THEORY EXAMINATION (SEM–VI) 2016-17 ADVANCE WELDING TECHNOLOGY

ADVANCE WELDING TECHNOLOGY (EME-022)

Section-Wise Solved Answers

SECTION – A (Short Answer Questions)

(10 × 2 = 20 Marks)

(a) Brazing

Brazing is a joining process in which two metals are joined by melting a filler metal having melting temperature above 450°C but lower than the base metals. The filler flows by capillary action and forms a strong joint without melting base metals.

(b) Arc blow

Arc blow is the deflection of welding arc from its normal path due to magnetic fields created by current flow. It causes uneven weld bead and poor quality weld.

(c) Spray welding

Spray welding is a process where molten metal droplets are sprayed onto a surface to form a coating. It is mainly used for surface repair and build-up.

(d) Importance of Schaeffler diagram

The Schaeffler diagram is used to predict microstructure of stainless steel weld metal based on chromium and nickel equivalents. It helps in selecting proper filler metal and avoiding cracks.

(e) Electroslag welding

Electroslag welding is a vertical welding process where heat is generated by electrical resistance of molten slag. It is used for welding thick plates in a single pass.

(f) Importance of welding

Welding provides permanent joints, saves material, reduces weight, and gives high structural strength. It is widely used in construction, automobiles, shipbuilding, and pipelines.

(g) Life prediction of welded structures

Life prediction involves estimating fatigue life of welded components by stress analysis, crack growth rate, and loading conditions to prevent failure.

(h) Modes of metal transfer

Modes of metal transfer in arc welding include:                        Short-circuiting transfer

Globular transfer                                                                        Spray transfer

Pulsed spray transfer

(i) TIG and MIG

TIG (Tungsten Inert Gas): Uses non-consumable tungsten electrode.

MIG (Metal Inert Gas): Uses consumable wire electrode.
Both use inert shielding gas to protect molten weld pool.

(j) Hard facing

Hard facing is the process of depositing a wear-resistant layer on a metal surface to increase hardness, abrasion resistance, and service life.

SECTION – B (Long Answer Questions)

(Attempt any FIVE – 5 × 10 = 50 Marks)

2(a) Principle of LASER and Laser Beam Welding

LASER stands for Light Amplification by Stimulated Emission of Radiation. Laser works on three principles:                                                        Stimulated emission

Population inversion                                        Optical resonator

Laser Beam Welding (LBW) uses a focused laser beam to melt and join metals.
Applications: aerospace, electronics, medical devices.
Advantages: deep penetration, narrow heat affected zone, high precision.

2(b) Electron Beam Welding (EBW)

EBW uses a high-velocity electron beam to produce heat on impact with workpiece. Welding is performed in vacuum.

Problems:

High equipment cost                                         Vacuum requirement

Solutions:

Partial vacuum systems                                      Improved beam control

2(c) Ultrasonic waves and Ultrasonic Welding

Ultrasonic waves are high-frequency mechanical vibrations above 20 kHz. They are generated using 
piezoelectric crystals.

Ultrasonic welding joins materials by frictional heat produced from vibrations.
Applications: plastics, thin metal foils, electronic components.

2(d) Explosive welding

Explosive welding uses controlled explosion to join two metals at high velocity.
The collision creates a wavy interface providing strong bond.

Parameters:                                                        Explosive velocity

Stand-off distance                                               Collision angle

Applications: cladding, heat exchangers, bimetal plates.

2(e) Hardfacing – theory and applications

Hardfacing is similar to simultaneous alloy steel making and heat treatment during arc welding. Alloying elements are deposited and rapidly cooled.

Processes used:                                                SMAW

TIG                                                                      MIG

Plasma arc welding

Advantages: increased wear resistance, longer component life.
Applications: crusher rolls, cutting tools, valves.

2(f) Plasma Arc Welding

Plasma is an ionized gas capable of conducting electricity.

Transferred mode: Arc between electrode and workpiece

Non-transferred mode: Arc between electrode and nozzle

Advantages: high temperature, stable arc, deep penetration.

2(g) Welding defects, distortion, causes and remedies

Defects: cracks, porosity, slag inclusion                                      Distortion: angular, longitudinal

Causes: improper heat input, poor joint design                          Remedies: preheating, proper sequence, controlled cooling.

2(h) Friction welding – advantages and limitations

Advantages:                                               No filler metal

High joint strength                                      No fusion defects

Limitations:

Only rotational or linear parts                     Equipment cost

Process parameters include pressure, speed, friction time.

SECTION – C (Very Long Answer Questions)

(Attempt any TWO – 2 × 15 = 30 Marks)

3. Lap joint and butt joint – design & life prediction

Lap joints overlap plates while butt joints join edges.
Design considerations include:                   Load type

Weld size                                                     Stress concentration

Life prediction is done using fatigue analysis, S-N curves, and crack propagation models.

4. Arc and power source characteristics

Arc characteristic shows relation between current and voltage of arc.
Power source characteristic may be:

Drooping                                                    Constant current

Constant voltage

Stability is achieved by matching arc and power source characteristics.

5. Temperature distribution, heating & cooling rate

Temperature distribution around weld shows highest temperature at weld center.

Heating rate: rate of temperature rise       Cooling rate: rate of temperature fall

They affect grain size, hardness, residual stress, and weld strength.