Process 1: Melting the glass

• The Silicon Dioxide is mixed with other chemicals then put into a furnace to be melted
• Oxygen and Hydrogen injected into the furnace to increase the heat transfer making the material melt faster
• Resulting glass(Alumino – Silicate) Contains Aluminium , Silicon, Oxygen , Sodium ions

Process 2: Mold the Glasses

• The molten glass is poured into the desired die and the required shape and thickness obtained.

Process 3: Ion Exchange

Manufacturing Process:

Gorilla glass starts as a mix of pure sand (silicon dioxide) and naturally occurring chemicals (resulting glass is termed as alumino-silicate) which splits the impurities and melting the sand. The molten glass fills up the bin and it is overflowed on each facet. During this "fusion draw" method, the resulting molten glass is pull down by a robust process to a long of 0.59 millimetre-thick sheets of Alumino-silicate Glass.

At this point, you have some very huge sheets of clear, clean, pure glass, however it’s not much stronger than regular glass. Gorilla gets its strength through a noteworthy action. Currently the glass sheet is dipped each into a molten salt bath where a chemical exchange happens. Potassium ions are infused into the glass. At the similar time, sodium ions exit from the glass compound. Here the potassium (K) ions are larger than the sodium (Na) ions. So a compressive stress occurs. That stress is really an honest factor and stops the glass from breaking on flaws.

Chemical Strengthening Process:

Chemical tempering strengthens the glass by putting the surface of the glass into compression by “stuffing” larger sized ions into the glass surface. During chemical tempering method, the glass is submersed in a bath of molten salt at prescribed temperatures. The heat causes the smaller ions to depart the surface of the glass and bigger ions present within the molten salts to enter it. Once the glass is off from the bath and cooled, they shrink. The larger ions that are currently present within the surface of the lens are crowded along. This creates a compressed surface, which results in stronger glass that’s more resistant to breakage.

Achieving the specified compressive stress characteristics is time/temperature dependent. Gorilla glass, in contrast to most soda lime glasses, is not self limiting thorough of layer, thus smart time/temperature management is important for a stable method. Although Gorilla glass could also be chemically tempered at temperatures up to 460°C (Temperature vary between 390°C and 420°C) with the target salt temperature maintained to +/- 2°C. Tempering time should be controlled at intervals +/- 5 min.

Fluorosilane coating:

As people’s lives become busier and workplaces transcend the boundaries of office walls, the demand for mobile technologies continues to grow. With this transformation comes the need for a cover glass that promotes clarity while, also protecting and promoting the lifespan of display devices. The primary objective for development of this coating has been to enable the continued ability of a coated glass surface to exhibit superior optical clarity and mechanical reliability, service life, and, most importantly, the glass surface must function for its purpose.

By applying Fluorosilane coating over the glass sheets prevent fingerprint appearance and enable ease of fingerprint removal from the everyday products like Dirt, Oil, Soap, Lotion, Butter, Ketchup etc. Now the coating improves the clarity and optical performance better than soda lime glass.

The latest generation of Gorilla Glass is claimed to be 20% thinner, and more responsive to touchscreen commands than its predecessor. This implies that the screen pictures are probably brighter and slimmer line in style.

Gorilla Glass’s development coincided somewhat fortunately with the increase of the touch-screen smartphone; the best example is arguably the iPhone. However, handsets are only the beginning. The product / merchandise is experimenting with more thin and flexible sheets, and printing on glass to be used on custom – built laptops, Liquid Crystal Display Televisions et al.

Characteristics of Gorilla Glass:

• Scratch resistance
• Slimness / Thinner
• Stronger
• Improved Touch Sensitivity

Comparatively perfect fit for today’s abundance touch-screen handsets.

Difference between ‘Scratch- Proof ‘ and ‘Scratch Resistant’ glass:

A Scratch- screen proof is impermeable resistant to scratches. This kind of technology is not on the market however – any glass may break if it is placed under enough stress. Gorilla Glass is NOT a scratch-proof.

