Laser Cutting With Controlled Fracture

INTRODUCTION: In the vast field of machining time and money are the matters of concern. This method uses the laser to scribe wafers along the desired separation line, after which the scribed wafers are immersed in an ultrasonic cell and broken along the scribed lines by ultrasonic energy .In recent years the laser scribing technique has been applied in the integrated circuit industry for cutting ceramic substrates .The complete of scribing process requires either that the continuous laser evaporates the material to form the deep groove or that the impulse laser drills the serial blind holes at the depths of 1\3 to 1\2 of the substrate thickness .For this it is necessary to concentrate the laser energy on a narrow line, with the workpiece on the focal plane . The scribed substrates are broken along the scribed line by applying a mechanical force induced from the cracking roller.

EVOLUTION AND WORKING: One of the important laser cutting method that has great potential for machining of ceramic substrates is the CONTROLLED FRACTURE TECHNIQUE proposed by “LOMLEY” .The laser energy absorbed in a local area produces a mechanical stress that will cause the material to separate along the path of the laser beam. The material separation is similar to a crack extension and the fracture controllable. LOMLEY successfully applied this technique in laser cutting of brittle materials such as alumina ceramic substrates and glass by using aCO2 laser. The laser power required for this method is less than that for cutting by laser evaporation, laser scribing and the cutting speed is much higher. The technology of laser cutting with controlled fracture has great advantage in the cutting of brittle material such as glasses it is a single step process and no mechanical snapping is required because the cutting edges are free of chipping and microcracks, the edges need not be further cleaned grinded. GROVE proposed a related method of controlled fracture for cutting glass, which the cutting speed is higher. The crack extension in laser cutting by controlled fracture technique is due to the thermal stress induced from the laser heat .In order to ensure accurate splitting; it is essential that the stress field must be symmetrical with crack. CHUI noted that the controlled fracture technique is not quite successful for fast cutting speeds and long cutting lengths, and attempted to increase the speed with higher power only results in losing control of crack propagation. Recently, KONDRATENKO, UNGER and WITTENBECHER have further investigated the use of lower power laser to separate glass without melting the material, an additional water jet coolant is applied to produce tensile stress along the cutting path. KONDRATENKO developed a method of splitting nonmetallic materials; especially glass based on the idea of controlled fracture. Here the surface of the glass plate is heated by CO2 laser and heated zone is subjected to local cooling to form a blind crack .The cooling is in the form of jet of water entrained with air .The method can increase the cutting speed and accuracy, and also can control the depth, shape and the angle of cut face formed by the crack. UNGER and WITTENBECHER proposed a system for cutting glass in a single step process, which combines a laser beam and a quenching jet. The travelling laser spot is closely followed by a quenching jet, which consists of a nozzle through which a cold air water jet is directed at glass surface. This rapid quench creates a tensile stress along the line defined by the sweeping of the laser spot. 1

In this paper, we investigate the fracture mechanism of the laser cutting with controlled fracture. When the moving laser beam is focused on the material surface, a shallow groove is formed by evaporative removal of material .The heat generation by laser induces a transient temperature distribution and a time dependent stress field .The fracture grows in a stable fashion along the moving path of the laser spot .The temperature field and the stress field for the 3 dimensional simulation are obtained by the finite element calculation. The mechanism by which the fracture is controlled is proposed from the analysis of the stress field in this paper crack detection technique using the image processing method proposed by “TSAI” and “LIOU” is applied to measure the position of the extending crack tip during the laser cutting process .In the present work, the machining parameters such as laser spot size, cutting speed, specimen size and the cutting quality are discussed. It is also found that the defocused laser beam is preferred for laser cutting and the larger spot size, the better the cutting quality and the higher cutting speed ..

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LASER CUTING SYSTEM AND SPECIMEN: The laser cutting system as shown in fig is composed of continuos wattage CO2 laser, an XYZ positioning table, and a computer .The laser beam moves along the upper surface of the workpiece mounted on the positioning table. The wavelength of the CO2 laser is 10.6 um and the min. dia. Of the focused spot of the CO2 laser is 127um.The focal plane of the CO2 laser is placed at a defocused distance 2mm in which the dia. Of the laser is about .18mm .The experimental specimen in this paper are alumina ceramic substrates produced by the Kyocera company Japan

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EXPERIMENTS OF LASER CONTROLLED FRACTURE:

