Electric Welding

Joining and Assembly

Joining - welding, brazing, soldering, and adhesive bonding

These processes form a permanent joint between parts

Assembly - mechanical methods (usually) of fastening parts together

Some of these methods allow for easy disassembly, while others do not

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

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

Classification of welding processes:(i) Arc welding

Carbon arc Metal arc

Shielded arc- coated electrode

Un-shield arc Metal inert gas Tungsten inert gas Plasma arc Hydrogen Submerged arc Electro-slag

(ii) Gas Welding Oxy-acetylene Air-acetylene Oxy-hydrogen

(iii) Resistance Welding Butt Spot Seam Projection Percussion

(iv) Thermit Welding(v)Solid State Welding

Friction Ultrasonic Diffusion Explosive

(vi)Newer Welding Electron-beam Laser

Types of Welding Processes

Welding processes can be divided into two major categories:Fusion welding

Arc welding (AW)

Resistance welding (RW)

Oxy-fuel gas welding (OFW)

Solid state welding: Joining processes using pressure alone or a combination of heat and pressure If heat is used, temperature is below melting point of metals being

welded

No filler metal is added in solid state welding

Five Types of Joints

(a) Butt joint, (b) corner joint, (c) lap joint, (d) tee joint, and (e) edge joint

Resistance Welding (RW)A group of fusion welding processes that use a combination of heat and pressure to accomplish coalescence Heat generated by electrical resistance to current flow at

junction to be welded The resistance of metal to the localized flow of current produces

heat Process variables

Current Time Force

Types of resistance welding Butt Spot Seam Projection Percussion

Components in Resistance Welding

Parts to be welded (usually sheet metal)

Two opposing electrodes

Means of applying pressure to squeeze parts between electrodes

Power supply from which a controlled current can be applied for a specified time duration

AC electric current (up to 100 000 A) is supplied through copper electrodes connected to the secondary coil of a welding transformer

Resistance Spot Welding (RSW)

Resistance welding process in which fusion of faying surfaces of a lap joint is achieved at one location by opposing electrodes

Used to join sheet metal parts

Widely used in mass production of automobiles, metal furniture, appliances, and other sheet metal products Typical car body has ~ 10,000 spot welds

Annual production of automobiles in the world is measured in tens of millions of units

(a) Spot welding cycle

(b) Plot of force and current

Cycle: (1) parts inserted between electrodes, (2) electrodes close, (3) current on, (4) current off, (5) electrodes opened

Spot Welding Cycle

Resistance Seam Welding (RSEW)

Uses rotating wheel electrodes to produce a series of overlapping spot welds along lap joint

Can produce air-tight joints Applications:

Gasoline tanks Automobile mufflers Various sheet metal containers

Resistance Projection Welding (RPW)

A resistance welding process in which coalescence occurs at one or more small contact points on the parts

Contact points determined by design of parts to be joined May consist of projections, embossments, or localized

intersections of parts

(1) Start of operation, contact between parts is at projections;

(2) when current is applied, weld nuggets similar to spot welding are formed at the projections

Projection Welding Operations

(a) Welding of fastener on sheetmetal and (b) cross-wire welding

Cross Section After Welding

Transformer

Movable Platen

Dies

Resistance Butt Welding is a Resistance Welding (RW) process, in which ends of wires or rods are held under a pressure and heated by an electric current passing through the contact area and producing a weld.

Resistance Butt\Flash Welding

A wide variety of materials in wire , tube, bar and strips can be welded by this method. The materials welded by this method are aluminum alloys, brass, copper, nickel alloys, stainless, low carbon and high carbon steels and gold

Typical applications of butt welding

(1) Butt welding of matching sections.

(2) Chain links.

(3) Railway lines.

(4) Window frames.

(5) Aero-engine rings.

(6) Car wheel rims.

(7) Metal strip in rolling mills.

Types of butt welding

Upset Butt Welding no arc

Flash Butt Welding arc between the to contacts

Percussion welding (PEW) is a type of resistance welding that blends

dissimilar metals together.

Percussion welding is similar to flash welding and upset welding

It is considered to be more complex because it uses an electric discharge at the

joint, followed by pressure being applied to join the materials together.

Percussion welding is used to join dissimilar metals together, or used when flash is

not required at the joint. Percussion welding is used on materials that have small

cross sectional areas.