A Scratch- screen resistant is much stronger than most screens. It is less probable to smash/crack if dropped, and less probably to scratch if scratched. Extreme force, sharp objects, and continual exposure to abrasive oils may leave scratches.

History of Gorilla Glass:

The Gorilla Glass is the trade name of “Corning”, an United States of America Glass maker. They form the toughened glass (Alkali – Alumino Silicate Sheet) for the portable electronic gadgets. This idea generated in the 60s period as a project name, “Project Muscle”. The glass invented was called as “Chemcor” glass, which are ultra strong and light weight. The product is developed for windshield glass for cars, But the product is very costly so they are not succeeding on that time.

In the period of 2005, they again started researching with the project name of “Gorilla Glass”. In this period touch screen cell phones are popular, and the product needs a resilient, scratch resistant cell phone cover glass. At that time, the company take the idea from Chemcor glass and they start building the Gorilla glass.

After completing the research, first production starts at the period of 2007 and they got the first order in the period of 2008. At that period nearly 200 million users (about 20% of Devices) uses the gorilla glass for their cell phones.

In 2012, the second generation of gorilla glass they built and launched, achieved a goal of one billion devices.

In 2013, the Third generation Gorilla Glass they launched, which are three times more resistant and stronger; 40% scratches which occur will not be visible to naked eye.

Currently Used Gorilla Glass products:

Phones

• Iphone
• HTC
• LG
• Motorolla
• Nokia
• Samsung

Tablets

• Samsung
• Blackberry
• Lenovo

Laptops

• Dell
• Sony
• Lenovo

TV’s

• Sony

Cameras

• Leica

BENEFITS:

• Glass designed for a high degree of chemical strengthening

- High compressive stress

- Deep compression layer

• High retained strength after use

• High resistance to scratch damage

• Superior surface quality

APPLICATIONS:

• Ideal protective covering for displays in

- Smart phones

- Laptop / Portable Computers and tablet computer screens

- Mobile devices

• Touchscreen devices

• Optical components

• High strength glass articles

Electro Gas Welding is an arc welding process that uses an arc between a continuous filler metal electrode and the weld pool, employing vertical position welding with backing to confine the molten weld metal. Electro gas welding is very much similar to electro slag welding except that an inert gas such as carbon di oxide is used to shield the weld from oxidation and there is a continuous arc as in the case of submerged arc welding to provide the heat for heating the weld pool. Again the flux instead of being supplied to the weld zone through a hopper is incorporated in electrode itself in the form of flux cored electrodes, or sometimes the process may be carried out without using the flux in which case there is no flux covering on the top of the molten metal pool.

Electro gas welding process is used for welding low and medium carbon steels, alloy steels and austenite stainless steels.

Plates from 12.5 to 75 mm thickness can be welded. For thicker plates it is preferable to use electro slag welding instead of electro gas welding because it may be difficult to obtain adequate shielding gas coverage with the latter process.

CO₂ gas is used as an inert gas to protect the welding from atmosphere contamination.

History:

First thick plate vertical welding method was electro slag welding. Demand arose immediately for equipment that would apply the process to thinner sections. Then in 1961, laboratory studies with an electro slag welding machine adapted to feed auxiliary gas shielding around a flux cored electrode that made the vertical welding of 13mm thin plates. This technique is called electro gas welding.

Features:

1. High deposition single pass welding with code quality welds

2. Carriage and rail system to handle vertical seams up to 3m

3. Linear oscillator to weld up to 40mm plate in a single pass

4. Powered lateral travel frame to create an “indoor” atmosphere for high quality site welding

5. Weld thickness ranges from 12mm to 75mm

6. Metals welded are steels, titanium, aluminium alloys

Applications:

Building of Storage tanks, Vertical Vessels, Blast furnaces, Chemical Furnaces, Ship Building, thick walled and large diameter pipes, Bridges etc

Advantages:

1. Weld is better visible to the operator

2. Restarting the weld is quicker

3. Welded joints have better mechanical properties such as impact strength

4. High Welding efficiency with high current / High deposition rate

5. Less angular distortion due to a small number of welding passes

6. The heat – affected zone can be softened and embrittle caused by the welding heat input.

Disadvantages:

