Vacuum brazing is widely used to manufacture aluminum plate-fin radiators and heat exchanger cores. However, internal cavity bulging and fin tearing can affect core dimensions, leak tightness, and long-term reliability.
These defects are often connected. Trapped gas can increase internal pressure during heating, while thermal stress, filler-metal erosion, and fixture restraint can further weaken thin fins.
Quick Answer: What Causes Bulging and Fin Tearing?
The main causes include:
- Moisture, oil, and other volatile residues inside the core
- Insufficient venting in closed flow channels
- Excessive brazing temperature or holding time
- Rapid heating or cooling
- Excessive fixture pressure
- Filler-metal erosion or incomplete brazing
- Thin fins and high stress concentration
- Material or surface-treatment defects
The basic improvement strategy is: remove gas → control brazing temperature → reduce stress → verify the finished core.
1. Understanding the Two Defects
Internal Cavity Bulging
After brazing, the flow-channel area may expand outward, causing the separator plate to deform and the internal cavity volume to increase. Local dimensions may exceed requirements, and the deformed area may leak during helium leak or water pressure testing.
Fin Tearing
Cracks may appear at the fin root, corrugated bends, or brazed joints between fins and separator plates.
Two typical failure modes are:
- High-temperature tearing: cracks appear immediately after brazing.
- Post-brazing tearing: cracks develop during pressure testing, thermal cycling, inspection, or customer operation.
Internal bulging and fin tearing can occur together because cavity expansion concentrates stress on thin fins.
2. Main Failure Mechanism
Aluminum 3003, clad separator plates, and thin fins with a thickness of 0.08–0.2 mm have very low strength at high temperatures.
During vacuum brazing, the core is exposed to four major loads:
- Internal gas pressure
- Thermal stress
- Brazing filler-metal erosion
- Assembly and fixture restraint
When gas is trapped inside a closed flow channel, pressure increases as the core heats. The separator plate may first bulge, while thermal and residual stresses can then cause thin fins to tear.
3. Main Causes of Failure
Trapped Gas and Insufficient Venting
Moisture, oil, and rolling lubricant may remain between fins and separator plates if cleaning is incomplete. At 400–550°C, these materials can rapidly vaporize and increase pressure inside closed channels.
Fully sealed bars, blocked passages, tightly stacked fins, or missing venting grooves can create isolated cavities. Magnesium in the clad brazing layer can also volatilize at high temperature and add to internal pressure.
Furnace or fixture leakage may allow oxygen and moisture to enter, increasing oxide formation and gas generation.
Uncontrolled Brazing Temperature
Before 450°C, a heating rate above 8°C/min can cause rapid vaporization of moisture and oil. A temperature difference of 50–100°C between thick side plates and thin fins can also create significant thermal stress.
If the brazing temperature exceeds 610°C or holding time exceeds 25 min, liquid Al-Si filler metal can severely erode the aluminum base material. Fin roots become thinner and more vulnerable to tearing.
Rapid cooling can create additional residual tensile stress because thick side plates and thin fins contract at different rates.
Furnace temperature uniformity is equally important. Local overheating may cause excessive erosion, while insufficient heating can produce weak brazed joints.
Fixture and Assembly Stress
Excessive clamping torque can restrict thermal expansion. Stainless-steel fixtures also behave differently from aluminum during heating, increasing the risk of restraint stress.
Uneven clamping, missing graphite support blocks, or unsupported areas can cause high-temperature bending and localized stress.
Poor dimensional matching can create pre-compression in the fins. When the core expands during brazing, insufficient clearance may result in fin deformation or tearing.
Material and Structural Factors
Fin thickness below 0.08 mm provides limited high-temperature load capacity. Small corrugation radii can further increase stress concentration.
Closed channels without venting, side plates without stress-relief grooves, and unsuitable filler-layer thickness can also contribute to failure.
Incomplete surface treatment may leave oxide films and cause weak brazed joints. Microcracks from fin stamping or rolling defects can also grow rapidly under brazing stress.
