ELECTRIC WELDING PART – 06 – FLASH WELDING - ADVANTAGES & DISADVANTAGES

FLASH WELDING

Flash welding is a resistance welding process in which joint is produced over the entire area of the abutting surfaces simultaneously. 

Flash welding is used for joining many ferrous and non-ferrous alloys except for alloys of cast iron, lead, zinc, antimony and bismuth. Flash is produced during the process hence it is named as flash welding.

PRINCIPLE

In this method two metals are butted together and current is passed through the joint which heats the ends to red hot temperature. The metals are then separated till an arc is developed between them and the metals begin to melt. The metals are butted under high pressure with the current still flowing.

EQUIPMENT

The flash welding system consists of

(a) The main frame to which the welding machine is attached. On this a fixed platen and movable platen is attached.

(b) Movable platen mounted on the machine

(c) Two clamps hold the job

(d) Welding transformer

(e) Controls for controlling the motion of the movable platen and temperature for flashing.

In flash welding process, the ends of the piece to be welded are connected to the secondary circuit of a step-down transformer to get a high current and low voltage.

PROCESS

1. One piece is held firmly by a clamping device attached to a stationary platen (test bench) and the other piece is clamped to a movable platen.

2. The surfaces to be welded are allowed to touch when heavy currents pass through the peaks of the edges which provide resistive heat to the edges.  

3. This duration of this process period is known as the flashing period.

4. The objective of is to make a suitable temperature distribution in the work piece and to assure proper forging action during the subsequent upset period of the cycle.

5. The platen travel is continuous starting at the time of flashing and progressing until upset.

6. The edges start melting and, at greater velocities, the molten bridges are broken and thrown off as flash particles from joint.

7. This cycle of the formation and collapse of bridges goes on as the movable platen advances.

8. When the conductive heat was sufficiently heated the metal behind the faying surfaces [the meaning of a faying surface is one of the surfaces that are in contact at a joint] on either side to ensure adequate plasticity, the flashing current is stopped and surfaces are butted against each other at greater force.

9. This portion of the operation is known as the upset period.

10. 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.

ADVANTAGES

1. Filler metals are not required.

2. The impurities are removed by flashing and metal ejcection.

3. This process eliminates corrugation (corrugation means the act of shaping into parallel ridges and grooves)

4.  Smaller heat affected and annealed zone

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

DISADVANTAGES

1. Similar cross section work pieces can be welded.

2. Metal is lost during flashing and upsetting.

3. Zinc and its alloys, cast iron, lead and its alloys are not flash welded.

4. For removal of flash and molten particles a special equipment is required which involves heavy expenses

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

APPLICATIONS

1. Used for producing joints in long tubes and pipes

2. In petroleum industry oil drilling pipe is attached with fittings by flash welding.

3. It is applied in automotive and aircraft industry.

4. It is applied in household appliances, refrigerators and framing implements.

5. Dissimilar metals may be welded provided their characteristics 
are similar.

ELECTRIC WELDING - PART - 05 - PROJECTION & SEAM WELDING - ADVANTAGES & DISADVANTAGES

PROJECTION WELDING

It is a modified form of spot welding. Projection welding is an electric resistance welding process that uses small projections, embossments, or intersections on one or both components of the weld to localize the heat and pressure.

INTRODUCTION

In this process localization of heat between the ends are affected by projection. The metal surfaces to be joined, have projection in which the current is passed. The welding takes place only at these points.

PROCESS

1. The machines are basically flat platens with T slots for the attachment of special tools as shown in fig.

2. In this method protrusions (protrusion means something that bulges out) are pressed on one of the sheets to be welded and exact location of the weld is determined. 

3. Sheet metal is first put through a punch press that makes small projections or buttons in the metal.

4. The projections are made with a diameter on the face equal to the thickness of the stock and extend from the stock about 60% of its thickness.  Such projection spots or ridges are made at all points where a weld is desired.

5. When current is passed and the electrode pressure is applied the projection collapses and the sheets are welded together. The projections are flattened during the welding.