1. The weld produced are not as clean and crack free as those produced by electro slag welding

2. It has more porosity particularly for the thicker jobs

3. Incomplete Fusion to One Sidewall is caused by asymmetric thermal conditions such as poor heat distribution and insufficient heat

4. Overlap is caused by weld metal flow out of the joint without melting the base metal

5. Hot cracking can be caused by the partial dissolution of the copper molding shoes, here the cracks are generally at near the surface

Parts to be joined are positioned approximately an inch apart and an electrode (weld wire) guide tube is positioned between the parts. Copper cooling shoes are clamped to the sides, bottom and top of the joint and contain the molten slag and metal during the weld.

After the components are assembled power is applied and the wire is fed through the guide tube. When the wire reaches the start block there is momentary arcing which melts the granulated flux, forms the slag pool and extinguishes the arc. The process is initiated by filling the joint with the flux and starting an arc by short circuiting. The consumable guide tube directs the electrode (welding wire) and conducts the welding current to the molten slag pool. The electrical resistance of the slag pool generates heat which melts the wire, the guide tube and the edges of the two components to be joined. The temperature obtained is approximately 1800 degree Celsius at the surface and 1930 degree Celsius inside under the surface. This much heat is sufficient to fuse the edges of the work pieces and the welding electrode.

As the wire and guide tube are melted by the flux the liquid metal sinks through the slag to the metal pool below and solidifies. Since the slag is less dense than liquid steel, it floats to the top and protects the metal from exposure to air. With continuing addition of weld wire the molten steel fills the gap, solidifies and fuses the two components. The weld is terminated when it reaches the top of the run-out cooling shoes above the rail running surface. Unnecessary weld reinforcement is removed immediately while the weld is hot.

A DC current of 750 – 1000 A is applied from a DC generator with flat volt-ampere. Load voltages generally range from 30 to 55 V, therefore the minimum open circuit voltage of the power source should be 60 V. Speed range of Electro slag welding are 17 to 150 mm/s.

History:

Single pass welding of heavy plates are desired one to avoid multi pass welding techniques. In the early 1950’s Russian scientists announced the single pass vertical welds by the principle of electrically conductive slag. In 1959 Electro slag welding was introduced in United states.

Applications:

• ESW is often used to weld stiffeners’ in structural box columns and wide flanges.
• Manufacture of large Presses and machine tools work with large heavy plates.
• Other machinery applications include kilns, gear blanks, motor frames, press frames, turbine rings, shrink rings, crusher bodies, rebuilding metal mill rolls and rims for road rollers
• Pressure vessels for the chemical, petroleum, marine, and power generating industries

Advantages:

• Electro slag welding can have extremely high deposition rates, but only one single pass is required no matter how thick the workpiece is.
• Unlike SAW or other arc welding processes, there is no angular distortion in ESW because the weld is symmetrical with respect to its axis.
• High Welding Speed and good stress distribution across the weld.

• Joint preparation is often much simpler than other arc welding processes.
• Residual stresses and distortion produced are low
• Flux composition as compared to submerged arc welding (SAW) is very low.

Disadvantages:

• However, the heat input is very high and the weld quality can be rather poor, including low toughness caused by the coarse grains in the fusion zone and the heat-affected zone.
• In Electro slag welding, there is some tendency toward hot cracking and notch sensitivity in the heat affected zone.
• Electro slag welding is restricted to vertical position welding because of the very large pools of the molten metal and slag.
• It is difficult to close cylindrical welds
• Electro slag welding tends to produce large grain sizes.
• Submerged Arc Welding is more economical than electro slag welding for joints below 60 mm.

Percussion or Percussive welding is similar to flash welding except that the arc is produced by the rapid discharge of stored electrical energy across an air gap between the ends of the work pieces to be welded. It belongs to a family of joining techniques generically referred to as percussion or stud welding. The weld is affected by the heat produced by the arc with pressure percussively (rapidly) applied immediately following the electric discharge.

