Al-Si vs. Al-Si-Mg Brazing Filler Metals for Aluminum Radiators & Heat Exchangers

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.

Al-Si and Al-Si-Mg: Key Differences

1. Why Al-Si-Based Filler Metals Are Widely Used

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.

2. Al-Si Binary Filler Metals: 4343, 4045 and 4047

Al-Si binary filler metals generally contain 7–12 wt.% Si.

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.

Advantages and Limitations

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.

3. Al-Si-Mg Filler Metals: 4004 and 4104

Adding 0.8–2.5 wt.% Mg to the Al-Si system creates a self-fluxing filler metal for vacuum brazing.

Under high-vacuum conditions, magnesium helps remove the aluminum oxide layer, allowing brazing without externally sprayed flux.

This provides several advantages:

• No flux spraying and drying process.
• No flux residue inside finished components.
• Excellent cleanliness for microchannels.
• Suitable for precision liquid cooling plates.
• Reduced risk of leakage caused by brazing defects.

Typical applications include AI server liquid cooling plates, energy-storage cooling modules, high-power electronics cooling systems, and high-end EV cooling plates.

Why Al-Si-Mg Is Not Suitable for NOCOLOK Brazing

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.

4. How to Choose the Right Aluminum Brazing Filler Metal

The choice should be based on both the product structure and brazing furnace:

• NOCOLOK atmosphere brazing: 4343 / 4045
• Vacuum brazing, cost-sensitive products: 4045 / 4047 with suitable flux
• Precision liquid cooling plates or microchannels: 4004 / 4104
• Pure aluminum and 3003 radiator products: 4343
• 6063 and medium/high-strength aluminum cooling parts: 4045 / 4004

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.

5. Match the Filler Metal with the Brazing Equipment

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.