ADVANTAGES

1. Filler metals are not used.

2. It is in effect, a form of multi-spot welding in which a number of welds are made simultaneously.

2. Welding current and pressure required is less and is .

3. This process is suitable for automation.

DISADVANTAGES

1. Equipment cost is very high.

2. It is not suitable for thin work pieces.

3. Thin work pieces cannot withstand the electrode pressure.

APPLICATIONS

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.

2. This welding process is used for steel radiator, coupling elements, brake shoes, tin-plate tank handle etc.

3. This process is also used for crosswire welding and for parts where the ridges are produced by machining.

SEAM WELDING

Spot welding are spaced so closely that they overlap each other, giving rise to seam welding. Seam means joint consisting of a line formed by joining two pieces.

INTRODUCTION

Seam welding is similar to the spot welding with the difference that the electrodes are in the form of rollers. This used for joining two metal sheets and to end in a continuous line.

PROCESS

1. To obtain sound weld the work pieces to be joined are cleaned to remove dirt, grease and other oxides either chemically or mechanically.

2. The welding electrodes are motor driven wheels as opposed to stationary rods.

3. The work pieces are overlapped and placed firmly between two wheels shaped copper alloy electrodes, which in turn are connected to a secondary circuit of a step-down transformer.

4. The electrode wheels are driven mechanically in opposite directions with the work pieces passing between them, while at the same time the pressure on the joint is maintained.

5. Welding current is passed in series of pulses at proper intervals through the bearing of the roller electrode wheels.

6. As the current passes through the electrodes, to the work piece, heat is generated in the air gap at the point of contact (spot) of the two work pieces.

7. It is desirable to have intermittent flow of current to prevent overheating which may occur resulting in burning of the sheets.

8. Under the pressure of continuously rotating electrodes and the current flowing through them, a series of overlapping spot welds are made progressively along the joint.

9. The number of spots per cm varies between 2 and 4. Such a weld is normally gas and liquid tight.

10. The weld area is flooded with water to keep the electrode wheels cool during welding.

ADVANTAGES

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

2. The production of single seam weld and parallel seams can be obtained simultaneously.  

3. Gas tight as well as liquid tight joints can be made and the overlap is less than spot or projection welding.

4. It can be automated.

DISADVANTAGES

1. It requires complex control system to regulate the travel speed of electrodes as well as the sequence of current to provide satisfactory overlapping welds.

2. The welding process is restricted to a straight line or uniformly curved line.

3. The metal sheets having thickness more than 3mm can cause problems while welding.

APPLICATIONS

1. Seam welding is extremely useful for fabricating all types of exhaust systems, barrels etc.

2. This process can produce continuous pressure tight seams at high speeds.

3. It is used for oil transformers, refrigerators, evaporators and condensers, aircraft tanks, paint and varnish containers etc. 

ELECTRIC WELDING – PART – 04 – SPOT WELDING & ADVANTAGES AND DISADVANTAGES

SPOT WELDING

Spot welding as the name implies, is carried out in which two or more metal sheets are welded by holding them between metal electrodes.

Spot welding is "the welding of overlapping pieces of metal at small points by application of pressure and electric current."

INTRODUCTION

Spot welding basically joins two pieces of metal using the heat from an electric current. The two pieces of metal are pressed together with electrodes on either side.

“It is a welding process that involves the generation of heat by passing current through the resistance induced by the contact between the surfaces of two or more metals. Small pools of molten metal are made at the weld area as high current passes through the metal”.

PROCESS

1. Spot Welding works by holding sheets of metal together with two copper alloy electrodes.

2. The welding current depends upon the thickness and the composition of the plates. It varies from 1,000 to 10,000 amperes and this current may be flow from a fraction of a second to several seconds.

3. Clamping (hold tightly to something else) two or more pieces of sheet metals between two copper or copper-alloy welding electrodes.

4. A large electric current flows through the electrodes and melts the metal into a “spot”, and welds the two pieces together.

5. The current and the heat generation are localized by the form of the electrodes.

6. The sheet metal's material type and thickness, along with the type of electrodes used, will help to determine the right amount of current required to create a spot weld.

