Process Analysis of NOCOLOK Brazing for Steel-Aluminum Composite Liquid Cold Plates

I. Process Fundamentals and Application Positioning

1. Structure of Steel-Aluminum Composite Liquid Cold Plates

Mainstream configuration: Carbon steel/stainless steel substrate clad with aluminum-silicon composite layer (4343/4045 filler metal layer + 3003 aluminum core). Liquid cooling channels are stamped or milled on the aluminum side; the steel layer provides structural rigidity, deformation resistance and pressure-bearing capacity, while the aluminum side delivers superior thermal conductivity and brazing compatibility.

2. Core Mechanism of NOCOLOK Brazing

The brazing flux is eutectic potassium fluoroaluminate salt (K₃AlF₆–KAlF₄) with a melting point ranging from 565℃ to 572℃. Workpieces are heated in a continuous nitrogen atmosphere furnace. Molten flux dissolves the Al₂O₃ oxide film on aluminum surfaces, enabling Al-Si filler metal to wet the steel-aluminum interface and form a metallurgical bond. This process leaves non-corrosive residues, suits mass continuous production, and features lower costs and higher throughput compared with vacuum brazing.

3. Inherent Difficulties of Dissimilar Steel-Aluminum Brazing (Specific to Liquid Cold Plates)

1. Fe-Al brittle intermetallic compounds (IMCs): At high temperatures, aluminum diffuses into steel to form hard brittle layers of FeAl₃ and Fe₂Al₅. Once the layer thickness exceeds 8 μm, joint strength drops sharply, cracking easily occurs under thermal cycling, and the risk of liquid cooling leakage under pressure rises drastically.
2. Mismatched thermal expansion coefficients: Aluminum has a linear expansion coefficient α ≈ 23.5×10⁻⁶/℃, while steel α ≈ 11.5×10⁻⁶/℃. Significant interfacial shear stress generates during heating and cooling, leading to thin-walled channel deformation, braze seam delamination, and reduced channel cross-section that increases flow resistance.
3. Dual oxide film removal requirement: Dense alumina on aluminum and iron oxide on steel. Conventional flux shows poor wettability on steel surfaces, readily causing insufficient filler spreading, incomplete bonding and unwelded areas.
4. Cleanliness constraints for internal cooling cavities: Residual flux, braze beads and metal debris block microchannels, elevating flow resistance, triggering air blockages and localized overheating. Soluble fluoride residues must be eliminated to prevent medium-induced channel corrosion.

5. Narrow processing temperature window: Filler metal melts at 577℃, while 3003 aluminum base metal melts at 650℃. Uneven heating of steel-aluminum composite plates easily causes local overburning and base metal erosion.

II. Complete Process Flow (Specialized for Steel-Aluminum Composite Liquid Cold Plates)

Step 1: Raw Material Inspection and Cutting

1. Inspect thickness uniformity of steel-aluminum composite strips; control the 4343 filler metal layer at 80–120 μm. Aluminum alloys with magnesium content >0.1% are prohibited, as MgO cannot be dissolved by NOCOLOK flux.
2. Stainless steel surfaces must be rust-free and oil-free; carbon steel requires pre-phosphating or passivation to mitigate high-temperature oxidation.

Step 2: Channel Forming (Stamping/Milling)

• Strictly control dimensional tolerances of channel gaps and fin heights within ±0.05 mm, resulting in a braze gap of 0.1–0.2 mm after assembly. Gaps wider than 0.2 mm cause filler metal runoff and insufficient bonding rate; gaps narrower than 0.08 mm weaken capillary action and incomplete gap filling.
• Deburr all formed parts to eliminate sharp protrusions inside channels, which otherwise raise flow resistance and induce localized stress cracks.

Step 3: Degreasing and Cleaning (Critical for NOCOLOK Brazing Success)

1. Process sequence: Alkaline degreasing → Pure water rinsing → Dilute nitric acid neutralization → Final pure water rinsing. Pass water break test (complete continuous water wetting without film rupture).
2. Requirements: No stamping oil, drawing agents or dust inside cavities. Residual grease isolates flux, hindering oxide film removal and resulting in incomplete bonding and blowholes.
3. Prioritize rust removal on steel surfaces; trace iron oxide consumes flux and impairs interfacial wettability.

Step 4: NOCOLOK Flux Coating and Drying

1. Flux suspension formulation: Pure water mixed with 5%–10% flux powder. Apply high-concentration slurry (15%–30%) locally on steel-aluminum joints and inner channel walls to enhance steel surface wettability.
2. Coating weight control: 4–6 g/m². Insufficient flux fails to fully remove oxide films; excessive flux generates abundant post-brazing slag residue that clogs microchannels and impairs corrosion resistance.
3. Stepwise drying: Pre-dry at 80℃ → Complete dewatering at 180℃. Residual moisture vaporizes during heating, forming weld blowholes and pressure leakage in liquid cold plates.

Step 5: Tooling Assembly and Clamping

1. Uniform compression via graphite/stainless steel tooling at 0.08–0.12 MPa to counteract warpage induced by steel-aluminum thermal expansion mismatch and prevent channel delamination.
2. Reserve exhaust passages at channel inlet/outlet to avoid localized underheating and suppressed filler flow caused by trapped furnace gas.
3. Avoid direct contact between tooling and channel zones to prevent localized overheating and erosion of aluminum cladding.

