welding sg

8/12/2019 Welding SG

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JOINING PROCESS

MECHANICAL BONDING ATOMIC BONDIN

temporary semi permanent

solid state liquid state solid / liquid state

Solid state cold welding friction welding diffusion welding ultrasonic welding

Liquid state Electric Chemical


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Electric

Arc welding Induction welding Resistance welding

Chemical Gas welding thermit welding

Solid/liquid state brazing soldering adhesive bonding


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Welding is the joining of two or more piecesOf metal by creating atom to atom bonds

Advantages of Welding provides a permanent joint welded jt. strength more than parent metal can be accomplished on field as well

Limitations manual and expensive in terms of labour cost involve high energy sources welding jt defects substantially reduce strength


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Five basic types of weld joints


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Types of edge preparations needed prior to welding

Edge prep.needed forButt weldingof sheets >9 m


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Different types of welding Symbols in use


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WELD BEAD TYPES


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Liquid state Welding

Electric Arc Welding

Temperature at the centre of the arc 6000 o CFor starting the arc 45 V for DC and upto 60 V for ACVoltage drops to 15 to 30 V after the arc establishedDCSP electrode is cathode deposition rate high,Heat penetration low thin sheet metals are welded


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Different types of Arc Welding Processes

Shielded Metal Arc Welding (SMAW) Submerged Arc Welding (SAW) Gas Metal Arc Welding (GMAW) Gas Tungsten Arc Welding (GTAW) Plasma Arc Welding (PAW) Carbon Arc Welding Flux cored Arc Welding

Electrodes used in Arc Welding

Consumable Non consumable

Consumable Bare coated


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SMAW

Coated consumable electrode used The coating melts to provide a protectiveatmosphere and slag for the welding operation

Current 30 to 300 amps at 15 to 45 V, Power < 10 kW Current can be DC or AC. DC preferred for better arc stability DC usedfor sheet metal and AC for thick section welding used for carbon steels, low alloy steels, CI Advanta es are better weld ualit arc stabilit


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SAW

Automatic process, weld area shielded by fusiblegranular flux

The flux forms a glass like slag which protectsthe weld joint

Currents used 3000 to 4000 amps high welding rate, high weld quality, deeper penetration suitable only for flat or horizontal position Cost is high.

( )


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GMAW (MIG)

solid continuous consumable electrode used Argon, He, Nitrogen, CO 2 used as shielding gas For welding Al alloys, stainless steels, Ar/He is used For low and medium carbon steels CO

2 is used

Process is automatic hi her de osition rate than SMAW


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Mode of Weld Metal transfer from Electrode to the Work

Short Circuiting Globular Spray Pulsed Spray

The mode of Metal transfer depends on the following parameters:

Welding currentElectrode sizeElectrode compositionElectrode stick outShielding gas (if any)

Short Circuiting mode:


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Short Circuiting mode:Produces very low heat due to the usage of low welding current and voltageMetal transfer takes place for a short duration at the time when welding rod is in contathe weld poolSurface tension of the melted metal pool helps in metal transfer electrode contacts the weld pool at random frequencies which range from 20 - 200 conSuch transfer results in les spatter and well suited for thin section welding

Globular TransferTransfer mode characterized by a drop two or three times larger than dia of the electrodeThe droplet gets detached from the electrode by some pinching forceWeak magnetic forces and strong gravity forces also aid in the metal transfer

Due to wobbling of the droplets at the electrode tip the arc stability is disturbedHeat affected zone is narrow but penetration becomes small and large spattering occurs

Spray Transfer


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Spray TransferDroplet size is small and directed axially in a straight line towards the weld poolThere is a transition current above which such transfer takes placeThere is less spatter and weld bead is smoothDroplets vary between hundred per second to several hundreds per second

Increase in current results in decrease in droplet size and increase in frequencyHigh deposition rateCannot be used in overhead or vertical welding since they cannot be supported bysurface tension

Pulsed Spray TransferType of spray transfer occurring at regularly spaced intervals

Power source should be capable of producing pulsed current, hence e xpensivDroplet size equal to diameter of the electrodeFlat weld beads are possible at comparatively low power


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16

ER70S- 1Electrode

Rod

70,000 psi Min . Tensile Stren gth

Sol idPos i t ion o f we ld ing

Chemis try, Am ou nt o f Deoxid izers (Si l icon ,

Manganese and /o r A lum inum , Zi rcon ium and

Titaniu m ) X=2,3,4,6,7 o r G

AWS Classification ofGMAW Electrodes

The "1" designates an all position electrode,"2" is for flat and horizontal positions only;while "4" indicates an electrode that can be used forflat, horizontal, vertical down and overhead.


