Module 4

Laser Beam Machining (LBM), Electron Beam Machining(EBM), Plasma arc Machining (PAM), Ion beamMachining(IBM) - Mechanism of metal removal, attributes ofprocess characteristics on MRR, accuracy etc and structureof HAZ compared with conventional process; application,comparative study of advantages and limitations of eachprocess.

Abrasive Jet Machining (AJM), Abrasive Water Jet Machining(AWJM) - Working principle, Mechanism of metal removal,Influence of process parameters, Applications, Advantages& disadvantages.

Laser Beam Machining – Introduction

Laser Beam Machining or more broadly laser material processing dealswith machining and material processing like heat treatment, alloying,cladding, sheet metal bending etc. Such processing is carried out utilizingthe energy of coherent photons or laser beam, which is mostly convertedinto thermal energy upon interaction with most of the materials.Nowadays, laser is also finding application in regenerative machining orrapid prototyping as in processes like stereo-lithography, selective lasersintering etc. Laser stands for light amplification by stimulated emission ofradiation. The underline working principle of laser was first put forward byAlbert Einstein in 1917 though the first industrial laser for experimentationwas developed around 1960s.Laser beam can very easily be focused usingoptical lenses as their wavelength ranges from half micron to around 70microns. Focussed laser beam as indicated earlier can have power densityin excess of 1 MW/mm2. As laser interacts with the material, the energy ofthe photon is absorbed by the work material leading to rapid substantialrise in local temperature. This in turn results in melting and vaporisation ofthe work material and finally material removal.

Laser Beam Machining – the lasing process

Lasing process describes the basic operation of laser, i.e. generation ofcoherent (both temporal and spatial) beam of light by “light amplification”using “stimulated emission”. In the model of atom, negatively chargedelectrons rotate around the positively charged nucleus in some specifiedorbital paths. The geometry and radii of such orbital paths depend on avariety of parameters like number of electrons, presence of neighbouringatoms and their electron structure, presence of electromagnetic field etc.Each of the orbital electrons is associated with unique energy levels. Atabsolute zero temperature an atom is considered to be at ground level,when all the electrons occupy their respective lowest potential energy. Theelectrons at ground state can be excited to higher state of energy by

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absorbing energy form external sources like increase in electronicvibration at elevated temperature, through chemical reaction as well asvia absorbing energy of the photon. Fig. 9.6.7 depicts schematically theabsorption of a photon by an electron. The electron moves from a lowerenergy level to a higher energy level.

On reaching the higher energy level, the electron reaches an unstableenergy band. And it comes back to its ground state within a very smalltime by releasing a photon. This is called spontaneous emission.Schematically the same is shown in Fig. 9.6.7 and Fig. 9.6.8. Thespontaneously emitted photon would have the same frequency as that ofthe “exciting” photon.

Sometimes such change of energy state puts the electrons in a meta-stable energy band. Instead of coming back to its ground stateimmediately (within tens of ns) it stays at the elevated energy state formicro to milliseconds. In a material, if more number of electrons can besomehow pumped to the higher meta-stable energy state as compared tonumber of atoms at ground state, then it is called “population inversion”.Such electrons, at higher energy meta-stable state, can return to theground state in the form of an avalanche provided stimulated by a photonof suitable frequency or energy. This is called stimulated emission. Fig.9.6.8 shows one such higher state electron in meta-stable orbit. If it isstimulated by a photon of suitable energy then the electron will comedown to the lower energy state and in turn one original photon, anotheremitted photon by stimulation having some temporal and spatial phasewould be available. In this way coherent laser beam can be produced.

Fig. 9.6.9 schematically shows working of a laser. There is a gas in acylindrical glass vessel. This gas is called the lasing medium. One end ofthe glass is blocked with a 100% reflective mirror and the other end is

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having a partially reflective mirror. Population inversion can be carried outby exciting the gas atoms or molecules by pumping it with flash lamps.Then stimulated emission would initiate lasing action. Stimulated emissionof photons could bein all directions. Most of the stimulated photons, not along the longitudinaldirection would be lost and generate waste heat. The photons in thelongitudinal direction would form coherent, highly directional, intenselaser beam.

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Lasing Medium

Many materials can be used as the heart of the laser. Depending on thelasing medium lasers are classified as solid state and gas laser. Solid-statelasers are commonly of the following type

• Ruby which is a chromium – alumina alloy having a wavelength of 0.7μm

• Nd-glass lasers having a wavelength of 1.64 μm

• Nd-YAG laser having a wavelength of 1.06 μm

These solid-state lasers are generally used in material processing.

