C. Karatas

Laser gas assisted nitriding of alumina surfaces

Abstract The laser control melting can be applied to improve the structural and tribological properties of alumina surfaces. In this case, the laser remelting provides a homogeneous structure while the assisting gas diffuses into molten layer forming the nitride compounds in the irradiated region. Since the process is rapid, local heating with controllable depth and cost effective, the laser remelting provides several advantages over the conventional surface treatment techniques. In the present study, laser melting and gas assisted nitriding of alumina pellets are carried out. Morphological and metallurgical changes after the laser treatment process are examined using scanning electron microscopy (SEM), optical microscopy and X-ray diffraction (XRD), and indentation tests. It is found that two regions are formed in the laser irradiated zone. The first region below the surface is dense and composes of α-Al2O3 and AlN while in the second region, which is below the first region, randomly stacked lamellae structure is observed.

Laser gas assisted melting of preprepared alumina surface including TiC particles at surface

Abstract Laser gas assisted remelting of prepared alumina surface is carried out. In the prepreparation cycle, carbon film containing TiC particles of 50 μm thickness is formed at the alumina surface. Laser controlled melting is carried out on the preprepared alumina surface at high pressure nitrogen environment. The microstructural and morphological changes in the laser treated section are examined using optical microscopy, scanning electron microscopy, energy dispersive spectroscopy and X-ray diffraction (XRD). The microhardness of the resulting surface is measured, and the residual stress developed in the surface is determined from the XRD technique. ABAQUS code is used to predict the temperature and stress fields in the laser treated region. It is found that a dense structure containing undissolved TiC particles is formed in the surface region of the laser treated layer. The nitride compounds of AlN and Al(C, N) are formed at the surface during the laser treatment process. The columnar structure is formed beneath the dense structure. The predictions of residual stress agree with the results obtained from the XRD technique.

Laser Remelting of Zirconia Surface: Investigation into Stress Field and Microstructures

Investigation into the laser remelting of zirconia surfaces in a nitrogen gas environment is carried out. The thermal stress fields during and after the laser treatment process are predicted numerically. The microstructural and morphological changes in the laser-treated region are examined using optical microscopy, scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), and X-ray diffraction (XRD). The residual stresses are determined using the XRD technique. It is found that the residual stress predicted remains high along a depth of 50 µm below the laser-treated surface. The predictions of residual stress agree well with the XRD data. A fine dendritic structure is formed in the vicinity of the surface, which contributes to the surface hardness. In addition, transformation of t-ZrO2 to c-ZrO2 at high temperature is accompanied by the formation of ZrN in the surface vicinity.

Laser gas‐assisted nitriding of steel: residual stress analysis

Purpose – The laser nitriding process is involved with high temperature heating and high cooling rates. This, in turn, results in high levels of thermal stresses in the heated region. Moreover, the residual stress in the heated region remains high after the completion of the heating process, which limits the application of the laser nitriding process. The purpose of this paper is to investigate thermal stresses development and residual stress levels in the nitrided region.Design/methodology/approach – The microstructural changes and residual stress development in the laser gas‐assisted nitrided zone are examined. Finite element modeling is carried out to predict temperature and stress fields in the laser nitrided layer. The indentation tests and X‐ray diffraction (XRD) technique are used to determine the residual stress levels while previously derived analytical formula is used to predict the residual stress levels in the nitrided region.Findings – The residual stress predicted attains values within 230 M...

Microstructure and Thermal Stress Distributions in Laser Carbonitriding Treatment of Ti–6Al–4V Alloy

The results of experiments into laser assisted gas carbonitriding of a Ti-6Al-4V alloy are reported. The temperature and thermal stress fields were simulated using finite element analysis. Microstructural changes in the laser treated region were examined using scanning electron microscopy, energy dispersive X-ray, and X-ray diffraction. In the process, a carbon film was formed at the workpiece surface prior to laser processing and the laser scanning speed was kept constant during the process. It was found that the laser treated layer extended uniformly along the surface; the depth of the layer was about 55 μm. The formation of TiC x N 1-x , TiN, and TiC in the surface region enhances the hardness significantly.

