Fabrication Methods(Welding)

8/11/2019 Fabrication Methods(Welding)

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Classification of fabrication

methods

Mechanical joining by means of bolts, screws & rivets.

Adhesive bonding by employing synthetic glues such as epoxy

resins Welding , brazing & soldering


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Various factors affect

during selection of

fabrication method Type of assembly : permanent, semipermanent & temporary

Materials being joined : steels, cast irons, similar or dissimilar

metals Economy achieved

Types of service required : such as assembly subjected to

heavy loading, impact loading, high temp. etc.


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Welding

(fabrication method)

Welding is a materials joining process which produces

coalescence of materials by heating them to suitable

temperatures with or without the application of pressure or

by the application of pressure alone, and with or without

the use of filler material.

Welding is used for making permanent joints.

It is used in the manufacture of automobile bodies, aircraft

frames, railway wagons, machine frames, structural works,

tanks, furniture, boilers, general repair work and ship

building.


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Classification of welding

process1. Arc welding:I. gas metal arc welding

II. Gas tungsten arc welding

III. Plasma arc welding

IV. Shielded metal arc weldingV. Submerged arc welding

VI. Carbon arc welding

VII. Flux cored arc welding

2. Gas welding:

I. oxyacetylene weldingII. Oxyhydrogen welding

III. Pressure gas welding

IV. Air acetylene welding


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Conti.

3. Resistance welding:I. Flash weldingII. Upset welding

III. Projection welding

IV. Resistance seam welding

V. Resistance spot welding

4. Other welding processes:I. Thermit welding

II. Electroslag welding

III. Electron beam welding

IV. Laser beam welding

V. Forge welding

VI. Friction welding

VII. Diffusion welding

VIII. Explosion welding

IX. Brazing

X. Braze welding

XI. soldering


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Welding terms

(Terminology)

Backing:It is the material support provided at the root side of

a weld to aid in the control of penetration.

Base metal: The metal to be joined or cut is termed the base

metal.

Bead or weld bead: Bead is the metal added during a single

pass of welding. The bead appears as a separate material from

the base metal.

Crater: In arc welding , a crater is the depression in the weld

metal pool at the point where the arc strikes the base metal.

Deposition rate: The rate at which the weld metal is deposited

per unit time is the deposition rate is normally expressed as

kg/h


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Conti

Fillet weld: The metal fused into the corner of a joint made bytwo pieces placed at approximately 900to each other is termedfillet weld.

Penetration: It is the depth up to which the weld metal

combines with the base metal as measured from the topsurface of the joint.

Puddle: The portion of the weld joint that is melted by the heatof welding.

Root: It is the point at which the two pieces to be joined bywelding.

Tack weld: A small weld , generally used to temporarily holdthe two pieces together during actual welding .


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Toe of weld: It is the junction between the weld face & base

metal.

Weld face: It is the exposed surface of the weld.

Weld metal: The metal that is solidified in the joint is calledweld metal. It may be only a base metal or a mixture of base

metal & filler metal.

Weld pass: A single movement of the welding torch or

electrode along the length of the joint which results in a bead

is a weld pass.


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Types of welding joints


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Welding Positions

FLAT

HORIZONTAL

VERTICAL

OVERHEAD

INCREASING DIFFICULTY


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Electr ic arc welding


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Electric arc welding


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Conti

Electric arc welding is one of the most widely

used welding process, mainly because of the

ease of use and high production rate that can

be achieved economically.

Uses an electric arc to join or coalesce metals.


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Arc welding Equipments

Direct current (D.C.) machines

I. Transformer with DC rectifier

II. Motor or engine driven generator

Alternating current (A.C.) machines

I. TransformerII. Motor or engine driven alternator

Two cables- one for work and one for electrode

Electrode holder

Electrode

Protective shield Gloves

Wire brush

Chipping hammer

Goggles


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Difference between

AC & DC welding

AC welding:

I. More efficiency

II. Power consumption less

III. Cost of equipment is less

IV. Higher voltagehence not safe

V. Not suitable for welding non ferrous metals

VI. Not preferred for welding thin sectionsVII.Any terminal can be connected to the work or electrode


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Conti.

DC Welding:

I. Less efficiency

II. Power consumption more

III. Cost of equipment is more

IV. Low voltagesafer operation

V. suitable for both ferrous non ferrous metals

VI. preferred for welding thin sectionsVII.Positive terminal connected to the work

VIII.Negative terminal connected to the electrode


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Advantages of arc welding

Most efficient way to join metals

Lowest-cost joining method

Affords lighter weight through better utilization of

materials Joins all commercial metals

Provides design flexibility


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Disadvantages of arc welding

Manually applied, therefore high labor cost.

