unit 1,2,3 of unconventional manufacturing process

7/29/2019 Unit 1,2,3 of Unconventional Manufacturing Process

1/62

Introduction:-

Since beginning of the human race, people have evolved tools and energy sources to power these tools to meet therequirements for making the life more easier and enjoyable. In the early stage of mankind, tools were made of stonefor the item being made. When iron tools were invented, desirable metals and more sophisticated articles could be

produced. In twentieth century products were made from the most durable and consequently, the most unmachinablematerials. In an effort to meet the manufacturing challenges created by these materials, tools have now evolved toinclude materials such as alloy steel, carbide, diamond and ceramics. A similar evolution has taken place with themethods used to power our tools. Initially, tools were powered by muscles; either human or animal. However as the

powers of water, wind, steam and electricity were harnessed, mankind was able to further extend manufacturingcapabilities with new machines, greater accuracy and faster machining rates.

Every time new tools, tool materials, and power sources are utilized, the efficiency and capabilities of manufacturersare greatly enhanced. Since 1940s, a revolution in manufacturing has been taking place that once again allows

manufactuers to meet the demands imposed by increasingly sophisticated designs and durable but in many casesnearly unmachinable, materials.

In the figure 1.1, Merchant had displayed the gradual increase in strength of material with year wise development ofmaterial in aerospace industry. This manufacturing revolution is now, as it has been in the past, centered on the useof new tools and new forms of energy. The result has been the introduction of new manufacturing processes used for

material removal, forming and joining, known today as non-traditional manufacturing processes.

The conventional manufacturing processes in use today for material removal primarily rely on electric motors andhard tool materials to perform tasks such as sawing, drilling and broaching. Conventional forming operations are

performed with the energy from electric motors, hydraulics and gravity. Likewise, material joining is conventionallyaccomplished with thermal energy sources such as burning gases and electric arcs.


2/62

In contrast, non-traditional manufacturing processes harness energy sources considered unconventional byyesterdays standards. Material removal can now be accomplished with electrochemical reaction, high temperature

plasmas and high-velocity jets of liquids and abrasives. Materials that in the past have been extremely difficult toform, are now formed with magnetic fields, explosives and the shock waves from powerful electric sparks. Material-

joining capabilities have been expanded with the use of high-frequency sound waves and beams of electrons andcoherent light.

During the last 55 years, over 20 different non-traditional manufacturing processes have been invented andsuccessfully implemented into production.

Classification of Unconventional Manufacturing Process:-

The non-conventional manufacturing processes are not affected by hardness, toughness orbrittleness of material and can produce any intricate shape on any workpiece material by suitablecontrol over the various physical parameters of the processes.

The non-conventional manufacturing processes may be classified on the basis of type of energy

namely, mechanical, electrical, chemical, thermal or magnetic, apply to the workpiece directlyand have the desired shape transformation or material removal from the work surface by usingdifferent scientific mechanism.

Thus, these non-conventional processes can be classified into various groups according to thebasic requirements which are as follows :

(i) Type of energy required, namely, mechanical, electrical, chemical etc.

(ii) Basic mechanism involved in the processes, like erosion, ionic dissolution, vaporisation etc.

(iii) Source of immediate energy required for material removal, namely, hydrostatic pressure,high current density, high voltage, ionised material, etc.

(iv) Medium for transfer of those energies, like high velocity particles, electrolyte, electron, hotgases, etc. On the basis of above requirements, the various processes may be classified as shownin table.


3/62

Conventional Machining VS Non-Conventional Machining:-

Conventional machining usually involves changing the shape of a workpiece using an implementmade of a harder material. Using conventional methods to machine hard metals and alloys meansincreased demand of time and energy and therefore increases in costs; in some casesconventional machining may not be feasible. Conventional machining also costs in terms of toolwear and in loss of quality in the product owing to induced residual stresses during manufacture.With ever increasing demand for manufactured goods of hard alloys and metals, such as Inconel718 or titanium, more interest has gravitated to non-conventional machining methods.

Conventional machining can be defined as a process using mechanical (motion) energy. Non-conventional machining utilises other forms of energy. The three main forms of energy used innon-conventional machining processes are as follows :

Thermal energy


4/62

Chemical energy

Electrical energy

One example of machining using thermal energy is laser. Thermal methods have many

advantages over conventional machining, but there are a few of disadvantages.

Inconel 718, titanium and other hard metals and alloys have a very high melting point. Usingthermal methods will require high energy input for these materials.

Concentrating heat onto any material greatly affects its microstructure and will normally cause

cracking, which may not be desirable.

Safety requirements for thermal methods, especially laser, are demanding in terms of time andcost.

Machining large areas or many surfaces at the same time using thermal methods is notnormally possible.

The methods using electrical energy are electrodischarge machining (EDM) and anodicmachining (AM), which are similar in practice. EDM, often refered to as spark erosion, usespulsed voltage to remove material from a workpiece and a non-conductive medium to clear thedebris. Because the medium is electrically inert the tool is a direct reverse of the workpiece andno complicated tool design criteria are required. But the shock of spark erosion can affect themicrostructure on the surface of the workpiece. Also, EDM has a lower material removal ratethan AM.

The chemicals used in AM are non-toxic and the energy required is less than othernonconventional machining processes. It has no effect on the microstructure of the workpiece.The electrolyte can even be common sea water, enabling AM to be used in a sub-sea capacity.The hardness and thermal resistivity of the workpiece material do not matter therefore hardmetals and alloys can be machined using tools made from softer materials. The onlydisadvantage is that tool design is a little more complex than that of EDM, but software is beingdeveloped to make this easier. The controllability, environmental versatility, speed, safety andabsence of change in workpiece microstructure make AM a competitive manufacturing process.

Introduction:-

Modern machining methods are also named as non-conventional machining methods. Thesemethods form a group of processes which removes excess material by various techniquesinvolving mechanical, thermal, electrical chemical energy or combination of these energies.There is no cutting of metal with the help of metallic tool having sharp cutting edge. The majorreasons of development and popularity of modern machining methods are listed below.


5/62

(a) Need of machine newly developed metals and non-metals having some special properties likehigh strength, high hardness and high toughness. A material possing the above mentionedproperties are difficult to be machined by the conventional machining methods.

(b) Sometimes it is required to produce complex part geometries that cannot be produced by

following conventional machining techniques. Non-conventional machining methods alsoprovide very good quality of surface finish which may also be an encouragement to thesemethods.

There can be a very long list of non-conventional machining methods. These methods can beclassified as the basis of their base principle of working.Objectives

After studying this unit, you should be able to understand

introduction of modern machining methods and their difference with conventionalmachining methods,

different classification criteria of modern machining methods and their classifications,and

working principle, process details, applications and advantages and disadvantagesmachining.

Principle of Working of Energy:-

The principle of working is the base of type of energy used to remove the material. Classificationalong with the principle of working is described below.

Use of Mechanical Energy

Mechanical energy is used for removing material from workpiece. In this process, cutting toolwith sharp edge is not used but material is removed by the abrasive action of high velocity ofstream of hard, tiny abrasive particles. The particles are kept vibrating with very high velocityand ultra high frequency to remove the material.

Electrical Energy

In this category of non-traditional machining electrical energy is used in the form ofelectrochemical energy or electro-heat energy to erode the material or to melt and vapourized it

respectively. Electrochemical machining, electroplating or electro discharge machining are theexamples work on this principle.

Use of Thermal Energy

According to this principle heat is generated by electrical energy. The generated thermal energyis focused to a very small portion of workpiece. This heat is utilized in melting and evaporatingof metal. The example based o this principle is electric discharge machining.


6/62

Use of Chemical Energy

According to this principle of working chemicals are used to erode material from the workpiece.Selection of a chemical depends upon the workpiece material. Example of this type of machiningis electrochemical machining. The dame principle can also be applied in reversed way in the

process of electrochemical plating.

Non-Conventional Mnachining Process:-

There can be one more way of classification of the non-conventional machining processes whichis mechanisms of metal removal.

Abrasion and Shear

When small and hard metallic particles are made vibrating against the workpiece to be machined,the material is removed by shear action and abrasion. These phenomenon take place in case of

ultrasonic machining.

Chemical Ablation and Ionic Dissolution

This is the dissolution of workpiece material into electrolyte solution (chemical) which takesplace atom by atom. This happens in case electrochemical machining.

Vapourization by Spark Erosion

Concentrated heat is focused at a point of the workpiece by electric spark which melts andevaporates the workpiece material like electric discharge machining and LBM.

