Fundamentals of Electric Resistance Welding

Fundamentals of Electric Resistance Welding –

The two factors or variables mainly responsible for resistance welding are 1.The generation of Heat at the place where two pieces are to be joined. 2. The application of pressure at the place where a weld joint is to be formed.

1. Heat The heat, H, for electrical resistance welding is generated by passing a large electrical current (of the order of 3000 to 100,000 Amps with a voltage between 1 and 25 volts) through two pieces of metal that are touching each other.

H α I2RT ...(i) where H is the heat generate Indicate In joules,I is the current in root mean square amperes,R is the resistance in ohms, T is the time (from fraction of a second to a few seconds) of current flow through the pieces to be welded.

Current, I With other parameters kept constant, the temperature in resistance welding is regulated by controlling the magnitude and timing of the welding current. Enough welding current is required to heat the metal pieces being welded to their plastic state

The current is obtained from a step down transformer. The magnitude of current may be controlled through taps on the primary of the transformer or by an autotransformer that varies the primary voltage supplied to the main transformer. Low welding current does not provide proper fusion whereas if welding current is too high, the entire thickness of the work metal between the electrodes is heated to the plastic state by the time the weld zone reaches the fusion temperature, and the electrodes embed themselves deeply into the metal. As the current/current density* is increased, the weld time can be decreased sufficiently to produce a weld without overheating the electrode contact surfaces. As the welding current increases, the nugget diameter, breaking load of welded joint and the electrode indentation into the workpieces, all, increase. In resistance welding, three types of current supply systems generally are used

(i) AC systems.(ii) DC systems.(iii) Stored energy current systems. By far the majority of resistance welding machines operate on single phase alternating current of the power line frequency, usually 50 cycles second. A single phase

transformer converts the power line voltage to a low voltage and provides the high currents needed for welding.

High frequency resistance welding is used for applications of continuous seam or butt seam welding. The welding current frequencies are of the order of 450,000 cycles per second. In DC systems, energy is delivered directly from the power line and rectifier to direct current on the secondary side of the welding transformer

Stored energy systems are: storage batteries, electromagnetic type, the homopolar generator and capacitor type. Capacitor stored-energy type involves charging a group of capacitors from a high-voltage rectifier unit and subsequent discharge of the energy from the capacitors through a welding transformer.

Resistance, R

The total resistance of the system between the electrodes consists of

(a) the resistance of the workpieceR1.

(b) The contact resistance between the electrodes and the work, R2, and

(c) The resistance between the faying surfaces of the two metal pieces to be welded

together, R3.

In order to obtain a sound weld and to avoid overheating of the welding electrodes, R1

and R2 should be kept as low as possible with respect to resistance R3.

* R1, the resistance of the workpiece, depends upon the nature of the material and its thickness. It cannot be changed otherwise. If the workpiece material has low electrical resistance, such as aluminium, it requires very high currents in order to produce the required welding temperature and hence a proper weld.

* R2, the contact resistance between the electrode and the workpiece can be minimized by (a) Keeping the electrode tip and the workpiece surface properly cleaned.(b) Using the welding electrodes of highly conductive materials such as Cu- Cd or Cu-Cr alloys.

(c) Controlling the shape and size of the electrodes.(d) Using the proper pressure between the electrodes and the workpieces.

* R3, the resistance between the contacting surfaces of the two workpieces, varies with the quality of the surfaces. Surfaces that have not been cleaned and possess scale, dirt or other contaminants on them offer more resistance to the flow of welding current. Smooth workpiece surfaces and high electrode pressures reduce resistance RJ. * Overheating of the welding electrodes is avoided by circulating either water or a refrigerant through them. The main aim is to obtain a sound weld without overheating either the electrodes or the workpieces.

Time, T

Four definite segments or periods of timing are set up on a resistance spot welding

machine during one welding cycle

1. Squeeze time.

2. Weld time.

3. Hold time.

4. off time.

1. Squeeze time. It is the time between the initial application of the electrode pressure

on the work and the initial application of current to make the weld. During this period the

upper electrode comes in contact with the workpiece and develops full electrode force.

At the end of the squeeze time, the welding current is applied.

2. Weld time. During this period the welding current flows through the circuit, i.e., it

enters from one electrode, passes through the work pieces and goes out from the

second electrode.

3. Hold time. It is the time during which force acts at the point of welding after the last

impulse of welding current ceases. The electrode pressure is maintained until the metal

has somewhat cooled.

4. Off time. It is the interval from the end of the hold time to the beginning of the

squeeze time for the next (resistance) welding cycle. In automatic machines all these

segments of times are controlled / automatically whereas in manually operated

machines, only the weld time is controlledautomatically and the remaining time periods

are adjusted by the operator himself.

Weld time can be controlled automatically by using a suitable (electronic) timer. Weld

times range from one-half cycle of 50 cycle frequency for thinnest sheets to as long as

several seconds for thicker plates, depending somewhat upon the metal being welded.

There are a number of types of spot welding cycles which incorporate different

elements. Some of them are explained below:

1. Just before the weld time the electrode pressure is reduced a little in order to

intentionally increase the resistance between the faying surfaces of the workpieces.

This will help raise temperature quickly at the place of welding.

2. During the Hold Time, electrode pressure is increased in order to forge the weld

metal as it cools. This will improve weld strength.

3. Post heat current is applied along with the forge pressure. This cycle, the welding

force usually is maintained until the post heat current is applied after which it is

increased to the forging force. Such a cycle is used chiefly for grain refinement on

hardenable carbon and alloy steels and is not used on low carbon steels.

Pressure or Electrode Force Electrode force is the force applied to the workpieces by the electrodes during the welding cycle. Pressure exerted on the workpieces by the welding electrodes does the following: (i) It brings the various interfaces into intimate contact and thus affects the contact resistance between the two workpieces.(ii) It ensures the completion of the electrical circuit between the electrodes and through the work.(iii) It permits the weld to be made at lower temperatures.(iv) It provides a forging action and thus reduces weld porosity. If too little electrode force is used, the contact resistance between the two workpieces is high and surface burning and pitting of the electrodes may result. If too high electrode force is used, it decreases the contact resistance of the work metal and therefore reduces the total heat generated between the faying surfaces of the work pieces by the welding current. Too high electrode force may also squeeze softened hot metal between the faying surfaces or the workmay be indented by the electrodes. Pressure on the work pieces is exerted by the electrodes extending from the arms of the welding machine. Besides this, the other junctions performed by electrodes are: (i) They carry the current which passes through and generates heat at the place where the two workpiece are in pressed contact. (ii) Depending upon the area of the electrodes face or tip, they determine the current density in the weld zone. *

(iii) They dissipate the heat from the weld zone and thus prevent surface fusion of the work.

Variables in Resistance Welding - Variables commonly encountered and considered carefully by both the design and welding engineers are: 1. Current.2. Electrode pressure. 3. Welding time.4. Human element.5. Welding machine characteristics.6. Type and condition of machines.7. Conditions of electrodes and arms.8. Condition of the material and surfaces of material. 9. Throat depth.10. Throat height.

Advantages of Resistance Welding - (i) Fast rate of production. (ii) No filler rod is needed. (iii) Semi automatic equipments. (iv) Less skilled workers can do the job. (v) Both similar and dissimilar metals can be welded. (vi) High reliability and reproducibility are obtained. (vii) More general elimination of warping or distortion of parts

Disadvantages of Resistance Welding - (i) The initial cost of equipment is high. (ii) Skilled persons are needed for the maintenance of equipment and its controls. (iii) In some materials, special surface preparation is required. (iv) Bigger job thicknesses cannot be welded

Applications of Resistance Welding -Resistance welding is used for(i) Joining sheets, bars, rods and tubes. (ii) Making tubes and metal furniture. (iii) Welding aircraft and automobile parts. (iv) Making cutting tools. (v) Making fuel tanks of cars, tractors etc. (vi) Making wire fabric, grids, grills, mash weld, containers etc.

Spot Welding - Spot welding came into use in the period 1900-1905. It is now the most widely used of resistance welding processes. Spot welding is employed for joining sheet to sheet, sheets to rolled sections or extrusions, wire to wire, etc. Spot welding is used for joining relatively light gauge parts (up to about 3 mm thick) superimposed on one another (as a lap joint).

Definition Spot welding is a resistance welding process in which overlapping sheets are joined by local fusion at one or more spots by the heat generated, by resistance to the flow of electric current through workpieces that are held together under force by two electrodes, one above and the other below the two overlapping sheets.

Procedure The steps involved in making a spot weld are listed below but before spot welding one must make sure that (i) The job is clean; i.e. free from grease, dirt, paint, scale, oxide etc.(ii) electrode tip surface is clean, since it has to conduct the current into the work with as little loss as possible. Very fine emery cloth may be used for routine cleaning.(iii) Water is running through the electrodes in order to (a) Avoid them from getting overheated and thus damaged,(b) Cool the weld. (iv) Proper welding current has been set on the current selector switch.(v) Proper time has been set on the weld timer

Step 1. Electrodes are brought together against the overlapping work pieces and pressure applied so that the surfaces of the two workpieces under the electrodes come in physical contact after breaking any unwanted film existing on the workpieces.