The generally used gas lasers are

• Helium – Neon

• Argon

• CO2 etc.

Lasers can be operated in continuous mode or pulsed mode. Typically CO2gas laser is operated in continuous mode and Nd – YAG laser is operatedin pulsed mode.

Laser Construction

Fig. 9.6.10 shows a typical Nd-YAG laser. Nd-YAG laser is pumped usingflash tube. Flash tubes can be helical, as shown in Fig. 9.6.10, or they canbe flat. Typically the lasing material is at the focal plane of the flash tube.Though helical flash tubes provide better pumping, they are difficult tomaintain.

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Fig. 9.6.11 shows the electrical circuit for operation of a solid-state laser.The flash tube is operated in pulsed mode by charging and discharging ofthe capacitor. Thus the pulse on time is decided by the resistance on theflash tube side and pulse off time is decided by the charging resistance.There is also a high voltage switching supply for initiation of pulses.Fig. 9.6.12 shows a CO2 laser. Gas lasers can be axial flow, as shown inFig.9.6.12, transverse flow and folded axial flow as shown in Fig. 9.6.13.The power of a CO2 laser is typically around 100 Watt per metre of tubelength. Thus to make a high power laser, a rather long tube is requiredwhich is quite inconvenient. For optimal use of floor space, high-poweredCO2 lasers are made of folded design.In a CO2 laser, a mixture of CO2, N2and He continuously circulate through the gas tube. Such continuousrecirculation of gas is done to minimize consumption of gases. CO2 acts asthe main lasing medium whereas Nitrogen helps in sustaining the gasplasma. Helium on the other hand helps in cooling the gases.

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As shown in Fig. 9.6.12 high voltage is applied at the two ends leading todischarge and formation of gas plasma. Energy of this discharge leads topopulation inversion and lasing action. At the two ends of the laser wehave one 100% reflector and one partial reflector. The 100% reflectorredirects the photons inside the gas tube and partial reflector allows apart of the laser beam to be issued so that the same can be used formaterial processing. Typically the laser tube is cooled externally as well.

As had been indicated earlier CO2 lasers are folded to achieve highpower.Fig. 9.6.13 shows a similar folded axial flow laser. In folded laserthere would be a few 100% reflective turning mirrors for manoeuvring thelaser beam from gas supply as well as high voltage supply as shown in Fig.9.6.13.

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Laser Beam Machining – ApplicationLaser can be used in wide range of manufacturing applications• Material removal – drilling, cutting and tre-panning• Welding• Cladding• AlloyingDrilling micro-sized holes using laser in difficult – to – machine materials is the most dominant application in industry. In laser drilling the laser beam is focused over the desired spot size. For thin sheets pulse laser can be used. For thicker ones continuous laser may be used.

Advantage of laser cuttingNo limit to cutting path as the laser point can move any path.

The process is stress less allowing very fragile materials to be laser cut without anysupport.Very hard and abrasive material can be cut.

Sticky materials are also can be cut by this process.

It is a cost effective and flexible process.

High accuracy parts can be machined.

No cutting lubricants required

No tool wear

Narrow heat effected zone

Limitations of laser cuttingUneconomic on high volumes compared to stamping

Limitations on thickness due to taper

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High capital cost

High maintenance cost

Assist or cover gas required

Electron Beam Machining (EBM)

In electron beam machining process there is a bombardment of highvelocity stream of electrons on the workpiece surface so this electrons arebombarded on the workpiece surface with a very high velocity, around66% velocity of the sunlight so because of this bombardment of electronson the workpiece surface the materials into a small area on the workpiecesurface it melts and vaporizes and temperature rises to a very hightemperature. So material on the workpiece surface melts and vaporizesand machining is going on.

So this process actually it is used for machining thousands of holes on a thin sheet which is used in aerospace industry, food processing industry, cloth industries and very high aspect ratio for making of very high aspect ratio holes, thousands of holes on a workpiece surface irrespective of the material property like metallurgical property, mechanical property of the material.So this material maybe electrically conducting or electrically non-conducting or maybe ceramics, metals, or any kind of metal, any kind of ceramics it works. So here there is a filament which is heated with a very high temperature. So because of this heating of this filament so electrons emanates from that cathode, cathode filament or these filament may be heated from a radiation from a another body, from the radiation from a another body on a solid block of cathode, on a solid block of filament also it can be generated. So these electrons emits from the cathode,cathode filament and it passes through a magnetic lens to coincide to concentrate or to reduce the diameter of the electron beam and it bombards on the workpiece surface.

