2026-08-27
Aluminum brazing filler metal is a key material in the production of automotive radiators, heat exchangers, battery coolers, liquid cooling plates, and other thermal management components. The two major systems are Al-Si binary filler metals and Al-Si-Mg self-fluxing filler metals.
Although both are aluminum-silicon based, they differ significantly in melting behavior, brazing process, and application. Selecting the right filler metal is essential for reliable joints, good wetting, and long-term leak resistance.
| Feature | Al-Si Filler Metal | Al-Si-Mg Filler Metal |
|---|---|---|
| Main Grades | 4343, 4045, 4047 | 4004, 4104 |
| Brazing Process | Vacuum / NOCOLOK | Vacuum only |
| Flux | Normally required | Self-fluxing under vacuum |
| Joint Cleanliness | Good | Excellent |
| Typical Applications | Radiators, heat exchangers | Precision liquid cooling |
| Cost | Lower | Higher |
Both systems are based on the Al-Si alloy system, using the Al-12.6Si eutectic point at 577°C to achieve relatively low-temperature aluminum brazing.
The filler metal should have a liquidus temperature at least 30°C lower than the solidus temperature of the base material. This helps reduce the risk of overheating, deformation, and grain deterioration.
Silicon also improves the wetting and spreading of molten aluminum, making Al-Si filler metals suitable for complex structures such as radiator cores, plate-fin heat exchangers, and liquid cooling passages.
Al-Si binary filler metals generally contain 7–12 wt.% Si.
| Grade | Si Content | Melting Range | Main Characteristic |
| 4343 | ≈7.5% | 577–615°C | Wider melting range |
| 4045 | ≈10% | 577–590°C | Balanced wetting |
| 4047 | ≈12% | 577–583°C | Very narrow melting range |
4343 is suitable for 1060, 1100 pure aluminum and 3003 aluminum, and is widely used in automotive heater cores, radiator cores, and standard heat exchangers.
4045 is compatible with 3003, 3004, 5005, 6063, and 6951 aluminum alloys. It is commonly used for battery coolers, stamped liquid cooling plates, and energy-storage cooling components.
4047, with its narrow melting range, is suitable for precision vacuum-brazed products such as liquid cooling plates and IGBT cooling chambers where sealing performance is critical.
Al-Si filler metals offer good corrosion resistance, lower cost, easy material availability, and broad compatibility. However, they do not provide a self-fluxing effect.
For NOCOLOK atmosphere brazing, potassium fluoroaluminate flux is normally required. Vacuum brazing also requires appropriate surface preparation and process control.
Adding 0.8–2.5 wt.% Mg to the Al-Si system creates a self-fluxing filler metal for vacuum brazing.
| Grade | Si | Mg | Melting Range |
| 4004 | ≈10% | ≈1.5% | 575–585°C |
| 4104 | ≈9–10% | ≈1.0% | 576–592°C |
Under high-vacuum conditions, magnesium helps remove the aluminum oxide layer, allowing brazing without externally sprayed flux.
This provides several advantages:
Typical applications include AI server liquid cooling plates, energy-storage cooling modules, high-power electronics cooling systems, and high-end EV cooling plates.
Al-Si-Mg filler metals are designed for vacuum brazing and should not be used in NOCOLOK atmosphere furnaces.
In an atmosphere containing trace oxygen, magnesium can oxidize into magnesium oxide, losing its oxide-removal effect and potentially causing poor wetting and unbrazed areas.
The choice should be based on both the product structure and brazing furnace:
For conventional automotive radiators and heat exchangers, Al-Si binary filler metals remain a practical choice. For high-cleanliness liquid cooling products, Al-Si-Mg filler metals are better suited to vacuum brazing.
Reliable aluminum brazing depends not only on filler metal selection but also on furnace atmosphere, temperature control, surface preparation, heating conditions, and cooling.
SUNHOPE supplies aluminum brazing furnaces, radiator manufacturing equipment, leak testing equipment, and related production solutions for different aluminum thermal management applications.
Choosing the correct combination of filler metal + brazing process + equipment helps manufacturers achieve stable production, reliable sealing, and consistent product quality.
Looking for an aluminum brazing furnace or complete radiator and heat exchanger production solution? Contact SUNHOPE for equipment and technical support.
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