In this article we will discuss about:- 1. Meaning of Laser Beam Machining (LBM) 2. Principle of Laser Beam Production 3. Advantages 4. Limitations 5. Applications 6. Laser in Production Techniques.
Meaning of Laser Beam Machining (LBM):
Laser is an electro-magnetic radiation. It produces monochromatic light which is in the form of collimated beam.
Through has been introduced recently but is making much head-way in industry. It finds uses for micro drilling it micro welding, etc. However it has low efficiency and high energy input requirements.
Light amplification by stimulated emission of radiation is called laser beam. In this process, use is made of laser beam (a narrow beam of coherent, monochromatic light) which is focused on the work-piece by lens to give extremely high energy density to melt and vaporise any material.
Principle of Laser Beam Production:
Electrons are arranged in different cells in an atom and each cell has a set number of electrons. If any atom is excited, i.e. we pump some amount of external energy (either by heat, or bombardment with protons), then the atomic cell will be in excited condition and some electrons will jump up to next energy level, i.e. electrons jump from one orbit to next one.
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When an atom gets some sort of energy from external source, three types of behaviours can be possible.
(a) Normally atom is not excited. (It remains at same energy level). Then it will absorb external energy, and shoot upto higher energy level.
(b) If nothing happens to that atom in the excited condition, it will radiate the energy received by it. This emission of energy will take some time but will be spontaneous.
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(c) If any photon bombards an atom at the excited condition, or at higher energy level, then it will emit the excess energy instantaneously.
For production of laser beam, we generally use Ruby rod in which aluminium is the main ingredient, chromium being present as impurity in the ratio of 1 to 5000 atoms. The chromium plays very important role for laser beam production and is most desirable.
If it was not there then no laser beam could be produced. When chromium receives any radiation, it absorbs it except red and blue light photons, and thereafter emission of various colours by it takes place. This emission in normal phenomena will not be instantaneous as the atoms are not at excited evil.
In the case of Ruby rods, chromium will impede external radiation and after some time emit spontaneously all the radiation and light can be seen coming out of it. For production of laser beam it is desirable that the energy be pumped in, and pumped energy should come out instantaneously, of course, it will be of great intensity.
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Thus we find that, laser beam technique involves:
(1) Pumping of energy, and
(2) Production of stimulation effect.
Fig. 10.34 shows the method of laser beam production. Flash lamps continuously bombard the chromium atoms of Ruby rod. These emit laser beam. The ends of Ruby rod are made flat and parallel within limits so that laser beam comes out with optical phenomena (i.e. wavelength is exact). Laser beam consists of photons having reddish appearance.
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For stimulation effect, the major portion of atoms must be at higher energy level so that the energy can be harnessed.
At normal condition, most of atoms will be at ground level and not at excited energy level as shown in Fig. 10.35. If energy is pumped continuously they will emit energy at their sweet will and energy will not be harnessed.
When atoms are excited to higher energy level and emit energy, at one stage these come to metastable state and they remain at this metastable state for 3/1000th sec. or even smaller time. (Refer Fig. 10.36). It is at this condition we try to bring stimulation of energy in case of chromium.
When chromium atom is bombarded, it jumps from ground level to higher energy level and instantaneously drops down to intermediate energy level. (Because upper position is most unstable). It has further tendency to return to ground level unless bombarded by other photons.
In case of chromium before jumping to ground level, it rests at intermediate level.
Intermediate level is metastable position and it remains at this stage for 3/1000 sec. and after that to ground level radiating out the energy absorbed by it.
Stimulation Effect:
At intermediate stage, atom is at high energy level. If a photon hits at this stage, then it will instantaneously emit the photon received by it (because it is at higher energy level) and this is stimulation effect. This energy is utilised for various purposes.
The major difficulty faced was of attaining metastable condition, i.e. artificially bringing at higher energy level and keeping at that level tor sufficient time so as to be bombarded by other photons.
After several reflections, all photons will come out from window with great intensity. If the two faces of the rod are perfectly parallel, then the condition of all the photons moving in same direction can be achieved. Otherwise the photons will move in random way.
