NONFUSION WELDING

Resistance Welding Resistance welding is a group of welding processes in which

coalescence is produced by the heat obtained from resistance of the work piece to electric current in a circuit of which the work piece is a part and by the application of pressure .

In resistance welding a low voltage (typically 1 V and very high current (typically 15000 A is passed through the joint for very short time typically 0.25 sec.

This high temperature heats the joint due to contact resistance at the joint and melts it.

The pressure on joint is continuously maintained and metal fuses together under this pressure.

The heat generated in resistance welding is expressed as H=KI2RT Where H= total heat generated in work J I=electric current,A t=time for which the electric current is passing through joint,s R= resistance of the joint,ohms k=constant on account of heat losses from the welded joint.

The resistance of the joint R is a complex factor to know as it is composed on

(a)resistance of electrode (b)the contact resistance between workpiece and electrode (c )contact resistance between two workpiece plates (d)the resistance of workplates The amount of heat released is directly proportional to

resistance. The main requirement of process is the low voltage and high

current which is obtained by means of step down transformer. The electric supply is to be closely controlled so that heat released is just enough to melt the joint and the subsequent fusion takes place due to force (forge welding) on the joint

The force required can be joined either mechanically or hydraulically or pneumatically

Variables in Resistance welding process Contact resistance between two workpiece plates

and their resistance themselves. Contact resistance is affected by

1. Surface finish on plates since rougher surface have higher contact resistance

2.Cleanliness of the surface. Oxides and other contaminants if present should be removed before resistance welding is done.

Forging Pressure It decreases contact resistance and there by increases the welding current for proper fusion.

Heat Balance The important characteristic of Resistance welding

is the Transfer of heat to two parts being joined differently so that proper fusion is obtained for dissimilar metal or of different thickness.

When two dissimilar metals with different electrical resistively or thermal conductivity are joined the heat liberated in lower resistively metal is less and therefore it is necessary to use electrode of higher resistively near metal or use electrode diameter of larger contact area near metal which has higher electrical conductivity

Electrodes For resistance WeldingThe function of electrodes are: 1.Carry high current required for fusion 2.Transmit the mechanical force to keep plates

under pressure and in alignment during fusion (high hardness)

3.Helps to remove heat from weld zone thus preventing over heating and surface fusion of work.

Hence electrodes should have High electrical conductivity High hardness

Metals Used Pure Copper----- Low strength Cu-Cd(Cd-0.5-1%)----High electrical

conductivity but low strength hence used for welding of non ferrous metals like Al, Mg

Cu-Cr(Cr-0.5-0.8%)---Lower electrical conductivity and high strength used for low strength steels as mild steel ,low alloy steel

Cu+(Co+Be)--- Low conductivity but high strength hence used for heat resisting alloys as stainless steel, steel with tungsten and other alloying elements.

Electrode Tip

Electrode tips are available in number of different configuration for different welding situations.

Since large amount of heat is liberated in electrode it is necessary to cool electrodes to maintain their strength. Hence cooling water is circulated through electrodes.

Advantages of Resistance Welding Very little skill is required to operate the

resistance welding. Mostly all m/cs are semiautomatic or automatic

Suitable for mass production and gives high production rate

No consumable are used except electric power and small electrode wear

Heating of w/p is confined to very small area so less distortion

Dis advantages Welding m/c is complex with various elements as

transformers, electrodes and heavy conductors for carrying high current. And m/c is expensive

Certain resistance welding machines are limited to lap joints. lap joints have inherent crevices which are responsible for stress concentration and loss of fatigue life. It also increases chances of corrosion.

Precaution:The electrode point should always be kept clean. If points are dirty or scaly there will be excessive heat in these points and cause burning or splitting.

Various types of Resistance Welding Processes Seven important resistance-welding processes.

These are1. Resistance spot welding, 2. Resistance seam welding3. Projection welding4. Flash welding5. Upset welding6.High-frequency resistance welding, 7.Percussion welding, .

Spot welding Spot welding is a type of resistance welding used to

weld various sheet metals. Typically the sheets are in the 0.5-3.0 mm thickness range.

