Manufacturing Technology – II 21-28

7/31/2019 Manufacturing Technology II 21-28

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ARC WELDING

Arc welding uses of the heat of the electric arc for fusion of platesfor welding.

Connect power supply to anode and cathode

Close the circuit by touching electrode together Short circuiting: thermo ionic emission followed by electro

magnetic field emission on separation

Increased conductivity of gap facilitates to establishing the arc

Ionization of gases is arc gap due to electron collision

Thus continuous flow of electron i.e. arc


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Effect of arc gap

Increase in gap increases theresistance for flow of current andso potential difference.

Excessive gap can extinguish thearc due to increased loss of

electrons from arc surfaces andheat by convection


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Stability

Depends on mode of metaltransfer

Gravity

Upward Downward

Surface tension

Electromagnetic interaction

Density

Size of metal


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Polarity DC allows better control overheat input using either

straight (DCEN) or reverse

polarity (DCEP).

DCEN is used for thicker

plates of higher thermalconductivity

DCEP for thinner sections

and shallow penetration DC is preferred for odd

(vertical, overhead) position

welding


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AC Ar c W eld i n g Set Up


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Function of electrode coatings To introduce alloying elements in

weld to Sp. properties. Control the slag viscosity for

vertical/overhead welds to avoidfalling down of molten metal andbetter control over puddle.

Coating extending beyond theelectrode core wire concentratesthe arc and directs the fillermaterial at desired position

Act as insulator to avoid shortcircuiting in narrow gap welding

Increase DR by adding ironpowders

Common stickElectrodes are Availablein diameters of1.6, 2.0,3.2, 4, 5, 6, 8 and 9 mm

and the length is 350 Or450 mm. Baking of electrode done

to remove Moisturefrom the Coating as theyare mostly hygroscopic.


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It is highly versatile and least expensive process and can be

used for both simple and sophisticated jobs.

Heat melts the electrode, base metal and coating on the

electrode, all of this weld in the weld bead

All position welding flat, horizontal vertical, overhead

SMAW can done by both AC or DC power source usinggeneral current rating from 50 to 500 A with voltages from 20

to 40 V.

Manual Metal Arc Welding (MMAW)

Shielded Metal Arc Welding (SMAW)


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Limitations

Slow welding speed due to low DR ranging from 1to 8 kg/hr in the flat position (further less in verticaland overhead positions)

Wastage in form of unused end of electrode, slagand gas besides chances of slag inclusions andmoisture pick-up in coating.


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Electrode designation

Electrode are designated using a set of 6 characterswhich combines alphabets and numbers

First is alphabet E/R indicates method ofmanufacturing (E: extruded and R: reinforced)

Second is number (1-9) indicates type of coating onelectrode

Third is number (1-9) indicates welding position onelectrode

Forth is number (1-9) indicates welding current onelectrode

Fifth is a set of three numbers indicating ultimatestrength of weld

Sixth is a alphabet indicating type of electrode


11/59MI -10 2 : Manufac tu r i ng Techn iques I . I . T . ROORKEE

Typ es o f W e ld ing Pos it i ons


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[ 4 .6 .6 ] El ect r o d e Desi gn at i on

Current

powder,

powder,

powder,


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Resistance And TIG Spot Weld


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Resistance Welding

A group of fusion welding processes that use a

combination of heat and pressure to accomplish

coalescence

Types- Resistance spot welding (Principal use)

Resistance seam welding

Resistance projection welding

Developed in early 1900

Less skill is required


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Resistance Welding

No shielding gas No flux

No consumable electrodes, no filler material

Heat required for welding is produced by means ofelectric resistance between members to be joined

H = I2Rt

Heat in Joules

I (current) in Amperes

R(resistance) in Ohms

T(time of current flow) in seconds

H = I2RtK

K energy losses through radiation andconduction


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Resistance Welding


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Resistance Welding


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Resistance Welding


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Resistance Welding


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Resistance Welding


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Resistance Welding


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Resistance Welding


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Resistance Welding


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Resistance Welding


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Resistance Welding


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Resistance Welding


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Resistance WeldingTop Electrode

Water

WeldNugget

Bottom Electrode

Resistance

Distance


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Cycle:

(1) parts inserted

between

electrodes,

(2) electrodes close

(3) current on

(4) current off

(5) electrodes opened

Spot Welding Cycle

(a) Spot welding cycle

(b) Plot of force and current


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Nugget

(a) (b)

Point 1

Point 2

Point 3

Microscopic irregularities - 20%contact

Three heat growth areas

at 2/60 of a second after

the weld current starts(1) On the surface of top

electrode where it makes

contact to the part.

