Visualization of thermal cutting fluid flows

Outside of the fields where flow visualization is traditionally applied, there exist many processes where fluid phenomena are critical. Here, we survey flow visualization work with a focus on two thermal metal cutting processes. These two processes – plasma-arc cutting and gas assisted laser cutting – account for a large fraction of the means by which steel is cut in our world. Plasma-arc cutting utilizes an electric arc transferred between a cathode and the steel being cut to produce a high temperature gas jet that melts and removes metal. In gas assisted laser cutting, the assist jet is often high-pressure supersonic nitrogen for stainless steel, or near-atmospheric pressure, low-speed oxygen for carbon steel. Visualization of these millimeter-range diameter jets helps to understand the different roles that the assist gas has in these cutting processes, particularly with how the jets interact with the metal being cut. We describe experimental techniques for visualization of the arc jet and gas assist jet, as well as the liquid metal flows being removed from the cut and the gas flow in the torch itself. These visualizations overcome the small physical scales of the process, the bright illumination from the arc itself, and harsh high-temperature environment. The results lend perspective and understanding of the physical phenomena important to process control.     

Thermal and efficiency analysis of the CO2 laser cutting process

In CO₂ laser gas-assisted cutting process, modeling of the interaction mechanism is important. Consequently, the present study treats the complete problem of the interaction of the melting surface with the boundary layer and describes the behavior of the melting layer. In the analysis, gas–liquid interface parameters are developed and relationships between the parameters influencing the cutting action are identified theoretically. To achieve this, effects of momentum and gas–liquid interface shear stress, due to the assisting gas jet, are considered. The approximate magnitude of the heat absorbed is estimated and melting layer thickness is predicted. An experiment is carried out and the theoretical predictions are compared with the experimental findings. First and second law efficiencies of the cutting process are predicted, which may, then, be used to improve the process. It is found that the assisting jet velocity increases the first and second law efficiencies of the CO₂ laser cutting process.     

Technological study of laser cutting silicon steel controlled by rotating gas flow

Using traditional laser cutting technology, it is easy to produce molten slag in laser cutting silicon steel sheet. The main reason is the inevitable oxidizing reaction in the process caused by the use of oxygen as the aided gas. As a common solution, high pressure and high purity N₂ or an inert gas is therefore used instead of oxygen. Although the cut quality is improved, the cutting efficiency is reduced because of the lack of energy generated from an exothermic oxidation reaction. The technology used in this paper is to employ a newly developed cyclone slag separator. The slag separator is located under the workpiece to form rotating gas flow for controlling the direction of the flowing slag gas. Adopting the new technology reported here, oxygen is still used as the aided gas. The experiments prove that, by controlling the technical parameters reasonably tightly, glossy and dross-free cutting kerfs are obtained for reduced laser power. The gas flow acting under the workpiece is simulated using the finite element method (FEM). The operating law of the rotating gas flow is verified by ANSYS, which provides an academic basis for controlling the flowing direction of the slag gas.     

Study into penetration speed during CO2 laser cutting of stainless steel

Laser cutting quality depends upon the proper selection of laser and workpiece parameters. Laser cut quality drops considerably when the size of the surface plasma increases. This plasma affects the speed of penetration, which in turn affects the cut quality. The present study examines the measurement of the penetration speed during CO₂ laser cutting of stainless steel workpieces. To achieve this, three different methods were employed, namely, optical, thermocouple and wire methods. Oxygen and an argon-oxygen gas mixture were used as assisting gases. Penetration speed was also predicted, using a one-dimensional heat transfer model. It was concluded that the cut quality improves when penetration speed is at a maximum.     

CO2 laser cutting of thin metal sheets with gas jet assist

A quantitative analysis of cutting thin stainless steel sheets with cw CO₂ lasers using an oxygen gas jet assist is reported. Results are interpreted theoretically using a point source model.     

Optimization of an off-axis nozzle for assist gas injection in laser fusion cutting

The aerodynamic features of the gas flow during laser fusion cutting are an essential factor influencing the cut performance. For this reason it has been a subject of some studies to explain the interactions of the gas jet with the workpiece and to design different gas injection systems with the aim of preventing the drawbacks of the conventional cutting heads.An off-axis cutting head with a de Laval nozzle to inject a supersonic gas jet has been previously demonstrated to be an effective design to achieve a complete removal of the molten material from the cutting front and to avoid the formation of the recast layer. In the present work, the fundamentals and procedures to adjust the main factors determining the efficiency of this gas injection system are described. Specifically, the gas flow inside the cut kerf is analysed by means of flow visualization using the Schlieren technique.     

