Welding and Joining Processes
Arc, gas, resistance and solid-state welding, heat input and arc power, resistance welding heat, heat-affected zone, weld defects, brazing, soldering and adhesive bonding.
Joining makes one part from several. Welding melts (or presses) the parts together; brazing and soldering use a filler that melts at a lower temperature than the parts.
Arc welding
An electric arc (about 6000 °C) melts the base and filler metals.
- Arc power P = V I; heat input per unit length = η V I / v (η = heat transfer efficiency, v = travel speed).
- Arc length–voltage characteristic: V = A + B l (voltage grows with arc length). With a drooping power source, the operating point is where the arc characteristic meets the source line.
- Processes: SMAW (stick, flux-coated electrode), GMAW/MIG (consumable wire + inert gas), GTAW/TIG (non-consumable tungsten electrode), SAW (submerged arc, thick plates), plasma arc.
- Polarity: DCEN (electrode negative) puts more heat into the work, giving deep penetration; DCEP gives more heat at the electrode.
Gas welding
Oxy-acetylene flame: neutral (equal O₂ and C₂H₂, most metals), oxidising (excess O₂, brass), carburising (excess acetylene, high-carbon steels). Hottest point ≈ 3200 °C at the tip of the inner cone.
Resistance welding
Heat is generated by the resistance of the joint:
H = I² R t
Spot, seam, projection and flash butt welding. Very fast and used for sheet metal in car bodies.
Solid-state welding
No melting: friction welding, friction stir welding (aluminium), ultrasonic, explosive and diffusion welding.
Heat-affected zone and defects
- The HAZ is base metal heated enough to change its structure but not melted. In steels it can become hard and brittle.
- Defects: porosity, slag inclusion, lack of fusion, incomplete penetration, undercut, and cracks (hot cracks while solidifying; cold cracks from hydrogen).
Brazing, soldering and adhesives
- Brazing: filler melts above 450 °C, below the base metal's melting point; joined by capillary action.
- Soldering: filler melts below 450 °C (electronic joints).
- Adhesive bonding: joins dissimilar materials without heat.
Worked example
An arc runs at 25 V and 200 A with heat transfer efficiency 0.8 and travel speed 5 mm/s.
- Arc power = 5000 W. Heat input = 0.8 × 5000/5 = 800 J/mm.
Practice
A spot weld uses 10,000 A for 0.1 s through a joint resistance of 100 µΩ. How much heat (J) is generated?
Show answer and explanation
Answer: 1000H = I²Rt = 10⁸ × 10⁻⁴ × 0.1 = 1000 J.
An arc welds at 20 V, 150 A, efficiency 0.75 and speed 4 mm/s. What is the heat input in J/mm?
Show answer and explanation
Answer: 562.5Heat input = ηVI/v = 0.75 × 3000/4 = 562.5 J/mm.
Which welding process uses a non-consumable electrode?
Show answer and explanation
Answer: BTIG uses a tungsten electrode that does not melt; filler is added separately if needed.
The arc characteristic is V = 20 + 4l (l in mm). What arc voltage results at an arc length of 3 mm?
Show answer and explanation
Answer: 32V = 20 + 4 × 3 = 32 V.