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FUNDAMENTALS OF WELDINGFUNDAMENTALS OF WELDING

1. Overview of Welding Technology2. The Weld Joint3. Physics of Welding4 F t f F i W ld d J i t4. Features of a Fusion Welded Joint

K.J.Sabareesaan | Lecturer | Nizwa College Of Technology.

Joining and Assembly DistinguishedJoining and Assembly Distinguished

Joining - welding, brazing, soldering, and dh i b diadhesive bonding

These processes form a permanent joint between partsbetween parts

Assembly - mechanical methods (usually) of fastening parts togetherSome of these methods allow for easy disassembly, while others do not


WeldingWelding

Joining process in which two (or more) parts are l d t th i t ti f bcoalesced at their contacting surfaces by

application of heat and/or pressureMany welding processes are accomplished byMany welding processes are accomplished by heat alone, with no pressure appliedOthers by a combination of heat and pressureStill others by pressure alone with no external heatIn some welding processes a filler material isIn some welding processes a filler material is added to facilitate coalescence


Why Welding is ImportantWhy Welding is Important

Provides a permanent jointWelded components become a single entity

Usually the most economical way to join parts in terms of material usage and fabrication costsin terms of material usage and fabrication costs

Mechanical fastening usually requires additional hardware components (e.g., screws and nuts) and geometric alterations of the parts being assembled (e.g., holes)

Not restricted to a factory environmentNot restricted to a factory environmentWelding can be accomplished "in the field"


Limitations and Drawbacks of WeldingLimitations and Drawbacks of Welding

Most welding operations are performed ll d i i t f l bmanually and are expensive in terms of labor

cost Most welding processes utilize high energy andMost welding processes utilize high energy and are inherently dangerous Welded joints do not allow for convenient di bldisassembly Welded joints can have quality defects that are difficult to detectdifficult to detect


Types of Welding ProcessesTypes of Welding Processes

Some 50 different types of welding processes h b t l d b th A ihave been catalogued by the American Welding Society (AWS)Welding processes can be divided into twoWelding processes can be divided into two major categories:

Fusion weldingSolid state welding


Fusion WeldingFusion Welding

Joining processes that melt the base metalsIn many fusion welding operations, a filler metal is added to the molten pool to facilitate the process and provide bulk and addedthe process and provide bulk and added strength to the welded joint A fusion welding operation in which no filler

l i dd d i ll d t ldmetal is added is called an autogenous weld


Some Fusion Welding ProcessesSome Fusion Welding Processes

Arc welding (AW) – melting of the metals is li h d b l t iaccomplished by electric arc

Resistance welding (RW) - melting is accomplished by heat from resistance to anaccomplished by heat from resistance to an electrical current between faying surfaces held together under pressure O f l ldi (OFW) l i iOxyfuel gas welding (OFW) - melting is accomplished by an oxyfuel gas such as acetyleney


Arc WeldingArc Welding

A man al arc elding A manual arc welding operation


Principal Applications of WeldingPrincipal Applications of Welding

Construction - buildings and bridgesPiping, pressure vessels, boilers, and storage tanksShipbuildingShipbuildingAircraft and aerospaceAutomotive Railroad


Welder and FitterWelder and Fitter

Welder - manually controls path or placement f ldiof welding gun

Often assisted by second worker, called a fitter, who arranges the parts prior to weldingwho arranges the parts prior to welding

Welding fixtures and positioners are used to assist in this function


The Safety IssueThe Safety Issue

Welding is inherently dangerous to human kworkers

High temperatures of molten metalsIn gas welding fuels (e g acetylene) are aIn gas welding, fuels (e.g., acetylene) are a fire hazard Many welding processes use electrical power, so electrical shock is a hazard


Special Hazards in Arc WeldingSpecial Hazards in Arc Welding

Ultraviolet radiation emitted in arc welding is i j i t h i iinjurious to human vision

Welder must wear a special helmet with a dark viewing windowdark viewing window

Filters out dangerous radiation but welder is blind except when arc is struck

Sparks, spatters of molten metal, smoke, and fumes add to the risks

Ventilation needed to exhaust dangerousVentilation needed to exhaust dangerous fumes from fluxes and molten metals


Automation in WeldingAutomation in Welding

Because of the hazards of manual welding, d t i d ti it d iand to increase productivity and improve

quality, various forms of mechanization and automation are used

Machine welding – mechanized welding under supervision and control of human operatoroperatorAutomatic welding – equipment performs welding without operator control g pRobotic welding - automatic welding implemented by industrial robot