Step 2. Welding current is switched on for a definite period of time. The current may be of the order of 3000 to 100,000 A for a fraction of second to a few seconds depending upon the nature of material and its thickness.As the current passes through one electrode and the workpieces to the other electrode, a small area where the workpieces are in contact is heated. The temperature of this weld zone is approximately 815°C to 930°C. To achieve a satisfactory spot weld, the nugget of coalesced metal should form with no melting of the material between the faying surfaces. Step 3. At this stage, the welding current is cut off. Extra electrode force is then applied or the original force is prolonged. This electrode force or pressure forges the weld and holds it

together while the metal cools down and gains strength. Step 4. The electrode pressure is released to remove the spot welded workpieces

Heat Shrinkage in Spot Welding - Spot welding usually leaves slight depressions or indentations on the workpieces and these are often undesirable on the show side of the finished products such as refrigerators. These depressions are distinctly different from the electrode marks that result when the electrodes embed themselves in the work because of improper control. These depressions are formed because of heatshrink age. As the work is heated it tends to expand in all directions. Electrodes being under pressure prevent any vertical movement of material, which, then, naturally expands in the horizontal plane and causes a slight ridge. After welding, as the work cools, contraction takes place in the vertical plane, i.e., along the line of the least resistance, thereby resulting in the concave surface or the depression. These depressions can be minimized by the following techniques.

(a) Use larger sized electrode tips on the show side.(b) On the show side use the electrode having depression. The hot metal conforms to this depression and, since it is above the surface of the surrounding material in the job, it is more easily removed. This method however produces a depression around the periphery of the weld.(c) Arrangement helps in obtaining a minimum of marking on show side by affecting current distribution in that part.

Spot Welding Equipment - IntroductionSpot welding machines may be classified as follows on the basis of mechanical construction: 1. Rocker-arm Machines, 2. Press-type machines, 3. Portable machines or Guns, and 4. Multiple-electrodes machines. A standard spot welding machine possesses the following: Basic Elements: 1. The frame, which is the main body of the machine and houses the transformer and tap switch. 2. An upper arm which is movable and a fixed lower arm.3. Welding electrodes. 4. The electrical circuit consisting of a step down transformer which reduces the voltage and proportionally increases the current. 5. The different controls that adjust the magnitude of current, length of welding time, the contact period and the flow of cooling water.

Power Sources for Spot Welders - Spot welding machines are of (a) Direct energy type(b) Stored energy type (a) Direct energy type In a direct-energy machine, single phase or three phase alternating current is drawn ordinarily from 220 volt or 440 volt in-plant power lines, is stepped down to about 2 to 20 volts and is fed directly to the electrodes as each weld is made. The choice between single-phase and three-phase supply is based mainly on the machine capability and on initial, operating and maintenance costs. Most spot welding machines belong to the single-phase, direct energy group. Single-phase machines are simpler than three phase machines and they cost less to buy, install and maintain. Single-phase machines, however, have two disadvantages: (i) Low power factor (about 40 to 50%) against 80% or more for three-phase machines. (ii) High kVA demand-about double that for three-phase machines of the same capacity. If the welding machine load is small, as in many cases, the amount of electric power consumed in welding may be insignificant. Single-phase machines are transformers that change line voltage to lower values in order to intensify the current. Since resistance welding must be done with current flowing in one direction through the workpiece, pulsed direct current is used. This is obtained by placing ignitron tubes in the primary supply line to the transformer. Three-phase machines draw electric power from all three phases of the power-supply line and are of two general types namely frequency-converter and rectifier. A three-phase machine is used where power supply is inadequate and it is required to weld thicker metal pieces.

Stored-energy Machines Prior to the introduction of the three-phase machines, stored energy machines were commonly used to reduce mains demand. These machines accumulate and store energy and then discharge it across the electrodes when making the weld.The energy may be stored: Electrostatically by charging capacitorsElectromagnetically in the transformer winding Electromechanically (rotating motor-generator set) and electrochemically (batteries). In stored energy machines, power demand is reduced because the energy is stored at lower power demand levels for longer times than used in making the weld.Single-phase power supply is used for small bench type machines and three phase-powers for larger stored-energy welding machines.

Heat Balance in Spot Welding - In order to obtain a proper spot weld between two workpieces, the fusion zones in the two pieces should experience the same degree of heat

and electrode force.The problem arises when two different thicknesses of the same material or same or different thicknesses of two different materials having different thermal and electrical conductivities are to be spot welded. Proper heat balance may be achieved by using the following techniques: (a) When welding two different thicknesses of the same material, e.g. mild steel, use a smaller (electrode) tip area on the side of bigger thickness. This will increase current density on the side of bigger thickness and thus help in obtaining equal degrees of fusion in the two pieces to be welded.

(b) When welding two dissimilar metal sheets of varying conductivity, e.g. those of a high copper content alloy and stainless steel, use a smaller (electrode) tip area on the side of the high conductivity alloy. This helps obtaining equal degrees of fusion by varying current density. Unequal thicknesses and many thickness combinations can be successfully spot welded; but in general, with the introduction of more than two pieces, the effects ofContact resistance,Heat loss,Heat transfer, etc. are encountered.

Spot Weldable Metals - (i) Low carbon steel (mild steel). (ii) Hardenable steels, which, after getting spot welded are treated in an annealing furnace. (iii) High speed steel bits are spot welded to tool shanks for use in lathes, shapers, etc. The tool, after A. Unequal thickness; Band C Multiple thickness getting welded, is annealed before final hardening. (iv) Stainless steelsFerritic stainless steels behave very much as mild steel, however, the pressure should be kept a little longer after welding. Martensitic (cutlery and similar qualities) stainless steel can be treated as hardenable steel as it has pronounced air hardening qualities

Austenitic (non-stabilized) stainless steel requires heating and eventual cooling to take place in the shortest possible time. High capacity spot welding machines are preferred for welding such steels. (v) Coated steels Paint should be removed from the place of spot welding as it is non-conductor of electricity. When tin, zinc and lead coated steel sheets are spot welded, there is a loss of protection in the neighbourhood of the weld as a result of melting and vaporization of these low melting point coated metals. Moreover, there is some fouling of the electrode tips also, which must be cleaned more frequently than when welding uncoated steel. Zinc if it gets alloyed with the weld nugget induces brittleness. For welding coated steel sheets, welding machines of higher capacities are preferred in order to obtain quicker heating and cooling rates.

Non-ferrous Metals (i)Aluminium(ii) Aluminium Magnesium Alloys(iii) Aluminium Manganese Alloys(i), (ii) and (iii) may be spot welded satisfactorily. Oxide film on them is removed and a high capacity machine is used as aluminium is a good conductor.(iv) Copper. For welding copper upto 1.5 mm thick, hardfaced or pure tungsten (welding) electrodes are necessary. For bigger thicknesses spot welding is not preferred.Copper and aluminium and their alloys being very good conductors of heat and electricity are difficult metals from the resistance welding stand point as compared to mild steel.(v) Nickel, Nickel alloys and Monel Metal require machine capacity and settings rather similar to those employed for spot welding stainless steels.

Spot Welding Methods - Different spot welding methods are (i) Direct(ii) Indirect (or series)(iii) Push pull (i) Direct welds. It is a welding method in which one or more electrodes oppose each other, contacting both sides of the work and with the current passing from the electrodes on one side directly through the work into the electrodes on the other side and back to the welding transformer.

(ii) Series welds. It is a welding method in which two or more spots are produced simultaneously with only one common but indirect current path

In series welding, a portion of the secondary current by passes (shunts) any weld nugget being formed. This shunt current passes through one of the panels being welded.(iii) Push pull welds A push pull system employs transformers with an electrically reversed polarity arrangement wherein two transformers complement each other to form circulating welding current circuit. Opposing electrodes are connected to different transformers and are of opposite polarity. Two spot welds may be obtained simultaneously.

Advantages of Spot Welding - (i) Low cost, (ii) High speed of welding, (iii) Dependability, (iv) Less skilled worker will do, (v) More general elimination of warping or distortion of parts, (vi) High uniformity of products, (vii) Operation may be made automatic or semiautomatic, and (viii) No edge preparation is needed

Applications of Spot Welding - (i) Spot welding of two 12.5 mm thick steel plates has been done satisfactorily as a replacement for riveting. (ii) Many assemblies of two or more sheet metal stampings that do not require gas tight or liquid tight joints can be more economically joined by spot welding than by mechanical methods. (iii) Containers such as receptacles and tote boxes frequently are spot welded. (iv) The attachment of braces, brackets, pads or clips to formed sheetmetal parts such as cases, covers, bases or trays is another application of spot welding. (v) Spot welding finds application in automobile and aircraft industries.

