Welding & Joining Processes — revision notes (GATE ME)
Welding is a steady GATE ME topic (~1–2 marks). The process classification, heat-affected zone concept, and defect/heat-input ideas are the reliably tested items.
Classification
- Fusion welding (melts the base metal): arc (SMAW/stick, TIG/GTAW, MIG/GMAW, submerged-arc), gas (oxy-acetylene), resistance (spot, seam — uses I²R heating), thermit, electron-beam, laser.
- Solid-state welding (no melting): friction, friction-stir, ultrasonic, diffusion, explosive — good for dissimilar metals.
- Brazing (filler melts above 450°C, base does not) and soldering (below 450°C): joining by a lower-melting filler, weaker than welds.
Key concepts
- Heat-affected zone (HAZ): the base metal near the weld whose microstructure changes without melting — often the weakest, crack-prone region.
- Heat input = (voltage × current × efficiency)/welding speed; higher heat input → wider HAZ, more distortion.
- Weld defects: porosity (gas), slag inclusion, lack of fusion/penetration, undercut, cracks (hydrogen/cold cracking), residual stress and distortion.
- TIG for thin/precision non-ferrous; MIG for speed; SMAW for versatility/field work.
Exam Tricks & Tips
- 🎯 Fusion melts the base metal; brazing/soldering do NOT (only the filler melts) — the key classification and a common MCQ.
- 🎯 Brazing filler melts above 450°C, soldering below 450°C — the defining temperature boundary.
- 🎯 Heat input = ηVI/speed; higher input → wider HAZ and more distortion — controls weld quality.
- 🎯 HAZ is often the weakest, most crack-prone region — its microstructure is altered without melting.
- 🎯 Resistance welding (spot/seam) uses I²R Joule heating; no filler or shielding gas needed.
- ❌ Common mistake: classifying brazing/soldering as fusion welding — the base metal never melts in brazing/soldering; only fusion processes melt the base.
Expected exam pattern
A 1-mark process-classification, brazing/soldering, or HAZ MCQ, and a 2-mark heat-input or weld-defect problem. The fusion-vs-solid-state and braze/solder temperature boundary are frequently tested.
Quick recap
Fusion (melts base: arc SMAW/TIG/MIG, gas, resistance I²R, EBW/LBW) vs solid-state (friction, ultrasonic — dissimilar metals) vs brazing (filler > 450°C)/soldering (< 450°C, base unmelted). HAZ = altered, weak zone. Heat input = ηVI/speed → wider HAZ. Defects: porosity, slag, lack of fusion, cracks.
Welding & Joining Processes — Flashcards
Cover the answer, recall, then check. 11 cards on welding for GATE ME.
Q1. Difference between fusion and solid-state welding?
A1. Fusion melts the base metal (arc, gas, resistance); solid-state joins without melting (friction, ultrasonic, diffusion).
Q2. How do brazing and soldering differ from welding?
A2. Only a lower-melting filler melts; the base metal stays solid. Brazing: filler above 450°C; soldering: below 450°C.
Q3. What is the heat-affected zone (HAZ)?
A3. Base metal near the weld whose microstructure changes from the heat without melting — often the weakest, crack-prone region.
Q4. Formula for welding heat input.
A4. Heat input = (η·V·I)/welding speed; higher input widens the HAZ and increases distortion.
Q5. What heating principle does resistance (spot) welding use?
A5. Joule (I²R) heating at the faying interface, with electrode pressure; no filler or shielding gas.
Q6. Which process suits thin, precision, non-ferrous welds?
A6. TIG (GTAW) — a non-consumable tungsten electrode gives a clean, controllable, high-quality weld.
Q7. Which arc process is fast and semi-automatic with a consumable wire?
A7. MIG (GMAW) — continuous wire feed with shielding gas, high deposition rate.
Q8. Name common weld defects.
A8. Porosity (gas), slag inclusion, lack of fusion/penetration, undercut, hydrogen (cold) cracking, and distortion/residual stress.
Q9. Advantage of solid-state welding for dissimilar metals?
A9. No melting means no brittle intermetallics from a molten pool, so dissimilar metals bond better.
Q10. What causes hydrogen (cold) cracking?
A10. Dissolved hydrogen combined with a hard martensitic HAZ and residual stress — mitigated by preheating and low-hydrogen electrodes.
Q11. Why is preheating used before welding thick/hardenable steel?
A11. It slows cooling, reducing HAZ hardness and residual stress, and lowers the risk of cracking.
Welding & Joining Processes
Welding permanently joins metals by fusion, and it is everywhere — structures, pipelines, pressure vessels. GATE tests the heat input, process characteristics, weld metallurgy (HAZ), and defects. Arc-welding energy balance underlies many numericals.
Core concept: fusion welding melts the joint faces (with or without filler) so they coalesce on solidification; the heat source, its intensity, and the resulting thermal cycle govern joint quality.
Deep explanation
Beginner — welding processes
- Arc welding: SMAW (stick), GMAW/MIG, GTAW/TIG, submerged arc (SAW). Heat from an electric arc melts the joint; shielding (flux/gas) protects the molten pool from oxidation.