So electron beam machining process there are 2 types of methods are there. One is thethermal type. Another one is the non-thermal type. So in normal in non-thermal type thiselectron beams are used for generating chemical reactions. So for generating chemicalreactions the electron beams are used. So this non-thermal type we are not going to discuss. nthermal type this cathode is heated to a very high temperature. So this stream of large numberof electrons comes as a small diameter beam, electron beam.So this stream of large number ofelectrons emits from the cathode, from a heated cathode. It comes out as a small diameterbeam. So it moves towards the workpiece with a very high velocity and it bombards,machining is going on due to the bombardment of this electrons on a very small localizedarea. So as it is bombarded on a very small localized area, huge amount of temperature isgenerated on the workpiece surface. So machining is going on due to the melting andvaporization of this material from the workpiece surface from a very localized area So thiskinetic energy of this electron beam is used for the machining to take place and whichconverts this kinetic energy of the beam, electron beam high velocity electron beam, convertsinto the heat energy. So it can produce holes of different shapes. So any kind of shapes otherthan circular also can be generated by deflecting the beam at certain angle. So workpiece

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material can be conducting as well as non-conducting, any kind of workpiece material can bemachined irrespective of their hardness, mechanical and metallurgical properties can bemachined by this process. Also another thing is that vacuum is required.

So this machining chamber should be vacuum chamber because this electron beams when itwhen it travels so if air is there into the chamber so it will collide on the air molecules. So itsstrength will be reduces. So that is why this vacuum is generated into the machining chamber.Also this cathode it is used as a tungsten cathode is used if air is there so it will immediatelyitwill be oxidized. So to reduce or to avoid the oxidation of this tungsten filament cathode sothis vacuum chamber is used. So beam size, this diameter of this beam size should be lessthan the diameter of the hole to be produced.

So ejection of the molten material, how this molten material is ejected or molten or vaporizedmaterial is ejected from the workpiece surface, so at the back side of the workpiece backingmaterial is used. So after machining on the top surface of the workpiece so when it reaches tothe bottom side of the workpiece it also vaporizes the backing material. So this backingmaterials also vaporizes so it comes out as a high pressure. So this backing materials it comesout as a very high pressure so while coming as a high pressure it also ejects the molten orvaporized material on the workpiece surface. So it expels the molten or vaporized materialfrom the workpiece surface so at the back side of the workpiece surface there is a backingmaterial is used. So that is why this backing material is used.

So non-circular holes also can be generated by electron beam machining process. Any kind ofshape, any shape, any kind of shape of the workpiece can be generated by this electron beammachining process. So you can make a you can keep a fixed beam and you can travel theworkpiece on a computer numerical CNC table. You can place your workpiece on a CNCtableand you can generate any kind of complex shape or you can move the beam on the workpiecesurface using a CNC table. So in both the way you can generate any kind of non-circularholeson the workpiece surface. So circular big holes can be generated. You can move the beamalongthe perimeter of the circle so you can generate the bigger size holes also.

Electron Beam Machining System

There are three important elements of EBM system, viz vacuum system, electron beam gunand power supply.

i) Electron Beam Gun

It is used to produce electron beam of desired shape and to focus at the predeterminedlocation. EBM gun is operated in pulsed mode. A super-heated cathode generates theelectrons cloud. Sometimes cathode may be used as a solid block indirectly heated byradiation emitted from a filament. Due to force of repulsion from the cathode, electronmove at a very high acceleration towards the anode which attracts them. The velocitywith which electrons pass through the anode is approximately 66% tat of light.On thepath of electrons, there is kind of switch which generates the pulses.

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A magnetic lens is used to shape the electron beam into a converging beam. This beam ispassed through a variable aperture to reduce the diameter of the focussed beam byremoving the stray electrons. Magnetic lenses are used to pin point the location of thebeam, deflect it, and make it a round beam falling on the workpiece.

ii) Power supply

Power supply generates a voltage as high as 150 kV to accelerate elecrtrons. The EBM gun ofa powerful system is usually operated at about 12 kW and individual pulse energy as 120J/pulse. The power density at the work surface is too high that is why it is cpable to melt andvaporize the workpiece material. Thus, material removal in EBM is basically due tovaporization.

iii) Vacuum System and Machining Chamber

The electron beam generation, its travel in space, and resulting machining take place in avacuum chamber. The vacuum does not allow rapid oxidation of incandescent filamentand there is no loss of energy of electrons as a result of collision with air molecules. Thevacuum in the chamber is of the order of 10*4 – 10-5 torr.