The minimum thickness of the metal to provide the required heat transfer to be welded is given be the relation.
The energy required to raise a given amount of metal to its melting temperature is given by the relation:
where ρ is the specific gravity (= 8.9 for Cu, 7.9 for Fe, 19.3 for W, and 6.4 for zirconium), v is the volume of metal melted, Cp is the specific heat = 1.0 for Cu, 1.8 for Fe, 0.5 for W and 0.05 for Zr.
θm = melting temperature. θA = ambient temperature
L is the heat of fusion, R = reflectivity = 0.15 for Cu and 0.27 for Fe.
The machining by laser beam is possible when the power density of beam is greater than what is lost by conduction, convection and radiation and further the radiation must penetrate and be absorbed into material.
The power density (P.D.) of laser beam can be calculated from the relationship:
where PD = power density of laser beam W/cm2.
P = laser energy output, watts (W)
L = focal length of lens, cm.,
α = beam divergence, radian
ΔT = pulse duration of laser, sec.
The size of the spot (diameter d) produced by laser is given by diameter d = Lα
The cutting rate (mm/min.) can be expressed as:
where C = is a constant dependent upon the material and conversion efficiency of laser energy into the material
P = laser power incident on surface, W,
E = vaporisation energy of material, W/mm3
A = area of laser beam at focal point, mm2,
t = thickness of material, mm.
It has been observed that material cutting rate is dependent upon the thickness of the workpiece, being maximum for thin materials and reduces exponentially for higher thickness. Thus while holes of 0.1 mm diameter may be drilled to a depth of 50 times the diameter, but hole of 1 mm diameter can be drilled through 2.5 mm thick material.
A big limitation of laser drilling is that it does not produce round and straight holes. This can however be overcome by rotating the workpiece as hole is being drilled. Other limitations are that taper of about 1:20 is introduced, the heat treated surface gets affected. Profiles upto an accuracy of ± 0.1 mm can be cut with numerical control or photo electric tracer techniques.
Advantages of Laser Beam Machining (LBM):
(a) There is no direct contact between tool and workpiece. As such no tool wear problems are faced. Metal, non-metal irrespective of their brittleness and hardness, and even soft metals like plastics and rubber can be machined.
(b) Laser beam can be sent to longer distances, without diffraction. It can also be focused at one place thereby generating lot of heat. It is thus possible to weld, drill and cut areas not readily accessible.
(c) The advantages of laser welding are that heat treated and magnetic material can be welded without losing their properties all over the material except a small region of heat-affection.
Laser welding is possible in any environment through transparent materials and magnetic fields as well. Distortion is negligible and any two materials can be joined together. However, it is important that the vaporisation of the metal must be avoided.
(d) Micro-sized holes can be laser drilled in difficult- to-machine or refractory materials. Precision location is ensured by focusing of the beam. Deep holes of very short diameter can be drilled by using unidirectional multiple pulses.
(e) Beam configuration and size of exposed area can be easily controlled.
Limitations of Laser Beam Machining (LBM):
LBM has the limitations of high initial cost, short life of flash lamp, safety procedures to be followed strictly, over-all low efficiency (0.3% to 0.5%), very low material removal rate, not able to drill too deep holes, machined holes not round and straight, and no possibility of machining some plastics which burn or char.
Applications of Laser Beam Machining (LBM):
(i) Used for making very small holes, difficult welding of non-conductive and refractory materials, cutting complex profiles in thin and hard materials. Also used for partial cutting or engraving.
(ii) To project intense energy to a small area—to illuminate, melt, weld, perforate or ignite.
(iii) Can be used for mass micro-machining production.
(iv) Can also be used for selective heat treating of materials.
(v) It is also sometimes used for dynamic balancing of rotating parts.
(vi) It is very useful for producing very fine and minute holes etc.
(vii) Micro-machining, micro-drilling, spectroscopic, metallographic sciences and photography have been developed.