The Weld JointThe Weld Joint

The junction of the edges or surfaces of parts that h b j i d b ldihave been joined by welding Two issues about weld joints:

Types of jointsTypes of jointsTypes of welds used to join the pieces that form the joints


Five Types of JointsFive Types of Joints

1. Butt joint 2. Corner joint 3. Lap joint 4 T j i t4. Tee joint 5. Edge joint


Butt Joint

Parts lie in same plane and are joined at their edgesedges

Figure 30.2 Five basic types of joints: (a) butt


Corner Joint

Parts in a corner j i t f i htjoint form a right angle and are joined at the corner of the angle

Figure 30.2 (b) corner


Lap Joint

Consists of two overlapping parts

ap Jo

overlapping parts

Figure 30.2 (c) lap


Tee Joint

One part is

ee Jo

perpendicular to the other in the approximate shape ofapproximate shape of the letter "T"

Figure 30.2 (d) tee


Edge Joint

Parts in an edge joint ll l ith t

dge Jo

are parallel with at least one of their edges in common, gand the joint is made at the common edge(s)edge(s)

Figure 30.2 (e) edge


Types of WeldsTypes of Welds

Each of the preceding joints can be made by ldiwelding

Other joining processes can also be used for some of the joint typessome of the joint types There is a difference between joint type and the way it is welded - the weld type


Fillet WeldFillet Weld

Used to fill in the edges of plates created by l d t j i tcorner, lap, and tee joints

Filler metal used to provide cross section in approximate shape of a right triangleapproximate shape of a right triangle Most common weld type in arc and oxyfuel weldingRequires minimum edge preparation


Fillet Welds

Figure 30.3 Various forms of fillet welds: (a) inside single fillet corner joint; (b) outside single fillet corner joint; (c) double fillet lap joint; and (d) double fillet tee joint. Dashed lines show the original part edges.


Groove WeldsGroove Welds

Usually requires part edges to be shaped into a t f ilit t ld t tigroove to facilitate weld penetration

Edge preparation increases cost of parts fabricationfabricationGrooved shapes include square, bevel, V, U, and J, in single or double sidesMost closely associated with butt joints


Groove WeldsFigure 30.4 Some groove welds: (a) square groove weld, one side; (b) single bevel groove weld; (c) single V-groove

Groove Welds

one side; (b) single bevel groove weld; (c) single V groove weld; (d) single U-groove weld; (e) single J-groove weld; (f) double V-groove weld for thicker sections. Dashed lines show original part edges.


Spot Weld

Fused section between surfaces of two plates U d f l j i t

p

Used for lap joints Closely associated with resistance welding

Figure 30.6 (a) ( )Spot weld


Physics of WeldingPhysics of Welding

Fusion is most common means of achieving l i ldicoalescence in welding

To accomplish fusion, a source of high density heat energy must be supplied to the fayingheat energy must be supplied to the faying surfaces, so the resulting temperatures cause localized melting of base metals (and filler metal if used)metal, if used)For metallurgical reasons, it is desirable to melt the metal with minimum energy but high heat gy gdensities


Power DensityPower Density

Power transferred to work per unit surface area, W/ 2 (Bt / i 2)W/mm2 (Btu/sec-in2)If power density is too low, heat is conducted into work, so melting never occursinto work, so melting never occurs If power density too high, localized temperatures vaporize metal in affected region There is a practical range of values for heat density within which welding can be performed


Comparisons Among Welding ProcessesComparisons Among Welding Processes

Oxyfuel gas welding (OFW) develops large t f h t b t h t d it i l ti lamounts of heat, but heat density is relatively

low because heat is spread over a large areaOxyacetylene gas, the hottest of the OFWOxyacetylene gas, the hottest of the OFW fuels, burns at a top temperature of around 3500°C (6300°F)

A ldi d hi hArc welding produces high energy over a smaller area, resulting in local temperatures of 5500° to 6600°C (10,000° to 12,000°F)( , , )


Power Densities for Welding ProcessesPower Densities for Welding Processes

Welding process W/mm2 (Btu/sec-in2)

Oxyfuel 10 (6)

Arc 50 (30)Arc 50 (30)

Resistance 1,000 (600)

Laser beam 9,000 (5,000)

Electron beam 10,000 (6,000)Electron beam 10,000 (6,000)


Power DensityPower Density

Power entering surface divided by corresponding fsurface area:

APPD =

where PD = power density, W/mm2 (Btu/sec-in2); P = power entering surface, W (Btu/sec); and A