Step 6: Nitrogen Continuous Furnace Brazing (Core Temperature Control Stage)

The furnace is divided into four zones: Preheating Zone → Heating Zone → Brazing Holding Zone → Cooling Zone. Maintain nitrogen atmosphere with oxygen content <10 ppm and dew point <−40℃ throughout the process.

Standard Recommended Thermal Cycle for Steel-Aluminum Composite Liquid Cold Plates

1. Preheating Zone (Room Temperature → 450℃, 60–90 min) Slow heating removes residual moisture and gradually activates flux to avoid workpiece deformation from thermal shock. Nitrogen purges internal cavities to carry away volatile oil contaminants.
2. Heating Zone (450℃ → 570℃, 30–45 min) Flux melts progressively to dissolve alumina and simultaneously remove steel surface oxides. Limit heating rate ≤4℃/min to avert stress cracking from excessive interfacial temperature difference between steel and aluminum.
3. Brazing Holding Zone (578–590℃, 3–6 min)

• Peak temperature strictly capped at 595℃: Every 10℃ temperature rise doubles the growth rate of Fe-Al intermetallic compounds. Holding durations exceeding 8 min lead to over-thick brittle layers.
• Short holding time is the core of dissimilar steel-aluminum brazing: It only enables full filler spreading and gap filling while restraining Fe-Al interdiffusion to control IMC thickness below 5 μm.

4. Rapid Cooling Zone (590℃ → 200℃, Forced Nitrogen Air Cooling)

   Fast cooling terminates elemental interdiffusion at the interface to halt continuous thickening of brittle layers. It also reduces residual stress originating from differential shrinkage of aluminum and steel, eliminating channel warpage and microcracks in braze seams.

Step 7: Post-Brazing Cleaning and Internal Cavity Purification

1. External surfaces: Remove flux slag via high-pressure hot air blast and mild acid washing.

2. Critical internal cavity treatment: Circulating high-pressure pure water flushing followed by compressed air blow-off to thoroughly eliminate detached flux slag and aluminum powder, preventing microchannel blockage and elevated flow resistance.

3. Dry and seal finished parts to avoid re-rusting of steel substrates.

Step 8: Specialized Performance Testing for Liquid Cold Plates

1. Gas tightness/Helium leak test: Leak rate ≤1×10⁻⁶ Pa·m³/s.
2. Metallographic sampling inspection: Steel-aluminum interfacial IMC thickness <5 μm, no continuous thick brittle layers.
3. Flow resistance measurement: Compare test data against design values to identify blockages caused by braze beads inside channels.
4. Thermal cycling test (−40℃ to 120℃, 500 cycles): No leakage or interfacial delamination permitted.

III. Matching Schemes for Key Materials

1. Flux: Standard non-corrosive NOCOLOK flux. Trace silicon-based activators may be added for steel-aluminum dissimilar joints to improve wettability on carbon steel and stainless steel surfaces.
2. Clad filler metal: 4343 (7.5% Si, melting point 577℃) is the primary choice. 4045 (10% Si, melting point 565℃) is adopted for high heat flux density liquid cold plates to ease restrictions on the brazing temperature window.
3. Steel Substrate

• Low-pressure lightweight applications: SPCC cold-rolled carbon steel (pre-passivation required to resist high-temperature oxidation).
• Corrosion-resistant and high-pressure applications: 304 stainless steel, which delivers superior NOCOLOK brazing wettability compared with carbon steel at higher material cost.

4. Aluminum Core Layer: 3003 Al-Mn alloy with good erosion resistance and medium corrosion resistance. 5xxx-series magnesium-containing aluminum alloys are strictly prohibited.

IV. Impacts of Core Process Parameters on Steel-Aluminum Interfaces and Liquid Cooling Performance

1. Brazing Temperature

• <575℃: Incomplete filler melting, poor steel surface wettability, incomplete bonding, and insufficient channel pressure resistance.
• 578–590℃: Optimal process window forming thin continuous metallurgical IMC layers.
• 595℃: Massive aluminum diffusion into steel generates thick brittle FeAl₃ layers, reducing joint shear strength by over 40%. Aluminum cladding erosion thins channel fins and weakens pressure resistance; excessive braze beads narrow channels and sharply increase flow resistance

2. Holding Time

Holding longer than 6 min accelerates IMC thickening. For thin-walled microchannel cold plates, limit maximum holding time to 4 min.

3. Nitrogen Atmosphere Purity

Oxygen content >15 ppm triggers persistent oxidation of aluminum and steel during heating, accelerating flux consumption and causing localized unwelded zones and weld blowholes. Oxidation slag trapped inside cavities blocks microchannels.

4. Tooling Clamping Pressure

• Insufficient pressure: Aluminum expands and separates from steel during heating, widening braze gaps, inducing filler runoff and interfacial voids that raise thermal resistance and degrade heat dissipation performance.
•  Excessive pressure: Channel cross-sections are extruded and constricted, leading to severely out-of-spec flow resistance or even crushed cooling fins.

