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ME0203 Manufacturing Technology

Chapter 1

Special welding processes

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Types of Welding

Fusion Welding Pressure Welding

Homogeneous Heterogeneous

Brazing SolderingGas Welding

Electroslag

High Energy Beam

Electric Arc

MIG

TIG

Shielded Metal ArcStick

Friction Welding

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WELDING PROCESSES

Basic Welding processes ( Revision- not insyllabus)

1. Arc Welding

2. Oxyfuel Gas Welding

Special Welding processes ( in syllabus)1. Other Fusion Welding Processes (Laser, Electron

Beam, Ultrasonic, Electro slag)

2. Resistance Welding (Friction welding)

3. Solid State Welding (electrical resistance welding)

4. Weld Quality

5. Weldability

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Two Categories of Welding Processes

Fusion welding - coalescence is accomplishedby melting the two parts to be joined, in somecases adding filler metal to the joint

Examples: arc welding, resistance spot

welding, oxyfuel gas welding Solid state welding - heat and/or pressure are

used to achieve coalescence, but no melting ofbase metals occurs and no filler metal is added

Examples: forge welding, diffusion welding,friction welding

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Joint Design

BUTT JOINT

STRAP JOINT

LAP JOINT

FILLET JOINT

CORNER JOINT

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Oxyfuel Gas Welding (OFW)

Group of fusion welding operations that burnvarious fuels mixed with oxygen

OFW employs several types of gases, which isthe primary distinction among the members of

this group Oxyfuel gas is also used in flame cutting

torches to cut and separate metal plates andother parts

Most important OFW process is oxyacetylenewelding

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Oxyacetylene Welding (OAW)

Fusion welding performed by a high temperatureflame from combustion of acetylene andoxygen

Flame is directed by a welding torch

Filler metal is sometimes added

Composition must be similar to base metal

Filler rod often coated with fluxto cleansurfaces and prevent oxidation

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Figure 31.21 A typical oxyacetylene welding operation (OAW).

Oxyacetylene Welding

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Maximum temperature reached at tip of innercone, while outer envelope spreads out andshields work surfaces from atmosphere

Figure 31.22 The neutral flame from an oxyacetylene torchindicating temperatures achieved.

Oxyacetylene Torch

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Other Fusion Welding Processes

FW processes that cannot be classified as arc,resistance, or oxyfuel welding

Use unique technologies to develop heat formelting

Applications are typically unique

Processes include:

Electron beam welding

Laser beam welding

Electroslag welding

Thermit welding

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Electron Beam Welding (EBW)

Fusion welding process in which heat for weldingis provided by a highly-focused, high-intensitystream of electrons striking work surface

Electron beam gun operates at:

High voltage (e.g., 10 to 150 kV typical) toaccelerate electrons

Beam currents are low (measured inmilliamps)

Power in EBW not exceptional, but powerdensity is

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EBW Advantages / Disadvantages

Advantages: High-quality welds, deep and narrow profiles

Limited heat affected zone, low thermaldistortion

High welding speeds No flux or shielding gases needed

Disadvantages:

High equipment cost

Precise joint preparation & alignment required Vacuum chamber required

Safety concern: EBW generates x-rays

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Laser Beam Welding (LBW)

Fusion welding process in which coalescence isachieved by energy of a highly concentrated,coherent light beam focused on joint

Laser = "light amplification by stimulated

emission of radiation" LBW normally performed with shielding gases

to prevent oxidation

Filler metal not usually added

High power density in small area, so LBW oftenused for small parts

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Comparison: LBW vs. EBW

No vacuum chamber required for LBW

No x-rays emitted in LBW

Laser beams can be focused and directed byoptical lenses and mirrors

LBW not capable of the deep welds and highdepth-to-width ratios of EBW

Maximum LBW depth = ~ 19 mm (3/4 in),whereas EBW depths = 50 mm (2 in)

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Advantages / Disadvantages of PAW

Advantages: Good arc stability

Better penetration control than other AW

High travel speeds

Excellent weld quality

Can be used to weld almost any metals

Disadvantages:

High equipment cost

Larger torch size than other AW

Tends to restrict access in some joints

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Resistance Welding (RW)

A group of fusion welding processes that use acombination of heat and pressure toaccomplish coalescence

Heat generated by electrical resistance to

current flow at junction to be welded Principal RW process is resistance spot

welding (RSW)

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Figure 31.12 Resistancewelding, showing thecomponents in spot

welding, the mainprocess in the RWgroup.

