Solid-state welding, brazing and soldering

Why it matters

Many joints cannot tolerate melting: aluminium to copper busbars in battery packs, titanium aerospace parts, electronic components on a PCB, carbide tips on saw blades, thin copper tubes in refrigeration coils. Solid-state welding, brazing and soldering join such parts with less heat, less distortion and often between dissimilar metals, and their choice decides joint strength, service temperature and cost.

Key ideas

Three families, one distinction: what melts?

• Fusion welding — base metal melts.
• Solid-state welding — nothing melts; coalescence comes from pressure, plastic deformation and diffusion, sometimes helped by heat below the melting point.
• Brazing and soldering — only a filler melts; the base metal stays solid and the molten filler is drawn into a narrow gap by capillary action and wets the surfaces.

Solid-state welding processes. The common requirement is clean, oxide-free surfaces brought within atomic distances, usually by breaking up the oxide film through plastic deformation.

• Forge welding — heated parts hammered together; the oldest process.
• Cold (pressure) welding — heavy deformation at room temperature; ductile metals such as Al and Cu (wire joining).
• Friction welding — one part rotates against the other under axial force; friction heats the interface to a plastic state, rotation stops and an upset (forging) force completes the joint, squeezing out an oxide-carrying flash. Joins shafts, valves, axle housings and dissimilar metals (Al–steel, Cu–Al).
• Friction stir welding (FSW) — a non-consumable rotating tool with a shoulder and pin traverses the joint line, stirring plasticised metal; excellent for aluminium alloys that crack in fusion welding.
• Ultrasonic welding — high-frequency (about 20–40 kHz) tangential vibration under modest clamping force scrubs away oxides; thin foils, wire bonding, battery tabs, and also thermoplastics.
• Diffusion bonding — clean surfaces held under pressure at about 0.5–0.8 of the absolute melting temperature for minutes to hours; atoms diffuse across the interface. Titanium aerospace structures, dissimilar-metal joints.
• Explosive welding — a controlled detonation drives one plate onto another at high velocity, forming a wavy metallurgical bond; cladding of large plates.
• Roll bonding — cladding by rolling sheets together under heavy reduction.

Advantages: no fusion defects (porosity, solidification cracking), narrow or no HAZ, dissimilar metals possible, little distortion. Limitations: specialised equipment, joint geometry restrictions (friction welding needs one part to be rotationally symmetric), careful surface preparation.

Brazing. The filler's liquidus is above 450 °C (the American Welding Society boundary) but below the solidus of the base metals. Common fillers: copper and copper–zinc (brass), silver-based alloys (about 600–800 °C), aluminium–silicon (about 577–600 °C) for aluminium, nickel-based for high-temperature parts. Heating by torch, furnace, induction, resistance or dip. Gaps are small, typically about 0.025–0.25 mm, to give good capillary flow; take the recommended clearance for a given filler from the data book. Joint types are lap and scarf rather than butt, because the filler is weaker than the base metal; a lap length of about three times the thinner section thickness is a common rule of thumb.

Braze welding is different: a brass filler is deposited in a groove like a weld bead, without capillary action (used for cast-iron repair).

Soldering. Filler liquidus below 450 °C. Tin–lead eutectic (63Sn–37Pb) melts at 183 °C; lead-free tin–silver–copper (SAC) solders melt at about 217–220 °C. Joints are weak mechanically and are used for electrical continuity and sealing, not load bearing. Methods: soldering iron, wave soldering and reflow soldering (surface-mount electronics).

Flux. Removes oxide films, prevents re-oxidation during heating and promotes wetting (low contact angle). Residues of active fluxes are corrosive and must be cleaned. Vacuum or controlled-atmosphere furnace brazing can be fluxless.

Wetting and capillarity. Good wetting means a small contact angle between molten filler and base metal; it is what lets filler run into a thin gap against gravity. Clean surfaces, the right flux and the right temperature are all needed.

Formulas

T = (2/3) · μ · F · R

• Friction torque on a solid circular face under uniform pressure: T (N·m), μ = coefficient of friction (–), F = axial force (N), R = bar radius (m). For uniform wear use T = μ·F·R/2.