The pieces to be welded are held in two clamps as for flash welding. One of the clamps in stationery while the other is mounted in a slide and backed up against a heavy spring. When the movable clamps is released it advances rapidly towards the fixed clamp carrying the work piece.

As the distance between the ends of the work pieces reduces to less than about 1.5 mm, then the stored electric energy causes intense arcing over the surfaces raising the temperature. As the two parts come together the arc is extinguished due to the percussion blow.

The energy required for causing the discharge may be built up either by the electrostatic method using a capacitor or by the electromagnetic method using a collapsing magnetic field linking the primary and secondary windings of an inductive device or transformer. A Protective gas shield around the weld may be provided when welds of very high quality are desired. The process is used in the butt welding of bars, rods, tubes and pipes. Theses welding machines are built for automatic operation and have pre-set controlled parameters at each stage of the cycle.

History of Percussion Welding:

Percussive arc welding has been around for years, but it’s only recently that technology capability and commercial demand have converged to expose some genuine component production advantages. Percussion welding has the best features of 19th century technology. It is a “heat it and beat it” method that can be readily automated using straight forward electromechanical tooling.

Welding Force:

Welding force may be applied by:

1. Pneumatic,

2. Electromagnetic,

3. Spring force or

4. Gravity (falling weights).

Power Supplies for Percussion Welding:

(i) Low voltage (10 to 150 volts DC) / High voltage (1000 to 6000 volts DC).

(ii) Electromagnetic, Capacitive storage or Inductive storage.

(iii) Low voltage (10 to 35 volts) AC that uses a transformer to furnish the welding voltage.

Advantages of Percussion Welding:

1. The action takes place in a very little time, usually less than 0.1 Seconds

2. It causes very little damage to material close to the weld.

3. Hardened surfaces may be welded without any danger of annealing.

4. As the heat is concentrated at the ends of the work pieces heat balance is not much of a problem.

5. Parts with different thermal conductivities or different masses can be easily welded.

Disadvantages of Percussion Welding:

1. The process cannot be used for welding heavy sections larger than 600 mm².

2. The process is limited to butt welded joint only

Applications of Percussion Welding:

1. Welding of satellite tips to tools

2. Welding of Steel to Cast Iron

3. Welding of Zinc to Steel

4. Welding of Copper to Aluminium etc.,

5. Welding of Studs

6. Join a stranded wire directly to a component pin on axis

Flash butt welding is one of the resistance welding processes employed to join metals. In flash butt welding process, the ends of the piece to be welded are connected to the secondary circuit of a transformer, while one piece is held firmly by a clamping device attached to a stationary platen; the other piece is clamped to a movable platen. The platen travel is continuous starting at the time of flashing and progressing until upset. At upset period the platens are rapidly squeezed together for upsetting, the current may be immediately terminated. The material being joined is clamped rigidly in the dies, and the specimens are separated by a suitable air gap. Then the movable platen is advanced slowly until contact is made.

The surfaces to be welded are allowed to touch when heavy currents pass through the peaks or asperities of the edges providing resistive heat (many short-circuits randomly located over the opposing interfaces) to the edges. This portion of the process is known as the flashing period, its objective being the establishment of a suitable temperature distribution in the work to assure proper forging action during the subsequent upset period of the cycle. These asperities start melting and, at greater velocities, the molten bridges are broken and thrown off as flash particles from joint.

This cycle of the formation and collapse of bridges goes on as the movable platen advances. When the conductive heat was sufficiently heated the metal behind the faying surfaces on either side to ensure adequate plasticity, the flashing current is stopped and surfaces are butted against each other at greater force. This portion of the operation is known as the upset period. This action ensures that the molten metal oxides and other impurities are extruded out of the surfaces to be joined and satisfactory welding takes place.

