraying coating, and the coating after laser remelting, tetragonal-monoclinic phase transformation happened.

Abstract: Characterization of top surface and microstructure of silicide coatings obtained during diffusion process of pack cementation type and laser remelting treatments was showed in this article. The basic materials were pure Mo sheet and TZM molybdenum alloy. The coatings were deposited in powder process and glazed by different types of lasers. In first step the phase’s compositions of coatings was described by XRD analysis in initial condition and after remelting process. The morphology of the coatings top surface was described as well. All types of coatings were characterized by network of cracks on top surface of the coatings. Laser treatment of the silicide top surface changed morphology of the top surface, especially their cracks network. LM and SEM analysis revealed that internal coatings morphology was very similar in all cases.

Abstract: In this study, Ni60/Ni-WC composite coatings were first prepared by plasma spraying. Then, the coatings were remelted successively with a CO₂ laser. The influences of laser power on the microstructural characterization and microhardness of the coatings were investigated. The results show that the defects of as-sprayed coating like lamellar stacking microstructure and pores were eliminated by laser remelting, and the remelted coating possessed a denser microstructure. With the increase of the laser power, the burning loss and dissolve of the WC particles is increased, while the dilution rate of the coating becomes large. The laser-remelted samples had higher hardness than the as-sprayed one. Laser power has a great impact on the coating and an optimized process parameter is helpful to achieve appropriate melting of WC particles, which leads to retain a high proportion of hard phase in the coating, good combination between the WC particles and Ni-base matrix alloy, and high microhardness.

Abstract: The microstructure and corrosion resistance of Fe-based amorphous coatings prepared by laser remelting after arc spraying were studied. The laser remelting process was carried out under different energy inputs, and the processing parameters varied with the different currents, pulse widths and scanning speeds. The corrosion behavior of the coatings in 1 mol/L NaCl solution was studied through potentiodynamic and potentiostatic polarization test. The morphology and microstructure of the coatings were characterized by using X-ray diffraction (XRD), scanning electron microscopy (SEM), and electrochemical analyzer. A Vickers hardness tester was also used to measure the microhardness of the coatings. The analysis of the microstructure shows that the amorphous coatings are composed of amorphous matrix and nanocrystalline phases. The diffusion of elements indicates a metallurgical bonding between the coating and substrate. The electrochemical corrosion results obtained from the Tafel polarization curves verify that the amorphous composite coatings prepared by different methods show no significant differences in their corrosion resistance, while the microhardness of laser remelting coatings increase obviously with the increase of laser currents. The corrosion resistance of laser remelting coatings is improved extensively due to the amorphous matrix and embedded nanocrystals, which popularizes the applications of amorphous coatings to a large extent.

Abstract: A Fe-based amorphous composite coating was deposited on a carbon steel substrate by arc spraying, and remelted with different laser energies by the Nd: YAG laser cladding system, in order to improve the mechanical properties of the coatings. The microstructure and microhardness of the composite coatings were investigated. The variation of harndness was measured as a function of the modified layer depth, which indicates that the laser remelting improves the bonding strength and hardness. Increasing the laser power, the quality of coating gets better, but the amorphous volume fraction decreases. It is obtained that the optimal laser electric current for the coating of 280 μm thickness is about 300 A, in which the remelted coating with medium energy densities has the highest average Vickers hardness of 741. Through the volume fraction change of the nanocrytals, the hardness of the composite coating is regulated by the laser power input, which amplified the application fields of the amorphous coatings.

Abstract: The Cylinder liner is one of the working environment of the worst parts in internal combustion engine ,it is required to have good wear resistance, corrosion resistance, resistance to high temperature and high pressure shock, etc. After tests showed, better comprehensive mechanical properties of cylinder liner is got by using Plasma Spray composite ceramic nanometer coating technology, and after laser remelting treatment.

Abstract: Welding is a main joint way of the alloy constructional steel. However, the welding HAZ is very sensitive to corrosion. In order to increase the corrosion resistance of the welding HAZ without reducing mechanical property of the parts simultaneously, the laser surface remelting was used to treat the welding HAZ in this study. According to the results of the orthogonal test, the optimized processing parameters were obtained. Based on the electrochemical analysis, the corrosion current of 30CrMnSi welding HAZ can be decreased by laser remelting technology. The corrosion resistance of the HAZ can be improved effectively. At the same time, the mechanical property of the treated welding HAZ was better compared with the untreated.