The process uses two copper alloy electrodes to concentrate welding current into a small "spot" and to simultaneously clamp the sheets together.

Forcing a large current through the spot will melt the metal and form the weld.

The attractive feature of spot welding is a lot of energy can be delivered to the spot in a very short time (say a few hundred milliseconds). That permits the welding to occur without excessive heating to the rest of the sheet.

Diameter of the weld spot is in the range 1/8” - 1/2” (3 - 12 mm).

The amount of heat (energy) delivered to the spot is determined by-

1. the resistance between the electrodes 2. amplitude of current 3. and duration of the current.

The amount of energy is chosen to match the sheet's 1.material properties,

2.its thickness, and 3. type of electrodes.

Applying too little energy won't melt the metal or will make a poor weld.

Applying too much energy will melt too much metal make a hole rather than a weld.

Energy delivered to the spot can be controlled to produce reliable welds.

Function of Electrode The electrodes do not take part in

Resistance-welding: They keep the parts aligned and in place. They apply the pressure required for developing the

correct surface resistance at the interface, for containing the molten metal avoiding weld expulsion and for forging the nugget near the end of the cycle.

They convey the electric welding current and, being water cooled, they also dissipate excess heat once the weld is completed, avoiding surface melting.

The diameter of electrode determines the diameter of fusion zone The diameter of fusion zone is given by de=(0.1+2t)mm

De = electrode diameter T=thickness of plate.

The Squence of Resistance spot welding consist of 1.Squeeze Time : It is time required for electrode to

align and clamp two workpiece together under them and provide necessary electrical contact.

2.Weld Time: is time of current flow through workpiece till they are heated to melting temperature.

3.Hold time: is time when pressure is maintained on molten metal w/o electric current. During this time pieces are expected to forge weld

4.Off time: is time during which pressure on electrodes is taken off so that plates can be positioned for next spot.

Process Variables:

Forging Pressure is applied to ensure that during the solidification nugget does not develop any porosity or crack. Porosity may also develop if forging pressure is released prematurely.

Forging pressure depends on (i)Thickness of metal (ii)Goemetry of weld.

Current1.Too high current will cause Weld explusion,cavitation and weld cracking Reduced mechanical properties Electrode embedment in the surface2. Too Low CurrentUnfused surface and poor weld.Time More time of application of current may lead

to boiling, porosity and growth of nugget upto electrode face.

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Advantages

It is non labor-intensive and can be easily automated.

Adaptable to a wide variety of electrically conductive materials

Applicable to a variety of thicknesses Very short cycle times A robust process Tolerant to fit-up variations

Application

In the automobile manufacturing industry, where it is used almost universally to weld the sheet metal to form a car. Spot welders can also be completely automated, and many of the industrial robots found on assembly lines are spot welders (the other major use for robots being painting).

Spot welding straps to nickel-cadmium or nickel-metal-hydride cells in order to make batteries

Advantages: Small HAZ Fast Welding speed Low cost per weld if large production is done Clean process Less operational skills required.Disadvantages:1.Limited to metal that range in thickness from

0.03-0.18 inch2.Mats with high thermal and electrical conductivity are

difficult to weld by Spot welding.3. Spot weld should not be made too close to the edge

otherwise metal will become overheated and cracking will result around the weld.

Resistance Seam Welding

Seam Welding is a Resistance Welding (RW) process of continuous joining of overlapping sheets by passing them between two rotating electrode wheels.It is specialised process of spot welding.

Here disc electrodes are used which are continoulsy rotated so that w/p gets advanced underneath them while at same time pressure on joint is maintained.

Heat generated by the electric current flowing through the contact area and pressure provided by the wheels are sufficient to produce a leak-tight weld.

The current is applied through heavy copper electrodes in series of pulses at proper intervals. The timing is adjusted so that pulses overlap each other and form a continuous seam joint.