Resistance Upset Butt Welding - Upset butt welding is a resistance welding process wherein coalescence is produced simultaneously over the entire area of abutting surfaces by the heat obtained from the resistance to electric current through the area of contact of those surfaces. Pressure is applied before heating is started and is maintained throughout the heating period. This pressure or force is later on increased to give a forging squeeze when the welding temperature* has been reached. When sufficient upset has been produced, the welding current is cut off and the force is removed

Principle of Operation The steps involved in making a resistance upset butt weld are given below: (i) The two pieces to be butt welded are gripped firmly, one in each clamp and are correctly aligned so that when brought into contact one with the other by sliding the movable clamp to the fixed one, they fit together exactly.

(ii) Force is applied so that the faces of two pieces touch together and remain under pressure. (iii) A heavy current is then passed from one piece to another. The resistance to the electrical current flow heats the faces to fusion temperature. (iv) Both pressure and current are applied throughout the weld cycle and when the faces (or ends) of the pieces become plastic, they are pressed together more firmly, upsetting the metal pieces to form a dense joint.Upsetting takes place while the current is flowing and continues until after the current is shut off. The upsetting action mixes the two metals homogeneously and pushes out many of the impurities of the atmosphere. It also reduces the heat affected zone to a minimum.

(v) The welding current is cut off.(vi) Upsetting force is released as the welded joint has cooled to the desired temperature.

(vii) Workpieces are unclamped. Good butt welding is obtained if(a) Faces to be butt welded are dean, parallel and reasonably smooth. (b) The two workpieces are equal in cross sectional area and of equal specific resistance. If the two pieces are of unequal specific resistance, the part having the lesser resistance should project farther from the clamping die than the other. Similarly, if the two pieces have equal specific resistance, but unequal area of crosssection, one with the larger crosssectional area should project from the damping die farther than the other part. (c) To facilitate heating at the abutting surfaces, the areas are sometimes restricted by bevelling the ends.

Metals Welded The following materials are butt welded in wire, bar (up to 30 mm diameter) strip or tube form. (i) Copper alloys,(ii) Low and high carbon steels(iii) Stainless steels,(iv) Aluminium,(v) Nickel alloys,(vi) Resistance alloys.

Applications of Butt Welding - (i) For welding of small ferrous and non ferrous strips and rods.

(ii) For welding of longitudinal butt joints in tubing and pipe and transverse butt joints in heavy steel rings.

(iii) In wire drawing industries, where wire drawing would be impossible without the upset butt welding process. Resistance butt welding has been largely replaced by flash butt welding.

Flash Butt Welding - The term flash welding derives its name from the flash produced during the process. Probably, flash welding process was developed from resistance butt welding by accident in attempts to increase the capacity of the butt welding machines by raising the voltage and applying pressure intermittently. Definition Flash welding is a resistance welding process wherein coalescence is produced, simultaneously over the entire area of abutting surfaces, by the heat obtained from resistance to electric current between the two surfaces, and by the application of pressure

after heating is substantially completed. Flashing and upsetting are accompanied by expulsion of metal from the joint

Principle of Operation The sequence of operations required for flash welding is given below: (i) Flash butt welds are made on a machine having one stationary and one opposing movable platen, on which are mounted the flash-welding dies or clamps. These clamps securely hold the two workpieces to be welded while simultaneously serving to conduct the welding current through these workpieces. (ii) The workpiece held in the movable platen is brought towards the one gripped in the stationary platen until the two come in light contact, and as the welding current (with voltage sufficiently high) is turned on, flashing is established.

While incandescent metal particles are being expelled by flashing, the movable platen keeps on moving constantly toward the stationary one at a carefully controlled and accelerated rate.As the flashing continues, the ends of the two workpieces burn off as they reach a higher and higher temperature until finally they attain the welding temperature. (iii) At this stage, the pressure of the moving clamp is quickly and greatly increased to (upset) forge the parts together and expel the molten metal and slag out of the joint thereby making a good solid weld. The metal expelled forms a ragged fin or flash round the joint which is removed later on by cutting or grinding. (iv) The welding current is cut off and the workpieces are unclamped as the fusion weld cools. Metals welded by flash welding

Difference Between Flash Welding and Upset Butt Welding - (i) In upset welding no arcing (and hence flashing) takes place between the surfaces being joined. Heat is produced solely by the electrical resistance at the abutting surfaces due to the passage of an electrical current. (ii) In flash welding, unlike upset welding, the movable platen keeps on moving constantly toward the stationary platen. (iii) Flash welding consumes much less welding current than consumed by upset butt weld process; the time allowed for weld to be completed is, however, more. (iv) In flash welding heat application precedes the pressure whereas in butt welding constant pressure is applied during the heating process which eliminates flashing.

Advantages of Flash Welding - (i) Many dissimilar metals with different melting temperatures can be flash welded. (ii) Flash welding offers strength factors up to 100%. (iii) Generally no special preparation of the weld surface is required.

(iv) Flash welding can be used for the welding of those highly alloyed steels which cannot be welded satisfactorily by any other process. This IS because, in flash welding, under correctly controlled conditions, the heating is not only even but extremely local, so that the cooling stresses are maintained at a minimum; this avoids hardening and cracking in highly alloyed steels. (v) The process is cheap, i.e., the cost of current per weld is small. (vi) Flash welding is faster than many other methods. (vii) Flash welding gives a smaller upset.

Disadvantages of Flash Welding -(i) The most undesirable feature of flash welding is the flashing operation during which it is impossible to protect the welding machine and the surrounding area from these particles, which can burn into slide way bearings, insulation etc. This necessitates more frequent maintenance.(ii) The process presents a considerable fire hazard. Operators need be protected from flying particles. (iii) Concentricity and straightness of workpieces during welding is often difficult to maintain.

(iv) Metal is lost during flashing and upsetting. (v) Shape of the workpieces to be flash welded should be similar.(vi) It is generally not recommended for welding zinc and its alloys, cast iron, lead and its alloys.

Applications of Flash Welding (i) Flash welding is applied primarily in the butt welding of metal sheets, tubing, bars, rods, forgings, fittings etc. (ii) Flash welding finds applications in automotive and aircraft products, household appliances, refrigerators and farm implements. (iii) The process is also used for welding the band saw blades into continuous loops, and joining of tool steel drill, tap and reamer bodies to low carbon steel and alloy steel shanks. (iv) Flash welding is used to produce assemblies that otherwise would require more costly forgings or castings.

Percussion Welding - It is a resistance welding process wherein coalescence* is produced simultaneously over the entire area of abutting surfaces by heat obtained from an arc produced by a rapid discharge of electrical energy, with pressure percussively (rapidly) applied during or immediately following the electrical discharge. Principle of Operation The following steps are involved in percussion welding. (i) The workpieces are cleaned by removing grease, dirt, paint, etc.(ii) The workpieces are clamped into machine or fixture. .

(iii) Light force is applied and the ends or faces of two work-pieces are brought near.(iv) Arc between the faces of workpieces is struck by using any of the following methods:

a) Workpieces are brought into light contact to establish a flow of current. The workpieces are then retracted to draw the arc. (b) A nib of small cross sectional area is formed (by cutting) on one of the pieces. Current as it flows through the nib, explodes the nib and establishes an ionized path for the welding current to flow. (c) A DC voltage (high enough to jump the gap between the workpieces as they are moved toward each other) when applied, ionizes the air gap between the workpieces and starts the flow of current.(d) Another method involves superimposing an auxiliary high-frequency, high voltage AC on a low voltage current across the gap between the workpieces. The high frequency AC ionizes the air in the gap and the low voltage DC maintains the arc

The arc as established above heats the faces of the two work-pieces to be joined to the welding temperature.(v) At this stage, welding force is applied. It extinguishes the arc find holds parts together while weld cools. Welding force may be: Pneumatic, Electromagnetic, Spring force or Gravity (falling weights).(vi) Welding force is released.(vii) Workpieces are unclamped. Power Supplies for Percussion Welding (i) Low voltage (10 to 150 volts DC), capacitive storage.(ii) High voltage (1000 to 6000 volts DC), capacitive storage. (iii) Electromagnetic or inductive storage.(iv) Low voltage (10 to 35 volts) AC that uses a transformer to furnish the welding voltage.

Metals Welded (i) Copper alloys,(ii) Aluminium alloys(iii) Nickel alloys,(iv) Low carbon steels, (v) Medium carbon steels,(vi) Stainless steels, and(vii) Copper to Molybdenum, etc.

Advantages, Disadvantages and Applications of Percussion Welding - (i) Because of the extreme brevity of the arc, fusion is confined to the surface of the parts being welded and there is almost complete absence of flash or upset.(ii) Heat treated or cold worked metals can be welded without annealing or

destroying the heat treatment. Limitations (i) The process is limited to butt welded joints only.(ii) Since control of the path of an arc is difficult, the joint used is limited to about 1.5 to 3 sq. cm.