Investigation of a novel hybrid process of laser drilling assisted with jet electrochemical machining

Recast layer and spatter are two inherent defects commonly associated with holes produced with laser drilling. This paper reports a novel hybrid process of laser drilling assisted with jet electrochemical machining (JECM-LD) that aims to minimize such defects and improve the quality of laser-drilled holes. The process based on the application of a jet electrolyte, being aligned coaxially with the focused laser beam, on the workpiece surface during laser drilling. The effect of the jet electrolyte mainly is an electrochemical reaction with materials. The jet electrolyte also cools the workpiece and transports debris during the process. On the basis of a measurement of laser attenuation in electrolyte, an experimental apparatus system is made and JECM-LD experiments have been performed on 0.5-mm-thick 321S20 stainless steel with two lasers at wavelength of 1064 and 532 nm. It is shown that recast layer and spatter have been effectively reduced during the JECM-LD compared with laser drilling in ambient atmosphere conditions.     

主流无载气N_2-氧碘化学激光配气方式的数值优化

针对主流无载气、副流以氮气为载气的氧碘化学激光(COIL),应用求解3维多组分化学反应流方程的数值方法,对流场和物理化学的耦合过程进行细致研究,对副流载气变化带来的问题及性能提升的手段、特别是合理的配气方式进行深入分析。结果表明:传统的在亚声速段进行喷流的配气方式不适用于主流无载气N2-COIL系统,必须采用超声速段射流方式;合理的流量配比条件下,超声速段射流方式COIL光腔位置处增益可达1.5%cm-1;N2-COIL流场边界层厚度明显减小,拓宽了增益的有效分布区域。     

Investigation of optimum condition in oxygen gas-assisted laser cutting

Laser cutting characteristics including power level and cutting gas pressure are investigated in order to obtain an optimum kerf width. The kerf width is investigated for a laser power range of 50–170 W and a gas pressure of 1–6 bar for steel and mild steel materials. Variation of sample thickness, material type, gas pressure and laser power on the average cut width and slot quality are investigated. Optimum conditions for the steel and mild steel materials with a thickness range of 1–2 mm are obtained. The optimum condition for the steel cutting results in a minimum average kerf width of 0.2 mm at a laser power of 67 W, cutting rate of 7.1 mm/s and an oxygen pressure of 4 bar. A similar investigation for the mild steel cutting results in a minimum average kerf width of 0.3 mm at the same laser power of 67 W, cutting rate of 9.5 mm/s, and an oxygen pressure of 1 bar. The experimental average kerf is about 0.3 mm, which is approximately equal to the estimated focused beam diameter of 0.27 mm for our focusing lens (f=4 cm and 100 W power). This beam size leads to a laser intensity of about 1.74×10⁹ W/m² at the workpiece surface. The estimated cutting rate from theoretical calculation is about 8.07 mm/s (1.0 mm thickness and 100 W power), which agrees with the experimental results that is 7.1 mm/s for 1.0 mm thickness of mild steel at the laser power of 88 W.     

In-situ laser material process monitoring using a cladding power detection technique

Progress in laser material processing may require real-time monitoring and process control for consistent quality and productivity. We report a method of in-situ monitoring of laser metal cutting and drilling using cladding power monitoring of an optical fibre beam delivery system—a technique which detects the light reflected or scattered from the workpiece. The light signal carries information about the quality of the process. Experiments involving drilling and cutting of two samples, a thin aluminum foil and a 2-mm thick stainless steel plate, confirmed the effectiveness of this method.     

Tuning of a parametric model for the laser cutting of steels

In this work the mathematical models developed to describe the gas jet laser cutting process are examined.

Some experimental results for the laser cutting of carbon and stainless steels are also shown. These results are then used to ‘tune’ the theoretical models in order to obtain a method for making predictions about the parameters of interest in the laser cutting of steels.     

Numerical analysis of supersonic gas-dynamic characteristic in laser cutting

The influence of the processing parameters on the dynamic characteristic of supersonic impinging jet in laser cutting is studied numerically. The numerical modeling of a supersonic jet impinging on a plate with a hole is presented to analyze the gas jet–workpiece interaction. The model is able to make quantitative predictions of the effect of the standoff distance and exit Mach number on the mass flow rate and the axial thrust. The numerical results show that the suitable cutting range is slightly different for different exit Mach number, but the optimal cutting parameter for certain exit total pressure is nearly changeless. So the better cut quality and capacity can be obtained mainly by setting the suitable standoff distance for a certain nozzle pressure.     

