2. LASER BEAM MACHINING (LBM) PROCESS CHARACTERISTICS

Transcription

1 61 2. LASER BEAM MACHINING (LBM) PROCESS CHARACTERISTICS 2.1 DESCRIPTION OF VARIOUS TYPES OF LASER MACHINING Laser beam machining process has various types of micro-machining applications such as laser cutting, fine cutting, laser drilling, micro-drilling, precision machining, micro-fabrication, laser marking and engraving etc. and these techniques are outlined in subsequent discussions LASER CUTTING Laser cutting, the most established laser materials processing technology, is a method for shaping and separating a workpiece into segments of desired geometry. The cutting process is executed by moving a focused laser beam along the surface of the workpiece with constant distance, thereby generating a narrow typically some tenths of a millimeter cut kerf. This kerf fully penetrates the material along the desired cut contour. Laser cutting is a thermal cutting process. During the process, part of the laser radiation is absorbed at the end of the kerf, called the cutting front. Fig.2.1 shows the cutting front of the Laser Beam and co-axial nozzle orifice with gas flow. The absorbed energy heats up and transforms the prospective kerf volume into a state (molten, vaporized, or chemically changed) which is volatile or which can be removed easily. Normally, the material removal is supported by a gas jet, impinging coaxially to the laser beam. This cutting gas accelerates the transformed material and ejects it from the kerf. Various types of laser cutting processes such as fusion cutting, oxidation cutting, and vaporization cutting, mixed process, new cutting processes with specialities, single and multi-dimensional cutting are described here in after [2].

2 LASER FUSION CUTTING In laser fusion cutting the kerf volume is transformed dominantly into the molten state and blown out of the kerf by a high-pressure (upto 2 MPa) inert gas jet. Therefore, this process is also called high-pressure or inert gas cutting. Fig.2.2 exhibits the Laser fusion cutting phenomenon. Fusion cutting is applicable to all metals, especially stainless steel and other highly alloyed steels and aluminium and titanium alloys, to many thermoplastic polymers and some ceramics. During laser fusion cutting, the laser beam is the only heat source. The inert gas jet mainly nitrogen or argon is responsible for melt ejection and for shielding the heated material from the surrounding air. The resulting cut edges are free of oxides. The main technical demand during this cutting process is to avoid adherent melt or dross attachment at the bottom edges of the kerf LASER OXIDATION CUTTING Laser oxidation cutting, sometimes also called laser oxygen cutting or simply laser gas cutting, uses oxygen as the cutting gas. The laser beam is mainly responsible for igniting and stabilizing a burning process within the kerf. Fig.2.3 represents the Laser Oxidation Cutting process. This exothermic reaction of the oxygen with the material such as mainly steel, especially mild and low alloyed steel supports the laser cutting process by providing additional heat input. In some cases, this heat may be dominant heat source. The result is higher cutting speeds compared to laser cutting with inert gases. The formation on an oxide layer on the cutting front increases the absorption of the laser radiation at angles of incidence below 80 compared to the absorption of a pure metallic melt. Especially with mild and low alloyed steel, the oxides reduces the viscosity at temperatures above 1900 K and surface tension of the melt and thereby simply melt ejection. The resulting cut edges are oxidized. It is important to balance the laser power and other process parameters to avoid burning of sharp corners, small circles, or narrow bridges LASER VAPORISATION CUTTING In laser vaporisation cutting, the kerf formation is mainly realized by vaporisation of the material. Fig.2.4 represents the Laser Vaporisation process.

