The following points highlight the main thirty one methods of welding used in industries. The methods are: 1. Plastic Welding 2. Fusion Welding 3. Solid-Phase Welding 4. Cold Welding 5. Forge Welding 6. Pressure Welding 7. Flash Welding 8. Resistance Welding 9. Gas Welding 10. Oxyhydrogen Welding 11. Air-Acetylene Welding 12. Pressure Gas Welding 13. Atomic Hydrogen Welding and Few Others.
Method # 1. Plastic Welding:
In it, the pieces of metal to be joined are heated to the plastic state and these forced together by external pressure without the addition of filler material. Forge welding, resistance welding and the thermit welding with pressure are the examples of this class.
Method # 2. Fusion Welding:
In this case, the metal at the joint is heated to a molten state and allowed to solidify. In this case filter material is used during welding process. This includes gas welding, arc-welding and the thermit welding.
Various fusion welding processes are:
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i. Oxy-Fuel Gas:
It is a versatile process used for welding sheet metal and small pipes.
ii. Shield Metal Arc:
It is used in all fields of engineering for all metals/alloys except copper and low melting materials.
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iii. Submerged Arc:
It is used for boilers, pressure vessels, ship building, automobile industry.
iv. Gas Tungsten Arc:
It is used for welding all engineering metals except zinc.
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v. Gas Metal Arc:
It is used for general engineering in all fields for welding carbon steels, alloy steels, stainless steel, aluminium, nickel alloys.
vi. Resistance Welding:
Used for welding to sheet metals of all engineering metals (except Cu, Ag) in automobile and air craft industries, pipe and tubing production.
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vii. Electro Slag Welding:
It is used for thick sections of pressure vessels, press frames, shafts of carbon, low and high alloy steels.
viii. Thermit Welding:
It is used for welding parts, copper conductors, copper/steel joints.
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ix. Plasma Arc:
It is used mainly for reactive metals and all engineering metals except zinc.
x. Electron Beam:
It is used in nuclear and aerospace industry for all metals, particularly reactive metals, Ni, Ti, Zirconium and stainless steel.
xi. Laser Beam:
It is used for deep hole drilling, repairs, in electronic industry and special applications.
Method # 3. Solid-Phase Welding:
These welds are made by the creation of metallic bond between the two surfaces being joined. The surfaces to be joined are brought so much close together that atoms are separated by less than the relaxation distance over which the inter-atomic forces act, thus creating a bond.
However it is very difficult to bring a sufficiently large area into intimate contact due to surface asperities on the metals and the oxide films and impurities on the surfaces which act as a barrier between the metals. Sufficient pressure has to be applied to cause plastic flow of the two surfaces and to make large areas to come into intimate contact.
Since such intimate contacts under cold conditions are difficult, most solid phase welding processes are performed hot. Heating also lowers field strength and enables lower pressures to be used, melts or evaporates the surface contaminants, permits growth and coalescence of grains across the interface, causes surface and volume diffusion across the interface.
Fig. 9.17 shows how the force between atoms varies with change in distance between them. Under equilibrium condition, distance is dc and force is nil. When external tensile force is applied, distance increases dc and on compression it decreases. It is interesting to note that inter atomic force increases considerably below dc and tends to be zero after a few multiples of dc.
Thus if two metallic surfaces are brought so much close together that their grain boundaries contact each other then the surfaces will adhere to each other with a very large force (solid phase welding). It is therefore, essential that the contaminated layers of oxides and adsorbed gases present on the surface of metal under normal atmosphere (usually a few hundred angstroms thick) are removed by scratch brushing.
The two metal surfaces are then brought into contact, the real contact taking place through a small area of asperities. The metallic bridging (having property of true grain boundary) occurs even in the presence of adsorbed surface layers.
Following variables play important role in solid phase welding:
(i) Surface Deformation:
It is expressed as ratio of change in thickness or diameter and original thickness/diameter. Strength of welded joint is proportional to surface deformation. If temperature of metal is increased, the amount of surface deformation for welding can be less (around 10% near melting temperature). Desired surface deformation is also dependent on the ratio of the oxide hardness and the parent metal hardness.
(ii) Surface Oxide Layers and Oil Films:
These pose big hurdle in solid phase welding and need to be cleaned/wiped properly.
