Solid-state welding, brazing, soldering and adhesive bonding

Why it matters

Not every joint can tolerate a fusion weld. Bimetallic engine valves, axle shafts and drive shafts are friction welded; aluminium battery trays and body structures are friction-stir welded or adhesively bonded; radiators, heat exchangers and AC lines are brazed; and every electronic control unit depends on soldered joints. These processes join dissimilar metals, heat-sensitive parts and thin sheets with little or no melting of the base metal, so the joint keeps most of its original properties.

Key ideas

Solid-state welding joins metals by bringing clean surfaces into atomic contact with pressure, often with heat, but below the melting point – no fusion zone, no filler, no shielding gas, and little or no cast structure. Surface oxides and contaminants must be broken up or expelled.

• Forge welding – heating and hammering (the oldest method).
• Cold (pressure) welding – large deformation at room temperature for ductile metals (Al, Cu).
• Roll bonding – cladding sheet by rolling two layers together (clad coins, bimetal strip).
• Friction welding (rotary or inertia) – one part rotates against the other under axial force; friction heats the interface to a plastic state, rotation stops and a forging (upset) force completes the joint, extruding a flash that carries away oxides. Fast, repeatable and good for dissimilar metals: bimetallic engine valves, axle shafts, turbocharger shaft-to-wheel joints. Mainly for round sections.
• Friction stir welding (FSW) – a non-consumable rotating tool with a shoulder and pin traverses the joint line, stirring plasticised metal from both sides. Excellent for aluminium alloys (which are hard to fusion weld): no porosity or hot cracks, low distortion. Exit hole at the end of the weld.
• Ultrasonic welding (metals) – high-frequency tangential vibration with moderate clamping force breaks oxide films; thin foils, battery tabs, wire harness splices. (Ultrasonic welding of plastics melts the interface and is a fusion process.)
• Explosive welding – a controlled detonation drives one plate onto another at high velocity; a characteristic wavy interface; cladding of large plates with dissimilar metals.
• Diffusion bonding – surfaces held together at high temperature (often about 0.5–0.7 of the absolute melting temperature) under modest pressure for a long time; atoms diffuse across the interface; aerospace titanium parts.

Brazing and soldering. The base metals are not melted; a filler metal with a lower melting point melts, wets the base metal and is drawn into a close-fitting gap by capillary action.

• Brazing uses fillers that melt above 450 °C (Cu, Cu–Zn brass, Ag-based, Al–Si). Joint clearances are small (typically a few hundredths to about a tenth of a millimetre; take from the filler maker's data). Fluxes such as borax dissolve oxides and promote wetting. Methods: torch, furnace (controlled atmosphere, many joints at once), induction, dip, vacuum brazing (aluminium radiators). Braze welding deposits filler in a groove like a weld, without relying on capillary action.
• Soldering uses fillers that melt below 450 °C: traditional Sn–Pb (the 63Sn–37Pb eutectic melts at 183 °C) and lead-free Sn–Ag–Cu alloys (about 217–220 °C). Rosin and acid fluxes; methods include iron, wave, reflow and dip soldering. Joints are weak mechanically and are used for electrical continuity and sealing.
• Joint strength depends on clearance (too wide or too narrow weakens it), wetting, and joint design – lap joints with enough overlap are preferred to butt joints because the thin filler layer is strong only over a large area.

Adhesive bonding. Polymer adhesives (epoxies, polyurethanes, acrylics, cyanoacrylates, anaerobics) bond by adhesion to prepared surfaces and cohesion within the adhesive.

• Advantages: joins dissimilar and thin materials (metal–composite, metal–glass), spreads load over a large area (good fatigue strength), seals against moisture, dampens vibration, no heat distortion and no holes.
• Limitations: surface preparation is critical; curing time; limited service temperature; creep and environmental degradation; difficult to inspect and disassemble.
• Design rule: load the adhesive in shear or compression; avoid peel and cleavage. In single-lap joints the stress is concentrated at the ends of the overlap, so very long overlaps add little strength.
• Car bodies use structural adhesives together with spot welds (weld-bonding) to raise stiffness and crash performance.