A

= surface area over which energy is entering, mm2 (in2)


Unit Energy for Melting

Quantity of heat required to melt a unit volume Q y qof metal Symbolized Um

It is the sum of: Heat to raise temperature of solid metal to melting pointto melting point

Depends on volumetric specific heatHeat to transform metal from solid to liquid phase at melting point

Depends on heat of fusion


Heat Transfer Mechanisms in WeldingHeat Transfer Mechanisms in Welding

Not all of the input energy is used to melt the ld t lweld metal

1. Heat transfer efficiency f1 - actual heat received by workpiece divided by total heatreceived by workpiece divided by total heat generated at source

2. Melting efficiency f2 - proportion of heat i d k f d f l ireceived at work surface used for melting;

the rest is conducted into work metal


Heat Available for WeldingHeat Available for Welding

Hw = f1 f2 Hw 1 2

where Hw = net heat available for welding; f1 = heat transfer efficiency; f2 = melting efficiency;heat transfer efficiency; f2 melting efficiency; and H = total heat generated by welding process


Heat Transfer Efficiency f1Heat Transfer Efficiency f1

Proportion of heat received at work surface l ti t t t l h t t d trelative to total heat generated at source

Depends on welding process and capacity to convert power source (e.g., electrical energy)convert power source (e.g., electrical energy) into usable heat at work surface

Oxyfuel gas welding processes are l i l i ffi irelatively inefficient

Arc welding processes are relatively efficient


Melting Efficiency f2Melting Efficiency f2

Proportion of heat received at work surface used f lti th t i d t d i t th kfor melting; the rest is conducted into the workDepends on welding process but also influenced by thermal properties of metal, jointinfluenced by thermal properties of metal, joint configuration, and work thickness

Metals with high thermal conductivity, such l i d blas aluminum and copper, present a problem

in welding because of the rapid dissipation of heat away from the heat contact area y


Energy Balance EquationEnergy Balance Equation

Net heat energy into welding operation equals h t i d t lt th l fheat energy required to melt the volume of metal welded

Hw = Um VHw Um V

where Hw = net heat energy delivered to operation J (Btu); Um = unit energy required tooperation, J (Btu); Um unit energy required to melt the metal, J/mm3 (Btu/in3); and V = volume of metal melted, mm3 (in3)


Typical Fusion Welded Joint

Fi 30 8 C ti f t i l f i ld d

yp ca us o e ded Jo

Figure 30.8 Cross section of a typical fusion welded joint: (a) principal zones in the joint, and (b) typical grain structure.


Features of Fusion Welded JointFeatures of Fusion Welded Joint

Typical fusion weld joint in which filler metal has b dd d i t fbeen added consists of: Fusion zoneWeld interfaceWeld interfaceHeat affected zone (HAZ) Unaffected base metal zone


Heat Affected ZoneHeat Affected Zone

Metal has experienced temperatures below lti i t b t hi h h tmelting point, but high enough to cause

microstructural changes in the solid metalChemical composition same as base metal, butChemical composition same as base metal, but this region has been heat treated so that its properties and structure have been altered

Eff h i l i i HAZ iEffect on mechanical properties in HAZ is usually negative, and it is here that welding failures often occur


WELDING PROCESSESWELDING PROCESSES

1. Arc Welding2. Resistance Welding3. Oxyfuel Gas Welding4 Oth F i W ldi P4. Other Fusion Welding Processes5. Solid State Welding6 Weld Quality6. Weld Quality7. Weldability8. Design Considerations in Weldingg g


Two Categories of Welding ProcessesTwo Categories of Welding Processes

Fusion welding - coalescence is accomplished b lti th t t t b j i d iby melting the two parts to be joined, in some cases adding filler metal to the joint

Examples: arc welding, resistance spotExamples: arc welding, resistance spot welding, oxyfuel gas welding

Solid state welding - heat and/or pressure are d hi l b l i fused to achieve coalescence, but no melting of

base metals occurs and no filler metal is added Examples: forge welding diffusion weldingExamples: forge welding, diffusion welding, friction welding


Arc Welding (AW)Arc Welding (AW)

A fusion welding process in which coalescence of th t l i hi d b th h t fthe metals is achieved by the heat from an electric arc between an electrode and the workElectric energy from the arc producesElectric energy from the arc produces temperatures ~ 10,000 F (5500 C), hot enough to melt any metalM AW dd fill l iMost AW processes add filler metal to increase volume and strength of weld joint


What is an Electric Arc?What is an Electric Arc?