3 63 Typical materials that are cut by this kind of processes are acrylic, thermoset polymers, rubber, some thermoplastic polymers, wood, paper, leather, and some ceramics. To avoid precipitation of the hot, gaseous emissions on the workpiece and to prevent them from condensation within the developing kerf, a process gas jet is used for blowing the material out of the kerf. Vaporisation cutting of metals is possible only if the relative contribution of the molten state is minimised by using repetitive, short laser pulses in conjunction with high power densities. If different laser cutting processes can be applied for cutting of metals, vaporisation cutting is the method with the lowest speed, but it is suitable for very precise, complex cut geometries in thin work-pieces MIXED PROCESS In practice, the cutting process is often not based on one individual transformation process but rather on two or more simultaneous transformations. For example, for reducing costs sometimes compressed air is used instead of oxygen or nitrogen as the cutting gas e.g. during cutting aluminium and plastics. The oxygen content within the air leads to partial burning of the kerf material or the emitted byproducts. On the other hand, this oxidation does not dominate the power and mass balance of the whole cutting process but only contributes to a mixed process. Another example of a mixed process is the laser inert gas cutting of thin sheet metal with high irradiance, resulting in partial evaporation of the superheated melt film of the cutting front LASER DRILLING The physical processes of laser hole drilling are fairly simple. The solid material absorbs laser energy, and the photon energy is converted to thermal energy. When the temperature rises above melting and vaporization temperatures, melting and vaporization occur. At high laser irradiance (>10 6 W/cm 2 ), materials are removed mostly through vaporization. The vapour also builds up pressure that pushes molten material out of the hole and this phenomenon is called as flushing. In most practical applications, a high-pressure assist gas jet is directed toward the hole,

4 64 further assisting material removal. For certain materials, such as polymers, photon absorption by the solid material can lead to direct breaking of bonds between atoms or molecules, this leads to photo-fragmentation and direct vaporization or ablation without melting. It is advantageous therefore to use pulsed lasers that heat up and remove a small volume of material very quickly with each pulse. To quantify such a statement, the time estimate, t v to reach the vaporization temperature is as follows [2]: t v = K c 4 l 2 (T 2 v T0 ) (2.1) where, T 0, ambient temperature, T v, latent heat of vaporization, K, thermal conductivity,, density, c, specific heat l, absorbed irradiance of the laser beam (in W/cm 2 ). The laser pulse duration may be longer than the t v. This equation can be used as a rough guide to verify whether effective drilling can take place with certain laser pulse duration and irradiance. For example, at 10 5 W/cm 2 irradiance, t v is 1.84 ms for nickel; while at 10 7 W/cm 2 irradiance, the time shortens to 184ns. Therefore vaporization happens very quickly for high irradiance. These times are similar to typical pulse durations of free-running and Q-switched Nd:YAG lasers, respectively. Conversely, the irradiances mentioned above as the threshold irradiances above which effective vaporization and drilling occur using these pulse durations. When a hole is drilled without relative motion of the laser beam to the workpiece, the process is called percussion drilling. The most popular drilling lasers are pulsed Nd:YAG lasers, and the maximum diameter of a percussion drilled hole is usually less than 1 mm. For holes that are larger, a technique called trepanning is used, which rotates either the workpiece or the laser beam to cut out a circular hole. Holes of rectangular or other shapes can also be pierced in this fashion. There are two ways of generating holes: percussion drilling and trepanning. Percussion drilling

5 65 is typically used for hole diameters less than 0.63mm, while trepanning is used for generating holes of larger diameter PERCUSSION DRILLING Percussion drilling is accomplished by transforming a cylindrical section of material from a solid to a vapour. Holes are drilled by focusing the laser beam to the approximate diameter required, and exposing the material to one or more pulses of energy. Small changes in hole diameter can be made by increasing or decreasing pulse energy. Even though nonmetals are poor thermal conductors, too much pulse energy can cause excessive melting in materials that are not readily sublimated, and become black after burning. Too much pulse energy can also distort the shape of the hole. Insufficient energy can cause the hole to be too small or tapered. In some cases the material type and thickness allows for the percussion drilling of holes as 0.8mm. examples of such materials are polyethylene and acetal. The shape or roundness of percussion-drilled holes is determined by the spatial characteristics of the laser beam. Even with lasers having very low M 2 (Mode Quality of laser beam) values, devices such as spatial filters and stops can be used to improve roundness. Repeatability of percussion-drilled holes is as good as the repeatability of laser pulse to pulse energy output. Percussion-drilled holes are typically small which is less than 0.5mm diameter, although larger diameter holes can be drilled on very thin materials. Lenses used for percussion drilling are selected according to the diameter of the holes to be drilled. For a given beam diameter input to the focus lens, shorter focal length lenses focus the laser beam to smaller diameters. For most optical systems, a 63.5 mm. focal length lens can be used when drilling holes 75 m to 25mm in diameter, and 127mm focal length lenses can be used when drilling holes 0.2mm to 0.5mm in diameter. Focal position can be optimal on, above, below the material surface, depending on the desired hole characteristics. The best focus is determined empirically following an evaluation of the hole quality. Percussion drilling can be accomplished with or without gas jet. Most often, compressed air is used as the process assist gas. Nozzle designs vary, but typical orifice diameters range from 1mm to 6.25mm, and nozzle standoff ranges from 0.5mm to 25.4mm. if a