Formulas

τ = F / (b·L) Average shear stress in a lap joint (Pa); F = load (N), b = joint width (m), L = overlap length (m). The true peak stress at the overlap ends is higher.

T = (2/3)·μ·p·π·R³ Friction torque for a solid round bar under uniform interface pressure (N·m); μ = friction coefficient at the interface (effective value from data), p = axial pressure (Pa), R = bar radius (m).

P = T·ω E = P·t Frictional power (W) and heat generated (J); ω = angular speed (rad/s), t = heating time (s).

E = ½·I·ω² Energy stored in the flywheel for inertia friction welding (J); I = moment of inertia (kg·m²).

h = 2·γ·cos θ / (ρ·g·c) Capillary rise of filler between two parallel plates with gap c (m); γ = surface tension (N/m), θ = contact (wetting) angle, ρ = filler density (kg/m³). Good wetting (small θ) and small gaps give strong capillary flow.

Worked examples

Example 1 (standard) – adhesive lap joint. A lap joint 20 mm wide must carry 15 kN. The adhesive's shear strength is 20 MPa and a factor of safety of 2 is required.

1. Allowable shear stress = 20/2 = 10 MPa.
2. τ = F/(b·L) ⇒ L = F/(b·τ) = 15 000/(20 × 10) = 75 mm of overlap.
3. Because stress peaks at the overlap ends, increasing the width is more effective than increasing the overlap further.

Example 2 (GATE level) – rotary friction welding. Two steel bars of 30 mm diameter are friction welded at 1500 rpm with an axial pressure of 60 MPa and an effective μ = 0.4. Heating lasts 5 s. Assume uniform pressure.

1. R = 0.015 m. T = (2/3)·μ·p·π·R³ = (2/3) × 0.4 × 60 × 10⁶ × π × (0.015)³ = 169.6 N·m.
2. ω = 2π × 1500/60 = 157.08 rad/s.
3. P = T·ω = 169.6 × 157.08 = 26.6 kW.
4. Heat generated E = 26.6 × 5 = 133 kJ during the heating phase.

Example 3 – inertia welding flywheel. A flywheel of I = 5 kg·m² spinning at 2000 rpm: ω = 209.4 rad/s, E = ½ × 5 × 209.4² = 109.7 kJ – all of it is converted to heat at the interface as the flywheel stops.

Common mistakes

• Saying brazing melts the base metal. Neither brazing nor soldering does; only the filler melts.
• Using 450 °C as the dividing line between soldering and welding – it divides soldering and brazing.
• Classifying ultrasonic welding of plastics as solid-state (it melts the interface); ultrasonic welding of metals is solid-state.
• Designing adhesive joints in peel or cleavage, or assuming strength grows in proportion to overlap length.
• Using diameter instead of radius in the friction-torque formula (R³ makes this an 8× error).
• Forgetting that the brazing gap must be small for capillary action; a wide gap needs braze welding instead.

For GATE ME

Expect conceptual and matching questions: which processes are solid-state, typical products of friction, friction-stir, explosive and diffusion welding, brazing vs soldering vs braze welding, the 450 °C criterion, flux functions, and adhesive joint design. Numericals include friction-welding torque, power and energy, flywheel energy for inertia welding, and lap-joint overlap. Practise the uniform-pressure torque formula.

Quick check

1. What temperature separates soldering from brazing?
2. Which solid-state process uses a rotating pin-and-shoulder tool?
3. Name one joint design an adhesive should avoid.
4. Does brazing melt the base metal?
5. Overlap needed for a 25 mm wide joint carrying 5 kN at an allowable 8 MPa?

Answers: 1. 450 °C; 2. Friction stir welding; 3. Peel (or cleavage); 4. No; 5. 25 mm.