Welding

Prepared By:Dhananjay PradhanAssistant Professor

Mechanical Engg. DepartmentKrishna Engineering College

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TYPES OF SOLID STATE WELDING

• COLD WELDING• ULTRASONIC

WELDING• FRICTION

WELDING

• RESISTANCE WELDING

• EXPLOSION WELDING

• DIFFUSION BONDING

COLD WELDING

• Pressure is applied to the work pieces, through either dies or rolls,.

• Both the parts should be ductile because of the plastic deformation involved

• It can be used to join small w/p made of soft, ductile metals.

ULTRASONIC WELDING• The faying surfaces are

subjected to a static normal force and oscillating shearing stresses.

• Shearing stresses are applied by the tip of a transducer.

• Range of frequency of oscillation is generally 10khz-75khz.

• Temp. required in the weld zone is usually in the range from 1\3 to 1\2 of the melting point of metals joined.

FRICTION WELDING

• Heat required for welding is generated through friction at the interface of the two components being joined.

• Oxides& other contaminants at the interface are removed by the radial outward movement of the hot metal at the interface.

• The shape of welded joint depends on the axial pressure applied.

WELD ZONE• The weld zone is usually

confined to a narrow region which depends on the following:

1. The amount of heat generated.

2. The thermal conductivity of the materials.

3. The mechanical properties of the materials at elevated temperatures.

4. The shape of the welded joint depends on the rotational speed and axial pressure applied.

INERTIA FRICTION WELDING

• The energy required for friction heating is supplied by the kinetic energy of a flywheel.

• The weld is completed when the flywheel has come to a stop.

LINEAR FRICTION WELDING

• The two components to be joined is subjected to a linear reciprocating motion, as opposed to a rotary motion.

• The process is capable of welding square or rectangular components, as well as rounded parts made of metals or plastics.

FRICTION STIR WELDING

• A third body is rubbed against the two surfaces to be joined in the form of a small rotating non consumable tool that is plunged into the joint.

• The thickness of the joint can be as little as 1mm and so much as 30mm.

• FSW welds are of high quality, with minimal pores and with uniform material structure.

• Used for welding of aerospace alloys, polymers & composite materials.

RESISTANCE WELDING

• Heat required for welding is produced by means of electrical resistance across the two components to be joined.

• These processes do not require consumable electrodes, shielding gases or flux.

• Actual temp.rise at the joint depends on the specific heat and on the thermal conductivity of the metals to be joined.

• It is mostly used process in sheet metal fabrication and in automotive-body assembly.

TYPES OF RESISTANCE WELDING

• SPOT WELDING• SEAM WELDING• HIGH FREQUENCY

RESISTANCE WELDING

• PROJECTION WELDING

• FLASH WELDING• STUD WELDING• PERCUSSION

WELDING

SPOT WELDING

• The tips of two opposing solid cylindrical electrodes touch a lap joint of two sheet metals.

• Pressure is applied until the current is turned off for strong bond

SEAM WELDING

• Electrodes are replaced by rotating wheels or rollers.

• Seam welding produces a joint that is liquid tight and gas tight.

HIGH FREQUENCY RESISTANCE WELDING

• This is similar to seam welding, except that high frequency current(up to 450khz) is employed.

• Typical example is production of butt welded tubing where the current is conducted through two sliding contacts.

PROJECTION WELDING

• High electrical resistance at the joint is developed by embossing one or more projections on one of the surface to be welded.

• The electrodes, made of copper- based alloys,and water cooled to keep their temp.low, are large and flat.

FLASH WELDING• Heat is generated from the arc as the ends of the

two members begin to make contact and develop an electrical resistance at the joint.

• An axial force is applied at a controlled rate.• Weld is formed by plastic deformation.• Suitable for end to end or edge to edge joining of

sheets of similar or dissimilar metals.• Impurities and contaminants are squeezed out

during this operation, so the quality of the weld is good.