Applications (i) Percussion welding is particularly applicable to the attachment of metal tips to valve stems usually of different compositions.(ii) The process makes large contact assemblies for relays, etc.(iii) Percussion welding finds use in telephone industry for connecting leaded components to terminals.(iv) The process joins similar and dissimilar metals that are not usually capable of being flash welded or stud welded. (v) The process is used for welding fine wire leads to filaments in lamps and to terminals of electrical and electronic components where a reliable joint is needed to withstand shock, vibration and extended service at elevated temperature.

Spot Welding Electrodes - Their Functions (i) To conduct the welding current to the workpieces.(ii) To transmit to the workpieces in the weld area the amount of force

needed to produce a satisfactory weld. (iii) To dissipate the heat from the weld zone and thus prevent surface

fusion of the work. Requirements of Spot Welding Electrodes A spot welding electrode must (i) Be a good conductor of electricity(ii) Be a good conductor of heat(iii) Have good mechanical strength and hardness at high temperatures.(iv)Have a minimum tendency to combine with the metal being welded.

Electrode Materials - Considering the above-mentioned requirements, the following materials are used for manufacturing spot (resistance) welding electrodes. (a) Group A 1. Copper 99%, Cadmium 1% alloy It has high strength and hardness coupled with high electrical and thermal conductivities. It is non-heat-treatable and is, therefore, hardened and strengthened by cold working. It is recommended for spot welding: (i) Low-carbon steel coated with tin, terne metal, chromium or zinc,

(ii) Scaly hot-rolled low-carbon steel,(iii) Aluminium and Magnesium alloys.

2. Copper 99.2%, Chromium 0.8% alloy It has high mechanical properties but lower thermal and electrical conductivities than Cu-Cd alloy. Optimum properties are developed by heat-treatment or by a combination of heat-treatment and cold work. It is used for spot welding. (i) Cold rolled low-carbon steels(ii) Hot rolled pickled low-carbon steels(iii) Nickel plated steel(iv)Stainless steel(v) Nickel alloys(vi)Copper-base alloys such as silicon bronze and nickel silver.

3. Beryllium 0.5%, Nickel 1%*, Cobalt 1%, and rest is copper It is hardenable alloy with higher mechanical properties, but lower electrical and thermal conductivities than Cu-Cd or Cu-Cr alloys. It is preferred where pressures and workpiece resistance are high. It is used for spot welding (i) Thick sections of low-carbon steel(ii) Stainless steel(iii) Monel and Inconel. (b) Group-B 4. Refractory-Metal Compositions. These materials are employed where high heat, long weld time, inadequate cooling or high pressure would cause rapid deterioration of the copper-base alloys discussed above. A typical refractory-metal composition is given below:

42% Cu, 58% W (by volume): This is used for spot welding of stainless steel. When a copper alloy is being spot welded to steel, a group B electrode is used to contact the copper alloys and a group A electrode of type (1) or (2) is used to contact the steel. 5. Special alloy electrodes They are made up of copper-zirconium and copper-cadmium zirconium. They find applications similar to alloy (1) but where resistance to softening of the electrode face is a must.

Electrode Shapes - Spot welding electrodes may be (i) Pointed,(ii) Domed,(iii) Flat.

(i) Pointed electrodes are widely used, since, with continued wear, they mushroom (expand and flatten) uniformly. By changing the area of the face of the pointed electrodes, it is possible to change current density and hence proper heat balance at the weld zone can be achieved.(ii) Domed electrodes can withstand heavy pressures and severe heating without mushrooming. The radius of the dome may be 50 to 100 mm. Many non-ferrous materials are welded using domed tip electrodes. (iii) Flat tip electrodes are preferred when invisible or inconspicuous welds are desired, or where the weld indentation is to be at a minimum e.g., in the welding of refrigerator body.

Seam Welding - Definition Seam welding is a resistance welding process wherein coalescence at the faying surfaces is produced by the heat obtained from resistance to electric current (flow) through the work parts held together under pressure by electrodes. The resulting weld is a series of overlapping resistance-spot welds made progressively along a joint by rotating the circular electrodes

Principle of Operation The seam welding is similar to spot welding, except that circular rolling electrodes are used to produce a continuous air-tight seam of overlapping welds. Overlapping (spot) welds are produced by the rotating electrodes and a regularly interrupted current

The workpieces to be seam-welded are cleaned, overlapped suitably and placed between the two circular electrodes, which clamp the workpieces together by the electrode force.A current impulse is applied through the rollers to the material in contact with them. The heat generated thus makes the metal plastic and the pressure from the electrodes completes the weld. As the first current impulse is applied, the power driven circular electrodes are set in rotation and the workpieces steadily move forward. Throughout the welding period, the electrodes revolve and the work passes through them at a specific speed. The current applied to welding electrodes is intermittent i.e. it is on for a definite length of time and then off for another definite and short period. 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.

On the other hand, if individual spot welds (or nuggets) are obtained by constant and regularly timed interruptions of the welding current, the process is known as Roll

(spot) welding. Roll welding simply joins two workpieces whereas stitch welding produces gas tight and liquid tight joints.

There are two seam welding methods. One involves continuous motion and the other intermittent motion during welding operation.

In continuous motion method, the electrodes rotate at a constant speed and the current flows continuously or are interrupted. In intermittent motion welding, the electrodes travel the distance necessary for each successive weld and then stop.

The current is then switched on and the weld made, the whole process being controlled automatically. Continuous motion is used for welding workpieces less than 4.5 mm thick and intermittent motion, above 4.5 mm thick.

The rotating welding electrodes are cooled to prevent over-heating with consequent wheel dressing and replacement problems reduced to a minimum. Moreover, employing water-cooling jets immediately before and after the electrodes reduce warping of the materials being joined.

Advantages and Disadvantages of Seam Welding -Advantages of Seam Welding (i) It can produce gas tight or liquid-tight joints. (ii) Overlap can be less than for spot or projection welds. (iii) A single seam weld or several parallel seams may be produced simultaneously. Disadvantages of Seam Welding (i) Welding can be done only along a straight or uniformly curved line. (ii) It is difficult to weld thicknesses greater than 3 mm. (iii) A change in the design of electrode wheels is required to avoid obstructions along the path of the wheels during welding

Applications of Seam Welding - (i) Girth welds can be made in round, square or rectangular parts.

(ii) Except for copper and high copper alloys, most other metals of common industrial use can be seam welded.

(iii) Besides lap welds, seam-welding can be used for making butt seam welds too.

Projection Welding - Definition Projection welding is a resistance welding process wherein coalescence is produced by the heat obtained from resistance to electric current flow through the work parts

held together under pressure by electrodes. The resulting welds are localized at predetermined points by projections, embossments or intersections. Principle of Operation Projection welding is similar to spot welding except that (i) The electrodes, instead of being tips as in spot welding, are flat and relatively large in surface area. Electrodes are cooled as in spot welding. (ii) Since a number of welds are made at each operation, both the electrical power (kVA) and mechanical pressure must be correspondingly greater, as compared with a spot welder.

The success of projection welding depends on the surface preparation of the work-pieces to be welded. Projections, small deformations that will touch the surface of the material to be welded are formed on the weld areas by embossing, stamping, casting or machining. These projections serve to concentrate (localize) the welding heat at these areas and facilitate fusion without the necessity of employing a large current. (i) The projection in the upper piece is held in contact with the lower piece under electrode pressure. (ii) The current flows and being localized to the region around the projection, heats the metal in that area to the plastic state. (iii) The heated and softened projection collapses under the pressure of the electrodes thereby forming the weld.

Projection Welding Equipment - Projection welding employs a press-type machine (with a single-phase or three-phase transformer. The welding head is guided by bearings or ways and moves in a straight line. Platens with T -slots are used for mounting the electrodes.

The welding head is actuated by air, spring or hydraulically. Welding machine controls are usually of the synchronous type. Phase shift and pulsation timing are often included to regulate welding current. Pulsation timings are helpful when welding thick materials.

Metals Welded The following metals are satisfactorily welded by projection welding: (i) Low carbon (0.20% C max) steels. (ii) Coated metals such as galvanized steel, terne plate, tin plate, etc. Considerable electrode maintenance is usually needed when welding coated metals because coating sticks to the face of the electrode. (iii) Naval brass, Monel (Ni-Cu) alloys. (iv) Stainless steel. (v) Titanium alloys.