(iii) Recrystallization and Grain Growth at the Interface:
Solid phase welding carried out at room temperature does not allow recrystallization and thus ductility is somewhat less. Increase in working temperature increases ductility and eliminates some other defects.
(iv) Diffusion:
Amount of diffusion modifies the shape and the size of the voids at the interface of solid phase welding.
Method # 4. Cold Welding:
Cold welding is a pressure welding process in which ductile materials can be welded without the application of heat or electric current. Weld is effected by making the metals to be joined to flow at room temperature by subjecting them to sufficient pressure. Required welding pressure is applied to overlapping surfaces with tool dies designed to cause controlled deformation of considerable extent.
Cold welds are characterised by deep indentation on the outer surfaces of the workpieces. This process is best suited to high purity and commercially pure aluminium, but can be used for many nonferrous metals. This can also be used to join metals of different hardness like copper to aluminium.
For satisfactory results there should be direct and intimate contact between the surfaces to be welded, and the tool dies must be of proper design to provide the degree of compression required for the weld. The two surfaces should be free of grease and oxides. Even fingermarks in some cases may render defective joints.
Usually surface is cleaned by a motor-driven rotary-brush to cut through the film of oxide and to expose the clean metal underneath. Symmetrical tool dies must apply pressure over a comparatively narrow strip and in such a manner that the metal can flow away from the weld on both sides.
Satisfactory welds may be produced with either an impact blow or a slow squeeze. For successful weld, the original thickness must be squeezed or reduced by a certain minimum percentage determined by the materials to be joined. For joining metals of different hardness, the width of the dies must be adjusted according to their respective hardness ratios, in order to share the final thickness at the indentation equally between the two metals.
Some of the cold-welding techniques are:
i. Trap Welding:
It is designed to permit welding of inserts into similar or dissimilar non-ferrous metals, by following metal around the insert somewhat like plastic moulding.
ii. Wave Welds:
Tool dies for wave welds consist of an element having a wave-form projection and a flat plate.
iii. Stagger Weld:
It is used to join thin sheets to heavy bar stock. It places dots, or short-line welds, along two or more parallel lines.
iv. Sandwich Welding:
It is best suited for applications where indentation of the surfaces of the work produced by dies is objectionable. In this method, a third piece (such as suitable piece of wire) is sandwiched between the two workpieces and all three are welded together in a single operation.
The insert flattens out between the two sheets producing a weld without indentation on either side of the joint.
Method # 5. Forge Welding:
This is oldest of all the methods of welding-processes. Generally this process is used in the blacksmith shop. In it, the work pieces are placed in a forge or other appropriate furnace and heated within the area to be joined to the condition of plasticity on the surface. The parts are then quickly super-imposed and worked into a complete union by hand or power hammering or by pressing together.
The quality of the weld depends to a great extent upon the amount of heating. If the ends to be joined are not heated enough, they will not stick together; if overheated, the metal becomes burned, brittle and has spongy appearance.
Wrought iron and low carbon steel are the materials most commonly joined by the forge welding. They oxidise very rapidly when exposed to the atmosphere after being heated to high temperature. Therefore, very little oxygen should be permitted to contact the metal being heated. The most commonly used forge-welding processes are hammer welding, die welding and roll-welding.
The principal difference between these processes is the manner in which pressure is applied. In case of hammer welding, the pressure is applied at high velocity in the form of blows. In die-welding, the pressure is exerted either by means of a bell or a mandrel and tube rolls. In roll-welding the work is forced longitudinally between plate-rolls, which supply required pressure.
The physical properties of force welds are influenced by several factors. Among these are the personal skill of the individuals making the weld, the weldability of the metal, the type of fuel used in forge, the atmospheric conditions, the amount of flux used and the time of application.
The application of forge welding is limited to country-side blacksmith shops on wrought iron, carbon steel and certain low alloy steels. It is not widely used as a production or manufacturing process as it is very costly and slow process.
The forge welding is of four types:
(1) Lap weld.
(2) Butt weld.
(3) Tee weld.
(4) Vee weld.
This classification of forge welding is based upon the fact, which way joints are made. In Table 9.1 are shown some of the joints normally used in the forge-welding.
The joint surfaces, in most of the cases are rounded or crowned slightly so that when brought together, they will unite in the centre first and thus force out any fused oxide, slag or dirt that may have stuck upon them.
