Concept Notes
Welding is a permanent joining process used to fuse two or more similar or dissimilar metals with the help of heat, pressure, or both, with or without filler material. It provides strong joints and is widely used in manufacturing, construction, and repair works.
🔹 Classification of Welding Processes
- Fusion Welding (No external pressure, high heat)
- Heat is supplied to melt the base metals + filler metal.
- Examples: Gas welding, Arc welding, Thermit welding, Electron beam welding, Laser welding.
- Pressure Welding (External pressure required, sometimes with heat)
- Metals are joined by applying force (plastic deformation) with/without heat.
- Examples: Forge welding, Resistance welding (spot, seam, projection), Cold welding, Ultrasonic welding, Friction welding.
🔹 Important Welding Processes
- Gas Welding
- Heat source: Oxy-acetylene flame (~3200°C).
- Used for thin plates, pipelines, repair works.
- Filler rod may/may not be used.
- Arc Welding
- Heat source: Electric arc (current passes between electrode & workpiece).
- Types:
- Shielded Metal Arc Welding (SMAW)
- Gas Tungsten Arc Welding (GTAW / TIG)
- Gas Metal Arc Welding (GMAW / MIG)
- Submerged Arc Welding (SAW)
- Used in fabrication, shipbuilding, automotive industry.
- Resistance Welding
- Heat produced by resistance to electric current at joint.
- Types: Spot, Seam, Projection, Butt.
- Fast process, widely used in automobile industry (joining thin sheets).
- Thermit Welding
- Chemical reaction between aluminum powder + iron oxide → produces molten iron at ~2500°C.
- Used for railway track joints, heavy sections.
- Friction Welding
- Heat generated by rubbing the surfaces + pressure applied.
- Used for joining dissimilar metals, automotive drive shafts, turbine rotors.
- Ultrasonic Welding
- Uses high-frequency vibrations (20–40 kHz) + pressure.
- Common in plastics, thin sheets, electrical/electronic components.
🔹 Advantages of Welding
- Permanent and strong joints.
- Can join dissimilar metals.
- Lighter than riveted/bolted joints.
🔹 Disadvantages of Welding
- High residual stresses & distortions.
- Needs skilled labor.
- Difficult to dismantle after joining.
🔹 Applications
- Construction (bridges, pressure vessels, pipelines).
- Automotive & Aerospace industry.
- Railways (thermit welding of tracks).
- Shipbuilding.
⚙️ Formulas (Quick Reference)
- Heat Input in Welding:
where
- VVV = Voltage (Volts)
- III = Current (Amps)
- SSS = Welding speed (mm/min)
- Arc Efficiency:
Resistance Welding Heat:
where
- III = Current
- RRR = Resistance
- ttt = Time
🔟 10 MCQs
Q1. In gas welding, the maximum temperature of oxy-acetylene flame is about:
a) 2200°C
b) 2700°C
c) 3200°C
d) 3600°C
Q2. In arc welding, heat is produced due to:
a) Chemical reaction
b) Electric arc between electrode and workpiece
c) Friction between surfaces
d) Gas combustion
Q3. The heat generated in resistance welding is proportional to:
a) III
b) I2I^2I2
c) VVV
d) RRR
Q4. Which of the following is NOT a fusion welding process?
a) Gas welding
b) Arc welding
c) Thermit welding
d) Friction welding
Q5. Thermit welding is commonly used for:
a) Thin sheet welding
b) Rail joints
c) Aircraft structures
d) Plastic welding
Q6. In TIG welding, the electrode used is:
a) Copper
b) Carbon
c) Tungsten
d) Steel
Q7. Ultrasonic welding is best suitable for:
a) Thick plates
b) Plastics and thin metals
c) Heavy forgings
d) Railway tracks
Q8. The efficiency of arc welding lies between:
a) 20–40%
b) 50–60%
c) 60–70%
d) 80–90%
Q9. The heat input per unit length in welding increases with:
a) Higher voltage and higher current
b) Lower voltage and higher speed
c) Higher voltage and higher speed
d) Lower current and higher speed
Q10. Spot welding belongs to which category?
a) Fusion welding
b) Solid-state welding
c) Resistance welding
d) Pressure gas welding
✅ Answer Key
| Q | Ans |
|---|---|
| 1 | c |
| 2 | b |
| 3 | b |
| 4 | d |
| 5 | b |
| 6 | c |
| 7 | b |
| 8 | c |
| 9 | a |
| 10 | c |
🧠 Explanations
- Q1: Oxy-acetylene flame reaches ~3200°C → (c).
- Q2: Arc welding heat source = electric arc → (b).
- Q3: Resistance welding heat = Q=I2RtQ = I^2 R tQ=I2Rt → depends on square of current → (b).
- Q4: Friction welding is solid-state, not fusion → (d).
- Q5: Thermit welding is ideal for rail joints → (b).
- Q6: TIG uses non-consumable tungsten electrode → (c).
- Q7: Ultrasonic welding is for thin sheets/plastics → (b).
- Q8: Arc welding efficiency ~60–70% → (c).
- Q9: Heat input H=VISH = \frac{VI}{S}H=SVI, higher V & I → more heat → (a).
- Q10: Spot welding = resistance welding process → (c).
🎯 Motivation / Why Practice Matters
Welding processes are repeatedly asked in ECET exams because they combine theory + practical applications.
By practicing conceptual + numerical problems, you’ll not only score in exams but also strengthen real-world job skills (fabrication, automobile, maintenance).
👉 Daily practice = higher accuracy, faster solving, stronger confidence.
📲 CTA
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