Need high energy causing danger Not convenient for disassembly.

Defects are hard to detect at joints.


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Manual metal-arc welding


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Conti..

Manual metal-arc welding also called shielded metal-arc

welding (SMAW).

It is the most extensively used manual welding process, which

is done with stick (coated) electrodes.

Weld by this process can be made in any position.

A job of any thickness can be welded by shielded metal-arc

welding.

SMAW can be done with either an AC or DC power source.

Current usage may vary from 50 to 500 A with voltage from

20 to 40 V.


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Carbon arc-welding


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1. Carbon Arc Welding (CAW) is a welding process, inwhich heat is generated by an electric arc struck betweenan carbon electrode and the work piece. The arc heats andmelts the work pieces edges, forming a joint.

2. Carbon arc welding is the oldest welding process.

3. If required, filler rod may be used in Carbon ArcWelding. End of the rod is held in the arc zone. The moltenrod material is supplied to the weld pool.

4. Shields (neutral gas, flux) may be used for weld poolprotection depending on type of welded metal.


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Advantages, disadvantages

& applications

Advantages of Carbon Arc Welding:

Low cost of equipment and welding operation;

High level of operator skill is not required;

The process is easily automated;

Low distortion of work piece.Disadvantages of Carbon Arc Welding:

Unstable quality of the weld (porosity);

Carbon of electrode contaminates weld material with carbides.

Applications: Carbon arc welding is often confused with the widely-used arc-air gouging process, which is used for metal removal rather than metal

joining. and is used for removing flawed welds, gouging out cracks for

welding, beveling the edges of plates, and other spot-removal problems.


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Inert-gas shielded arc

welding


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Conti..

The endeavour of the welder is always to obtain a joint which

is as strong as the base metal and at the same time, the joint is

as homogeneous as possible. To this end, the complete

exclusion of oxygen and other gases.

In inert gas shielded arc welding processes, a high pressure

inert gas flowing around the electrode while welding would

physically displaced all the atmospheric gases around the weld

metal to fully protect it.

The shielding gases most commonly used are argon, helium,carbon dioxide and mixture of them.

Argon are normally preferred over helium.


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Conti.

Tungsten Inert Gas Arc Welding (Gas Tungsten ArcWelding) is a welding process, in which heat is generated byan electric arc struck between a tungsten non-consumableelectrode and the work piece.

The weld pool is shielded by an inert gas (Argon, helium,Nitrogen) protecting the molten metal from atmospheric

contamination.

The heat produced by the arc melts the work pieces edgesand joins them. Filler rod may be used, if required.

Tungsten Inert Gas Arc Welding produces a high qualityweld of most of metals. Flux is not used in the process.


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& applications

Advantages of Tungsten Inert Gas Arc Welding (TIG, GTAW):

Weld composition is close to that of the parent metal;

High quality weld structure

Slag removal is not required (no slag);

Thermal distortions of work pieces are minimal due to concentration of

heat in small zone.

Disadvantages of Tungsten Inert Gas Arc Welding (TIG, GTAW):

Low welding rate;

Relatively expensive;

Requires high level of operators skill.

Applications:

auto mobile industries(bus, car, trucks etc )

aircrafts etc


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Conti..

Metal Inert Gas Welding (Gas Metal Arc Welding) is a arc

Welding process, in which the weld is shielded by an external gas

(Argon, helium, CO2, argon + Oxygen or other gas mixtures).

Consumable electrode wire, having chemical composition similar to

that of the parent material, is continuously fed from a spool to the

arc zone. The arc heats and melts both the work pieces edges and the

electrode wire. The fused electrode material is supplied to the

surfaces of the work pieces, fills the weld pool and forms joint.

Due to automatic feeding of the filling wire (electrode) the process

is referred to as a semi-automatic. The operator controls only the

torch positioning and speed.


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Advantages,disadvantages

Advantages of Metal Inert Gas Welding (MIG, GMAW):

Continuous weld may be produced (no interruptions);

High level of operators skill is not required;

Slag removal is not required (no slag);

Disadvantages of Metal Inert Gas Welding (MIG, GMAW):

Expensive and non-portable equipment is required;

Outdoor application are limited because of effect of wind,dispersing the shielding gas.


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Flux-cored arc welding


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Conti.

The flux-cored arc welding(FCAW) process is a modificationof the GMAW, where the solid electrode wire is replaced bytubular electrode containing a flux at center of the electrodethroughout its length.

