welding. fabrication methods introduction: joining two or more elements to make a single part is...

welding

Fabrication methods

Introduction:• Joining two or more elements to make a single part is termed

as a fabrication process. Large no. of industrial components are made by fabrication processes.

Ex:- Aircraft & ship bodies, bridges,building trusses etc

• It is also known as secondary manufacturing process.

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

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.

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.

Classification of welding process

1. Arc welding: I. gas metal arc welding

II. Gas tungsten arc welding

III. Plasma arc welding

IV. Shielded metal arc welding

V. Submerged arc welding

VI. Carbon arc welding

VII. Flux cored arc welding

2. Gas welding: I. oxyacetylene welding

II. Oxyhydrogen welding

III. Pressure gas welding

IV. Air acetylene welding

Conti……. 3. Resistance welding:I. Flash weldingII. Upset weldingIII. Projection weldingIV. Resistance seam weldingV. Resistance spot welding

4. Other welding processes:VI. Thermit weldingVII. Electroslag weldingVIII. Electron beam weldingIX. Laser beam weldingX. Forge weldingXI. Friction weldingXII. Diffusion weldingXIII. Explosion weldingXIV. BrazingXV. Braze weldingXVI. soldering

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

Conti………

• Fillet weld: The metal fused into the corner of a joint made by two pieces placed at approximately 90 0 to each other is termed fillet weld.

• Penetration: It is the depth up to which the weld metal combines with the base metal as measured from the top surface of the joint.

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

• Root: It is the point at which the two pieces to be joined by welding.

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

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 called

weld 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.

Types of welding joints

Welding Positions

FLAT

HORIZONTAL

VERTICAL

OVERHEAD

INCREASING DIFFICULTY

Weld joints edge preparation methods

Electric arc welding

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.

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. Transformer

II. 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

Difference between AC & DC welding

AC welding:I. More efficiency

II. Power consumption less

III. Cost of equipment is less

IV. Higher voltage – hence not safe

V. Not suitable for welding non ferrous metals

VI. Not preferred for welding thin sections

VII.Any terminal can be connected to the work or electrode

Conti….

DC Welding:I. Less efficiency

II. Power consumption more

III. Cost of equipment is more

IV. Low voltage – safer operation

V. suitable for both ferrous non ferrous metals

VI. preferred for welding thin sections

VII.Positive terminal connected to the work

VIII.Negative terminal connected to the electrode

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

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.

Manual metal-arc welding

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.

Conti…..

Advantages of Shielded Metal Arc Welding (SMAW): • Simple, portable and inexpensive equipment;• Wide variety of metals, welding positions and electrodes are

applicable;• Suitable for outdoor applications.

Disadvantages of Shielded Metal Arc Welding (SMAW): • The process is discontinuous due to limited length of the electrodes;• Weld may contain slag inclusions;• Fumes make difficult the process control.

Application• Widely used in the fabricating industries for the construction and

repair of plain carbon and low alloy steels.

Carbon arc-welding

Conti…….

1. Carbon Arc Welding (CAW) is a welding process, in which heat is generated by an electric arc struck between an carbon electrode and the work piece. The arc heats and melts 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 Arc Welding. End of the rod is held in the arc zone. The molten rod material is supplied to the weld pool.

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

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.

Inert-gas shielded arc welding

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.

Tungsten inert-gas welding(TIG)

Conti….

• Tungsten Inert Gas Arc Welding (Gas Tungsten Arc Welding) is a welding process, in which heat is generated by an electric arc struck between a tungsten non-consumable electrode 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 edges and joins them. Filler rod may be used, if required.

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

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

Gas metal-arc welding(GMAW)

or (MIG)

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.

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.

Flux-cored arc welding

Conti….

• The flux-cored arc welding(FCAW) process is a modification of the GMAW, where the solid electrode wire is replaced by tubular electrode containing a flux at center of the electrode throughout its length.

• Since the electrode is not a solid wire, the feeding units should be properly designed so as not to put excess on the electrode resulting in its flattening.

• flux availability to remove the oxide and other contaminations in the form of slag, which covers the weld bead during solidification and cooling, and protects it for longer periods.

Advantages,disadvantages& applications

Applications: FCAW is normally used for the welding of structural steels and alloy steels etc.

Submerged arc-welding(SAW)

Conti……

• Submerged Arc Welding is a welding process, which utilizes a bare consumable metallic electrode producing an arc between itself and the work piece within a granular shielding flux applied around the weld.

• The arc heats and melts both the work pieces edges and the electrode wire. The molten electrode material is supplied to the surfaces of the welded pieces, fills the weld pool and joins the work pieces.

• Since the electrode is submerged into the flux, the arc is invisible. The flux is partially melts and forms a slag protecting the weld pool from oxidation and other atmospheric contaminations.