(2) Part to part contact

which is directly betweenthe electrodes.

(3) On the surface of

lower electrode where it

makes contact to the

part.

Microscopic irregularities - 20%contact


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Nugget

(a) (b)

Nugget grows to a cross sectional area of up to the electrode tip contact size

Thickness is just less than twice the thickness of thinner of the two pieces being welded.

Time is programmed

to be just less than

the time it takes for

the molten nugget to

reach an electrode

contact area

4/60th of a second after

the welding current

starts.


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Electrode Force

Welding Current

Welding Cycle

Squeeze Time Weld Time HoldTime

OffTime

Welding Handbook, Volume 2, AWS, p.538]

Squeeze time - Time interval between timer initiation and the first applicationof current.

Weld time - Time that welding current is applied to the work in making a weld.

Hold time - Time during which force is maintained on work after the last

impulse of current ends.

Off time - Time during which the electrodes are off the work and the work ismoved to the next weld location

Single Impulse Welding Cycle


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Resistance Welding


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Resistance Welding


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Resistance Welding


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Pulsing

Cool Time

Pulse 1 Pulse 2 Pulse 3

PulseTime


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Without Pulsing With Pulsing


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Electrode

Electrode tips are made of copper alloys and other

materials

Classification:

Group A Copper based alloys

Group B Refractory metal tips

Size of the electrode tip point controls the size ofthe resistance spot weld

Weld nugget diameter should be slightly less than

the diameter of the electrode tip point.


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Resistance Welding


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Resistance Seam Welding (RSEW)

Uses rotating wheel electrodes to produce a

series of overlapping spot welds along lap

joint

Can produce air-tight joints

Applications:

Gasoline tanks

Automobile mufflers

Various sheet metal containers


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Resistance Seam Welding (RSEW)


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Resistance Projection Welding (RPW)

A resistance welding process in which coalescence

occurs at one or more small contact points on the

parts

Contact points determined by design of parts to be

joined

May consist of projections, embossments, or

localized intersections of parts


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(1) Start of operation, contact between parts is at projections

(2) when current is applied, weld nuggets similar to spot

welding are formed at the projections



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Resistance Welding


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Resistance Welding


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(a) Welding of fastener on sheetmetal

(b) cross-wire welding


Oth R i t P j ti


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Other Resistance ProjectionWelding Operations

(a) Welding of fastener on sheetmetal and(b) cross-wire welding

Ad t


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Advantages

Adaptability for Automation in High-Rate

Production of Sheet Metal Assemblies

High Speed

Economical

Dimensional Accuracy


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Resistance Welding


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Submerged Arc Welding


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Submerged Arc Welding ( SAW)

Welding arc is shielded by a granular

flux consisting of lime,silica,manganeseoxide,calcium fluoride

Flux: insulates weld area,allows deepthermal penetration

Prevents spatter and spark over moltenmetal

Shielded glass etc is unnecessary

3002000 Amp (max 5000Amp for multi-arc)

Speed 5 m/min


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Submerged Arc Welding ( SAW)

Thick plate welding 4 to 10 times more

weld material than SMAW. SAW Deposition rate 45 kg/h

SMAW Deposition rate 5 kg/h

Generally automatic or mechanized Pressurized or gravity flux feed.

DC or AC power

SAW filler material Standard wire (1.6 - 6 mm)

Twisted wire (oscillating movement)


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Advantages

High deposition rates No arc flash or glare

Minimal smoke and fumes

Flux and wire added separately - extra dimension

of control

Easily automated

Joints can be prepared with narrow grooves

Can be used to weld carbon steels, low alloy steels,

stainless steels, chromium-molybdenum steels,

nickel base alloys


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Limitations

Flux obstructs view of joint during welding

Flux is subject to contamination porosity

Normally not suitable for thin material

Restricted to the flat position for grooves - flat andhorizontal for fillets

Slag removal required

Flux handling equipment


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Electroslag Welding (ESW)


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Electroslag Welding(ESW)

Arc is created

Flux is added and melted by arc

Electrode submerges in molten slag and arc isextinguished

600 A at 40-50 V 50 mm 900 mm

Travel speed 12 36 mm/min

Used for heavy structural steel, Pressure vessels

etc.


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Thank you

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Manufacturing Technology – II 21-28