Process Parameters

The important parameters in EBM process are beam current, duration of pulse, lens currentand signals for the deflection of beam. The values of these parameters during EBM arecontrolled with the help of computer.

Beam current it maybe it varies from 100 microampere to 1 ampere. So this beam currentactually governs what is the energy per pulse. It governs the energy, beam energy per pulseon the workpiece surface. Higher value of this beam current will generate high beam energy

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per pulse and it will generate the high material removal rate. So by changing this beamcurrent you can change the beam energy or you can change the material removal rate on theworkpiece surface. Now pulse duration, this duration of this pulse varies from 50microsecond to 10 millisecond. So longer pulse duration will generate wider and deeperholes. So if we put this pulse on a for a longer duration of time on the workpiece surface soobviously it will generate a wider shallow hole but deeper holes because it is kept on theworkpiece surface at a certain time for a longer period of time. So it will generate wider anddeeper holes and it affects this pulse duration also affects the heat affected zone on theworkpiece surface because of this heating of this surrounding materials it generates a higherheat affected zone if this pulse duration is increased. So this recast layer the dimension of therecast layer in case of electron beam machining process is 25 micron around 25 micron.

Now lens current, it determines the working distance and focused beam size. So beamdiameter it determines the lens current determines the beam diameter and also it determinesthe working distance and also it determine the what is the diameter of the beam. So beamdeflecting signals it affects the hole shape straight or tapered hole determined by the positionof the focal point below the top surface of the work piece.This holes whether it will bestraight or tapered it depends on the this focal position focal point of the beam whether it isjust top surface of the workpiece or just below the workpiece surface. For noncircular holebeam movement can be programmed. So other than circular other than circular hole thisbeam movement can be programmed.

Characteristics of the process

Now this material removal rate it is calculated as eta P by W. So material removal rate in caseof electron beam machining process it is calculated from eta P by W where material removalrate is rate at which material workpiece material is vaporized. So eta is the cutting efficiency.

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It is around 20% or less than 20% in case of electron beam machining process. P is the powerin J/S and W is the specific energy J/cm cube required to vaporize the workpiece material orW is the energy specific energy required joule per J/cm cube to vaporize the workpiecematerial.

Now this W can be calculated as C PS TM minus Ti. CP is the specific heat, TM is themelting point temperature of the workpiece material minus Ti, Ti is the initial temperature ofthe work piece plus C Pl, C Pl is the specific heat, TB minus TM, TB is the boiling pointtemperature of the workpiece material minus TM is the melting point temperature of theworkpiece material plus HF plus HV. HF is the latent heat of fusion and HV is the latent heatof vaporization and this S and l C PS is the specific heat of the solid and C Pl specific heat inliquid. So CP is assumed to be constant although it varies with the temperature but it isassumed to be constant and suffix S and L indicate the solid and liquid states respectively. Sofrom this equation MRR can be calculated eta into P by W, P is the power and W is the energyrequired per volume per unit volume of the workpiece.

Now process characteristics electron beam machining process characteristics, so it can beused for conducting as well as non-conducting material like nickel, copper, aluminium,ceramics, leather, plastics. So any kind of material can be machined by this process. So at theentry side of the beam it is observed that there is a burr. Because this recast layer or vaporizedmaterials are coming outside, vaporized while this vaporized materials are coming outside, soit actually generates a recast layer at the entry side also.

So this because of this recast layer small amount of burrs are actually visible at the entry sideof the workpiece. So this at the entry side of the beam a small sized burr solidified layer leftout. So workpiece properties physical, mechanical, and metallurgical properties do not havesignificant effect on performance of the electron beam machining process. So workpieceproperties does not have any effect on the performance of the electron beam machiningprocess. KTUNOTES.IN

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Hole diameter as low as 0.1 to 1.4 mm and large depth 10 mm and high aspect ratio of 15:1 can be generated from the workpiece surface. So there is no mechanical force because this electron beam machining process this tool does not touch the workpiece surface so there is nomechanical force. So any kind of thin and fragile materials can be machined, low strength components can be machined, easily machined by this process.

