High Power CW CO2 Laser Research Articles

تأثير استعمال ليزر النيوديميوم ياك (Nd-YAG) على البنية الدقيقة وصلابة سطح الفولاذ AISI1012

The unique characteristics of the laser beam such as directionality, monochromaticity, and brightness make it a very important tool in the surface treatment engineering. The concentrated heat in the laser beam can be precisely directed toward the processed region on the surface. The laser beam power, beam size, and scanning velocity are easily controlled during the different processing methods. Laser surface modifications of metallic materials, such as laser surface melting, laser surface alloying, and laser cladding are some of the most important methods to improve the mechanical properties of the processed layer on the surface. A pulsed Nd-YAG laser surface melted specimens of low carbon steel with a maximum power of 90W at different processing parameters. The rapid melt and solidification of the treated layer produced a refinement in the microstructure. The original microstructure of this steel (Ferrite + Pearlite) has been transformed to new phases of martensitic and bainitic structures. The melted layer consists of the melted pool, the heat affected zone, and the substrate. The maximum hardness of the laser melted layer is more than 400 HV. The effect of different lasers such as high power CW CO2 laser on the mechanical properties and corrosion resistance of steel alloys will be carried out in future studies. Keywords: Nd-Yag Laser, Laser surface treatment, Microstructure and hardness of AISI1012 steel

Experimental study of laser pyrolysis of coal and residual oil

A lab-scale laser pyrolysis platform was set up to study the pyrolysis of coal and residual oil to light gases combining high accuracy, good repeatability and ultra-high temperature, fast heating rate. With a high-power CW CO2 laser used as heat source, the laser pyrolysis took place as a procedure-controlled reaction process connected with a specially designed GC to analyze the gas products on line. The temperature of samples was measured by a two-color infrared pyrometer in milliseconds time-step, and the measured ultimate temperature and heating rate exceeded 2900 K and 3 × 104 K/s respectively. Through a series of experiments, the accuracy and reliability of the laser pyrolysis platform are verified and the effects of sample weight, environmental atmosphere, laser irradiation time and power, hydrogen content in atmosphere on the yields of gas products were investigated. The appropriate operating conditions are: the weight of residual oil less than 1.0 mg, the laser irradiation time longer than 3.0 s, laser irradiation power stronger than 90 W. The relationship between laser pyrolysis and plasma pyrolysis processes has also been discussed in this work. This work established an efficient, easy-to-use method for the preliminary feedstock selection of plasma pyrolysis process because C1 + C2 yield from laser pyrolysis could be used as a criterion for plasma pyrolysis.

The formation of trinitite-like surrogate nuclear explosion debris (SNED) and extreme thermal fractionation of SRM-612 glass induced by high power CW CO2 laser irradiation

We describe a new approach to the bench top production of surrogate nuclear explosion debris by employing high power continuous wave CO2 laser irradiation. High surface temperatures >2,500 K can be rapidly attained, allowing virtually any combination of materials to be fused into a glassy matrix that can display high levels of elemental fractionation. Examples of the laser fused glasses will be presented and compared to trinitite nuclear explosion glass along with the elemental fractionation effects that were induced in the NIST glass standard SRM-612 by this method.

高功率轴快流CO2激光器的流体动力学模拟

To increase the photoelectronic conversion efficiency of the single discharge tube and to meet the requirements of the laser cutting system, optimization of the discharge tube structure and gas flow field is necessary. We present a computational fluid dynamic model to predict the gas flow characteristics of high-power fast-axial flow CO2 laser. A set of differential equations is used to describe the operation of the laser. Gas flow characteristics, are calculated. The effects of gas velocity and turbulence intensity on discharge stability are studied. Computational results are compared with experimental values, and a good agreement is observed. The method presented and the results obtained can make the design process more efficient.

高功率轴快流CO2激光器的流体动力学模拟

高功率轴快流CO₂激光器的流体动力学模拟

Computational fluid dynamic modeling of gas flow characteristics of the high-power CW CO2 laser

Computational fluid dynamic modeling of gas flow characteristics of the high-power CW CO2 laser

Keyhole depth instability in case of CW CO2 laser beam welding of mild steel

The study of keyhole (KH) instability in deep penetration laser beam welding (LBW) is essential to understand welding process and appearance of weld seam defects. The main cause of keyhole collapse is the instability in KH dynamics during the LBW process. This is mainly due to the surface tension forces associated with the KH collapse and the stabilizing action of vapour pressure. A deep penetration high power CW CO2 laser was used to generate KH in mild steel (MS) in two different welding conditions i.e. ambient atmospheric welding (AAW) and under water welding (UWW). KH, formed in case of under water welding, was deeper and narrower than keyhole formed in ambient and atmospheric condition. The number and dimensions of irregular humps increased in case of ambient and under water condition due to larger and rapid keyhole collapse also studied. The thermocapillary convection is considered to explain KH instability, which in turn gives rise to irregular humps.

