Meta Description: Learn why aluminum plate-fin radiator cores sag after vacuum brazing and how fin material, core design, tooling, and brazing parameters affect deformation.
Primary Keywords: plate-fin radiator core sagging, vacuum brazing deformation, aluminum fin sagging, 3003 aluminum fin, radiator core deformation
Introduction
Aluminum plate-fin radiator cores are widely used in new energy thermal management, air separation, mechanical engineering, and energy storage liquid cooling. During vacuum brazing, however, large or thin cores may develop fin collapse and center sagging, affecting heat-transfer performance, joint reliability, and dimensional accuracy.
The main cause is the sharp reduction in aluminum strength at high temperature. During brazing at 590–605°C, self-weight, thermal stress, tooling pressure, and cooling stress can act together and deform the core.
This article explains the main causes of plate-fin radiator core sagging, practical inspection methods, and ways to reduce deformation during production.
Why Does an Aluminum Plate-Fin Core Sag?
A typical plate-fin core consists of parting sheets, corrugated fins, and sealing bars assembled and joined by vacuum brazing.
At elevated temperature, the strength of aluminum decreases rapidly. For commonly used 3003 aluminum fins, material recovery begins to reduce work hardening after approximately 200°C. At around 600°C, the yield strength can fall to less than one-tenth of its room-temperature value.
The deformation generally develops through three stages:
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Room temperature–200°C: Fin stiffness remains relatively stable.
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200°C–recrystallization temperature: Work hardening decreases and sagging increases rapidly.
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Above recrystallization temperature: Grain growth and plastic flow allow deformation to continue.
As a result, the original U-shaped corrugated fin may become compressed toward a V-shaped profile, while the center of the core moves downward.
Four Main Loads Behind Core Sagging
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Self-weight: Large unsupported cores develop a downward bending moment.
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Thermal stress: Temperature differences between the core surface and center can reach 50–100°C.
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Tooling pressure: Uneven or excessive clamping can compress thin fins.
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Cooling stress: Different contraction rates during cooling create residual stress.
What Problems Can Core Sagging Cause?
1. Reduced Heat-Transfer Performance
Collapsed fins have lower effective height and surface area, reducing the designed heat-transfer capacity.
2. Brazed Joint Leakage
Deformation can generate internal stress and damage fin-to-sheet brazed joints, potentially causing failed helium leak testing.
3. Dimensional Failure
Excessive deformation may cause core flatness and diagonal dimensions to exceed drawing tolerances, preventing final assembly.
4. Lower Long-Term Reliability
Residual stress may continue to affect the core during repeated heating and cooling cycles, increasing the risk of deformation or leakage.
Key Factors Affecting Sagging Resistance
1. Fin Material, Thickness and Forming Condition
The fin material is one of the most important factors in aluminum radiator core deformation.
For 3003 aluminum fins, higher cold-rolling reduction generally provides stronger work hardening. Under the supplied test conditions, 50% rolling reduction reduced sagging by more than 30% compared with 20% reduction.
Fin geometry also matters:
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Thinner fins deform more easily.
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Higher fin height increases the height-to-thickness ratio.
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Under the same conditions, 0.12 mm fins show significantly greater sagging than 0.15 mm fins.
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For large cores, increasing fin thickness from 0.08 mm to 0.10–0.12 mm can improve structural resistance.
Accurate fin forming is therefore important not only for heat-transfer performance but also for maintaining stable core geometry during brazing.
2. Core Structure
A high length-to-width ratio creates a larger unsupported area and increases the bending moment.
For cores with an effective length above 1000 mm, an intermediate support bar can be added approximately every 500 mm along the width direction.
The sealing-bar height should also closely match the fin height. If it is too low, clamping pressure can concentrate on the fins. If it is too high, proper contact between the fin and parting sheet may be affected.
3. Vacuum Brazing Parameters
Brazing temperature and holding time have a direct effect on fin collapse during vacuum brazing.
At around 600°C, longer holding increases the time available for high-temperature plastic deformation. Above 605°C, fin softening can become significantly more severe.
Under the supplied process recommendation, the 600°C holding time can be reduced from 10 minutes to 4–6 minutes, provided brazed-joint formation remains satisfactory.
Temperature uniformity is equally important. A surface-to-center temperature difference of 50–100°C can increase thermal stress and deformation.