Since the electrode is not a solid wire, the feeding units shouldbe properly designed so as not to put excess on the electroderesulting in its flattening.

flux availability to remove the oxide and other contaminationsin the form of slag, which covers the weld bead duringsolidification and cooling, and protects it for longer periods.


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& applications

Applications: FCAW is normally used for the welding of

structural steels and alloy steels etc.


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Submerged arc-welding

(SAW)


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& applications

Advantages of Submerged Arc Welding (SAW):

Very high welding rate;

The process is suitable for automation;

High quality weld structure.Disadvantages of Submerged Arc Welding (SAW):

Weld may contain slag inclusions;

Limited applications of the process - mostly for welding

horizontally located platesApplications:


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OTHER ARC-WELDING

PROCESSES


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Atomic hydrogen welding


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& applications

Advantages:

This process can be used for any job

This welding gives extremely clean weld

Excellent quality of weldDisadvantages:

This welding is used for the flat positions only

High cost prohibits its general usage

Only Ac power supply is usedApplications:

It is generally used for welding of tools steels containing

tungsten, nickel & molybdenum

Also for hard surfacing & repairing of moulds, dies & tools.


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Plasma arc welding


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Plasma Arc Weldingis the welding process utilizing heat generatedby a constricted arc struck between a tungsten non-consumableelectrode and either the work piece (transferred arc process) orwater cooled constricting nozzle (non-transferred arc process).

Plasmais a gaseous mixture of positive ions, electrons and neutralgas molecules.

Transferred arc processproduces plasma jet of high energy densityand may be used for high speed welding and cutting of Ceramics,steels, Aluminum alloys, Copper alloys, Titanium alloys, Nickel

alloys.

Non-transferred arc processproduces plasma of relatively lowenergy density. It is used for welding of various metals and for

plasma spraying (coating). Since the work piece in non-transferredplasma arc welding is not a part of electric circuit, the plasma arctorch may move from one work piece to other without extinguishingthe arc.


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& applications

Advantages of Plasma Arc Welding (PAW):

Requires less operator skill due to good tolerance of arc to

misalignments;

High welding rate;

High penetrating capability (keyhole effect);

Disadvantages of Plasma Arc Welding (PAW):

Expensive equipment;

High distortions and wide welds as a result of high heat input.

Applications:


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Stud arc welding


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Arc stud welding (SW) is a welding process in which a metalfastener (weld stud) is joined to a workpiece. This process isgenerally referred to as stud welding. The metal fastener is joinedunder pressure once sufficiently heated with an electric arc.

The fastener or weld stud is positioned for welding through the useof a stud gun. When the operator activates the stud gun trigger, thefastener (electrode) is welded to the workpiece without the use offiller metal. The welding duration of SW is typically one second orless.

One end of a SW fastener is prepared for welding. A ceramic ferrule

surrounding the weld end of the fastener provides partial shieldingof the weld. The ferrule also dams the molten metal to form a fillettype weld. Shielding gases or flux may or may not be used to protectthe weld.


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&applications

Advantages:.

Capable of producing a full cross section weldment with a minimum of base

metal distortion or surface disruption.

Can be used in locations which do not permit use of other fastening methods.

Process lends itself to the automated concept or high production shops. Capable of producing welds which are less costly per unit than other methods.

Process may be effectively utilized with a minimum of time spent in personnel

training.

Applications:

The arc stud welding process lends itself to a wide range of applications such asautomotive, ship building, appliance, aero-dynamics, boilers, light or heavy

construction, preassembled fabrication designs and fit-up parts design.

The process may be utilized on a range of material thickness from about .060 in

to several inches.


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Fire-cracker welding

Fire-cracker welding is a variation of the manual metal-arc

welding(SMAW).

The joint to be welded is filled with a stick electrode, which is

kept in place by means of a glass fibre tape or a copper

retaining bar.

An arc is initiated by short circuiting the electrode to the

workpiece. once initiated, it is an automatic process till the

electrode is completely burned off.

Flux is provided by the coating on the electrode as well as theshielding gas.

A i


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&applications

Advantages The process is semi-automated

The equipment required is simple and cheap, the same as for manual arc.

Welding is quicker than manual arc, as electrode changing is reduced.

Porosity and slag inclusion of the finished weld is reduced, as electrode

positioning is consistent and accurate. The process can be applied in inaccessible areas, with poor visibility, and

with less operator skill.

Disadvantages

The one-pass bead is limited in cross section to that of the originalelectrode, i.e. there is no opportunity for weaving. Multiple passes are not

generally used. The process is limited to straight welds in horizontal position.