Flash-Welding Process

STUD WELDING

• It is also called stud arc welding.

• The stud serves as one of the electrodes while being joined to another component.

• A disposable ceramic ring(ferrule is placed around the joint to concentrate heat generated.

EXPLOSION WELDING

• Pressure is applied by detonating a layer of explosive that has been placed over one of the components being joined.

• Cold pressure welding by plastic deformation takes place.

• Bond strength is very high.

ARC WELDING• The heat required is obtained from

electrical energy.• An arc is produced between the tip of the

electrode and the w/p to be welded, by the use of an AC or a DC power supply.

• This flow of current between the electrode and the work piece is better described as a column of ionized gas called plasma.

TYPES-1. Arc welding(consumable electrode)2. Arc welding( non consumable electrode)

POLARITY

• In straight polarity work piece is the positive pole and electrode is negative.

• In reverse polarity arrangement is just reversed.

• Polarity can be used to control the location of the liberated heat.

• If large deposits are to be made on heavy work piece, straight polarity is used, while if it is necessary to keep the w/p cool reverse polarity is used.

ARC WELDING(CONSUMABLE ELECTRODE)

1. Shielded metal arc welding

2. Submerged arc

3. Gas metal arc (MIG)

4. Flux-cored arc

5. Electro gas

6. Electro slag

ARC WELDING( NON CONSUMABLE ELECTRODE

• Typically it use a tungsten electrode.as one pole of the arc,it generates the heat required for welding.a shielding gas is supplied from an external source.

1. Gas tungsten arc

2. Atomic hydrogen

3. Plasma arc

SHIELDED METAL ARC WELDING(SMAW)

• Shielded arc welding is a fusion process where heat is obtained from an electric arc which is formed between a base metal and an electrode.

• The electric arc is generated by touching the tip of a coated electrode against a w/p and then withdrawing it quickly to a distance sufficient to maintain the arc.

• Electrodes are in the shape of a thin, long stick, so this process is also known as stick welding.

• The electrode coating deoxidizes the weld area and provides a shielding gas to protect it from oxygen in the environment.

• The polarity of a DC,i.e. the direction of current flow, its selection depends on such factors as type of electrode , the metals to be welded, and the depth of the heated zone.

CHARACTERISTICS+ Equipment is simple, very portable and less

expensive.+ Welding can be done in virtually any position.- Rate of metal deposition is not high.- Welding is interrupted each time an electrode is

consumed, causing downtime and a wasted stub.- Slag must be chipped and wire brushed from the

weld surface.- Electrode selection and care are more critical for

welding hardenable steels than by some other processes.

SUBMERGED ARC WELDING(SAW)

• The weld arc is shielded by a granular flux.it acts as a thermal insulator, promoting deep penetration of heat into the w/p.

• Then flux is fed into the weld zone by gravity flow through a nozzle.

• It is fed automatically through a tube(welding gun).

• It is used to weld carbon, alloy steel and stainless steel sheet or plate.

CHARACTERISTICS

+ Very high current can be used.+ High deposition rate and good penetration.+ The process is thermally efficient.+ Each size of wire can be used over a wide

range of current settings.+ The weld beads are extremely smooth.- Weld can not be seen, it is more difficult to

guide it..- It is largely limited to flat-position welding.

GAS METAL-ARC WELDING(MIG)

• Weld area is shielded by an effectively inert atmosphere of argon, helium, carbon dioxide, or various gas mixtures.

• Suitable for welding ferrous and nonferrous metals and is used extensively in the metal-fabrication industry.

CHARACTERISTICS+ It can produce high quality welds at high

speeds .+ There is no flux to remove.+ It is a very versatile process, used on both

light and heavy structural plates.+ It is possible to weld in all positions.+ Deep penetration is possible.+ Process can be used on out-of-position

welding in pulsed mode.

