Cut Front Research Articles

Automatic control of soil puncture

Among the existing methods of making horizontally directed wells for trenchless laying of utilities, the method of static soil puncture is the most popular. The main disadvantage of this method is poor accuracy of the piercing head movement in the soil. This drawback requires constant adjustment of the trajectory of this movement. The movement of the head is controlled by using a head with an asymmetric tip and the action of translational and translational-rotational motion on it. The transmission of these movements from the power plant is accomplished by means of a set of fixed pushing rods. If you know how the process of off and on deviation of the head happens, you can make it possible to automatically control the puncture of the soil by timely changing the direction of the head tip shift.
 To achieve this goal, the following issues have been resolved. It is determined that for automated control of the soil puncture process by static method it is necessary to use a puncture head with an asymmetric tip. Changing its position in the soil space can be controlled by rotating the tip and orienting it according to the direction of deviation.
 The obtained mathematical model of correcting the trajectory of the head with a truncated cylindrical tip makes it possible to predict the process of soil puncture in time depending on different soil conditions and to get a solution to the problem of deviation from the specified design trajectory of the tip head which has a recognized angle of the frontal plane cut to the horizon with successive build-up of rods.Changing the orientation in the space of the cut front surface of the cylindrical tip, taking into account the obtained mathematical model of correcting the trajectory of the head in the soil, allows to increase the span length to 100 m due to the proposed automated principle of correcting its movement.

Controlling nozzle and kerf gas dynamics to manage hazardous laser cutting fume

Conventional laser cutting is optimized for cut speed and quality but does not consider process fume. When processing hazardous materials in environments where global extraction is not practical, such as during in situ nuclear decommissioning, consideration of process fume is critically important. Fume generation in laser cutting is governed by forces applied to the molten cut front; dynamic interactions between the assist-gas and the cut front control these forces. This paper investigates the causal links between key gas-dynamic features and fume generation mechanisms in laser cutting. Its aim is to provide the preliminary understanding required to design process setups targeted at reducing the fine particle fraction. During laser cutting, the assist-gas boundary layer separates (BLS) from the molten cut front and, therefore, downstream forces applied on the melt are significantly changed. In this work, the BLS point is analyzed, captured via schlieren imaging, in an idealized environment. This separation point is related to features on the cut wall of laser-cut samples, validating the gas-dynamic interactions. The samples were cut from 3 mm thick 304 stainless steel, using a 1 kW Nd:Yb fiber laser. Control of the surface stagnation pressure and gas flow boundary conditions were used to relocate the point of BLS within the cut front, and the relationship of these parameters was analyzed. The results demonstrate that shifting the point of BLS along the cut front can be used to control fume generation mechanisms. This study successfully provided the preliminary understanding necessary to manage fume generation during the laser cutting process.

Analysis of Gas Flow Dynamics in Thermal Cut Kerf Using a Numerical and Experimental Approach for Nozzle Selection

Consistency in gas flow behavior under various operating conditions is expected for uniform cutting performance in the thermal cutting process. The slope of the cut front in the kerf slot of a sample cutting material varies with the operating condition which affects the gas flow pattern. Therefore, how the nozzle exit diameter and the slope of the cut front effects gas flow behavior has been studied using the Reynolds averaged Navier–Stokes (RANS) based k–ω turbulence model. Convergent–straight-type nozzles with exit diameters φexit of 1.5 mm, 2 mm and 2.5 mm were used to study the flow patterns through the kerf slots of variable cut front slopes. The numerical simulation results were then compared with the results obtained from the Schlieren experiments. In addition, image processing was performed in the Schlieren images for clear visualization and precise comparison of the numerical and experimental data. The results confirm that a nozzle with an exit diameter of 2 mm shows a higher consistency in flow behavior in variable operating conditions.

