Dynamic characteristics of gas jets from subsonic and supersonic nozzles for high pressure gas laser cutting

During laser cutting of stainless steels, titanium and aluminum alloys, a coaxial and high pressure inert gas jet is used to improve the cut edge quality. The process normally consumes a large amount of inert gas and has a poor tolerance to variation in process parameters. This is solely because the gas nozzles are mostly of the conical and convergent type in which the gas jets are subsonic. Based on two dimensional steady state gas dynamic theory, computer simulation and shadowgraphic techniques, the gas jet patterns from conical nozzles and the newly designed supersonic nozzles are analyzed. The distribution of pressure, momentum, gas density and existence of shock waves are predicted and mapped. Based on these characteristics, the effect of the gas jets upon the cut quality is explained. It is concluded that a supersonic gas jet offers the best flow characteristics for high pressure laser cutting.     

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.     

毛细喷孔超高速水射流的脉动及破碎

水切割射流的动力学特性的诸多方面尚未得到认识和理解。本文对毛细喷孔产生的超高压水射流展开可视化研究,分析了常规压力及超高压条件下毛细水射流的液体破碎机制并对超高压毛细射流的脉动现象进行了讨论。常规条件下的毛细射流遵从经典的破碎模式;在超高压条件下,射流完整段呈瑞利模式,完整段以下呈雾化破碎模式,射流集束性呈现周期性变化。结果表明,传统理论不能够表达小孔径时超高速毛细水射流的破碎特性;喷孔内部流动情况如流动分离及空化成为该条件下射流破碎和脉动的重要原因。     

Plasma generation for the plasma cutting process

This study is an attempt to estimate the overall properties, viz. the thermal power and force, of an intense plasma jet produced by a plasma cutting torch, and to relate the properties of the plasma to the diameter of the nozzle of the plasma torch and the flow rate of plasma-forming gas. For cutting metallic plates using a thermal plasma, a narrow plasma jet is produced by means of a transferred electric are between an electrode in a plasma torch and the material to be cut. The power density and pressure exerted by the plasma jet on the material at the region of cut needs to be high so that a straight cut, without dress at the bottom of the plate, can be obtained. A simple theory to describe the behavior of the arc in a plasma cutting torch has been developed to predict the are radius, pressure, and arc voltage at the nozzle exit as a function of are current for a range of nozzle sizes and air flow rates. The results obtained are in good agreement with the measured values for an air plasma cutting torch nominally rated for 100-A operation. The relationships between the mass flow rate of plasma gas, plasma power, and arc force have been discussed in the light of design of plasma torches for plasma cutting     

Effects of processing parameters on laser cutting of aluminium-copper alloys using off-axial supersonic nozzles

Conventional laser cutting involves the utilization of converging coaxial nozzles to inject the assist gas used to remove the molten material. This processing system prevents the utilization of this technique to cut aluminium alloys for aerospace applications. The inefficient removal of molten material by the assist gas produces cuts with poor quality; very rough cuts, with a large amount of dross, and a large heat affected zone (HAZ) are obtained. An alternative to increase the assist gas performance is the utilization of off-axial supersonic nozzles. Removal of molten material is substantially increased and cuts with high quality are obtained. On the other hand, pulsed laser cutting offers superior results during the processing of high reflectivity materials as aluminium alloys. However, there are no experimental studies which explore the pulsed laser cutting of aluminium alloys by means of a cutting head assisted by an off-axis supersonic nozzle.The present work constitutes a quantitative experimental study to determine the influence of processing parameters on the cutting speed and quality criteria during processing by means of off-axial supersonic nozzles. Cutting experiments were performed in pulsed mode and the results explained under the basis of the molten material removal mechanisms. Performed experiments indicate a reduction in cutting speed as compared to continuous wave (CW) mode processing and the existence of two processing regimes as a function of the pulse frequency. Best results are obtained under the high pulse frequency one (f > 100 Hz) because the superior capabilities of molten material removal of the supersonic jets are completely exploited in this processing regime.     

