Thermochemical modelling in CO2 laser cutting of carbon steel

A thermochemical heat transfer model in oxygen-assisted laser cutting of carbon steel has been developed in terms of the laser mode pattern, the power density, combustion reaction, kerf width and cutting speed. This model emphasizes the chemical combustion effect as well as the laser mode pattern, which are usually neglected by most existing laser cutting models. Good agreement was obtained between theoretical and experimental results, indicating that approximately 55–70% of the cutting energy is supplied by the combustion reaction of the steel with oxygen, which is consistent with experimental data obtained by other investigators.Nomenclature a Focused laser beam diameter (m) - A Absorptivity - H Heat of combustion (J kg^–1) - I Power density (Wm^–2) - k Thermal diffusivity (m²s^–1) - K Thermal conductivity (Wm^–1K^–1) - K ₀ Modified Bessel function of the second kind and zeroth order - l Workpiece thickness (m) - P Laser power (W) - q Heat rate (W) - q q/l heat rate per unit length (Wm^–1) - R Half the kerf width (m) - s VR/2 normalized cutting speed - T _mp Melting temperature (K) - T _rm Room temperature (K) - T(x, y) Temperature at (x, y) (K) - V Cutting speed (ms^–1) - W 2R kerf width (m) - x, y, z Cartesian co-ordinates - Thermal diffusivity (m² s^–1) - Average thickness of liquid melt film (m) - Combustion efficiency - , Polar co-ordinates - Material density (kgm^–3)     

CO2 laser annealing of plasma-deposited high-T c superconducting thick films

Recent advances in the fabrication of high-T _c superconducting thick films demand processing techniques which can eliminate film/substrate interdifiusion that occurs during subsequent post-annealing heat treatment after the film is deposited, thereby limiting the application of the thick films for devices. The present study evaluates laser annealing techniques for plasma-deposited Y-Ba-Cu-O thick films using a high-energy CO₂ laser (10.6Μm) in a continuous wave mode. The results are compared with those obtained by conventional furnace annealing techniques necessary for post-heat treatment of as-deposited superconducting thick films. The high-T _c superconducting phase is recovered by cationic diffusion during subsequent post-annealing heat treatment. Crystallographic phases and microstructural characterization have been performed using XRD, SEM, and EPMA analytical techniques. The significance of the technology lies in the elimination of film/substrate interdiffusion problems, thereby resulting in high-quality superconducting thick films. The technology will significantly reduce the post-annealing times usually required by conventional furnace annealing techniques.     

CO2激光辅助加工PMMA微流控芯片中的重铸物现象

根据实验现象分析了CO2激光制作PMMA微流控芯片储液池过程中,材料的流动与变形过程,以及凸起重铸物形成对芯片制作的影响.考察了激光功率、扫描速度以及材料取向对流变行为的作用及其机理.实验中采用不同的激光光束功率以及扫描速度,分析热量吸收的大小对材料流动与变形的影响,比较了挤出PMMA在顺着挤出方向与垂直于挤出方向切割时实验结果的不同.CO2激光加工PMMA过程中,材料在激光的热作用下发生软化、熔化及汽化.软化材料在汽化材料产生的气体压力作用下发生流动与变形,形成了沟道表面的凸起重铸物.实验结果表明热量及材料内部存在的取向对其流动与变形及凸起重铸的形成起着重要的作用.     

Infrared optical constants for CO2 laser waveguide materials

The infrared optical constants of a number of commercially available glasses and ceramics have been determined by Kramers-Krönig analyses of the reflectance spectra of the materials at near normal incidence. The data derived, the refractive indexn, and extinction coefficientK, have been further analysed to provide an assessment of these materials as hollow waveguides at mid-infrared frequencies. The results demonstrate that the frequency dependence of waveguide transmission is a feature of anomalous dispersion. Furthermore, in the case of beryllia and alumina ceramics, the high transmissions predicted in straight guide at 10.6m can be attributed to the low refractive indices measured for these materials. Some experimental data are presented which substantiate the frequency characteristics of the predicted waveguide transmission.     

Theory of cutting materials with a laser beam

In strict three-dimensional statement the article investigates the steady and nonsteady problems of the shape and depth of cut originating on the surface of material under the effect of a powerful laser beam in dependence on the speed of the specimen, the direction of the beam, the power and polarization of the radiation. The known local approximation is used where the energy of destruction of unit volume is taken to be constant. The theoretical results are compared with known experimental data.Translated from Problemy Prochnosti, No. 2, pp. 56–62, February, 1991.     

CO2 laser deposition of diamond thin films on electronic materials

Laser-induced pyrolysis has been utilized to create gas-surface chemical reactions necessary for diamond deposition on electronic materials. A 1200 W CO₂ gas laser has been used as an energy source for depositing diamond thin films from a gas mixture of CH₄ and H₂ in a chemical vapour deposition chamber. The substrate temperature was about 500°C. The laser beam energy was largely absorbed by the gases that lead to their excitation and decomposition on contact with the nearby hot substrate. Raman spectroscopy and scanning electron microscope analysis revealed high quality, fine crystalline diamond structures.     