Method # 6. Pressure Welding:
This process is used for making butt welds on small tubes with outside diameter upto 75 mm. It makes butt welds without filler metal by not forging, under protective atmospheres. Ends to be welded are raised to the required temperature by induction heating. Each tube is fixed between two half jaws on which a hydraulic clamping pressure is applied.
One jaw is fixed. The other one moves along the axis of the tube. An induction equipment in two parts permits to heat ends of tubes under gas protection. Air is purged by gas insulation into the protective surrounding walls. When desired metal temperature is attained, upsetting is caused by means of axial press.
Method # 7. Flash Welding:
This process is also used to make butt welds on small tubes with outside diameter upto 75 mm. The ends to be welded are raised to the required temperature by flashing. The air inside the tubes is purged by circulating formigas. Preheating is obtained by joule effect and discontinuous flashing.
When the temperature necessary to start the flashing is obtained, the voltage is increased and a series of electric arcs is maintained between the two ends of the tube by bringing them together at a constant rate over a given length, the formigas circulation being kept on. When the expected flashing duration is reached, the two edges are forged by a sharp forward movement of the welding head.
Method # 8. Resistance Welding:
In resistance welding a heavy electric current is passed through the metals to be joined over a limited area, causing them to be locally heated to plastic state and the weld is completed by the application of pressure for a prescribed period of time. No additional filler metal is required. In this process two copper electrodes are used and the metals to be welded are pressed between the electrodes.
The current is passed through the electrodes which incorporate very low resistance in the circuit and the resistance at the joints of the metals is very high. Thus maximum heating is produced at the point of contact where weld is to be made. Generally alternating current is used and the voltage is stepped down to about 4—12 volts by a transformer in order to have high amperage and good heating effect.
The amount of current needed for welding is inversely proportional to the duration of time and directly proportional to the area in contact. The pressure necessary to effect the weld varies from 250 to 1000 kgf/cm2.
The heat generated in the resistance welding is expressed by H = I2Rt, where H is the heat, I. The welding current in amperes, R the resistance of metal being welded and T the time or duration of current flow. The voltage needed in resistance welding depends on the composition, area and thickness of metal being welded.
Therefore, in order to provide possible variation of the secondary current, the transformer is equipped with a regulator on the primary side which varies the number of turns on the primary coil.
In resistance welding, the time for which current flows is very important. Usually automatic arrangements are devised which switch off the supply after a predetermined time from applying of pressure (starting of weld). The pressure may be applied manually, by air pressure, by springs or by hydraulic means.
After switching off the current, the pressure is maintained on the electrodes until the weld cools. Referring to Fig. 9.18 it will be seen that there are four stages controlled in some machines automatically. The electrodes are cooled by circulating water through hollow electrodes.
Resistance welding is employed mainly for mass production. It is easily adapted to those components which can be moved to the machine and are light. The operation is extremely rapid and simple. This is the only process where heat can be controlled and which permits a pressure action at the weld. This process is suited to nearly all metals except tin, lead, zinc etc. which exhibit many problems.
Method # 9. Gas Welding:
Gas welding is a process in which the required heat to melt the surfaces is supplied by a high temperature flame obtained by a mixture of two gases. The gases are mixed in proper proportions in a welding blowpipe (torch). For controlling the welding flame, there are two regulators on the torch by which the quantity of either gas can be regulated.
Usually the mixture of oxygen and acetylene is used for welding purposes. It produces temperature in the range of 3200—3300°C. However the mixture of hydrogen and other combustible gases can also be employed to some extent. Other gases used are MAPP (methyl acetate propediene) (2600°— 2900°C), propylene (2500°—2850°C), propane (2450°— 2775°C), natural gas/methane (2350°—2750°C).
In gas welding the two surfaces to be welded are properly prepared and placed near each other. The metal in the joint is brought to melting temperature by heat from the flame and then weld is completed by supplying additional metal as the filler metal obtained by a filler rod.
Advantages and Limitations of Gas Welding:
Advantages:
The oxyacetylene flame can be easily controlled because it is not as piercing as arc welding. It is generally more suitable for thin sheets. The equipment is portable and thus gas welding can be easily done outdoor and for repair works. By changing the nozzle, torch can be used for heating, welding, brazing, and cutting purposes.
