MODULE 5

OUTLINES

Welding processes

Cutting processes

Applications

Joining two or more elements to make a single part is termed as a fabrication process.

Various fabrication process are

1. Mechanical joints- bolts, screws & rivets2. Adhesive bonding- synthetic glue3. Welding, Brazing & Soldering

Selection depends on….

1. Type of assemble-permanent, semi-permanent or temporary2. Material being joined3. Economy4. Service conditions

Advantages and disadvantages of welding compared to other types of assembly

operations

Advantages:

(1)It provides a permanent joint, (2)Joint strength is as high as the strength of base metals,(3)It is most economical in terms of material usage, and (4)It is versatile in terms of where it can be accomplished.

Disadvantages:

1)It is usually performed manually, so labor cost is high and the skilled labor to perform it is sometimes scarce,

2)Welding is inherently dangerous, 3)A welded joint is difficult to disassemble, and 4)Quality defects are sometimes difficult to detect.

● Welded Joints are permanent joints.

● Welding joints are obtained by localized heating and are based on molecular attraction.

● The properties of the welded components are closest to those of a solid member.

● Members of unlimited size can be fabricated.

● Strength of welded joins in terms of static and impact loads is equal to the base metal.

● All Structural steel's including high alloy grades, non ferrous alloys can be efficiently welded.

Introduction - Welding

Name AWS Characteristics Applications

Carbon arc welding CAW Carbon electrode, historical Copper, repair (limited)

Flux cored arc welding

FCAWFCAW-S

Continuous consumable electrode filled with flux Industry, construction

Gas metal arc welding

GMAW Continuous consumable electrode and shielding gas Industry

Gas tungsten arc welding

GTAW Nonconsumable electrode, slow, high quality welds

Aerospace,Construction(piping),Tool and Die

Plasma arc welding PAW Nonconsumable electrode, constricted arc Tubing, instrumentation

Shielded metal arc welding

SMAWConsumable electrode covered in flux, can weld any metal as long as they have the right electrode

Construction, outdoors, maintenance

Submerged arc welding

SAW Automatic, arc submerged in granular flux

ARC WELDING

Name AWS Characteristics Applications

Oxyacetylene welding

OAWCombustion of acetylene with oxygen produces high-temperature flame, inexpensive equipment

Maintenance, repair

Oxy fuel gas welding

Name AWS Characteristics Applications

Resistance spot welding

RSWTwo pointed electrodes apply pressure and current to two or more thin workpieces

Automobile industry, Aerospace industry

Resistance seam welding

ERWTwo wheel-shaped electrodes roll along workpieces, applying pressure and current

Aerospace industry, steel drums, tubing

Projection welding

PW

Flash welding FW

Upset welding RSEW Butt joint surfaces heated and brought together by force

Resistance welding

Name AWS Characteristics Applications

Explosion welding EXW Joining of dissimilar materials, e.g. corrosion resistant alloys to structural steels

Transition joints for chemical industry and shipbuilding. Bimetal pipelines

Electromagnetic pulse welding

Tubes or sheets are accelerated by electromagnetic forces. Oxides are expelled during impact

Automotive industry, pressure vessels, dissimilar material joints

Forge welding FOW The oldest welding process in the world. Oxides must be removed by flux or flames. Damascus steel

Friction welding FRW Thin heat affected zone, oxides disrupted by friction, needs sufficient pressure

Aerospace industry, railway, land transport

Friction stir welding FSW A rotating consumable tool is traversed along the joint lineShipbuilding, aerospace, railway rolling stock, automotive industry

Hot pressure welding HPWMetals are pressed together at elevated temperatures below the melting point in vacuum or an inert gas atmosphere

Aerospace components

Hot isostatic pressure welding

HPW A hot inert gas applies the pressure inside a pressure vessel, i.e. an autoclave Aerospace components

Roll welding ROW Bimetallic materials are joined by forcing them between two rotating wheels Dissimilar materials

Ultrasonic welding USW High-frequency vibratory energy is applied to foils, thin metal sheets or plastics.

Solar industry. Electronics. Rear lights of cars.

Solid-state welding

Name AWS Characteristics Applications

Electron beam welding

EBW Deep penetration, fast, high equipment cost

Electroslag welding

ESWWelds thick workpieces quickly, vertical position, steel only,continuous consumable electrode.

