This article throws light upon the nine types of metal joining processes. The types are:- 1. Solid Phase Welding at Elevated Temperature 2. Arc Welding 3. Resistance Welding 4. Gas Welding 5. Thermite Welding 6. Ultrasonic Welding 7. Electron Beam Welding 8. Laser Beam Welding 9. Explosive Welding.
Types and Classification of Metal Joining Processes # 1. Solid Phase Welding at Elevated Temperature:
Depending on the nature of the heat source and the method of application of the pressure, the solid phase welding processes can be classified as:
(i) Forge Welding:
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In forge welding, the parts to be welded are heated in a furnace and then hammered together to form the weld. In the case of tubes, the welding pressure is applied by the forging rolls. The amount of bulk deformation varies in a wide range.
(ii) Butt Welding:
In butt welding, the surfaces to be joined are first brought in contact. Then, they are heated by the passage of an electric current. Once the required temperature is attained, the parts are subjected to an axial compression. This compression results in a lateral flow of the surface layers (e.g., oxide) and brings the clean metal surfaces in contact. The applied pressure is controlled accurately; it is held almost constant with a sharp rise near the end of the operation.
(iii) Oxyacetylene Pressure Welding:
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The oxyacetylene pressure welding process is similar to butt welding. The only difference is that here the joint is heated by an oxyacetylene ring burner. In both the processes, a bulk deformation of the order of 50% may take place.
(iv) Flash Butt Welding:
In flash butt welding, the parts are brought in contact with a light pressure. The interface is heated by the passage of an electric current, as in butt welding. However, the heating is continued till the interface melts when the dies are brought closer. Thus, the liquid metal, along with the oxide film, is driven into the die cavity. This brings the solid, clean metal faces in contact under high pressure to make the weld. It should be noted that the form of the die cavity eases both the flow of the liquid metal into it and the final trimming off of the flash.
(v) Friction Welding:
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In the friction welding process, the parts to be welded are kept in contact and rotated relative to each other. The interface is heated up due to friction. After the desired temperature is attained, an axial pressure is applied to complete the welding. After the welding is complete, the welded parts rotate together as one piece until stopped. One obvious limitation of this process is that the parts to be welded must have a rotational symmetry.
It should also be noted that the interface never melts as the softening of the material reduces the friction, and consequently the heat input, making the process self-regulating.
Types and Classification of Metal Joining Processes # 2. Arc Welding:
Each of the following fusion welding processes uses an electric arc as the source of heat:
(i) Arc welding with coated electrodes.
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(ii) Tungsten-inert gas welding.
(iii) Consumable metal-inert gas welding.
(iv) Submerged arc welding.
Though all these processes use an electric arc as the source of heat, they differ in many aspects, including the power source and the arc characteristics. This will be apparent from the discussion that follows.
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(i) Arc Welding with Coated Electrodes:
Manual arc welding with coated electrodes is the most common welding process. The process is applicable for almost all metals and alloys with the exception of pine copper, pure aluminium, and some low melting point and reactive metals.
The coating serves the following purposes:
(i) It shields the weld pool from an atmospheric contamination by creating a suitable gaseous atmosphere and a slag. The slag also refines the molten metal.
(ii) It acts as a carrier for alloying elements, de oxidants, and other elements necessary to produce the desired arc and metal transfer characteristics.
The thickness of the coating governs the size of the weld pool. With a high thickness, the weld pool is found to be narrower and deeper.
The four most common coatings used in practice are – (i) cellulosic coating, (ii) rutile coating, (iii) iron oxide coating, and (iv) basic or low hydrogen coating. The first two coatings are similar, the only difference being that in the rutile coating the content of TiO2 (rutile) is somewhat more. For the same thickness, the cellulosic coating penetrates more than the rutile coating.
On decomposition, the organic (cellulose) material generates hydrogen and carbon monoxide at a sufficient rate to form the protective gaseous atmosphere. Since the rate of hydrogen evolution is high, the chances of an embrittlement of the joint due to hydrogen are more.
In the iron oxide coating, the gas evolved is less than that in the cellulosic and rutile coatings, and the protection of the weld pool depends heavily on the slag.
The basic or low hydrogen coating avoids an embrittlement due to hydrogen. Here, the coating contains mainly calcium carbonate and calcium fluoride. On decomposition, these generate a CO-CO2 mixture which serves as the protective atmosphere. The rate of gas evolution is substantially lower in this case. Moreover, since CO-CO2 reduces less (than H2), a short arc length is maintained for full protection.
