Welding
Welding joins metals by melting and fusing them. Learn arc processes (SMAW, MIG/MAG, TIG, SAW), heat input, joint types and symbols, weld strength calculations, heat-affected zone, distortion and defects.
Ships, bridges, pipelines, car bodies, pressure vessels and machine frames are held together by welds. A weld melts the edges of two parts (usually with filler metal) so they fuse into one piece on cooling.
A good weld is as strong as the base metal; a bad one can crack, corrode or fail in fatigue — so procedures, qualified welders and inspection are strictly controlled.
An analogy
Like soldering electronics but at far higher temperature, and the parts themselves melt — not just a filler between them.
Arc welding processes
An electric arc (≈ 6000 °C) melts the metal.
- SMAW (stick): flux-coated electrode, portable, outdoor/site work.
- GMAW (MIG/MAG): continuous wire + shielding gas, fast, most common in fabrication and robots.
- GTAW (TIG): tungsten electrode, separate filler — highest quality, thin sheet, stainless, aluminium.
- SAW: arc under granular flux, very high deposition for thick plate, pipes, wind towers.
Joints, symbols and design
Butt, fillet (T and lap), corner and edge joints. Drawings use standard weld symbols (ISO 2553 / AWS A2.4) for type, size, length and location. Fillet welds are sized by leg length; the throat carries the stress.
Heat-affected zone and distortion
Metal near the weld (HAZ) is heated but not melted; its microstructure changes — hardening in steels (risk of cracking) or softening in heat-treated aluminium. Uneven heating and shrinkage cause distortion; fixtures, balanced welding sequences and preheat control it.
Quality, codes and advanced processes
Welding procedures (WPS) are qualified by testing (ISO 15614, ASME IX). Defects — porosity, lack of fusion, undercut, cracks, slag inclusions — are found by visual, dye penetrant, magnetic particle, ultrasonic and radiographic testing. Laser, electron-beam and friction-stir welding give deep, narrow or solid-state joints for aerospace, EV batteries and aluminium structures.
Formulas
- Heat input:
HI = η · V · I · 60 / (1000 · v) [kJ/mm, v in mm/min] - Fillet weld throat:
a = 0.707 · s - Fillet weld capacity (shear):
F = τ_allow · a · L - Butt weld (full penetration):
σ = F / (t · L)
Worked example: Heat input
Given: MAG welding: 25 V, 200 A; Travel 300 mm/min, η = 0.8
Find: Heat input.
- HI = 0.8 × 25 × 200 × 60/(1000 × 300) = 0.80 kJ/mm
Answer: 0.8 kJ/mm
Worked example: Fillet weld capacity
Given: 8 mm leg fillet, 200 mm long; τ_allow = 140 MPa
Find: Load capacity.
- a = 0.707 × 8 = 5.66 mm
- F = 140 × 5.66 × 200 = 158 kN
Answer: ≈ 158 kN
Common mistakes
- Wrong: Oversizing welds 'to be safe'. — Right: Excess weld adds heat, distortion and cost — size to the calculated throat.
- Wrong: Placing welds at high-stress, high-fatigue locations. — Right: Move welds to low-stress areas; weld toes are fatigue hot spots.
Applications
- Structures & ships: Beams, columns, hull blocks — often with robotic MAG and SAW.
- Pipelines & pressure vessels: Code-controlled welds with 100 % radiography or ultrasonic testing.
- Automotive & EV: Resistance spot welding of bodies; laser welding of battery tabs.
Key takeaways
- MIG/MAG for speed, TIG for quality, SAW for thick plate.
- HI = ηVI·60/(1000v).
- Fillet capacity = τ × 0.707s × L.
- Control HAZ, distortion and defects with procedures and inspection.
Going further
- Carbon equivalent: CE = C + Mn/6 + (Cr+Mo+V)/5 + (Ni+Cu)/15 predicts hardenability and preheat requirements.
- Friction-stir welding: Solid-state joining of aluminium with no melting — used in rockets, rail cars and EV trays.
- Fatigue of welds: Design by detail category (FAT 71, 80…) from Eurocode 3 / IIW, independent of steel strength.