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Purposes of Electrode CoatingsTo stabilize the arc to provide shielding to the weld zone to control electrode melting rate

to act as flux to protect weld zone from oxides, nitrides and inclusions anproduce a slag for weld pool protection to add alloying elements / deoxidizers to improve weld strength

The deposited electrode coating is removed after each pass with the help of wire brush

Electrode Coating Materials:Claylike materials like silicate binders and powdered materials likeOxides, fluorides, metal alloys and cellulose


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GTAW (TIG)

Non consumable electrode Thorium or Zirconium used to improveelectron emission & inert gas is used for shielding

can be used to weld all types of engg. Matls. Thin sheets of Al, Mg alloys welded by DCRP (200 amps) AC (500 amps) is preferred for the above alloys since oxides remove DCSP used for steels, copper & Ni alloys All t es of oints can be roduced

PAW


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PAW

Special type of GTAW where a plasma arc is directedat the weld area

Arc can be transferred / non transferred type Advtgs are arc stability, better penetration control

high travel speeds, excellent weld quality major limitation is the high equipment cost

Transferred Non Transferred


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Thermit Welding

A mixture of fine alumina and iron oxide mixedIn the ratio 1:3.Temp. achievable 3000 oC8Al+3Fe 3O 4 = 9Fe + 4Al 2O 3 + heatReaction time is around 30 sIt is used in joining railroad rails, pipes and in

Repairing heavy castings No finishing reqd.


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Gas Welding Process

Oxyacetylene flame weldingFlame temperature around 3150 oCNeutral, reducing and oxidising flamesReducing flame employed in welding CIOxidising flame employed in welding brass, bronzeThe flame has three distinct zones Inner cone having higher temp. acetylene feather of temp around 2000 oC Outer envelope 1250 oC Combustion Equations are


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Oxyacetylene Welding Torch


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Oxyacetylene Welding Torch


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Resistance Welding Process

It is a fusion welding process that utilizes aCombination of heat and pressure to accomplishCoalescence.Current used is very high 5000 to 20,000 amps,Voltage is low (10 V), time duration is small(0.1-0.4s)Functions of pressure are1. To ensure contact between electrodes and work2. To press the surfaces when welding temp reached

Electrode materials are generally Cu or Cu alloysDesirable properties are high electrical conductivity high hardness

Spot Welding Process Weld nugget 6 to 10 mm


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Spot Welding Process

Application of RSWIn automobile manufacturing unit (10,000 spot weld present in the bIn fabrication of various sheet metal partsIn metal furniture making

Current 3000 to 40,000

Special Electrodes for Spot Welding in an air operated


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Special Electrodes for Spot-Welding in an air operatedrocker arm spot welding machine

Press type spot welding machines are used for welding larger workpie

Seam Welding Process


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Seam-Welding ProcessOverlapping nuggets Roll spot welds

Advantages Low cost High production rates Suitable for automation

Limitationso Well designed fixture necessaryo Sheets of thickness > 4 mm

of carbon steel cannot be seam welbecause of the requirement of high am

Resistance Projection Welding


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Resistance Projection Welding

Projections produced by Embossing

A number of welds in one pass can be obtainedElectrode life is enhanced

Metals of different thicknesses can be welded


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Projection Resistance weldingof nuts and studs


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Flash-Welding Process

Flash-welding process for end-to-end welding of solid rods or tubular parts.Characteristics:o Impurities squeezed out during operation so weld quality is goodo Machines are automated and largeo Power requirement ranges from 10 kVA to 1500 kVA


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High frequency current in the range of 10 kHz to 300 kHzThe primary source of heat is the eddy current inducedinto the workpiece.

An important phenomenon in induction welding is theskin effect. This refers to the fact that electric currentflows superficially. The penetration can be effectivelycontrolled by proper choice of frequency.Industrial application include butt welding of pipes and

continuous seam welding of tubes.

InductionWelding

Cold pressure welding


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Joining mechanism plastic deformation Applications thin sheets, wires and small electrical components Normally accomplished by special purpose machine Effectively used for non ferrous matls especially

Al, Cu, and Al-Cu alloys Post joining process like annealing and

machining may be required Diffusion Welding


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Diffusion Welding

Mate the surfaces intimately after cleaning Keep them together at elevated temperature

and appropriate pressure for a long time mechanism of joining is by diffusion major process parameters temp. press., time Applications joining of superalloys Major Advantage less distortion and residual

stressesPressure applied by dead weight, gas press.Heating done by resistance, furnace.