So hence as there is no workpiece tool material does not touch the workpiece surface. Thereis no distortion of the workpiece due to the mechanical forces. Off axis holes also can beeasily generated by deflecting the beams but there is a thermal residual stress is generatedbecause there is a huge temperature gradient is there because there is a huge amount oftemperature is generated locally at the on the workpiece surface where this beam falls.So there is a huge temperature gradient is there so because of this temperature gradientthermal residual stress is generated by electron beam machined surface. One disadvantage ofthis process is there. Cost of equipment is very high. So this electron beam machining cost,equipment cost is very high and also it requires skilled operator to machine on the workpiecesurface and machine edge quality depends on the thermal properties of the workpiecematerial and pulse energy, how much pulse you are giving, how much energy you are givingper pulse also affects the quality of the machine edge. Heat affected zone in case of electronbeam machining process depends on the pulse duration and the hole diameter. So this heataffected zone depends on the pulse duration and the hole diameter.

Applications

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These are the applications of electron beam machining process. So it is most popular foraerospace, insulation, food processing and chemical, and clothing industries. Hundreds ofhundreds to thousands of holes can be generated on the workpiece surface, in a simple andcomplex shape holes can be generated on the workpiece surface. Perforation of sheets on acomplex shaped and difficult to machine material can be generated. Examples are drillings ofthousands of holes diameter less than 1 mm in a thin plates used for turbine engine combustordome.Many thousands of holes less than, diameter less than 1 mm in cobalt alloy fibre spinningheadof thickness 5 mm can be generated. Drilling by electron beam machining is claimed 100timesfaster than EDM process. Holes and holes in filters and screens in food processing industry.Fine gas orifice in space nuclear reactor. Holes in wire drawing dies. Cooling holes in turbineblades. Metering holes injection nozzles of diesel engines can be made by electron beammachining process.

Plasma Arc Welding (PAW)It is a fusion welding process wherein the coalescence is produced by heating the work with aconstricted arc established between a non consumable tungsten electrodeand work piece orbetween a non consumable electrode and constricted nozzle. The shielding of the weld pool isobtained by the hot ionized gas produced by passing inert gas through the

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arc and constrictednozzle. Filler material may or may not be applied.

Principles of Operation:

In the PAW process, the workpiece is cleaned and edges are prepared. An arc is established between a non-consumable tungsten electrode and workpiece or between a non-consumable electrode and constricted nozzle.An inert gas is passed through the inner orifice surrounding the tungsten electrode and subsequently the gas is ionized and conducts electricity. This state of ionized gas is known as plasma. The plasma arc is allowed to pass through the constricted nozzle causing high energy and current density. Subsequently high concentrate heat and very high temperatures are reached. The low flow rate (0.25 to 5 l/min) of the orifice gas is maintained as excessive flow rate may cause turbulence in the weld pool. However the orifice gas at this flow rate is insufficient to shield the weld pool effectively. Therefore inert gas at higher flow rate (10- 30 l/min) is required to pass through outer gas nozzle surrounding the inner gas nozzle to protect the weld pool. A typical manual torch used in PAW is as shown in Fig. 4.5.2.

Plasma arc welding is of two types: Non-transferred plasma arc weldingprocess and transferred arc welding process. In the former, the arc isestablished between the electrode and the nozzle and in the latter processthe arc is established between the electrode and the workpiece. Thedifferences between these two processes are presented in the Table 4.5.1.

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Equipment and Consumables:

Power source: A conventional DC current power supply with drooping V-I characteristics is required. Both rectifier or generator type power source may be used; however, rectifier type power source is preferred. The general range of the open-circuit voltage and current is 60-80V and 50-300A respectively.

Plasma torch: It consists of non-consumable tungsten electrode, inner nozzle (constricting nozzle) and outer gas nozzle. The torch is water cooled to avoid heating of the nozzle. It is of two types: transferred arc and non-transferred arc welding torch.

Filler material and shielding gases:

Filler material used in this process is the same as those used in the TIG and MIG welding processes. The selection of the gases depends upon the martial to be welded. The orifice gas must be an inert gas to avoid contamination of the electrode material. Active gas can be used for shielding provided it does not affect the weld quality. In general, the orifice gas is the same as the shielding gas.

PAW Operation:

In the PAW process, arc can not be initiated by touching the work piece as electrode isrecessed in the inner constricted nozzle. Therefore, allow current pilot arc is established in theconstricted inner nozzle and electrode. The pilot arc is generally initiated by the use of highfrequency AC or high voltage DC pulse superimposed of the main welding current. It causesthe ionization of the orifice gas and high temperature which contributes to easy initiation ofthe main arc b/w the electrode and work piece. After the initiation of the main arc,the pilotarc may be extinguished. this is followed by adding the filler material as in TIG weldingprocess. Next, the welding torch is moved manually or automatically in the direction ofwelding. There are two techniques i) Key hole technique and ii) Non key hole technique. Inthe key hole technique, due to constricted arc, high temperature and high gas flow; small

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weld pool with high penetration to width is obtained, resulting in complete melting of thrbase material beneath the arc. As the arc moves forward, the material is melted and fills thehole produced due to arc force. The power supply and gas flow rate are turned off once thekey hole is filled appropriately in the end of welding. The workpiece is suitably cleaned aftercooling.