Basic components of the machine:

1. Clamping Mechanism

2. Forging Mechanism

3. A transformer

(This will reduce the mains supply voltage from 400/500 Volts to a suitable welding voltage between 4 and 12 Volts and make available sufficient current to heat the components being welded. The welding current required varies between 30,000 amps to 80,000 amps depending on cross sectional area of the rail being welded)

History of Flash Welding:

Flash butt welding technique spread too many countries during the 1930s but much of this development work came to a standstill during the war years particularly in the U.K. and on the Continent. However, by 1950 the flash butt welding of rails was common place in all major railroads throughout the world.

Features of Flash Welding:

Basic Metallurgy – Forging Operation

Heat Affected Zone – 40 – 60 mm

Nominal welding transformers power – 600 kVA

Upsetting and stripping force – 800 kN

Typical welding cycle within – 150 s

Advantages of flash welding:

1. Joint obtained is clean, as filler metal is not use in this process.

2. Produces defects free joint. Oxides, scales and other impurities are thrown out of the weld joint due to high pressure applied at elevated temperature.

3. Reduces maintenance costs

4. Faster installation

5. Lowest life cycle cost

6. Saves track time

7. Eliminates corrugation

8. No weld filler material

9. Smaller heat affected zone

10. Smaller annealed zone

11. Consistent hardness

12. Highest fatigue resistance

13. Average life equal to the rail

14. 25% savings over Thermite welding

15. Large cross sectioned shape materials can be welded in a short time

Disadvantages of flash butt welding:

1. The process is suitable for parts with similar cross sectional area

2. Joint preparation is must for proper heating of work pieces to take place

Applications of Flash Butt Welding:

1. Used for producing joints in long tubes and pipes

2. Flash butt welding is widely employed in the automotive, air craft, and several other engineering industries. Some examples of its use on wheel rims for automobiles, long welded rails, etc.

The projection welding process is similar to spot welding except that the welding pressure, and welding current. Hence the welding heat are localized by making projection or embossments on one or both of the work pieces to be joined. Such projections are made at all points where a weld spot is desired.

The projections have a diameter on the face equal to about the thickness of the stock and extend about 60 percentage of the stock thickness above the stock.

Operation:

The welding current is passed through the joint, welding heat is generated at these projections. Under the welding pressure the projections flatten allowing the two surfaces to be joined to come together. The melted projection becomes the weld.

The number of projections made in a joint should permit proper contact between the work pieces at the projections. The ideal number is three as the two sheets will always be in contact at three points. The maximum number of projections that can be satisfactorily handled is about six.

Special attention must be paid to the selection of correct pressing force at the beginning of the welding process. Use of excessive force causes the projection to collapse before the weld pool is created, which increases the contact surface and reduces current density. Variation in tensile strength of the workpiece may make welding more difficult, because it may result in projections of different sizes, in addition to which they flatten in different ways during welding.

When welding several projections at the same time, problems may occur in the heat balance of the joint or in the flattening of the projections. Problems can often be avoided by increasing the distance between projections. The recommended distance is four times the diameter of the projection.

Welding soft materials may be difficult if the workpiece thickness is less than 0.50 mm, because projections may collapse before welding current is applied.

For a successful projection weld the projections made on the parts should have the following characteristics:

1. The projections should be stiff enough to take the squeeze force before the current is passed.

2. The projections should have sufficient mass to heat a spot in the plane surface to welding temperature

3. The projections should collapse during welding without splashing between the sheets being welded.

4. The projections should be properly formed without any partial shearing.

5. It should be possible to form the projections without disturbing the other portions of the component.

Advantages of Projection Welding:

1. More than one spot weld can be made in a single operation, so the operation is very fast.

2. Welding current and pressure required is less

3. It helps in obtaining a satisfactory heat balance in welding of difficult to weld combinations of metals and thickness.

4. Closer spacing of welds is possible

5. Electrodes can be shaped to act as assembly fixtures for mass welding of parts

6. Uniform welds with good finish are produced.

7. Suitable for automation

8. Filler metals are not used. Hence clean weld joints are obtained

Disadvantages of projection welding:

1. Projections cannot be made in thin work pieces.

2. Thin work pieces cannot withstand the electrode pressure

3. Additional operation is required after the welding process is over.

4. Equipment is costlier

Applications of projection welding:

1. A very common use of projection welding is the use of special nuts that have projections on the portion of the part to be welded to the assembly. Also, used for welding parts of refrigerator, condensers, refrigerator racks & grills, bushings, studs, nuts, handles etc..