Advantages and Disadvantages of Projection Welding - Advantages of Projection Welding (i) A number of welds can be made simultaneously. (ii) Projection welds can be made in metals that are too thick to be joined by spot welding. (iii) Scale, rust, oil and work-metal coatings interfere less with projection welding than with spot welding. (iv) Projection welding electrodes possess longer life than spot welding ones because of less wear and maintenance resulting from fusion and overheating. (v) Show-sides of the jobs can be produced with no electrode marking, thus making it possible to paint or plate them without grinding.

vi) Any two surfaces which can be brought together to give line or point contact can be projection welded; projection welding is not limited to sheet -sheet joints; (vii) Projection welding locates the welds at certain desired points.(viii) A better heat balance can be obtained in difficult-to-weld combinations of compositions and thicknesses. (ix) Projection welding lowers the amount of current and pressure needed to form a good bond between two surfaces. This reduces the chances of shrinkage and distortion around the weld zone

Disadvantages (i) The projection welding is limited to combinations of metal thickness and composition which can be embossed. (ii) Metals that are not strong enough to support projections (e.g., some brasses, or copper) cannot be projection welded satisfactorily. (iii) Forming of projection on one of the workpieces is an extra operation.(iv) For proper welding, all projections must be of the same height.(v) Since there is no possibility of using post-weld current pulses for heat-treatment (as is possible in spot welding) weld assemblies in hardenable materials must be heat-treated in a furnace

Brazing - Brazing is defined as a group of joining processes where in coalescence is produced by heating to a suitable temperature and by using a filler metal having a liquid us above 800°F (470°C) and below the solid us of the base metals. In brazing, metallic parts are joined by a non-ferrous filler metal or alloy. The filler metal is distributed between the closely fitted surfaces of the joint by capillary attraction.

Brazing Principle of Operation - Brazing involves the melting of a comparatively low melting point filler material* against the base metal pieces to be joined while they are clean and free from oxides, oil, grease, etc. It is not necessary to melt the base metal. The molten (brazing) filler material(i) Wets the base metal surfaces,

(ii) Spreads along the joint (to be brazed) by capillary action,(iii) Adheres and solidifies to from the brazed joint.Capillary flow plays a major role in producing good brazements, provided the base metal surfaces are wet by the molten filler material. The flux which is employed during brazing melts at a lower temperature than the brazing filler material, wets the surfaces to be brazed, removes the oxide film and gives clean surfaces.

Since the capillary attraction between the base metal and the filler material is at least several times higher than that between the base metal and the flux, the filler material replaces the flux and flows into the narrow space or joint between the surfaces by capillary attraction. The narrower the joint the better will be the capillary flow. The joint (thus filled with liquid filler material) upon cooling to room temperature, will be found filled with solid filler material and the flux, now also solidified, will be found on the joint periphery. The high fluidity of the molten filler material is also an important factor in obtaining successful brazing joints.

Brazing Processes -A number of processes are employed to heat the workpieces to brazing temperature which is somewhat above the melting point of the filler metal.  The method of heating is a major factor that governs the economics of the brazed joint as well as the quality of the job. The method of heating is selected after considering the following factors:(i) Type of job. (ii) Number of jobs to be brazed. (iii) Availability of required equipment.

On the basis of method of heating, various brazing processes are:

1.Torchbrazing 2. Furnace brazing3.Vacuumbrazing 4.Inductionbrazing 5.Dipbrazing(metalandsalt-bathbrazing)6.Resistancebrazing 7.Infraredbrazing 8.Carbonarcbrazing 9.Flowbrazing 10. Block brazing.Difference Between Brazing and Welding - 1. Surfaces to be brazed are not raised to fusion point, rather the joint is produced by the solidification and adhesion of a thin layer of molten filler metal to closely adjacent mating surfaces; whereas in welding, the two surfaces to be joined are actually melted. 2. The brazing alloy spreads by capillary action between very closely adjacent surfaces. The molten brazing filler alloy spreads along the joint whereas molten filler metal in welding solidifies almost at the same place where it melts. For this reason, welded bead shows

ripples whereas a brazed joint does not. 3. A very important difference between a welded joint and a brazed joint is the penetration. A welded joint achieves penetration into the base metal whereas there is no penetration into base metal in the case of brazed joint.Comparison Between Soldering, Brazing and Welding - Soldering and Brazing processes differ from welding in the sense that there is no direct melting of the base metal(s) being joined. Rather, the brazing or soldering filler alloy flows between the two closely adjacent surfaces of the workpieces by capillary action. Both soldering and brazing processes are particularly useful for joining two dissimilar metals. Both, the brazing alloy and the solder, have lower melting points than the metals to be joined. In order to obtain a satisfactory brazed or soldered joint, it is necessary for the filler alloy to

(i) Wet the base metal,

(ii) Spread and make contact with the joint opening,

(iii) Be drawn into the joint by capillary action

(iv) Brazing produces joints stronger than those made by soldering and they can be used in service at higher temperatures.

(iv)Soldered joints do not resist corrosion to the same extent that brazed or welded joints do.

Soldering Definition - Soldering is a common process for joining steel, copper and other materials at a low temperature. Soldering is defined as a group of joining processes wherein coalescence is produced by heating to a suitable temperature and by using a filler metal having a liquid us not exceeding 800°F (427°C) and below the solid us of the base metals.

Soldering Methods - Soldering methods are best classified by the method of heat application. The heat should be applied in such a manner that the solder melts while the surface is heated to permit the molten solder to wet and now over the surface.Various soldering methods are 1. Soldering iron method. 2. Torch method.3. Dip and wave methods.4. Induction method.5. Resistance method.6. Furnace and hot plate method. 7. Spray method.8. Ultra-sonic method.9. Condensation method.

The filler metal (i.e., the solder) is usually distributed between the properly fitted surfaces of the joint by capillary attraction.

Soldering Fluxes - A soldering flux is defined as any solid, liquid or gaseous compound which, when heated, is capable of promoting, or accelerating, the wetting of metals with solders. The purpose of a soldering flux is to remove and exclude small amounts of oxides and other surface compounds from the surfaces being soldered and prevent reoxidation of the surfaces during the soldering process. It is absolutely necessary to remove grease, surface dirt and oxides before a flux can be fully active in a soldering operation.

Types of Soldering fluxes are categorized as 1. Inorganic or acid 'corrosive' fluxes. 2. Organic acid 'mild' fluxes. 3. Rosin 'non-corrosive' fluxes.

Brazing and soldering processes differ in the following ways: (i) Filler metal used in soldering has a melting point below 800°F (427°C) whereas that in brazing has a melting point above 800°F (427°C). (ii) The extent of diffusion. In brazing process, bonding conditions are set up so that a large amount of diffusion will take place in order to strengthen and improve the bond, whereas, in soldering, diffusion is secondary in importance

Soldering - Soldering, brazing and braze welding are those metal joining techniques which make use of (i) Heat, and (ii) A filler metal whose melting point is lower than the melting point of the metals or alloys being joined.

In soldering, the melting point of the filler metal is less than 427°C. In brazing and braze welding the melting point of the filler metal is above 427°C but below the melting point of the base metals.

Gas Welding Processes and Equipments - Gas welding is a fusion welding process. It joins metals, using the heat of combustion of an oxygen/air and fuel gas (i.e acetylene, hydrogen, propane or butane) mixture. The intense heat (flame) thus produced melts and

fuses together the edges of the parts to be welded, generally with the addition of a filler metal

Oxy Acetylene Welding Principle of Operation - When acetylene is mixed with oxygen in correct proportions in the welding torch and ignited, the flame resulting at the tip of the torch is sufficiently hot to melt and join the parent metal.

The oxyacetylene flame reaches a temperature of about 3200°C and thus can melt all commercial metals which, during welding, actually flow together to form a complete bond.

A filler metal rod is generally added to the molten metal pool to build up the seam slightly for greater strength. Oxyacetylene welding does not require the components to be forced together under pressure until the weld forms and solidifies

Types of Welding Flames - Open the acetylene control valve of the welding torch and after the system has been flushed clean of air the gas is ignited. At this stage, enough of oxygen is drawn in from the atmosphere to burn acetylene partially. The acetylene control valve is then adjusted until the flame ceases to smoke.

The oxygen control valve of the welding torch is then opened in order to adjust the proportions in which acetylene and oxygen are required to mix and burn. This results in three distinct types of flames as discussed under.

Types of Flames 1. Neutral Flame (Acetylene oxygen in equal proportions)2. Oxidising Flame (Excess of oxygen)3. Reducing Flame (Excess of acetylene)In oxyacetylene welding, flame is the most important tool. All the welding equipment simply serves to maintain and control the flame. The correct type of flame is essential for the production of satisfactory welds. The flame must be of the proper size, shape and condition in order to operate with maximum efficiency.

Neutral Flame - A neutral flame is produced when approximately equal volumes of oxygen and acetylene are mixed in the welding torch and burnt at the torch tip. (More accurately the oxygen-to-acetylene ratio is 1.1 to 1). The temperature of the neutral flame is of the order of about 5900ºF (326ºC)

The flame has a nicely defined inner cone* which is light blue in colour. It is surrounded by an outer flame envelope, produced by the combination of oxygen in the air and superheated carbon monoxide and hydrogen gases from the inner cone. This envelope is usually a much darker blue than the inner cone

A neutral flame is named so because it effects no chemical change in the molten metal and therefore will not oxidize or carburize the metal.

The neutral flame is commonly used for the welding of:(i) Mild steel (ii) Stainless steel (iii) Cast Iron(iv) Copper(v) Aluminium

Oxidising Flame - If, after the neutral flame has been established, the supply of oxygen is further increased, the result will be an oxidising flame. An oxidising flame can be recognized by the small white cone which is shorter, much bluer in colour and more pointed than that of the neutral flame.