An electric arc is a discharge of electric current i i itacross a gap in a circuit

It is sustained by an ionized column of gas (plasma) through which the current flows(plasma) through which the current flows To initiate the arc in AW, electrode is brought into contact with work and then quickly

d f i b h diseparated from it by a short distance


Arc Welding

A pool of molten metal is formed near electrode ti d l t d i d l j i t

Arc Welding

tip, and as electrode is moved along joint, molten weld pool solidifies in its wake

Figure 31 1 Basic configuration of an arc welding processFigure 31.1 Basic configuration of an arc welding process.


Manual Arc Welding and Arc TimeManual Arc Welding and Arc Time

Problems with manual welding:Weld joint qualityProductivity

A Ti (ti i ) di id d b (hArc Time = (time arc is on) divided by (hours worked)

Also called “arc-on time”Manual welding arc time = 20%Machine welding arc time ~ 50%


Two Basic Types of AW ElectrodesTwo Basic Types of AW Electrodes

Consumable – consumed during welding process

Source of filler metal in arc weldingNonconsumable not consumed duringNonconsumable – not consumed during welding process

Filler metal must be added separately


Consumable ElectrodesConsumable Electrodes

Forms of consumable electrodesWelding rods (a.k.a. sticks) are 9 to 18 inches and 3/8 inch or less in diameter and must be changed frequentlymust be changed frequentlyWeld wire can be continuously fed from spools with long lengths of wire, avoiding f i ifrequent interruptions

In both rod and wire forms, electrode is consumed by arc and added to weld joint asconsumed by arc and added to weld joint as filler metal


Nonconsumable ElectrodesNonconsumable Electrodes

Made of tungsten which resists melting Gradually depleted during welding (vaporization is principal mechanism) Any filler metal must be supplied by a separateAny filler metal must be supplied by a separate wire fed into weld pool


Arc ShieldingArc Shielding

At high temperatures in AW, metals are h i ll ti t it dchemically reactive to oxygen, nitrogen, and

hydrogen in air Mechanical properties of joint can beMechanical properties of joint can be seriously degraded by these reactions To protect operation, arc must be shielded f di i i AWfrom surrounding air in AW processes

Arc shielding is accomplished by: Shielding gases e g argon helium COShielding gases, e.g., argon, helium, CO2

Flux


FluxFlux

A substance that prevents formation of oxides d th t i t i ldiand other contaminants in welding, or

dissolves them and facilitates removalProvides protective atmosphere for weldingProvides protective atmosphere for weldingStabilizes arcReduces spattering


Various Flux Application MethodsVarious Flux Application Methods

Pouring granular flux onto welding operationStick electrode coated with flux material that melts during welding to cover operationTubular electrodes in which flux is contained inTubular electrodes in which flux is contained in the core and released as electrode is consumed


Power Source in Arc WeldingPower Source in Arc Welding

Direct current (DC) vs. Alternating current (AC) AC machines less expensive to purchase and operate, but generally restricted to ferrous metalsferrous metalsDC equipment can be used on all metals and is generally noted for better arc control


Consumable Electrode AW ProcessesConsumable Electrode AW Processes

Shielded Metal Arc WeldingGas Metal Arc WeldingFlux-Cored Arc WeldingEl t W ldiElectrogas WeldingSubmerged Arc Welding


Shielded Metal Arc Welding (SMAW)Shielded Metal Arc Welding (SMAW)

Uses a consumable electrode consisting of a filler t l d t d ith h i l th t idmetal rod coated with chemicals that provide

flux and shielding Sometimes called "stick welding"Sometimes called stick weldingPower supply, connecting cables, and electrode holder available for a few thousand d lldollars


Shielded Metal Arc Weldingg

Figure 31.3 Shielded metal arc welding (SMAW).


Welding Stick in SMAWWelding Stick in SMAW

Composition of filler metal usually close to base metalbase metalCoating: powdered cellulose mixed with oxides, carbonates, and other ingredients, gheld together by a silicate binder Welding stick is clamped in electrode holder connected to power sourceconnected to power sourceDisadvantages of stick welding:

Sticks must be periodically changedSticks must be periodically changedHigh current levels may melt coating prematurely


Shielded Metal Arc Welding

Figure 31.2 Shielded metal arc welding (stick welding) performed by a g) p y(human) welder (photo courtesy of Hobart Brothers Co.).


