week7

WEEK -7

S E P 2 3 R D 2 0 1 4

2 0 1 4 - 2 0 1 5 S E M E S T E R - I

TA201 Manufacturing Processes

2014-15 Semester-I

Announcements

TA 201 Dr. Shashank Shekhar

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Upload Project report on website (check group names and project names on website)

TA201 midsem copies will be distributed at the end of labs from Thursday onwards

No re-checking will be done Only unchecked questions and totaling mistake will be

considered Midsem performance ?

Endsem + Project Marks

80% and above A

70% to 80% B

60% to 70% C

50% to 60% D

Along with attendance

W E L D I N G

Fusion Welding

Physics of fusion welding

Examples: Arc Welding, Resistance Welding, Gas Welding

Solid state Welding

Weld Quality and Weldability

B R A Z I N G

S O L D E R I N G

A D H E S I V E B O N D I N G

Dr. Shashank Shekhar TA 201

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Joining [Chap-30, 31, 32 & 33 :Groover]

2014-15 Semester-I

Welding


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Welding involves localized coalescence of two metallic parts at their faying surface. The faying surfaces are the part surface in contact or close proximity that are to be joined. Welding is usually performed on parts made of the same metal but some welding operations can be used to join dissimilar metals

2014-15 Semester-I

5 Types of Welding

Fusion welding Solid state welding (Pressure or no pressure)

Arc welding

Gas welding

Resistance welding

Other Fusion Welding (Laser, Electron beam)

Diffusion Welding

Friction Welding

Ultrasonic Welding

No filler metal; No melting


2014-15 Semester-I

Melting base metal and coalescing two work pieces, using heat is called Fusion Welding It involves: • Melting of localized regions • Solidification of molten pool when the heat is extracted • Source of heating is usually in moving condition

Heat source: • Chemical, as in gas welding • Electrical, as in arc welding • Electron & Laser beam welding • Resistance welding

Fusion Welding 6

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Advantages 1. It provides a permanent joint

2. Joint can be stronger than the parent materials if the strength of filler metal is more than that of the parent metals and proper welding techniques are used.

3. Most economical way of joining.

4. Not restricted to factory environment.

Disadvantages 1. Manual welding requires skilled trade.

2. High energy requirement.

3. Difficult to dismantle if required.

4. Possibility of defect formation.


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8 Types of Joint Nature

Butt Joint

Corner Joint

Lap Joint Tee Joint

Edge Joint


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Types of Welds

Fillet weld

Spot weld

Plug weld

Groove weld


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Fillet weld

Groove weld

Plug weld

Spot weld


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11 Physics of Fusion Welding

Power Density (PD) = P/A P = Power entering the surface (Watt) A = Surface area over which energy is entering Minimum power density required to melt most metals in welding is about 10 W/mm2

High power density and low energy are preferable because 1. Quick melting of base metal or filler metal is crucial 2. To avoid metallurgical damage (Defects) What are the defects that can arise because of prolong heating?


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Example-1: Variation of PD 12

A heat source is capable of transferring 3000W to the surface of a metal part. The heat impinges the surface in a circular area, with intensities varying inside the circle. The distribution is as follows: 70% of the power is transferred within a circle of diameter = 5mm, and 90% is transferred within a concentric circle of diameter =12mm. What are the power densities in

(a) the 5mm dia inner circle

(b) 12 mm dia ring that lies around the inner circle

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13 Physics of Welding

Quantity of heat required to melt a given volume of metal

is the sum of

1. Heat to reach melting point

2. Latent heat of fusion

To a reasonable approximation

Quantity of heat (Um) = KTm2

K: Constant (3.33x10-6) when Kelvin scale is used

Um = Heat required (Joules/mm3)

Tm = Melting temperature


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14 Heat transit

Total heat (amount = H)

f1: Heat transfer factor (ratio of the actual heat received by the workpiece and the total heat generated at the source) f2: Melting factor: Fraction of heat received by the workpiece available for melting (This is due to the conduction of heat away from weld zone)

Heat received by workpiece (amount = f1H)

Heat for melting (amount = f1f2H = Hw)

f1H

f2f1H

Hw = f1f2H …………… Hw: net heat available for welding (Joule) For Aluminium f2 is very small and welding becomes difficult…why?


2014-15 Semester-I

15 Energy Balance in Welding

Heat input for welding (Hw) = Energy needed for welding (Um*V)

Um: Unit energy required to melt unit volume of metal (J/mm3) V: Volume of the metal to be melted (mm3)

Hw = Um*V

RH: Heat generated at the source per unit time

RHw: Rate of heat energy delivered to the weld (J/s)

Rwv : Volume rate of the metal welded (mm3/s)

=Aw*v

Where, Aw: Area of the weld

v: Travel speed of arc or flame

along the weld line

RHw = Um*Rwv = Um*Awv = f1f2RH

Rate balance


2014-15 Semester-I

Example-2 16

The power source in a particular welding setup is capable of generating 3500 W that can be transferred to the work surface with an efficiency f1=0.7. The metal to be welded is low carbon steel, whose melting temperature, is Tm = 1760K. Melting efficiency, f2 = 0.5. A continuous fillet weld is to be made with a cross-sectional area Aw =20 mm2. Determine the travel speed at which the welding operation can be accomplished

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17 Cross section of Fusion weld

Different zones

Grain structure in different zones


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18 Arc Welding

Coalescence of the metals is achieved by the heat from an electric arc between the tip of an electrode and the workpiece to be welded

Electric arc: Discharge of electric current across a gap in a circuit

Electric arc is sustained by plasma (ionized gas)

Touch it and separate it and maintain the distance

Temperature ~ 5500°C

Filler metal is added to increase the strength and volume of the weld metal


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Electrode nature

Consumable: Filler metal acts as electrode Non-consumable: W (tungsten rod): Separate filler metal is needed

Wire electrode (Consumable) Non-consumable electrode

(separate Filler metal)


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20 Arc Shielding

Need: To avoid chemical interaction between molten

metal with O2, H2 and N2.