Applications of PAW:

This process is comparatively new and hence the potential of the process is yet to be understood/accepted. This process can be used to join all the materials those can be welded by welding TIG process. Present applications of the process include:

1) Piping and tubing of stainless and titanium,

2) Submarine, aeronautical industry and jet engine manufacturing,

3) Electronic components.

Advantages of PAW:

1) Welding speed is higher.

2) Penetration is more.

3) Higher arc stability.

4) The distance between torch and workpiece does not affect heat concentration on the work

up to some extent.

5) Addition of filler material is easier than that of TIG welding process.

6) Thicker job can be welded.

7) Higher depth to width ratio is obtained resulting in less distortion.

Disadvantages of PAW:

1) Higher radiations.

2) Noise during welding.

3) Process is complicated and requires skilled manpower.

4) Gas consumption is high.

5) Higher equipment and running cost.

6) Higher open circuit voltage requiring higher safety measures to take.

Ion Beam Machining

Ion beam machining (IBM) takes place in a vacuum chamber using charged ions fired froman ion source toward the workpiece by means of an accelerating voltage. The mechanism ofmaterial removal in IBM differs from that of EBM. It is closely related to the ejection ofatoms, from the surface, by other ionized atoms (ions) that bombard the work material. The

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process is, therefore, called ion etching, ion milling, or ion polishing. The machining system,shown in Fig. 5.56, has an ion source that produces a sufficiently intense beam, with anacceptable spread in its energy for the removal of atoms from the workpiece surface byimpingement of ions. A heated tungsten filament acts as the cathode, from which electronsare accelerated by means of high voltage (1 kV) toward the anode. During the passage ofthese electrons from the cathode toward the anode, they interact with argon atoms in theplasma source, to produce argon ions.

Ar + e− → Ar+ + 2e

A magnetic field is produced between the cathode and anode that makes the electrons spiral.The path length of the electrons is, therefore, increased through the argon gas, which, in turn,increases the ionization process. The produced ions are then extracted from the plasmatoward the workpiece, which is mounted on a water-cooled table having a tilting angle of 0°to 80°. Machining variables such as acceleration voltage, flux, and angle of incidence areindependently controlled.

Material removal rate

As shown in Fig. 5.57, if the ions strike the machined surface obliquely, atom ejection islikely to occur from the primary collision. Under such conditions the incident momentumvector is reported to have the greatest influence on the ejection process .The sputtering yield,that is, the number of atoms yielded per incident ion, may be one order of magnitude greaterfor oblique cutting than normal incidence. The material is, therefore, removed by the transferof momentum from the incident ions to atoms on the surface of the material. The atom,removed from the surface, is also deflected away from the material. Energies greater than thebinding energy of 5 to 10 eV are needed to effect removal of atoms. At higher energiessufficient momentum may cause the removal of several atoms from the surface (cascade-typeeffect). Furthermore, the incident ion will become implemented deeper into the material,damaging it, by displacement of atoms. In IBM material is removed by the transfer ofmomentum from the incident ions to the atoms, in the surface of the material. As a result anatom is removed from the surface, while the ion is deflected away from the material. Theamount of yield and hence the machining rate depend on material being machined.

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where V(q) = etch rate, atoms per min/(mA⋅cm-2)n = density of target material, atoms per cm3S(q) = yield, atoms per ion

The cos (q) term takes into account the reduced current densities at angles away from normalincidence. The yield and hence the machining rate depend on the binding energy of atoms inthe material being machined. The amount of yield varies with the introduction of gases,whichreact with the surface of the material, varying its binding energy and hence the rate ofmaterial removal. The amount of yield and hence the machining rate present suitable indicesof machinability in IBM. Figure 5.58 and 5.59 arrange some materials in descending orderwith respect to their machinability index. Metallic materials seem to have highermachinability than non-metallic ones.

Accuracy and surface effectsSmall dimensions as 10 to 100 nm are possible using IBM. The slope of the sidewalls of the machined surface and its surface finish are determined by the angle of incidence of the ion beam. Accuracy levels of ±1.0 percent, with a repeatability of ±1 percent have been reported. Surface texturing produces a cone-and-ridge-like configuration on the order of 1 µm in size. However, smoothing to a surface finish less than 1 µm can be obtained.