6 66 gas jet is not used, an alternative lens protection method must be employed. Vapour produced by laser drilling will condense on the lens surface, creating a film that readily absorbs laser energy. The heat resulting from absorption will cause the lens to fracture spontaneously TREPANNING During Laser Trepanning operation, the holes up to 6.25mm diameter can be drilled using a rotating optical device. So-called boring heads rotate the focused laser beam at very high rates. Holes are drilled by either a single pass or multiple passes of the laser beam. Drilling by trepanning is to cut a hole around its periphery. Depending on the hole diameter, a slug may be produced. Boring heads usually use 63.5mm focal length lenses and are equipped with gas jets similar to those used for laser cutting applications. Roundness of the holes produced by boring heads is exact, and repeatability of hole diameter is excellent. Boring-head-hole diameter is established either manually or by use of a programmable controller. Trepanned holes can also be drilled by interpolation of linear axes, moving either the material or the laser focusing device. Speed of drilling by interpolation is dictated by the size of the linear axes. The linear axes servo system must be properly tuned to produce circular holes. Specially beam-manipulation devices with very small linear axes are used to move the focusing device in a circle. The system controller can be programmed to establish desired hole diameters [2] NEWER LASER DRILLING TECHNIQUES FOR METALS AND NON-METALS The conventional methods used to drill fine holes using solid state lasers involve single pulse drilling, the superposition of a series of pulses over the same focal area (percussion drilling), and the rotation of the work piece under the laser focal spot for producing larger-diameter holes (trepanning). The trepanning method involves direct scanning of the laser beam around the hole circumference. For smaller-diameter hole drilling, the optical trepanning technique can be used. In these case the focusing lens is rotated in the beam path. The drilling on-the-fly process is a newer technique, which has a slight variation of the percussion drilling technique. During this process the pulses are

7 67 delivered while the laser beam is moving relative to the work-piece. The multiple pulses are required to make a hole, the motion system and laser pulse repetition frequency are synchronized so that a predetermined number of pulses are delivered to the same location. The method is used to increase the hole production rate. The edge quality, circularity and the aspect ratio of the holes depend on the method of drilling and the laser beam properties. The high quality of holes produced due to the short pulse ( s) laser interaction with the material, leading to the rapid creation of vapour and plasma phases, negligible heat conduction and the absence of a liquid phase. The latter allows better control during process with further enhancement in reproducibility. Another newer drilling technique is the use of diffractive optical elements (DOE) in the path of the laser beam to produce an array of micro-holes. When conventional beam focusing geometries are used, the beam intensity in the focal plane is so high that ionisation, plasma formation and other nonlinear effects such as self-focusing, self-phase modulation, and filamentation can occur in the surrounding air. These effects are detrimental and cause a distorted spatial profile of the beam. The use of diffractive optical elements in the beam path can minimize these distortions and provide accurate beam delivery to the work piece. In effect, significant improvement in the resultant quality of the holes can be obtained in terms of reproducibility, burr-free edges, and high-aspect-ratio drilling in a wide range of metals and nonmetals. Another benefit of using diffractive optical elements, is high energy throughput compared to a typical masking-imaging system. A particularly challenging example of an application using this technique is quartz glass drilling. At pulse widths less than 200 femtoseconds, highly reproducible 20 m diameter holes can be drilled in 1mm thick quartz substrates. Drilling of micro-hole arrays in polyimide material for ink-jet printers and micro-hole drilling of glass woven epoxy material are some of the other applications where this method is particularly effective.