Electro -Discharge Machining (EDM):-

It is also known as spark erosion machining or spark machining. Material of workpiece removeddue to erosion caused by electric spark. Working principle is described below.

Working Principle of Electric Discharge Machining:-

Electric discharge machining process is carried out in presence of dielectric fluid which createspath for discharge. When potential difference is created across the two surfaces of die electricfluid, it gets ionized. An electric spark/discharge is generated across the two terminals. The

potential difference is developed by a pulsating direct current power supply connected across thetwo terminals. One of the terminal is positive terminal given to workpiece and tool is madenegative terminal. Two third of the total heat generated is generated at positive terminal soworkpiece is generally given positive polarity. The discharge develops at the location where twoterminals are very close. So tool helps in focusing the discharge or intensity of generated heat atthe point of metal removal.


7/62

Application of focused heat raise the temperature of workpiece locally at a point, this way twometal is melted and evaporated.

Electric Discharge Machining Process Details:-

The working principle and process of EDM is explained with the help of line diagram in Figure.The process details and components are explained below serially.

Line Diagram Indicating Working Principle and Process Details of EDM

Base and Container:-

A container of non-conducting, transparent material is used for carrying out EDM. The containeris filled with dielectric solution. A base to keep workpiece is installed at the bottom of container.The base is made of conducting material and given positive polarity.

Tool:-

Tool is given negative polarity. It is made of electrically conducting material line brass, copperor tungeten. The tool material selected should be easy to machine, high wear resistant. Tool ismade slightly under size for inside machining and over sized for cut side machining. Tool isdesigned and manufactured according to the geometry to be machined.

Dielectric Solution:-

Dielectric solution is a liquid which should be electrically conductive. This solution provides twomain functions, firstly it drive away the chips and prevents their sticking to workpiece and tool.It enhance the intensity of discharge after getting ionized and so accelerates metal removal rate.


8/62

Power Supply:-

A DC power supply is used, 50 V to 450 V is applied. Due to ionization of dielectric solution anelectrical breakdown occurs. The electric discharge so caused directly impinges on the surface ofworkpiece. It takes only a few micro seconds to complete the cycle and remove the material. The

circuit cam be adjusted for auto off after pre-decided time interval.

Tool Feed Mechanism:-

In case of EDM, feeding the tool means controlling gap between workpiece and the tool. Thisgap is maintained and controlled with the help of servo mechanism. To maintain a constant gapthroughout the operation tool is moved towards the machining zone very slowly. The movementspeed is towards the machining zone very slowly. The movement speed is maintained by the helpof gear and rack and pinion arrangement. The servo system senses the change in gap due to metalremoval and immediately corrects it by moving the tool accordingly. The spark gap normallyvaries from 0.005 mm to 0.50 mm.

Workpiece and Machined Geometry:-

The important point for workpiece is that any material which is electrical conductor can bemachined through this process, whatever be the hardness of the same. The geometry which is tobe machined into the workpiece decides the shape and size of the tool.

Application of Electric Discharge Machining:-

This process is highly economical for machining of very hard material as tool wear isindependent of hardness of workpiece material. It is very useful in tool manufacturing. It is also

used for broach making, making holes with straight or curved axes, and for making complicatedcavities which cannot be produced by conventional machining operations. EDM is widely usedfor die making as complex cavities are to be made in the die making. However, it is capable todo all operations that can be done by conventional machining.

Advantages of EDM:-

(a) This process is very much economical for machining very hard material.

(b) Maintains high degree of dimensional accuracy so it is recommended for tool and diemaking.

(c) Complicated geometries can be produced which are very difficult otherwise.

(d) Highly delicate sections and weak materials can also be processed without nay risk of theirdistortion, because in this process tool never applies direct pressure on the workpiece.

(e) Fine holes can be drilled easily and accurately.


9/62

(f) Appreciably high value of MRRR can be achieved as compared to other non-conventionalmachining processes.

Disadvantages and Limitations of EDM Process:-

There are some limitations of EDM process as listed below :

(a) This process cannot be applied on very large sized workpieces as size of workpiece isconstrained by the size of set up.

(b) Electrically non-conducting materials cannot be processed by EDM.

(c) Due to the application of very high temperature at the machining zone, there are chances ofdistortion of workpiece in case of this sections.

(d) EDM process is not capable to produce sharp corners.

(e) MRR achieved in EDM process is considerably lower than the MRR in case of conventionalmachining process so it cannot be taken as an alternative to conventional machining processes atall.

Wire Cut Electric Discharge Machining (WCEDM):-

This is a special type of electric discharge machining that uses a small diameter wire as a cuttingtool on the work. Working a principle of wire cut electric discharge machining is same as that ofelectric discharge machining.

Process Details of WCEDM:-

Process details of WCEDM are almost similar to EDM with slight difference. The details of theprocess are indicated in the line diagram shown in Figure. Its major difference of process detailswith EDM process details are described below.

Tool Details:-

The tool used in WCEDM process is a small diameter wire as the electrode to cut narrow kerf inthe workpiece. During the process of cutting the wire is continuously advanced between a supply

spoil and wire collector. This continuous feeding of wire makes the machined geometryinsensitive to distortion of tool due to its erosion. Material of wire can be brass, copper, tungstenor any other suitable material to make EDM tool. Normally, wire diameter ranges from 0.076 to0.30 mm depending upon the width of kerf.


10/62

Line Diagram for Process Details of Working of Wire Cut Electric Discharge Machining


Two type of movements are generally given to the total (wire). One is continuous feed from wiresupply spoal to wire collector. Other is movement of the whole wire feeding system, and wirealong the kerf to be cut into the workpiece. Both movements are accomplished with ultraaccuracy and pre-determined speed with the help of numerical control mechanism.

Dielectric Fluid and Spray Mechanism:-

Like EDM process dielectric fluid is continuously sprayed to the machining zone. This fluid isapplied by nozzles directed at the tool work interface or workpiece is submerged in the dielectricfluid container.

Rest of the process details in case of WCEDM process are same as that in case of EDM process.

Application of WCEDM:-

WCEDM is similar to hand saw operation in applications with good precision. It is used to makenarrow kerf with sharp corners. It does not impose any force to workpiece so used for verydelicated and thin workpieces. It is considered ideal for making components for stamping dies. Itis also used to make intricate shapes in punch, dies and other tools.

Advantages of WCEDM:-

Advantages are listed below :


11/62

(a) Accuracy and precision of dimensions are of very good quality.

(b) No force is experienced by the workpiece.

(c) Hardness and toughness of workpiece do not create problems in machining operation.

Disadvantages and Limitations of WCEDM:-

The major disadvantages of this process are that only electrically conducting materials canmachined. This process is costly so recommended for use specifically at limited operations.

Ultrasonic Machining (USM):-

Ultrasonic machining (USM) is one of the non-traditional machining process. Working principleof this process resembles with conventional and metal cutting as in this process abrasivescontained in a slurry are driven at high velocity against the workpiece by a tool vibrating at low

amplitude and high frequency. Amplitude is kept of the order of 0.07 mm and frequency ismaintained at approximately 20,000 Hz. The workpiece material is removed in the form ofextremely small chips. Normally very hard particle dust is included in the slurry like, Al 2O2,silicon carbide, boron carbide or diamond dust.

Working principle of USM is same as that of conventional machining that is material ofworkpiece is removed by continuous abrasive action of hard particles vibrating in the slurry.Abrasive slurry acts as a multipoint cutting tool and does the similar action as done by a cuttingedge.

Process Details:-

USM process is indicated in line diagram shown in Figure. Details of the process are discussedbelow.


12/62

Details of USM Process

Abrasive Slurry:-

Abrasive slurry consists of dust of very hard particles. It is filled into the machining zone.Abrasive slurry can be recycled with the help of pump.

Workpiece:-

Workpiece of hard and brittle material can be machined by USM. Workpiece is clamped on thefixture I the setup.

Cutting Tool:-

Tool of USM does not do the cutting directly but it vibrates with small amplitude and highfrequency. So it is suitable to name the tool as vibrating tool rather than cutting tool. The tool ismade of relatively soft material and used to vibrate abrasive slurry to cut the workpiece material.The tool is attached to the arbor (tool holder) by brazing or mechanical means. Sometimeshollow tools are also used which feed the slurry focusing machining zone.


13/62

Ultrasonic Oscillator:-

This operation uses high frequency electric current which passes to an ultrasonic oscillator andultrasonic transducer. The function of the transducer is to convert electric energy into mechanicalenergy developing vibrations into the tool.

Feed Mechanism:-

Tool is fed to the machining zone of workpiece. The tool is shaped as same to the cavity of beproduced into the workpiece. The tool is fed to the machining area. The feed rate is maintainedequal to the rate of enlargement of the cavity to be produced.