SMAW ApplicationsSMAW Applications

Used for steels, stainless steels, cast i d t i f llirons, and certain nonferrous alloys Not used or rarely used for aluminum and its alloys, copper alloys, andand its alloys, copper alloys, and titanium


Arc Welding Design GuidelinesArc Welding Design Guidelines

Good fit-up of parts - to maintain di i l t l d i i i di t tidimensional control and minimize distortion

Machining is sometimes required to achieve satisfactory fit-upachieve satisfactory fit up

Assembly must allow access for welding gun to reach welding area Design of assembly should allow flat welding to be performed as much as possible since this is fastest and mostpossible, since this is fastest and most convenient welding position


Arc Welding Positions

Flat welding is best positionO h d ldi i t diffi lt

Arc Welding Positions

Figure 31.35 Welding positions (defined here for groove welds): (a) flat (b) horizontal (c) vertical and (d)

Overhead welding is most difficult

welds): (a) flat, (b) horizontal, (c) vertical, and (d) overhead.


Design Guidelines - RSWDesign Guidelines RSW

Low-carbon sheet steel up to 0.125 (3.2 mm) is id l t l f RSWideal metal for RSW How additional strength and stiffness can be obtained in large flat sheet metal componentsobtained in large flat sheet metal components

Spot welding reinforcing parts into them Forming flanges and embossments

Spot welded assembly must provide access for electrodes to reach welding area S ffi i t l f h t t l t i dSufficient overlap of sheet metal parts required for electrode tip to make proper contact


Oxy fuel Gas Welding (OFW)


Oxyfuel Gas Welding (OFW)Oxyfuel Gas Welding (OFW)

Group of fusion welding operations that burn i f l i d ithvarious fuels mixed with oxygen

OFW employs several types of gases, which is the primary distinction among the members ofthe primary distinction among the members of this group Oxyfuel gas is also used in flame cutting

h d l l dtorches to cut and separate metal plates and other partsMost important OFW process is oxyacetyleneMost important OFW process is oxyacetylene welding


Oxyacetylene Welding (OAW)Oxyacetylene Welding (OAW)

Fusion welding performed by a high temperature fl f b ti f t l dflame from combustion of acetylene and oxygen Flame is directed by a welding torchFlame is directed by a welding torchFiller metal is sometimes added

Composition must be similar to base metalFiller rod often coated with flux to clean surfaces and prevent oxidation


Oxyacetylene WeldingO yace y e e e d g

Figure 31.21 A typical oxyacetylene welding operation (OAW).


Acetylene (C2H2)Acetylene (C2H2)

Most popular fuel among OFW group because it i bl f hi h t t thit is capable of higher temperatures than any other - up to 3480°C (6300°F) Two stage chemical reaction of acetylene andTwo stage chemical reaction of acetylene and oxygen:

First stage reaction (inner cone of flame): C2H2 + O2 → 2CO + H2 + heat

Second stage reaction (outer envelope): 2CO H 1 5O 2CO H O h t2CO + H2 + 1.5O2 → 2CO2 + H2O + heat


Oxyacetylene TorchMaximum temperature reached at tip of inner cone while outer envelope spreads out and

Oxyacetylene Torch

cone, while outer envelope spreads out and shields work surfaces from atmosphere

Figure 31.22 The neutral flame from an oxyacetylene torch indicating temperatures achieved.


Safety Issue in OAWSafety Issue in OAW

Together, acetylene and oxygen are highly fl blflammableC2H2 is colorless and odorless

It is therefore processed to haveIt is therefore processed to have characteristic garlic odor


OAW Safety IssueOAW Safety Issue

C2H2 is physically unstable at pressures much b 15 lb/i 2 ( b t 1 t )above 15 lb/in2 (about 1 atm)

Storage cylinders are packed with porous filler material (such as asbestos) saturatedfiller material (such as asbestos) saturated with acetone (CH3COCH3)Acetone dissolves about 25 times its own

l f lvolume of acetyleneDifferent screw threads are standard on the C2H2and O2 cylinders and hoses to avoid accidentaland O2 cylinders and hoses to avoid accidental connection of wrong gases


Alternative Gases for OFWAlternative Gases for OFW

Methylacetylene-Propadiene (MAPP)HydrogenPropylenePPropaneNatural Gas


Gas Metal Arc Welding (GMAW)(GMAW)

Uses a consumable bare metal wire as electrode and shielding accomplished byelectrode and shielding accomplished by flooding arc with a gasWire is fed continuously and automatically y yfrom a spool through the welding gun Shielding gases include inert gases such as argon and helium for aluminum welding andargon and helium for aluminum welding, and active gases such as CO2 for steel welding Bare electrode wire plus shielding gases p g geliminate slag on weld bead - no need for manual grinding and cleaning of slag