Shields: Gas blanket or flux or both

1. Gas (as in GMAW/ MAG/ TIG) : Argon, Helium or Mixture of Ar + CO2

2. Flux: Flux forms slag which is to be removed

A. Using a stick coated with flux material (SMAW/ MMAW)

B. Flux can be delivered by pouring granular flux onto the welding operation (submerged AW)

C. Using tubular electrode in which flux is contained in the core (as in the case of Flux-cored AW)

• Provides protective atmosphere for welding

• Stabilizing arc

• reducing spattering


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21 Arc Shielding


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Power source and analysis

Both AC and DC arc are used

f1: Heat transfer factor (ratio of the actual heat received by the workpiece and the total heat generated at the source) f2: Melting factor: Fraction of heat received by the workpiece available for melting (This is due to the conduction of heat away from weld zone)

RHw = Um*Awv = f1f2RH = f1f2E*I E: Voltage (V) I: Current (Amp)

RHw: Rate of heat energy delivered to the weld (J/s)


2014-15 Semester-I

23 Resistance Welding

Heat + Pressure : Joining by fusion due to electrical resistance to the current flow at the junction to be welded

H = I2Rt H: Heat generated I: Current R: Resistance t: Time


2014-15 Semester-I TA 201 Dr. Shashank Shekhar

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Resistance Welding

Advantage: 1. No filler metal 2. High production rate 3. Good repeatability and reliability Disadvantage: 1. High cost


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Oxyfuel Gas Welding

C2H2 + O2 = 2CO +H2 + Heat CO + H2 + 1.5O2 = 2CO2 + H2O + Heat

Acetylene is the fuel and Combustion of C2H2 by O2 generates heat and temperature can reach 34800C.


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• Cylinders contain oxygen and acetylene gas at extremely high pressure.

• C2H2 + O2 mixture is flammable and hazardous.

• It can be used for structural sheet metal fabrication, automotive bodies, repair work

• f1 is relatively low because of large spread of flame (0.1 to 0.3)

• Pure C2H2 colorless and odorless gas mixed with commercial acetone (why?).

(Hint: Acetone + Acetylene: Acetylene is unstable beyond 1 atm pressure. And Acetone can dissolve 25 times acetylene…and the cylinder is filled with porous materials such as asbestos…)

OxyAcetylene Welding (OAW)


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Typical Oxyacetylene Gas Welding apparatus


2014-15 Semester-I TA 201 Dr. Shashank Shekhar

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There are three distinct types of oxy-acetylene flames, usually termed:

Neutral

Carburizing (or “excess acetylene”)

Oxidizing (or “excess oxygen” )

The type of flame produced depends upon the ratio of oxygen to acetylene in the gas mixture which leaves the torch tip.

Flame Settings

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Advantage: Inexpensive Portable Economical and versatile process Well suited for low quality production and repair job Disadvantage: Longer time for welding Larger HAZ Problem with stainless steel welding

OxyAcetylene Welding (OAW)


2014-15 Semester-I

Radiation: Electron Beam Welding

It is a fusion welding process in which heat for welding is provided by a highly focused and high-intensity stream of electrons impinging against the work surface

• The electron bum operates at high voltage to accelerate the electrons

• The power in EBW is not exceptional, but it has high power density

• It is operated in the vacuum chamber to minimize the disruption of the electron beam by air molecules

Disadvantages

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2014-15 Semester-I

Radiation: Laser Beam Welding

It is a fusion welding process in which coalescence is achieved by an energy of a highly concentrated, coherent light beam focused on the joint to be welded

• Focuses energy onto a small area

• Produce weld of high quality, deep penetration, and narrow HAZ

• Single pass weld penetration up to 3/4” in steel

• Materials need not be conductive

• No filler metal required • Low heat input produces low

distortion • Does not require vacuum

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2014-15 Semester-I

Examples

Laser beam

Plasma plume

Molten material

shielding gas nozzle (optional)

Keyhole welding

Gillette razor cartridge with laser beam welds. Welded joints are of 0.5 mm in diameter

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34 Solid State Welding

Application of Heat or Pressure or only Pressure leads to welding

Localized melting is possible or melting may not even occur

Filler metal is not used

Metallurgical bond is created with almost no melting

Advantages:

Welding with little melting or no melting

Metallurgical purity is maintained

No HAZ

Dissimilar metals can be bonded

Example: Forge welding, Roll welding, Cold welding, Diffusion welding

Explosion welding (Cladding)


week7

Documents

fraction of heat

conduction of heat

weldingquantity of heat

welding um

rate of heat energy

latent heat of fusion

welding hw

net heat available