Applications:

Application in ship building industry


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RESISTANCE WELDING


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Resistance welding


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Ad t di d t


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& applications

Advantages of Resistance Welding: High welding rates;

Low fumes;

Cost effectiveness;

Easy automation;

No filler materials are required; Low distortions.

Disadvantages of Resistance Welding: High equipment cost;

Low strength of discontinuous welds;

Thickness of welded sheets is limited - up to 1/4 (6 mm);applications

Resistance Welding (RW) is used for joining vehicle body parts, fuel tanks,domestic radiators, pipes of gas oil and water pipelines, wire ends, turbine

blades, railway tracks.


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Resistance spot welding


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Conti.

Spot Weldingis a Resistance Welding (RW) process, in which

two or more overlapped metal sheets are joined by spot welds.

The method uses pointed copper electrodes providing passage

of electric current. The electrodes also transmit pressure

required for formation of strong weld.

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

mm).

Spot welding is widely used in automotive industry for joining

vehicle body parts.


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Projection welding


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Ad t di d t


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& applications

Advantages of Projection Welding: More than one spot weld can be made in a single operation.

Welding current and pressure required is less.

Suitable for automation.

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

Disadvantages of Projection Welding:

Projections cannot be made in thin work pieces.

Thin work pieces cannot withstand the electrode pressure.

Equipment is costlier.

Applications of Projection Welding:

A very common use of projection welding is the use of special nutsthat have projections on the portion of the part to be welded to theassembly. Also, used for welding parts of refrigerator, condensers,refrigerator racks, grills etc


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Upset welding

Upset weldingis a form of resistance welding in which coalescenceoccurs simultaneously across the entirety of the abutting surfacearea, or progressively along a joint, using heat from the base metalsresistance to the current passing through them.

OR

In upset welding(UW), the w/p to be joined are brought together tomate with each other in a butt joint compared to lap joint that hasbeen used so far. The two w/p are held tightly together & current isapplied, so that the heat is generated through the contact area betnthe two plates. Because of the joint being under pressure, the ends ofthe two pieces get slightly upset & hence its name upset.

This is useful for joining the two ends of rods or similar pieces.

Advantages disadvantages


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&applications

Advantages: suitable for joining many alloys that are difficult to weld using fusion

processes.

Since no melting takes place, the weld metal retains many of the

characteristics of the base metal.

Resulting welds have a hot worked structure, and thereby have higherstrength than fusion welds in the same mate.

Since the material being joined is not melted, compositional gradients are

not introduced, second phase materials are minimally disrupted.

Disadvantages:

Applications:

upset welding has been adapted for fabrication of structures considered

very large compared to typical resistance welding applications. The process

has been used for closure of capsules, small vessels, and large containers.


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Flash welding

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Flash Welding is a Resistance Welding (RW) process, in which

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 isstationary and the second is movable.

Flash Welding method permits fast (about 1 min.) joining of large

and complex parts.

Welded part are often annealed for improvement of toughness of the

weld.

Steels, Aluminum alloys, Copper alloys, Magnesium alloys, Copper

alloys and Nickel alloys may be welded by Flash Welding.



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&applications

Applications:

Thick pipes, ends of band saws, frames, aircraft landing gears areproduced by Flash Welding.

R i ldi


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Resistance seam welding


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Conti

Seam Welding is a Resistance Welding (RW)

process of continuous joining of overlapping

sheets by passing them between two rotating

electrode wheels. Heat generated by theelectric current flowing through the contact

area and pressure provided by the wheels are

sufficient to produce a leak-tight weld.


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OTHER

FABRICATION WELDING)

METHODS

Thermit welding


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Thermit welding


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1.Thermit Welding is a welding process utilizing heat generated byexothermic chemical reaction between the components of the thermit (amixture of a metal oxide and aluminum powder). The molten metal,

produced by the reaction, acts as a filler material joining the work piecesafter Solidification.

2.Thermit Welding is mainly used for joining steel parts, therefore commonthermit is composed from iron oxide (78%) and aluminum powder (22%).

3.The proportion 78-22 is determined by the chemical reaction of combustionof aluminum:

8Al+3Fe3O4=9Fe+4Al2O3+3.01MJ/Mol

4.The combustion reaction products (iron and aluminum oxide) heat up to4500F (2500C). Liquid iron fills the sand (or ceramic) mold built aroundthe welded parts, the slag (aluminum oxide), floating up, is then removedfrom the weld surface.