Heavy plate fabrication, constructionConstruction, shipbuilding.

Induction welding IW

Laser beam welding

LBW Deep penetration, fast, high equipment cost Automotive industry

Laser-hybrid welding

Combines LBW with GMAW in the same welding head, able to bridge gaps up to 2mm (between plates), previously not possible with LBW alone.

Automotive, Shipbuilding, Steelwork industries

Percussion welding

PEW Following an electrical discharge, pressure is applied which forges the materials together

Components of switch gear devices

Thermite welding TW Exothermic reaction between alumnium powder and iron oxide powder Railway tracks

Other welding Processes

Oxy Acetylene welding

• OAW is a manual process in which the welder must personally control the torch movement and filler rod.

• Cylinders contain oxygen and acetylene gas at extremely high pressure.

• Acetylene is stored in the form of dissolved acetone under a pressure of 16 to 22 atm gauges.

Oxyacetylene Welding (OAW)

Regulator Hoses

• Hoses are fabricated from rubber

• Oxygen hoses are green in color and have right hand thread.

• Acetylene hoses are red in color with left hand thread.

• Left hand threads can be identified by a grove in the body of the nut and it may have “ACET” stamped on it

Flame Settings

• There are three distinct types of oxy-acetylene flames, usually termed:– Neutral– Carburizing (or “excess acetylene”)– Oxidizing (or “excess oxygen” )

• The type of flame produced depends upon the ratio of oxygen to acetylene in the gas mixture which leaves the torch tip.

• The neutral flame is produced when the ratio of oxygen to acetylene, in the mixture leaving the torch, is almost exactly one-to-one. It’s termed ”neutral” because it will usually have no chemical effect on the metal being welded. It will not oxidize the weld metal; it will not cause an increase in the carbon content of the weld metal.

• The excess acetylene flame, as its name implies, is created when the proportion of acetylene in the mixture is higher than that required to produce the neutral flame. Used on steel, it will cause an increase in the carbon content of the weld metal.

• The oxidizing flame results from burning a mixture which contains more oxygen than required for a neutral flame. It will oxidize or ”burn” some of the metal being welded.

Flame profiles

Flame Profiles

Type of flame Application

1.Neutral flame Steel, Cast-Iron ,Copper, Aluminum

2.Carburizing flame Low carbon steels, alloy steels, non ferrous metals

3.Oxidizing flame Brass, Bronze

Filler rod• Used to supply additional metal to the weld zone during

welding• Available as filler rods or wire and may be bare or

coated with flux• Purpose of the flux is to retard oxidation of the surfaces

of the parts being welded by generating a gaseous shield around the weld zone

Advantages of Gas welding1.Temp.of flame cane be controlled easily.2.The amount of filler metal deposits can be controlled easily.3.Flame can be used for welding and cutting.4.All types of metal can be welded.5.Cost of equipment is less.6.Portable equipment.7.Low maintenance cost.

Limitations of Gas welding1.Not suitable for thick plates.2.Slow process.3.Handling and storing of gas cylinders needs more care.4.Strength of weld is not so good as arc welding.5.Gas flame takes up a longer time to heat up the metal than arc welding.

Gas cutting Ferrous metal is heated in to red hot condition and a jet of

pure oxygen is projected onto the surface, which rapidly

oxidizes

Melt are blown away by the force of the jet, to make a cut

Fast and efficient method of cutting steel to a high degree

of accuracy

Torch is different from welding

Cutting torch has preheat orifice and one central orifice for

oxygen jet

Flame Cutting

Fig. (a) Flame cutting of steel plate with an oxyacetylene torch, and a cross-section of the torch nozzle.

(b) Cross-section of a flame-cut plate, showing drag lines.

• Equipments:• A welding generator (D.C.) or Transformer (A.C.)• Two cables- one for work and one for electrode• Electrode holder• Electrode • Protective shield• Gloves • Wire brush• Chipping hammer• Goggles

Arc welding

Arc Welding Equipments

• At high temperatures in AW, metals are chemically reactive to oxygen, nitrogen, and hydrogen in air – Mechanical properties of joint can be degraded by

these reactions – To protect operation, arc must be shielded from

surrounding air in AW processes • Arc shielding is accomplished by: – Shielding gases, e.g., argon, helium, CO2

– Flux

Arc Shielding

A substance that prevents formation of oxides

and other contaminants in welding, or dissolves

them and facilitates removal

Provides protective atmosphere for welding

Stabilizes arc

Reduces spattering

Flux

DC arc welding is more expensive than AC welding.