4. Cause and Defect Quick Reference
| Possible Cause |
Typical Result |
| Moisture or oil residue |
Gas pressure and internal bulging |
| Poor venting |
Local cavity expansion |
| Temperature above 610°C |
Excessive filler-metal erosion |
| Holding time above 25 min |
Fin-root thinning |
| Rapid cooling |
Residual tensile stress |
| Excessive fixture pressure |
Fin deformation or tearing |
| Insufficient brazing |
Weak fin-to-plate joints |
| Material microcracks |
Crack propagation |
5. How to Identify the Root Cause
A systematic inspection helps separate different failure mechanisms:
Visual and dimensional inspection: Record the bulging position, tearing location, and bulge height.
Leak testing: Use helium leak or water pressure testing to determine whether the damaged area corresponds to leakage.
Metallographic analysis: Severe erosion may indicate excessive temperature; separation along a brazed joint may indicate incomplete brazing; through-thickness tearing may be related to internal pressure or thermal stress.
Process traceability: Review furnace temperature curves, vacuum level, heating rate, holding time, and cooling parameters.
Cleanliness and fixture tests: Compare residual oil or moisture levels and reproduce defects using different fixture clamping forces.
6. Process Improvements
Improve Cleaning and Venting
A controlled cleaning process can include:
Alkaline cleaning → acid cleaning → multiple pure-water rinses → hot-air drying
Dry components at 120°C for ≥15 min and load them into the furnace within 4 hours after drying.
Provide venting grooves at both ends of each flow channel and avoid completely sealed cavities. Before 450°C, maintain a vacuum level of ≤1×10⁻³ Pa and use controlled heating for sufficient degassing.
Proper pre-brazing cleaning is also important. Ultrasonic cleaning equipment can help control contaminants before the brazing process.
Optimize the Brazing Curve
A suitable process should control both temperature and holding time:
- Room temperature–500°C: 3–5°C/min
- Brazing stage: 590–605°C for 10–20 min
- Maximum brazing temperature: strictly below 610°C
- Below 550°C during cooling: ≤4°C/min
- Optional stress relief: 250°C for 2 h
A properly controlled aluminum vacuum brazing furnace helps maintain consistent heating, holding, vacuum, and cooling conditions.
Control Fixtures and Core Structure
Use standardized clamping torque according to core dimensions. Graphite fixtures can reduce thermal expansion differences compared with stainless steel.
Provide graphite support blocks across the upper, lower, and corner areas. For fin height, reserve a 0.02–0.03 mm positive tolerance to reduce assembly pre-compression.
For pressure-bearing applications, use fins with a thickness of ≥0.12 mm and larger transition radii. Large cores can also use intermediate support fins to improve rigidity.
Strengthen Final Quality Control
Before loading, check component drying and venting conditions. During brazing, monitor temperature differences between furnace zones.
After brazing, inspect cores for bulging and fin cracks. Leak testing equipment can then verify core tightness before the next production stage.
Manufacturers can also classify defects by gas generation, overheating, fixture stress, and incoming material to support continuous process improvement.
7. Conclusion
Internal bulging in aluminum plate-fin radiator cores is mainly associated with trapped gas and internal pressure, while fin tearing results from a combination of internal pressure, high-temperature filler-metal erosion, thermal stress, and fixture restraint.
The most effective approach is to:
Remove gas and improve venting → control brazing temperature and erosion → reduce thermal and assembly stress → verify the finished core.
For radiator and heat exchanger manufacturers, cleaning, core assembly, fixture design, vacuum brazing, and leak testing should be managed as one integrated quality process.
SUNHOPE supplies equipment for radiator and heat exchanger manufacturing, including aluminum brazing furnaces, fin forming machines, ultrasonic cleaning equipment, leak testing equipment, and related core manufacturing equipment, supporting both new production lines and existing factory upgrades.
FAQ
Why do aluminum plate-fin radiator cores bulge during vacuum brazing?
The main causes are trapped moisture or oil, insufficient venting, furnace leakage, excessive heating rates, and thermal stress.
What causes fin tearing after brazing?
Fin tearing can result from internal pressure, filler-metal erosion, thermal stress, fixture restraint, incomplete brazing, or material defects.
What brazing temperature is used for this process?
The referenced process controls the brazing stage at 590–605°C for 10–20 min, while temperatures above 610°C should be avoided.
How can fin tearing be reduced?
Improve cleaning and drying, provide effective venting, control the brazing temperature curve, reduce fixture stress, and optimize fin thickness and core structure.
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