5. Flux Coating Weight

• Excess flux: Post-brazing fluoride slag residues trapped inside cavities block microchannels and induce long-term aluminum wall corrosion.
• Insufficient flux: Incomplete removal of steel surface oxides causes localized non-bonded interfaces prone to leakage under thermal cycling.

V. Typical Defect Root Causes and Process Control Countermeasures (Exclusive to Liquid Cold Plates)

1. Thick Brittle IMC Layers at Steel-Aluminum Interfaces, Cracking and Leakage after Thermal Cycling

Root causes: Excessive peak temperature, prolonged holding time, excessively slow heating rate.

Countermeasures: Cap peak temperature below 590℃, shorten holding time to 3–5 min, apply forced rapid cooling after furnace exit, and adopt low-melting-point 4045 filler metal to lower peak brazing temperature.

2. Poor Steel Surface Wettability, Incomplete Bonding and Local Unwelded Zones

Root causes: Inadequate rust removal and degreasing on steel surfaces, insufficient flux concentration, high furnace dew point. Countermeasures: Pre-passivate steel parts, locally brush high-concentration flux on steel-aluminum joints, and stabilize furnace dew point below −40℃.

3. Internal Channel Braze Beads, Blockages and Excessive Flow Resistance

Root causes: Over-thick filler metal cladding, prolonged holding time, uneven tooling pressure, excessive flux coating. Countermeasures: Control cladding thickness, shorten holding duration, conduct high-pressure internal flushing post-brazing, and optimize tooling for uniform compression.

4. Liquid Cold Plate Warpage and Channel Cross-Section Constriction

Root causes: Steel-aluminum thermal expansion mismatch, slow cooling rate, unbalanced tooling pressure. Countermeasures: Stepwise heating and cooling, rapid nitrogen cooling after brazing, symmetric uniform-pressure tooling, and zoned temperature control to minimize temperature gradients.

5. Dense Weld Blowholes and Helium Leakage

Root causes: Residual moisture from incomplete cleaning, insufficient flux drying, high furnace oxygen content.

Countermeasures: Extend the 180℃ drying stage, cool workpieces fully before furnace loading, and maintain continuous high-purity nitrogen purging.

6. Long-Term Channel Corrosion and Pitting Leakage

Root causes: Unremoved NOCOLOK flux residues trapped inside cavities.

Countermeasures: Multi-pass circulating pure water flushing post-brazing, followed by compressed air purging of dead zones in channels before drying.

VI. Advantages and Disadvantages of NOCOLOK Brazing vs. Vacuum Brazing for Steel-Aluminum Liquid Cold Plates

Advantages

1. Mass production via continuous mesh belt furnaces delivers high throughput, with unit cost only half that of vacuum brazing.
2. Continuously adjustable thermal cycles minimize deformation of thin-walled steel-aluminum composite plates.
3. Flux actively removes dual oxide films on steel and aluminum, exhibiting superior wettability for dissimilar metals compared with flux-free vacuum brazing.
4. Suitable for mass manufacturing of integrated liquid cold plates with complex serpentine and microchannel structures

Disadvantages

1. Extra internal cavity flushing procedures increase cleanliness control costs.
2. Magnesium-series aluminum alloys are incompatible, limiting material selection range.
3. Strict control over flux residues is mandatory; microchannels are susceptible to blockage, raising the difficulty of flow resistance specification management.
4. Ultra-high vacuum hermetic liquid cold plates for aerospace applications require additional post-brazing passivation treatment.

VII. Process Optimization Design Concepts (Balancing Weld Strength and Low Flow Resistance)

1. Interfacial Metallurgy Optimization: Adopt low-temperature short-holding thermal cycles to limit Fe-Al IMC thickness within 5 μm, securing joint strength and thermal cycling resistance.
2. Channel Dimension Preservation: Apply uniform tooling pressure and control peak temperature strictly to avoid channel extrusion deformation and cross-section constriction from base metal erosion.
3. Closed-Loop Internal Cavity Cleanliness Control: Full-process management covering cleaning, drying, low-weight flux coating, and multi-stage post-brazing flushing to eliminate slag blockages.
4. Residual Stress Mitigation: Stepwise heating and cooling combined with rapid post-brazing cooling reduce residual stress induced by steel-aluminum thermal expansion mismatch, preventing braze seam microcracks and channel deformation.
5. Material Matching Optimization: Pair stainless steel substrates with 4045 filler metal to lower peak brazing temperature, suppress interfacial elemental interdiffusion and reduce channel wall erosion.

Translation Notes

1. Professional terms adopt internationally recognized welding/thermal management industry standards (NOCOLOK, IMC, flow resistance, helium leak test, etc.).
2. Metallurgical formulas, alloy grades, temperature/pressure parameters, and tolerance values retain original numerical specifications.
3. Long Chinese compound logic sentences are restructured into concise English technical paragraphs conforming to international process document writing conventions.
4. Specialized cold plate manufacturing terminology (microchannel, fin, braze bead, dew point, mesh belt furnace) follows automotive thermal management industry customary expressions.