Limitations:
It is slower process than are welding. Heat affected zone and distortion are more compared to arc welding. Gases are expensive and there may be some safety problems in storing and handling the gases.
Method # 10. Oxyhydrogen Welding:
In this process, hydrogen is used in place of acetylene and the temperature of flame is very low (1980°C). It is therefore, best suited for welding thin sheets, low melting alloys and for brazing work. An advantage of this process is that no oxides are formed on the surface of the weld if a reducing atmosphere is used. The flame adjustment is very difficult in this welding as there is no distinguishing colour to judge the gas proportions.
Method # 11. Air-Acetylene Welding:
This is generally used for lead welding, low-temperature brazing and soldering operations. The torch used for this process is similar in construction to a Bunsen burner. In this case probably lowest temperature is produced in comparison to all other types of gas welding processes. The acetylene is applied to the torch from a cylinder and air is drawn into the torch from atmosphere and its quantity can be adjusted for proper combustion by varying the opening of air inlet to torch.
Method # 12. Pressure Gas Welding:
In this process, coalescence is produced simultaneously over the entire area of abutting surface by heating with multiple oxy-acetylene flames and then applying pressure. In it, no filler metal is required. The process is of two types depending upon the way of applying pressure to effect weld. The first type is known as closed-joint pressure welding or closed-butt welding.
In this method, the weld faces are in contact during the complete welding cycle. Before bringing the faces in contact with each other, they are carefully machined and cleaned. After butting they are heated by multiple oxyacetylene flames to a high temperature below melting point (plastic state).
Pressure is then applied in the direction normal to the weld faces resulting in an up-setting of the metal in the weld zone. The bond is formed by the action of diffusion and recrystallization across the interface. The resultant weld has a smooth surfaced bulge at the weld zone. Low initial pressure is also applied in order to assure intimate contact: or it may be maximum from beginning till upsetting occurs.
The second method of pressure gas welding is open joint pressure welding or fusion pressure welding. In this process the parts to be welded have square faces and are spaced a short distance apart during the heating cycle.
When the melting temperature is reached, the parts are rapidly brought together causing the upsetting of the metal in weld zone. The weld is thus effected in the molten state, most of the metal being squeezed from the interface by the impact pressure.
Oxyacetylene Torch Cutting:
This is the most commonly and convenient method in use now-a-days for cutting metals. The principle on which it is based is that oxygen has a strong affinity for iron and steel at elevated temperatures (760—870°C) the action being less at low temperatures. At high temperature oxygen forms iron oxide (rust) with iron which has low melting temperature and at high temperatures it melts out and the metal is cut.
Thus if steel is first heated to a red colour and then a jet of pure oxygen is blown on the surface, instantaneously the steel is burned in an iron oxide slag-like appearance which under pressure falls down and steel is cut. The process is very rapid and metals upto thickness of 7.5 cm can be cut by this process. The nozzle is kept about 4 mm above the work.
For 6 mm thick mild steel plate, 1.2 mm size nozzle and cutting speed of 30-45 m/hr. is employed with acetylene consumption of 0.42 m3/hr and O2 consumption of 1.38 to 2.7 m3/hr. As thickness increases all parameters increase and corresponding values of 150 mm thick sheet are 2.0 mm size nozzle, cutting speed 3.6-5.4 m/hr, acetylene 0.95 m3/hr and oxygen, 15.3 to 19.5 m3/hr.
The torch for flame cutting has several small holes for reheating the steel to a red colour. There is a main central hole in the torch which carries pure oxygen for cutting action. The preheating flame is just like the welding flame.
The torch used for cutting purposes is shown in Fig. 9.28. However, this process is not suited to cast-iron, non-ferrous alloys and high manganese steels. With correct cutting speed, sharp and smooth edges can be obtained.
This process has become so versatile now due to its simplicity that several cutting machines have been developed bared on this principle which automatically controls the movement of the torch to cut any desired shape. These machines utilise some template or drawing of the shape to be cut in steel and a sensing device to guide the torch along a predetermined path.
Flame Machining:
In this process metal is removed with a cutting blowpipe in the same way as any other machining operation. The torch is held at a small angle to the work surface and cuts out a groove as it progresses. The process is rapid, requires no power and work set up need not be rigid. But surface finish is very poor, therefore, it is best suited for initial rough machining operation.