FLUX-CORED ARC WELDING

• It is similar to gas metal arc welding, with the exception that the electrode is tubular in shape and filled with flux.

• Cored electrodes produce a more stable arc, improve weld contour, and produce better mechanical properties of the weld metal.

CHARACTERISTICS+ High deposition rate can be achieved.+ It can be used for a wide range of metal

thicknesses.+ Self-shielding process is relatively simple, and the

welding gun is relatively light and easy to use.+ The deposition rate is about twice that of SMAW

for a comparable setup.+ It can be used in any position in smaller-diameter

Wires.- Flux-cored wire emits a large amount of welding

fume during operations.- A equipped gun is required to draw off exhaust

fumes and improve visibility.

ELECTROGAS WELDING

• EGW is used primarily for welding the edges of sections vertically in one pass, with the pieces placed edge to edge(butt joint).

• It is classified as a machine-welding process, because it requires special equipment.

• The shielding is done by means of an inert gas.the gas may be provided from an external source, or it may be produced from flux cored electrode.

ELECTROSLAG WELDING

• It is similar to electro gas welding.the main diff. Is that the first arc is started and then flux is added and then melted by the heat of the arc.

• The electrode may be consumable or nonconsumable.

CHARACTERISTICS

+ It is automatic.

+ Heating is uniform, which keeps warpage to a minimum.

+ Joint preparation is not required.

ELECTRODE COATINGS Electrodes are coated with claylike materials that

include binders and powdered materials.the coating,has the following basic functions-

1. To stabilize the arc.2. To generate gases to act as a shield against the

surrounding atmosphere.3. To control the rate at which the electrode melts.4. To act as a flux to protect weld against

formation of oxides, nitrides and other inclusions.

5. To add alloying elements to the weld zone to enhance the properties of the joint

GAS TUNGSTEN ARC-WELDING (GTAW)

GTAW formerly known as TIG welding.• The filler metal is supplied from a filler wire.it is

similar to metals to be welded, and flux is not used.

• A constant and stable arc gap is maintained at a constant current level.

• Welding with GTAW can be done without filler metals-ex.close fit joints.

• Contact of the electrode with the molten metal pool should be avoided.

• It is suitable for thin metals like aluminum.

CHARACTERISTICS+ It is used to make high quality weld in almost all

metals and alloys, including high temp. alloys.+ No flux is used, so almost no weld clean up is

required.+ Very little weld spatter since the weld metal is not

carried across the arc.+ Welding can be done in all positions.+ The process is particularly well suited for welding

thin materials where a high quality finish is desired,.

- It is relatively slow and not well suited for welding heavier metals.

ATOMIC HYDROGEN WELDING (AHW)

• AHW uses an arc in a shielding atmosphere of hydrogen.

• The arc is between two tungsten or carbon electrodes.

• W/p is not a part of the electrical circuit.

• The hydrogen gas also cools the electrode.

PLASMA-ARC WELDING (PAW)

• A plasma is ionized hot gas, composed of nearly equal numbers of electrons and ions.

• The arc is stable and reaches temp. as high as 33,000degc.

• PAW has higher energy concentration, less thermal distortions and higher welding speeds.

• There are two methods of PAW-

1.Transferred arc- the w/p being welded is part of the electrical circuit. The arc transfers from the electrode to the w/p.

2.Non transferred arc- the arc occurs between the electrode and the nozzle, and the heat is carried to the w/p by plasma gas.

CHARACTERISTICS

+ Greater energy concentrations, improved arc stability, higher welding speeds and lower width-to-depth ratio of the weld bead for a given penetrations.

+ More arc length flexibility is possible without changing the bead width.

+ It ensures complete penetration and weld uniformity.

+ Close-fitting backing bars are not required.- Higher equipment cost compared to GMAW, short

life, need for greater welder knowledge, and a high rate of inert gas consumption.

THERMIT WELDING(TW)

• The process involves exothermic reactions between metal oxides and metallic reducing agents.