CUTTING

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In order to understand the mechanism of symmetrical cutting with controlled fracture technique, the cross sections along the cutting path are investigated. Using the CO 2 laser to cut an alumina ceramic substrate the laser beam transverse a distance but does not go through the material completely, then the cutting is stopped. The separation of the specimen is similar to the crack propagation along the cutting path, but the crack is invisible to the naked eye .In order to distinguish the different fracture regions, the dark ink is painted along the cutting path to he final position of the laser spot, the ink will penetrate to the separated surfaces. Then the specimen is stripped in 2 pieces along the cutting path. The dark ink only exists in the fracture surface induced from the controlled fracture. Photographs of the specimen from the cutting speed of 2mm\s to 10mm\s and the laser output power 30Ware shown. Photograph shows following 3 regions 1. heat affected region due to laser heat 2. the controlled fracture region throughout the thickness 3. the fracture region The dark region indicates the heat affected region and the lighter region indicates the controlled fracture region. It is found that the depths of the heat-affected region are .2mm to.1mm for cutting speeds of 2mm\s and 10mm\s. Some of the laser heat is absorbed to melt the material and decrease the bonding strength, and then the material will resolidify and induce a primary groove, which closely follows the laser spot path. The controlled fracture is due to the groove existing in the affected region extending from up to down after the passage of the laser

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NON UNIFORMLY EXTENDING CRACK: In order to understand the crack propagation during the laser cutting with controlled fracture the image processing system of crack detection proposed by TSAI and LIOU is applied to obtain the crack image continuously during the laser cutting. ACCD camera is placed below the specimen, the images of the crack tip are grabbed by camera and computer and analyzed by image processing program .The distance between the crack tip and the laser spot can be measured by using the crack detection 5

system .For cutting condition of laser power 30Watts and cutting speeds 2mm\s,4mm\s and 10mm\s, the distance from the crack tip to laser spot are obtained from the detection method as shown. The crack leaves behind the laser spot. The interesting phenomenon is that crack propagation is not at the constant speed. The distance between the crack tip and the laser spot varies from 2.2mm to 2.8mm for the cutting speed of 10mm\s, varies from 1.3mm to 2mm for the cutting speed4mm\sand varies from 0.2mm to 1.5mm for cutting speed 2mm\sec

FRACTURE EXTENSION MODEL OF ASSYMETRICAL CUTTING: 1.STRAIGHT LINE CUTTING: The stress state at the crack tip is not pure mode I for the asymmetrical cutting. If the stress is not symmetrical with the crack, the actual cutting fracture trajectory will deviate from the desired cutting path. 2.CURVE CUTTING: In the curve cutting process, the cutting is symmetrical .The crack tip behaves as the mixed mode stress condition and the crack growth does not exactly follow the path of laser spot. Thus the actual fracture trajectory will deviate from the applied path of the laser spot. A sine curve cutting of the alumina ceramic substrate is shown in fig. Indicating that the laser beam moves to the turning point of curve line, but the crack tip lags a short distance behind the laser spot at the same time the laser beam begins to turn a direction tangential to the primary cutting path, resulting in a mixed mode stress intensity factor, which could make the crack tip deviate from the primary cutting path. The wavelength half is 90mm and wave magnitude is 14mm for the sine curve cut .The max. Deviation is about 1.9mm .The root mean square deviation is 0.8mm. The crack tip begins turning to the curved path too early to follow the desired path, and the deviation is severe if the cutting speed is higher. 3.RIGHT ANGLE CUTTING: Using the laser cutting technique with controlled fracture to cut aright angle, the actual fracture trajectory will deviate from the desired right angle and become around angle. The deviation is due to lag of the separated crack tip from the laser spot, so that when the laser spot arise at the turning point, the crack tip has not yet reached the turning point at the instant the laser beam turns to the perpendicular direction and the crack tip will also turn to the same direction. Because the crack tip has not yet reached the turning point of the right angle, so the actual fracture trajectory deviates severely from the desired right angle.

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CUTTING SPEED, SPECIMEN WIDTH AND OTHER PARAMETERS: In order to investigate the relation of the cutting speed, laser power and width of the specimen, three sizes alumina ceramic substrate are selected as specimens the specimen sizes are wide 101.6 x 101.6 x 0.635 mm3 medium 50.8x 101.6 x0.635mm3 and narrow 25.4 x 101.6 x 0.635 mm3. The experimental results of the cutting speed and the required laser power are shown in fig .The symbols represent the cutting speed and required laser power of which the substrate can be separated .The solid lines represent the max, cutting speed, which can be reached for the different laser power outputs .It can be seen that if the specimen is narrow, then the max. Cutting speed is higher, the Stress State and the extending speed of the crack are strongly affected by the specimen size.

CUTING SPEED AND LASER SPOT SIZE: In the laser cutting operation with controlled fracture, the specimen must be placed below the focal plane .The spot size is min. in the focal plane. If the distance between the specimen and the focus lens is longer than then focal length of the lens, the spot size will be the enlarged and the cutting speed is affected by the spot size. The alumina ceramic substrates with thickness 0.635-mm are cut with variable spot size and with constant laser power output of 30Watt-.The dia. Of the laser spot equals the product of the distance from the focal plane and the diverged angle .The max. Cutting speed that can be reached for variable laser spot sizes are shown in fig. For larger spot sizes, the cutting speed is higher. For the larger spot sizes, the laser energy is diverged to a larger area, so less material will melt and be evaporated, so that the most heat energy can be absorbed to induce higher thermal stress. Strong light occurs in the cutting process for the small laser spot size, but for the large spot size, the laser energy is not as concentrated, so local burning in the specimen does not occur.

