CO2 laser processing for reducing core loss of cold-rolled grain-oriented silicon steel

Non-contact processing technique involving the use of CW and pulsed CO₂ laser irradiation has been used for reducing the core loss of cold-rolled grain-oriented silicon steel. Laser scribing perpendicular to the rolling direction resulted in a refinement of domain wall spacing which subsequently reduced the loss of silicon steel. It was found that laser irradiation was more effective in a specimen with higher magnetic induction (Hi-B) and the loss was reduced by more than 10% under optimum conditions of the laser irradiation and the scribing speed. Since laser processing is a non-contact technique, it can be easily applied to the production line of the silicon steel.     

Amorphous ribbon domain refinement by scribing

Domain control has been applied to amorphous metal ribbon by mechanical and laser scribing. Mechanically scribed lines were made using a number 88 tungsten carbide scriber with minimum applied pressure. The scribing interval was varied from 0.6 to 10 mm. Laser scribing was performed by using a well-focusedQ-switch YAG laser. The laser power level and scribing interval were varied to obtain optimal magnetics properties. Core loss reductions of 20-25 percent were measured for mechanically and laser scribed Metglas 2605S-2 ribbon. A mechanism of domain refinement is suggested.     

Gas lasers

The important gas lasers which find wide applications in material processing are the CO₂ laser, the argon-ion laser and the excimer lasers. This paper briefly describes the basic concepts and the technology of these lasers.     

Technologie von kompakten Excimerlasern

Technology of Compact Excimer Laser Systems Excimer lasers are pulsed gas lasers. Depending on the gas mixture laser emission is generated on single lines in the UV and VUV portion of the electromagnetic spectra. These emission lines are at wavelengths between 355 nm and 157 nm. The pulse length is normally 5 – 20 ns. Compact Excimer lasers typically have single pulse energy of 10 mJ. The maximum repetition rate is 1 kHz in the compact class. Large Excimer lasers can operate with repetition rates less than 300 Hz, but with single pulse energies up to 1 J. The laser emission is initiated by a pulsed electrical discharge in high pressure gas, having a mixture of rare gases and a halogen. The materials of the laser vessel must be resistant against the Fluorine, or Chlorine. Contaminations must be avoided during assembly and operation of the laser system. Therefore, the newest generation of Excimer lasers is completely metal sealed. All surfaces of the laser vessel – which are exposed to the corrosive laser gas ‐ are specially cleaned and partly coated. By introducing this new technique in the past decade, the lifetime of the laser chamber rose from approximately hundred million pulses up to 10 billion and more. Main applications of compact Excimer lasers are the ophthalmology, the vision correction (LASIK), and metrology for semiconductor manufacturing. Large Excimer lasers are used in industrial applications like surface treatment and as light source in wafer steppers producing highly integrated processors and memory chips.     

Metal Surface Treatment and Reduction in Pitting Corrosion of 304L Stainless Steel by Excimer Laser

Abstract

The use of excimer lasers to produce thin rapidly quenched melts on metal surfaces is discussed. Results obtained using a XeCl laser on a variety of metal samples are given. The surface refinement and smoothing induced by laser treatment, giving improved resistance to pitting corrosion and higher etch resistance suggest significant potential for this method as a means of ‘micro-engineering’ surfaces.     

Microstructure and Transport Properties of Bi-2212 Prepared by CO2 Laser Line Scanning

A novel Laser Line Scanning method has been applied to process rectangular cross-section Bi-2212 monoliths containing 2.9 % Ag using a CO₂ Laser. Although previous work has suggested the use of nIR lasers (≈0.8–1.1 μm) for melt-processing metal oxide superconductors, the results obtained here demonstrate that mid-IR radiation from a CO₂ laser (10.6 μm) may be just as convenient for such a purpose while it enables processing large surface areas. The samples described here were processed at traverse rates ranging between 15 and 60 mm/h, exhibited a complex textured microstructure and yielded highest I _c values of 71 A at 77 K.     

Nano-second UV laser processed micro-grooves on Ti6Al4V for biomedical applications

Laser surface texturing can be used to produce well defined micro-grooves on biomedical materials such as Ti-6Al-4V. Such micro-grooves can be optimized to improve the integration with surrounding tissue. This paper examines the effects of Gaussian shaped beam profiles for nano-second laser processing on the laser micro-groove geometry, topography, and micro-structure of Ti-6Al-4V under atmospheric conditions. Laser and machining parameters such as pulse rate, scan speed, wavelength, groove width and pitch are shown to affect the resulting micro-groove geometries. In contrast to prior micro-groove studies using top-hat beam profiles with ultra-violet (UV) Excimer lasers or large area masking techniques, grooves produced with Nd:YVO₄ exhibit improved roughness parameters and reduced heat-affected zones. Initial processing parameters are established for the fabrication of micro-groove geometries on flat geometries that are relevant to biomedical implants and devices.     

