The effect of material thickness,laser power and cutting speed on cut path deviation in high-power diode laser chip-free cutting of glass

In the laser cleaving of brittle materials, using controlled fracture technique, thermal stresses are used to induce the crack and the material is separated along the cutting path by extending the crack. One of the problems in laser cutting of glass with this technique is the cut path deviation at the leading and the trailing edges of the glass sheet. Previous work has shown this deviation to be partly due to the high magnitudes of thermal stresses generated near the edges of the sheet. This paper reports on the experimental results of the effects of glass thickness, laser power and the cutting speed on cut path deviation in diode laser cutting of glass. Finite element modelling of the cutting process has also been used to simulate the transient effects of the moving beam and predict thermal fields and stress distributions. These predictions are validated against the experimental data and are used to explain the process mechanisms. It is shown that an increase in the thickness of the glass sheet for the same power and cutting speed or an increase in the cutting speed with constant power and a given sheet thickness results in smaller cut path deviations at the leading and trailing edges of the glass sheet.     

Effect of thermal stresses on chip-free diode laser cutting of glass

In laser cleaving of brittle materials using controlled fracture technique, thermal stresses are used to induce a crack and the material is separated along the cutting path by extending this crack. In this study, a glass sheet is stressed thermally using a 808-940 nm diode laser radiation. One of the problems in laser cutting of glass with controlled fracture technique is the cut deviation at the leading and the trailing edges of the glass sheet. In order to avoid this damage it is necessary to understand the stress distributions which control crack propagation. A study is conducted here to analyse the cut deviation problem of glass by examining the stress fields during diode laser cutting of soda-lime glass sheets. Optical microscope photographs of the breaking surface are obtained to examine the surface quality and cut path deviation while the latter is explained from the results of the stress fields which are obtained from a finite element simulation.     

Cutting glass substrates with dual-laser beams

In order to improve the cutting quality, a dual-laser-beam method was proposed to cut glass substrates in the current study, where a focused CO₂-laser beam was used to scribe a straight line on the substrate, and a defocused CO₂-laser beam was used to irradiate on the scribing line to generate a tensile stress and separate the substrate. The finite-element-method (FEM) software ANSYS was applied to calculate the temperature distribution and the resulting thermal stress filed. Through experimental study, it concluded that the glass substrate can be separated along an expected path with dual-laser beams and the cutting quality can be improved comparing with the cutting using a defocused laser beam alone. The relation between the cutting speed and the defocused laser power was also investigated in cutting glass with this method.     

Wedge cutting of mild steel by CO2 laser and cut-quality assessment in relation to normal cutting

In some applications, laser cutting of wedge surfaces cannot be avoided in sheet metal processing and the quality of the end product defines the applicability of the laser-cutting process in such situations. In the present study, CO₂ laser cutting of the wedge surfaces as well as normal surfaces (normal to laser beam axis) is considered and the end product quality is assessed using the international standards for thermal cutting. The cut surfaces are examined by the optical microscopy and geometric features of the cut edges such as out of flatness and dross height are measured from the micrographs. A neural network is introduced to classify the striation patterns of the cut surfaces. It is found that the dross height and out of flatness are influenced significantly by the laser output power, particularly for wedge-cutting situation. Moreover, the cut quality improves at certain value of the laser power intensity.     

Laser cutting of steel and thermal stress development

In laser cutting of sheet metals, thermal stresses are developed in the region of the cutting section. Depending on the cutting conditions and substrate material properties, the thermal stress levels can attain high values. In the present study, thermal stress developed in the region of the laser cut edges is modeled and temperature as well as stress fields are predicted. Temperature predictions are validated through the experimental results. It was found that the temporal variation of the maximum temperature along y-axis follows the laser heating source. However, temporal variation of von-Mises stress deviates slightly from the temporal variation of temperature along the cutting direction. Increase in scanning speed enhances the von-Mises stress levels due to the attainment of high temperature gradients in the substrate material.     