7. As the current passes through one electrode and the job to the other electrode, the area in contact of the electrodes gets heated to 800 to 900 degree centigrade.

8. When the temperature is reached the welding current cut off.

9. Then mechanical pressure is then applied by the electrodes to forge the weld. Finally the electrodes open.

10. The weld nugget size is usually defined by the electrode tip contact area.

ADVANTAGES

1. Spot welding is quick and easy.

2. There is no need to use any fluxes or filler metal to create a joint by spot welding, and there is no dangerous open flame.

3. This process does not require skilled labours.

4. Spot welding can be used to join many different metals, and can join different types to each other.

5. Sheets as thin as 1/4 inch can be spot welded, and multiple sheets may be joined together at the same time.

DISADVANTAGES

1. If the current is not strong enough, hot enough or the metal is not held together with enough force, the spot weld may be small or weak.

2. Warping and a loss of fatigue strength can occur around the point where metal has been spot welded.

3. The appearance of the join is often rather ugly, and there can be cracks. The metal may also become less resistant to corrosion.

APPLICATIONS

1. Steel, brass, copper and light alloys can be joined by this method, which forms a cheap satisfactory substitute for riveting.

2. Spot welding is the predominant joining process in automotive industry for assembling the automobile bodies and large components.

3. It is also widely used for manufacturing of furniture and domestic equipment etc.

ELECTRIC WELDING PART – 03 – RESISTANCE BUTT WELDING & ADVANTAGES AND DISADVANTAGES

Resistance welding technology is widely used in manufacturing industry for joining metal sheets and components.

BUTT WELDING

The meaning of butt is to place a work piece end to end without overlapping.

In Butt welding the faces of the components are placed end to end, heated electrically and pressed together.

INTRODUCTION

A butt-welding joint is made by gradually heating up by using an electric current to weld the two ends and then joining them under a specific pressure. 
This process is very suitable for prefabrication and producing special fittings.

PROCESS

1. In this welding process the one part of the work piece is held in a fixed clamp and the other part of the same piece is held in a moving clamp.

2. Let two rods be brought together such that they touch themselves.

3. Then heavy current is passed from the welding transformer which creates the current flowing through the contact resistance between the ends bringing them closer due to the weld. Since the melting of carbon is higher than the rods, the clamps will not melt.

4. Extra pressure is now applied, so that they are pushed into each other. Due to pressure of the spring, molten metal is forced out producing bulged joint.

5. Then, on disconnecting the supply, the rods cool and the weld is formed.

ADVANTAGES

1. Butt welding joint is permanently leak proof and strong.

2. The butt welding commonly have a smooth surface and gradual directional changes

3. It will decrease the pressure losses and turbulence of the system, and protect from the corrosion and erosion.

4. The butt welding have a lower cost in budget.

5. The butt welding joints have a smaller space in the system.

DISADVANTAGES

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

1. This method is used for welding rods, pipes, tubes, strips and wires.

2. The materials which can be welded by this method are aluminum alloys, brass, copper, nickel, stainless, low carbon and high carbon steels and gold.

3. This method is highly used in making shopping trolley and metal basket. 

ELECTRIC WELDING PART – 02 – WELDING PROCESSES

There are two methods of Electric Welding.

(a) Non-fusion or pressure welding

This process involves heating of work-pieces to the temperature range in which the base metal of the work-pieces becomes plastic state and the work-pieces joined together by applying pressure them. Filler metal is not used in pressure welding.

(b) Fusion or non-pressure welding                      

In this process heating of work-pieces to be joined to a temperature above the melting point of the metal of the work-pieces and the work-pieces are joined together without the application of any pressure. This process requires additional filler metal through an electrode.

There are about 35 different welding and brazing process and several soldering methods, in use by the industry today.