Resistance Welding

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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 partsbetween electrodes

Power supply from which a controlled currentcan be applied for a specified time duration

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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

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Resistance Spot Welding (RSW)

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

Used to join sheet metal parts using a series of

spot welds Widely used in mass production of

automobiles, appliances, metal furniture, andother products made of sheet metal

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

Spot Welding Cycle

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Resistance Seam Welding (RSEW)

Uses rotating wheel electrodes to produce aseries of overlapping spot welds along lap

joint

Can produce air-tight joints

Applications: Gasoline tanks

Automobile mufflers

Various other sheet metal containers

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Figure 31.15 Resistance seam welding (RSEW).

Resistance Seam Welding

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Resistance Projection Welding (RPW)

A resistance welding process in whichcoalescence occurs at one or more smallcontact points on 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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Equipment used in Electroslag welding

Fig : Equipment used forelectroslag weldingoperations.

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Solid State Welding (SSW)

Coalescence of part surfaces is achieved by: Pressure alone, or

Heat and pressure

If both heat and pressure are used, heat

is not enough to melt work surfaces

For some SSW processes, time is also afactor

No filler metal is added

Each SSW process has its own way of creatinga bond at the faying surfaces

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Success Factors in SSW

Essential factors for a successful solid stateweld are that the two faying surfaces must be:

Very clean

In very close physical contact with each

other to permit atomic bonding

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SSW Advantages over FW Processes

If no melting, then no heat affected zone, sometal around joint retains original properties

Many SSW processes produce welded jointsthat bond the entire contact interface between

two parts rather than at distinct spots or seams Some SSW processes can be used to bond

dissimilar metals, without concerns aboutrelative melting points, thermal expansions,

and other problems that arise in FW

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Solid State Welding Processes

Forge welding Diffusion welding

Explosion welding

Friction welding

Ultrasonic welding

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Diffusion Welding (DFW)

SSW process uses heat and pressure, usually ina controlled atmosphere, with sufficient time fordiffusion and coalescence to occur

Temperatures 0.5 Tm

Plastic deformation at surfaces is minimal Primary coalescence mechanism is solid state

diffusion

Limitation: time required for diffusion can range

from seconds to hours

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DFW Applications

Joining of high-strength and refractory metalsin aerospace and nuclear industries

Can be used to join either similar and dissimilarmetals

For joining dissimilar metals, a filler layer ofdifferent metal is often sandwiched betweenbase metals to promote diffusion

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Friction Welding (FRW)

SSW process in which coalescence is achievedby frictional heat combined with pressure

When properly carried out, no melting occurs atfaying surfaces

No filler metal, flux, or shielding gases normallyused

Process yields a narrow HAZ

Can be used to join dissimilar metals

Widely used commercial process, amenable toautomation and mass production

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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.

Friction Welding

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Applications / Limitations of FRW

Applications: Shafts and tubular parts

Industries: automotive, aircraft, farmequipment, petroleum and natural gas

Limitations:

At least one of the parts must be rotational

Flash must usually be removed

Upsetting reduces the part lengths (which mustbe taken into consideration in product design)

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Ultrasonic Welding (USW)

Two components are held together, oscillatoryshear stresses of ultrasonic frequency areapplied to interface to cause coalescence

Oscillatory motion breaks down any surface

films to allow intimate contact and strongmetallurgical bonding between surfaces

Although heating of surfaces occurs,temperatures are well below Tm

No filler metals, fluxes, or shielding gases

Generally limited to lap joints on soft materialssuch as aluminum and copper

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USW Applications

Wire terminations and splicing in electricaland electronics industry

Eliminates need for soldering

Assembly of aluminum sheet metal panels

Welding of tubes to sheets in solar panels

Assembly of small parts in automotiveindustry

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Weldability

Capacity of a metal or combination of metals tobe welded into a suitably designed structure,and for the resulting weld joint(s) to possessthe required metallurgical properties to perform

satisfactorily in intended service Good weldability characterized by:

Ease with which welding process isaccomplished

Absence of weld defects

Acceptable strength, ductility, andtoughness in welded joint

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Weld Defects

Undercuts/Overlaps

Grain Growth

A wide T will exist between base metal and HAZ.

Preheating and cooling methods will affect the brittlenessof the metal in this region

Blowholes

Are cavities caused by gas entrapment during thesolidification of the weld puddle. Prevented by proper weldtechnique (even temperature and speed)

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