Cutting copper sheets using CO2 lasers

In this work various problems concerning cutting copper sheets using CO₂ laser are reported. First, all copper thermophysical properties, that regulate the process dynamics, and then the weight of each parameter has been evaluated numerically, even though only approximately. The surface absorption value of copper at room temperature and near the melting point and the order of laser power that is necessary to cause a gradual raise of the workpiece temperature from room to melting have been estimated. Then, the order of the cutting speed at which a sheet of a known thickness can be cut has been calculated. The analysis of all these problems, and the process dynamics and state of the art seem to confirm the validity of the current thesis on the impossibility of cutting copper by CO₂ laser. In the second part of the work the experimental data relating to the first ever tests on 0.2–4.0 mm thick copper sheet cutting by 2 kW CO₂ laser are reported. These first interesting results have been obtained thanks to the possibility of making overlapped layers of cupric oxide CuO, mixed with a small quantity of cuprous oxide Cu₂O grown under laser beam irradiation (CuO and Cu₂O, together, allow the laser cutting to be carried out). This has been confirmed by the analyses of the cutting edges with a computerized X-ray diffractometer. We have also seen that the per cent absorption of laser radiation at 10.6 micron does not increase in the presence of just cuprous oxide while, when the experimental conditions allow it, the growth of cupric oxide increases the absorption value to around 52–58 per cent, giving rise to the loop process with three variable quantities (temperature-oxide-absorption) that has been the winning clue of cutting process. The behaviour of the critical cutting speedV, the cutting widthsb and the productV·b versus the thickness for 2 kW CO₂ laser using a 4″ ZnSe and 3.5″ KCl focusing lens have been tested. Moreover, the influence of different gases and flows on the cutting process have been experimented. The work-speed turned out to be significant and various micrographic sections, performed on the workpieces have shown that the laser cutting quality is quite good. A first analysis of the results has shown that laser cutting is not comparable to the one of steel, so much so that the mathematical formalism developed for steels has proved to be unsuitable for copper. This paper was done under research contract with Italian Governmental Agency ISMEZR-INTERVENTO STRAORDINARIO NEL MEZZOGIORNO, which is supporter and financing organization within the project P.S. 35-105 IND. The first, the second and the third paper on this topic has been published on:-Review LA MECCANICA ITALIANA, n. 190, 1985, 45–47, ITALY (all rights reserved)-Review LASERS & APPLICATIONS, n. 3, Vol. 5, 1986, 59–64, High Tech. Publications Inc., Torrance, CA-USA (all rights reserved)-Proceedings of ECOOSA'86-European Conference on Optics, Optical Systems and Applications, Sept. 30–Oct. 3, 1986, Florence, ITALY (all rights reserved)     

Features of Laser Through-Cutting of Silicate Glass by a Fiber Ytterbium Laser

Some principal aspects of silicate glass cutting by controlled laser through thermal cleavage are considered. In particular, it is shown that the cutting speed in the case of ytterbium fiber laser radiation with a wavelength of 1.065 μm lying in fact in the glass transmission range (more precisely, in the low absorption region) depends linearly on the laser power. It is shown that the glass end face takes various geometrical shapes under various conditions of bulk heating and cooling. Therefore, to obtain a homogeneous end face, it is required to stabilize both the laser radiation power and the laser beam speed at a corresponding laser beam geometry in the cut region. Methods for obtaining various cross section shapes of the glass end face and methods for obtaining blunt edges of end faces are presented.     

Laser cleaving on glass sheets with multiple laser beams

A multiple laser system consisting of CO₂ line-shaped and Nd-YAG pulsed lasers was applied to cleave a soda-lime glass substrate in this study. Due to an increase of absorption coefficient of the wavelength of 1.06 μm for Nd-YAG laser on the soda-lime glass at high temperatures, the glass sheets were preheated by the CO₂ line-shaped laser and followed with the pulsed Nd-YAG laser to generate a mixture fracture mode on the substrate. The stress distribution on the glass substrate cleaved by the multiple laser beams has been analyzed. An uncoupled thermal–elastic analysis based on the finite-element method (FEM) was made. The numerical results show that the stress field of the fracture region is caused by a complex stress state and the cleavages are significantly affected by the pulsed laser. A clean cut of the soda-lime glass substrate could be obtained due to a large shear stress state on the cutting direction with the pulsed laser radiated on the glass substrate.     

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.     

Laser cutting of thick ceramic tile

This paper details developments in the CO₂ laser cutting of thick ceramic tiles, that is thicknesses of 8.5 mm and 9.2 mm. These tiles were cut at a combination of different cutting speeds to determine the necessary cutting parameters for various tile geometries. Different cutting modes were used in conjunction with different cutting speeds to investigate cut quality after laser processing. The work also looked into the effects on cutting through using various shield gases. Multipass cutting and underwater cutting were performed to examine their effects on thermal load during processing.     

Application of iterative path revision technique for laser cutting with controlled fracture

Laser cutting using the controlled fracture technique has great potential to be employed for the ceramic substrate machining. The heat produced on the surface of a ceramic substrate by the laser separates the substrate controllably along the moving path of the laser beam. Because the extension of the breaking frontier is lager than the movement of the laser spot, the actual fracture trajectory deviates from the desired trajectory when cutting a curve or cutting an asymmetrical straight line. To eliminate this deviation, the iterative learning control method is introduced to obtain the optimal laser beam movement path. The fracture contour image is grabbed by a CCD camera after laser cutting completion. A new image processing system is proposed to detect the deviation between the desired cutting path and the actual fracture trajectory. The laser-movement path for the next trial can then be determined according to the iterative path revision algorithm. The actual fracture trajectory converging to the desired cutting path is assured after a few path revisions. The experimental materials used in these experiments are alumina ceramics and the laser source is CO₂ laser. The proposed system can achieve a machining precision of about 0.1 mm.     