CLASSIFICATION OF WELDING PROCESSES

(1) RESISTANCE WELDING

(a) Spot welding                (b) Seam welding

(c) Projection welding       (d) Resistance butt welding

(e) Flash butt welding       (f) Percussion welding

(2) ARC WELDING

(a) Carbon Arc welding             (b) Plasma Arc welding

(c) Shield Metal Arc Welding     (d) T.I.G. Welding

(e): M.I.G. Welding

(3) GAS WELDING

(a) Air Acetylene               (b) Oxy Acetylene

(c) Oxy Hydrogen Welding

(4) THERMO CHEMICAL WELDING

(a) Thermit welding           (b) Atomic welding

(5)  RADIANT ENERGY WELDING

(a) Electric Beam Welding        (b) Laser Beam Welding

(6) SOLID STATE WELDING

(a) Cold welding                (b) Diffusion welding

(c) Forge welding              (d) Fabrication welding

(e) Hot pressure welding   (f) Roll welding

RESISTANC WELDING

PRINCIPLE OF ARC WELDING

Resistance welding is a fairly simple heat generation process. The passage of current through a resistance generates heat.

The general heat generation formula for resistance welding is: Heat = I² x R x t x K

where, “I” is the weld current through the work-pieces, “R” is the electrical resistance in ohms of the work-pieces and “t” is the weld time, and “K” is a thermal constant.

The weld current (I) and duration of current (t) are controlled by the resistance welding power supply.

The resistance of the work-pieces (R) is a function of the weld force and the materials used. 

The thermal constant “K” can be affected by part geometry, fixturing and weld force.

ADVANTAGES OF RESISTANCE WELDING

1. Very short process time.

2. No consumables, such as brazing materials, solder, or welding rods.

3. Operator safety because of low voltage.

4. Clean and environmentally friendly.

5. A reliable electro-mechanical joint is formed.

6. Parent metal is not harmed.

7. Both similar and dissimilar metals can be welded.

8. Heat is localized when required.

9. Lesser skill is required.

10. Difficult shapes and sections can be welded.

DISADVANTAGES OF RESISTANCE WELDING

1. High initial cost

2. High maintenance cost

IMPORTANT TERMS IN WELDING

1. Amperage - The strength of an electrical current measured in amperes or the magnitude of an electric current measured in amperes, especially the rated current of an electrical component or device

2. Arc voltage – The voltage across the welding arc. Arc voltage has an important effect on the weld bead shape and the depth of penetration; the precise effect being dependent on the joint preparation. 

3. Arc Current – The current that flows through a circuit while a weld is being made.

4. Arc Length – The distance from the tip of the electrode or wire to the work piece.

5. Arc Blow – The deflection of an electric arc from its normal path because of magnetic forces.

6. Duty cycle – The percentage of time during a time period that a power source can be operated at rated output without overheating.

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

8. Arc Time – The time during which an arc is maintained.

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

10. Hold time – The time during which pressure is applied at the point of welding after the welding current has created to flow.

ELECTRIC WELDING PART – 01 – SHORT HISTORY OF ELECTRIC WELDING

Electric arc welding was first patented in France in mid 1800s, by a Russian scientist, Nikolai N. Benardos who was working in the Cabot laboratory with Auguste De Meritens. Later, Benardos and his colleague Stanislaus Olszewski patented an electrode-holder in Britain, and in the USA in late 1800s.

During the middle Ages, the art of blacksmithing was developed and many items of iron were produced which were welded by hammering.

The production of an arc between two carbon electrodes using a battery was invented by Sir Humphry Davy in 1800.

Edmund Davy of England is credited with the discovery of acetylene in 1836.

Edmund Davy was born on 1785 in UK.

He was a professor of Chemistry at the Royal Cork Institution from 1813 and professor of chemistry at the Royal Dublin Society from 1826.

In 1836, Edmund Davy discovered a gas which he recognized as "a new carburet of hydrogen."

It was an accidental discovery while attempting to isolate potassium metal.

By heating potassium carbonate with carbon at very high temperatures, he produced a residue of what is now known as potassium carbide, which reacted with water to release the new gas. 

He died on 5th November, 1857 in Ireland.

He lived for 72 years in this planet and even today he lives in the form of acetylene. 

CASTING

Casting is a 6000-year-old process.