Applications of USM:-

This process is generally applied for the machining of hard and brittle materials like carbidesglass, ceramics, precious stones, titanium, etc. It is used for tool making and punch and die

making. The workpeice material is normally removed in the form of very find chips so generatedsurface quality is extremely good. It is widely used for several machining operations like turning,grinding, trepanning and milling, etc. It can make hole of round shape and other shapes.

Advantages of USM:-

Advantages of USM process are listed below :

(a) Its main advantage is the workpiece after machining is free from any residual stress as toconcentrated force or heat is subject to it during the machining process.

(b) Extremely hard and brittle materials can be machined, their machining is very difficult byconventional methods.

(c) Very good dimensional accuracy and surface finish can be obtained.

(d) Operational cost is low.

(e) The process is environmental friendly as it is noiseless and no chemical and heating is used.

Disadvantages of USM:-

The process of USM have some disadvantages and limitations as described below :

(a) Its metal removal rate (MRR) is very low and it can not be used for large machining cavities.

(b) Its initial setup cost and cost of tool is very high, frequency tool replacement is required astool wear takes place in this operation.

(c) Not recommended for soft and ductile material due to their ductility.


14/62

(d) Power consumption is quite high.

(e) Slurry may have to be replaced frequently.

Chemical Machining Processes (CHM):-

Chemical machining is one of the non-conventional machining processes where material isremoved by bringing it in contact of a strong chemical enchant. There are different chemicalmachining methods base on this like chemical milling, chemical blanking, photochemicalmachining, etc.

Working Principle of CHM:-

The main working principle of chemical machining is chemical etching. The part of theworkpiece whose material is to be removed, is brought into the contact of chemical calledenchant. The metal is removed by the chemical attack of enchant. The method of making contact

of metal with the enchant is masking. The portion of workpiece where no material is to beremoved, is mashed before chemical etching.

Process Details of CHM:-

Following steps are normally followed in the process of CHM :

Cleaning:-

The first step of the process is a cleaning of workpiece, this is required to ensure that materialwill be removed uniformly from the surfaces to be processed.

Masking:-

Masking is similar to masking action is any machining operation. This is the action of selectingmaterial that is to be removed and another that is not to be removed. The material which is not tobe removed is applied with a protective coating called maskant. This is made of a materials areneoprene, polyvinylchloride, polyethylene or any other polymer. Thinkers of maskent ismaintained upto 0.125 mm. The portion of workpiece having no application of maskent is etchedduring the process of etching.

Etching:-

In this step the material is finally removed. The workpiece is immersed in the enchant where thematerial of workpiece having no protective coating is removed by the chemical action ofenchant. Enchant is selected depending on the workpiece material and rate of material removal;and surface finish required. There is a necessity to ensure that maskant and enchant should bechemically in active. Common enchants are H2SO4, FeCL3, HNO3. Selection of enchant alsoaffects MRR. As in CHM process, MRR is indicated as penetration rates (mm/min).


15/62

Demasking:-

After the process is completed demasking is done. Demasking is an act of removing maskentafter machining.

Application of CHM:-

The application and working of CHM process are indicated in Figure, various applications ofCHM are discussed below.

Chemical Milling:-

It is widely used in aircraft industry. It is the preparation of complicated geometry on theworkpiece using CHM process.

Application and Working of CHM

Chemical Blanking:-

In this application cutting is done on sheet metal workpieces. Metal blanks can be cut from verythin sheet metal, this cutting may not be possible by conventional methods.

Photochemical Machining:-

It is used in metal working when close (tight) tolerances and intricate patterns are to be made.This is used to produce intricate circuit designs on semiconductor wafers.

Advantages of CHM:-

Advantages of CHM process are listed below :


16/62

(a) Low tooling cost.

(b) Multiple machining can be done on a workpiece simultaneously.

(c) No application of force so on risk of damage to delicate or low strength workpiece.

(d) Complicated shapes/patterns can be machined.

(e) Machining of hard and brittle material is possible.

Disadvantages and Limitations of CHM:-

(a) Slower process, very low MRR so high cost of operation.

(b) Small thickness of metal can be removed.

(c) Sharp corners cannot be prepared.

(d) Requires skilled operators.

Electrochemical Machining (ECM):-

Electrochemical machining (ECM) process uses electrical energy in combination with chemicalenergy to remove the material of workpiece. This works on the principle of reverse ofelectroplating.

Working Principle of ECM:-

Electrochemical machining removes material of electrically conductor workpiece. Theworkpiece is made anode of the setup and material is removed by anodic dissolution. Tool ismade cathode and kept in close proximity to the workpiece and current is passed through thecircuit. Both electrodes are immersed into the electrolyte solution. The working principle andprocess details are shown in the Figure. This works on the basis of Faradays law of electrolysis.The cavity machined is the mirror image of the tool. MRR in this process can easily becalculated according to Faradays law.

Process Details:-

Process details of ECM are shown in Figure and described as below :


17/62

Working Principle and Process Details of ECM

Workpiece:-

Workpiece is made anode, electrolyte is pumped between workpiece and the tool. Material ofworkpiece is removed by anodic dissolution. Only electrically conducting materials can beprocessed by ECM.

Tool:-

A specially designed and shaped tool is used for ECM, which forms cathode in the ECM setup.The tool is usually made of copper, brass, stainless steel, and it is a mirror image of the desiredmachined cavity. Proper allowances are given in the tool size to get the dimensional accuracy ofthe machined surface.

Power Supply:-

DC power source should be used to supply the current. Tool is connected with the negativeterminal and workpiece with the positive terminal of the power source. Power supply supplieslow voltage (3 to 4 volts) and high current to the circuit.

Electrolyte:-


18/62

Water is used as base of electrolyte in ECM. Normally water soluble NaCl and NaNO3 are usedas electrolyte. Electrolyte facilitates are carrier of dissolved workpiece material. It is recycled bya pump after filtration.


Servo motor is used to feed the tool to the machining zone. It is necessary to maintain a constantgap between the workpiece and tool so tool feed rate is kept accordingly while machining.

In addition to the above whole process is carried out in a tank filled with electrolyte. The tank ismade of transparent plastic which should be non-reactive to the electrolyte. Connecting wires arerequired to connect electrodes to the power supply.

Applications of ECM Process:-

There are large number of applications of ECMs some other related machining and finishing

processes as described below :

(a) Electrochemical Grinding : This can also be named as electrochemical debrruing. This is usedfor anodic dissolution of burrs or roughness a surface to make it smooth. Any conductingmaterial can be machined by this process. The quality of finish largely depends on the quality offinish of the tool.

(b) This is applied in internal finishing of surgical needles and also for their sharpening.

(c) Machining of hard, brittle, heat resistant materials without any problem.

(d) Drilling of small and deeper holes with very good quality of internal surface finish.

(e) Machining of cavities and holes of complicated and irregular shapes.

(f) It is used for making inclined and blind holes and finishing of conventionally machinedsurfaces.

Advantages of ECM Process:-

Following are the advantages of ECM process :

(a) Machining of hard and brittle material is possible with good quality of surface finish anddimensional accuracy.

(b) Complex shapes can also be easily machined.

(c) There is almost negligible tool wear so cost of tool making is only one time investment formass production.


19/62

(d) There is no application of force, no direct contact between tool and work and no applicationof heat so there is no scope of mechanical and thermalresidual stresses in the workpiece.

(e) Very close tolerances can be obtained.

Disadvantages and Limitations of ECM:-

There are some disadvantages and limitations of ECM process as listed below :

(a) All electricity non-conducting materials can not be machined.

(b) Total material and workpiece material should be chemically stable with the electrolytesolution.

(c) Designing and making tool is difficult but its life is long so recommended only for massproduction.

(d) Accurate feed rate of tool is required to be maintained.

Laser Beam Machining (LBM):-

Laser beam have wide industrial applications including some of the machining processes. A laseris an optical transducer that converts electrical energy into a highly coherent light beak. Onemust know the full name of laser, it stands for light amplification of stimulated emission ofradiation. Laser being coherent in nature has a specific property, if it is focused by conventional

optical lenses can generate high power density.

Working Principle of LBM:-

LBM uses the light energy of a laser beam to remove material by vaporization and ablation. Theworking principle and the process details (setup) are indicated in Figure. In this process theenergy of coherent light beam is focused optically for predecided longer period of time. Thebeam is pulsed so that the released energy results in an impulse against the work surface thatdoes melting and evaporation. Here the way of metal removing is same as that of EDM processbut method of generation of heat is different. The application of heat is very finely focused incase of LBM as compared to EDM.