Gas Metal Arc Weldingg

31.4 Gas metal arc welding (GMAW).


GMAW Advantages over SMAWGMAW Advantages over SMAW

Better arc time because of continuous wire l t delectrode

Sticks must be periodically changed in SMAWSMAW

Better use of electrode filler metal than SMAWEnd of stick cannot be used in SMAW

Higher deposition ratesEliminates problem of slag removalCan be readily automated


Nonconsumable Electrode ProcessesNonconsumable Electrode Processes

Gas Tungsten Arc WeldingPlasma Arc WeldingCarbon Arc Welding St d W ldiStud Welding


Gas Tungsten Arc Welding (GTAW)Gas Tungsten Arc Welding (GTAW)

Uses a nonconsumable tungsten electrode and an inert gas for arc shieldingand an inert gas for arc shielding Melting point of tungsten = 3410°C (6170°F)A.k.a. Tungsten Inert Gas (TIG) weldingA.k.a. Tungsten Inert Gas (TIG) welding

In Europe, called "WIG welding" Used with or without a filler metal

When filler metal used, it is added to weld pool from separate rod or wire

A li ti l i d t i l t lApplications: aluminum and stainless steel most common


Gas Tungsten Arc WeldingGas Tungsten Arc Welding

Figure 31.9 Gas tungsten arc welding.


Advantages / Disadvantages of GTAWAdvantages / Disadvantages of GTAW

Advantages:High quality welds for suitable applicationsNo spatter because no filler metal through arcarcLittle or no post-weld cleaning because no fluxflux

Disadvantages:Generally slower and more costly than consumable electrode AW processes


Plasma Arc Welding (PAW)Plasma Arc Welding (PAW)

Special form of GTAW in which a constricted l i di t d t ldplasma arc is directed at weld area

Tungsten electrode is contained in a nozzle that focuses a high velocity stream of inert gasthat focuses a high velocity stream of inert gas (argon) into arc region to form a high velocity, intensely hot plasma arc stream T i PAW h 28 000 CTemperatures in PAW reach 28,000°C (50,000°F), due to constriction of arc, producing a plasma jet of small diameter and p g p jvery high energy density


Resistance Welding (RW)Resistance Welding (RW)

A group of fusion welding processes that use a bi ti f h t d tcombination of heat and pressure to

accomplish coalescence Heat generated by electrical resistance toHeat generated by electrical resistance to current flow at junction to be welded Principal RW process is resistance spot

ldi (RSW)welding (RSW)


Resistance Weldingg

Figure 31 12 ResistanceFigure 31.12 Resistance welding, showing the components in spot welding, the main g,process in the RW group.


Components in Resistance Spot WeldingComponents in Resistance Spot Welding

Parts to be welded (usually sheet metal)Two opposing electrodesMeans of applying pressure to squeeze parts between electrodesbetween electrodesPower supply from which a controlled current can be applied for a specified time duration


Advantages / Drawbacks of RWAdvantages / Drawbacks of RW

Advantages:No filler metal requiredHigh production rates possibleL d it lf t h i ti d t tiLends itself to mechanization and automationLower operator skill level than for arc weldingGood repeatability and reliabilityGood repeatability and reliability

Disadvantages:High initial equipment costg q pLimited to lap joints for most RW processes


Resistance Spot Welding (RSW)Resistance Spot Welding (RSW)

Resistance welding process in which fusion of faying surfaces of a lap joint is achieved at onefaying surfaces of a lap joint is achieved at one location by opposing electrodes Used to join sheet metal parts using a series of j p gspot welds Widely used in mass production of automobiles appliances metal furniture andautomobiles, appliances, metal furniture, and other products made of sheet metal

Typical car body has ~ 10,000 spot welds yp y , pAnnual production of automobiles in the world is measured in tens of millions of units


Spot Welding Cycleg y

Figure 31.13 (a) Spot welding cycle, (b) plot of squeezing force & current in cycle (1) parts inserted between electrodes (2) electrodes closein cycle (1) parts inserted between electrodes, (2) electrodes close, force applied, (3) current on, (4) current off, (5) electrodes opened.