Gouging:
This technique is used to remove bulk material and to form groove. It is also used for preparing the edges of thick plate prior to welding. For preheating, torch is held at 30° – 40° and for gouging, torch is held at 10° – 15°. It utilises specially shaped nozzles.
CNC Flame Cutting System:
The traditional method of flame cutting uses optical 1:1 machine in which a full scale paper template is re-traced by an optical line follower, and the cutting torch thus reproduces the profile drawn on the template. This method is slow because the follower loses the line if attempts are made to cut at speed more than a meter per minute.
Further the imperfections in template are reflected on sheet being cut. Also the cartridge paper being sensitive to humidity any change in humidity results in variations in component size. The template life is also limited.
For faster operation with high repeatability and reliability of component shape and size, and minimum rejects, computer numerically controlled (CNC) flame cutting system is employed.
Such a system comprises of flame cutting hardware (comprising of NC flame cutter equipped with plasma torches, oxy-fuel torches, chamfering head driven by a controller, rectifiers, water tables and water softening unit) and computer hardware [comprising of central processing unit, disk drives, floppy disk, VDU (visual display unit), graphics terminal, printer, tape punch, tape reader, plotter].
The cutting torch is used for thickness greater than 25 mm and upto 110 mm and is equipped with the necessary ducts, mixing chamber and control valves to supply no oxy- fuel gas mixture and a pure oxygen steam to the cutting tip.
Plasma torch is used for thickness upto 25 mm at a speed to upto 6 m/min. It is usually carried out under water to reduce atmospheric pollution and reduce noise.
The user interface with the system consists of a multilevel menu sub-system. The system is configured around adequate number of primary input files and system generated files. All orders for flame cutting are fed into the system. The system keeps a full record of data missing for any particular order. Those parts for which geometry is defined and no restriction has been flagged are made available for welding.
Detailed information about the material, its length and width, density type of cutting process etc. are fed. Computer itself creates material record, new requirements, record of usable offcuts from plate. Information in respect of plain cut or chamfered cut, geometry of shapes by co-ordinate information are made available and stored in a file.
The system generates reports on VDU and printer, like asking for the geometry of parts for which it does not exist, material requirements, plate cutting schedule, material usage summary, chamfered tag reports, etc.
Method # 13. Atomic Hydrogen Welding:
It is an arc welding process in which a single phase A.C. arc is maintained between two tungsten electrodes and a stream of hydrogen gas introduced into the arc at the pressure of 0.5 kg/cm2. As the hydrogen enters the arc, the molecules are broken into atoms which recombine into molecules of hydrogen outside the arc.
This reaction is accompanied by the liberation of intense heat, attaining a temperature of about 4200°C. Weld metal may be added in the joint in the form of welding rod, the operation being very similar to the oxyacetylene process. This intense heat is used in making fusion welds.
The principal difference between other arc welding processes and atomic hydrogen arc welding process is that in the former case, arc is formed between the electrode and the work piece whereas in the latter case arc is formed between the two electrodes as shown in Fig. 9.40. This makes the electrode holder a noble tool.
The importance of this process over other process lies in the availability of high heat concentrations. In addition, it also acts as shield and protects the electrodes and molten metal from oxidation, so that smooth, uniform, strong and ductile welds can be obtained.
This process had a wide use in die repairs, welding of heat-resistant alloys particularly stainless-steel. It finds its use as an excellent means of applying carbides and many other hard-surfacing alloys, and in production work where special ferrous and nonferrous alloys are used.
Method # 14. CO2 Shielded Welding:
The CO2 shielded-welding process may be used with advantage for joining plain low-carbon alloy steels, and is very suitable for continuous automatic operation.
A bare metal wire electrode is fed to the work with a special gun. An arc is struck and is immediately surrounded by constant supply of carbon dioxide gas which protects the weld area from the harmful effects of nitrogen, hydrogen, and oxygen.
The use of a continuous wire electrode is economical, eliminates the necessity for frequent electrode changes, and enables a high welding rate to be achieved. The completed welds are very dense and free from slag. The carbon dioxide gas, supplied in cylinder, is relatively inexpensive.
The main limitations are that the process is unsuitable for use on some steels, and that the operator must be safeguarded against the inhalation of excessive carbon dioxide fumes when working in confined spaces.