Process Details of LBM:-

Process details of LBM are shown in line diagram shown in Figure, description of the details isgiven below.


20/62

Working Principle and Process Details of LBM

Laser Tube and Lamp Assembly:-

This is the main part of LBM setup. It consists of a laser tube, a pair of reflectors, one at eachend of the tube, a flash tube or lamp, an amplification source, a power supply unit and a coolingsystem. This whole setup is fitted inside a enclosure, which carries good quality reflectingsurfaces inside. In this setup the flash lamp goes to laser tube, that excites the atoms of the insidemedia, which absorb the radiation of incoming light energy. This enables the light to travel toand fro between two reflecting mirrors. The partial reflecting mirror does not reflect the totallight back and apart of it goes out in the form of a coherent stream of monochromatic light. Thishighly amplified stream of light is focused on the workpiece with the help of converging lense.

The converging lense is also the part of this assembly.

Workpiece:-

The range of workpiece material that can be machined by LBM includes high hardness andstrength materials like ceramics, glass to softer materials like plastics, rubber wood, etc. A goodworkpiece material high light energy absorption power, poor reflectivity, poor thermalconductivity, low specific heat, low melting point and low lotent heat.


21/62

Cooling Mechanism:-

A cooling mechanism circulates coolant in the laser tube assembly to avoid its over heating inlong continuous operation.


There is no tool used in the LBM process. Focusing laser beam at a pre-decided point in theworkpiece serve the purpose of tool. As the requirement of being focused shifts during theoperation, its focus point can also be shifted gradually and accordingly by moving theconverging lense in a controlled manner. This movement of the converging lense is the tool feedmechanism in LBM process.

Applications of LBM:-

LBM is used to perform different machining operations like drilling, slitting, slotting, scribingoperations. It is used for drilling holes of small diameter of the order of 0.025 mm. It is used forvery thin stocks. Other applications are listed below :

(a) Making complex profiles in thin and hard materials like integrated circuits and printed circuitboards (PCBS).

(b) Machining of mechanical components of watches.

(c) Smaller machining of very hard material parts.

Advantages of LBM:-

(a) Materials which cannot be machined by conventional methods are machined by LBM.

(b) There is no tool so no tool wear.

(c) Application of heat is very much focused so rest of the workpiece is least affected by theheat.

(d) Drills very find and precise holes and cavities.

Disadvantages of LBM:-

Major disadvantages of LBM process are given below :

(a) High capital investment is involved. Operating cost is also high.

(b) Recommended for some specific operations only as production rate is very slow.


22/62

(c) Cannot be used comfortably for high heat conductivity materials light reflecting materials.

(d) Skilled operators are required.

Plasma Arc Machining (PAM):-

Working Principle of PAM:-

In this process gases are heated and charged to plasma state. Plasma state is the superheated andelectrically ionized gases at approximately 5000oC. These gases are directed on the workpiece inthe form of high velocity stream. Working principle and process details are shown in Figure.

Working Principle and Process Details of PAM

Process Details of PAM:-

Details of PAM are described below.


23/62

Plasma Gun:-

Gases are used to create plasma like, nitrogen, argon, hydrogen or mixture of these gases. Theplasma gun consists of a tungsten electrode fitted in the chamber. The electrode is given negativepolarity and nozzle of the gun is given positive polarity. Supply of gases is maintained into the

gun. A strong arc is established between the two terminals anode and cathode. There is acollision between molecules of gas and electrons of the established arc. As a result of thiscollision gas molecules get ionized and heat is evolved. This hot and ionized gas called plasma isdirected to the workpiece with high velocity. The established arc is controlled by the supply rateof gases.

Power Supply and Terminals:-

Power supply (DC) is used to develop two terminals in the plasma gun. A tungsten electrode isinserted to the gun and made cathode and nozzle of the gun is made anode. Heavy potentialdifference is applied across the electrodes to develop plasma state of gases.

Cooling Mechanism:-

As we know that hot gases continuously comes out of nozzle so there are chances of its overheating. A water jacket is used to surround the nozzle to avoid its overheating.

Tooling:-

There is no direct visible tool used in PAM. Focused spray of ho0t, plasma state gases works as acutting tool.

Workpiece:-

Workpiece of different materials can be processed by PAM process. These materials arealuminium, magnesium, stainless steels and carbon and alloy steels. All those material which canbe processed by LBM can also be processed by PAM process.

Applications of PAM:-

The chief application of this process is profile cutting as controlling movement of spray focuspoint is easy in case of PAM process. This is also recommended for smaller machining ofdifficult to machining materials.

Advantages of PAM Process:-

Advantages of PAM are given below :

(a) It gives faster production rate.

(b) Very hard and brittle metals can be machined.


24/62

(c) Small cavities can be machined with good dimensional accuracy.

Disadvantages of PAM Process:-

(a) Its initial cost is very high.

(b) The process requires over safety precautions which further enhance the initial cost of thesetup.

(c) Some of the workpiece materials are very much prone to metallurgical changes on excessiveheating so this fact imposes limitations to this process.

(d) It is uneconomical for bigger cavities to be machined.

Electrodischarge Grinding:-

Electrodischarge grinding (EDG) removes conductive materials by rapid spark dischargesbetween a rotating tool and workpiece that are separated by a flowing dielectric fluid Fig..

EDG schematic

The spark gap is normally held at 0.013 to 0.075 mm by the servomechanism that controls themotion of the workpiece. The dc power source has capabilities ranging from 30 to 100 A, 2 to500 kHz, and 30 to 400 V. The conductive wheel, usually made of graphite, rotates at 30 to 180m/min in a dielectric bath of filtered hydrocarbon oil. The workpiece is usually connected to thepositive terminal of the dc power supply. As can be seen from Fig.4, the workpiece is machinedusing a stream of electric sparks. Each spark discharge melts or vaporizes a small amount ofmetal from the workpiece surface. Higher machining currents produce faster rates of machining,


25/62

rougher finishes, and a deeper heat-affected zone (HAZ) in the workpiece. Less current is usedfor the production of smoother and less damaged surfaces. Additionally, higher pulse frequenciesmake smoother surfaces. Wheel wear ranges from 100:1 to 0.1:1 with an average of 3:1depending upon the current density, workpiece material, wheel material, dielectric, andsharpness of corner details. Material removal rates range from 0.16 to 2.54 cm3/min. Surface

finishes in the range of 1.6 to 3.2 micro m Ra are possible.

Elements of EDG


26/62

Removal rate and surface roughness in EDG

Above Fig.shows the relationship between removal rate andsurface roughness in EDG. Thecorner radius depends on the overcut and ranges from 0.013 to 0.130 mm. Greater voltagespermit larger gaps, which makes the process suitable for plunge grinding where ease of dielectricflushing is ensured. Tolerances of 0.005 mm are normal with 0.001 mm possible. The surfacefinish improves with an increase in pulse frequency and is typically 0.4 to 0.8 micro m Ra. Theselayers must be removed or modified in case of highly stressed applications. Bellow Fig.showsthe main elements of EDG. Abrasive electrodischarge grinding (AEDG) employs the interactiveeffect of EDE and MA in order to enhance the machining productivity. In the AEDG process themetallic or graphite electrode used in electrodischarge grinding is replaced by a metallic bondgrinding wheel. Therefore, electro erosion in addition to the MA action occurs as shown in Figs.

An increase in performance measures of the machining process becomes evident whenmachining super hard materials (plates with synthetic polycrystalline diamond), engineeringceramics, sintered carbides, and metallic composites.


27/62

AEDG machining system components

Apart from the above-mentioned effects, the electric discharge causes a considerable decrease ingrinding forces, lowers the grinding wheel wear, and provides an effective method for dressingthe grinding wheel during the machining process. The relative material removal rate for the EDGand AEDG processes was compared to the material removal rate of the electrodischarge process(under the same conditions). Accordingly, the increase in productivity of the EDG process isattributed to improvements in hydrodynamic conditions of dielectric flow. This improvementresults from the Introducing mechanical effects into the AEDG process leads to a further increasein the metal removal rate by about 5 times that of the EDM process and about twice that of the

EDG process. As the number of wheel revolutions increases, the effect of abrasive action is alsoincreased. This may be evidence of better utilization of electrical discharge energy.

