fabrication methods for students

7/27/2019 Fabrication Methods for Students

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welding


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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 welding

II. 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 90 0 to 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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Conti

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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Electr ic arc welding


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Electric arc welding


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Conti

Arc welding is a welding process, in which heat is

generated by an electric arc struck between an

electrode and the work piece

Electric arc welding is one of the most widely usedwelding 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.
http://www.substech.com/dokuwiki/doku.php?id=classification_of_welding_processes&DokuWiki=7e0bbe3b2236d0a1c6d7b2675ae0b922http://www.substech.com/dokuwiki/doku.php?id=classification_of_welding_processes&DokuWiki=7e0bbe3b2236d0a1c6d7b2675ae0b922


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Arc welding Equipments

Power supply (AC or Dc)

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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Polarity


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Weld Fluxes

Typical fluxes

SiO2, TiO2, FeO, MgO, Al2O3

Produces a gaseous shield to prevent contamination

Act as scavengers to reduce oxides

Add alloying elements to the weld

Influence shape of weld bead during solidification


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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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OTHER ARC-WELDING

PROCESSES


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RESISTANCE WELDING


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Resistance welding


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Conti

Resistance Welding is a welding process, in which work pieces are welded

due to a combination of a pressure applied to them and a localized heat

generated by a high electric current flowing through the contact area of the

weld.

Heat produced by the current is sufficient for local melting of the work

piece at the contact point and formation of small weld pool (nugget). The

molten metal is then solidifies under a pressure and joins the pieces.

AC electric current (up to 100 000 A) is supplied through copper electrodes

connected to the secondary coil of a welding transformer.

The following metals may be welded by Resistance Welding:

Low carbon steels - the widest application of Resistance Welding

Aluminum alloys

Medium carbon steels, high carbon steels and Alloy steels (may be welded,

but the weld is brittle)


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Advantages,disadvantages

& 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);

applicationsResistance 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 Welding is 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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Advantages, disadvantages

& applications


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Projection welding


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Conti


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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 nuts

that 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 welding is 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 has

been 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.


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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 higher

strength 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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Conti

Flash Welding is a Resistance Welding (RW) process, in whichends 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 theweld.

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 are

produced by Flash Welding.


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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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OTHERFABRICATION(WELDING)METHODS


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Thermit welding


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Conti..

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+3Fe3

O4

=9Fe+4Al2

O3

+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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&applications

Advantages of Thermit Welding:

No external power source is required (heat of chemical reaction is utilized);

Very large heavy section parts may be joined.

Making a fast weld

Provides a reasonably strong weld

Disadvantages of thermit Welding:

Only ferrous (steel, chromium, nickel) parts may be welded;

Slow welding rate;

High temperature process may cause distortions and changes in Grain

structure in the weld region.

Weld may contain gas (Hydrogen) and slag contaminations.

Applications:

Thermit Welding is used for repair of steel casings and forgings, for joining railroad

rails, steel wires and steel pipes, for joining large cast and forged parts


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Electroslag welding

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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 a

welding 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.


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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 by

means 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 examined

for 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 obtained

by 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

Soldering is 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).

A i


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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).


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Tungsten inert-gas welding

(TIG)


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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

as me a -arc we ng


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g(GMAW)

or

(MIG)


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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 tothat 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.

Ad di d


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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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