NOCOLOK Brazing for Steel-Aluminum Liquid Cooling Plates: Process and Quality Control

Steel-aluminum composite liquid cooling plates combine the structural strength of steel with the thermal conductivity of aluminum. However, joining these dissimilar metals requires precise control of temperature, surface preparation, and interfacial reactions to ensure reliable bonding and prevent leakage.

NOCOLOK brazing offers a potential solution for continuous production when the material combination and brazing process are properly validated.

NOCOLOK brazing uses a potassium fluoroaluminate flux system (K₃AlF₆–KAlF₄), with a melting range of 565–572°C. Under a controlled nitrogen atmosphere, the molten flux removes the aluminum oxide film, allowing the Al-Si filler metal to wet and join prepared surfaces.

Common materials for the described composite structure include:

• 4343 filler alloy: Approximately 7.5% Si, melting point 577°C.

• 4045 filler alloy: Approximately 10% Si, melting point 565°C.

• 3003 aluminum alloy: Used as the aluminum core material.

• Steel substrate: SPCC carbon steel or 304 stainless steel, depending on application requirements.

Compared with vacuum brazing, NOCOLOK brazing can offer higher throughput and lower processing costs in suitable high-volume applications. However, reliable steel-aluminum bonding depends on the steel surface condition and the compatibility of the complete material system.

Steel and aluminum have different thermal expansion coefficients: approximately 23.5 × 10⁻⁶/°C for aluminum and 11.5 × 10⁻⁶/°C for steel. This mismatch can cause residual stress, plate distortion, and joint cracking during heating and cooling.

Temperature control is equally important. Excessive heat exposure promotes brittle Fe-Al intermetallic compounds, including FeAl₃ and Fe₂Al₅, which can weaken joints and increase leakage risks.

For the described composite structure, the recommended process parameters are:

The target intermetallic layer thickness is below 5 μm. For thin-wall microchannel plates, a holding time of no more than 4 minutes may be considered, subject to process validation.

These values apply to the specified process concept and should not be treated as universal settings for every steel-aluminum composite.

Surface cleanliness directly affects brazing quality. Oil, forming lubricants, dust, and oxides can interfere with wetting and cause incomplete joints.

A typical preparation sequence includes alkaline degreasing, pure-water rinsing, dilute nitric acid neutralization, and final rinsing. The surfaces should pass the applicable water-break test.

The recommended NOCOLOK flux suspension contains 5–10% flux powder in pure water, with an application quantity of 4–6 g/m². Localized application at 15–30% may be considered for selected steel-aluminum joints after compatibility testing.

Drying at 80°C followed by 180°C helps remove moisture before brazing.

After brazing, internal channels require thorough circulating pure-water rinsing and compressed-air drying. Excess flux residue, metal particles, or filler metal can obstruct microchannels, increase flow resistance, and compromise long-term reliability.

Several process issues can affect liquid cooling plate performance:

• Brittle intermetallic layers: Excessive temperature or holding time accelerates Fe-Al diffusion. Control the thermal cycle and verify the interface through metallographic inspection.

• Incomplete wetting: Poor cleaning, surface oxidation, or inadequate flux application can prevent reliable bonding. Improve surface preparation and atmosphere control.

• Blocked channels: Excess filler metal, flux residue, or uneven fixture pressure can restrict coolant flow. Optimize joint clearance and clean internal passages thoroughly.

• Plate distortion: Thermal expansion differences can cause warping and channel deformation. Use uniform fixtures, controlled heating, and appropriate cooling.

• Leakage and porosity: Residual moisture and oxidation can contribute to joint defects. Ensure complete drying and validate furnace atmosphere conditions.

Effective process control must balance joint strength, dimensional stability, and internal channel cleanliness.

Liquid cooling plates require inspection of both joint integrity and coolant flow performance.

Key quality checks include:

• Leak testing: A target leak rate of ≤1 × 10⁻⁶ Pa·m³/s where specified.

• Metallographic inspection: Intermetallic layer thickness below 5 μm.

• Flow resistance testing: Confirm that internal passages meet design requirements.

• Thermal cycling: Testing from -40°C to 120°C for 500 cycles, with no leakage or interface separation.

These are specified targets for the process described here. Actual acceptance criteria should be established according to product requirements and validated test methods.

A suitable continuous aluminum brazing furnace should provide stable temperature control, a consistent nitrogen atmosphere, uniform heating, and controlled cooling.

NOCOLOK brazing can support efficient, repeatable production, but it also requires strict flux management and post-braze cleaning. Aluminum alloys with magnesium contents above 0.1% should be avoided under the specified process conditions because standard NOCOLOK flux does not effectively remove magnesium oxide.

SUNHOPE supplies aluminum brazing furnaces and heat exchanger manufacturing equipment for industrial production. When selecting equipment, manufacturers should consider component dimensions, production capacity, material compatibility, atmosphere requirements, and post-braze cleaning needs.

NOCOLOK brazing can be a practical option for steel-aluminum composite liquid cooling plates when material compatibility and process conditions are carefully controlled.

Maintaining the correct brazing temperature, limiting intermetallic compound growth, ensuring clean internal channels, and verifying leak tightness are essential to reliable performance. A properly selected aluminum brazing furnace and a validated production process help manufacturers achieve consistent brazing quality.