In principle seam welding is similar to spot welding except that it uses disc shaped electrodes. A current impulse is applied through the rollers to the material in contact with them. The heat generated thus rollers to the material in the pressure from the electrodes completes the weld. If the current is put OFF and ON quickly, a continuous fusion zone made up of overlapping nuggets is obtained and the process is known as Stitch Welding Process.

Unlike spot welding the disc shaped electrodes are not separated after each weld, but maintain continues pressure over the work pieces. The electrode current is timed to flow in pulses so that a row of welds is produced along the interface. Copper alloy electrodes are used to keep the heat at the electrode contact surface to a minimum. The motion of the electrodes and current impulses are so arranged that the weld nuggets overlap forming a gas or liquid pressure tight weld.

The amount of overlap depends on the ratio of current ON and OFF time. The normal overlap is generally 25 to 50 percent.

After the welding is over the electrodes and work pieces are flooded with water to dissipate the heat.

Types of seam welding:

Seam welding may be divided in two main categories namely:

1. Continuous motion seam welding

2. Intermittent motion seam welding

Continuous motion seam welding:

In the continuous motion method of seam welding the electrodes are rotated at a predetermined constant speed and the current impulses timed to get an overlapping weld. Alternatively the work pieces may be moved at constant speed with the electrodes idling under the welding pressure.

Intermittent motion seam welding:

In the intermittent method the work pieces move till the weld position stop for the welding to take place automatically move to the next weld position, stop again for welding and so on. This method is suitable for seam welding in thicker sheets which are too thick to be properly welded by the continuous motion method.

Resistance Seam Welding Wheel:

Types of seam welding joints:

The common types of seam welding joints are:

1. Lap seam weld (Common type weld)

2. Butt seam weld

Advantages of seam welding:

1. A continuous overlapping weld produced by the process makes it suitable for joining liquid or gas tight containers and vessels

2. Efficient energy use

3. Filler metals are not required. Hence no associated fumes or gases. This results in clean welds.

4. Roll welding simply joins two work pieces whereas stitch welding produces gas tight and liquid tight joints.

Disadvantages of seam welding:

1. Requires complex control system to regulate the ravel speed of electrodes as well as the sequence of current to provide satisfactory overlapping welds. The welding speed, spots per inch and timing schedule are all dependent on each other

2. Difficult to weld metals having thickness greater than 3mm

3. Relatively higher current is thus required for seam welding than for spot welding.

4. The work pieces to be welded are over lapped sufficiently to prevent metal flowing out from the edges of the pieces during welding under pressure.

Applications of seam welding:

1. Used to fabricate liquid or gas tight sheet metal vessels such as gasoline tanks, automobile mufflers and heat exchangers.

2. The production of seam welded pipes and tubing (Butt seam weld).

In spot welding the weld is effected by the heat produced due to resistance to the flow of current through two or more overlapping work pieces held pressed together between the electrodes. This is the simplest form of resistance welding and does not pose any problem for welding sheets ranging u to 12.5 mm in thickness. The majority of spot welding is however done with metal pieces less than 6 mm thick.

For best results the surfaces to be welded must be free from scales and foreign matter. Spot welds should not be made too close to the end of a workpiece or to each other. When a spot weld is attempted too close to the edge of the work pieces molten metal may flow out of the weld zone. Similarly if two spot welds are made too close to each other electric current may be shunted through a parallel path provided by an adjacent weld.

The resulting weld nugget is typically 5 to 10 mm in diameter, with a heat affected zone extending slightly beyond the nugget into the base metal.