Applications

1. IBM is used in smoothing of laser mirrors as well as reducing the thickness of thin films without affecting their surface finish. In this regard thinning of samples of silicon to a thickness of 10 to 15 µm has been reported using argon ions impinging at normal incidence.

2. Using two opposing beams, a thin circular region on a rotating sample can produce samples for transmission electron microscopy.

3. Polishing and shaping of optical surfaces by direct sputtering of preforms in glass, silica, and diamond is performed using patterning masks.

4. The process can produce closely packed textured cones in different materials including copper, nickel, stainless steel, silver, and gold (Fig. 5.60). Sputter etching can also create microscopic surface texture using the sputter deposition of a lower yield material on the surface.

5. Atomically clean surfaces can be produced by IBM that are used in the adhesion of gold films to silicon and aluminum oxide substrate.Higher ion energies can be used to remove a layer of the surface oxide.

6. IBM can mill a line width of 0.2 µm, which is used in the fabrication of bubble memory devices of depth-to-width ratios of 2:1.Problems associated with the alternative chemical etching method such as the lack of undercutting are avoided since masking is only needed to shadow the beam.

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7. Further applications regarding two-dimensional polymer base diffraction grating by IBM can be seen in the website www.blkbox.com/ while ion beam deposition (IBD) in the website www.skion.com/.

Abrasive Jet Machining

In abrasive jet machining (AJM) a focused stream of abrasive grains of Al2O3 or SiC carriedby high-pressure gas or air at a high velocity is made to impinge on the work surface througha nozzle of 0.3- to 0.5-mm diameter. The process differs from sandblasting (SB) in that AJMhas smaller diameter abrasives and a more finely controlled delivery system. The workpiecematerial is removed by the mechanical abrasion (MA) action of the high-velocity abrasiveparticles. AJM machining is best suited for machining holes in super hard materials. It istypically used to cut, clean, peen, deburr, deflash, and etch glass, ceramics, or hard metals.Thehigh velocity stream of abrasive is generated by converting the pressure energy of thecarrier gas or air to its kinetic energy and hence high velocity jet. The nozzle directs theabrasive jet in a controlled manner onto the work material, so that the distance between thenozzle and the work piece and the impingement angle can be set desirably. The highvelocity abrasive particles remove the material by micro-cutting action as well as brittlefracture of the work material. Fig. 9.1.3 schematically shows the material removal process.

In AJM, generally, the abrasive particles of around 50 μm grit size would impinge on thework material at velocity of 200 m/s from a nozzle of I.D. of 0.5 mm with a stand off distanceof around 2 mm. The kinetic energy of the abrasive particles would be sufficient to providematerial removal due to brittle fracture of the work piece or even micro cutting by theabrasives.

Equipment

In AJM, air is compressed in an air compressor and compressed air at a pressure of around 5 bar is used as the carrier gas as shown in Fig. 9.1.4. Fig. 9.1.4 also shows the other major parts of the AJM system. Gases like CO2, N2 can also be used as carrier gas which may directly be issued from a gas cylinder. Generally oxygen is not used as a carrier gas. The carrier gas is

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first passed through a pressure regulator to obtain the desired working pressure. The gas isthen passed through an air dryer to remove any residual water vapour. To remove any oilvapour or particulate contaminant the same is passed through a series of filters. Then thecarrier gas enters a closed chamber known as the mixing chamber. The abrasive particlesenter the chamber from a hopper through a metallic sieve. The sieve is constantly vibratedby an electromagnetic shaker. The mass flow rate of abrasive (15 gm/min) entering thechamber depends on the amplitude of vibration of the sieve and its frequency. The abrasiveparticles are then carried by the carrier gas to the machining chamber via an electromagneticon-off valve. The machining enclosure is essential to contain the abrasive and machinedparticles in a safe and eco-friendly manner. The machining is carried out as high velocity(200 m/s) abrasive particles are issued from the nozzle onto a work piece traversing under thejet.

Principle of AJM The principle of machining / cutting by abrasive jet process is explainedthrough the

following steps:

1. Abrasive particles of size between 10 m to 50 m (depending upon the requirement ofeither cutting or finishing of the workpiece) are accelerated in a gas stream (commonly usedgas stream is air at high atmospheric pressures).

2. The smaller abrasive particles are useful for finishing and bigger are used for cuttingoperations.

3. The abrasive particles are directed through the nozzle, towards the workpiece surfacewhere-ever cutting or finishing is to be done. The distance between the tip of the nozzle andthe work surface is normally within 1 mm.