8 LASER MARKING OR ENGRAVING Laser marking often takes the form of an alphanumeric code imprinted on the label or on the surface of the product to describe date of manufacture, best-before date, serial number or part number, etc. but the mark can also be a machine-readable bar code or 2D symbol (ID matrix). As well as coding, laser marking sometimes takes the form of functional marking such as gradation lines on a syringe or decorative marking such as a logo or graphic image on an integrated circuit. Laser marking is often one of the final processes in the assembly of a product, taking place during the final filling cycle on a finished product before it is boxed for shipment. Compared to other on-line marking techniques, such as inject, hot stamping, or mechanical scribing, laser marking offers many advantages such as indelibility, reliability, no consumables, cleanliness and high speed. Laser marking is usually the best marking solution with one proviso. All materials can not be marked with every laser [2] THE BASICS OF LASER MARKING Laser marking can take a number of forms as follows: (i) Black carbonisation, (ii) Bleaching or changing the colour of the material, (iii) Physical modification of the surface finish, (iv) Scribing a shallow groove into the material by vaporization, (v) Highly controlled modification of the surface by melting, or (vi) A combination of any of the above. In some cases, a surface mark by colour change with little material removal is desired. On the other hand, noncoloured marks that scribe a shallow groove into the material are sometimes desired to provide resistance to abrasion. Laser marking is a surface process. Typically, the light absorbed during the optical-pulse, which can be very short, e.g., less than 0.1 s is transformed into heat, thereby creating a high instantaneous temperature rise in the material, resulting in surface melting and resolidification, carbonisation, chemical decomposition, or explosive ejection of the material. The resultant mark consists typically of a crater of shallow depth, surface

9 69 modification within the crater and around the heat-affected zone, a raised ridge or kerf around the crater, and debris scattered nearby as shown in Fig.2.5. It is evident that melting and material removal would add other terms to the energy balance. Absorption may be strongly temperature dependent, as, for example, in silicon marking at wavelength 1.06 m, or strongly time dependent, as, for example, in marking metals reflective at the laser wavelength, where plasma formation and initiation of surface damage may occur before a significant portion of the energy can be absorbed. Absorption of radiation from lasers such as pulsed CO 2 lasers with very high peak power can be highly nonlinear. In fact, the marking interaction of light at various wavelengths with different materials can be quite complex. Reasonably strong absorption of the laser light is the key not only efficient, cost-effective marking, but also to aesthetically attractive marking. Light that penetrates too deeply will either not raise the surface temperature enough to have any effect, or it may cause incomplete or noncontinuous marking e.g., bubbling in plastics. For this reason, it is important to choose the laser wavelength to interact best with the material to be marked. As for example the radiation from the Nd:YAG laser at 1.06 m passes through a transparent PET (polyethylene terephthalate) water bottles, but radiation from the CO 2 laser at 1.06 m marks clear PET bottles very nicely. Excimer lasers have extremely shallow penetration depth in most organic materials such as plastics; so they can provide very shallow in white fluoropolymers with relatively low laser energy EVALUATION OF THE MARK AND MARKING PROCESS Many techniques are available to evaluate the readability of the laser mark. Often a subjective evaluation by the customer is all that is required. However, quantitative techniques are available to aid in this evaluation and also to ensure that the marking process quality is maintain from run to run. The use of an optical contrast meter to measure the contrast between scattered light from the marked region compared to the unmarked region of the samples provides a simple quantitative estimate of readability. However, for marks that cut a shallow groove into the surface, edge sharpness and depth can also have a strong effect on readability. In this case, it may be necessary also to use surface profiling equipment