• The most commonly used mixture is fine particles of iron oxide, aluminum oxide, iron and aluminum.

• This non explosive mixture produces a max. theoretical temp.of 3200degc in less than a minute.

ELECTRON BEAM WELDING(EBW)

• The heat is generated by high velocity narrow beam electrons.

• The kinetic energy of electrons is converted into heat as they strike the w/p.

• The higher the vacuum, the more the beam penetrates and the greater the depth-to-width ratio.

• Typical application include welding of aircraft, missile, nuclear and electronic components and of gears and shaft for automotive industry.

LASER BEAM WELDING(LBW)

• LBW utilizes a high power laser beam as the source of heat, to produce a fusion weld.

• This process is particularly suitable for welding deep and narrow joints.

• Used in automotive industry,welding of transmission components and electronic industry.

• Laser beam may be pulsed for application such as spot welding of thin materials.continuous beam are used for deep welds on thick sections.

TYPES of Solid/Liquid Joining

• SOLDERING

• BRAZING

• ADHESIVE BONDING

BRAZING• When filler material is copper alloy, the process is called

brazing.• Temp. is raised enough to melt the filler metal but not

the w/p.• The molten metal fills the closely fitting space by

capillary action.• In braze welding the filler metal is deposited at the joint

with a technique similar to oxyfuel gas welding.• The strength of the brazed joint depends on 1. Joint design2. The adhesion at the interfaces between the w/p’s and the

filler metal.

FILLER METALS

• The choice of filler metal and of its composition are important, in order to avoid embrittlement of the joint, formation of brittle intermetallic compounds at the joint and galvanic corrosion in the joint.

• Diffusion between the filler metal & the base metal, the mechanical and metallurgical properties of a joint can change in subsequent processing or during the service life of a brazed component.

FLUXES

• To prevent oxidation and to remove oxide films from work piece surfaces, flux is essential in brazing.

• Brazing fluxes are generally made of borax, boric acid fluorides & chlorides.

• Wetting agents may also be added to improve both the wetting characteristics of the molten filler metal and the capillary action.

SOLDERING

• In soldering, the filler metal, called solder, melts at a relatively low temp.

• Important characteristics for solders are high wetting capability and low surface tension.

• Heat sources for soldering are usually soldering irons, torches or ovens.

TYPES OF SOLDERS AND FLUXES

• Meaning of solder is to make solid melt at a temp. that is the eutectic point of the solder alloy.

• Solders have tin-lead alloys in various proportions.• Fluxes are used as they are in brazing, and for same

purpose.

Types1. Inorganic acids or salts– which clean the surface rapidly

after the soldering, the flux residues should be removed by water to avoid corrosion.

2. Noncorrosive resin based fluxes, used in electrical applications.

WELD DEFECTS

A welded joint may develop various discontinuities.• The microstructure and grain size of the welded

joint depends on the amount of heat applied and the consequent temp. rise, on the degree of prior cold work of the metals, and on the rate of cooling after the weld is produced.

• Weld quality depends on many factors ,among them the geometry of weld bead and the presence of cracks, residual stresses, inclusions and oxide films, etc.

Fusion Weld Zone

POROSITY

• Porosity in weld is caused-1. by gases released during melting of the weld

area but trapped during solidification,2. by chemical reactions during welding,3. by contaminants.• Porosity is generally in the shape of spheres or

of elongated pockets, and distribution of porosity in the weld zone may be random, or concentrated in a certain region.

• Porosity in welds can be reduced by the following practices:-

1. Proper selection of electrodes and filler metals.

2. Improved welding technique, such as preheating of the weld area or an increase in the rate of heat input.

3. Proper cleaning, and the prevention of contaminants from entering the weld zone.

4. Reduced welding speeds, to allow time for gases to escape.

SLAG INCLUSIONS• Slag inclusions are compounds such as oxides,

fluxes, and electrode-coating materials.• If shielding gases are not effective during

welding, then contamination from environment may also contribute in inclusions.