Application

Seam welding is restircted to welding of thin materials thickness range from 0.10-0.187inch

Can be carried out for steels, Al, Mg and Ni alloys but not for Copper or its alloys

Advantages

Adaptable to a variety of electrically conductive materials

Economical Ability to produce leak-tight welds Fast processing times

Projection welding Projection welding is a variation of resistance

welding in which current flow is concentrated at the contact surfaces of interest by an embossed, cold headed, or machined projection.

The projections are generally very small of theorder of 0.8mm and are obtained by means of embossing.

The projection(s) effectively localize the current, forcing the parts to heat predominately at the mating surfaces.

This rapid interfacial heating allows for the application of resistance projection welding across a wide range of applications not feasible by conventional resistance spot welding.

Advantages

The advantages of projection welding include It is versatile process high speed and ability to automate, the ability to make a number of welds

simultaneously and minimization of marking on one side of joints in

sheet materials. Welds can be placed closer than possible in spot

welding. The electrodes can be made to take shape of

assembly fixture.

Proper heat balance can be easily obtained in projection welding by making projection in thicker plate while welding sheet of different thicknesses. Ability to join sheets of widely dissimilar thicknesses For welding dissimilar metals the projections are made on material having higher electrical conductivity to provide proper heat balance.

Because of larger sized electrodes used the life of electrodes is much longer than that of spot welding electrodes

Can be used to minimize surface marking when joining sheets Increased energy efficiency Minimal cycle time Due to the efficiency of power transfer, thicker materials can be

successfully welded. Materials as thick as 3 mm (0.125 in) can be successfully welded

Solid state welding

It is a group of welding processes which produces coalescence at temperatures essentially below the melting point of the base materials being joined, without the addition of brazing filler metal. Pressure may or may not be used.

Cold Welding (CW)

Cold welding is a solid state welding process which uses pressure at room temperature to produce coalescence of metals with substantial deformation at the weld.

Welding is accomplished by using extremely high pressures on extremely clean interfacing materials.

Sufficiently high pressure can be obtained with simple hand tools when extremely thin materials are being joined.

When cold welding heavier sections a press is usually required to exert sufficient pressure to make a successful weld

Indentations are usually made in the parts being cold welded. The process is readily adaptable to joining ductile metals.

Aluminum and copper are readily cold welded. Aluminum and copper can be joined together by cold welding.

Flash welding

Flash welding is a development of resistance butt welding and is particularly suitable for butt welding complex or larger sections.

It is used for a wide range of component shapes and sizes from bicycle wheel rims to rails.

Process In flash welding, the components are clamped between

dies and brought together slowly with the current switched on, see Fig.1.

In this process ends of rods (tubes, sheets) are heated and fused by an arc struck between them and then forged (brought into a contact under a pressure) producing a weld.The welded parts are held in electrode clamps, one of which is stationary and the second is movable

After a pre-set material loss has occurred, sufficient to heat the material behind the interface to its plastic state, the components are forged together to expel melted material and contaminants.

Precautions Careful setting up and control of the welding sequence,

particularly flashing and upset, are required to ensure freedom from weld discontinuities.

Automatic monitoring of process parameters, particularly current, force and displacement, can be used to verify conformance with the set sequence, and provide a guide to weld quality.

Flash welds in steels have a low impact toughness performance and require post-weld heat treatment for applications where impact toughness is specified. Certain weld interface flaws, such as flat spots, are not readily detected by non-destructive testing techniques

Advantages

Flash welding is ideally suited to producing butt welds in large or complex sections.

Weld time is relatively short, from a few seconds for the thinnest sections to a few minutes for the largest.

Flash Welding can be used for joining many ferrous and nonferrous alloys and combinations of dissimilar metals. Practically Flash-welding can be applied to any metal that can be forged

Economical in operation and in the use of metal.

Suitable for mass production. Unskilled workforce needed. Strong welds obtained. Good fatigue properties available. No special symmetry requirements (different

from for Friction welding).

Limitations: Removal of flash required (same as for Friction welding). Process may affect or remove any strain hardening (cold

work) properties. Heat treatment after Flash-welding may be needed to

develop full properties. Costly maintenance of equipment due to flashing. Fire hazards present. Electric power and upsetting force in available equipment

limit the weldable size. Heat unbalance from different materials may cause

upsetting problems. Shunt effect for closed rings rolled and flash welded may

need attention. High accuracy alignment is required.