10 70 such as an optical interferometer or a profilometer to evaluate the depth of the mark, the edge sharpness, and the change in surface texture. This equipment would also be essential if the depth of the mark must be carefully controlled, for example, in the case of very thin integrated circuit packages. Bar codes are commonly coded using lasers. Bar codes are sensitive to edge quality and line thickness; so issues such as dot overlap in the laser writing technique and surface roughness can have a major effect. Bar code readers will not use light at the same wavelength as room light; so mark contract must be optimised for the bar code reader wavelength. Because bar codes can be difficult to mark reproducibly in some situations with lasers, some users, particularly in the electronics sector, have moved to two-dimensional identification codes, such as a matrix. Laser marking systems are often chosen because they provide an indelible and durable mark. Testing procedures such as scratch resistance, life testing for ultraviolet degradation in plastics, and testing for mark permanence under various environmental conditions are standard. One major factor in the acceptance of laser marking systems has been their inherent reliability. In applications such as marking integrated circuits, lasers provide the only potential route to very high marking reliability. In order to ensure a repeatable marking process month after month, companies that use laser marking equipment expect the lasers to perform reproducibly and reliably within a specified operating range. However, it should not be forgotten that the material being marked represents another important factor. Material properties of the part being marked must also be controlled to ensure mark repeatability from batch to batch [2].

11 IDENTIFICATION OF THE IMPORTANT PROCESS PARAMETERS Laser machining of any material is a complex process involving many different parameters, all of which need to work in consort to produce a quality machining operation. The most important parameters in the laser machining process are peak power or threshold intensity, pulse width, pulse repetition frequency, cutting speed, focal length, assisted gas pressure and types of assisted gas etc. as described below: (a) Peak Power or Threshold Intensity The peak power must be large enough to vaporize the work piece. There exists a threshold value of laser beam intensity below which, no melting/vaporization will occur. When a laser without gas jet heats up a metal target, the energy absorbed is conducted into surrounding colder metal. The minimum amount of power impact necessary to initiate evaporation when it is exposed to laser radiation is called threshold intensity [12, 23]. 2E 2 2 { R (0) ( Z ) } T m i I t.(2.2) Where: Z m = maximum depth of penetration. R(0)=maximum intensity laser beam spot radius I t = threshold intensity. T i = pulse duration. E = single pulse energy. θ = half angle of focusing laser beam or beam divergence. At higher absorption rates, the surface region of the metal will melt and perhaps begin to vaporize. At even higher absorption rate, vaporization becomes the dominant mechanism of material removal from the target [50]. When the power density is too high, the gas near the work piece interactions are instantly transform into plasma. It may cause such as micro cracks. Lamp current is the predominant parameter for the solid state Pulsed Nd:YAG laser beam machining system. The krypton arc lamp stimulated YAG host to emit the photon. The intensity of the lamp, which is modulated only by the lamp

12 72 current, is directly involve to improve power density in the laser beam. For pulsed Nd:YAG laser, the peak power will be achieved at low pulse frequency as the time for accumulation per pulse is more, hence the high peak power is available on high lamp current and low pulse frequency. (b) Pulse Width Pulse duration or pulse width defined as the time needed to vaporize the material. It should not be shorter then the penetration time of the laser beam. It can be roughly calculated by using the relation [12]: 1 Ti. (2.3) 2 f where, T i = pulse duration. f p = pulse repetition frequency. p V Considering for a continuous and smooth machined surface: d(0) (2.4) f Where, d(0) = diameter of the focused spot. V = cutting speed. As the pulse energy increases the penetration time decreases. (c) Pulse Repetition Frequency Because of the periodic nature of the heating, the mechanism of pulsed laser cutting is different from that of CW laser cutting. The overall effect of laser cutting in pulsed mode is similar to overlapping of a series of drilling operations. Every pulse peak makes a hole in the work piece. Cutting action is the process of an accumulation of the action of a series of single pulses. Higher frequency will increase the overlapping number and reduces the cut roughness. However, there is an upper limit of pulse repetition frequency beyond which the pulse duration will be limited and pulse will approach a continuous wave. The inverse the pulse frequency, that is, the pulse period should be larger than the pulse duration. (d) Cutting Speed Sound and safe cutting results are practicable at feed rates of about 80 to 90% of the maximum possible cutting speed. For certain quality demands, the speed p