Determination of the geometry of laser-cutting fronts with high spatial and temporal resolution

The melt flow velocity and the local surface angles of the cutting front during laser fusion cutting of 10 mm AISI 304 were determined for a laser power of 8 kW and a feed rate of 2 m/min. The cut front was recorded with a polarization goniometer, which uses the polarization of the process emission to determine the local surface angles, allowing to calculate the orientation of the normal vector of the surface. The records in this work were carried out with a frame rate of 75 kHz and a spatial resolution of about 30 µm. This allowed to identify big and small structures moving down the cutting front and to determine their velocities. The approximate velocity of the small structures was 9.1 m/s and for the big structures approx. 2.5 m/s. The information of a usual high-speed video was compared with the additionally obtained geometry information.

Influence of mill scale on oxygen laser cutting processes

Mill scale formed on the surface of hot rolled steels consists of magnetite (Fe3O4), hematite (Fe2O3) and wustite (FeO) layers, which can protect the steels from corrosion and other atmospheric effects. Existence of mill scale on the specimens’ surface has shown to be able to decrease the cut edge quality. Since the mechanism behind influence of mill scale on the laser cutting process is unknown, this work performs direct observation of oxygen laser cutting processes on specimens with and without removed mill scale layers. Oxygen laser cutting processes were carried out using Ytterbium fibre laser 1070 nm along the edge of 20-mm-thick-steel specimens which were attached to a borosilicate glass. Focal point of the laser beam was positioned to be 0.7 mm below the specimens’ surface. A high speed imaging system was arranged to face the glass, recording the cut front and kerf dynamics during cutting processes. It was found that cut front inclination angle increase when the mill scale was removed from the specimens’ surface. This implies that mill scale on the specimens’ surface seem to contribute in increasing the exothermal energy during laser cutting processes.

Understanding the Changed Mechanisms of Laser Beam Fusion Cutting by Applying Beam Oscillation, Based on Thermographic Analysis

The latest research on applying beam oscillation in laser beam fusion cutting revealed significant process improvements regarding speed and quality. The reason for this increasing process efficiency remains unexplained; however, theoretical investigations suggest the change in energy deposition (respectively heat conduction) as the cause. The present paper aims to analyze the energy deposition by a novel temperature measurement method. For this purpose, a conventional laser beam cutting setup was equipped with beam oscillation technology and a high-speed temperature measurement setup. Various characteristics of the temperature distribution in the process zone (spatial and temporal resolved temperature profiles, maximum and average values, as well as melt pool size) were evaluated for different conditions of beam oscillation (amplitude, frequency, cutting speed). Additionally, the geometrical properties of the process zone, defining the absorptivity have been measured. The comparison with static beam shaping reveals strong temperature volatility, which is induced by the way of energy deposition and an improved absorptivity over a substantial part of the cut front, with the overall result of enhanced heat conduction. For the first time, changed mechanisms applying beam oscillation instead of static beam shaping have been experimentally identified. Based on these measurements, a previously developed explanatory model was not only confirmed but also extended.

Local Vaporization at the Cut Front at High Laser Cutting Speeds

High-speed videos of the cut front and spectrometric measurements were applied to detect local vaporization on the cut front at high cutting speeds to show that with increasing feed rate, temporally short and intense flashes are generated by vaporization phenomena on the upper part of the cut front. The latter are accompanied by the emergence of an interrupted striation pattern on the surface of the cutting edge. The findings support the assumption that local vaporization at the cut front might be the cause for reduced quality of the cutting process at elevated cutting speeds. The observation of vaporization serves as a diagnostic method to anticipate a fail cut and the interrupted striation pattern.

Laser Cutting with annular intensity distribution

An annular intensity distribution has been assumed theoretically with great potential to provide high quality improvements to the cut flank during laser cutting process. We introduce several optical concepts to realize an annular intensity distribution with a high-power laser beam, such as an optical fiber with an annularly formed fiber core, a diffractive optical element, and axicons. Furthermore, cutting experiments for an axicon telescope with stainless steel are presented. With a specific optical setup and appropriate cutting parameters, a burr free cut flank with a surface roughness comparable to the state of art could be achieved. By means of cut front analysis, we assume that the burr free cut flank is a result of an improved gas coupling and the change of cut front geometry at the bottom side of the sheet.