Upset welding (UW) It is a resistance welding process which produces

coalescence simultaneously over the entire area of abutting surfaces or progressively along a joint, by the heat obtained from resistance to electric current through the area where those surfaces are in contact.

Pressure is applied before heating is started and is maintained throughout the heating period

The equipment used for upset welding is very similar to that used for flash welding

The difference from flash welding is that the parts are clamped in the welding machine and force is applied bringing them tightly together.

High-amperage current is then passed through the joint, which heats the abutting surfaces.

When they have been heated to a suitable forging temperature an upsetting force is applied and the current is stopped.

The high temperature of the work at the abutting surfaces plus the high pressure causes coalescence to take place.

After cooling, the force is released and the weld is completed

Many different shapes like wire, bar, strip and tubing of various materials can be joined end to end by Upset-welding

Advantages Fast process Ease of parameters control (only current, time and

force) High quality, absence of typical fusion defects Metallurgical properties comparable to those of hot

worked material. Simple, sturdy and reliable equipment operated by

unskilled workers Tolerance for minor alloy deviations Large selection of materials, including difficult to

weld ones.

Limitations:

Equipment generally suitable to one type of applications only

High-frequency resistance welding, High Frequency current in range of 300-1000KHz is

applied to the welding area and squeeze force is added to workpiece heated from the heat resistance.

In this process the high frequency welding current is introduced into the metal at the surfaces to be welded but prior to their contact with each other.

The High Frequency electric resistance welding method is classified, depending on method applied in application of High frequency current on workpiece:

1.High frequency Induction 2.High Frequency Resistance Welding.

In High Frequency Induction ERW method Induction coil is used in inducting high frequency current to generate heat.

It is used for joining small diameter steel pipes.

In High Frequency Resistance Welding, contactor is applied to work piece to directly provide current. Used for large diameter steel pipes.

This process is ideally suited for making pipe, tubing, and structural shapes.

It is used for other manufactured items made from continuous strips of material.

Percussion Welding

Percussion Welding uses electrical energy stored in a condenser to produce an intense momentary power discharge to provide the localized heating at the interface.

It is suitable for joining dissimilar metals that are not weldable by flash butt welding, or when flash is not desirable at the weld joint

Pressure is applied progressively during or immediately following the electrical discharge. This process is quite similar to flash welding and upset welding, but is limited to parts of the same geometry and cross section

Arc is produced at the abutting surfaces by the very rapid discharge of stored electrical energy across a rapidly decreasing air gap.

This is immediately followed by application of pressure to provide an impact bringing the two parts together in a progressive percussive manner.

Advantages

There is an extremely shallow depth of heating and time cycle is very short.

It is used only for parts with fairly small cross-sectional areas.

Diffusion welding

It is a solid state welding process which produces coalescence of the faying surfaces by the application of pressure and elevated temperatures.

The process does not involve microscopic deformation melting or relative motion of the parts

The process is used for joining refractory metals at temperatures that do not affect their metallurgical properties.

Successful welds have been made on refractory metals at temperatures slightly over half the normal melting temperature of the metal.

To accomplish this type of joining extremely close tolerance joint preparation is required and a vacuum or inert atmosphere is used.

The process is used quite extensively for joining dissimilar metals.

The process is considered diffusion brazing when a layer of filler material is placed between the faying surfaces of the parts being joined.

These processes are used primarily by the aircraft and aerospace industries.

The characteristic process parameters are the welding temperature, the compression the welding time.

To get a surface free of oxidation the process is done in vacuum or inert gas atmosphere.

Explosion welding

It is a solid state welding process in which coalescence is effected by high-velocity movement together of the parts to be joined produced by a controlled detonation.

Even though heat is not applied in making an explosion weld it appears that the metal at the interface is molten during welding.

Advantages: Explosion welding creates a strong weld

between almost all metals. It has been used to weld dissimilar metals

that were not weldable by the arc processes. The weld apparently does not disturb the

effects of cold work or other forms of mechanical or thermal treatment .