Applications:-

EDG and AEDG can be used on

1. Steel and carbide at the same time without wheel loading

2. Thin sections on which abrasive wheel pressures might cause distortion

3. Brittle materials or fragile parts on which abrasive materials might cause fracturing

4. Form tools and tungsten carbide throw away bits for which diamond wheel costs would beexcessive.

Electric Discharge Wire Cutting Process:-


28/62

Wire Electric Discharge Machining (Wire EDM) is a special form of EDM that uses a smalldiameter wire as the electrode to cut a narrow kerf in the work. Wire EDM is illustrated in thefigure:

The setup of Wire Electric Discharge Machining (WEDM) process

The workpiece is fed continuously and slowly past the wire in order to achieve the desiredcutting path. Numerical control is used to control the work-part motions during cutting. As itcuts, the wire is continuously advanced between a supply spool and a take-up spool to present afresh electrode of constant diameter to the work. This helps to maintain a constant kerf widthduring cutting. As in EDM, wire EDM must be carried out in the presence of a dielectric. This is

applied by nozzles directed at the tool-work interface as in the figure, or the workpart issubmerged in a dielectric bath. Wire diameters range from 0.08 to 0.30 mm, depending onrequired kerf width. Materials used for the wire include brass, copper, tungsten, andmolybdenum. Dielectric fluids include deionized water or oil. As in EDM, an overcut in therange from 0.02 to 0.05 mm exists in wire EDM that makes the kerf larger than the wirediameter. This process is well suited to production of dies for sheet metalworking, cams, etc.Since the kerf is so narrow, it is often possible to fabricate punch and die in a single cut, asillustrated in the figure:


29/62

Punch and die fabricated in a single cut by WEDM

Power generator Circuits of EDM:-

Fig. depicted general nature of voltage pulses used in electro-discharge machining. Differentpower generators are used in EDM and some are listed below:

Resistance-capacitance type (RC type) Relaxation generator

Rotary impulse type generator

Electronic pulse generator

Hybrid EDM generator


30/62

Basic circuits for different types of EDM generators

Mechanism of Material Removal:-

In EDM, the removal of material is based upon the electrodischarge erosion (EDE) effect ofelectric sparks occurring between two electrodes that are separated by a dielectric liquid asshown in Fig.

EDM components


31/62

Metal removaltakes place as a result of the generation of extremely high temperatures generatedby the high-intensity discharges that melt and evaporate the two electrodes.

Electrode Material:-

Electrode material should be such that it would not undergo much tool wear when it is impingedby positive ions. Thus the localised temperature rise has to be less by tailoring or properlychoosing its properties or even when temperature increases, there would be less melting. Further,the tool should be easily workable as intricate shaped geometric features are machined in EDM.Thus the basic characteristics of electrode materials are:

High electrical conductivity electrons are cold emitted more easily and there is less bulkelectrical heating

High thermal conductivity for the same heat load, the local temperature rise would be lessdue to faster heat conducted to the bulk of the tool and thus less tool wear

Higher density for the same heat load and same tool wear by weight there would be lessvolume removal or tool wear and thus less dimensional loss or inaccuracy

High melting point high melting point leads to less tool wear due to less tool material meltingfor the same heat load

Easy manufacturability

Cost cheap

The followings are the different electrode materials which are used commonly in the industry:

Graphite

Electrolytic oxygen free copper

Tellurium copper 99% Cu + 0.5% tellurium

Brass

Dielectric Fluid:-

In EDM, as has been discussed earlier, material removal mainly occurs due to thermalevaporation and melting. As thermal processing is required to be carried out in absence ofoxygen so that the process can be controlled and oxidation avoided. Oxidation often leads topoor surface conductivity (electrical) of the workpiece hindering further machining. Hence,dielectric fluid should provide an oxygen free machining environment. Further it should haveenough strong dielectric resistance so that it does not breakdown electrically too easily but at thesame time ionise when electrons collide with its molecule. Moreover, during sparking it should


32/62

be thermally resistant as well. Generally kerosene and deionised water is used as dielectric fluidin EDM. Tap water cannot be used as it ionises too early and thus breakdown due to presence ofsalts as impurities occur. Dielectric medium is generally flushed around the spark zone. It is alsoapplied through the tool to achieve efficient removal of molten material.

Machine Tool Selection of EDM:-

Material - Metals with a high melting point and good electrical conductivity are usually chosenas tool materials for EDM. Graphite is the most common electrode material since it has fair wearcharacteristics and is easily machinable and small flush holes can be drilled into graphiteelectrodes. Copper has good EDM wear and better conductivity. It is generally used for betterfinishes in the range of 0.5m Ra. Copper tungsten and silver tungsten are used for making deepslots under poor flushing conditions especially in tungsten carbides. It offers high machiningrates as well as low electrode wear. Copper graphite is good for cross-sectional electrodes. It hasbetter electrical conductivity than graphite while the corner wear is higher. Brass ensures stablesparking conditions and is normally used for specialized applications such as drilling of small

holes where the high electrode wear is acceptable.

Electron Beam Machining (EBM):-

Introduction

Electron Beam Machining (EBM) and Laser Beam Machining (LBM) are thermal processesconsidering the mechanisms of material removal. However electrical energy is used to generatehigh-energy electrons in case of Electron Beam Machining (EBM) and high-energy coherentphotons in case of Laser Beam Machining (LBM). Thus these two processes are often

Classified as Electro-Optical-Thermal Processes:-

There are different jet or beam processes, namely Abrasive Jet, Water Jet etc. These two aremechanical jet processes. There is also thermal jet or beams. A few are oxyacetylene flame,welding arc, plasma flame etc. EBM as well as LBM are such thermal beam processes. Fig.shows the variation in power density vs. the characteristic dimensions of different thermal beamprocesses. Characteristic length is the diameter over which the beam or flame is active. In case ofoxyacetylene flame or welding arc, the characteristic length is in mm to tens of mm and thepower density is typically low. Electron Beam may have a characteristic length of tens ofmicrons to mm depending on degree of focusing of the beam. In case of defocused electronbeam, power density would be as low as 1 Watt/mm2 . But in case of focused beam the same can

be increased to tens of kW/mm

2

. Similarly as can be seen in Fig.1, laser beams can be focusedover a spot size of 10 100 m with a power density as high as 1 MW/mm2 . Electricaldischarge typically provides even higher power density with smaller spot size.

EBM and LBM are typically used with higher power density to machine materials. Themechanism of material removal is primarily by melting and rapid vaporisation due to intenseheating by the electrons and laser beam respectively.


33/62

Process:-

Electron beam is generated in an electron beam gun. The construction and working principle ofthe electron beam gun would be discussed in the next section. Electron beam gun provides highvelocity electrons over a very small spot size. Electron Beam Machining is required to be carried

out in vacuum. Otherwise the electrons would interact with the air molecules, thus they wouldloose their energy and cutting ability. Thus the workpiece to be machined is located under theelectron beam and is kept under vacuum. The high-energy focused electron beam is made toimpinge on the workpiece with a spot size of 10 100 m. The kinetic energy of the highvelocity electrons is converted to heat energy as the electrons strike the work material. Due tohigh power density instant melting and vaporisation starts and melt vaporisation frontgradually progresses, as shown in Fig.. Finally the molten material, if any at the top of the front,is expelled from the cutting zone by the high vapour pressure at the lower part. Unlike inElectron Beam Welding, the gun in EBM is used in pulsed mode. Holes can be drilled in thinsheets using a single pulse. For thicker plates, multiple pulses would be required. Electron beamcan also be manoeuvred using the electromagnetic deflection coils for drilling holes of any

shape.

Mechanism of Material Removal in Electron Beam Machining


34/62

Equipment:-

Fig. shows the schematic representation of an electron beam gun, which is the heart of anyelectron beam machining facility. The basic functions of any electron beam gun are to generatefree electrons at the cathode, accelerate them to a sufficiently high velocity and to focus them

over a small spot size. Further, the beam needs to be manoeuvred if required by the gun. Thecathode as can be seen in Fig is generally made of tungsten or tantalum. Such cathodefilaments are heated, often inductively, to a temperature of around 2500 0 C. Such heating leadsto thermo-ionic emission of electrons, which is further enhanced by maintaining very lowvacuum within the chamber of the electron beam gun. Moreover, this cathode cartridge is highlynegatively biased so that the thermo-ionic electrons are strongly repelled away form the cathode.This cathode is often in the form of a cartridge so that it can be changed very quickly to reducedown time in case of failure.

Just after the cathode, there is an annular bias grid. A high negative bias is applied to this grid sothat the electrons generated by this cathode do not diverge and approach the next element, the

annular anode, in the form of a beam. The annular anode now attracts the electron beam andgradually gets accelerated. As they leave the anode section, the electrons may achieve a velocityas high as half the velocity of light.