4. Tooling and Furnace Loading
Common tooling-related causes include:
For large cores, removable high-temperature support pads can be installed under the center area to counteract the bending effect of self-weight.
Single-layer independent furnace loading is preferred. Vertical placement can also help eliminate stacking loads.
How to Test Plate-Fin Core Sagging
A practical quality-control system should check both the fin material before production and the finished brazed core.
Fin Material Sagging Test
Recommended sample:
Simulated brazing cycle:
Room temperature → 577°C in 10 min → 600°C in 6 min → hold for 5 min → air cool
Calculate:
Sagging value H = H₁ − H₂
A smaller H indicates better resistance.
For new-energy liquid-cooling radiator fins, the supplied criteria are:
Finished Core Inspection
After brazing, place the core on a granite inspection platform and measure 9 points: four corners, four edge midpoints, and the center.
Recommended tools include a dial indicator, level, steel ruler, and granite inspection plate.
The supplied dimensional limits are:
| Application |
Maximum Sagging |
| Passenger vehicle small radiator |
≤1.5 mm |
| Construction machinery / energy storage large core |
≤3 mm |
| More than 3 mm over tolerance |
Reject |
For complex or large cores, finite element analysis can also be used during development to predict the temperature, stress, and deformation fields before production.
Practical Ways to Reduce Core Sagging
Optimize Fin Material
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Consider Mn/Zn-modified 3003 aluminum alloys where higher high-temperature strength is required.
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Control final rolling reduction around 45–50% where appropriate.
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Use low-temperature stress-relief annealing instead of complete high-temperature recrystallization annealing.
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For large cores, consider 0.10–0.12 mm fins instead of 0.08 mm.
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Reduce excessive fin height when structural rigidity is a priority.
Improve Core Design
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Add intermediate support bars when the effective core length exceeds 1000 mm.
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Control fin and sealing-bar height tolerance to approximately ±0.03 mm.
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For heavy-duty applications, consider straight or reinforced corrugated fins where greater rigidity is required.
Optimize the Brazing Process
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Keep peak brazing temperature at or below 600°C.
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Reduce the 600°C holding period to 4–6 minutes, provided joint quality remains satisfactory.
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Use controlled heating stages around 200°C and 550°C to improve temperature uniformity.
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In the supplied process recommendation, cooling above 400°C should be limited to ≤3°C/min to reduce residual stress.
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Below 400°C, cooling can be accelerated.
Improve Tooling and Assembly
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Add removable high-temperature center support pads for large cores.
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Standardize clamping torque with a torque wrench.
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Maintain tooling flatness at ≤0.03 mm/m.
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Avoid multi-layer stacking during furnace loading.
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Perform 100% height inspection of fins, sealing bars, and parting sheets before assembly.
What Should Manufacturers Prioritize?
Based on the engineering assessment presented in this study, the relative contribution of the major factors is:
Fin material: 40% > Tooling support: 28% > Brazing process: 22% > Core structure: 10%
This indicates that manufacturers should first focus on fin material performance and center-support tooling, followed by brazing-process control and structural optimization.
Sagging should therefore be controlled throughout the complete production flow:
Fin forming → Core assembly → Tooling → Vacuum brazing → Cooling → Flatness inspection
SUNHOPE: Equipment for Stable Radiator Core Production
Controlling plate-fin heat exchanger sagging requires more than furnace temperature control. Consistent fin dimensions, accurate core assembly, proper tooling, and stable brazing conditions all contribute to final core quality.
SUNHOPE supplies equipment and components for radiator and condenser manufacturing, including:
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Radiator & condenser core assembly machines
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Fin forming machines
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Aluminum brazing furnaces
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Leak testing equipment
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Radiator and condenser components
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Production line and factory setup support
Our technical support covers equipment selection, installation guidance, operator training, and production-line solutions.
Planning a radiator or heat exchanger production line? Send us your core size, fin specifications, production capacity, and brazing requirements for a suitable equipment solution.
Conclusion
Plate-fin radiator core sagging after vacuum brazing is mainly caused by high-temperature aluminum softening combined with self-weight, thermal stress, tooling pressure, and cooling-induced residual stress.
The most effective approach is to control the complete process:
Select suitable fin material → optimize core structure → control brazing temperature and time → improve center support → standardize inspection.
For large aluminum plate-fin cores, early material testing and tooling validation can reduce deformation-related scrap while protecting heat-transfer performance and brazed-joint reliability.
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