Uticaj brzine rezanja i dubine reza na geometriju prednje linije reza pri obradi abrazivnim vodenim mlazom

Abrasive water jet machining is an unconventional machining process, which is increasingly being used. It is suitable for cutting complex contoured elements, in all types of materials. The major disadvantages of this machining technique are ununiform quality of the machined surface at different depths of cut, lagging of the cut front of the machined surface from the vertical plane and the phenomenon of striation patterns on the cut surface. The striation patterns on machined surface are the trail of the abrasive water jet through the material, and as such, they represents the cut front. Striation may result in different flaws occurring at cutting closed contours and may cause irregularities at cutting angles. The objective of this paper was to determine traverse speed and depth of cut effect on the cut front geometry i.e. on the striation geometry. The experiments were carried out on 6 mm and 10 mm thick samples of aluminum alloy AA 6060 (EN AW-6060; ISO AlMgSi). Samples were machined with several traverse speeds. The lagging of the cut front, at different depths of cut, was measured and analyzed. The obtained results enabled us to define the influence of traverse speed on striation geometry.

Influence of the Real Geometry of the Laser Cut Front on the Absorbed Intensity and the Gas Flow

The numerical investigations on experimentally determined cut front geometries presented in this paper show that the absorbed intensity reacts more sensitively to small local changes of the angle of the cut front than the gas velocity and the pressure. It is also found that the absorbed intensities near the top and the bottom of the cut front increase significantly when the initially regular process at high cutting speeds turns into the regime with interrupted striation patterns on the surface of the cutting edge.

The geometry of the cutting front created by Fibre and CO2 lasers when profiling stainless steel under standard commercial conditions

Cutting fronts created by CO2 and fibre lasers in stainless steel at thicknesses between 2 mm and 10 mm have been ‘frozen’ and their geometry has been measured. Standard commercial cutting parameters were used to generate the cuts for both types of laser. The resulting three-dimensional cutting front shapes have been curve fitted as polynomials and semicircles. Various features of the cutting front geometry are discussed including the lack of correlation of the cut front inclination with either the relevant Brewster angle or the inclination of the striations on the cut edge.

Characterization of the melt flow direction and cut front geometry in oxygen cutting with a solid state laser

Laser oxygen cutting is one of the most established laser based manufacturing processes and mainly used for cutting thick metal sheets of mild steel up to a thickness of 25 mm. Besides local absorption of the laser beam, the exothermal FeO oxidation reaction of iron and oxygen provides additional heat input to the cutting process. A lack of energy input can lead to dross formation and decreased cut quality. A detailed, phenomenological description of the melt flow direction and cut front geometry can help understanding dross formation and improving the stability of the cutting process at the cutting limit. Until now, the correlation of the melt flow direction and cut front geometry in case of dross formation is not completely understood. Therefore, high speed videos and metallographic cross and longitudinal sections have been prepared and correlated to dross formation. In order to describe the transition from a quality cut with optimized energy input to a fail cut with dross formation caused by a lack of energy input, three characteristics have been identified: The melt flow direction, characterized by the angle of melt expulsion, the local cut front angle in the lower part of the cut front and the width of the cut kerf at the bottom side of the sheet.

Fibre laser cutting stainless steel: Fluid dynamics and cut front morphology

In this paper the morphology of the laser cut front generated by fibre lasers was investigated by observation of the ‘frozen’ cut front, additionally high speed imaging (HSI) was employed to study the fluid dynamics on the cut front while cutting. During laser cutting the morphology and flow properties of the melt film on the cut front affect cut quality parameters such as cut edge roughness and dross (residual melt attached to the bottom of the cut edge). HSI observation of melt flow down a laser cutting front using standard cutting parameters is experimentally problematic because the cut front is narrow and surrounded by the kerf walls. To compensate for this, artificial parameters are usually chosen to obtain wide cut fronts which are unrepresentative of the actual industrial process. This paper presents a new experimental cutting geometry which permits HSI of the laser cut front using standard, commercial parameters. These results suggest that the cut front produced when cutting medium section (10mm thick) stainless steel with a fibre laser and a nitrogen assist gas is covered in humps which themselves are covered by a thin layer of liquid. HSI observation and theoretical analysis reveal that under these conditions the humps move down the cut front at an average speed of approximately 0.4m/s while the covering liquid flows at an average speed of approximately 1.1m/s, with an average melt depth at the bottom of the cut zone of approximately 0.17mm.