A numerical simulation of machining glass by dual CO2-laser beams

In the flat panel display (FPD) industry, lasers may be used to cut glass plates. In order to reduce the possibility of fracture in the process of cutting glass by lasers, the thermal stress has to be less than the critical rupture strength. In this paper, a dual-laser-beam method is proposed, where an off-focus CO₂-laser beam was used to preheat the glass sample to reduce the thermal gradients and a focused CO₂-laser beam was used to machine the glass. The distribution of the thermal stress and the temperature was simulated by using finite element analysis software, Ansys. The thermal stress was studied both when the glass sample was machined by a single CO₂-laser beam and by dual CO₂-laser beams. It was concluded that the thermal stress can be reduced by means of the dual-laser-beam method.     

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.     

Estimating cutting front temperature difference in disk and CO2 laser beam fusion cutting

A three-dimensional, semi-stationary, simplified thermal numerical model was developed. The average cutting front temperature difference in disk and CO₂ laser beam fusion cutting of 90MnCrV8 was estimated by computing the conductive power loss. Basing on heat affected zone extension experimentally measured and using an inverse methodology approach, the unknown thermal load on the cutting front during laser cutting was calculated. The accuracy of the numerical power loss estimation was evaluated comparing the results from simulation with the ones from analytical models. A good agreement was found for all the test cases considered in this study. The conduction losses estimation was used for justifying the lower quality of disk laser cuts due to the lower average cut front temperature. This results in the increase of viscosity of molten material and in the subsequent more difficult ejection of the melted material from the cut kerf.     

Designing of an efficient multi-aperture,edge pumped thin-disk laser

In this article the operation of a composite Yb:YAG/YAG asymmetric hexagonal thin disk laser is optimized. This structure consists an Yb:YAG thin sheet as laser material and a diffusion bonded undoped material on top of thin sheet. Three diodes are placed outside the asymmetric hexagonal shape disk, and the emitting light is coupled via three lens ducts through its three edges and propagates through the disk along the zigzag path and repeatedly passes the gain medium, thus pump uniformity and absorption efficiency are improved. A Monte Carlo ray tracing code and Finite Element Analysis is utilized for calculation of absorbed pump density and volume temperature distribution and also thermal and mechanical stresses, respectively. Finally the output power is calculated. The triplet designing parameters, namely absorbed power efficiency, absorption uniformity and temperature distribution are optimized. Results show this edge-facet pumped geometry is a high efficient configuration for thin gain lasers.     

Laser cutting of holes in thick sheet metals: Development of stress field

Laser cutting of hole in a mild steel thick sheet metal is investigated. Temperature and stress fields developed around the cutting section are simulated using the finite element method. An experimental is carried out accommodating the simulation parameters. The residual stress developed in the cutting section is measured using the XRD technique and findings are compared with the predictions. Optical microscopy and SEM are carried out to examine the morphological changes in the cutting sections. It is found that temperature decays sharply in the region of the laser heat source, which results in high temperature gradient in this region. This causes the development of high stress levels around the cut edges. The residual stresses predicted are in agreement with the measured results.     

CO2 sensor for mainstream capnography based on TDLAS

The setup and signal processing for a mainstream capnography sensor is presented in this paper. The probe exhibits an optical path length of 2.5 cm and is equipped with a vertical-cavity surface-emitting laser at 2 μm. The sensor does not need any calibration, since the CO₂ absorption line as well as the laser background is measured using direct tunable diode laser absorption spectroscopy. Unavoidable optical fringes are reduced with a self-developed fringe rejection method. The sensor achieves a concentration resolution <300 ppmv at 4 vol% and a measurement rate >30 Hz.     

Formation and removal of multi-layered fluorescence patterns in gold-ion doped glass

We report the formation of fluorescence patterns inside gold-doped glass medium by femtosecond-laser fabrication. Strong fluorescence images appeared from the irradiated multi-layered region after low temperature annealing. We removed the images by exposing the glass to an electric furnace or a CO₂ laser beam for high temperature annealing. The method was also applied to recording, reading, and erasing of fluorescence data by a femtosecond laser, a 405-nm laser diode, and a CO₂ laser respectively.     

Laser cutting of Kevlar laminates and thermal stress formed at cutting sections

Laser cutting of Kevlar laminates is carried out and thermal stress field developed in the cutting region is predicted using the finite element code. Temperature predictions are validated through the thermocouple data. The morphological changes in the cutting section are examined by incorporating optical and scanning electron microscopes. It is found that temperature predictions agree well with the thermocouple data. High values of von Mises stress are observed at the cutting edges and at the mid-thickness of the Kevlar laminate due to thermal compression formed in this region. The laser cut edges are free from whiskers; however, striation formation and some small sideways burning is observed at the kerf edges.