ELECTRIC WELDING
It is a process of joining two pieces of metal at faces rendered plastic or liquid by the application of heat with or without application of pressure and addition of filler material.
Advantages:
- Saving in time and cost
- Strong joints
- Less consumption of materials etc.
Disadvantages:
- Radical improvements in the manufacture a large variety of machines, devices and structures.
- Prompted mechanization and automation in their manufacture.
Uses:
Used in automobile industry, aircraft machine frames, structural work, tanks, machine repair work, shipbuilding, pipe-line fabrication in thermal power plants and refineries, fabrication of metal structure etc.
TYPES OF WELDING
Gas welding:
- Oxy-acetylene welding
- Air acetylene welding
- Oxy-hydrogen welding
Resistance welding:
- Butt welding
- Spot welding
- Projection welding
- Seam welding
- Percussion welding
Arc welding:
- Carbon arc welding
- Metal arc welding
- Gas metal arc welding
- Gas tungsten arc welding
- Atomic hydrogen arc welding
- Plasma arc welding
- Submerged arc welding
- Flux-cored arc welding
- Electro-slag welding
Solid state welding
- Friction welding
- Ultrasonic welding
- Diffusion welding
- Explosive welding
Newer welding:
- Electron beam welding
- Laser welding
RESISTANCE WELDING
In resistance welding, a sufficiently strong electric current is sent through the two metal pieces in contact to be welded which melts the metal pieces by the resistance they offer to the flow of electric current.
Applications:
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. In this process, heat can be controlled, and it permits a pressure action at the weld. Metals of medium and high resistances, such as steel, stainless steel, Monel metal and silicon bronze are easy to weld.
Parts of resistance welding plants:
- Heating unit consisting of transformer
- Timing unit for controlling the length of time of welding current flow
- Mechanical unit for providing the necessary electrode force.
Types of resistance welding:
Butt welding: It is of two types
(a) Upset butt welding:
This welding includes end to end welds, lap welds and butt welds. It has the advantage of creating no flash spatter and a smooth and symmetrical upset, is used principally on non-ferrous materials for welding bars, rod wire, tubing, formed parts etc.
(b) Flash-butt welding:
It creates a joint which has practically
full strength of the parent metal under static loading conditions
strength slightly less than the parent metal under dynamic loads.
Flash butt welding is better than upset butt welding and is used for welding chains, rail ends, rolled sections, shift axles etc.
Spot welding:
In this welding, parts of pieces are joined in spots, accompanied by heated relatively small sections of the parts or pieces between suitable electrodes under pressure. Such a welding only provides mechanical strength and is neither airtight nor watertight. Spot welding is used for joining components made from plate material, the plate thickness being limited (usually to 10 or 12 mm) by the pressure and current capacity of the available spot-welding machines.
Projection welding:
It is a modified form of spot welding and has several advantages over spot welding such as simple welding process, more output, increased electrode life, good finished appearance etc. Projection welding used for punched. formed or stamped parts, where the projections automatically exist.
Seam welding:
These can be defined as series of continuous Spot welds. This process is employed for making a continuous between two overlapping pieces of sheet metal. This type of welding is very important as it provides pressure tight or leak proof joint.
Seam welding is used for welding pipes, conduits, tanks, transformers, refrigerators, gasoline tanks, air Crafts and various types of containers.
Percussion Welding
This welding depends on the arc effect for heating and not on the resistance. This is a self-timing spot welding method. In this process a current impulse is obtained by the discharge from a capacitor or from a magnetic field. The action of process is so rapid that there is little heating effect in the material adjacent to the weld.
Percussion welding is used for welding satellite tips to tools, copper to aluminium or stainless steel, silver contact tips to copper. The equipment used is quite expensive as it must be extremely rugged and provided with accurate holding fixtures and sensitive timing devices etc.
ELECTRIC ARC WELDING
In arc welding, the pieces of the metal to be welded are brought to the proper welding temperature at a point of contact by the heat liberated at the arc terminals and in the arc stream so that the metal pieces are completely fused into each other, forming a single solid homogeneous mass, after it solidifies. In this process an electric arc is produced by bringing two conductors (electrode and metal piece) connected to a suitable source of electric current, momentarily in contact and then separating by a small distance. The current continues to flow across the small gap and gives intense heat. The heat developed is to melt the part of the work piece, and the filler metal and thus form the joint. So, arc welded joint is a union of metal parts made by localized heating without any pressure. Thus, this type of welding is also called non-pressure welding. The heat developed by arc is also used for cutting of metal. The voltage necessary to strike the arc is higher than that required to maintain it. The temperature is of the order of 3600oC, at which mechanical pressure is not required for jointing.
According to B.S.S. 638 maximum voltage specified for welding is about 100 volts for the safety of operator. The arc voltage varies from 20 to 40 volts and current from 50 A in sheet metal work to 1000 A in heavy automatic welding.
Uses:
It is widely used for joining of metal parts, repair of fractured castings and fillings by the deposition of new metal on the worn-out parts.
Parts of a welding circuit:
A welding machines
Two leads
An electrode holder
An electrode and work
TYPES OF ELECTRIC ARC WELDING:
Carbon arc welding:
There are of two types.
When no flux ¡s used to prevent the weld from oxidation. This method is confined to non-ferrous metals. It is usually used for ferrous metals.
When flux either in the form of powder or paste is used to prevent the weld from oxidation.
For this type of welding only dc can be used.
Carbon arc welding is used for welding sheet steel, copper alloys, brass, bronze and aluminium. It is not suitable for vertical and overhead welding.
Metal arc welding:
In this type of welding, a metal rod of the same metal as being welded forms one of the electrodes and serves as a filler and no filler rod is used separately. For this type of welding both ac and dc can be used. For de supply 50-60 V and for ac 70-100 V are used.
Automatic hydrogen arc welding:
In this method, an arc is maintained between the two 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. This process can produce smooth, uniform, strong and ductile welds. This method is very flexible. In this method any metal or alloy, ferrous and non-ferrous can be welded.
This method is used for welding of thin sheets, production of tubing and the repairing of expensive tools and dies.
Inert gas metal arc welding:
It is a gas shielded metal arc welding process and makes use of intense heat of an electric arc between a continuously fed, consumable electrode wire and the material to be welded. 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. 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 is used.
Submerged arc welding:
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 process. The welds obtained by this process have good ductility, impact strength, uniformity, low nitrogen content and high corrosion resistance.
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