Characteristics of Laser Beam Machining (LBM) Process:
(1) Material Removal Technique:
Heating, melting and vaporisation.
(2) Tool:
Laser beams in wavelength range of 0.4—0.6 µm.
(3) Power Density:
As high as 107 W/mm2.
(4) Output Energy of Laser and Its Pulse Duration:
20 J, 1 milli second.
(5) Peak Power:
20 kW.
(6) Medium:
Normal atmosphere.
(7) Material Removal Rate:
5 mm3/min.
(8) Specific Power Consumption:
1000 W/mm3/min.
(9) Material of Workpiece:
All materials except those with high thermal conductivity and high reflectivity.
(10) Applications:
Drilling micro holes (upto 250 µm) and cutting very narrow slots.
(11) Dimensional Accuracy:
± 0.025 mm.
(12) Efficiency:
0.3—0.5%.
(13) Limitations:
Taper of 0.05 mm when work thickness is more than 0.25 mm. Very large power consumption.
Laser in Production Techniques:
The laser is uniquely versatile tool, useful in many areas from precision watchmaking to heavy metalwork. The key to the laser’s effectiveness lies in its ability to produce a tremendous quantity of highly concentrated power, as high as 1012 Watts in some cases, within a very short span of time, on a very small area. Such concentration of power enables an industrial worker to increase his output potential multifold.
Laser beams can be pulsed to deliver the equivalent energy intensity as the core of the Sun for a millisecond or less to perform a specific piece of work. Using different lasers—or the unique “tunable” laser—it is possible to deliver just the right concentration of power for the right amount of time to a piece of work.
A unique advantage of laser in industrial applications is its capability to do selective work. The conventional heat treatment process involves heating up the entire workpiece and then quenching it under controlled conditions in order to get the required hardening.
The process hardens the entire piece. In applications like hardening of gear, only tooth edge of the gear needs to be hardened. With the help of a properly tuned laser such preferential hardening can be done. As a result of this the amount of energy required can be reduced and the use of expensive materials can also be reduced.
It is possible to make automotive engine block out of aluminium, with a thin layer of hard metal on the inside of the cylinders, by preferential heat treatment with laser. Thus weight of engine is reduced considerably.
The laser’s divisibility without loss of power opens the way to a new manufacturing set-up where a single laser’s beam is divided to perform many functions simultaneously. Significant progress has also been made in integrating robot technology with lasers in a device called a flexible machine station.
Flexible machine stations are installed on a platform which moves to accommodate doing work in three dimensions digitally and under automated control. Each station is equipped with one laser, whose beam is split into as many beams as are required to perform various procedures simultaneously or otherwise.
Stations are designed to take a stock piece and, without a change of tool, and basically without any manual effort, cut out according to designed shape, finish, machine and heat treat the piece.
Types of Laser Sources:
For industrial use, we are concerned with the power output of the laser beam, the size of its different wavelengths, and the mode of the laser beam—pulsed or in a continuous wave. Basically five types of lasers in terms of their lasing media prevail in metal working processes.
One is a gas laser, the CO2 laser, another is an ion laser, the argon laser; and the others are solid state lasers—the ruby laser, the neodymium-doped yitrium- aluminium-garnet (Nd-YAG) laser commonly known as the YAG laser, and the neodymium-doped glass laser (Nd-glass) commonly known as the glass laser.
Out of these the CO2 and YAG lasers are considered as workhorses. The CO2 laser, which operates at a wavelength of 10.6 micron, is the most powerful laser used in industry. The YAG laser operates at a wavelength of 1.06 micron and generates power outputs close to 1 kW.
Both the glass and ruby (0.694 micron wavelength) lasers are pulsed lasers capable of power outputs of about 100 joules. Laser wavelength is a useful measure since it determines the amount of incidental energy that the workpiece will absorb.
The laser beam is directed to the workpiece via an optical delivery system. The focused-beam radius is directly proportional to the laser wavelength and the lens focal length and inversely proportional to the unfocused radius.