DC W is generally preferred because of the control of heat input offered by it.

70 % of heat is liberated near the anode ,30% cathode.

If more heat is required at w/p ( thicker w/p, high thermal conductivity metals such as Al, Copper ) w/p can be

connected to anode –Straight polarity or DCEN ( Direct current Electrode negative)

It produces welds that are narrow and deep.

Power Source in Arc Welding

If less heat is required at w/p, (thinner w/p) w/p can be connected to negative. This is referred as reverse polarity, or DCEP(direct current Electrode positive )

The weld zone is shallower and widerDCEPDCEN

Fig. The effect of polarity on weld beads:

(a) dc current straight polarity; (b) dc current reverse polarity; (c) ac current.

AC machine ( Transformer) DC machine (Generator)1.Efficency is more (80 to 85 %) Efficiency is less (30 to 60 %)2.Power consumption is less Power consumption is more3.Cost of equipment is less Cost of equipment is more 4. Any terminal can be connected to work or electrode

Polarity is significant

5.Voltage is higher, not safe Voltage is low, safer operation6.Not suitable for welding nonferrous metals

Very much suitable for both ferrous & nonferrous metals

7.Not preferred for welding thin sections

Preferred for welding thin sections

Comparison of AC & DC welding machines

Arc welding

Advantages– 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

Limitations• Manually applied, therefore

high labor cost.• Need high energy causing

danger• Not convenient for

disassembly.• Defects are hard to detect at

joints.

Heat transfer in arc welding

• Heat input is

• Heat input to melt a certain volume of material is

• Welding speed is

v

VIe

l

H

H = heat input l = weld lengthV = voltage applied I = currentv = welding speed e = efficiency

uA

VIev

uAluVH m u = specific energy required for meltingVm = volume of material meltedA = cross section of the weld

Consider the situation where a welding operation is being performed with V = 20volts, I = 200A and the cross-sectional area of the weld bead is 30 mm2. Estimate the welding speed if the work piece and electrode are made of (a) aluminium, (b) carbon steel, and (c) titanium. Use an efficiency of 75%.

Solutiona)For aluminium,

b)For carbon steel,

c)For titanium,

mm/s 5.34

309.2

2002075.0

uA

VIev

mm/s 1.8v

mm/s 0.7v

Consumable Electrode

AW Processes • Shielded Metal Arc

Welding• Gas Metal Arc

Welding(MIG)• Flux Cored Arc Welding‑• Electrogas Welding• Submerged Arc Welding

• Gas Tungsten Arc Welding (TIG)

• Plasma Arc Welding• Carbon Arc Welding • Stud Welding

Non consumable Electrode Processes

MIG (Metal Inert Gas) or

GMAW (Gas Metal Arc Welding )

Weld materials: Carbon steels, low alloy steels, stainless

steels, most aluminum alloys, zinc based copper alloys

MIG or GMAW

Consumable electrode is in the form of a wire reel which is fed at constant rate.

Weld area is shielded by an external source of gas.

Deoxidizers are present in the electrode to prevent oxidation.

Process is rapid, versatile and economical.

Shielding gas : ArgonNitrogenHelium

TIG (Tungsten Inert Gas)or

Gas Tungsten Arc Welding ( GTAW)

As the tungsten electrode is not consumed, a constant and stable arc gap is maintained at a constant current level

GTAW process is used for applications with aluminium, magnesium, titanium and the refractory metals

Cost of the inert gas is more expensive but provides high quality welds and surface finish

Gas Tungsten Arc Welding

Advantages:• High quality welds for suitable applications• No spatter because no filler metal through arc• Little or no post-weld cleaning because no flux

Disadvantages:• Generally slower and more costly than consumable

electrode AW processes

Gas Tungsten Arc Welding

Shielded Metal Arc Weld (SMAW):Most popular welding technique (stick welding).The electrode coating performs the following:

A – Produce gaseous shield to exclude oxygen.B – Introduce dioxider material to improve grain.C – Produce a blanket of slag to retard cooling and prevent oxidation.

The SMAW process isdesignated by AWS as“E6OXX” or “E7OXX” e.g. (E 6013)

Submerged Arc Weld (SAW):In this process the automatically fed arc (spool) is

protected by a blanket of granular material called “flux”.