Resistance Seam Welding (RSEW)Resistance Seam Welding (RSEW)

Uses rotating wheel electrodes to produce a i f l i t ld l lseries of overlapping spot welds along lap

jointCan produce air-tight jointsCan produce air tight joints Applications:

Gasoline tanks Automobile mufflers Various other sheet metal containers


Resistance Seam Weldinges s a ce Sea e d g

Figure 31.15 Resistance seam welding (RSEW).


Other Fusion Welding ProcessesOther Fusion Welding Processes

FW processes that cannot be classified as arc, i t f l ldiresistance, or oxyfuel welding

Use unique technologies to develop heat for meltingmeltingApplications are typically unique Processes include:

Electron beam weldingLaser beam weldingElectroslag weldingThermit welding


Roll Welding (ROW)Roll Welding (ROW)

SSW process in which pressure sufficient to l i li d b fcause coalescence is applied by means of

rolls, either with or without external heat Variation of either forge welding or coldVariation of either forge welding or cold welding, depending on whether heating of workparts is done prior to process

If l h ll d ld ll ldiIf no external heat, called cold roll weldingIf heat is supplied, hot roll welding


Roll Weldingg

Fi 31 26 R ll ldi (ROW)Figure 31.26 Roll welding (ROW).


Roll Welding ApplicationsRoll Welding Applications

Cladding stainless steel to mild or low alloy t l f i i tsteel for corrosion resistance

Bimetallic strips for measuring temperature"Sandwich" coins for U S mintSandwich coins for U.S mint


Diffusion Welding (DFW)Diffusion Welding (DFW)

SSW process uses heat and pressure, usually in t ll d t h ith ffi i t ti fa controlled atmosphere, with sufficient time for

diffusion and coalescence to occur Temperatures ≤ 0.5 TmTemperatures ≤ 0.5 Tm

Plastic deformation at surfaces is minimal Primary coalescence mechanism is solid state diffusionLimitation: time required for diffusion can range from seconds to hoursfrom seconds to hours


DFW ApplicationsDFW Applications

Joining of high-strength and refractory metals i d l i d t iin aerospace and nuclear industriesCan be used to join either similar and dissimilar metalsmetalsFor joining dissimilar metals, a filler layer of different metal is often sandwiched between b l diff ibase metals to promote diffusion


Explosive Welding

Commonly used to bond two dissimilar metals, in particular to clad one metal on top of a

g

in particular to clad one metal on top of a base metal over large areas

Figure 31.27 Explosive welding (EXW): (1) setup in theFigure 31.27 Explosive welding (EXW): (1) setup in the parallel configuration, and (2) during detonation of the explosive charge.


Friction Welding (FRW)Friction Welding (FRW)

SSW process in which coalescence is achieved b f i ti l h t bi d ithby frictional heat combined with pressure When properly carried out, no melting occurs at faying surfacesfaying surfacesNo filler metal, flux, or shielding gases normally usedProcess yields a narrow HAZCan be used to join dissimilar metalsWid l d i l bl tWidely used commercial process, amenable to automation and mass production


Friction Weldingc o e d g

Figure 31.28 Friction welding (FRW): (1) rotating part, no contact; (2) parts brought into contact to generate friction heat; (3) rotation stopped and axial pressure applied; and (4) weld created.pp p pp ; ( )


Weld QualityWeld Quality

Concerned with obtaining an acceptable weld joint that is strong and absent of defects, and the methods of inspecting and testing the joint to assure its qualitythe joint to assure its qualityTopics:

Residual stresses and distortionWelding defectsInspection and testing methods


Welding DefectsWelding Defects

Cracks CavitiesSolid inclusions I f t h t bl tImperfect shape or unacceptable contour Incomplete fusion Miscellaneous defectsMiscellaneous defects


Welding CracksWelding Cracks

Fracture-type interruptions either in weld or in b t l dj t t ldbase metal adjacent to weld Serious defect because it is a discontinuity in the metal that significantly reduces strengththe metal that significantly reduces strength Caused by embrittlement or low ductility of weld and/or base metal combined with high

i d i irestraint during contraction In general, this defect must be repaired


Welding Crackse d g C ac s

Figure 31.31 Various forms of welding cracks.