Since the size of the focused spot and the depth of focus vary directly with each other, very small spot sizes can be achieved at the expense of depth of focus. The spot size can be varied by varying the wavelength.
Once the wavelength and depth of focus are determined, the laser beam is usually delivered to the workpiece in the transverse excitation mode (TEM). The common mode of optical configuration is TEM10 mode, which produces a Gaussian output beam with low beam divergence; this mode gives the most uniform beam profile.
The other modes widely used are TEM10, in cases where a broader and less intense spot is required in some cutting and welding, and TEM01, which is useful for surface hardening and producing large holes.
If the laser is very close to the workpiece, the divergence of the beam, particularly in the case of cutting, will result in formation of a bevelled edge on either side of the cut. One way of dealing with this problem is to use a gas jet between the laser and the workpiece. The laser beam does not hit the workpiece directly and it just helps to heat up the gas jet intensely to do the cutting. Helium or argon gas can be used for such work.
Laser in Metal Cutting:
A laser beam can be used in cutting metals, plastics, ceramics, textile, cloth and even glass, when its surface is coated with radiation- absorbing material such as carbon. Usually laser cutting starts by drilling a hole through the workpiece, then moving along a pre-determined path of the shape to be cut. An argon laser, however, does not actually burn but ionizes the epoxy resin board in order to cut it.
What actually occurs is the disassociation of chemical bonds, and it leaves behind an ash or charred remains. Carbon dioxide and YAG lasers, on the other hand, burn through the material. A jet of oxygen is used to obtain exothermic reaction with metals to produce a clean-cut kerf and rapid rate of cutting or drilling. Of the industrial lasers, the CO2 laser yields the highest depth-to- diameter ratio in most metals, using a gas-jet assist.
Cutting speeds depend on the material being cut, its thickness and physical characteristics, and the output power of the laser beam. Steel, titanium, nickel, certain refractory materials and plastics cut easily. Cutting aluminium metal and copper has been especially problematic, since these metals tend to absorb the applied heat.
But during the past year, a YAG laser with enhanced laser focusing has been developed that can cut these metals. It is superior as a cutting instrument to the more high-powered CO2 laser, because of the shorter wavelength beam it emits.
The cutting speeds that can be attained using lasers are impressive. A large flat stock can be manipulated by a programmable X-Y table beneath a fixed CO2 laser beam capable of providing 475 W. Cutting speeds range from 4.25 m per minute for 1 mm thick bare or galvanized steel to 0.5 mpm for 6 mm bare steel, from 1.9 mpm for 1.5 mm stainless steel to 0.9 mpm for 3 mm stock. With 1 kW cutting power, speeds essentially double.
Beside speed-cutting the laser has an additional advantage in cutting complex shapes with sharp corners and slots. The advantage is that the heat affected zones are just 10—12 per cent of kerf width, and distortion is minimal.
Laser machining techniques are best suited:
i. for applications demanding high accuracy for high technology application;
ii. for machining jobs in which the heat affected zone is to be as narrow as possible to avoid adverse mechanical effects;
iii. where for setting up a new operation the cost of tooling requirements using traditional machining processes are unacceptably high;
iv. for avoiding distortion in thin metals, for obtaining a high quality edge finish, for applications where, only a part of the component is to be processed without adversely affecting the remainder;
v. for welding job that has to be carried out adjacent to heat sensitive components, or one which requires that there is no additional weld material, and for very intricate profile cutting.
Laser in Drilling:
Laser drilling was one of the first practical applications of laser technology in industry and the demand for laser drilling is increasing phenomenally. Further, not much precision can be attained in laser drilling in the sense of perfectly cylindrical holes; laser drilled holes tend to be conical. However laser drilling is not suited for deep- hole drilling and for producing perfectly cylindrical holes.
Laser radiation exceeding a certain power density produces melting and vaporization of material and ejection of solid particles. With increasing diameter and depth of the hole the ejected solid particles melt and deposit on the walls and the bottom of the holes, thus not suiting for deep-hole drilling.