In the case of CO2 welding, spattering is influenced by arc voltage, the spatter losses being low at low voltage short-circuit transfer at 24 volts. Weld spattering occurs either by droplet explosions in dip/short-circuit metal transfer or by expulsion of the formed molten droplets away from the arc in the free-flight transfer. Weld spattering is lowest with dip transfer. Weld spatter is also observed to reduce by replacing CO2 with mixture of 25% CO2 and 75% Argon.
Method # 15. Pulsed Gas Tungsten Arc Welding:
This process is similar to simple GTAW process but it uses a low current with high pulsation. It produces same weld as the standard GTAW but with much less heat input into the metal due to the pulsating action. Fig. 9.45 shows the nature of square wave pulse.
It will be seen that as the pulse peak level is reached, penetration and heat are very quickly put into the place but before the plate is saturated with excess heat, the pulsed GTAW drops to the background current where arc is maintained but heat input is much less. This results into smaller heat-affected zones (HAZ) and a less brittle and more ductile weld. Distortion is reduced due to lesser heat.
Thick-to-thin sections are joined easily. It requires less foot pedal control manipulation. It also offers excellent puddle control for out-of-position welding.
The size and shape of pulses can be controlled by varying pulse current, background current, pulse frequency, and pulse duty cycle depending on the type of metal and its properties, and the thickness of metal.
For instance for thicker material it is desirable use high pulse duty cycle. i.e. heat with occasional drops to background. For welding of thick-to-thin sections, it is advisable to used high pulse peak-low pulse duty cycle so that high heat is produced but for shorter duration.
For producing rapid welds, fast pulsing to obtain puddle stimulation and uniform bead, it is advisable to used fast high frequency control. For welding extremely thin or low melting temperature metals, it is better to use slow pulse frequency and low pulse duty cycle.
Method # 16. Stud Welding:
It is a special welding development that quickly and efficiently welds studs and other fastening devices to plates and other surfaces without drilling or punching holes in the main structure. In this process the fluxed end of the stud is placed in contact with the work and the threaded end is held by the special collet on the welder gun.
Spring pressure from the gun holds the stud in place against the work, (Refer Fig. 9.51). An arc is struck between the stud and the main plate automatically by depressing the operating trigger which energises a solenoid which overcomes the spring tension and retracts the stud. At the end of an automatically timed interval, the stud is plunged into pool of molten metal—thus welding the stud to the plate.
A ceramic ferrule or collar is arranged around the stud so that it holds the molten metal in place and helps to form a good fillet. The flux on the end of the stud aids the arc control, and enables the operator to make stud welds in any position. For accurate positioning of studs, suitable jigs and fixtures may be used. The power for welding is supplied from a welding transformer.
Method # 17. Arc Spot Welding:
This process is also known as button welding. In this process, metallic arc welding process is employed; the arc is struck and held in one place until the top sheet of metal melts through and fuses with molten portion of the sheet or structural member underneath.
To start, a trigger moves the tungsten electrode upto the sheet metal, when it is withdrawn, the arc is created and timer controls the duration. This process is extensively used for automobile body assembly.
Method # 18. Self-Shielded Flux Cored Arc Welding:
In this process the electrode incorporates a flux core inside a tubular filler metal sheath. The force of the electrode consists of powdered metals, vapour forming materials, slag formers, deoxidisers, and scavengers. These materials protect the metal as it is transferred across the arc.
This process, thus results in production of a properly deoxidised arc stream which controls weld porosity, corrosion, cracking and hot shortness susceptibility. In addition, alloying elements are added in order to produce the desired mechanical properties such as strength, elongation, notch toughness and crack propagation resistance. Due to no built-in work break at the end of each electrode, operator efficiency increases and start- stop defects are eliminated.
The system consists of a wire feeder, power source and the gun and cable assembly. The process becomes simple and more flexible as no external shield gas or flux is required. The weld deposit by this process is low in hydrogen. The surface of the root pass is virtually flat. This results in deep penetration into the root bead and excellent fusion into the side walls.
Method # 19. Underwater Shielded Metal Arc Welding:
This type of welding is done under water using well insulated electrodes suitably protected from damage by the water. Usually a close arc and DCSP is used and current setting is kept 10% higher. Generally a telephone communication system is used and the arc welding current turned on remotely outside the water only after receiving the diver’s orders when he is actually welding.