The nature of coating also controls the mode of metal transfer. In general, the cellulosic, rutile, and basic coatings give rise to a short circuit transfer, whereas the iron oxide coating results in a projected transfer.
The coating on the electrode can be applied in various ways. The wire electrode can be dipped in the paste of flux which, on drying, results in the coating. The coating can also be applied by extrusion. Sometimes, the base wire is fed through a magnetic flux which is attracted towards the wire due to the electric field generated on the passage of current. A flux is also projected through a tubular electrode to serve the purpose of the coating.
(ii) Tungsten-Inert Gas Welding:
In the tungsten-inert gas welding process, the arc is maintained between a non- consumable tungsten electrode and the work piece in a protective inert gas atmosphere. Any filler material needed is supplied externally. Normally, a dc arc is used with tungsten as the negative pole. This is not possible for metals, such as Al and Mg, where the oxide layer persists if the work piece is used as the anode. This layer prevents the formation of the weld pool.
The mobile cathode spot can disperse the oxide layer but excessive heat is generated at the tungsten electrode if this is used as the anode. Hence, an ac arc is used for such materials. To avoid the melting of the electrode, thorium or zirconium is added to the tungsten (to increase the melting point).
Argon is most commonly used to provide the inert atmosphere. Nitrogen is sometimes used for welding copper. This is a special type, costly welding process used only for aluminium, magnesium, and other reactive metals. To prevent the possible little contamination, an argon de oxidant is added to the filler rod.
(iii) Metal-Inert Gas Welding:
Here, the arc is maintained between a consumable electrode and the work piece in an inert gas atmosphere. The coiled electrode wire is fed by drive rolls as it melts away at the tip. Except for aluminium, a dc source is used with the consumable electrode as the positive terminal. The difference in this respect with the tungsten-inert gas welding should be noted. For welding steel, a shielding is provided by CO2 for lower cost.
Normally, a high current density in the electrode (of the order of 10,000 amp/cm2) is used so that a projected type of metal transfer results. The welding current is in the range 100-300 amp. The process is primarily meant for thick plates and fillet welds.
In submerged arc welding, the arc is maintained underneath a mass of fusible, granular flux. First, the flux containing calcium oxide, calcium fluoride, silica is sintered to form a coarse powder. This granulated flux is then spread over the joint to be made.
The consumable electrode is fed into this flux. A. portion of the flux melts to protect the liquid weld pool, whereas the rest of the flux shields the arc. Both ac and dc sources are used with a welding current in the range 200-2000 amp. The process is used to have a thick welding in a single run. Obviously, it is unsuitable for overhead welding.
Types and Classification of Metal Joining Processes # 3. Resistance Welding:
The following processes use an ohmic resistance heating as the source of heat:
The electro slag welding process is particularly suitable for welding thick plates. Initially, the plates to be welded are set up vertically with a gap of about 2-3 cm. Also, the filler wires and flux are kept in this gap. Here, the filler wires are used as the electrodes. To start with, an arc is created which melts the flux, and thereafter the molten flux short circuits the arc and heat is generated due to the ohmic heating of the slag. The slag circulates and melts the work pieces and the filler wires.
As the process continues, a little flux is added and the weld pool formed is covered by a layer of liquid slag of almost constant depth. The layer of liquid slag and the weld pool is retained by a water cooled dam. Since the weld pool formed is large and the welding speed is slow, the cooling rate is quite low. This results in a coarse grain size, and a follow up heat treatment is normally required to restore the strength.
(ii) Spot Welding:
In spot welding, the parts to be joined are normally overlapped. The work pieces are clamped between two water cooled copper electrodes. On the passage of a high transient current, the interface melts over a spot and forms the weld. The cooling at the electrode limits the size of the spot. A very high current (40 amp or more) is needed for a very short duration (of the order of a fraction of a second) to complete the welding.
The interfaces to be joined are initially cleaned by various methods, including scratch brushing and vapour degreasing. A spot weld normally contains some porosity at the weld centre, which, unless excessive, is harmless. The spot welding process is difficult to use for highly conductive materials such as aluminium and magnesium. If a series of spot welds are to be made, obviously then a higher current is necessary for each subsequent spot in view of the short circuiting provided by the preceding welds.