等离子体射流产生与特性的实验研究

本文报道热等离子体射流产生及射流特性的实验研究结果。采用同一个直流等离子体发生器,工作气体流量小时产生出层流等离子体长射流,射流长度随气体流量或弧电流的增加而明显增加;工作气体流量大时则产生出湍流等离子体短射流,此时射流长度几乎与工作气体流量或弧电流无关;在层流与湍流等离子体射流工况之间,存在一个流动状况不稳定的过渡区,此时等离子体射流的平均长度随气流量的加大而减小,但随弧电流的加大而明显加大。层流等离子体长射流有相当好的刚性。     

Oxygen assisted laser cutting mechanism—a laminar boundary layer approach including the combustion process

The present study examines the combined effects of chemical reactions taking place between a gas jet and molten metal, the cooling effect of the jet and the evaporation of metal, during a CO₂ laser cutting process. A laminar boundary layer approach was used to develop a theoretical model for the oxygen gas jet laser cutting mechanism. An experiment was carried out to monitor the keyhole formation using a video recorder and detect the light emitted from the entrance and exist surfaces of the workpiece using a fibre-optic probe during the cutting process. The experimental study was extended to employ two different workpiece materials (stainless steel and mild steel) at two thicknesses, and varying oxygen assisting gas pressures. It is found that the theoretical model developed in the present study is valid for a cutting speed of about 30 mm s⁻¹ and all jet velocities up to sonic, since the effect of shock is excluded in the model.     

Metal and polymer melt jet formation by the high-power laser ablation

The laser-induced metal and polymer melt jets are studied experimentally. Two classes of physical phenomena of interest are: first, the process of explosive phase change of laser induced surface ablation and second, the hydrodynamic jetting of liquid melts ejected from a beamed spot. We focus on the dynamic link between these two distinct physical phenomena in a framework of forming and patterning of metallic and polymer jets using a high-power Nd:YAG laser. The microexplosion of ablative spot on a target first forms a pocket of hot liquid melt and then it is followed by a sudden volume change of gas-liquid mixture leading to a pressure-induced spray jet ejection into surrounding medium.     

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.     

Production of long,laminar plasma jets at atmospheric pressure with multiple cathodes

Plasma jets from conventional non‐transferred arc plasma devices are usually operated in turbulent flows at atmospheric pressure. In this paper, a novel non‐transferred arc plasma device with multiple cathodes is introduced to produce long, laminar plasma jets at atmospheric pressure. A pure helium atmosphere is used to produce a laminar plasma jet with a maximum length of >60 cm. The influence of gas components, arc currents, anode nozzle diameter, and gas flow rate on the jet characteristics is experimentally studied. The results reveal that the length of the plasma jet increases with increasing helium content and arc current but decreases with increasing nozzle diameter. As the gas flow rate increases, the length of the plasma jet initially increases and then decreases. Accordingly, the plasma jet is transformed from a laminar state to a transitional state and finally to a turbulent state. Furthermore, the anode arc root behaviours corresponding to different plasma jet flows are studied. In conclusion, the multiple stationary arc roots that exist on the anode just inside the nozzle entrance are favourable for the generation of a laminar plasma jet in this device.     

Ultraviolet imaging of the anode attachment in transferred-arcplasma cutting

The anode phenomena occurring at the location of current transfer from the plasma jet to the plate affects cut quality in plasma cutting of mild steel plate. To understand these phenomena, an ultraviolet imaging technique was used to visualize the anode attachment spot under various cutting conditions. This technique has provided a unique view and fostered a better understanding of the plasma-arc cutting process     

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.     

Effect of CO2 laser cutting process parameters on edge quality and operating cost of AISI316L

Laser cutting is a popular manufacturing process utilized to cut various types of materials economically. The width of laser cut or kerf, quality of the cut edges and the operating cost are affected by laser power, cutting speed, assist gas pressure, nozzle diameter and focus point position as well as the work-piece material. In this paper CO₂ laser cutting of stainless steel of medical grade AISI316L has been investigated. Design of experiment (DOE) was implemented by applying Box–Behnken design to develop the experiment lay-out. The aim of this work is to relate the cutting edge quality parameters namely: upper kerf, lower kerf, the ratio between them, cut section roughness and operating cost to the process parameters mentioned above. Then, an overall optimization routine was applied to find out the optimal cutting setting that would enhance the quality or minimize the operating cost. Mathematical models were developed to determine the relationship between the process parameters and the edge quality features. Also, process parameters effects on the quality features have been defined. Finally, the optimal laser cutting conditions have been found at which the highest quality or minimum cost can be achieved.     