Laser power coupling efficiency in conduction and keyhole welding of austenitic stainless steel

Laser welding of thin sheets of AISI 304 stainless steel was carried out with high power CW CO2 laser. The laser power utilized in the welding process was estimated using the experimental results and the dimensionless parameter model for laser welding; and also the energy balance equation model. Variation of laser welding efficiency with welding speed and mode of welding was studied. Welding efficiency was high for high-speed conduction welding of thin sheets and also in keyhole welding process at high laser powers. Effect of pre-oxidization of the surface and powder as filler material on laser power coupling is also reported. The paper also discusses effect of microstructure on the cracking susceptibility of laser welds.

New technology for high rate synthesis of PC-diamond coatings in air with photon plasmatron

In this paper the development of a new plasma-CVD method for synthesis of PC-diamond coatings in air is presented. The method does not require any vacuum or reaction chamber and is based on plasma-CVD processes taking place in a photon-initiated stationary plasma in the open-air atmosphere maintained in the electromagnetic optical field of a high-power cw-CO2 laser radiation. To use this plasma as an activation source for the CVD process with different gas precursors, a special photon plasmatron operating in air has been designed and constructed. The high temperature in the photon-initiated plasma (15000–20000 K) as well as the high pressure (≥1 atm) of the precursor gases, are prerequisites for a high-rate synthesis of PC-diamond coatings. For this purpose, different gases like Ar, CO2, CH4, H2, N2 and their mixtures have been used. The properties of the deposited coatings in dependence on the experimental conditions have been investigated with SEM, EDX, AUGER and RAMAN techniques. With this new technology uniform and continuous PC-diamond films on molybdenum, Si and cemented carbides have been locally deposited onto an area of approximately 2 cm2 with a deposition rate of up to 2 μm/min for the present. With optimisation of the process, a further significant increase in deposition rate can be expected.

High-speed simultaneous observation of plasma and keyhole behavior during high power CO2 laser welding: Effect of shielding gas on porosity formation

Laser welding can produce a deeply penetrated bead at high speed. However, in high power cw CO2 laser welding, the characteristic porosity is easily formed in the weld metal, but its formation mechanism has not been well understood. Therefore, the authors have conducted systematic studies of the elucidation of porosity formation mechanism and the development of preventive remedies. They have revealed that many bubbles are formed, mainly from the bottom tip of the keyhole by intense evaporation of the metal. It has also been revealed that the keyhole fluctuates frequently and changes its size and shape, corresponding to the intermittent bubble formation. The majority of bubbles are trapped at the solidifying front in the rear part of the molten pool. However, there are few reports that deal with the simultaneous observation of keyhole and plasma dynamic behavior as well as the formation of bubbles and porosity. In this study, therefore, the interrelationship between keyhole and plasma behavior was examined by using two synchronized high-speed cameras and an x-ray transmission observation system. Especially, the effect of shielding gas on porosity formation was investigated in terms of plasma and keyhole behavior. In the case of He shielding gas, metallic plasma emanated from a keyhole, and the keyhole was open continuously. On the other hand, in the case of N2 shielding gas, a big gas plasma was formed above the weld bead periodically, and metallic plasma and keyhole disappeared just like in the pulsed laser welding. Such periodical interval and duty were different depending on the materials used and exerted an effect on porosity formation tendency.

Laser Pyrolysis of Hydrocarbons in Synthesis of Soot Containing Fullerenes

Using a high power CW CO2 laser, the pyrolysis in a flow reactor of different hydrocarbons/oxidizer mixtures with/without sensitizer lead to soots containing weightable quantities of fullerenes which were identified by FTIR spectroscopy, high performance liquid chromatography (HPLC) and time-of-flight mass spectrometry. The C60 and C70 quantification was done by HPLC. The FTIR spectroscopy of toluene extracts from the soots and of exhaust gases allowed to discuss some aspects concerning the influence of sensitizer (SF6) addition as well as of the oxidiser (O2/N2O) on the fullerene production in laser pyrolysis of benzene-based mixtures.

Design considerations and scaling laws for high power convective cooled CW CO2 lasers

Various criteria for designing high power convective cooled CO2 lasers have been discussed. Considering the saturation intensity, optical damage threshold of the optical resonator components and the small-signal gain, the scaling laws for designing high power CW CO2 lasers have been established. In transverse flow CO2 lasers having discharge of square cross-section, the discharge lengthL and its widthW for a specific laser powerP (Watt) and gas flow velocityV (cm/s) can be given byL = 1.4 x 104 p 1/2 V -1 cms andW = 0.04P 1/2 cms. The optimum transmitivity of the output coupler is found to be almost constant (about 60%), independent of the small signal gain and laser power. In fast axial flow CO2 lasers the gas flow should be divided into several discharge tubes to maintain the flow velocity within sonic limit. The discharge length in this type of laser does not depend explicitly on the laser power, instead it depends on the input power density in the discharge and the gas flow velocity. Various considerations for ensuring better laser beam quality are also discussed.