In industry laser drilling is widely used for so-called rough drilling—for instance, in watch jewels, diamond dies and other machine parts for various industries where a particularly high level of precision is not demanded.
The laser has the greatest advantage in its ability to make small and very small holes of shallow depth. The laser can, moreover, make these holes in a wide variety of materials such as plastics, ceramics and many metals and alloys, including those alloys that are the most difficult to drill otherwise. Laser machines now in use can drill holes that are 75 micron in diameter. A drill bit would break if it is applied to such small holes.
One application that is under proprietary wraps is the making of semi-conductor chips for computers and microprocessors, which benefits from the capability of a laser to etch lines at hair thickness. At the moment lasers are operating in the range of a couple of micrometers. What is needed is an etching line less than 1 micron in thickness, which requires going beyond the ultra-violet range. Exclaimer lasers are now producing cuts 0.5 micron in width.
In aircraft-turbine industry, laser drilling is used to make holes for air bleeds, air cooling or the passage of other fluids.
Other applications include making holes in hypodermic needles, automotive fuel plates, various lubrication devices, ceramic substrates for electronic circuits, holes in tungsten-carbide tool plate, and so on.
Laser in Welding:
Welding materials with the help of a laser beam require more precise control of the input laser power than is necessary in the case of drilling. Laser welding is especially useful when it is essential to control the size of the heat affected zone, to reduce the roughness of the welded surface and to eliminate mechanical effects.
Lasers are generally used for welding multilayer materials in which there are discontinuities in thermal properties at the interfaces where the layers come into contact.
There are two general kinds of laser welds:
i. Conduction-limited welds and
ii. Deep-penetration welds.
In conduction-limited welds, normally used to join thin components, the metal absorbs the laser beam at the work surface, and the area below the surface is heated by conduction.
Deep-penetration welds require greater power and the CO2 laser is used for this purpose, in the case of deep-penetration welds, thermal conduction does not limit penetration, because the beam energy is delivered to the depth of the weld. Neither type of weld requires any filler material.
There are certain basic parameters for laser welding.
The most important are:
(a) The wave length of the laser beam must be compatible with the material being welded;
(b) The focus of the beam must be adjusted to the thickness of the material;
(c) A pulsed mode of operation is usually better than the continuous-wave;
(d) A pulse shape of the laser beam must be controlled precisely from weld to weld; and
(e) Close contact with the surf aces of materials to be joined must be maintained.
The most important components of a laser system designed for welding consist of a source of alternating current, a step-up transformer, a rectifying system, a pulse-forming mechanism, a lasering element, a cooling system, and an optical system for controlling the beam-profile projected to the specimen.
To weld any two pieces of metal together, the temperature of the weld area must be raised to the melting point of the metals. If both the metals are similar, then the problem is nominal. However, if two different metals need to be welded then a compromise of the laser energy must be made. In this case the configuration of the joint, thermal conductivity, diffusivity, latent neat, etc. are also taken into consideration to determine the amount of laser energy required.
Laser welding has become popular for joining sheet metal or stock pieces of about 2.5 mm thick or less. Besides eliminating the need for filler material, the extremely narrow continuous welds or spot welds can be made at fast speeds with very narrow heat-affected zones, in fact, laser welding is not only replacing arc welding and resistance seam and spot welding; it has become a viable alternative even to electron beam welding.
The issue is not only quality but also cost. Laser welding has four times the productivity of electron beam welding, and the capital cost of laser equipment is about half that of a comparable electron beam welding set-up. Besides, the maintenance costs of a laser system are less than that of the electron beam welding.
With laser welding heat generation is least, weld quality excellent, weld speed moderate, and ease of automation is excellent.
One of the major criteria for laser welding is the proper joint preparation. The two surfaces being welded must remain in close contact with each other. Since no filler material is used in laser welding, no gap in the joint exceeding the beam- spot size can be tolerated.
To establish an intimate contact between the surface, a pressure is applied. Alternatively, for sheet metal welds a transparent plastic is often used. Many metals and alloys can be laser-welded.