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Electroslag welding


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Electroslag welding

Conti.


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Conti.

Electroslag Welding is a welding process, in which the heat is

generated by an electric current passing between the consumableelectrode (filler metal) and the work piece through a molten slag

covering the weld surface.

ESW process is developed essentially to weld very large plates

without any edge preparation.

Prior to welding the gap between the two work pieces is filled with awelding flux.

The slag reaches a temperature of about 3500F (1930C). This

temperature is sufficient for melting the consumable electrode and

work piece edges. Metal droplets fall to the weld pool and join thework pieces.

Electroslag Welding is used mainly for steels.

Advantages disadvantages &


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Advantages, disadvantages &

applications

Advantages of Electroslag Welding: High deposition rate - up to 45 lbs/h (20 kg/h);

Low slag consumption (about 5% of the deposited metal weight);

Low distortion;

Unlimited thickness of work piece.

Disadvantages of Electroslag welding: Coarse grain structure of the weld;

Low toughness of the weld;

Only vertical position is possible.

Applications:

Fabrication of high pressure vessels

Frames of heavy mechanical & hydraulic presses

Rolling mill frames

Ship hulls & locomotive frames etc.


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Brazing

Brazing is a method of joining two metal work pieces bymeans of a filler material at a temperature above its melting

point but below the melting point of either of the materials

being joined.

Flow of the molten filler material into the gap between the

work pieces is driven by the capillary force. The filler material

cools down and solidifies forming a strong metallurgical joint,

which is usually stronger than the parent (work piece)materials. The parent materials are not fused in the process.



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&applications

Advantages of Brazing :

It's easy to learn.

You can join virtually any dissimilar metals.

The bond line can be very neat in appearance.

Joint strength is strong enough for most non-heavy-duty use applications.

Disadvantages of Brazing :

A badly brazed joint looks similar to a good joint, and can have a VERY low

strength.

The metal used to bond the two parts may be different in color than the parts being

bonded. This may or may not be a problem.

Long-term effects of dissimilar metals in constant contact may need to be examinedfor special applications.

Since the filler material (typically bronze) melts at a relatively low temperature,

brazed parts may not be put in an environment which exceeds the melting point of

the filler metal.

Applications:


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Braze welding

Braze welding is similar to brazing in that the joint is obtainedby means of a filler metal, whose liquidus temperature is

above 4500 C & below the solidus temperature of the base

metal. But the difference is that in braze welding, the filler

metal reaches the joint without the capillary action since thejoint gap is bigger.

Typical filler metals used in braze welding are brasses with

zinc content up to 40%.



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&applications

Advantages of braze welding:

This process requires less preheating, permits greater welding speed, demands a

shorter cooling-off period, and is less likely to crack metals, such as cast iron,

during the braze welding operation.

There is no splash or weld spatter to worry about and low temperatures minimize

distortion. The completed joint requires little or no finishing.

Brazing technique does not require as much skill as the technique required for

fusion welding.

Disadvantages of braze welding:

If the joint is to be exposed to corrosive media, the filler metal must have therequired corrosion-resistant characteristics.

All brazing alloys lose strength at elevated temperatures.

If the joint is to be painted, all traces of the flux must be removed


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Soldering

Solderingis a method of joining two metal work pieces by means of athird metal (solder) at a relatively low temperature, which is above themelting point of the solder but below the melting point of either of thematerials being joined.

Flow of the molten solder into the gap between the work pieces isdriven by the capillary force. The solder cools down and solidifiesforming a joint. The parent materials are not fused in the process.

Soldering is similar to Brazing. The difference is in the melting pointof the filler alloy: solders melt at temperatures below 840F (450C);

brazing filler materials melt at temperatures above this point.

Soldering joints have relatively low tensile strength of about 10000 psi(70MPa).



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&applications

Advantages of soldering:

Low power is required;

Low process temperature;

No thermal distortions and residual stresses in the joint parts;

Microstructure is not affected by heat;

Easily automated process;

Dissimilar materials may be joined;

High variety of materials may be joined;

Thin wall parts may be joined;

Moderate skill of the operator is required.

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Conti.

Disadvantages of soldering: Careful removal of the flux residuals is required in order to prevent

corrosion;

Large sections cannot be joined;

Fluxes may contain toxic components;

Soldering joints can not be used in high temperature applications; Low strength of joints.

Applications:

Soldering is used in plumbing, in electronics and metalwork fromflashing to jewelry.

Electronic soldering connects electrical wiring and electroniccomponents to printed circuit boards (PCBs).

Fabrication Methods(Welding)

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