Microstructure and properties of aluminium tungsten oxide ceramics synthesized by a high power cw CO2 laser

Laser powder sintering has been used as a new technique to synthesize electronic ceramics consisting of the Al2O3-WO3 system. Examination of the products by scanning electron microscopy and energy dispersive X-ray spectroscopy showed that cell structure and lateral intergrowth crystals formed in the specimens. The depth distribution of the elemental composition of the sample changed in an amplitude modulated manner. A comparison of the properties of the samples synthesized by laser and/or conventional sintering methods showed that the laser technique can produce new substances with some unique properties, such as a linear decrease in the resistivity of Al2O3-50 mol% WO3 specimens with increasing temperature over the range 10 °–150 °C. X-ray photographic powder analysis indicated that the conducting phase in the specimen is non-equilibrium product: AlxWO3 tungsten bronze. The experiments also showed that the conductivity of samples was related to their composition and microstructure.

Analysis of aluminum tungsten oxide ceramics synthesized by a high-power cw CO2 laser

Ceramics in the Al2O3-WO3 system have been produced by laser irradiation. Examination of the products by scanning electron microscopy showed that a cell structure had formed in the specimens. A comparison of the properties of the samples synthesized by laser and by conventional sintering methods showed that the laser technique can produce new materials with some unique properties such as the linear decrease in resistivity of Al2O3-50 mol % WO3 specimens with increasing temperature over the range of 10–150 °C. X-ray photographic powder analysis indicated that the conducting phase in the specimen is a nonequilibrium product: AlxWO3 tungsten bronze.

Transverse mode selection in a convectively cooled cw-CO2 laser with a cyclonic flow configuration

Strong cyclonic flow induced discharge confinement has been used for the first time to efficiently select the lower order transverse modes in a fast axial flow high power cw-CO2 laser. The laser performance with and without the cyclonic flow has been compared. Comparison has also been made with the conventional aperture method of selection of lower-order transverse modes.

Maximum depth of keyhole melting of metals by a laser beam

A calculation is reported of the maximum depth and diameter of a narrow crater formed in a stationary metal target exposed to high-power cw CO2 laser radiation. The energy needed for erosion of a unit volume is assumed to be constant and the energy losses experienced by the beam in the vapor–gas channel are ignored. The heat losses in the metal are allowed for by an analytic solution of the three-dimensional boundary-value heat-conduction problem of the temperature field in the vicinity of a thin but long crater with a constant temperature on its surface. An approximate solution of this problem by a method proposed earlier by one of the present authors was tested on a computer. The dimensions of the thin crater were found to be very different from those obtained earlier subject to a less rigorous allowance for the heat losses.

Unstable resonator with phase-unifying coupler for high-power lasers

A novel unstable resonator with a phase-unifying output coupler has been developed. The output coupler structured with a partial-reflection region in its center surrounded by an antireflection region unifies the phases of laser beams passing through these regions. Thus, a near-Gaussian far-field pattern beam has been obtained with a divergence full-angle of 1.08 mrad on the application to a high-power cw CO2 laser.

Influence of turbulence and convection on the output of a high-power CO2 laser with a fast axial flow

A high-power CW CO2 laser with a fast axial flow is investigated experimentally and theoretically. The gas flow is controlled by several different designs of the discharge tube which is either made of glass of cascaded metal discs. A high laser-gas purity is achieved by a Pt-Rh catalyst placed in the main flow. A laser power of 1.2 kW m-1 with 30% efficiency is reached. The active medium is described by assuming a five-temperature model and balancing the quantum densities of vibrational states of the CO2 and N2 molecules. This system of basic equations is completed by the mass, momentum and energy continuity equations. A model, method of solution and theoretical and experimental results are described and presented. The experimental results are in very good agreement with the theoretical predictions within a broad range of parameters.

Glow Discharge for High Power cw CO2 Laser Excitation

This paper deals with the characteristics of glow discharge for high power cw CO2 laser excitation. It is found that J/P2 (J: current density, P: gas pressure) on the cathode surface has the constant value of 6.25×10-3 mA/Torr2-cm2 for the Mo cathode. The CO2 dissociation in the gas mixture is dominated by the phenomena near the cathode, since the steady-stage CO2 concentration after discharge closely depends on the kind of cathode materials The discharge gap length gave the effect on the maximum discharge power. The optimum gap length was about 4 cm at the pressure of 100 Torr. For the gap length above 4 cm, type of the silent-discharge assisted glow discharge is recommended.

Unstable–stable resonators with toroidal mirrors

A resonator with toroidal mirrors is described. This resonator behaves like an off-axis unstable confocal resonator in one transverse dimension and like an on-axis concave-convex stable resonator in the other orthogonal dimension. Some experimental results are reported for a fast flow high power cw-CO(2) laser whose transverse cross section is restricted in the stable-resonator direction. These cavities allow an output laser beam with a fully illuminated cross section which is well suited for focusing. Moreover, the fraction of the available laser power which may be concentrated in the central lobe of the focal plane intensity distribution is 2.5-4.5 times higher than other unstable resonators with similarly restricted modal volumes. Finally the alignment requirements are examined.