Resistance Welding (RW) - ivipin.comivipin.com/wp-content/uploads/2017/02/WELDING-Part-2.pdf · during solidification and contraction of the weld bead and the welded structure . Fig.

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Resistance Welding (RW)

A group of fusion welding processes that use a

combination of heat and pressure to

accomplish coalescence

• Heat generated by electrical resistance to

current flow at junction to be welded

• Principal RW process is resistance spot

welding (RSW)

Resistance Welding

• Resistance

welding, showing

components in

spot welding, the

main process in

the RW group

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Components in Resistance Spot

Welding

• Parts to be welded (usually sheet metal)

• Two opposing electrodes

• Means of applying pressure to squeeze parts

between electrodes

• Power supply from which a controlled current

can be applied for a specified time duration

Advantages and Drawbacks of

Resistance Welding

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

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Types of Resistance welding

1. Spot welding

2. Seam welding

3. Butt welding

4. Projection welding

5. Flash welding

6. Percussion welding

1. Resistance Spot Welding (RSW)

Resistance welding process in which fusion of surfaces of a lap joint is achieved at one location by opposing electrodes

• Used to join sheet metal parts

• Widely used in mass production of automobiles, metal furniture, appliances, and other sheet metal products – Typical car body has ~ 10,000 spot welds

– Annual production of automobiles in the world is measured

in tens of millions of units

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• (a) Spot welding cycle

• (b) Plot of force and

current

• Cycle: (1) parts inserted

between electrodes, (2)

electrodes close, (3)

current on, (4) current

off, (5) electrodes

opened

Spot Welding Cycle

2. Resistance Seam Welding

(RSEW)• Uses rotating wheel electrodes to produce a

series of overlapping spot welds along lap

joint

• Can produce air-tight joints

• Quicker operation than spot welding & gives

a stronger joint

• Applications:

– Gasoline tanks

– Automobile

– Various sheet metal containers

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Resistance Seam Welding

Limitations of Seam Welding

• Cannot be applied where abrupt changes in

contours are there

• It is necessary that the weld should proceed in

straight line or uniform curve

• Stock thickness above 3 mm cannot be weld

easily

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3. Resistance Butt/ Upset Welding• Used to join components

of similar cross section by

making a weld in a single

operation

• Solid state process

Steps for Butt welding

• Two work pieces to be welded are first brought

together under pressure.

• Current is then applied

• Heat is produced in the weld region by resistance to

the passage of the current through the parts

• As the material heats, the force forges the material

to consolidate and complete the joint

• The pressure and current are applied throughout the

weld cycle

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Application of Butt welding

• wire and rod joints up to about 16mm

diameter

• Making chains

• Welding of high electrical conductivity

material

4. 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

• The surfaces of the w/p are in contact only at the

projections.

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• (1) Start of operation, contact between parts

is at projections by pressing the upper

electrode downwards; (2) when current is

applied, weld nuggets similar to spot welding

are formed at the projections

Resistance Projection Welding

5. Resistance Flash Welding

• Two-stage process

• Used for joining metal

pieces end to end

• Does not use any filler

material

• AC power supply is

generally used

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Steps for Flash Welding-

1. First the current is switched ON

• Weld pieces are slowly brought together to

make end contact

• produces a flashing or arcing across the

interface of the two butting ends

2. Pieces are then forcibly press against each

other.

• This forces the molten metal & slag to be

squeezed out in the form of sparks

Application of Flash Welding

• Automobile industry

• Tubular and solid structures

• Welding of pipes

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Advantages of Flash welding-

• Much quicker than Butt welding

• As only a small portion of w/p is heated,

current consumption is less

• Stronger than the butt welding joints

Limitations-

• Upsetting pressure and power limit the size &

area of w/p

6. Resistance Percussion welding

• Striking of one solid

object against another

with some degree of

force.

• Used for joining of

dissimilar metals having

small cross sectional

areas.

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Steps for Percussion Welding-

• First hold the parts at a small distance

• Switch ON the current & bring them closer at

fast speed

• An arc will be generated just before the

contact

• Then the weld will be completed after impact

Soldering

• It is a low temperature joining process by using another

metal or alloy which has fairly low melting pt. as compared to

the metals to be joined. It is performed at temperatures

below 450° C for joining.

• Soldering is used for,

• Sealing, as in automotive radiators or tin cans

• Electrical Connections

• Joining dissimilar metals

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Soldering

• Soft soldering: is used for sheet metal works

that are not subjected to excessive loads,

these melt at temp below 350° C

– Alloy of lead & tin

• Hard Soldering: employed solders whose

melting temp. is higher than soft solders.

– Alloy of Cu & Zn

Solder = Filler metal

Soldering

Applications:

• Printed Circuit Board (PCB) manufacture

• Pipe joining (copper pipe)

• Jewelry manufacture

Easy to solder: copper, silver, gold

Difficult to solder: aluminum, stainless steels

• Alloys of Tin (silver, bismuth, lead)

Flux used to clean joint & prevent oxidation

• Typically non-load bearing

Tinning = pre-coating with thin layer of solder

• Zinc chloride

• separate or in core of wire

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• Low temperature joining process.