Relationship between temperature at cut front edge and kerf quality in fiber laser cutting of Al–Cu aluminum alloy

The characteristics of the temperature at cut front edge (Tce) during fiber laser cutting of AA2219 aluminum alloy were studied. A temperature monitoring system employing an infrared laser thermometer was established to record the time-domain Tce. A series of experiments was carried out to explore the relationship between the Tce and kerf quality. There was an optimal Tce range of 1800–1950°C for accepted kerf. By calculating the forces acting on molten pool, the existence of the optimal Tce range was demonstrated, and the effect of the Tce on kerf quality was discussed. The kerf quality was mostly dependent on the angle of the resultant to the downward (θ). Only when the θ was around the minimum of 38° that is corresponding to optimal Tce range, the kerf was accepted because the liquid metal was pushed away smoothly, although the resultant value was small. When increasing or decreasing the Tce outside the optimal range, the kerf was bad because the increased θ intended to push the liquid metal backward to attach on the kerf rather than push them away.

Determination of cut front position in laser cutting

Laser cutting has a huge importance to manufacturing industry. Laser cutting machines operate with fixed technological parameters and this does not guarantee the best productivity. The adjustment of the cutting parameters during operation can improve the machine performance. Based on a coaxial measuring device it is possible to identify the cut front position during the cutting process. This paper describes the data analysis approach used to determine the cut front position for different feed rates. The cut front position was determined with good resolution, but improvements are needed to make the whole process more stable.

Visualization of flow separation inside cut kerf during laser cutting of thick sections

The behavior of oxygen gas not only determines the oxidation heat input but also affects the removal of molten metal during laser-oxygen cutting. Flow separation inside the cut kerf is the major problem for the deterioration of cut ability and cut quality, especially for cutting thick steel sections, because it significantly weakens the shear stress on molten slag and changes the pressure gradient along the cut front. There have been some researches on flow separation using numerical simulation and model kerf. However, many experimental details and results are unclear, and the numerical simulation is questionable due to lack of experimental verification. In this paper, a model kerf is set up based on the previous results of observation and cutting trials. High resolution and high speed Schlieren visualization is used to observe the formation of flow separation in the model kerf. Stand-off distance, inlet pressure, cut front slope, nozzle displacement, kerf width, and nozzle tilting angle are used as variables for Schlieren visualization of flow separation for both a conical subsonic and a minimum length nozzle supersonic nozzle. The effect of the above parameters on the position and the pattern of flow separation is discussed based on the gas dynamic theory. It is found that nozzle displacement, kerf width, and nozzle tilting angle are the key factors for flow separation. Possible strategies for controlling flow separation are also proposed for both subsonic and supersonic nozzles.

Performance optimization of water-jet assisted underwater laser cutting of AISI 304 stainless steel sheet

Recent development of water-jet assisted underwater laser cutting has shown some advantages over the gas assisted underwater laser cutting, as it produces much less turbulence, gas bubble and aerosols, resulting in a more gentle process. However, this process has relatively low efficiency due to different losses in water. Scattering is reported to be a dominant loss mechanism, which depends on the growth of vapor layer at cut front and its removal by water-jet. Present study reports improvement in process efficiency by reducing the scattering loss using modulated laser power. Judicious control of laser pulse on- and off-time could improve process efficiency through restricting the vapor growth and its effective removal by water-jet within the laser on- and off-time, respectively. Effects of average laser power, duty cycle and modulation frequency on specific energy are studied to get an operating zone for maximum efficiency. Next, the variation in laser cut quality with different process parameters are studied within this operating zone using Design of experiment (DOE). Response surface methodology (RSM) is used by implementing three level Box-Behnken design to optimize the variation in cut quality, and to find out the optimal process parameters for desired quality. Various phenomena and material removal mechanism involved in this process are also discussed.