13 73 may have to be reduced. If the speed is too low, during fusion cutting dross formation and during oxidation cutting burnouts can occur. These two defects can be avoided by pulsing the laser. In pulsed laser cutting, the displacement of the work-piece during a pulse cycle should be much smaller than the diameter of the focused spot, so that a continuous and smooth machined surface can be obtained. Therefore the pulsed laser cutting can be considered approximately as an accumulation of the actions of a series of single pulses. The pulse over lap coefficient is defined as [12, 41]: K ovl d(0) f V p.(2.5) where, f p = pulse repetition frequency, V = feed rate, d(0) = diameter of the focused spot. The overlapping number should lie between 4 and 6 to ensure an overlapping area of about 85-90%. Higher frequency will increase the overlapping number and reduces the cut roughness. As the cutting speed increases surface roughness decreases up to certain limit after that again it will show increasing tendency because, as a result of reduced overlapping between single pulses. (e) Focal Length In laser fusion cutting, the focal position should be near the bottom plane of the workpiece to simplify dross prevention and near or above the middle to maximize speed. In laser oxidation cutting, the focal point should be positioned in the upper half of the material. In the thick section range of 10mm or more the optimum focal position is often some millimeters above of the workpiece. It is the distance between the work- piece and the focusing lens. It determines the diameter of focused spot and therefore the light concentration on the work surface. It is important parameter to obtain kerfs widths. (f) Assisted Gas Pressure and Types of Assisted Gas In fusion cutting, the pressure has to be high (up to 2Mpa), increasing with workpiece thickness. On the other hand, a certain upper limit must not be exceeded

14 74 to avoid a shielding plasma resulting in a kerf collapse. In oxidation cutting, typical pressure values are in the range of 0.1 to 0.5 MPa. In the case of thick (10mm or more) mild steel, the oxygen pressure should be below 0.08 MPa to avoid burn-outs. The functions of gas pressure during laser machining are mentioned as follows [12, 23, 41]: (I) The primary purpose of a gas jet is to remove the melt / vapour generated by high- energy laser beam, and possibly to provide exothermic heat. (II) Acceleration and ejection of transformed kerf material by momentum transfer via shear forces (friction) and pressure gradients (III) Protection of focusing optics against vapour or spatter emitted from the interaction zone (IV) Protection of the interaction zone against oxidation from ambient air, or in contrast to this (V) Providing the interaction zone with reactive gas (oxygen) to burn the kerf material (oxidation cutting) The settings of gas pressure during laser machining are important due to the following reasons: (i) The gas pressure must be increased when the cutting speed increases in order to improve the material removal capability. (ii) The gas pressure cannot be high because the violent behaviour of turbulence will influence the cut quality. Most cutting machines are now equipped with proportional valves and electronic sensors to allow the numerical controller to control the gas pressure. Generally the effect of the cutting gas jet can be enhanced by increasing the gas pressure or the nozzle orifice cross-section and by decreasing the clearance. In addition the process gas efficiency can be improved by a wider top kerf width. The kerf width is mainly dependent on the dimensions and the position of the laser focus. Even though considerable research works have been done in this field, still it need much focus on parametric analysis for finding out the optimal parameter settings for different materials.

15 75 FIG.2.1. CUTTING FRONT OF THE LASER BEAM WITH COAXIAL GAS FLOW

16 FIG LASER FUSION CUTTING 76

17 FIG LASER OXIDATION CUTTING 77

18 FIG.2.4. LASER VAPORISATION CUTTING 78

19 79 KERF DEPTH DISCOLOURATION FIG SCHEMATIC REPRESENTATION OF A LASER-MARKED SURFACE