fabrication methods for students
TRANSCRIPT
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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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Advantages,disadvantages
& 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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Advantages,disadvantages
&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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Advantages,disadvantages
&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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Advantages, disadvantages
&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
C i
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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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Advantages, disadvantages
&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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Advantages,disadvantages
&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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Advantages, disadvantages
&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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Advantages, disadvantages
& 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,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.