Phase transitions in thin titanium oxide films under the action of excimer laser radiation

We have studied the action of pulsed vacuum ultraviolet (VUV) radiation of an ArF excimer laser (wavelength, 193 nm; pulse duration, 20 ns; pulse energy density, 40–50 J/cm²) on thin amorphous films of titanium oxide (TiO₂) grown by light-assisted molecular beam epitaxy. A comparison of the Raman spectra of samples measured before and after exposure shows that VUV irradiation at increasing fluence induces the crystallization of amorphous TiO₂ to anatase, which is further transformed into rutile.     

On the possibility of increasing and controlling the radiation pulse duration in gas-discharge lasers

The experimental results demonstrating the possibility to increase and control the pulse duration in molecular lasers by means of quasistationary energy pumping into the active laser media are presented. It is shown that the pulse duration in the XeCl, N₂, and CO₂ lasers can be controlled by matching parameters of the energy source and the gas-discharge plasma in the active laser media pumped in a quasistationary regime.     

Laser surface alloying of 1045 AISI steel using Ni–CrB2 powder

Abstract

Laser surface alloying is a process whose purpose is to improve the surface properties by incorporating alloying elements into the surface. The advantages of using laser for surface treatment are: formation of a non-equilibrium or amorphous phase as well as homogenisation and refinement of the microstructure, all without affecting the substrate properties. Powder (50 wt-%Ni–50 wt-%CrB₂) was injected into a melt pool created by a CW–CO₂ laser on AISI1045 steel plates. In order to alloy the entire surface, the sample was scanned at scan speeds in the range of 600–6000 mm min^–1 and the laser power was in the range of 1750–2500 W. The powder feed rate was 1·6 g min^–1, the laser beam was 2 mm in diameter, with 60% overlap between successive laser paths. Metallographic cross-sections were made of the samples. For each sample the following properties were characterised: layer depth, microhardness (HV), layer microstructure and composition. It has been found that the scan speed and the laser power affect the depth of the melt pool, the microstructure, the hardness and the treated layer composition. The laser boronised surface exhibits better wear resistance than D2 tool steel hardened to 59 ± 1 HRC. This will be discussed based on numerical analysis of the laser/material interaction.     

Pulsed laser annealing of sodium super ionic conductor for carbon dioxide sensors

This paper discussed a way to improve solid electrolyte carbon dioxide (CO₂) sensor by excimer laser annealing of sodium super ionic conductors (NASICON). The CO₂ sensor used in this paper consists of a thin NASICON layer. We additionally annealed the NASICON to improve its electrical conductivity by pulsed excimer laser. The laser annealing results in re-crystallization of the NASICON thin film and changes the conductivity, grain sizes, and the structure of grain boundaries. From the scanning electron microscope pictures, we saw that NASICON grain sizes were enlarged after laser annealing. Grain sizes were also correlated to laser annealing energy and annealing times. After 2 times annealing of 420 mJ laser energy with 7 pulses each time at 1 Hz repetition rate, the conductivity of NASICON was increased by 90%. When the CO₂ concentration was changed from 1000 ppm to 5000 ppm, the sensor resolution was enhanced up to 66%. These results suggested that appropriate laser annealing treatment not only enlarges NASICON grain sizes but also reduces its resistance. Therefore, the NASICON CO₂ sensor resolution can be improved accordingly.     

LASER PROCESSING OF DIAMOND AND DIAMOND-LIKE FILMS

Laser processing of polycrystalline diamond and amorphous carbon films is shown to be a promising technology for micropatterning of these materials in electronics and other applications. By using excimer lasers, holes and pits have been formed in 10-60 μm thick diamond films by physical etching with ablation rates of up to 300 nm/pulse. The channels of micrometer scale width were created in diamond-like carbon films on silicon by chemical etching in oxygen by the scanning with a cw Ar⁺ laser beam. At laser powers below the etching threshold, a transformation of amorphous carbon to graphitic carbon allows the formation of conductive lines of different geometry in dielectric carbon layers.     

Laser Gas Nitriding of Ti-6Al-4V Part 1: Optimization of the Process

A multi-variable test method, known as the Orthogonal Array, was used to optimize the parameters for the laser gas nitriding process (LGNP) to avoid surface cracking. Based on the fundamental requirement of a crack-free condition, the processing parameters were further optimized to improve the surface finish and to obtain a reasonable hardened depth. The effects of processing parameters have also been investigated with respect to the characteristics of the laser nitrided layer. Two types of lasers, i.e., CO₂ and Nd:YAG lasers were used. The CO₂ laser was operated in both the continuous as well as the pulse mode, while the Nd:YAG laser was used only in the pulse mode. A Nd:YAG laser in the pulse mode provided a better surface finish and lower cracking severity.     