• It is performed at temperatures above 450° C and it generally

affords strengths comparable to those of the metal which it

joins.

• It is low temperature in that it is done below the melting point

of the base metal.

• It is achieved by diffusion without fusion (melting) of the base

Brazing

Brazing

Advantages

• Dissimilar metals which canot be welded can be joined by brazing

• Very thin metals can be joined

• Metals with different thickness can be joined easily

• In brazing thermal stresses are not produced in the work piece. Hence there is no distortion

• Using this process, carbides tips are brazed on the steel tool holders

Disadvantages

• Brazed joints have lesser strength compared to welding

• Joint preparation cost is more

• Can be used for thin sheet metal sections

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Use of low melt point filler metal to fill thin gap between

mating surfaces to be joined utilizing capillary action

Brazing

Applications:

• Pipe/Tubing joining (HVAC)

• Filler metals include Al, Mg & Cu alloys

• Automotive - joining tubes

• Electrical equipment - joining wires

• Jewelry Making

• Flux used (Borax)

• Types of brazing classified by heating

method:– Torch, Furnace, Resistance

• Joint can possess significant

strength

WELDING

DEFECTS

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Defects affect the quality of weld

• Porosity

• Poor penetration

• Warping

• Undercut & Underfill

• Distortion

• Cracked welds

• Poor appearance

• Poor fusion

• Brittle welds

• Spatter

• Magnetic blow

• Weld stress

Causes and cures of common welding troubles

Presence of small pores or voids in the weld, due to entrapement of gases.

When these pores are quite small – porosity

Bigger void- Blow Hole

Due to:-

1. Presence of oil, grease, moisture

2. Faulty electrodes

3. Improper gas shielding

4. Wrong welding techniques

5. Low welding current

Porosity

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Failure of weld metal to reach the Root of the joint.

Weakens the joint

Due to:-

1. Faster welding speed

2. Incorrect size Electrodes

3. Current too low.

4. Faulty preparation.

Poor penetration


Undercutting is a groove that occurs in the parent metal near the toe of

the weld.

When this weld flaw occurs, the weld metal fails to fill in that grooved

area, resulting in a weak weld that is prone to cracking along the

toes.

why?

1. Faulty electrode manipulation

2. Faulty electrode usage.

3. Current too high.

Undercut/Underfill

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Causes and cures of common welding

troubles

Examples of various discontinuities in fusion welds

Fig. a) Underfill

Fig. a) Undercut


troubles

• Slag got entrapped in the weld metal specially in case of Multi pass welds and is known as Slag Inclusion

• Such inclusion may also be added due to dirt, rust etc.

• Such inclusions weakens the weld.

Inclusions

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Due to:-

1. Uneven heat

2. Improper sequence.

3. Deposited metal shrinks.

Distortion


troublesDistortion of parts after

welding

Fig. a) in butt joints

Fig. a) in fillet joints

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Crack is a discontinuity of metal.

• Macro crack

• Micro crack

Makes the joint weak and finally lead to fracture.

Due to:-

1. Wrong electrode.

2. Weld and part sizes unbalanced.

3. Faulty welds.

4. Faulty preparation.

5. Rigid joints.

Cracked welds

Cracks caused by thermal stresses

Types of cracks in welded joints caused by thermal stresses that develop

during solidification and contraction of the weld bead and the welded

structure .

Fig. a) Crater cracks

Fig. b) various types of cracks

in butt & T-joints

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troubles

Tiny electrode metal particles are blown out of the arc which get deposited over the bead and base material.

Gives Poor appearance

Does not have any effect on weld strength

Due to:-

1. Too high welding current

2. Wrong composition of flux coating

3. Discontinuous Arc

Spatter


troubles

Deposited weld metal by the electrode does not fuse fully with the base metal because the latter is relatively cooler.

Due to:-

1. Presence of oxide, dirt, scale, slag b/w to fusing surfaces

2. Wrong speed.

3. Current improperly adjusted.

Poor fusion

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Examples of various discontinuities

in fusion welds

Poor

fusion


troubles

why?

1. Wrong electrode.

2. Faulty preheating.

3. Metal hardened by air.

What to do?

1. Preheat at 135 to 260º C if welding on medium-carbon steel or certain alloy steel.

2. Make multiple-layer welds.

3. Anneal after welding.

4. Use stainless or low-hydrogen electrodes for increasing weld ductility.

Brittle welds

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troubles

Due to fixed polarity in D.C. Welding, magnetic lines form in the W.P. When

welding at the centre of W.P. these lines are equally distributed on both sides so

Arc will be straight. But while welding at the edges, the magnetic lines will try to

pull back the arc and it will be deflected towards the W.P., as these lines will be

formed only in the material. This phenomenon is called arc blow and causes

spatter and improper bead geometry.

Arc/ Magnetic blow

What to do?

1. Use steel blocks to alter magnetic path around arc.

2. Divide the ground into parts.

3. Weld in same direction the arc blows.

4. Use a short arc.

5. Locate the ground properly on the work.

6. Use a-c welding

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Thank you

Resistance Welding (RW) - ivipin.comivipin.com/wp-content/uploads/2017/02/WELDING-Part-2.pdf · during solidification and contraction of the weld bead and the welded structure . Fig.

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