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.     

层/湍流等离子体射流波动特性实验研究

本文应用电压传感器、光电倍增管及水冷皮托管,对产生射入空气中的纯氩层流和湍流等离子体射流的弧电压波动、发生器出口处的射流光强波动以及沿射流轴线的滞止压力波动进行了测量。结果显示层流等离子体射流各参数的波动幅度远小于湍流射流的对应值;弧电压的波动幅度随气流量的变化明显,但随电流的变化很小;弧电压的波动幅度与其平均值之比随电流增加呈下降的趋势。     

一种新型大气压毛细管介质阻挡放电冷等离子体射流技术

介绍一种结构设计简单、操作运行方便的新型毫米量级大气压冷等离子体射流发生技术.这种射流可以在大气压条件下，利用多种工作气体(如Ar,He,N₂)，通过毛细管介质阻挡放电(DBD)的方式实现.使用频率为33kHz，峰值电压为1—12kV的双向脉冲电源，利用Ar,He,N₂等工作气体，在毛细管内形成了稳定的冷等离子体射流.放电区域的光辐射空间分布利用商用CCD摄像机记录，从中研究放电形态和空间分布，观察到了在DBD区域的流动气体放电和在毛细管出口处形成的等离子体射流 关键词： 冷等离子体射流 毛细管介质阻挡放电 射流射程 射流激发温度     

A knowledge-based adaptive control environment for an industrial laser cutting system

A hierarchically structured environment that integrates a knowledge- based expert system, adaptive process control and pattern recognition techniques for controlling a laser cutting process is described. Knowledge of the laser cutting process for different materials is organised and encoded into a rule-based system. An adaptive control algorithm based on on-line recursive parameter estimation and on-line control law synthesis was adopted for the highly non-linear cutting process control. Cutting speed was selected as the major control variable. Irradiance emitted from the cut front is used for the feedback signal to this adaptive controller. The irradiance signal feeds the recursive parameter estimator for system identification. Techniques of pattern recognition, which have been well developed in coherent optics, were applied to assess cut quality by characterising the exit spark cone images of the gas assisted laser cutting process. Images from the cutting processes were grabbed, edge enhanced and correlated with a synthetic discriminant function filter which was synthesised from reference images to give good cut quality. Results from digital simulations based on these pattern recognition algorithms are also presented.     

射流式氧发生器的三维模拟研究

实现了对射流式氧发生器的三维仿真模拟,给出了氧发生器内部流场结构、各组分的分布状态等信息。研究了射流孔结构对氧发生器性能的影响。指出即便是具有相同比表面积的不同射流孔排布方式,也会对发生器性能产生影响。此外,逆向射流式氧发生器反应器中气体从双侧进入对于减小发生器对气体的阻力具有重要作用。     

射流式氧发生器的三维模拟研究

实现了对射流式氧发生器的三维仿真模拟,给出了氧发生器内部流场结构、各组分的分布状态等信息。研究了射流孔结构对氧发生器性能的影响。指出即便是具有相同比表面积的不同射流孔排布方式,也会对发生器性能产生影响。此外,逆向射流式氧发生器反应器中气体从双侧进入对于减小发生器对气体的阻力具有重要作用。     

血管支架光纤激光切割技术

采用光纤激光器对血管支架进行了激光切割工艺研究,通过实验获得了聚焦透镜焦距及焦点位置、输出功率、切割速度、脉冲频率、脉冲宽度、辅助气体种类及压强等工艺参数对切缝宽度和缝面质量的影响规律.结果表明:缝宽随输出功率、频率、脉宽及辅助氧压的增大而增加,随着切割速度的增加而减小.在实验的基础上找出了血管支架切割的最佳工艺参数,在316LVM不锈钢细管上(管壁厚度为0.12 mm,直径为2 mm)获得了切缝均匀,缝宽小于20 μm网状结构的血管支架.