Effect of laser incidence angle on cut quality of 4 mm thick stainless steel sheet using fiber laser

Fiber laser has potential to outperform the more traditionally used CO2 lasers in sheet metal cutting applications due to its higher efficiency, better beam quality, reliability and ease of beam delivery through optical fiber. It has been however, reported that the higher focusability and shorter wavelength are advantageous for cutting thin metal sheets up to about 2mm only. Better focasability results in narrower kerf-width, which leads to an earlier flow separation in the flow of assist gas within the kerf, resulting in uncontrolled material removal and poor cut quality. However, the advarse effect of tight focusability can be taken care by shifting the focal point position towards the bottom surface of work-piece, which results in a wider kerf size. This results in a more stable flow within the kerf for a longer depth, which improves the cut quality. It has also been reported that fiber laser has an unfavourable angle of incidence during cutting of thick sections, resulting in poor absorption at the metal surface. Therefore, the effect of laser incidence angle, along with other process parameters, viz. cutting speed and assist gas pressure on the cut quality of 4mm thick steel sheet has been investigated. The change in laser incidence angle has been incorporated by inclining the beam towards and away from the cut front, and the quality factors are taken as the ratio of kerf width and the striation depth. Besides the absorption of laser radiation, beam inclination is also expected to influence the gas flow characteristics inside the kerf, shear force phenomena on the molten pool, laser beam coupling and laser power distribution at the inclined cut surface. Design of experiment has been used by implementing response surface methodology (RSM) to study the parametric dependence of cut quality, as well as to find out the optimum cut quality. An improvement in quality has been observed for both the inclination due to the combined effect of multiple phenomena.

Craterlike structures on the cut surface after oxygen-assisted laser cutting of steel

A new type of defects on the laser cut surface after oxygen-assisted laser cutting of low-carbon steel is reported. The defects look like craters or hollows. The craters are predominantly observed in the bottom part of the cut. At the sheet thickness of 16 mm, the craters have a characteristic diameter and depth of about 20 μm. The craters are presumably formed by the melt drops that are separated by the gas flow and fall onto the liquid melt film on the side wall near the cut front.

An investigation of energy loss mechanisms in water-jet assisted underwater laser cutting process using an analytical model

The water-jet assisted underwater laser cutting processes has relatively low overall efficiency compared to gas assisted laser cutting process due to high convective loss in water-jet from the hot melt layer and scattering loss of laser radiation by the water vapour formed at the laser–workpiece–water interaction region. However, the individual contribution of different losses and their dependency on process parameters are not fully investigated. Therefore, a lumped parameter analytical model for this cutting process has been formulated considering various laser–material–water interaction phenomena, different loss mechanisms and shear force provided by the water-jet, and has been used to predict various output parameters including the maximum cutting speed, cut front temperature, cut kerf and the loss of laser power through different mechanisms as functions of laser power and water-jet speed. The predictions of cutting speed, kerf-width and cut front temperature were validated with the experimental results. The modeling revealed that the scattering in water vapour is the dominant loss mechanism, causing ~40–50% of laser power loss. This also predicted that the percentage losses are lower for higher laser powers and lower water-jet speeds. In order to minimize the deleterious effect of vapour, dynamics of its formation due to laser heating and its removal by water-jet was experimentally studied. And, the cutting was done with modulated power laser beam of different pulse on- and off-times to determine the pulse on-time sufficiently short to disallow growth of vapour layer, still cutting be effected and the off-time enough long for water-jet to remove the vapour layer from the interaction zone before next pulse arrives. Compared to CW laser beam the modulated laser beam of same average power yielded higher process efficiency.