Induction Brazing & Soldering
Localized, flameless joining using filler metal heated above its liquidus by electromagnetic induction. Controlled, repeatable, and ideal for automation.
How It Works
An alternating magnetic field induces eddy currents in the base metal (the workpiece, not the filler). The base metal heats by resistance, and the filler metal melts by thermal conduction from the heated joint area. The filler flows into the joint gap by capillary action.
Brazing uses filler metals with liquidus above 450°C — silver-based alloys (BAg), copper-phosphorus (BCuP), and nickel-based fillers for high-temperature service. Soldering uses fillers below 450°C, typically tin-silver or tin-lead compositions.
Coupling distance and coil geometry must deliver heat uniformly around the joint so filler flows evenly. Joint gap is typically 0.025–0.15 mm for optimal capillary action. Flux or controlled atmosphere prevents oxidation during the short heating cycle.
Induction is particularly suited to brazing because the heat is fast and localized — minimizing the oxidation window, reducing distortion in adjacent areas, and enabling precise cycle time control for automated production.
Typical Parameters
| Joint Type | Frequency | Power Density | Temp Range | Cycle Time |
|---|---|---|---|---|
| Steel-to-steel (silver braze) | 10 – 100 kHz | 0.5 – 2 kW/cm² | 620 – 720°C | 5 – 30 s |
| Carbide-to-steel (tool tips) | 30 – 100 kHz | 1 – 3 kW/cm² | 680 – 750°C | 5 – 20 s |
| Copper-to-copper (CuP braze) | 100 – 400 kHz | 0.3 – 1 kW/cm² | 650 – 820°C | 3 – 15 s |
| HVAC copper assemblies | 100 – 400 kHz | 0.5 – 1.5 kW/cm² | 700 – 800°C | 5 – 20 s |
| Aluminium (flux-core, Al-Si) | 50 – 200 kHz | 0.5 – 2 kW/cm² | 570 – 610°C | 5 – 30 s |
| Electronics soldering (Sn-Ag) | 200 kHz – 2 MHz | 0.1 – 0.5 kW/cm² | 220 – 280°C | 1 – 5 s |
Key Considerations
- Joint fit-up is critical — gap too wide and capillary flow fails; gap too tight and filler cannot penetrate. Optimal gap is 0.025–0.15 mm for most silver and copper-phosphorus fillers.
- Filler alloy selection must account for service temperature, corrosion environment, and base metal compatibility. BAg alloys offer the widest range; BCuP is self-fluxing on copper but not on steel or nickel.
- Overheating causes base metal grain growth and filler erosion. Pyrometer or thermocouple feedback is essential for consistent quality, especially in automated production.
- Flux residue must be removed after brazing to prevent long-term corrosion. Consider self-fluxing alloys (CuP on copper) or atmosphere brazing (N₂/H₂) to eliminate post-braze cleaning.
- Aluminium brazing has a narrow window — the liquidus-solidus range is often less than 30°C, demanding tight thermal control and fast response power supplies.
- Dissimilar metal joints require matching thermal expansion coefficients to avoid cracking on cool-down. Pre-heat and controlled cooling rates reduce residual stress.
Common Coil Geometries
Solenoid (Helical)
Most common for tube-to-tube and fitting joints. Wraps around the joint for uniform circumferential heating. Single or multi-turn depending on joint length.
Split-Return / Clamshell
Opens for part loading on automated lines. Two halves close around the joint, enabling fast load/unload cycles without threading the part through.
Pancake (Flat Spiral)
For flat-surface brazing such as heat exchangers or plate joints. Heats one face of the assembly; useful when access is limited to one side.
Hairpin (U-Coil)
For selective heating of one side of a joint, such as carbide tip brazing on cutting tools. Concentrates flux in a narrow zone.