Unburned carbon measurement in fly ash using laser-induced breakdown spectroscopy with short nanosecond pulse width laser

The unburned carbon in fly ash is one of the important factors for the boiler combustion condition. Controlling the unburned carbon in fly ash is beneficial for fly ash recycle and to improve the combustion efficiency of the coal. Laser-induced breakdown spectroscopy (LIBS) technology has been applied to measure the fly ash contents due to its merits of non-contact, fast response, high sensitivity, and real-time measurement. In this study, experimental measurements have been adopted for fly ash flows with the surrounding gases of N₂ and CO₂, while the CO₂ concentration varified to evaluate the CO₂ effect on the unburned carbon signal from fly ash powder. Two kinds of pulse width lasers, 6?ns and 1?ns, were separately adopted to compare the influence of laser pulse width. Results showed that compared with that using 6?ns pulse width laser, plasma temperature was lower and had less dependence on delay time when using 1?ns pulse width laser, and spectra had more stable background. By using 1?ns pulse width laser, the emission signal from surrounding CO₂ also decreased because of the less surrounding gas breakdown. The solid powder breakdown signals also became more stable when using 1?ns pulse width laser. It is demonstrated that 1?ns pulse width laser has the merits for fly ash flow measurement using LIBS.     

Microstructural studies in low specific energy laser surface treated Al(A356)-SiCP composites

SiC particulate reinforced A356 Al metal matrix composites were laser treated using pulsed Nd-YAG laser beam. The processing was carried out in air atmosphere at varying pulse energy (5 to 20 J) and scan rates (30 to 150 mm/sec). The samples were cut perpendicular to the track and they were characterized using optical microscopy, scanning electron microscopy and X-ray diffraction. Microstructure of laser treated region consists of regular succession of coarse and fine microstructure signifying the presence of low velocity bands. Transition from cellular/columnar dendritic to equiaxed dendritic structure has been observed. Microstructure of samples laser treated with specific energy greater than 13 kJ/cm² show presence of Al₄C₃ platelets.     

Shack-Hartmann sensor for fast infrared wave-front testing

Abstract

A Shack-Hartmann sensor has been designed for testing the wave front of CO₂ lasers. Fabrication of a lens array and a detector array with tight tolerances on position accuracy are essential steps. Parallel electronics allow for high-speed wave-front measurements with 1 kHz sampling frequency. The device has been used to investigate the behaviour of a high-power CO₂ laser. Besides the expected thermal drifts of beam direction at the beginning of laser action, periodic changes of beam direction, have been detected. The Shack-Hartmann sensor seems the appropriate device for controlling adaptive optics in high-power laser applications.     

Fatigue performance and residual stresses in laser treated 50CrV4 steel

The effects of continuous CO₂ laser surface quenching of one side of a rectangular section of 50CrV4 steel samples are studied. The treatment is carried out in one or several passes, working with a square beam with a uniform power density of 2550 W cm^–2 at scan rates between 700 and 1400 mm min^–1, with either a single or overlapped sweep of the beam. The magnitude and distribution of internal longitudinal stresses created by the uniform treatment of the surface, relating them to the harness gradients created in depth, conditioned by structural variations, are considered. The uniform treatment of the surface leads to considerable improvement in fatigue performance. In the presence of the thermally affected zones produced by successive sweeps of the beam, however, the noted improvement is largely lost.     

Laser Gas Nitriding of Ti-6AI-4V Alloy

Laser gas nitriding of Ti-6A1-4V has been investigated with both CO₂ and Nd:YAG lasers. Results indicate that Nd:YAG laser in pulse mode provides a better surface finish and a lower cracking severity than CO₂ laser. A crack-free nitrided layer has been obtained by optimizing the processing parameters. Titanium nitride (TiN) significantly increases the hardness of the nitrided surfaces. The amount of titanium nitride produced depends on the processing parameters, such as laser pulse energy and nitrogen concentration. With optimized parameters, the nitrided surface is somewhat rougher than the polished base metal but much smoother than the shot peened surface. The shrinkage effect in the laser melt zone produces surface residual tensile stresses regardless of the processing environment. Preheating or stress relieving after laser nitriding can significantly reduce the residual tensile stress level.     

Raman and Atomic Resonance Lasers in Optically Excited Thallium (6P 3/2 1/2)-xenon Mixtures

Raman and atomic resonance lasers have been observed in optically excited thallium (6P_1/2 ?6P _3/2)-xenon mixtures. T1(6P _1/2) or T1(6P _3/2) atoms were produced by selective photodissociation of TlCl at 308 or 193 nm respectively in the presence of a high pressure of xenon. A pulsed dye laser was used to optically excite the resulting Tl-Xe quasi-molecules via the quasi-molecular absorptions in the vicinity of the Tl(6P_1/2, 6P_3/2-7S_1/2) atomic resonance transitions. The possibility of realizing an optically pumped Tl-Xe excimer laser is discussed.     

Dual-beam CO2 laser cutting of thick metallic materials

A dual-beam technique involving two CO₂ gas lasers with a power capacity of 1.5 kW each, was used to cut steel and superalloy. A comparison with single-beam CO₂ laser cutting showed that dual beams were capable of enhancing the cutting thickness and speed without deteriorating the quality of cut. Heat-conduction models, assuming the laser beams as line sources, were used to estimate the cutting thickness and speed as a function of distance between the two laser beams. Experimental data, coupled with theoretical modelling, have provided a new concept, namely stretching the width of the laser beam in the direction of cutting to cut thicker section solids at moderate speeds.