High-speed laser cutting of superposed thermoplastic films: thermal modeling and process characterization

Common thermoplastic films used in the packaging industry have a thickness lower than 100 μm, and present low absorption to CO₂ laser radiation. This characteristic renders the use of cutting parameters, predicted by models developed for thicker thermoplastics inappropriate. In addition, the usual procedures involve the use of an assisting gas, responsible for removing the melted material, which, when processing thin films, induces changes in position in the material. A new theoretical model describing the temperature distribution on thin thermoplastic material during laser cutting was later developed. The heat conduction was solved analytically by the Green function method and heating and cooling thermal stress evolution was taken into consideration. The laser beam diameter over the samples provides the possibility of obtaining two cut operations: a simple cut, on beam focus, and a cut with welding, defocusing the beam. Engineering parameters predicted by the model were applied to cutting superposed high- and low-density polyethylene and polypropylene samples, transparent and white, with thicknesses between 10 and 100 μm, and experimentally validated.Proper modeling and the introduction of a reflective substrate under the samples allowed the improvement of process efficiency and the achievement of cutting operations up to 20 m s⁻¹, and cut with welding up to 14 m s⁻¹; an order of magnitude of improvement on industrial speeds previously attained for this operation.     

On cutting and penetration welding processes with high power lasers

In this paper cutting and continuous welding laser processes are examined. Experiments with CO₂ and YAG lasers were carried out on carbon and stainless steels. Two distinct regimes were identified in the gas jet assisted cutting process and in both cases predictions of working parameters can be made. Penetration welding results, when represented on a mathematical model, were seen to be similar to those of cutting ones. Finally, there is a great difference between the above processes and the conduction welding one.     

Investigation of cutting of engineering ceramics with Q-switched pulse CO2 laser

In this paper, the cutting of Si₃N₄ engineering ceramics with Q-switched pulse CO₂ laser is studied. Considering the influence of the cut front shape on the absorption of the laser beam, a simplified 2D mathematic model is developed based on a pulsed laser vaporization cut process. This model is based on the conservation of energy. The experimental results show that it would realize crack-free cutting by using high-speed and multi-pass feed cutting process.     

DYNAMICS OF CUTTING TOOL TEMPERATURES DURING CUTTING PROCESS

An experimental study is performed to measure temperature variation of a carbide insert during metal cutting by means of fine thermocouples connected to a data acquisition system. A semiempiriad equation is developed based on transient conduction solutions combined with experimental data to predict temperature-time history in the cutting tool during operation. The governing parameters in the equation are derived, using AISI 1045 steel as the workpiece, to be the cutting speed, feed rate, and depth of cut at fixed rake and lead angles. Emphasis is placed on analyzing effects of the important factors on the cutting process, such as frictional heat generated at the tool-workpiece interface and wear of the tool. Dynamic behavior of the metal cutting process as predicted by the direct thermocouple measurement agreed with phenomenological observations reported in the existing literature.     

Gases for increased productivity of laser processing

To achieve a high return on investment, laser systems must be used to their fullest capacity, avoiding power losses and downtimes. High-quality laser gases are therefore needed to run the laser. But if the quality of the gas cannot be guaranteed all the way from the cylinder to the laser cavity, the risk of impurities such as water vapour and hydrocarbons or particles being entrained into the laser system is large. Unstable laser operation and damage to the resonator optics can result, needing costly repairs.The profitability of laser operations is also affected by the selection of the assist gas. High-purity oxygen, for example, results in a correspondingly high cutting speed in mild steel. In cutting stainless steel, on the other hand, any oxidation of the cut surface must be avoided in order to preserve the corrosion resistance.In contrast, different assist gases are used for laser welding depending on the wavelength of the laser radiation, the material or the energy per unit length of weld. Helium is often the most convenient choice for CO₂ laser welding of mild steel and helium-argon mixtures for aluminium; argon is suitable for Nd:YAG laser weiding and productivity is increased by small additions of oxygen.Consequently, high-purity gases and suitable gas distribution equipment are the basis for a satisfactory return on investment.     

Theoretical and experimental investigation into laser melting of steel samples

Among the available laser applications, laser melting has turned out to be a powerful technique for the production of mechanically improved surfaces. To enhance the understanding of the laser melting process investigations into modeling of the heating mechanism initiating the laser melting are necessary. In the present study, a mathematical modeling of the laser melting process is introduced and power require ments for the laser melting are predicted as functions of workpiece properties and velocity. Maximum melt width is predicted analytically and compared with the experimental results. In this regard, an experiment is conducted to melt the mild steel samples with a cw CO₂ laser at different power settings and workpiece velocities. It is found that the melt variables predicted from theory are in agreement with the experimental results.     

底部气流对CO2激光切割硅钢切口质量的影响

激光切割相对冲压及线切割工艺具有自由度大、加工效率高、不会影响铁心质量等优点,但由于残渣及切口粗糙问题一定程度上影响了激光切割硅钢片的质量和应用范围.通过采用在工件底部增设辅助侧吹喷嘴,控制熔渣流向,保证成品切割质量激光切割工艺,试验证明,合理控制工艺参量,可获得良好效果.利用有限元法进一步对工件底部气流状况进行了数值模拟,分析了流动过程中在不同的角度和流速下光斑移动和气流场变化的情况,初步确定工件底部侧吹气流以20°吹入的辅助侧吹工艺,为进一步合理控制熔渣流向获得光滑的精细切口提供了实践和理论依据.