• Slag inclusions can be prevented by:-1. cleaning of weld bead surface before the next

layer is deposited.2. providing enough shielding gas.3. permit enough space for proper manipulation of

the puddle of molten weld metal.

INCOMPLETE FUSION AND PENETRATION

• Incomplete fusion produces poor weld beads. A better weld can be obtained by:-

I. Raising the temp. of the base metal.

II. Cleaning the weld area, prior to welding.

III. Changing the joint design and type of electrode.

IV. Providing enough shielding gas.

• Incomplete penetration occurs when the depth of the welded joint is insufficient. Penetration can be improved by :-

1. Increasing the heat input.

2. Reducing the travel speed during the welding.

3. Changing the joint design.

4. Ensuring that the surface to be joined fit properly.

Incomplete Fusion

WELD PROFILE• It effects on the strength and appearance of the

weld.• It can signal incomplete fusion or the presence

of slag inclusions in multiple-layer welds.1. UNDERFILLING:- results when the joint is not

filled with the proper amount of weld metal.2. UNDERCUTTING:- results from the melting

away of the base metal and the consequent generation of a groove in the shape of a sharp recess or notch.it may lead to premature failure.

3. OVERLAP:- is a surface discontinuity caused by poor welding practice and by the selection of improper materials.

Discontinuities in Fusion Welds

CRACKS

• Cracks may occur in various locations and directions in the weld area.

• These cracks generally result from the following factors:-

1. Temp. gradients.2. variations in the composition of the weld zone .3. Embrittlement of grain boundaries.4. Hydrogen embrittlement.5. Inability of the weld metal to contract during

cooling.

Basic crack prevention measures

1. Change the joint design,to minimize stresses from shrinkage during cooling.

2. Change the parameters, the procedures, and the sequence of the welding process.

Cracks in Welded Joints

LAMELLAR TEARS

• It may develop because of shrinkage of the restrained components in the structure during cooling.

• Such tears can be avoided by providing for shrinkage of the members or by changing the joint design to make the weld bead penetrate the weaker component more deeply.

RESIDUAL STRESSES

• Because of localized heating and cooling during welding expansion and contraction of the weld area causes residual stresses in the w/p.

• Residual stresses can cause the following defects:-1. Distortion, warping, and buckling of the welded

parts.2. Stress corrosion cracking.3. Reduced fatigue life.4. Further distortion, if a portion of the welded

structure is subsequently removed.

Residual Stresses Developed During Welding

STRESS RELIEVING OF WELDS

• The problem caused by residual stresses, can be reduced by preheating the base metal or the parts to be welded.

• Preheating reduces distortion by reducing the cooling rate and the level of thermal stresses.

• For multilayer welds , the first and last layers should not be peened , in order to protect them against possible peening damage.

• Residual stresses can also be relieved or reduced by plastically deforming the structure by a small amount.

WELDABILITY

• It is defined as its capacity to be welded into a specific structure that has certain properties and characteristics and will satisfactorily meet service requirements.

• Mechanical and physical properties affects weldability.

TESTING WELDED JOINTS

• The quality of a welded joint is established by testing.

• Standard test procedures are:-

1. DESTRUCTIVE TECHNIQUES

2. NONDESTRUCTIVE TECHNIQUES

DESTRUCTIVE TECHNIQUES

1. Tension test

2. Tension-shear test

3. Bend test

4. Fracture toughness test

5. Corrosion and creep tests

NONDESTRUCTIVE TECHNIQUES

• Welded structures often have to be tested nondestructively, particularly for critical applications where weld failures can be catastrophic, such as pressure vessels, load bearing structural members and power plants.

• NDT consists of 1. visual2. radiographic3. magnetic-particle4. liquid-penetration5. ultrasonic