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 plates

Applications:

OTHER ARC-WELDING PROCESSES

Atomic hydrogen welding

Conti…..

• The atomic hydrogen welding(AHW) is an inert gas shielded arc-welding process done with non-consumable electrodes.

• The main difference bet’n TIG (or GTAW) welding & this process is that in AHW, the arc is obtained bet’n two tungsten electrodes rather than bet’n the tungsten electrode & work piece.

• The shielding gas is used here is hydrogen(H2), which is reactive in nature compared to argon

Advantages,disadvantages & applications

Advantages:• This process can be used for any job• This welding gives extremely clean weld• Excellent quality of weld

Disadvantages:• This welding is used for the flat positions only• High cost prohibits its general usage• Only Ac power supply is used

Applications:• It is generally used for welding of tools steels containing

tungsten, nickel & molybdenum• Also for hard surfacing & repairing of moulds, dies & tools.

Plasma arc welding

Conti…..

• Plasma Arc Welding is the welding process utilizing heat generated by a constricted arc struck between a tungsten non-consumable electrode and either the work piece (transferred arc process) or water cooled constricting nozzle (non-transferred arc process).

Plasma is a gaseous mixture of positive ions, electrons and neutral gas molecules.

Transferred arc process produces plasma jet of high energy density and may be used for high speed welding and cutting of Ceramics, steels, Aluminum alloys, Copper alloys, Titanium alloys, Nickel alloys.

Non-transferred arc process produces plasma of relatively low energy density. It is used for welding of various metals and for plasma spraying (coating). Since the work piece in non-transferred plasma arc welding is not a part of electric circuit, the plasma arc torch may move from one work piece to other without extinguishing the arc.


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:

Stud arc welding

Conti….

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

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

• One end of a SW fastener is prepared for welding. A ceramic ferrule surrounding the weld end of the fastener provides partial shielding of the weld. The ferrule also dams the molten metal to form a fillet type weld. Shielding gases or flux may or may not be used to protect the weld.

Advantages,disadvantages&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 as

automotive, 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.

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 the shielding gas.


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 original electrode,

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

RESISTANCE WELDING

Resistance welding

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)


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.

Resistance spot welding

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.

Projection welding

Conti……


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 the assembly. Also, used for welding parts of refrigerator, condensers, refrigerator racks, grills etc

Upset welding

• Upset welding is a form of resistance welding in which coalescence occurs simultaneously across the entirety of the abutting surface area, or progressively along a joint, using heat from the base metals’ resistance to the current passing through them.

OR • In upset welding(UW), the w/p to be joined are brought together to

mate 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 is applied, so that the heat is generated through the contact area bet’n the two plates. Because of the joint being under pressure, the ends of the two pieces get slightly upset & hence its name upset.

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


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.

Flash welding

Conti……

• 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 is stationary 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.


Applications:• Thick pipes, ends of band saws, frames, aircraft landing gears are

produced by Flash Welding.

Resistance seam welding

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

OTHER FABRICATION(WELDING)

METHODS

Thermit welding

Conti……..

1.Thermit Welding is a welding process utilizing heat generated by exothermic chemical reaction between the components of the thermit (a mixture of a metal oxide and aluminum powder). The molten metal, produced by the reaction, acts as a filler material joining the work pieces after Solidification.

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

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

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

4.The combustion reaction products (iron and aluminum oxide) heat up to 4500°F (2500°C). Liquid iron fills the sand (or ceramic) mold built around the welded parts, the slag (aluminum oxide), floating up, is then removed from the weld surface.

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

Electroslag welding

Conti….• Electroslag Welding is a welding process, in which the heat is

generated by an electric current passing between the consumable electrode (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 3500°F (1930°C). This temperature is sufficient for melting the consumable electrode and work piece edges. Metal droplets fall to the weld pool and join the work pieces.

• Electroslag Welding is used mainly for steels.


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.

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.


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:

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 the joint gap is bigger.

• Typical filler metals used in braze welding are brasses with zinc content up to 40%.


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 the

required 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

Soldering

• Soldering is a method of joining two metal work pieces by means of a third metal (solder) at a relatively low temperature, which is above the melting point of the solder but below the melting point of either of the materials being joined.

• Flow of the molten solder into the gap between the work pieces is driven by the capillary force. The solder cools down and solidifies forming a joint. The parent materials are not fused in the process.

• Soldering is similar to Brazing. The difference is in the melting point of the filler alloy: solders melt at temperatures below 840°F (450°C); brazing filler materials melt at temperatures above this point.

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


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.

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 from flashing to

jewelry.• Electronic soldering connects electrical wiring and electronic components to

printed circuit boards (PCBs).