Plate-Fin Radiator Brazing Leakage: Analysis of Typical Cases and Countermeasures for Aluminum Brazing Defects

Leakage in aluminum plate-fin radiators after the brazing process—whether vacuum brazing or controlled atmosphere brazing—is a critical quality issue typically caused by process, structure, or cleanliness problems. The common causes of brazing leakage can be grouped into the following categories.

I. Root Cause Analysis of Common Brazing Leakage Defects

Improper Brazing Process Parameters

• Excessive temperature / prolonged time: Leads to filler metal runoff and base metal erosion (thinning or perforation of fin edges).
• Insufficient temperature / inadequate time: Incomplete melting of the filler metal and poor fluidity result in unfilled gaps or interrupted joint continuity.

Part Assembly and Joint Clearance Issues

• Excessive clearance (>0.06 mm): Capillary action fails, preventing the filler metal from filling the braze joint and forming voids.
• Insufficient clearance or direct contact: The filler metal cannot penetrate, causing localized lack of bonding (non-brazed areas).

Material and Surface Cleanliness Issues

• Surface oxide film: The aluminum oxide layer on aluminum alloy surfaces has a high melting point and is non-wettable. If not effectively removed before brazing or if the flux activity is insufficient, pseudo-brazing defects will occur.
• Oil / moisture: Vaporization at high temperatures generates gas porosity and may even cause localized oxidation.
• Base metal matching: If both fins and separator plates are made of low-melting-point alloys, collapse may occur at high temperature; if both are made of high-melting-point alloys, the filler metal becomes difficult to melt.

Fixture and Furnace Loading Issues

• Uneven pressure: Insufficient fixture pressure or fixture deformation leads to locally enlarged clearances or poor contact.
• Uneven heating: Large temperature variations within the brazing furnace create thermal non-uniformity, resulting in overburning in some areas and incomplete melting in others.

Structural Design Issues

• Insufficient stiffness: Large-size brazed cores warp under their own weight or thermal stress at high temperatures, pulling the braze joints apart and causing cracks.
• Mismatch between side bars and fins: Inconsistent thermal expansion between side bars and fins generates cracks during the cooling stage after brazing.

II. Typical Brazing Leakage Cases and Countermeasures

Case 1: Micro-porosity Leakage Caused by Fin Edge Erosion — A Common Aluminum Brazing Defect

• Phenomena: X-ray inspection reveals black voids at the fin-to-separator interface; metallographic examination shows rounded and thinned fin edges.

• Cause: Brazing peak temperature too high (exceeding 610°C) or holding time too long, causing the low-melting-point eutectic phases to melt and flow away.

• Countermeasures: Lower the peak temperature (recommended 600±3°C) and shorten the holding time; verify the accuracy of thermocouples for proper brazing thermal profile control.

Case 2: Linear Leakage at Side Bar Corners

• Phenomena: Leak testing reveals a linear gas leak at the joint between the side bar and separator plate, with a smooth fracture surface.

• Cause: Excessive assembly clearance (>0.06 mm) combined with insufficient filler metal; or low fixture pressure causing side bar displacement at high temperature.

• Countermeasures: Control the assembly clearance within 0.02–0.06 mm; add localized filler metal (e.g., pre-placed brazing foil); optimize the brazing fixture design for uniform pressure.

Case 3: Batch Porosity Leakage Due to Poor Cleaning and Vacuum Brazing Atmosphere Control

• Phenomena: Random leakage points; circular gas pores visible on the fracture surface under stereomicroscope.

• Cause: Incomplete cleaning of parts (residual stamping oil, fingerprints), moisture absorption, or low vacuum level in the vacuum brazing furnace.

• Countermeasures: Strictly implement ultrasonic cleaning plus thorough drying before brazing; control the vacuum atmosphere quality and leak-tightness.

Case 4: Warpage Leakage in Large-Size Cores — Thermal Stress and Design Optimization

• Phenomena: Leakage at the four corners or in the middle of the long edges of the core; overall bow-shaped deformation visible.

• Cause: Significant difference in the thermal expansion coefficients of separator plates and side bars; cooling shrinkage stresses pull the brazed edges apart.

• Countermeasures: Add edge reinforcement strips; apply segmented fixtures for constraint during heating and cooling; optimize the cooling rate (slow cooling recommended above 400°C to minimize residual stress).

III. Leak Detection and Metallographic Analysis Methods for Brazed Components

1. Non-destructive location: Helium mass spectrometer leak testing (helium spray method) can achieve millimeter-level accuracy; the compressed air and soap bubble method can be used for preliminary leak screening.
2. Destructive examination: Section the leakage area and observe the braze joint fracture morphology under a stereomicroscope (lack of fusion, gas pores, erosion, cracks).
3. Metallographic analysis: Measure the braze joint fill rate (acceptable value >90%) and the intermetallic compound layer thickness (should be <5 μm) for quality assessment.
4. Process traceability: Review the complete brazing thermal profile (heating, holding, and cooling stages), vacuum level records, and fixture pressure records to identify root causes.

IV. Key Preventive Measures for Reliable Aluminum Brazing Quality

• Brazing process window control: For 3003/4104 aluminum alloys, the peak brazing temperature shall be 598–605°C, with a holding time of 3–8 minutes. Strict process control prevents both overburning and incomplete fusion defects.

• Cleanliness management: All parts must be thoroughly degreased (alkaline cleaning or ultrasonic cleaning) and dried before brazing, and assembled as soon as possible to avoid recontamination.

• Assembly and fixturing: Ensure a uniform joint clearance of 0.02–0.06 mm. The brazing fixture must have sufficient rigidity and deliver uniformly distributed pressure.

• Process monitoring and quality control: Regularly verify furnace temperature uniformity (within ±3°C); use test coupons to check filler metal wettability and braze joint integrity as part of ongoing quality assurance.