Diffusion Bonding and Superplastic


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Diffusion Bonding and SuperplasticForming

Structures made by this process are thin and have high stiffness to weight ratiosHence the process is employed in aircraft and aerospace applications

It produces parts with good dimensional accuracy and low residual stresses


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Friction welding is asolid state joiningprocess that producescoalescence by the heatdeveloped between twosurfaces bymechanically inducedsurface motion.

Definition of Friction Welding


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Linnert, WeldingAWS, 1994


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AWS Weldi

F i Z i F i ti


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Fusion Zone in FrictionWelding

Shape of the fusion zone in friction welding as a function of the forceapplied and the rotational speed.

Continuous Drive


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The joint face of atleast one of the workpiece must havecircular symmetry(usually the rotatingpart).

Typical jointconfigurations shownat right.

Rod Tube Rod t

Rod to plate Tube to plate Tube

Friction Welding Joint Design

R di l F i i W ldi


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Radial Friction Welding

Used to join collars to shafts

and tubes. Two tubes are clamped in

fixed position. The collar tobe joined is placed betweenthe tubes.

The collar is rotatedproducing frictional heat. Radial forces are applied to

compress the collar tocomplete welding.

F

+

FF F

F

FF

F

Friction Welding


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Limitations one of the two parts to be joined must be a bodyof revolution

only forgeable materials can be friction welded

Advantages high efficiency in terms of

energy utilisation dissimilar metals can be joined

F i ti Sti


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Friction StirWelding

Parts to be joined are clampedfirmly.

A rotating hardened steel tool isdriven into the joint and traversedalong the joint line between theparts.

The rotating tool produces frictionwith the parts, generating enough

heat and deformation to weld theparts together. Al alloy plates upto 75 mm thick

can be welded by this process

Butt welds

Overlap welds

Friction Welding forMounting Ti Alloy Shielding Gas &


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Mounting Ti AlloyRotor Blades

Shielding Gas &Induction Pre-heat

Weld Nub

Linear Friction Weld

Force

Schneefeld, D,et al. Friction Welding Process for Mounting Blades of a Rotor for a FlowMachine, US Patent 6,160,237 Dec 12, 2000


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A solid state welding process in which coalescence iproduced at the faying surfaces by the application ofhigh frequency vibratory energy while the work piecare held together under moderately low staticpressure.

Definition of Ultrasonic Welding


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(a) Components of an ultrasonic-welding machine for lap welds. The lateralvibrations of the tool tip case plastic deformation and bonding at the interface ofthe workpieces. (b) Ultrasonic seam welding using a roller.

Mechanism


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A static clamping force is applied

perpendicular to the interfacebetween the work pieces. The contacting sonotrode

oscillates parallel to theinterface.

Combined effect of static andoscillating force producesdeformation which promoteswelding.

Anvil

Mass

Sonotrodetip

Clampingforce

wedge Tra

Force

workpiec

Mechanism

10-75 KHz


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No heat is applied and no melting occurs. Permits welding of thin to thick sections. Welding can be made through some surface coatings. Pressures used are lower, welding times are shorter, and the thickne

of deformed regions are thinner than for cold welding.

Advantages of Ultrasonic Welding


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The thickness of the component adjacent to the sonotrode tip mustnot exceed relatively thin gages because of power limitations of theequipment.

Process is limited to lap joints. Butt welds can not be made because there is no means of supportin

the workpieces and applying clamping force.

Limitations of Ultrasonic Welding


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Assembling of electronic components such as diodes andsemiconductors with substrates.

Electrical connections to current carrying devices including motors,field coils, and capacitors.

Encapsulation and packaging. Plastic parts

Applications of Ultrasonic Welding

E l i W ldi P


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Explosion-Welding Process

Schematic illustration of the explosion-welding process: (a) constaninterface clearance gap and (b) angular-interface clearance gap.