The strength of the weld joint is equal to or greater than the strength of the weaker of the two metals joined.

Application One of the most widely used applications of

explosion welding has been in the cladding of base metals with thinner alloys.

Joining of tube-to-tube sheets for the manufacture of heat exchangers.

The process is also used as a repair tool for repairing leaking tube-to-tube sheet joints.

Another and new application has been the joining of pipes in a socket joint.

Forge welding

It is a solid state welding process which produces coalescence of metals by heating them in a forge and by applying pressure or blows sufficient to cause permanent deformation at the interface

This is one of the older welding processes and at one time was called hammer welding.

Forge welds made by blacksmiths were made by heating the parts to be joined to a red heat considerably below the molten temperature

Friction welding It is a solid state welding process which produces

coalescence of materials by the heat obtained from mechanically-induced sliding motion between rubbing surfaces.

The work parts are held together under pressure. This process usually involves the rotating of one

part against another to generate frictional heat at the junction.

When a suitable high temperature has been reached, rotational motion ceases and additional pressure is applied and coalescence occurs.

Advantages

Ability to produce high quality welds in a short cycle time.

No filler metal is required and flux is not used.

The process is capable of welding most of the common metals.

It can also be used to join many combinations of dissimilar metals

Factors affecting Friction Welding There are three important factors involved in making a friction

weld: The rotational speed which is related to the material to be

welded and the diameter of the weld at the interface. The pressure between the two parts to be welded. Pressure

changes during the weld sequence. At the start it is very low, but it is increased to create the frictional heat. When the rotation is stopped pressure is rapidly increased so that forging takes place immediately before or after rotation is stopped.

The welding time. Time is related to the shape and the type of metal and the surface area. It is normally a matter of a few seconds. The actual operation of the machine is automatic and is controlled by a sequence controller which can be set according to the weld schedule established for the parts to be joined

Application Of Friction Welding Lightweight tanks from aluminium alloys or

aluminium based metal matrix composites (MMCs) Military bridges and amphibious personnel carriers Steel tanks that could be friction stir welded from

extra high strength armour plate using two passes from two sides after further tool and parameter development

Titanium lightweight field howitzers

Hot pressure welding It is a solid state welding process which produces

coalescence of materials with heat and the application of pressure sufficient to produce macro-deformation of the base metal.

In this process coalescence occurs at the interface between the parts because of pressure and heat which is accompanied by noticeable deformation.

The deformation of the surface cracks the surface oxide film and increases the areas of clean metal. Welding this metal to the clean metal of the abutting part is accomplished by diffusion across the interface so that coalescence of the faying surface occurs.

This type of operation is normally carried on in closed chambers where vacuum or a shielding medium may be used. It is used primarily in the production of weldments for the aerospace industry

Roll welding It is a solid state welding process which

produces coalescence of metals by heating and by applying pressure with rolls sufficient to cause deformation at the faying surfaces.

This process is similar to forge welding except that pressure is applied by means of rolls rather than by means of hammer blows.

Coalescence occurs at the interface between the two parts by means of diffusion at the faying surfaces

Applications

One of the major uses of this process is the cladding of mild or low-alloy steel with a high-alloy material such as stainless steel.

It is also used for making bimetallic materials for the instrument industry.

Ultrasonic welding It is a solid state welding process which

produces coalescence by the local application of high-frequency vibratory energy as the work parts are held together under pressure.

Welding occurs when the ultrasonic tip or electrode, the energy coupling device, is clamped against the work pieces and is made to oscillate in a plane parallel to the weld interface.

The combined clamping pressure and oscillating forces introduce dynamic stresses in the base metal.

This produces minute deformations which create a moderate temperature rise in the base metal at the weld zone.

This coupled with the clamping pressure provides for coalescence across the interface to produce the weld.

Ultrasonic energy will aid in cleaning the weld area by breaking up oxide films and causing them to be carried away.

This process is used extensively in the electronics, aerospace, and instrument industries. It is also used for producing packages and containers and for sealing them.