Method # 20. Atomic Hydrogen Welding:
This process combines gas welding with electric arc welding. The electrode holder incorporates two tungsten electrodes arranged in inclined position and hydrogen is ejected from the hydrogen nozzle in between the tips of these electrodes. The electric arc between the electrodes breaks down the molecular hydrogen into atomic hydrogen.
This atomic hydrogen, when touches the relatively cold metal, recombines into molecular hydrogen, thus liberating considerable heat which melts the metals to be welded and creates molten puddle into which a consumable welding rod to supply the material for welding may be added.
The hydrogen also provides a reducing gas atmosphere under which the fusion takes place. Usually alternating current supply is used and temperature of the order of 4200°C is achieved. This process is best suited for tool and die parts where alloy control and heat input are important.
Method # 21. Self-Generating Oxyhydrogen Gas Welding:
In this process distilled water is fed into electrolytic reactor where it is changed into hydrogen and oxygen by electrolysis. The two gases are then fed to a torch via a booster containing methyl alcohol.
As the gas mixture bubbles through the booster, it carries alcohol vapour along with it and the mixture burns in the torch flame. The alcohol vapour provides a slightly reducing flame for soldering, brazing or welding.
Method # 22. Cold Welding:
In this process the cleaned metal is forced together under considerable pressure, and the ductility of the metals produces a true fusion condition. This is process best suited for aluminium and its alloys, copper, alloys of cadmium, nickel, lead, zinc, etc. The pressure applied is so much that the original thickness is reduced to nearly one-fourth.
The metal to be joined must be completely freed from oxides and other contaminants. The tool must be designed to compensate for varying hardness of the metals. The welds obtained can be of straight, ring, or seam design.
Method # 23. Ultrasonic Welding:
In ultrasonic welding of metals, the high frequency and high intensity vibrations are imposed upon the metals and in passing through them, these cause a fusion of the surfaces and thus metals are joined under pressure.
There is no need of cleaning surfaces, and only low pressure of the order of 2—3 kg/cm2 is applied to hold the material to be welded. The other advantages are no grain growths, no gas absorption, no porosity; embrittlement is at a minimum.
Method # 24. Electron Beam Welding:
In this process (which can be performed in vacuum), the metals to be joined together are brought rather close together and a concentrated stream of high energy electrons is directed into the gap between the metal causing fusion to take place.
The kinetic energy of the electrons is converted into intense heat energy when the electrons are absorbed by the metal piece over a small area of the weld, producing deep penetration weld with a depth/width ratio as high as 15.
This results in a narrow, almost parallel weld with very little distortion and a small width of the heat affected zone. There is no possibility of contamination by atmospheric gases because process is carried out in vacuum.
Almost any metal (even reactive ones like titanium) can be welded. Refractory metals such as molybdenum or tungsten can be readily welded because of high power density. No flux or shielding gas is required.
This process employs an electron gun in which electrons are emitted by a hot filament of tungsten or tantalum usually connected to a 12 volts supply (Fig. 9.54).
The electrons emitted from filament by thermionic emission are accelerated to a high velocity to the anode (metal ring) fed with a D.C. high voltage supply at 10 kV to 150 kV.
The fast electrons then move through a diaphragm whose opening determines the beam width. The electron beam is then focused by a magnetic lens system (consisting of powerful electromagnets whose magnetising current can be adjusted) on the workpieces to be welded.
This process is best suited for reactive metals (nuclear reactor components), titanium, zirconium stainless steel, etc. for automotive and aero-space industries.
Problem:
Calculate the kinetic energy of electrons, beam current, power density of beam (0.5 mm diameter) of electron beam welding unit operating at 100 kV and rated at 1 kW.
Solution:
Kinetic energy of an electron accelerated by 1 volt is called 1 electron volt (1 eV) and 1 eV = 1.602 x 10-19 Joules.
Method # 25. Friction Welding:
In this process the metals to be joined are mounted in a device with one surface stationary and other joint surfaces revolved under pressure. Due to the heat of friction the joint surfaces are raised to a fusion temperature and at that point a higher pressure is applied and rotation is stopped, and thus efficient joint is produced.
