Types of Arc Welding: 5 Types | Electrical Engineering

The following points highlight the five main types of arc welding. The types are: 1. Carbon Arc Welding 2. Metal Arc Welding 3. Atomic Hydrogen Arc Welding 4. Inert Gas Metal Arc Welding 5. Submerged Arc Welding.

Type # 1. Carbon Arc Welding:

Carbon arc welding differs from metal arc welding in that it is a puddling process and is somewhat similar to the gas welding process. The carbon arc is very stable and easy to maintain. The length of the arc can be varied over wide limits without causing the arc to go out. There is no tendency for the electrode to freeze or stick, as in the case of the metallic electrode. Accordingly the arc can be struck without difficulty at any point, and rapidly moved over the surface of the work to the point where the weld is to be made. High melting speed and efficient welds are the main advantages claimed.

In this process, a carbon or graphite rod is used as a negative electrode and the work being welded as positive. Mostly graphite electrodes are used as they yield longer life and have low resistance and thus capable of conducting more current. The arc produced between the two electrodes heats the metal to the melting temperature. This is about 3,200°C on the negative electrode, and 3,900°C on the positive electrode. The reason to use carbon rod as negative electrode is that less heat will be generated at the electrode tip than that at the work-piece, and carbon from the electrode will not fuse and mix up with the job.

If this so happens the resultant weld will be rich in carbon, and consequently very much brittle and unsound. For this type of welding only dc can be used. The use of ac is not recommended because no fixed polarity can be maintained. This process is normally adopted where addition of filler material is not required, e.g., flange or edge joints, but if filler metal is required, it is provided by the welding rod made of the similar metal to that of the metal to be welded.

Some protection for the molten weld metal may be provided by using a long arc which produces a carbon monoxide gas envelope. There are two methods of carbon arc welding. In one method no flux is used and in the other method flux either in the form of powder or paste is used to prevent the weld from oxidation. Former method is confined to non-ferrous metals and the later method is usually employed for ferrous metals.

Electrodes up to 25 mm in diameter with currents up to 600 or 800 A are used, so that the carbon arc process is well suited to rapid work, or cases such as arise in the repair of castings, where large quantities of filler metals have to be deposited.

Carbon arc welding is employed for welding sheet steel, copper alloys, brass, bronze, and aluminium. It is not suitable for vertical and overhead welding.

In another process of carbon arc welding, known as twin carbon arc welding, two carbon electrodes are used between which the arc is formed, as illustrated in Fig. 6.10 (b). In this way the arc generates heat which is applied close to the parts to be welded, causing the edges to fuse. In heavy welds, a filler rod is melted by the arc and deposited in the weld. An ac supply is recommended for twin carbon arc welding. In case dc supply is used, the positive electrode will disintegrate and consume at a much faster rate as compared to the -ve electrode because two-thirds of the total heat is generated at the positive pole.

This will produce an unstable arc and need more frequent adjustment of the electrodes. The electrodes used for twin carbon arc welding are approximately of the same diameter as the thickness of work-piece. The magnitude of arc current required depends upon both, electrode diameter and plate thickness. Twin carbon arc welding, though, more complex than single carbon arc welding, possesses the advantage that arc is independent of the job and can be moved anywhere without getting extinguished. Moreover, the work-piece is not a part of the electrical circuit.

Type # 2. Metal Arc Welding:

In this type of welding, a metal rod of the same metal as being welded forms one of the electrode and also serves as a filler and no filler rod is used separately. The arc struck between the work being welded and the electrode causes the melted rod to flow across the arc into the metal pool of the parent metal. This depo­sition of metal is accomplished by contact made between the molten metal and the globules formed on the end of the elec­trode filler rod.

The temperature produced is about 2,400°C and 2,600°C on the negative electrode and positive electrode re­spectively. The concentration of heat energy at the terminals of the electrode causes a small part of the work being welded to melt almost instantaneously and an intermittent flow of metal across the arc stream. The metal in the arc stream is in both the liquid and gaseous forms, the liquid metal being transferred across the arc by molecular attraction, adhesion, cohesion, sur­face tension or a combination of these.

For this type of welding both ac and dc can be used. For dc supply 50-60 volts and for ac supply 70-100 volts are used for welding. For current above 750 A ac equipment is preferred as it has high efficiency, negli­gible loss at no load and minimum maintenance and initial cost. Welding with dc also gives rise to “arc blow”, owing to the magnetic forces created by the current, a trouble which does not exist with ac welding as each magnetic force is immedi­ately followed by one in opposite direction and the arc-deflect­ing forces are automatically cancelled.