Casting is a manufacturing process in which a liquid material is usually poured into a mould, which contains a hollow cavity of the desired shape, and then allowed to solidify.

The solidified part is also known as a casting, which is ejected or broken out of the mould to complete the process.

METHODS OF JOINING METALS

Soldering, brazing, and welding are different methods of joining two or more pieces of metal and selected other materials. They are also methods used to fill gaps in metal parts.

SOLDERING

Soldering is a process in which two or more metal items are joined together by melting and then flowing a filler metal into the joint—the filler metal having a relatively low melting point.

Soldering is a group of joining processes that produces joining of base materials by heating to the soldering temperature and using a filler metal with a liquidus not exceeding 450°C (840°F) and below the solidus of the base materials. When soldering, the base material is not melted.

Metals that can be soldered include gold, silver, copper and iron. The filler, called solder, melts. When it solidifies, it is bonded to the metal parts and joins them.

[The liquidus temperature is above which a material is completely liquid]

BRAZING

Brazing joins two metals by heating and melting a filler (alloy) that bonds to the two pieces of metal and joins them.

The filler obviously must have a melting temperature below that of the metal pieces.

Brazing is a group of joining processes that use a filler metal with a liquidus above 450°C (840°F) and below the solidus of the base material.

When brazing, the base material is not melted.

Brazing can join dissimilar metals such as aluminum, silver, copper, gold, and nickel.

Flux is often used during brazing.

[Solidus is the highest temperature at which an alloy is completely solid]

WELDING

Welding is a process by which two similar or dissimilar metals may be joined by heating them to a suitable temperature with or without the application of pressure and with or without the use of filler metal.

The filler metal has its melting point either approximately the same as that of the workpiece metal or little lower, but is normally above 430 degree centigrade.

REVETTING

A rivet is a permanent mechanical fastener.

A rivet consists of a smooth cylindrical shaft with a head on one end.

The end opposite to the head is called the tail.

On installation the rivet is placed in a drilled hole, and the tail is upset, or deformed, so that it expands to about 1.5 times the original shaft diameter, holding the rivet in place.

To distinguish between the two ends of the rivet, the original head is called the factory head and the deformed end is called the shop head.

COMPARISON BETWEEN WELDING TO REVETTING AND CASTING

1. Welding is economical than revetting and casting  

2. Welding design involves less cost.                        

3. Welding structure is much lighter is weight.

4. Old and worn out structure can be easily renovated and reinforced by welding.

5. Welding is more effective in pressure vessels and tanks.

CLASSIFICATION OF METALS

(a) Ferrous metals and (b) Non Ferrous metals.

FERROUS METALS -The metals in which iron is the principle element is known as ferrous metal. These are directly attracted by magnets. Iron is extracted from iron ore.

The important ores are, Haematite, Magnetite, Iron pyrites, Siderite, Limonite etc. E.g: Cast iron, wrought iron, etc.

NON FERROUS METALS - The metals which do not contain iron is called non-ferrous metals. E.g: copper, Aluminium, Tin, Lead

TYPES OF WELDING

1. AUTOGENOUS WELDING

Similar metals are joined with the help of filler rod of the same metal.

In this method mild steel is joined with mild steel and cast iron with cast iron only.

2. HOMOGENEOUS WELDING

In homogeneous welding processes, the filler material used to provide the joint is the same as the parent material.

3. HETEROGENEOUS WELDING

Dissimilar metals are joined. The metals being joined are brought upto the critical temperature or plastic state and the filler rod is used. The filler rod is of such metal which has its melting point less than the parent material.

ADVANTAGES OF WELDING

1. Freedom in choice of design is possible in welding.

2. Welding can be automatic and mechanized.

3. Portable welding equipment is available and can be taken to the site easily.

4. Similar and dissimilar metals can be welded easily.

5. With modern welding techniques all alloys can be welded with good finish.

DISADVANTAGES OF WELDING

1. The ultra violet rays of welding are very harmful for the eyes.

2. The fumes and gases are harmful for the human being.

3. For having good weld the operator must be a skilled one otherwise the job once it gets spoil it cannot be rectified.