The nature of biasing just after the cathode controls the flow of electrons and the biased grid isused as a switch to operate the electron beam gun in pulsed mode. After the anode, the electronbeam passes through a series of magnetic lenses and apertures. The magnetic lenses shape thebeam and try to reduce the divergence. Apertures on the other hand allow only the convergentelectrons to pass and capture the divergent low energy electrons from the fringes. This way, theaperture and the magnetic lenses improve the quality of the electron beam.

Then the electron beam passes through the final section of the electromagnetic lens anddeflection coil. The electromagnetic lens focuses the electron beam to a desired spot. Thedeflection coil can manoeuvre the electron beam, though by small amount, to improve shape ofthe machined holes. Generally in between the electron beam gun and the workpiece, which isalso under vacuum, there would be a series of slotted rotating discs. Such discs allow the electronbeam to pass and machine materials but helpfully prevent metal fumes and vapour generatedduring machining to reach the gun. Thus it is essential to synchronize the motion of the rotatingdisc and pulsing of the electron beam gun.


35/62

Electron Beam Gun

Introduction

The fact that electric arc could operate was known for over a 100 years. The first everunderwater welding was carried out by British Admiralty Dockyard for sealing leaking shiprivets below the water line. Underwater welding is an important tool for underwater fabricationworks. In 1946, special waterproof electrodes were developed in Holland by Van der

Willingen. In recent years the number of offshore structures including oil drilling rigs, pipelines,platforms are being installed significantly. Some of these structures will experience failures of itselements during normal usage and during unpredicted occurrences like storms, collisions. Anyrepair method will require the use of underwater welding

Classification

Underwater welding can be classified as


36/62

1) Wet Welding

2) Dry Welding

In wet welding the welding is performed underwater, directly exposed to the wet environment. In

dry welding, a dry chamber is created near the area to be welded and the welder does the job bystaying inside the chamber.

Wet Welding

Wet Welding indicates that welding is performed underwater, directly exposed to the wetenvironment. A special electrode is used and welding is carried out manually just as one does inopen air welding. The increased freedom of movement makes wet welding the most effective,efficient and economical method. Welding power supply is located on the surface withconnection to the diver/welder via cables and hoses.

In wet welding MMA (manual metal arc welding) is used.Power Supply used : DCPolarity : -ve polarity

When DC is used with +ve polarity, electrolysis will take place and cause rapid deterioration ofany metallic components in the electrode holder. For wet welding AC is not used on account ofelectrical safety and difficulty in maintaining an arc underwater.

The power source should be a direct current machine rated at 300 or 400 amperes. Motorgenerator welding machines are most often used for underwater welding in the wet. The weldingmachine frame must be grounded to the ship. The welding circuit must include a positive type ofswitch, usually a knife switch operated on the surface and commanded by the welder-diver. Theknife switch in the electrode circuit must be capable of breaking the full welding current and is


37/62

used for safety reasons. The welding power should be connected to the electrode holder onlyduring welding.

Direct current with electrode negative (straight polarity) is used. Special welding electrodeholders with extra insulation against the water are used. The underwater welding electrode holder

utilizes a twist type head for gripping the electrode. It accommodates two sizes of electrodes.

The electrode types used conform to AWS E6013 classification. The electrodes must bewaterproofed. All connections must be thoroughly insulated so that the water cannot come incontact with the metal parts. If the insulation does leak, seawater will come in contact with themetal conductor and part of the currentwill leak away and will not be available at the arc. In addition, there will be rapid deterioration ofthe copper cable at the point of the leak.

Principle of operation of Wet Welding

The process of underwater wet welding takes in the following manner:

The work to be welded is connected to one side of an electric circuit, and a metal electrode to theother side. These two parts of the circuit are brought together, and then separated slightly. Theelectric current jumps the gap and causes a sustained spark (arc), which melts the bare metal,forming a weld pool. At the same time, the tip of electrode melts, and metal droplets areprojected into the weld pool. During this operation, the flux covering the electrode melts toprovide a shielding gas, which is used to stabilize the arc column and shield the transfer metal.The arc burns in a cavity formed inside the flux covering, which is designed to burn slower thanthe metal barrel of the electrode.

Developments in Under Water Welding

Wet welding has been used as an underwater welding technique for a long time and is still beingused. With recent acceleration in the construction of offshore structures underwater welding hasassumed increased importance. This has led to the development of alternative welding methodslike friction welding, explosive welding, and stud welding. Sufficient literature is not available ofthese processes.

Scope for further developments

Wet MMA is still being used for underwater repairs, but the quality of wet welds is poor and are

prone to hydrogen cracking. Dry Hyperbaric welds are better in quality than wet welds. Presenttrend is towards automation. THOR 1 (TIG Hyperbaric Orbital Robot) is developed wherediver performs pipefitting, installs the trac and orbital head on the pipe and the rest process isautomated. Developments of diverless Hyperbaric welding system is an even greater challengecalling for annexe developments like pipe preparation and aligning, automatic electrode and wirereel changing functions, using a robot arm installed. This is in testing stage in deep waters.Explosive and friction welding are also to be tested in deep waters.


38/62

Hyperbaric Welding (dry welding)

Hyperbaric welding is carried out in chamber sealed around the structure o be welded. Thechamber is filled with a gas (commonly helium containing 0.5 bar of oxygen) at the prevailingpressure. The habitat is sealed onto the pipeline and filled with a breathable mixture of helium

and oxygen, at or slightly above the ambient pressure at which the welding is to take place. Thismethod produces high-quality weld joints that meet Xray and code requirements. The gastungsten arc welding process is employed for this process. The area under the floor of the Habitatis open to water. Thus the welding is done in the dry but at the hydrostatic pressure of the seawater surrounding the Habitat.

Risks Involved

There is a risk to the welder/diver of electric shock. Precautions include achieving adequateelectrical insulation of the welding equipment, shutting off the electricity supply immediately thearc is extinguished, and limiting the open-circuit voltage of MMA (SMA) welding sets.

Secondly, hydrogen and oxygen areproduced by the arc in wet welding.

Precautions must be taken to avoid the build-up of pockets of gas, which are potentiallyexplosive. The other main area of risk is to the life or health of the welder/diver from nitrogenintroduced into the blood steam during exposure to air at increased pressure. Precautions includethe provision of an emergency air or gas supply, stand-by divers, and decompression chambers toavoid nitrogen narcosis following rapid surfacing after saturation diving.

For the structures being welded by wet underwater welding, inspection following welding maybe more difficult than for welds deposited in air. Assuring the integrity of such underwater welds

may be more difficult, and there is a risk that defects may remain undetected.

Advantages of Dry Welding

1) Welder/Diver Safety Welding is performed in a chamber, immune to ocean currents andmarine animals. The warm, dry habitat is well illuminated and has its own environmental controlsystem (ECS).

2) Good Quality Welds This method has ability to produce welds of quality comparable toopen air welds because water is no longer present to quench the weld and H2 level is much lowerthan wet welds.

3) Surface Monitoring Joint preparation, pipe alignment, NDT inspection, etc. are monitoredvisually.

4) Non-Destructive Testing (NDT)NDT is also facilitated by the dry habitat environment.

Disadvantages of Dry Welding


39/62

1) The habitat welding requires large quantities of complex equipment and much supportequipment on the surface. The chamber is extremely complex.

2) Cost of habitat welding is extremely high and increases with depth. Work depth has an effecton habitat welding. At greater depths, the arc constricts and corresponding higher voltages are

required. The process is costlya $ 80000 charge for a single weld job. One cannot use the samechamber foranother job, if it is a different one.

Advantages of Wet Welding

Wet underwater MMA welding has now been widely used for many years in the repair ofoffshore platforms.

The benefits of wet welding are: -

1) The versatility and low cost of wet welding makes this method highly desirable.

2) Other benefits include the speed. With which the operation is carried out.

3) It is less costly compared to dry welding.

4) The welder can reach portions of offshore structures that could not be welded using othermethods.

5) No enclosures are needed and no time is lost building. Readily available standard weldingmachine and equipments are used. The equipment needed for mobilization of a wet welded job is

minimal.

Disadvantages of Wet Welding

Although wet welding is widely used for underwater fabrication works, it suffers from thefollowing drawbacks: -

1) There is rapid quenching of the weld metal by the surrounding water. Although quenchingincreases the tensile strength of the weld, it decreases the ductility and impact strength of theweldment and increases porosity and hardness.

2) Hydrogen Embrittlement Large amount of hydrogen is present in the weld region, resultingfrom the dissociation of the water vapour in the arc region. The H2 dissolves in the Heat AffectedZone (HAZ) and the weld metal, which causes Embrittlement, cracks and microscopic fissures.Cracks can grow and may result in catastrophic failure of the structure.