4. As the abrasive particles impact the surface of the workpiece, it causes a small fracture atthe surface of the workpiece. The material erosion occurs by the chipping action.

5. The erosion of material by chipping action is convenient in those materials that are hardand brittle.

6. As the particles impact the surface of workpiece, it causes a small fracture and wear, whichis carried away by the gas along with the abrasive particles.

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7. The abrasive particles once used, cannot be re-used as its shape changes partially and theworkpiece material is also clogged with the abrasive particles during impingement andsubsequent flushing by the carrier gas.

Advantages

AJM process is a highly flexible process wherein the abrasive media is carried by aflexible hose, which can reach out to some difficult areas and internal regions.

AJM process creates localized forces and generates lesser heat than the conventionalmachining processes.

There is no damage to the workpiece surface and also the process does not have tool-workpiece contact, hence lesser amount of heat is generated.

The power consumption in AJM process is low.

Disadvantages

The material removal rate is low The process is limited to brittle and hard materials The wear rate of nozzle is very high The process results in poor machining accuracy The process can cause environmental pollution

Applications:

Metal working:

De-burring of some critical zones in the machined parts. Drilling and cutting of the thin and hardened metal sections. Removing the machining marks, flaws, chrome and anodizing marks.

Glass:

Cutting of the optical fibers without altering its wavelength. Cutting, drilling and frosting precision optical lenses. Cutting extremely thin sections of glass and intricate curved patterns. Cutting and etching normally inaccessible areas and internal surfaces. Cleaning and dressing the grinding wheels used for glass.

Grinding:

Cleaning the residues from diamond wheels, dressing wheels of any shape and size.

Abrasive water jet machining (AWJM)

Abrasive water jet machining (AWJM) is a mechanical material removal process used toerode holes and cavities by the impact of abrasive particles of the slurry on hard andbrittle materials. Since the process is non- thermal, non-chemical and non-electrical itcreates no change in the metallurgical and physical properties of the work piece.

Abrasive Water Jet Machining have potential for cost reduction andspeeding up the process through considerable reduction in secondaryprocesses of machining. The cut edges are clean with fewer burrs as

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there is no heat application. In this process the subsequent problemsfaced in other processes such as crystallization, edge defects,hardening, reduction in weldability and machinability are considerablyreduced.

The term water jet is made used for describing equipment which uses ahigh pressurewater stream for cleaning and cutting applications. In someapplications no abrasives areused, therein the process is termed as Pure-Water Jet machining. Theblock diagram ofwater jet machine is schematically shown in Fig. 3.5.1 along with atypical pure water jetmachining nozzle in Fig. 3.5.2

Abrasive Water Jet Machining (AWJM) is a subcategory of water jetmachining in whichabrasive is introduced in the water to accelerate the process. In AWJMprocesses, whichis considered as an extension of water jet cutting, abrasive particlessuch as aluminiumoxide or silicon carbide are added, which increases the materialremoval rate further. Theabrasive water jet cutting process is suitable for machining differenttypes of materialsranging from hard, brittle ceramics and glass to soft metals such asrubber and foam.The abrasives are separately mixed in the nozzle with the water-stream, making it distinctfrom water jet machining process.

ApplicationsAbrasive water jet cutting is highly used in aerospace, automotive and electronicsindustries. In aerospace industries, parts such as titanium bodies for military aircrafts,engine components (aluminium, titanium, heat resistant alloys), aluminium body partsand interior cabin parts are made using abrasive water jet cutting.

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In automotive industries, parts like interior trim (head liners, trunk liners, door panels) and fibre glass body components and bumpers are made by this process. Similarly, in electronics industries, circuit boards and cable stripping are made by abrasive water jet cutting.Advantages of abrasive water jet cuttingIn most of the cases, no secondary finishing required

No cutter induced distortion

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Low cutting forces on workpieces

Limited tooling requirements

Little to no cutting burr

Typical finish 125-250 microns

Smaller kerf size reduces material wastages

No heat affected zone

Localises structural changes

No cutter induced metal contamination

Eliminates thermal distortion

No slag or cutting dross

Precise, multi plane cutting of contours, shapes, and bevels of any angle.Limitations of abrasive water jet cuttingCannot drill flat bottom

Cannot cut materials that degrades quickly with moisture

Surface finish degrades at higher cut speeds which are frequently used for roughcutting.The major disadvantages of abrasive water jet cutting are high capital cost and high noise levels during operation.

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