Spot welding machines are available in three different varieties:

1. Stationary single spot welding machines

             a. Rocker arm type

             b. Direct pressure type

2. Portable single spot welding machines

3. Multiple spot welding machines

Rocker arm type:

The rocker arm type is the simplest and cheapest but is limited to smaller sizes. It employs rocking motion of the upper arm for applying pressure and raising and lowering of the upper electrode.

Direct pressure type:

The direct pressure type employs a straight line motion of the upper electrode along the face of the machine column and can be used for larger sizes.

Electrode Tip shape:

The size and shape of the weld spot is determined by the electrode tip, the most common electrode shape being:

1. Round (most common)

2. Hexagonal

3. Square and other shapes

Spot Gun Machines:

When the size of the work pieces to be welded increases, then spot welding machines are used on that occasion. A large number of welding guns are available for the purpose. These portable spot welders are connected to the transformers through a long cable and may be taken to the spot where welding is to be done. The electrodes used should have a contact area which gives the desired current density through the work pieces.

Welding pressure in these guns may be applied manually, pneumatically or hydraulically depending up on the size and shape of the gun.

Advantages of Resistance Spot Welding:

1. Adaptability for automation in high rate production of sheet metal assemblies

2. High speed

3. Economical

4. Dimensional accuracy

Limitations of Resistance Spot Welding:

1. Difficulty for maintenance or repair

2. Adds weight and material cost to the product, compared with a butt joint

3. Generally have higher cost than most arc welding equipment’s

4. Produces unfavourable line power demands

5. Low tensile and fatigue strength

6. The full strength of the sheet cannot prevail across a spot welded joint

7. Eccentric loading condition

Resistance welding is a process in which two or more parts are welded by the coordinated and regulated use of heat and pressure. The heat in the resistance welding process is generated by the resistance offered by the work pieces to the flow of low voltage high density electric current.

The pressure for welding is applied through the contacting electrodes. It may be obtained by mechanical means, springs, air pressure or hydraulically with the help of a pressure cylinder and piston arrangement. The pressure generally used range from 30 to 55 MPa.

The high density current is produced by using transformer in the welding machine circuit. The transformer reduces the voltage to around 4 to 12 volts and raises the amperage to produce a current density of 45 to 52 VA / mm² of the area to be welded based on a time of around 10 seconds.

AC current has been found to be most convenient for resistance welding because it is possible to obtain any desired combination of current and voltage by using suitable transformer with different settings.

History:

Sometime in the year 1885, Professor Thompson invented a process called electric resistance welding. He discovered that to weld metals together, one could fire an electric current through the metals while they were tightly clamped together. When the current passed through the metals, it would create such a high heat that the metals would melt and run together and a weld would be made.

The total heat generated in the work pieces and electrodes can be expressed as:

H = I² R t K

H = I V t K

Where,

H – Heat generated in the workpiece, Joules

I – Current, Amperes (5000 to 20000 A), although voltage is low (below 10 V)

R – Resistance, Ohms

t – Time over which the current is supplied, seconds (0.1 to 0.4 sec)

V – Voltage, volts

K – A correction factor to account for the loss of heat due to radiation, conduction and convection from the electrodes and work pieces.

The current flowing to develop the joint determines the rate of heat generation at the joint. High welding currents are required in resistance welding to develop the necessary heat as the ohmic resistance of any resistance welded joint is low.

Process Variables:

A typical resistance welding cycle consists of:

1. Squeeze

2. Weld

3. Hold periods

Operation:

The pressure is applied and built up to the desired value over a period of time. After the proper pressure value has been attained, current of required magnitude is passed for pre-set period of time. Further the interface resistance between upper sheet and lower electrode and lower sheet are kept low by proper contact pressure. Heat is rapidly generated at this interface where it is trapped and slowly dissipated. In a properly controlled weld the welding heat is first generated at pin points on sheet interfaces and then subsequently a weld nugget is formed gradually.

Weld Defects:

Advantages:

1. Resistance welding is a production welding process most suitable for light gauge sheets which can be overlapped

2. Operation is quite fast, practically all metals can be resistance welded.

3. Cleaner workspace with less contaminants