P = T · ω, ω = 2πN / 60

• Frictional heating power: P (W), ω (rad/s), N (rpm). Energy E = P · t if conditions are constant over time t (s).

P_joint = τ_f · A_lap

• Load capacity of a brazed or soldered lap joint: τ_f = shear strength of filler joint (Pa), A_lap = bonded area (m²). For a sleeve joint on a tube, A_lap = π · D · L (D = tube outside diameter, L = lap length, m).

L = σ_t · t / τ_f

• Lap length for a flat joint to be as strong as the sheet: σ_t = tensile strength of the sheet (Pa), t = sheet thickness (m). Width cancels.

Q = m · c · ΔT

• Sensible heat to bring a part to brazing temperature: m (kg), c = specific heat (J/(kg·K)), ΔT (K). Ignores losses and latent heat.

Worked examples

Example 1 (standard) — friction welding. Two steel bars of 20 mm diameter are friction welded at 1500 rpm under an axial force of 30 kN. Take an average friction coefficient of 0.3 and uniform pressure. Find the friction torque, heating power and the energy delivered in a 4 s heating phase.

1. R = 0.010 m. T = (2/3) μ F R = (2/3)(0.3)(30 000)(0.010) = 60 N·m.
2. ω = 2π × 1500 / 60 = 157.08 rad/s.
3. P = T ω = 60 × 157.08 = 9425 W.
4. E = P t = 9425 × 4 = 37 700 J.
5. Final: T = 60 N·m, P ≈ 9.42 kW, E ≈ 37.7 kJ. (In practice μ changes as the interface heats, so this is an estimate.)

Example 2 (GATE level) — brazed sleeve joint length. A copper tube of outside diameter 20 mm and inside diameter 17 mm has an ultimate tensile strength of 220 MPa. It is brazed into a socket; the brazed joint shear strength is 100 MPa. Find the minimum lap length so that the tube fails before the joint.

1. Tube tensile capacity: P_tube = σ_t · (π/4)(D² − d²).
2. A_tube = (π/4)(20² − 17²) = (π/4)(111) = 87.18 mm².
3. P_tube = 220 × 87.18 = 19 180 N.
4. Joint capacity: P_joint = τ_f · π · D · L = 100 × π × 20 × L = 6283 L (N, with L in mm).
5. Set P_joint ≥ P_tube: L ≥ 19 180 / 6283 = 3.05 mm.
6. Final: L ≥ about 3.05 mm. A designer would use a larger lap (about 3 times the wall or more, plus allowance for incomplete filling), but this is the strength-equality value.

Common mistakes

• Saying the base metal melts in brazing. Only the filler melts; if the base melts, it has become fusion welding.
• Using the 450 °C boundary for the process temperature rather than for the filler's liquidus.
• Designing brazed butt joints; filler strength is lower than base metal, so lap joints with enough overlap are used.
• Using too large a joint clearance; capillary flow fails and the joint is weak.
• Confusing brazing with braze welding (no capillary action in braze welding).
• Using uniform-wear torque μFR/2 when the question states uniform pressure, or vice versa.
• Forgetting that friction welding needs at least one part to be roughly round and rotatable.

For GATE ME

Mostly concept questions: classify processes by what melts, temperature limits for brazing and soldering, role of flux and capillary action, matching solid-state processes to applications (FSW for aluminium, explosive welding for cladding, ultrasonic for foils and wire bonding). Numerical questions appear as friction-welding torque and power, lap-joint length from strength equality, and heat requirements. Practise the torque formulas from clutch theory, which transfer directly.

Quick check

1. What is the temperature boundary between soldering and brazing, and what does it refer to?
2. Why is friction stir welding preferred for high-strength aluminium alloys?
3. Name the solid-state process used to clad large steel plates with titanium.
4. A flat lap joint of 1.5 mm sheet (σ_t = 300 MPa) uses filler of shear strength 150 MPa. Find the lap length for equal strength.
5. State two functions of flux.

Answers: 1. 450 °C, referring to the filler's liquidus temperature; 2. No melting, so no solidification cracking or porosity and better retained strength; 3. Explosive welding; 4. L = 300 × 1.5 / 150 = 3 mm; 5. Removes oxides, prevents re-oxidation and promotes wetting.