- Resistance welding: spot/seam — heat from I²R at the interface (Joule heating), no filler.
- Other: oxy-acetylene (gas), and solid-state (friction, ultrasonic, explosion) that join below melting.
Intermediate — heat input and energy balance
Arc power = V·I. Heat input per unit length H = (V·I·η)/v, where η = arc efficiency (SAW ~0.9, SMAW ~0.7–0.8, GTAW ~0.5) and v = travel speed. Melting efficiency and the heat needed to melt a unit volume (∝ melting temperature squared, from Ryan's/quantity-of-heat relations) set the weld size. Higher heat input → wider bead, larger heat-affected zone, slower cooling.
Advanced — weld metallurgy and defects
- Heat-affected zone (HAZ): base metal near the weld that is thermally altered (grain growth, hardening) but not melted — often the weakest/most crack-prone region.
- Cooling rate controls HAZ microstructure; fast cooling in hardenable steels can form brittle martensite → preheat to slow cooling and avoid cracking.
- Defects: porosity (gas), slag inclusion, lack of fusion/penetration, undercut, cracks (hot/cold), residual stress and distortion. Preheating, correct heat input, and post-weld heat treatment control these.
- Weldability decreases with carbon/alloy content (carbon equivalent).
Worked example
An arc weld uses 25 V and 200 A with an arc efficiency of 0.8, travelling at 5 mm/s. Find the net heat input per unit length.
Arc power = VI = 25 × 200 = 5000 W.
Net power = η × VI = 0.8 × 5000 = 4000 W.
Heat input H = net power/travel speed = 4000/0.005 = 800,000 J/m = 800 kJ/m (= 0.8 kJ/mm).
GATE relevance
Welding heat input (VIη/v), process/arc-efficiency comparisons, HAZ and preheat concepts, and defect identification are recurring Manufacturing questions. The energy-balance numerical and the effect of heat input on weld/HAZ size are frequently tested.
Exam tricks & shortcuts
- Heat input H = VIη/v — higher speed means less heat per length (narrower bead, faster cooling).
- Preheat slows cooling to avoid brittle martensite and cracking in hardenable steels.
- SAW has the highest arc efficiency (
0.9); TIG the lowest (0.5). - Mnemonic: "Volts times amps times efficiency, over speed — that's the heat you feed."
Ignoring arc efficiency η in the heat-input calculation, or forgetting that travel speed is in the denominator. Not all electrical power reaches the workpiece; only η·VI does, and dividing by speed gives energy per unit length — faster travel deposits less heat.
- ✓- Fusion welding melts joint faces; shielding protects the pool.
- ✓- Heat input H = VIη/v (J/length); η varies by process.
- ✓- HAZ is thermally altered, often crack-prone; controlled by cooling rate.
- ✓- Preheat/PWHT reduce cracking and residual stress.
- ✓- Defects: porosity, slag, lack of fusion/penetration, undercut, cracks.
- ✓Welding quality is a heat story: net heat input VIη/v sets bead and HAZ size and cooling rate. Fast cooling in hardenable steels risks brittle martensite, so preheat and correct heat input, and watch the HAZ as the joint's weakest link.
Welding & Joining Processes — Formula Sheet
Key formulas
- Heat input (arc welding): H = η·VI/v (V = voltage, I = current, v = travel speed, η = efficiency).
- Resistance welding heat: Q = I²Rt.
- Weld cross-section / deposition rate relates to current.
- Cooling rate affects HAZ (heat-affected zone) properties.
- Weldability depends on carbon equivalent CE.
- ✓- Arc heat input H = ηVI/v.
- ✓- Resistance welding Q = I²Rt.
- ✓- Higher heat input → wider HAZ.
Heat input (per unit length) governs weld penetration and HAZ size; resistance welding follows Joule heating I²Rt.
Welding & Joining Processes — Worked Example
Worked Example
Problem: In an arc-welding operation the arc voltage is 25 V, the welding current is 300 A, and the travel speed is 5 mm/s. If the arc (heat-transfer) efficiency is 0.9, find the net heat input per unit length of weld.
Solution:
Electrical power supplied by the arc:
P = V·I = 25 × 300 = 7500 W.
Net (effective) power reaching the workpiece after arc losses:
P_net = η·V·I = 0.9 × 7500 = 6750 W.
Heat input per unit length of weld is the net power divided by the travel speed:
H = P_net/v = 6750 W /(5 mm/s) = 1350 J/mm.
(Equivalently 6750 / 0.005 m/s = 1.35 × 10⁶ J/m.)
Answer: Net heat input ≈ 1350 J/mm (1.35 MJ/m).
- ✓- Heat input per length H = ηVI/v; it rises with current/voltage and falls as travel speed increases.
- ✓- The arc efficiency η accounts for heat lost to the surroundings — it varies with the process (higher for submerged arc, lower for TIG).
- ✓- High heat input gives a wider heat-affected zone and slower cooling, affecting weld microstructure and distortion.