Some of the most readily processed are:
(a) Low carbon steels,
(b) Stainless steel,
(c) Titanium and titanium alloy,
(d) Tantalum,
(e) Zirconium,
(f) Silicon bronze, and
(g) Some nickel alloys.
Some aluminium alloys, copper, tool steel, medium-to-high-carbon steels, and nickel alloy are somewhat less suitable. The least suitable are zinc, galvanized steel, brass, silver, and gold. Brasses generally weld poorly because of the volatility of zinc.
Techniques used for deep-penetration welds, welding sheets and precision welds are somewhat different. In the case of deep-penetration welds, the laser melts a small cylindrical volume of metal in the workpiece. This cylindrical molten hole is surrounded by hot metal in the workpiece. This cylindrical molten hole is surrounded by hot metal plasma. As the beam moves along the joint, the metal on the ad van cine side of the hole melts and that along the rear solidifies.
Beam power and travel speed are obviously important factors for determining deep-penetration welds. However, another factor is the stabilization of the plasma column formed. This, again, can be achieved by marrying the beam density with the appropriate travel speed.
Excessive energy density will form an unstable hole that could drop through the joint, while excessive travel speed can cause incomplete penetration. Stabilizing the plasma column is essential but not sufficient. Being opaque to the input laser beam, the plasma column reduces effective power and therefore must be reduced as much possible. One way to retain this is by directing inert gases at high velocity to the interaction zone.
Laser welding of sheets-up to 6 mm is rather routine work. For sheets thicker than 6 mm laser welding is still in a developmental stage. A 6-kW laser output power can weld 12 mm plates at 25 mpm travel speed and a 13-kW beam at 0.4 mpm can be used for plates as thick as 18 mm. The problem of laser welding thicker plates does not lie with the laser itself but in the close fit-up requirements. It is difficult to have long sheets meeting the fit-up requirements of 0.25 mm or less over the entire length.
Fast and precision welds, as are used in the automotive industry worldwide, are just the kind of welds for which the laser is extremely productive. Today fast and precision welds extend well beyond the automotive industry.
Another advantage of the laser weld is the elimination of grinding from the entire process. In conventional as well as electron beam welding and plasma welding, excess filler material is removed by grinding.
Laser for Surface Treatment:
Various techniques of surface treatment by laser, the areas of application and advantages arc- as follows. By the preferential use of laser radiation, gears, saw teeth, valve wear pads, and cylinder liners can be strengthened. In all these cases the input heat is confined to a localized area and the mass of the rest of the surrounding metal serves as the quench.
The laser is used to deposit a thin layer of cobalt alloy on the turbine blade shroud contact areas. Again gas is used for shielding during deposition of the cobalt alloy and for cooling afterwards. It is possible to hard face the shroud wear pad of nickel-alloy turbine blades with a cobalt-base alloy.
With the conventional gas-tungsten-arc welding method building up the hard face required a double pass, involving grinding the pad, depositing and finish grinding. With the help of lasers, the total input of heat to the blades has been considerably reduced.
Other advantages are:
(a) The nickel dilution is minimal,
(b) The deposit is uniform,
(c) Consumption of cobalt alloy is reduced by almost 50 per cent,
(d) Process time is reduced to one tenth.
It is possible to apply thin ceramic coatings on metal substrates for heat and wear resistance. Laser has also been used to seal micro cracks which are usually present in hard- chromium electroplates. The cracks, potential sites for corrosion and spelling, are removed by softening the plate and forming new grains by recrystallization and grain growth.
Transformation hardening is used for transformable alloys, resulting in improved hardness and wear resistance. Surface homogenisation treatment is used for coarse grained materials, carbide-containing material, tool steel and cast steel to produce improved corrosion resistance, improved hardness and wear.
Laser glazing treatment is used on deep eutectic material to produce no grains, less brittle, corrosion and wear resistant surface. Cladding treatment is used for hard faced cosmetic coating, corrosion protection to obtain new surface material.