(iii) Projection Welding:
Projection welding is a variation of the spot welding process in which small projections are made in one of the surfaces. Then, the parts to be welded are clamped between the flat copper alloy electrodes. On the passage of a high current, the projections melt and form the weld. The process is obviously suitable for a sheet metal assembly, and, unlike spot welding, leaves no indentation mark on the free surface.
(iv) Seam Welding:
Seam welding is a continuous spot welding process where the overlapped parts to be welded are fed through a pair of copper alloy electrodes to form a continuous seam.
Type # 4. Gas Welding:
Acetylene, if kept in a confined space, decomposes into carbon and hydrogen. This decomposition results in a high pressure. When this pressure reaches a value around 0.2 N / mm2, the mixture of C and H becomes violently explosive even in the absence of oxygen. This happens when the mixture is subjected to a spark or shock. To avoid this problem, acetylene is dissolved in acetone.
At a pressure of 0.1 N / mm2, one volume of acetone dissolves twenty volumes of acetylene, and the solubility increases almost linearly to 300 volumes at a pressure of 1.2 N/mm2. An excess of oxygen or acetylene is used, depending on whether a decarburizing or a carburizing flame is desired. In the welding of brass, bronze, and copper-zinc-tin alloys, a decarburizing flame is used, whereas a carburizing flame is used for the welding of high carbon steel.
Types and Classification of Metal Joining Processes # 5. Thermite Welding:
The thermite welding process utilizes a chemical heat source and is normally used for an on-site welding of rails. The chemical reaction, indicates that the temperature obtained is much higher than is necessary and the reaction product is iron and not steel. To avoid these, the reaction mix is added with carbon, ferromanganese, and ferrosilicon to cool off the reaction and to produce steel at the end of the reaction. The reaction is started by burning a magnesium ribbon dipped in the reaction mix.
Types and Classification of Metal Joining Processes # 6. Ultrasonic Welding:
The ultrasonic welding process is used only for the welding of thin strips and foils. The core of a magnetostriction ultrasonic generator is coupled to the work through a bar having a suitably-shaped tip. The tip applies a transverse pressure between the work pieces and the simultaneous application of ultrasonic vibration to the tip results in a spot weld.
The welding takes place due to a combination of fracturing of the brittle oxide layers and softening of the asperities because of localized heating by the high velocity rubbing. In this process, no bulk heating, with the consequent bad effects (e.g., metallurgical changes and mechanical deformation), takes place.
Types and Classification of Metal Joining Processes # 7. Electron Beam Welding:
In electron beam welding, the heat for fusion is obtained from the kinetic energy of a dense beam of high velocity electrons. The electrons are emitted by a cathode and thereafter accelerated by a ring-shaped anode, and focused by means of an electromagnetic field to finally impinge upon a very narrow area (a few microns in diameter) of the work piece. The entire operation is carried out in vacuum with a pressure of 10-3 mm of Hg. The accelerating voltage is in the range 20-200 kV with a welding current of the order of milliamperes.
When the applied voltage crosses the critical value, the electron beam penetrates into the metal. Under such a condition, if the work piece is traversed relative to the beam, an extremely narrow bead of weld is formed. The process is not suitable for a metal which vaporizes excessively or emits a lot of gas when melted. The process can be used to join dissimilar materials and reactive metals, and also for joints requiring a precise control of the weld profile and penetration (as in the fabrication of gas turbine parts).
Types and Classification of Metal Joining Processes # 8. Laser Beam Welding:
A laser beam, instead of a highly focused electron beam, can also be used for welding. A major advantage in this process is that the operation need not be carried out in vacuum. A laser beam is capable of producing a power density as high as 107 W / cm2.
Types and Classification of Metal Joining Processes # 9. Explosive Welding:
The explosive welding process is used to join two plates, face to face. One of the work pieces, called the target plate, is held fixed. The other one, called the flyer plate, is kept at an angle to the target plate.
The minimum distance between the two plates is of the order of 1/4 to 1/2 of the flyer plate thickness. An explosive charge is kept on the top of the flyer plate with an intervening layer of rubber spacers. When the explosive charge is detonated, the flyer plate comes and hits the target plate with a huge velocity and the two plates are welded face to face. This process can be used to join dissimilar materials and the weld interface is seen to be wavy.