The resultant joint is always characterised by an upset annulus around the weld which may be subsequently removed. No joint preparation is required for most non-ferrous metals since the rotating action is self-cleansing. For certain non-ferrous metals, preparatory cleansing of the joint faces is important.
At least one part should be of circular section. The process is best suited to pipe, tubing, or solid round rods made of carbon steel, stainless steel, tool steel, copper, aluminium, alloy steel, titanium etc. Bars of dissimilar metals and varying cross-section can be joined by this process.
However, it is essential that the ratio of thermal conductivities should be in approximately inverse proportion to the ratio of the melting points. Various requirements can be met by varying the initial pressure, heating time and final upsetting pressure.
Servo Controlled Friction Welding Machine:
Friction Welding is a solid phase of two similar or dissimilar materials using the heat generated by rubbing of the two materials and when the impurities at the interphase are removed as flash, stopping the relative motion of the jobs and applying an axial forge welding force to form a very strong metallic bond throughout the interface.
Material combinations as diverse as copper to aluminium, Aluminium and copper to steel, nylon to PVC etc can be successfully welded with weld joints as strong as the parent material.
Typical applications are:
i. IC engine valves,
ii. Twist drills,
iii. Taps,
iv. Rear axle housing for automobiles,
v. CV joints,
vi. End fittings of hose pipes,
vii. Cable lugs.
Advantages of Friction Welding are:
Very short cycle time (few seconds) and hence suitable for mass production. No edge preparation, no filler material or shielding gas, no spattering, fumes or radiation and joints as strong or stronger than the parent material.
Servo controlled friction welder uses a load cell to directly measure the axial forces on the jobs during welding cycle and is controlled by a servo valve (instead of a proportional relief valve to control the hydraulic pressure). The spindle is driven by the an AC spindle motor and the carriage moves on linear motion guide ways.
A linear scale mounted on the carriage measures the burn-off and upset shrinkage. Spindle speed, axial movement, spindle torque and axial thrust are monitored and plotted against time. Braking of spindle is accomplished by line regenerative braking system. The machine is controlled by an industrial computer and data of every weld is recorded. There is provision for retrieval of old welds data and printing.
Method # 26. Explosive Welding:
In this process, the metals to be joined are placed at an angle to each other. The energy derived from an explosion forces the plates together at high velocity causing surface ripples in the metal. As the force is dissipated the ripples lock or weld the two metals together.
The process is quite dangerous and should be performed by experts in specially designed chambers or water-filled chambers. This process has been successfully used to weld steel to steel, aluminium to aluminium, copper to steel, and many other metals.
Method # 27. Indent Lap Welding:
In this process the two non-ferrous metals to be welded are cleaned, lapped and pressed between two platens to reduce thickness causing plastic deformation and resulting in cold welding. This process is used for joining foil and thin strip materials.
Method # 28. Diffusion Welding:
This process is time-consuming and is hardly used. The two parts to be joined are finished to a high standard of flatness and wrung together under pressure in inert atmosphere. In order to promote diffusion, temperature is raised.
Method # 29. High Frequency Welding:
In this process, temperature is raised by high frequency eddy currents and then a deformation force is applied. It is possible to heat the components locally by high frequency current. This process is commonly used for manufacture of axially or helically welded tubes.
Method # 30. Laser Welding:
The laser beam is produced by firing a brilliant light (capacitor discharge into Xenon tubes or almost instant ignition of aluminium or magnesium foil of wire) and directing this intense light into the ruby by using parabolic mirrors.
The electrons in the ruby rod are stimulated by this light course and in returning to their normal energy level, release energy of a constant wavelength. The laser method can weld at a 200 to 1 ratio, i.e. the weld can be 200 times deeper than wide. In other words, the width of the weld for 25 mm thick metals can be only 0.125 mm wide.
Method # 31. Cold Metal Transfer Welding:
In this technique workpieces to be joined and all their weld zones remain colder compared to conventional gas metal arc welding thus enabling low distortion, high precision, welding of light gauge sheet and capability of joining galvanised sheet and steel to aluminium.
This process is based on dip-transfer arc (systematic discontinuity of arc, i.e. alternating hot-cold-hot-cold sequence). Wire motion is controlled by machine, with low heat and no spatter. On occurrence of short circuit, power supply is interrupted and wire retracted, thus arc inputs heat into materials to be joined for a very short time.