Metal in the molten state has an affinity for foreign elements such as oxygen, nitrogen etc.; and in order to prevent impurities from entering the weld modern arc welding electrodes are covered with a coating which, under the welding heat, generates a gas shield round the arc, and also forms a slag which is depos­ited on and around the molten metal, protecting it against foreign matter dur­ing the cooling stages. The correct welding current, voltage and welding speed are very important. Various welding currents can have a deciding effect on the forming of proper beads.

A welding shows bead characteristics under different conditions:

(i) Welding current too low excessive piling of the metal

(ii) Welding current too high causing excessive spatter,

(iii) Voltage too high bead too small

(iv) Welding speed too low excessive piling up of weld metal.

Proper current and timing create a smooth, regular, well- formed bead. Undercutting is also a result of too much current. No enough current results in overlapping and a lack of fusion with the metal.

Type # 3. Atomic Hydrogen Arc Welding:

The principle of atomic hydrogen welding is based on:

(i) The possibility of obtaining atomic hydrogen by means of an electric arc between the two tungsten electrodes, in an atmosphere of hydrogen at atmospheric pressure,

(ii) The very high temperature produced by the recombination of the atoms, which occurs in the cooler regions immediately outside the arc, and

(iii) The very great heat conductivity of hydrogen at high temperatures, owing to the dissociation of the hydrogen molecules into the atomic state, and resulting in an extremely high rate of delivery of heat to the surfaces to be welded-roughly twice that for the oxyacetylene welding flame.

In this method of arc welding an ac arc is maintained between the two non-consumable tungsten electrodes while a stream of hydrogen gas, under a pressure of about 0.5 kg/cm², is passed through the arc and around the electrodes. AC supply is used in order to obtain equal consumption of the electrodes. The hydrogen is usually supplied from steel cylinders and serves two-fold purpose—firstly as protective screen for the arc, secondly as cooling agent for the glowing tungsten electrode points.

As the molecules of hydrogen pass through the electric arc, they are changed into the atomic state and thus absorb a considerable amount of energy—resulting in cooling of glowing tungsten electrodes. But when the atoms of hydrogen recombine into molecules just outside the arc, a large amount of heat is liberated. This extra heat, added to the intense heat of the arc itself, produces a temperature of about 4,000°C, as compared to 2,000°C produced by the combination of normal hydrogen and oxygen.

This heat is used in making fusion welds. When additional metal is required, filler rods are melted into the joint. Due to the decarbonizing character of the welding flame (with the formation of hydrocarbons) the filler rod material should have a carbon content higher than that of the material to be welded, if the finished weld metal is to have the same carbon content as the parent material. Owing to the concentrated heat, high welding speeds may be obtained keeping distortion small.

Furthermore hydrogen excludes all oxygen and other gases which might combine with the molten metal to form oxides and other impurities. It also removes oxides from the surface of the work. Thus, this process is capable of producing smooth, uniform, strong, and ductile welds. Automatic equipment for this process has been developed, the hydrogen being fed through a water-cooled nozzle block.

Arc currents up to 150 A can be used and the power supply arrangements are similar to those for ordinary ac arc welding except that the transformer voltages are higher, being about 300 V on open circuit for striking the arc and between 80 and 100 V while operating.

This method is very flexible—practically any metal or alloy, ferrous and non-ferrous, can be welded. Its greatest usefulness is for the fusion welding of certain special ferrous alloys, such as chrome nickel steels, aluminium, and duralumin, but it must use flux to weld some of these materials. Welding of thin sheets, production of tubing, and the repairing of expensive tools and dies are some of the common uses of this method.

Type # 4. Inert Gas Metal Arc Welding:

It is a gas shielded metal arc welding process which uses the intense heat of an electric arc between a continuously fed, consumable electrode wire and the material to be welded. A consumable bare electrode wire producing the filler metal is automatically fed from a reel on to the job through a welding gun which also carries a nozzle. Through this nozzle helium or argon is blown around the arc and on to the weld. Because of low arc voltage for given welding current argon is suitable for thin materials as tendency to burn through is reduced.

