Welding & Joining Processes

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.

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 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."

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.

Heat input (per unit length) governs weld penetration and HAZ size; resistance welding follows Joule heating I²Rt.

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).