Advantages of using percussion welding types include a shallow heat affected

zone, and the time cycle involved is very short. Typical times can be found to be

less than 16 milliseconds.

Percussion welding (PEW)

(1) Rotating part, no contact; (2) parts brought into contact to generate friction heat; (3) rotation stopped and axial pressure applied; and (4) weld created

Advantages and Drawbacks of Resistance Welding

Advantages: No filler metal required High production rates possible Lends itself to mechanization and automation Lower operator skill level than for arc welding Good repeatability and reliability High welding rates Low fumes Cost effectiveness Low distortions.Disadvantages: High initial equipment cost Limited to lap joints for most RW processes High equipment cost; Low strength of discontinuous welds; Thickness of welded sheets is limited -up to 1/4 (6 mm);

Arc Welding (AW)

A fusion welding process in which coalescence of the metals is achieved by the heat from an electric arc between an electrode and the work

Electric energy from the arc produces temperatures ~ 10,000 F (5500 C), hot enough to melt any metal

Most AW processes add filler metal to increase volume and strength of weld joint

What is an Electric Arc?

An electric arc is a discharge of electric current across a gap in a circuit

It is sustained by an ionized column of gas (plasma) through which the current flows

To initiate the arc in AW, electrode is brought into contact with work and then quickly separated from it by a short distance

A pool of molten metal is formed near electrode tip, and as electrode is moved along joint, molten weld pool solidifies in its wake

Arc Welding

Arc welding Equipments:

A welding generator (D.C.) or Transformer (A.C.)

Two cables- one for work and one for electrode

Electrode holder

Ground clamp

Electrode

Welding helmet and hand shield

Protective clothing including hand gloves

Protective shield

Gloves

Wire brush

Chipping hammer

Goggles

Types of arc welding

Carbon arc welding

Shielded metal arc welding

Gas metal arc welding

Submerged arc welding

Carbon Arc Welding

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.

The temperature produced is about 2,400C and 2,600C on the negative

electrode and positive electrode respectively.

For dc supply 50-60 volts and for ac supply 70-100 volts are used for

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.

This process is used for welding satellite tips to tools, copper to

aluminium or stainless steel, silver contact tips to copper, cast iron to

steel, lead-in-wires on electric lamps and zinc to steel. Gold, silver,

copper- tungsten, silver-tungsten and silver-cadmium oxide percussion

welded to copper alloys for commonly used assemblies for electric

contacts.

Metal-Arc Welding .

Shielded metal arc welding

This method flux coated electrodes are used. The coating have 3 functions

It creates inert atmosphere which shields the molten metal from oxygen and nitrogen present in the air

It forms a fast hardening slag

It adds deoxidizer or scavangers

Gas metal - arc welding

Here the consumable electrode is shielded from the atmosphere by an externally supplied protective atmosphere (usually CO2, ARGON-CO2 mixture, helium-base gases)

Submerged arc welding

A bare metal electrode is used and the arc is separately supplied blanket of

granular fusible flux. The flux, when cold, is non conductor but in molten

state it has high conductivity.

The arc as such is not visible from outside

Arc welding

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.

S. No Particular Resistance welding Arc welding

1 Supply. Usually ac only. AC or dc.

2 Voltage. Very low. The striking voltage is high so requiresvoltage control.

3 Power factor. Very low. Poor.

4 Additional material requirement.

No material is added in any form to get the two pieces joined.

Suitable filler metal electrodes are necessary to get proper strength.

5 External pressure External pressure is required. No external pressure is required hence the equipment is more simple and easy to control.

6 Development of heat.

Heat is developed due to flow of current through the contact resistance mainly.

Heat is developed due to arc between electrode and the work piece.

7 Temperature The temperature attained is not very high as in case of arc welding.

The temperature of the arc is very high and so likely to damage the work if not properly handled.

8 Applications. It cannot be used for repair work. It is most suitable for mass production.

It is not suitable for mass production. It is most suitable for repair work and where more metal is to be deposited.

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

Alternating Current (from Transformer)

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

Direct Current (from Generator)Less efficiency

Power consumption more

Cost of equipment is more

Low voltage safer operation

suitable for both ferrous non ferrous metals

preferred for welding thin sections

Positive terminal connected to the work

Negative terminal connected to the electrode

Comparison between dc welding and ac welding is given below in tabular form.