The outer flame envelope is much shorter and tends to fan out at the end on the other hand the neutral and carburizing envelopes tend to come to a sharp point. An oxidising flame burns with a decided loud roar. An oxidising flame tends to be hotter than the neutral flame. This is because of excess oxygen and which causes the temperature to rise as high as 6300°F.

The high temperature of an oxidizing flame (O2: C2H2 = 1.5: 1) would be an advantage if it were not for the fact that the excess oxygen, especially at high temperatures, tends to combine with many metals to form hard, brittle, low strength oxides. Moreover, an excess of oxygen causes the weld bead and the surrounding area to have a scummy or dirty appearance For these reasons, an oxidising flame is of limited use in welding. It is not used in the welding of steel. A slightly oxidising flame is helpful when welding most(i) Copper base metals (ii) Zinc base metals, and

(iii) A few types of ferrous metals, such as manganese steel and cast ironThe oxidizing atmosphere, in these cases, creates a base metal oxide that protects the base metal. For example, in welding brass, the zinc has a tendency to separate and fume away. The formation of a covering copper oxide prevents the zinc from dissipating.

Reducing Flame - If the volume of oxygen supplied to the neutral flame is reduced, the resulting flame will be a carburising or reducing flame, i.e. rich in acetylene. A reducing flame can be recognized by acetylene feather which exists between the inner cone and the outer envelope. The outer flame envelope is longer than that of the neutral flame and is usually much brighter in colour.

A reducing flame does not completely, consume the available carbon; therefore, its burning temperature is lower and the left over carbon is forced into the molten metal

With iron and steel it produces very hard, brittle substance known as iron carbide. This chemical change makes the metal unfit for many applications in which the weld may need to be bent or stretched. Metals that tend to absorb carbon should not be welded with reducing flame.

A reducing flame has an approximate temperature of 5500°F (3038°C). A reducing flame may be distinguished from a carburizing flame by the fact that a carburizing flame contains more acetylene than a reducing flame. A carburizing flame is used in the welding of lead and for carburizing (surface hardening) purposes

A reducing flame, on the other hand, does not carburize the metal, rather it ensures the absence of the oxidizing condition. It is used for welding with low alloy steel rods and for welding those metals, (e.g. non ferrous) that do not tend to absorb carbon. This flame is very well used for welding high carbon steel

To conclude, for most welding operations the Neutral Flame is correct, but the other types of flames are sometimes needed for special welds, e.g., non-ferrous alloys and high carbon steels may require a reducing flame, whilst zinc bearing alloys may need an oxidising flame for welding purposes.

Leftward Technique - The welder holds welding torch in his right hand and filler rod in the left hand. The welding flame is directed away from the finished weld, i.e., towards the unwelded part of the joint. Filler rod, when used, is directed towards the welded part of the joint.

The weld is commenced on the right hand side of the seam, working towards the left hand side. The blowpipe or welding torch is given small sideways movements, while the filler rod is moved steadily across the seam. The filler rod is added using a backward and forward movement of the rod, allowing the flame to melt the bottom edges of the plate just ahead of the weld pool.

Since the flame is pointed in the direction of the welding, it preheats the edges of the joint. Good control and a neat appearance are characteristics of the leftward method. Leftward technique is usually used on relatively thin metals, i.e., having thicknesses less than 5 mm.

When work piece thickness is over 3 mm, it is necessary to bevel the plate edges to produce a V-joint so that good root fusion may be achieved. The included angle of V-joint is 80-90°. This large volume weld is uneconomical in terms of time, weld metal deposited and quantity of gases used and may also over distort the weldment when welding thick materials

Long welding time also leads to overheating of the weld area and thus the weld metal may have coarse grains. When welding materials over 6.5 mm thick, it is difficult to obtain even

penetration at the bottom of the V and therefore the quality of the weld decreases as plate thickness increases.

The leftward technique requires careful manipulation to guard against excessive melting of the base metal, which results in considerable mixing of base metal and filler metal. The influence of the base metal on the properties of the weld metal can be very great.

Another disadvantage associated with leftward technique is that the view of the joint edges is interrupted and it is necessary to remove the end of the rod, (which slows down this welding method) this itself resulting in the oxides formed on the tip of the rod being deposited into the weld pool when the weld is recommenced.

Rightward Technique - Here again the welding torch is held in the right hand of the welder and the filler wire in the left. Welding begins at the left hand end of the joint and proceeds towards the right, hence the name rightward technique.

The direction of welding is opposite to that when employing the leftward technique. The torch flame in rightward technique is directed towards the completed weld and the filler rod remains between the flame and the completed weld section,

Since the flame is constantly directed on the edges of the V ahead of the weld puddle, no sidewise motion of the welding torch is necessary. As a result a narrower V -groove (30° bevel or 60° included angle) can be utilized than in leftward welding. This provides a greater control and reduced welding costs.

During welding, the filler rod may be moved in circles (within the puddle) or semicircles (back and forth around the puddle.) In rightward welding, the weld puddle is less fluid and this result in a slightly different appearance of the weld surface. The ripples are heavier and spaced further apart

The rightward technique is one used on heavier or thicker (above 5 mm) base metals, because in this technique the heat is concentrated into the metal. Welds with penetrations of approximately 12 mm can be achieved in a single pass. Rightward technique has got certain. Advantages over the left ward one, as listed below:

(i) Up to 8.2 mm plate thickness no bevel is necessary. This saves the cost of preparation and reduces the consumption of filler rod(ii) For welding bigger thicknesses, where beveling of plate edges becomes necessary, the included angle of V need be only 60°, which requires less filler metal against 80°V preparation used in leftward welding technique(iii) The welder's view of the weld pool and the sides and bottom of the V groove is unobstructed. This results in better control and higher welding speeds.

(iv) The smaller total volume of deposited metal, as compared to leftward welding, reduces shrinkage and distortion.(v) The weld quality is better than that obtained with the leftward technique(vi) Owing to less consumption of the filler metal,

Welding Filler Metal Rods - Filter metal is the material that is added to the weld pool to assist in filling the gap (or groove). Filler metal forms an integral part of the weld. Filler metal is usually available in rod form. These rods are called Filler Rods. Filler rods have the same or nearly the same chemical composition as the base metal. Welding filler rods are available in a variety of compositions (for welding different materials) and sizes

Welding Fluxes - During welding, if the metal is heated/melted in air, oxygen from the air combines with the metal to form oxides which result in poor quality, low strength welds or, in some cases, may even make welding impossible. In order to avoid this difficulty, a flux is employed during welding. A flux is a material used to prevent, dissolve or facilitate removal of oxides and other undesirable substances

A flux prevents the oxidation of molten metal. The flux (material) is fusible and non metallic. During welding, flux chemically reacts with the oxides and a slag is formed that floats to and covers the top of the molten puddle of metal and thus helps keep out atmospheric oxygen and other gases.

Common Welding Rod Composition -

S.No Filler Rod Size (mm)Melting Point, °C

Flux Required

Applications i.e. for

1.

Copper coated Mild Steel

IS: 1278 Type 4.1

1.6,3.15, 5,6,.3

1490 NoMild steel and wrought iron welding

2.High carbon steel

1.6,3.15,5 1350 YesBuilding up and repair of cutting edges of paper and leather cutters.

3.

3% nickel steel

IS: 1278 Type 4.4

1.6, 2.5, 3.15,5

1450 YesBuilding up worn cam shafts, shafts, gears, etc.

4.

Wear resisting alloy steel

BS: 1453 A5

5,6.3 1320 No

Building up worn crossings and rail ends on railway or tramway tracks, crushing tools, etc.

5. Pipe welding 2.5, 3.15,5 1450 No Welding steel pipe.

rods

IS: 1278 Type 4.2

6.

Stainless steel Decay Resistant

IS: 1278 Type 4.6

1.6, 2.5, 3.15, 5, 6.3

1440 YesWelding austenitic stainless steel tubes, sheets, tanks, etc.

7.

Super silicon cast iron

IS: 1278 Type 5.1 (square)

5, 6.3, 8,10 1147 Yes

Welding high grade castings where subsequent machining is necessary such as lathe beds, cylinder blocks, etc.

8.

Copper silver alloy

IS: 1278 Type 6.1

1.6,3.15, 5,6.3

1068 YesWelding copper fire boxes and electrical work.

9.

Nickel bronze

IS: 1278 Type 6.4

3.15, 5,6.3 910 Yes

Braze welding steel or malleable iron. Building up worn surfaces and welding Cu-Zn-Ni alloys of similar composition.

10.Aluminium alloy 5% copper

1.6,3.154, 5,6.3 (square)

640 Yes Welding aluminium castings.

11.Aluminium alloys 5% silicon

1.6, 3.15, 5,6.3

635 Yes

Welding pure aluminium sheet, tube and extruded sections and aluminium alloy castings not containing zinc.