CUTTING QUALITY: The surface quality of laser cutting with controlled fracture is very smooth and has very few little defects. Photographs obtained by scanned electron microscopy of the cutting surface from the controlled fracture, laser scribing, and conventional laser cutting are shown in fig crossection of controlled fracture is under the cutting conditions of laser output 20 W and cutting speed 15mm\sec. The result in max. Surface roughness RMax =26µm .The specimen of laser scribing is performed by using the pulsed CO2 laser. The conventional laser cutting is 40Watt with cutting speed 1mm\sec. In the 2 methods of controlled fracture and laser scribing, the surface are much better than the conventional laser cutting by evaporating the material.

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UNSTABLE FRACTURE: In some conditions of special stress distribution, the material separation becomes unstable and cutting fails. A specimen containing a small hole with a dia. Of 0.2mm is shown in fig where the main crack extension is influenced by the small hole and becomes unstable. After cutting 20mm length the main crack linked with the hole and ruptured suddenly. These unstable fractures occur under the condition of tensile stress or shear stress of the mixed mode condition. For the controlled stable fracture the stress near the cracktip is compressive and the crack propagation is silent. But in the final stage of cutting, when the crack tip propagates close to the edge of the specimen

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CONCLUSION: Laser cutting technique of controlled fracture is successfully applied for the cutting of alumina ceramic substrate .The cutting process is divided into three stages .In the first stage the fracture is initiated due to tensile stress .In second stage the stable growth of fracture takes place when stress behaves comparatively while laser passing, this stress is then relaxed and tensile stress is induced in the substrate. In the third stage the unstable fracture takes place. The factors affecting the process are distance between the crack tip and laser spot that affects stress, cutting speed, which is greatly affected by specimen size .The efficiency of the process depends highly on the specimen size, laser used and its range, type of the specimen and the type of cut to be obtained. Various types of cuts can be obtained by this process, so can be one of the widely used advanced manufacturing techniques. Mainly used in Japan now a days .The process drawbacks are that high perfection requiring products cannot be manufactured, as the expected tolerance level is more. High cost investment for laser and that having tendency of lasers constricts the prospects. In near future it’s expected that drawbacks will be overcome by further development in the process, which will widen the heights soared by the laser wings

REFERRENCES: 1. D.K.PATEL & J.D.BAKER - METHOD FOR LASER SCRIBING 2. GROVE , WRIGHT & HANMER – CUTTING OF GLASS WITH LASER 3. E.H & LIOU - THE CUTTING EDGE OF LASER 4. JOURNAL OF MANUFACTURING SCIENCE & TECHNOLOGY – AUG’03 , MAY’03

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Abstract: In the increasing and vast field of manufacturing in less time we have to go at a speed of supersonic jet. In this paper we will understand fracture mechanism using laser cutting with controlled fracture. Laser cutting using the controlled fracture technique has great potential to be used for the machining of brittle materials. In this technique, the applied laser energy produces a mechanical stress that causes the material to separate along the moving path of laser beam .The material separation is similar to a crack extension and the fracture growth is controllable. The laser heat first induces compressive stress around the laser spot. After the passage of the laser beam, the compressive stress is relaxed, and then a residual tensile stress is induced. The stable separation of the brittle material is due to the local residual tensile stress. The experimental material in this study are alumina ceramic and the laser source is CO2 laser. The relationship between the laser power, cutting speed, diameter of the laser spot, and specimen geometry are obtained from the experimental analysis, and the phenomenon are also explained from the result of the stress analysis. This method is a single step process and no mechanical snapping is required. Because the cutting edges are free of chipping and micro cracks, the edge need no further cleaning or grinding. The paper discusses the use of lower power laser to separate glass without melting the material. An additional water jet coolant is applied to produce tensile stress along the cutting path. This improvement of the controlled fracture method has been recognized as having good prospects of fracture use. We investigate the fracture mechanism of laser cutting with controlled fracture when the moving laser beam is focused on material surface; a shallow groove is formed by evaporative removal of material. The heat generation by laser induces a transient temperature distribution and a time dependent stress field. The fracture grows in a stable fashion along the moving path of the laser spot . It is clear by the study that mechanism has wide prospects in the future and will be amongst the leading manufacturing technique.

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ADVANCED MANUFACTURING TECHNIQUES

LASER CUTTING WITH CONTROLLED FRACTURE

BAPURAO DESHMUKH COLLEGE OF ENGINEERING PRESENTED BY: ASHWINI PARAGE - VIII SEM PRODUCTION SANCHITA PARADKAR- VIII SEM PRODUCTION 12

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