ELECTRIC WELDING

Vineet Mediratta

Deptt. Of Electrical Engg.

COES , UPES

Welding is a materials joining process which produces coalescence of materials by heating them to suitable temperatures with or without the application of pressure , and with or without the use of filler material.

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WELDING PROCESS

Coalescence : To join into a single mass .

Welding is used for making permanent joints.

It is used in the manufacture of automobile bodies, aircraft frames, railway wagons, machine frames, structural works, tanks, furniture, boilers, general repair work and ship building.

WELDING PROCESS

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Plastic Welding or Pressure Welding

The piece of metal to be joined are heated to a plastic state and forced together by external pressure

(Ex) Resistance welding

Fusion Welding or Non-Pressure Welding

The material at the joint is heated to a molten state and allowed to solidify

(Ex) Gas welding, Arc welding

WELDING TYPES

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(iv)Thermit Welding

(v)Solid State Welding

Friction

Ultrasonic

Diffusion

Explosive

(vi)Newer Welding

Electron-beam

Laser

(vii)Related Process

Oxy-acetylene cutting

Arc cutting

Hard facing

Brazing

Soldering

Classification of Welding Processes:

Factors influencing heat or welding temperatures

• the proportions of the work pieces ,
• the coating or the lack of coating,
• the electrode materials,
• electrode geometry,
• electrode pressing force,
• weld current and weld time.

Squeeze Time :

It is the time that elapses between the initial application of the electrode pressure of the work and the first application of current.

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Weld Time

The time for which the welding current flows through the parts being welded . It is usually expressed in cycles of supply.

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Hold Time

The time during which pressure is applied at the point of welding after the welding current has ceased to flow

Welding Terminology

There are two methods of Electric Welding

• Resistance welding
• Electric Arc Welding

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These two methods are to be studied in Detail

Electric resistance welding (ERW) refers to a group of welding processes such as spot and seam welding that produce coalescence of faying surfaces where heat to form the weld is generated by the electrical resistance of material Vs the time and the force used to hold the materials together during welding.

The heat produced H = I²Rt , where I is the current through electrodes , R is contact resistance of the inter-face and ‘t’ is the time for which current flows

Electric Resistance Welding

Resistance welding is conducted as follows: Apply force and current through electrodes contacted metal parts to be welded; and resistance heat is generated at the interface of metal parts and makes a nugget, resulting in melt joint. Though a large current flows, there is no danger of an electric shock because only low voltage is impressed.

Principle of Resistance Welding

Small pools of molten metal are formed at the point of most electrical resistance (the connecting surfaces) as a high current ( 100–100,000 A ) is passed through the metal. In general, resistance welding methods are efficient and cause little pollution,

ERW STEEL PIPES

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• No flux such as solder is necessary, so welded parts can be easily recycled.
• Spatter and ultraviolet ray are most unlikely to be generated; consequently,

clean and neat worksite is realized.

• Easy operation as only pressing buttons facilitates process automation and

does not require trained skills unlike arc welding and gas welding.

• As this welding is performed efficiently in a short period of time, it is suited

for a high-volumes production of low-cost products.

• Since welding is done in short time duration, it gives less heat-affected

area on work pieces, resulting in beautiful appearance with less

indentation.

Features of Resistance Welding

Applications of Resistance Welding

Spot welding is a resistance welding method used to join two to three overlapping metal sheets, studs, projections, electrical wiring hangers, some heat exchanger fins, and some tubing. Usually power sources and welding equipment are sized to the specific thickness and material being welded together. The thickness is limited by the output of the welding power . Care is taken to eliminate contaminants between the faying surfaces.

SPOT WELDING

Usually, two copper electrode are simultaneously used to clamp the metal sheets together and to pass current through the sheets. When the current is passed through the electrodes to the sheets, heat is generated due to the higher electrical resistance where the surfaces contact each other. As the electrical resistance of the material causes a heat buildup in the work pieces between the copper electrodes, the rising temperature causes a rising resistance, and

results in a molten pool contained most of the time between the electrodes. As the heat dissipates throughout the work piece in less than a second (resistance welding time is generally programmed as a quantity of AC cycles or milliseconds) the molten or plastic state grows to meet the welding tips.

SPOT WELDING

When the current is stopped the copper tips cool the spot weld, causing the metal to solidify under pressure. The water cooled copper electrodes remove the surface heat quickly, accelerating the solidification of the weld, since copper is an excellent conductor .

SPOT WELDER

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Resistance spot welding typically employs electrical power in the form of direct current, alternating current, medium frequency half-wave direct current, or high-frequency half wave Direct current.

• Efficient Energy use
• Limited work piece deformation
• High production rates,
• Easy automation ,
• No filler material required.

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When high strength in shear is needed, spot welding is used in preference to more costly mechanical fastening, such as riveting

It is used extensively in the automotive industry— cars can have several thousand spot welds. A specialized process, called shot welding, can be used to spot weld stainless steel .

The advantages of the method include

Resistance seam welding is a process that produces a weld at the faying surfaces of two similar metals. The seam may be a butt joint or an overlap joint and is usually an automated process. It differs from butt welding in that butt welding typically welds the entire joint at once and seam welding forms the weld progressively, starting at one end..