Gas Welding Equipment - The basic equipments used to carry out gas welding are:1. Oxygen gas cylinder.2. Acetylene gas cylinder.3. Oxygen pressure regulator.4. Acetylene pressure regulator.5. Oxygen gas hose(Blue).6. Acetylene gas hose(Red).7. Welding torch or blow pipe with a set of nozzles and gas lighter

8. Trolleys for the transportation of oxygen and acetylene cylinders9. A set of keys and spanners.10. Filler rods and fluxes.11. Protective clothing for the welder (e.g., asbestos apron, gloves, goggles, etc.)

Oxygen Gas Cylinder - Oxygen cylinders are painted black and the valve outlets are screwed right handed. The usual sizes of oxygen cylinders are 3400, 5200 and 6800 litre. Oxygen cylinder is a solid drawn cylinder out of mild steel or alloy steel. Mild steel cylinder is charged to a pressure of 13660 KN/m2 (136.6 bar) and alloy steel cylinders to 17240 KN/m2 (172 bar).

The oxygen volume in a cylinder is directly proportional to its pressure. In other words, if the original pressure of a full oxygen cylinder drops by 5% during welding, it means 1/20 of the cylinder contents have been consumed

Because of the possibility of the oxygen pressure becoming high enough to rupture the steel cylinder in case the temperature rises, an oxygen cylinder is equipped with a safety nut that allows the oxygen to drain slowly in the event the temperature increases the gas pressure beyond the safety load of the cylinder.

An oxygen cylinder has an inside diameter of 8.5" (21.6 cm), wall thickness 0.260"(0.650 mm) and length 51" (127.5 cm). In order to protect cylinder valve from getting damaged, a1emovable steel cap is screwed on the cylinder at all times When the cylinder is not in use. The cylinder valve is kept closed when the cylinder is not in use and even when cylinder is empty.

Acetylene Gas Cylinder - An acetylene cylinder is painted maroon and the valves are screwed left handed; to make this easily recognisable they are chamfered or grooved. An acetylene cylinder is also a solid drawn steel cylinder which is charged to a pressure of 1552 KN/m2 (15.5 bar).

The usual size of acetylene cylinders are 2800 and 5600 litre. An acetylene cylinder has an inside diameter of 12" (30 cm), wall thickness 0.175" (0.438 mm) and a length of 40.5" (101.25 cm). An acetylene cylinder is filled with a spongy (porous) material such as balsa wood or some other absorptive material which is saturated with a chemical solvent called acetone.

Since high pressure acetylene is not stable, it is dissolved in acetone, which has the ability to absorb a large volume of the gas and release it as the pressure falls. The small compartments in the porous material (filled in the cylinder) prevent the sudden

decomposition of the acetylene throughout the mass, should it be started by local heating or other causes.

An acetylene cylinder is always kept upright for safety reasons. The acetone in the cylinder must not be permitted to enter the blowpipe, otherwise an explosion could result. The acetylene cylinder valve can only be opened with a special wrench and this wrench is kept in place whenever the cylinder is in use.

An acetylene cylinder has a number of fusible plugs, at its bottom, designed to melt at 220°F (104°C). These plugs melt and release the pressure in case the cylinder is exposed to excessive heat.

Advantages of Gas Welding - 1. It is probably the most versatile process. It can be applied to a wide variety of manufacturing and maintenance situations.2. Welder has considerable control over the temperature of the metal in the weld zone. When the rate of heat input from the flame is properly coordinated with the speed of welding, the size, viscosity and surface tension of the weld puddle can be controlled, permitting the pressure of the flame to be used to aid in positioning and shaping the weld.3. The rate of heating and cooling is relatively slow. In some cases, this is an advantage.4. Since the sources of heat and of filler metal are separate, the welder has control over filler metal deposition rates. Heat can be applied preferentially to the base metal or the filler metal

5. The equipment is versatile, low cost, self sufficient and usually portable. Besides gas welding, the equipment can be used for preheating, post heating, braze welding, torch brazing and it is readily converted to oxygen cutting.6. The cost and maintenance of the welding equipment is low when compared to that of some other welding processes

Disadvantages of Gas Welding - 1. Heavy sections cannot be joined economically.2. Flame temperature is less than the temperature of the arc.3. Fluxes used in certain welding and brazing operations produce fumes that are irritating to the eyes, nose, throat and lungs.4. Refractory metals (e.g., tungsten, molybdenum, tantalum, etc.) and reactive metals (e.g., titanium and zirconium) cannot be gas welded.5. Gas flame takes a long time to heat up the metal than an arc.

6. Prolonged heating of the joint in gas welding results in a larger heat affected area. This often leads to increased grain growth, more distortion and, in some cases, loss of corrosion resistance.7. More safety problems are associated with the handling and storing of gases.

8. Acetylene and oxygen gases are rather expensive.9. Flux shielding in gas welding is not so effective as an inert gas shielding in TIG or MIG welding

Applications of Gas Welding - 1. For joining thin materials.2.For joining materials in whose case excessively high temperatures or rapid heating and cooling of the job would produce unwanted or harmful changes in the metal.3. For joining materials in whose case extremely high temperatures would cause certain elements in the metal to escape into the atmosphere.

4. For joining most ferrous and nonferrous metals, e.g., carbon steels, alloy steels, cast iron, aluminium, copper, nickel, magnesium and its alloys, etc.5. In automotive and aircraft industries. In sheet metal fabricating plants, etc.

Gas Welding Techniques - Base Metal PreparationJoints used in gas welding are(i) Butt (ii) Lap. (iii) Edge(iv) T and(v) Corner joints

Either fillet or groove welds are used, depending on the work piece and on strength requirements

Work piece edges may be prepared by(i) Gas cutting. (ii) Plasma cutting.(iii) Milling.(iv) Shaping.(v) Planing, etc

In joint preparation, it should be ensured that plate edges are free from rust or oil. This prevents excessive fumes and helps improve the appearance of finished weld. A strong wire brush is used to dispose of rust and small burrs.Even on thin gauge sheets, it is better to tack weld the parts or to hold them in a fixture to ensure maintenance of the correct gap. The parts to be welded may be preset to counteract distortion.

Defects in Welds -IntroductionThe significance of defects in welds, which can be assumed to occur in normal fabrication,

is of importance to Design Engineers.A defective weldment fails under service conditions and causes damage to property and loss of human lives. This makes it necessary to study defects in welded joints and analyses their causes.Improper welding parameters and base metal and wrong welding procedures introduce defects or faults in the weld metal and around (i.e., in the heat-affected zone). The tolerance of welded joints to defects ranges from acceptance of gross defects (lack of penetration, gross slag inclusions, lack of fusion) under static loading at low stress levels, to sensitivity to extremely small defects, such as cracks, which are almost impossible to detect by non destructive testing.

The significance of individual defects depends on (i) The microstructure in which the defect occurs.(ii) The mechanical properties of the material with particular reference to notch toughness.(iii) The type of general loading (static, cyclical or shock). (iv) The environment (corrosive or non-corrosive). (v) Section thickness. (vi) Type and size of defect, and (vii) The stress pattern local to the defect. Some of the common weld defects along with their causes will be discussed below.

1. Cracks.2. Distortion.3. Incomplete penetration.4. Inclusions.5. Porosity and blow holes.6. Poor fusion.7. Poor weld bead appearance.8. Spatter.9. Undercutting.10. Overlapping.

Welding of Alloy Steel - Alloy steel contains elements such as chromium, nickel, vanadium, molybdenum, tungsten, cobalt, boron and copper; and manganese, silicon, phosphorus and sulphur in amounts greater than normally are present.The purpose of adding alloying elements into steel is to achieve Strengthening of the ferriteCorrosion resistanceBetter hardenabilityGrain size controlGreater strengthImproved machinability

Improved high or low temperature stabilityImproved ductility, etc.

Given below is the composition of a typical alloy steel1. Carbon in steel affects hardness, tensile strength, machinability and melting point.2. Manganese contributes markedly to strength and hardness. It lowers both ductility and weldability, if present in high percentage with high carbon content in steel.3. Silicon improves oxidation resistance and strengthens low alloy steels.4. Nickel increases toughness and resistance to .impact. It renders high chromium iron alloy austenitic. It strengthens steels and lessens distortion in quenching.5. Chromium adds to depth hardenability with improved resistance to abrasion. It helps preventing corrosion and oxidation.6. Molybdenum promotes harden ability of steel, makes it fine grained, counteracts tendency towards temper brittleness, raises tensile and creep strength at high temperatures, etc.

7. Vanadium promotes fine grains in steel, increases strength while retaining ductility, etc.8. Tungsten improves hardness and strength at high temperatures, resists heat and promotes fine grain.9. Cobalt contributes to red-hardness by hardening ferrite.10. Copper (0.20.5%) when added to steel increases resistance to atmospheric corrosion and acts as a strengthening agent.11. Aluminium produces fine austenitic grain size and acts as a deoxidizer.12. Sulphur imparts free machining properties.13. Boron (0.0010.003%) is a powerful hardenability agent.14. Titanium reduces martensitic hardness in chromium steels.