Because of low arc voltage and low power for given welding current in case of argon gas welding tendency for metal fusion and metal sag or run is reduced and, therefore, it is preferred for in position welding. On the other hand, because of high arc voltage and high power in case of helium gas welding, it is used for welding of thick materials and metals with high heat conductivity. Mixture of the two gases is useful to get intermediate characteristics. AC and dc both can be used.

This type of welding has the following advantages:

(a) In this method concentration of heat is possible and thus distortion is minimised.

(b) In this method no flux is required since atmosphere is inert and air does not come in contact with the molten metal.

This process is particularly employed for welding light alloys, stainless steel and non-ferrous metals such as copper, aluminium and their alloys. For welding aluminium and aluminium alloys ac supply and argon gas is used whereas for welding magnesium and magnesium alloys either ac supply and argon gas or dc supply, reversed polarity and either argon or helium gas is used. For the welding of stainless steel, mild steel, copper and copper alloys, dc and either argon or helium or ac high frequency stabilized with argon or helium used.

Type # 5. Submerged Arc Welding:

It is an arc welding process that uses an arc between a bare metal electrode and the weld pool. The arc and molten metal are shielded by a blanket of granular flux on the work-pieces. The process is used without pressure and with the filler metal from an electrode, and sometimes from a supplemental source such as a welding rod, flux, or flux with metal granules.

It is an automatic process developed primarily for the production of high quality butt welds in thicker steel plate than is normally suited to other manual arc welding processes.

As in open arc welding, the source of heat in the submerged arc welding process is an electric arc or arcs between a metal electrode, or electrodes and the work. The welding zone is shielded by a blanket of flux, so that the arc is not visible. Hence the name “submerged arc welding”. The arc melts the parent metal, the electrode, and the flux. The fused flux produces a blanket of liquid slag which forms a protective envelope both around the arc and the welding zone.

Right beneath the electrode tip a weld pool is formed, holding an amount of molten metal. On melting, the electrode metal forms globules which go over into the puddle and mix with the molten parent metal. The pressure of the arc forces the mix out of the puddle so that it forms the weld. This action continues as long as the arc advances. The shape and size of the puddle have a strong effect on those of the weld.

The method of making a weld by submerged arc welding is illustrated diagrammatically in Fig. 6.13. Flux is fed continuously onto the work just ahead of the advancing arc. Since the arc is not visible, the operator can wear goggles with ordinary glasses, using neither a hand screen nor a helmet. Since molten flux is lighter than molten metal, it floats to the surface and solidifies as a crust of brittle slag which is readily removable from the weld surface. Much of the flux spread on the work remains un-fused. It is recovered by suction into the hopper for re-use.

The flux may be made of silica, metal oxides, and other compounds fused together and then crushed to proper size. Another group of fluxes is made of similar material “bonded” and formed into granules.

The bare electrode is fed from a reel down through the gun or nozzle. The operator can move it slowly, a little at a time for the start of the arc and then set the proper feed rate on the control box. Wire of several alloys for steel, stainless steel, copper etc. is available in diameters from 3 to 6 mm. Voltage employed is from 25 to 40 volts. Current employed depends considerably on work-piece thickness. Normally dc is used employing 600 to 1,000 A current for welding alloy and stainless steel, although ac is preferable particularly for low carbon steel. The current for ac is usually 2,000 A.

The advantages of submerged arc welding are:

(i) Partly because it is often automated, it is much faster than regular arc welding. Speeds up to 3,800 mm/minute are possible on 3 mm thick steel at 100% efficiency.

(ii) Deep penetration with high quality weld is possible.

(iii) Less distortion occurs from high speed and uniform heat input, especially when automated.

(iv) No edge preparation is required.

(v) Operator is not exposed to usual spatter and can work more easily without helmet and other safety equipment.

(vi) The welds obtained have good ductility, impact strength, uniformity, low nitrogen content and high corrosion resist ance.

(vii) Manipulative skills normally not involved.

(viii) High utilisation of electrode wire.

(ix) Easily automated for high operator factor.

(x) Little or no smoke.

The submerged arc welding is widely used in heavy steel plate fabrication work. This includes the welding of structural shapes, the longitudinal seam of larger diameter pipe, the manufacture of machine components for all types of heavy industry, the manufacture of vessels and tanks for pressure and storage use. It is widely used in the ship building industry for splicing and fabricating subassemblies, and by many other industries where steels are used in medium to heavy thickness.

It is also used for surfacing and build-up work, maintenance, and repair.