Laser Cutting of Small Diameter Holes Into Alumina Tiles: Thermal Stress Analysis

Laser cutting of small diameter holes in alumina tiles is carried out. Temperature and stress fields are predicted numerically using the A B AQUS finite element code. The cut sections are examined by incorporating scanning electron microscope and optical microscope. The residual stress developed in the cutting section is determined using the X-ray diffraction technique. It is found that high residual stresses are formed in the cutting section, and predictions agree well with the experimental results. The laser cut edges are found to be free from the large cracks. However, interconnected shallow cracks are observed at the hole cut surface.

Laser Cutting of Triangular Geometry into Alumina Tiles: Morphological Changes and Thermal Stress Analysis

Laser cutting of triangular geometry into 5-mm-thick alumina tile is carried out. Temperature and stress fields are predicted by using ABAQUS finite element code in line with the experimental conditions. Experiments are carried out to validate the predictions of temperature and the residual stress in the cutting section. Thermocouples are incorporated in temperature measurements while X-ray diffraction technique is accommodated to obtain the residual stress at the kerf surface. The morphological changes in the cutting section are examined by using optical and scanning electron microscopy, energy dispersive spectroscopy, and X-ray diffraction. It is found that temperature and residual stress predictions agree well with their counterparts obtained from the experiments. The optical and SEM micrographs reveal that the cut sections are free from large size defects such as large-scale cracks and sideways burnings. The maximum value of von Mises stress occurs at the mid-thickness of the workpiece due to the formation of high magnitude of thermally induced strain in this region.

Corrosion Properties and Morphology of Laser Melted Aluminum Alloy 8022 Surface

Laser surface melting of aluminum alloy 8022 is considered and electrochemical studies of the laser-melted and as-received alloy surface are carried out. The surface morphology and metallurgical changes in the laser-melted region are examined using optical microscopy, electron scanning microscopy (SEM), and atomic force microscopy (AFM). Elemental changes in the specimens after the laser-melting process are examined using energy dispersive spectroscopy (EDS), and x-ray diffraction (XRD) is used for assessment of the compound formed after the laser-treatment process. Nitrogen is used as an assisting gas during the laser-melting process to prevent high-temperature oxidation reactions. It is found that the laser-melted surfaces is free from cracks and deep cavities. The oxygen diffusion in the surface region of the melt layer forms Al2O3 compound in the surface vicinity. The corrosion current increases significantly for the laser-melted specimens due to the irregular surface structure. AC impedance results showed a decrease in pores resistant and an increase in pores capacitance. In addition, the surface morphology resulting from the laser melting gives rise to pitting sites at the surface.

Laser assisted nitriding of nickel–chromium-based superalloy surface: Heating and diffusion analysis

Inconel 718 is a nickel–chromium-based superalloy, and it is widely used in power industry because of its resistance to high-temperature environments. Treatment of the alloy becomes essential to prevent niobium segregation at the surface. Laser controlled melting and gas assisted nitriding is one of the methods to minimize changes in the elemental composition of the alloy surface. In general, high pressure nitrogen assisting gas is used coaxially with the laser beam to form a nitride layer and avoiding high-temperature exothermic oxidation reactions in the laser-irradiated region. The present study is carried out to model and simulate sequentially coupled thermal-diffusion process during laser assisted surface nitriding of nickel–chromium-based superalloy in line with experimental conditions. High pressure nitrogen gas jet is considered to impinge onto a workpiece surface coaxially with the laser beam during the treatment process. Finite element model is incorporated to predict the nitrogen concentration and temperature in the laser treated layer. It is found that the predictions of surface temperature and nitriding are found to be in close agreement with the experimental data. The study is extended to include the effect of laser intensity on the nitriding behavior.

Laser Treatment of Rene-41: Thermal and Microstructural Analysis

Laser treatment of Rene 41 surface is carried out at high pressure environment of nitrogen. Temperature and stress fields are predicted using abaqus finite element code. Metallurgical and morphological changes in the laser treated layer are examined using optical and scanning electron microscopes (SEM). The residual stress formed at the surface vicinity is obtained by X-ray diffraction (XRD) technique. It is found that the predictions of the residual stress agree well with the results obtained from the XRD technique. Cellular or cellular dendritic structures with fine secondary dendrites are formed in the laser treated surface due to high cooling rates. In addition, γ′ particles formed are generally in cubic morphology with varying sizes.