SEAM WELDING

Like spot welding, seam welding relies on two electrodes, usually made from copper, to apply pressure and current. The electrodes are disc shaped and rotate as the material passes between them. This allows the electrodes to stay in constant contact with the material to make long continuous welds. The electrodes may also move or assist the movement of the material

SEAM WELDING

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A transformer supplies energy to the weld joint in the form of low voltage, high current AC power. The joint of the work piece has high electrical resistance relative to the rest of the circuit and is heated to its melting point by the current. The semi-molten surfaces are pressed together by the welding pressure that creates a fusion bond, resulting in a uniformly welded structure.

SEAM WELDING

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Most seam welders use water cooling through the electrode, transformer and controller assemblies due to the heat generated. Seam welding produces an extremely durable weld because the joint is forged due to the heat and pressure applied. A properly welded joint formed by resistance welding is typically stronger than the material from which it is formed.

SEAM WELDING

Uses an electric arc to coalesce metals

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Arc welding is the most common method of welding metals

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Electricity travels from electrode to base metal to ground

ARC WELDING

• Arc welding is a welding process, in which heat is

generated by an electric arc struck between an electrode

and the work piece.

• Electric arc is luminous electrical discharge between two

electrodes through ionized gas.

Any arc welding method is based on an electric circuit

consisting of the following parts:

Power supply (AC or DC); Welding electrode;

Work piece;

Welding leads (electric cables) connecting the electrode and work piece to the power supply.�

PRINCIPLE OF ARC WELDING

Electric arc between the electrode and work piece closes the electric circuit. The arc temperature may reach 10000°F (5500°C), which is sufficient for fusion the work piece edges and joining them.

When a long join is required the arc is moved along the joint line. The front edge of the weld pool melts the welded surfaces when the rear edge of the weld pool solidifies forming the joint.

WORKING OF ARC WELDER

Equipments:

• A welding generator (D.C.) or Transformer (A.C.)
• Two cables- one for work and one for electrode
• Electrode holder
• Electrode
• Protective shield
• Gloves
• Wire brush
• Chipping hammer
• Goggles

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Arc Welding

Metal Arc Welding

Carbon Arc Welding

Carbon arc welding differs from common metal arc welding in that it uses non – consumable carbon – graphite electrodes instead of consumables flux-coated electrodes .

Refer Fig , Arc is formed in the gap , filling metal is supplied by fusing rod or wire into the arc

The voltage required for striking an arc with carbon electrodes is about 30 volts (A.C.) and 40 volts(D.C.)

Advantages

• Most efficient way to join metals
• Lowest-cost joining method
• Affords lighter weight through better utilization of materials
• Joins all commercial metals
• Provides design flexibility

Limitations

• Manually applied, therefore high labor cost.
• Need high energy causing danger
• Not convenient for disassembly.
• Defects are hard to detect at joints.

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Arc Welding

• More efficiency
• Power consumption less
• Cost of equipment is less
• Higher voltage – hence not safe
• Not suitable for welding non ferrous metals
• Not preferred for welding thin sections
• Any terminal can be connected to the work or electrode

Comparison of A.C. and D.C. arc welding

Alternating Current (from Transformer)

Direct Current (from Generator)

• safer operation
• suitable for both ferrous non ferrous metals
• preferred for welding thin sections
• Positive terminal connected to the work
• Negative terminal connected to telectrode
• Power consumption more
• Cost of equipment is more
• Less efficiency

Comparison of A.C. and D.C. arc welding

Shielded metal arc welding (Stick welding, Manual metal arc welding) uses a metallic consumable electrode of a proper composition for generating arc between itself and the parent work piece. The molten electrode metal fills the weld gap and joins the work pieces�����

Shielded Metal Arc Welding

The electrodes are coated with a shielding flux of a suitable composition. The flux melts together with the electrode metallic core, forming a gas and a slag, shielding the arc and the weld pool. The flux cleans the metal surface, supplies some alloying elements to the weld, protects the molten metal from oxidation and stabilizes the arc. The slag is removed after solidification.

Advantages

• Simple, portable and inexpensive equipment;
• Wide variety of metals, welding positions and electrodes are applicable;
• Suitable for outdoor applications.� Disavantges

The process is discontinuous due to limited length of the electrodes;

Weld may contain slag inclusions;

Fumes make difficult the process control.

Advantages and Disadvantages of Shielded Metal Arc Welding

(SMAW):

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Types of weld joints are shown in the figure:

Submerged Arc Welding

A bare metal electrode is used and the arc is shielded by a separately supplied blanket of granular fusible flux . The flux , when cold , is non – conductor of electricity but in molten state it is highly conductive . The arc as such is not visible from outside. The arc , molten electrode and weld puddle are completely submerged in the conductive high resistance flux . In order to start an arc ; steel rod is placed in between the electrode and the work piece

Submerged arc welding is either semi-automatic or an automatic process . A The variety of metals and alloys can be welded by this process. This method of welding is primarily a production process and is used for straight line welds specially in box framing