CavitiesCavities

Two defect types, similar to defects found in ticastings:

1. Porosity - small voids in weld metal formed by gases entrapped during solidificationby gases entrapped during solidification

Caused by inclusion of atmospheric gases, sulfur in weld metal, or surface contaminants

2. Shrinkage voids - cavities formed by shrinkage during solidificationshrinkage during solidification


Solid InclusionsSolid Inclusions

Solid inclusions - nonmetallic material t d i ld t lentrapped in weld metal

Most common form is slag inclusions generated during AW processes that use fluxgenerated during AW processes that use flux

Instead of floating to top of weld pool, globules of slag become encased during

lidifi isolidificationMetallic oxides that form during welding of certain metals such as aluminum whichcertain metals such as aluminum, which normally has a surface coating of Al2O3


Incomplete Fusion

Also known as lack of fusion, it is simply a weld b d i hi h f i h t d

co p e e us o

bead in which fusion has not occurred throughout entire cross section of joint

Figure 31 32 Several forms of incomplete fusionFigure 31.32 Several forms of incomplete fusion.


Weld Profile in AW

Weld joint should have a certain desired profile to maximize strength and avoid incomplete fusion and lack of penetration

Figure 31.33 (a) Desired weld profile for single V-groove weld joint.


Weld Defects in AWe d e ec s

Figure 31.33 Same joint but with several weld defects: (b) undercut, in which a portion of the base metal part is melted away; (c) underfill, a depression in the weld below the level of the adjacent base metala depression in the weld below the level of the adjacent base metal surface; and (d) overlap, in which the weld metal spills beyond the joint onto the surface of the base part but no fusion occurs.


Inspection and Testing MethodsInspection and Testing Methods

Visual inspection Nondestructive evaluation Destructive testing


Visual InspectionVisual Inspection

Most widely used welding inspection method Human inspector visually examines for:

Conformance to dimensions WWarpage Cracks, cavities, incomplete fusion, and other surface defects

Limitations: Only surface defects are detectable Welding inspector must also determine if additional tests are warranted


Nondestructive Evaluation (NDE) Tests

Ultrasonic testing - high frequency sound waves directed through specimen - crackswaves directed through specimen cracks, inclusions are detected by loss in sound transmission Radiographic testing - x-rays or gamma radiation provide photograph of internal flawsDye-penetrant and fluorescent-penetrantDye penetrant and fluorescent penetrant tests - methods for detecting small cracks and cavities that are open at surfaceMagnetic particle testing – iron filings sprinkled on surface reveal subsurface defects by distorting magnetic field in partdefects by distorting magnetic field in part


Destructive TestingDestructive Testing

Tests in which weld is destroyed either during t ti t t t itesting or to prepare test specimen Mechanical tests - purpose is similar to conventional testing methods such as tensileconventional testing methods such as tensile tests, shear tests, etc Metallurgical tests - preparation of metallurgical

i ( h i h ) fspecimens (e.g., photomicrographs) of weldment to examine metallic structure, defects, extent and condition of heat affected ,zone, and similar phenomena


WeldabilityWeldability

Capacity of a metal or combination of metals to b ld d i t it bl d i d t tbe welded into a suitably designed structure, and for the resulting weld joint(s) to possess the required metallurgical properties to perform q g p p psatisfactorily in intended service Good weldability characterized by:

E i h hi h ldi iEase with which welding process is accomplishedAbsence of weld defectsAbsence of weld defectsAcceptable strength, ductility, and toughness in welded joint


Weldability Factors – Welding ProcessWeldability Factors Welding Process

Some metals or metal combinations can be dil ld d b b t diffi ltreadily welded by one process but are difficult

to weld by othersExample: stainless steel readily welded byExample: stainless steel readily welded by most AW and RW processes, but difficult to weld by OFW


Weldability Factors – Base MetalWeldability Factors Base Metal

Some metals melt too easily; e.g., aluminumMetals with high thermal conductivity transfer heat away from weld, which causes problems; e.g., copperproblems; e.g., copper High thermal expansion and contraction in metal causes distortion problemsDissimilar metals pose problems in welding when their physical and/or mechanical properties are substantially differentproperties are substantially different


Other Factors Affecting WeldabilityOther Factors Affecting Weldability

Filler metal Must be compatible with base metal(s) In general, elements mixed in liquid state that form a solid solution upon solidificationthat form a solid solution upon solidification will not cause a problem

Surface conditions Moisture can result in porosity in fusion zone O id d th fil t l fOxides and other films on metal surfaces can prevent adequate contact and fusion


Design Considerations in WeldingDesign Considerations in Welding

Design for welding - product should be d i d f th t t ld d bldesigned from the start as a welded assembly, and not as a casting or forging or other formed shape pMinimum parts - welded assemblies should consist of fewest number of parts possible

E l ll ffi iExample: usually more cost efficient to perform simple bending operations on a part than to weld an assembly from flat plates y pand sheets


Documents

workshop oil and gas handout.pdf