Alloy steels can be classified as(a) Low alloy steels (total alloy content up to 5%)(b) Medium alloy steels (total alloy content from 5 to 8%).(c) High alloy steels (total alloy content above 8%).

Some of the popular alloy steels are(i) Low alloy, high strength steels. (A typical composition is C 0.12%, Mn 0.60%, Si 0.30%, Cu 1.1%, Ni 0.55%, Fe balance).(ii) Chromium steels.(iii) Nickel steels.(iv) High nickel chromium steels.(v) Low carbon molybdenum steels(vi) Tools steels.(vii) Stainless steels

Welding of Low Alloy High Strength Steels - Low alloy steels (under 0.3% carbon) are 10 to 30% stronger than the straight carbon steels and are used where resistance to corrosion and heat is desired. Low alloy steels are of course slightly more expensive than straight carbon steels.

Low alloy high strength steels are characterized by good resistance to atmospheric and other mildly corrosive environments.  Such steels have yield strength values between 50,000 and 80,000 psi (3500 and 5600 kglcm2) and tensile strength values between 70,000 and 110,000 psi (4900 and 7700 kglcm2).Such steels may find applications as sheets and thin plates in trucks, railroad cars, road bui1ding equipment, etc.

WeldabilityWeldability of low alloy steels is dependent upon the composition and the harden ability, those exhibiting low hardenability being welded with relative ease, while those of high harden ability require preheating and post heating. Welding of such steels is carried out on much the same lines as that of carbon steels of equivalent carbon contents.Sections of 6 mm or less may be welded with mild steel filler metal and may secure joint strengths approximating base metal strength by virtue of alloy pickup in the weld metal and weld reinforcement. Alloys of higher strength require filler metals of mechanical properties matching the base metal.Special alloys with creep resistant or corrosion resistant properties must be welded with filler metals of the same chemical analysis.

An important consideration in welding many high strength low alloy steels is the prevention of under bead or cold cracking; which can be minimized by using low hydrogen type electrodes (either mild or alloy steel analyses) and a slower rate of cooling.Welding of low alloy high strength steels with high hydrogen types of covered mild steel electrodes, however, usually requires that the assembly be preheated. Welding with the low hydrogen electrodes generally does not require preheating except for highly restrained sections.

Welding ProcessesThe low alloy high strength steels can be welded readily by an the common welding processes. However, low alloy high strength (hot-rolled) steels have the following chemistry limitations:

(a) For resistance welding, 0.12% C max and 1% Mn max.(b) For other welding processes, 0.2% C max, 1.25% Mn max, 0.05% S Max, 0.15% P max and 0.90% Si max.1. Oxyacetylene Welding

The type of filler rod employed depends upon the mechanical properties required. A high tensile steel rod will prove effective. For corrosion resistance, etc., the weld metal must match with the parent metal.A flux is used to counteract the oxidation of alloying elements. After welding, a post heat treatment is necessary for the heat treatable low alloy steels to refine the grain structure.

2. Flux shielded Metal Arc WeldingMild steel electrodes will work very well with steels having a carbon content under 0.14%. Weld develops tensile strength as high as 80,000 psi (5600 kg/cm2) as the result of alloy pickup from the base steel.Where higher strength at better ductility is desired, low alloy steel electrodes may be required. Because of greater crack sensitivity of the low alloy steel electrodes, preheating may be necessary.Where corrosion is a factor, it may be advisable to use core wires of the same composition as the base steel. Given below are the recommendations for welding some typical low alloy high strength steels.

3. Submerged Arc WeldingBoth hot rolled and heat treated grades of low alloy steels are welded by using the method very similar to that used for welding low carbon steels.Because of deep penetration characteristics of this process, mild steel filler rods are usually satisfactory. Preheating is generally not necessary.

4. Thermit WeldingLow alloy high strength steels can be Thermit welded. Metallic elements are added to the thermit mixture to obtain composition close to that of the parent metal. Metallic elements are added either as metallic pieces or in the form of combinations of oxides of the required elements with aluminium. Stress relieving, heat treatments, when required, should be carried out between 595 and 675°C.

5. Resistance Spot WeldingSpot welding can be carried out satisfactorily. For alloys having high hardenability, special treatments such as preheating, grain refinement and tempering heat treatments may be incorporated in the welding cycle

6. Other joining processes includeMIG WeldingAtomic Hydrogen weldingSeam welding. Brazing

Welding of Cast Iron -Cast irons include a number of iron base materials that contain Carbon 1.7 to 4.5%

Silicon 0.5 to 3%Manganese 0.2 to 1.3% Phosphorus0.8% Max.Sulphur 0.2% Max.Alloy cast irons contain the following elements in addition to the ones mentioned aboveMolybdenumNickel ChromiumCopper Cast Irons can be classified as(i) White Cast Iron (ii) Gray Cast Iron(iii) Malleable Cast Iron(iv) Nodular Cast Iron

(i) White Cast Iron Welding or brazing of white Cast Iron is rarely required or done. (ii) Gray Cast Iron It shows a grayish surface when fractured. It contains C 2.5-3.8%Si 1.1-2.8%Mn 0.4-1.0%P 0.15%S 0.10%Fe Balance It is marked by the presence of flakes of graphite in a matrix of ferrite or pearlite. It contains a portion of carbon in free form as graphite flakes and the rest in the combined form as cementite, pearlite, etc

Gray cast iron possesses lowest melting point of all the ferrous alloys. Gray cast iron is easy to machine. Cast iron is brittle in nature. Gray cast iron is probably the most difficult of all metals to weld economically. Nevertheless, the number of gray iron castings made and the inevitably high percentage of breakages that occur in service make welding necessary.The three major areas of application of welding to cast iron are:(i) Repair of casting defects in the foundry.(ii) Repair of castings that have become damaged or worn in service.(iii) To join together separately cast sections.(iii) Malleable Cast IronWhereas in gray cast iron, the graphite has a flake appearance, it possesses a quasi-spheroidal (temper carbon) appearance in malleable iron. Malleable iron is obtained when

white cast iron is heated to 760°C for 24 hours per each 25 mm of thickness and then cooled slowly. This heat treatment converts graphite from flake form to quasispheroidal shape. Welding malleable iron destroys this heat treatment and turns the metal into white or gray cast iron.

Malleable iron containsC 2-3%Si 0.6-1.3%Mn 0.2-0.6%P 0.15%S 0.10%Malleable iron has a solidification range of 1400-1130°C.(iv) Nodular Cast IronWhereas in gray cast iron, the graphite has a flake appearance, it possesses a spheroidal appearance in nodular cast iron. The spheroidal graphite, which is the result of small additions of magnesium, cerium or cadmium to the cast iron, gives to it superior ductility and toughness.

Nodular, Ductile or Spheroidal Cast Iron contains C 3.2-4.2% Si 1.1-3.5%Mn 0.3-0.8% P 0.08% S 0.02% Most of the welding is done on gray cast iron. Malleable iron welding and ductile iron welding are comparatively less common.

Welding of Tool Steel and Carbon Steels - Welding of Tool Steel Tools steels range from plain carbon steels (C 1.41.5%) to high alloy high speed steels, some of which have a total alloy content that exceeds 25%. Given below are the typical compositions (%) of a few tool steels (i) W-high speed steelC 0.75, Cr 4, V 1, W 18, Co 5 (ii) Mo-high speed steelC 0.8,Cr 4, V 1.5, W 4, Mo 5, Co 12 (iii) High C High Cr SteelC 1.5, Cr 12, Mo.1, Co 3 (iv) Air hardening steelC 2.25, Cr 5.25, V 4.75, W 1, Mo 1 (v) Oil hardening steelC 0.9, Mn 1, Cr 0.5, W 0.5

vi) Water hardening steelC 0.6/1.4, V 0.25(vii) Hot work steelC 0.4, Cr 3.25, V 0.4, Mo 2.5(viii) Shock resisting steelC 0.5, Cr 3.25, Mo 1.4

Welding of Carbon Steels Carbon steels differ from cast iron as regards the percentage of carbon. They contain carbon from 0.10 to 1.5% whereas cast iron possesses carbon from 2.0 to 4.2%.Carbon steels are classified asLow carbon steels - C 0.05-30%Medium carbon steels - C 0.30-50%High carbon steels - C 0.50-5%Carbon steels find uses as follows:

Low carbon steel0.05-15% C - Rivets, screws, press sheets, pipe, nail and chain0.15-30% C - Plates, structural shapes and bars.Medium carbon steel0.30 - 50% C - Shafts, axles, connecting rods.High carbon steel0.50 - 0.75% C - Crankshaft, scraper blades, automobile springs, anvils, band saws, etc.0.75 - 0.90% C - Punches and chisels.0.90 - 1.0% C - Shear blades and knives.1.00 - 1.2% C - Taps, dies, milling cutters, lathe tools, etc.1.20 - 1.3% C - Reamers and files.1.30 - 1.5% C - Wire drawing dies and metal cutting saws.