Experimental study on the CO2 laser cutting of carbon fiber reinforced plastic composite

Promotion of massive application of carbon fiber reinforced plastics (CFRPs) in the industry can be accomplished by using faster and more flexible technologies such as laser cutting. The anisotropic and heterogeneous features of the CFRP make laser processing very challenging.A comprehensive study on the cut performance of a CO₂ laser to process sheets (3 mm thick) of a CFRP composite is presented. A high-beam quality CO₂ laser has been used in order to ascertain the capabilities of CO₂ laser cutting machines, widely used in metalworking applications, on the machining of this material. On the other hand, the influence of processing parameters, in both CW and pulsed mode, on the cut quality was studied.Cuts with a minimum heat affected zone, about 540 μm, were achieved using a high-beam quality CO₂ laser working in pulsed mode. In consequence, the CFRP strength remains practically unaffected compared to more conventional mechanical machining.     

On chip formation in cutting metallic materials using tools with a large negative rake

It has been noted that the chip formation mechanism in metal cutting with edge tools with a large negative rake angle γ is not fully understood. Methods for assessment of some chip formation characteristics are put forward and examples are given of how they are implemented during the comparison between calculated and experimental values for the case of cutting lead workpieces using tools with a rake angle γ ranging from zero to −60°.     

A review of laser fabrication of metallic engineering components and of materials

Abstract

The state of the art and directions for the future development of two laser based technologies, direct laser fabrication in which powder is fed into the focal point of a laser, and a laser powder bed technology are outlined in this review. The areas in which these technologies have made significant contributions are: the manufacture, directly from powder, of alloys and of functionally graded materials which enables a range of compositions to be assessed rapidly and the manufacture of net shape and the repair of engineering components. It is suggested that rapid assessment of structure/property relationships in a range of compositions will continue to be a useful application of laser fabrication. It is further concluded that the two approaches, direct laser fabrication and laser powder bed will continue to be developed since each has its own advantages and disadvantages; direct laser fabrication is the preferred technique for alloy development work and for component repair but laser bed technology is currently the preferred technology for the manufacture of small components which require a good surface finish. Improvements in surface finish, in dimensional accuracy, in microstructural control and in process control with real time feedback to control properties are nevertheless required if these technologies are to increase their impact in the area of the manufacture of net shape components.     

Modification of SiO2 nanowires with metallic nanocrystals from supercritical CO2

Through hydrogen reduction of metal precursors in supercritical CO2, Cu, and Pd, nanocrystals were deposited onto SiO2 nanowires to form different types of nanostructured materials, including nanocrystal-nanowire, spherical aggregation-nanowire, shell-nanowire composites, and "mesoporous" metals supported by the framework of nanowires. This supercritical fluid deposition technique is an attractive approach for modifying nanowires because of its generality and simplicity; the modified nanowires could be useful as catalysts and for further fabrication of multifunctional composites.     

CO2 laser annealing of Al-GaAs structure

A CO₂ laser treatment of Al contacts onto GaAs is presented which leads to ohmic contacts. Electron microscopy shows however that the surface is not homogeneous.     

Additional possibilities offered by using porous metallic materials for cooling laser diode bars

A new scheme of using porous metallic materials (PMs) for cooling laser diode bars is proposed, which provides for a two-dimensional regime of heat removal and reduces the effective filtration length of a cooling agent. The values of characteristic limiting thermal fluxes are calculated for the heat sink made of a permeable PM, operating under conditions of a maximum permissible temperature of the active layer in a laser diode bar.     

Comparative study of jetting machining technologies over laser machining technology for cutting composite materials

There has been a great interest for improving the machining of composite materials in the aerospace and other industries. This paper focuses on the comparative study of jetting techniques and laser machining technics. This paper concentrates on the machining of composite materials like epoxy pre-impregnated graphite woven fabric and fibre reinforced plastic materials that are used in aerospace industries. While considering machining these materials with the traditional machining there are many disadvantages projected. One of these advantages is that all the traditional machining processes involve the dissipation of heat into the workpiece. This serious shortcoming has been dealt by the jetting technologies, which, contrary to the traditional machining, operate under cold conditions. The two methods in the jetting technologies used for processing materials are water jet machining and abrasive water jet machining. The first of these, water jet machining, has been around for the past 20 years and has paved the way for abrasive water jet technology. Water jet machining and abrasive water jet machining have been used for processing composite materials because of the advantages offered by this technologies as compared to traditional techniques of processing. The high surface and structural integrity required of any technique used for processing composite materials has created an opportunity for abrasive water jet machining. Cutting of composites using laser is also an option, and experiments were also conducted to reveal the extent of using laser technique.     

Semisolid processing of metallic materials

Abstract

Semisolid processing involves forming metallic alloys between the solidus and the liquidus. For the process to operate, the microstructure must consist of solid spheroids in the liquid matrix, rather than dendrites. The material then flows when it is sheared but thickens again when allowed to stand, i.e. it behaves thixotropically. This type of behaviour was first discovered by Flemings and co-workers in the 1970s and is utilised in a family of processes, some now applied commercially. Here, the current status of semisolid processing, both technologically and from a research point of view, will be reviewed.     

Aluminium foaming using a CO2 laser

Abstract

This paper explores the possibility of laser assisted aluminium foaming from a foamable precursor material. A preliminary study of the experimental results shows a unidirectional and localised expansion of the foam. A pore size gradient and a density gradient exist in the structure as the processing conditions change. The foam has large pores and lower density for slow processing speeds, in contrast to the case of fast processing speed where there is a small pore size but higher density. On the top surface as the melt collapses, there is a closure of pores.