3) Another disadvantage is poor visibility. The welder some times is not able to weld properly.

Introduction


40/62

The plasma welding process was introduced to the welding industry in 1964 as a method ofbringing better control to the arc welding process in lower current ranges. Today, plasma ret ainsthe original advantages it brought to industry by providing an advanced level of control andaccuracy to produce high quality welds in miniature or precision applications and to provide longelectrode life for high production requirements.

The plasma process is equally suited to manual and automatic applications. It has been used in avariety of operations ranging from high volume welding of strip metal, to precision welding ofsurgical instruments, to automatic repair of jet engine blades, to the manual welding of kitchenequipment for the food and dairy industry.

Plasma arc welding (PAW)

Plasma arc welding (PAW) is a process of joining of metals, produced by heating with aconstricted arc between an electrode and the work piece (transfer arc) or the electrode and theconstricting nozzle (non transfer arc). Shielding is obtained from the hot ionized gas issuing from

the orifice, which may be supplemented by an auxiliary source of shielding gas.

Transferred arcprocessproduces plasma jet of high energy density and may be used for highspeed welding and cutting of Ceramics, steels, Aluminum alloys, Copper alloys, Titanium alloys,Nickel alloys.

Non-transferred arcprocessproduces plasma of relatively low energy density. It is used for

welding of various metals and for plasmaspraying(coating).

Equipment:

(1) Power source:- A constant current drooping characteristic power source supplying the dcwelding current is required. It should have an open circuit voltage of 80 volts and have a dutycycle of 60 percent.


41/62

(2) Welding torch:- The welding torch for plasma arc welding is similar in appearance to a gastungsten arc torch but it is more complex.

(a) All plasma torches are water cooled, even the lowest-current range torch. This is because thearc is contained inside a chamber in the torch where it generates considerable heat.During thenon

transferred period, the arc will be struck between the nozzle or tip with the orifice and thetungsten electrode.

(b) The torch utilizes the 2 percent thoriated tungsten electrode similar to that used for gastungsten welding.

(3) Control console:- A control console is required for plasma arc welding. The plasma arctorches are designed to connect to the control console rather than the power source. The console

includes a power source for the pilot arc, delay timing systems for transferring from the pilot arcto the transferred arc, and water and gas valves and separate flow meters for the plasma gas andthe shielding gas. The console is usually connected to the power source. The high-frequencygenerator is used to initiate the pilot arc.

Principles of Operation

The plasma arc welding process is normally compared to the gas tungsten arc process. But in theTIG-process, the arc is burning free and unchanneled, whereas in the plasma-arc system, the arcis necked by an additional water-cooled plasma-nozzle. Aplasma gas almostalways 100 %argonflows between thetungsten electrode andthe plasma nozzle.

The welding process involves heating a gas called plasma to an extremely high temperature andthen ionizing it such that it becomes electrically conductive. The plasma is used to transfer anelectric arc called pilot arc to a work piece which burns between thetungsten electrode and theplasma nozzle. By forcing the plasma gas and arc through a constricted orificethe metal, which isto be welded is melted by the extreme heat of the arc. The weld pool is protected by the shieldinggas, flowing between the outershielding gas nozzle and the plasma nozzle. As shielding gas pureargon-rich gas-mixtures with hydrogen or helium are used.


42/62

The high temperature of the plasma or constricted arc and the high velocity plasma jet provide anincreased heat transfer rate over gas tungsten arc weldingwhen using thesame current. Thisresults in faster welding speeds and deeper weld penetration. This method of operation is usedfor welding extremely thin material and for welding multi pass groove and welds and filletwelds.

Uses & Applications

Plasma arc welding machine is used for several purposes and in various fields. The common

application areas of the machine are:

1. Single runs autogenous and multi-run circumferential pipe welding.

2. In tube mill applications.

3. Welding cryogenic, aerospace and high temperature corrosion resistant alloys.

4. Nuclear submarine pipe system (non-nuclear sections, sub assemblies).

5. Welding steel rocket motor cases.

6. Welding of stainless steel tubes (thickness 2.6 to 6.3 mm).

7. Welding of carbon steel, stainless steel, nickel, copper, brass, monel, inconel, aluminium,titanium, etc.

8. Welding titanium plates up to 8 mm thickness.


43/62

9. Welding nickel and high nickel alloys.

10. or melting, high melting point metals.

11. Plasma torch can be applied to spraying, welding and cutting of difficult to cut metals and

alloys.

Plasma Arc Machining (PAM)

Plasma-arc machining (PAM) employs a high-velocity jet of high-temperature gas to melt anddisplace material in its path called PAM, this is a method of cutting metal with a plasma-arc, ortungsten inert-gas-arc, torch. The torch produces a high velocity jet of high-temperature ionizedgas called plasma that cuts by melting and removing material from the work piece. Temperaturesin the plasma zone range from 20,000 to 50,000 F (11,000 to 28,000 C).

It is used as an alternative to oxyfuel-gas cutting, employing an electric arc at very high

temperatures to melt and vaporize the metal.

Equipment:

A plasma arc cutting torch has four components:

The electrode carries the negative charge from the power supply. The swirl ring spins the plasma gas to create a swirling flow pattern. The nozzle constricts the gas flow and increases the arc energy density. The shield channels the flow of shielding gas and protects the nozzle from metal spatter.


44/62

Principle of operation

PAM is a thermal cutting process that uses a constricted jet of high-temperature plasma gas to

melt and separate metal. The plasma arc is formed between a negatively charged electrode insidethe torch and a positively charged work piece. Heat from the transferred arc rapidly melts themetal, and the high-velocity gas jet expels the molten material from the cut.

Applications

The materials cut by PAM are generally those that are difficult to cut by any other means, suchas stainless steels and aluminum alloys. It has an accuracy of about 0.008".

Plasma Arc Cutting

Plasma arc cutting employs an extremely high-temperature, high-velocity, constricted arcbetween an electrode contained within the torch and the piece to be cut. The arc is concentratedby a nozzle onto a small area of the workpiece. The metal is continuously melted by the intenseheat of the arc and then removed by the jetlike gas stream issuing from the torch nozzle. Becauseplasma arc cutting does not depend on a chemical reaction between the gas and the work metal,because the process relies on heat generated from an arc between the torch electrode and theworkpiece, and because it generates very high temperatures (28,000 C, or 50,000 F, comparedto 3000 C, or 5500 F, for oxyfuel), the transferred arc cutting mode can be used on almost anymaterial that conducts electricity, including those that are resistant to oxyfuel gas cutting. Usingthe nontransferred arc method, nonmetallic objects such as rubber, plastic, styrofoam, and woodcan be cut with a good quality surface to within 0.50 to 0.75 mm (0.020 to 0.030 in.) tolerances.

The past decade has seen a great increase in use of plasma arc cutting, because of its high cuttingspeed (Fig.). The process increases the productivity of cutting machines over oxyfuel gas cuttingwithout increasing space or machinery requirements.


45/62

Operating Principles and Parameters

The basic plasma arc cutting torch is similar in design to that of a plasma arc welding torch. Forwelding, a plasma gas jet of low velocity is used to melt base and filler metals together in thejoint (see the article "Plasma Arc Welding" in Welding, Brazing, and Soldering, Volume 6 of theASM Handbook). For the cutting of metals, increased gas flows create a high-velocity plasmagas jet that is used to melt the metal and blow it away to form a kerf. The basic design andterminology for a plasma arc cutting torch are shown in Fig.


46/62

Components of a plasma arc cutting torch.

All plasma arc torches constrict the arc by passing it through an orifice as it travels away fromthe electrode and toward the workpiece. As the orifice gas passes through the arc, it is heatedrapidly to high temperature, expands, and accelerates as it passes through the constricting orifice.

The intensity and velocity of the arc plasma gas are determined by such variables as the type oforifice gas and its entrance pressure, constricting orifice shape and diameter, and the plasmaenergy density on the work.

The basic plasma arc cutting circuitry is shown in Fig.. The process operates on direct current,straight polarity (dcsp), electrode negative, with a constricted transferred arc. In the transferredarc mode, an arc is struck between the electrode in the torch and the workpiece. The arc isinitiated by a pilot arc between the electrode and the constricting nozzle. The nozzle is connectedto ground (positive) through a current-limiting resistor and a pilot arc relay contact. The pilot arcis initiated by a high-frequency generator connected to the electrode and nozzle. The weldingpower supply then maintains this low current arc inside the torch. Ionized orifice gas from the

pilot arc is blown through the constricting nozzle orifice. This forms a low-resistance path toignite the main arc between the electrode and the workpiece. When the main arc ignites, the pilotarc relay may be opened automatically to avoid unnecessary heating of the constricting nozzle.