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Mechanical interlockingproduced duringExplosive welding

It is popular forManufacturing of heatExchangers andChemical processing

EquipmentLimitation of the process is that it cannot be usedfor welding hard, brittle materials

Explosion-Welded Joints


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p

Cross-sections of explosion-welded joints: (a) titanium (top) on low-carbsteel (bottom) and (b) incoloy 800 (iron-nickel-base alloy on low-carbonThe wavy interfaces shown improve the shear strength of the joint. Somecombinations of metals, such as tantalum and vanadium, produce a muchwavy interface. If the two metals have little metallurgical compatibility, ainterlayer may be added that has compatibility with both metals.Courtes of DuPont Com an

Typical Fusion-Weld Zone in Gas or Arc


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Typical Fusion Weld Zone in Gas or Arcwelding

Grain Structure


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Grain Structure

Deep Weld Shallow Weld

Weld Bead Microhardness


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Bead produced by Laser Beam Welding

Profile

WELDING DEFECTS VARIOUS DISCONTINUITIES IN WELD


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Defects in Fusion Welds


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UNDERCUTTING:Caused by excessive current in arc welding

UNDERFILLING:

Caused due to inadequate filler metal deposition

OVERLAP:More material deposition

LACK OF PENETRATION:Penetration refers to the depth that the weld extends


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pinto the base metal of the joint. Lack of penetrationmeans that fusion has not penetrated deeply enoughto the root of the joint.Causes are low energy input, wrong polarity and highwelding speed

ARC STRIKES:Caused when the welder accidentally strikes theelectrode onto the base metal resulting in deep scaron the part.

CRACKS:Fracture type interruptions in the weld itself or in the


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HAZ. Cracks developed in the weld zone is alsoknown as hot cracks. Such cracks occur in ferrousalloys with high percentages of sulfur andphosphorous and also in alloys having highfreezing range.

HAZ cracks are called cold cracks. They are formedbecause of the brittleness of the HAZ. Such cracks

can be eliminated by Avoiding hydrogen during weldingControlling cooling rateEmploying multipasswelding technique

Types of Cracks in WeldedJ i


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Joints

LAMELLAR TEARING:


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A long and continuous visual separation line between

the HAZ and the base metal. It is caused by Presence of Mn, S in the base metal Improper weld configuration which led to

development of high residual stresses in thetransverse direction

Distortion of Parts After


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Welding

Residual Stresses in Straight Butt Joint


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CONCAVE OR CONVEX WELD BEADS


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Excessive concavity or convexity should be avoided. The avoidance can be accomplished byo Proper selection of current in arc weldingo Proper selection of electrode size.

Misalignment: occur mostly when two different thickness plate

Photographic view of porosity defect in welding Photographic view of Spatter defect in welding


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WELDABILITY OF DIFFERENT MATERIAL


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Steels: They are readily weldable than copper or aluminium. However increase in carbonin the steels result in a decrease in its weldability.Austenitic steels are readily weldable

Ferritic and martensitic steels are not that weldable and they often require preheating andSpecial electrodes.Ferritic steels are more susceptible to hot cracking

Aluminium and its alloys: More susceptible to hot cracking, oxide inclusioHydrogen porosity

Copper and its alloys: Because of high thermal conductivity of copper preIs required to counteract the heat sink effect. TIG or MIG is preferred.For copper alloy Brass presence of Zinc is the main problemBronze require careful cleaning and deoxidization to prevent porosity

BRAZING


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It is a joining process in which a filler metal is meltedand distributed by capillary action between the

Faying surfaces of the metal parts being joined.No melting of the base metals occur.Filler metal (brazing metal) has a m.p. 450 oC butbelow the m.p. of the base metal.The clearance gap between the mating surfacesis an imp. Parameter deciding strength of theJoint. Clearances are small.Use of flux like borax, fluorides, chlorides isEssential.

Different Brazing methods


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Torch brazing

Furnace brazing

Resistance brazing

Advantages of brazing over welding


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any metals can be joined faster and more consistent multiple joints can be brazed simultaneously it can be used to join thin parts HAZ and associated problems are eliminated less heat and power required

Limitations of Brazing

joint strength is less high service temperatures weaken the joint colour of the metal at the brazed part not same

as the colour of the base metal

SolderingSoldering is similar to brazing No melting of base


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Soldering is similar to brazing. No melting of baseMetals occur. Surfaces to be soldered must beprecleaned. An appropriate flux is applied to theFaying surfaces and then the surfaces are heated.The filler metal called solder is added to the jointwhich distributes itself between the closely fit

Parts. The filler metal is mostly an alloy of lead tinHaving a lower strength and m.p. than base metalsSoldering is mostly carried out by electricalResistance heating method.


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Reflow Soldering


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g

Screening solder paste onto a printed circuit board inreflow soldering.

Wave-Soldering Process


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g

(a) Schematic illustration of the wave-soldering process.(b) SEM image of a wave-soldered joint on a surface-mount

device.

Advantages of Soldering


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very low energy input good electrical and thermal conductivity in the joint air tight and liquid tight joints possible easy to repair and rework

Limitations of Soldering

very low joint strength possible weakening of the joint in elevated temp.

welding sg

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