This flux material acts to improve weld quality and to protect it

from the air.

Submerged arc welding (SAW)

Welded Joints

Fig. Examples of welded joints and their terminology.

Resistance Welding

• RW- heat required is produced by electrical resistance across the two components to be joined

• Heat generated is

• By including a factor K, which denotes energy losses through conduction and radiation, we have RtKIH 2

RtIH 2H = Heat I = CurrentR = Resistance t = Time of current flow

The desirable properties of a metal that would provide good weld ability for resistance welding are

High resistivity,Low electrical conductivityThermal conductivity and Low melting point.

Resistance Welding

• Total resistance is the sum of:1. Resistances of the electrodes2. Electrode–workpiece contact resistance3. Resistances of the individual parts to be

welded4. Contact resistance between the two

workpieces to be joined (faying surfaces)• Temperature rise at the joint depends on the

specific heat and the thermal conductivity of the metals to be joined

Resistance Spot Welding

• Tips of 2 opposing solid, cylindrical electrodes touch a lap joint of two sheet metals, and resistance heating produces a spot weld

• To obtain a strong bond in the weld nugget, pressure is applied until the current is turned off and the weld has solidified

• Surface of the spot weld has a slightly discolored indentation

• Current level depends on the materials thicknesses

Resistance Spot Welding

Heat generated = I2Rt

Heat required for melting

Volume of nugget Vm = (3.14*D2 )/4 *d

Where D =dia of nugget d= thickness of nugget

muV

Heat distribution through conduction and radiation is = Heat generated-heat required for melting.

Resistance Spot Welding

• Simplest and most commonly used• May be performed by means of single or

multiple pairs of electrodes • Required pressure is supplied through

mechanical or pneumatic means• Variety of electrode shapes are used to spot-

weld areas that are difficult to reach

An RSW operation is used to make a series of spot welds between two pieces of aluminum, each 2.0 mm thick. The unit melting energy for aluminum = 2.90 J/mm3. Welding current = 6,000 amps, and time duration = 0.15 sec. Resistance = 75 micro-ohms. The resulting weld nugget measures 5.0 mm in diameter by 2.5 mm thick. How much of the total energy generated is used to form the weld nugget? How much of heat is dissipated into the surroundings?

Solution: H = I2Rt = (6000)2(75 x 10-6)(0.15) = 405 W-sec = 405 J Weld nugget volume V = πD2d/4 = π(5)2(2.5)/4 = 49.1 mm3

Heat required for melting = UmV = (2.9 J/mm3)(49.1 mm3) =142.4 J Proportion of heat for welding =142.4/405 = 0.351 = 35.1%

The remaining heat 405 J-142.4 J= 262.6 J is dissipated into the metal surrounding through conduction and radiation.

Resistance Spot Welding

Resistance Spot Welding

Testing Spot Welds• Spot-welded joints may be tested by:1. Tension-shear2. Cross-tension3. Twist4. Peel

Advantages &Drawbacks of RW

Advantages:• No filler metal required• High production rates possible• Lends itself to mechanization and automation• Lower operator skill level than for arc welding• Good repeatability and reliability Disadvantages:• High initial equipment cost• Limited to lap joints for most RW processes

Resistance Seam Welding

• Electrodes are replaced by rotating wheels or rollers

• Using a continuous AC power supply to rollers• In roll spot welding, current is applied

intermittently and a series of spot welds at specified intervals

• In mash seam welding, overlapping welds are about one to two times the sheet thickness

Resistance Seam Welding

High-frequency RW• High-frequency current (up to 450 kHz) is used• Used for production of butt-welded tubing or

pipe• For high-frequency induction welding (HFIW),

the roll-formed tube is subjected to high-frequency induction heating

Resistance Projection Welding• High electrical resistance is developed by

embossing one or more projections on one of the surfaces to be welded

• Used for resistance projection welding by modifying the electrodes

RW:Flash Welding

• Heat is generated from the arc as the ends of the two members begin to make contact and develop an electrical resistance at the joint

• Quality of the weld is good• Suitable for end-to-end or edge-to-edge

joining of sheets of similar or dissimilar metals• Can be automated• Can be used in operating rolling mills and

feeding of wire-drawing equipment

RW:Flash Welding