Plasma arc cutting was originally developed for severing nonferrous metals using inert gases.Modifications of the process and equipment to allow the use of oxygen or compressed air in theorifice gas permitted the cutting of carbon and alloy steel with improved cutting speeds and a cutquality similar to that obtained with oxyfuel cutting.

Because the plasma constricting nozzle is exposed to the high plasma flame temperatures(estimated at 10,000 to 14,000 C, or 18,000 to 25,000 F), the nozzle is sometimes made ofwater-cooled copper. In addition, the torch should be designed to produce a boundary layer ofgas between the plasma and the nozzle.


47/62

Several process variations are used to improve the plasma arc cutting quality for particularapplications. They are generally applicable to materials in the 3 to 38 mm ( 1/8 to1(1/2) in.)thickness range, depending on the current rating of the plasma machine. Auxiliary shielding inthe form of gas or water is used to improve cutting quality.

Applications

The ECG process is particularly effective for

1. Machining parts made from difficult-to-cut materials, such as sintered carbides, creepresisting(Inconel, Nimonic) alloys, titanium alloys, and metallic composites.

2. Applications similar to milling, grinding, cutting off, sawing, and tool and cutter sharpening.

3. Production of tungsten carbide cutting tools, fragile parts, and thin walled tubes.

4. Removal of fatigue cracks from steel structures under seawater. In such an application holesabout 25 mm in diameter, in steel 12 to 25 mm thick, have been produced by ECG at the ends offatigue cracks to stop further development of the cracks and to enable the removal of specimensfor metallurgical inspection.

5. Producing specimens for metal fatigue and tensile tests.

6. Machining of carbides and a variety of high-strength alloys. The process is not adapted tocavity sinking, and therefore it is unsuitable for the die-making industry.

Advantages and disadvantages

Advantages

Absence of work hardening Elimination of grinding burrs Absence of distortion of thin fragile or thermosensitive parts Good surface quality Production of narrow tolerances Longer grinding wheel life

Disadvantages

Higher capital cost than conventional machines Process limited to electrically conductive materials Corrosive nature of electrolyte Requires disposal and filtering of electrolyte Electrochemical Honing Introduction


48/62

Electrochemical honing (ECH) combines the high removal characteristics of ECD andMA of conventional honing. The process has much higher removal rates than eitherconventional honing or internal cylindrical grinding. In ECH the cathodic tool is similarto the conventional honing tool, with several rows of small holes to enable the electrolyteto be introduced directly to the interelectrode gap. The electrolyte provides electrons

through the ionization process, acts as acoolant, and flushes away chips that are sheared off by MA and metal sludge that resultsfrom ECD action. The majority of material is removed by the ECD phase, while theabrading stones remove enough metal to generate a round, straight, geometrically truecylinder. During machining, the MA removes the surface oxides that are formed on thework surface by the dissolution process. The removal of such oxides enhances further theECD phase as it presents a fresh surface for further electrolytic dissolution. Sodiumnitrate solution (240 g/L) is used instead of the more corrosive sodium chloride (120g/L)or acid electrolytes. An electrolyte temperature of 38C, pressure of 1000 kPa, and flowrate of 95 L/min can be used. ECH employs dc current at a gap voltage of 6 to 30 V,which ensures a current density of 465 A/cm2 . Improper electrolyte distribution in the

machining gap may lead to geometrical errors in the produced bore. Process characteristics The machining system shown in Fig.8 employs a reciprocating abrasive stone (with

metallic bond) carried on a spindle, which is made cathodic and separated from theworkpiece by a rapidly flowing electrolyte. In such an arrangement, the abrasive stonesare used to maintain the gap size of 0.076 to 0.250 mm and, moreover, depassivate themachining surface due to the ECD phase occurring through the bond. A different toolingsystem (Fig.) can be used where the cathodic tool carries nonconductive honing sticksthat are responsible for the MA. The machine spindle that rotates and reciprocates isresponsible for the ECD process. The material removal rate for ECH is 3 to 5 times fasterthan that of conventional honing and 4 times faster than that of internal cylindricalgrinding. Tolerances in the range of 0.003 mm are achievable, while surfaceroughnesses in the range of 0.2 to 0.8 m Ra are possible. To control the surfaceroughness, MA is allowed to continue for a few seconds after the current has been turnedoff. Such a method leaves a light compressive residual stress in the surface. The surfacefinish generated by the ECH process is the conventional cross-hatched cut surface that isaccepted and used for sealing and load-bearing surfaces. However, for stress-freesurfaces and geometrically accurate bores, the last few seconds of MAaction should beallowed for the pure ECD process


49/62

ECH schematic

ECH machining system components

Applications As a result of the rotating and reciprocating honing motions, the process markedly

reduces the errors in roundness through the rotary motion. Moreover, through toolreciprocation both taper and waviness errors are also reduced as shown in Fig.10.

Because of the light stone pressure used, heat distortion is avoided. The presence of theECD phase introduces no stresses and automatically deburrs the part. ECH can be usedfor hard and conductive materials that are susceptible to heat and distortion. The processcan tackle pinion gears of high-alloy steel as well as holes in cast tool steel components.Hone forming (HF) is an application that combines the honing and electro depositionprocesses. It is used to simultaneously abrade the work surface and deposit metal. Insome of its basic principles the method is the reversal of ECH.


50/62

ECH effects on bore errors.

According to the Metals Handbook (1989), this method is used in case of salvaging parts

that became out-of-tolerance and reconditioning worn surfaces by metal deposition andabrasion of the new deposited layers.

Electrochemical Deburring When machining metal components, it is necessary to cross-drill holes to interconnect

bores. Hydraulic valve bodies are a typical example where many drilled passages areused to direct the fluid flow. The intersection of these bores creates burrs, which must beremoved (Fig.) to avoid the possibility of them breaking off and severely damaging thesystem.


51/62

Burrs formed at intersections of holes

In this method, burrs are hit by 2760C blast of heat for milliseconds, which burns them

away, leaving everything else including threads, dimensions, surface finish, and thephysical properties of the part intact. Parts subjected to TEM should be cleaned of oil and

metal chips to avoid the formation of carbon smut or the vaporization of chips. Burrs canbe removed using several other methods including vibratory and barrel finishing,tumbling, water blasting, and the applicationof ultrasound and abrasive slurry. Abrasiveflow machining (AFM) provides a reliable and accurate method of deburring for theaerospace and medical industries. AFM can reach inaccessible areas and machinemultiple holes, slots, or edges in one operation. It was originally devised in the 1950s fordeburring of hydraulic valve spools and bodies and polishing of extrusion dies. Thedrawbacks of these methods include lack of reliability, low metal removal rates, andcontamination of surfaces with grit.

Hole deburring.

In electrochemical deburring (ECDB), the anodic part to be deburred is placed in a

fixture, which positions the cathodic electrode in close proximity to the burrs. Theelectrolyte is then directed, under pressure, to the gap between the cathodic deburring


52/62

tool and the burr. On the application of the machining current, the burr dissolves forminga controlled radius. Since the gap between the burr and the electrode is minimal, burrs areremoved at high current densities. ECDB, therefore, changes the dimensions of the partby removing burrs leaving a controlled radius. Figure 8 shows a typical EC holedeburring arrangement. ECDB can be applied to gears, spline shafts, milled components,

drilled holes, and punched blanks. The process is particularly efficient for hydraulicsystem components such as spools, and sleeves of fluid distributors. Mechanism of Deburring Faradays laws of electrolysis dictate how the metal is removed by ECDB. The deburring

speed may be as high as 400 to 500 mm/min. ECDB using a rotating and feeding toolelectrode (Fig.) enhances the deburring process by creating turbulent flow in the interelectrode gap. The spindle rotation is reversed to increase the electrolyte turbulence.Normal cycle times for deburring reported by Brown (1998) are between 30 to 45 s afterwhich the spindle is retracted and the part is removed. In simple deburring when the toolis placed over the workpiece, a burr height of 0.5 mm can be removed to a radius of 0.05to 0.2 mm leaving a maximum surface roughness of 2 to 4 m.

Electrochemical deburring using a rotating tool.

When burrs are removed from intersections of passages in housing, the electrolyte is

directed and maintained under a pressure of 0.3 to 0.5 MPa using a special tool. That toolhas as many working areas as practical so that several intersections are deburred at a

time. P

unit 1,2,3 of unconventional manufacturing process

Documents