Introduction

For aluminum heat exchangers manufactured by vacuum brazing, summer high humidity is one of the most challenging production conditions. Increased moisture in the air can cause hidden problems such as incomplete brazing, poor filler metal spreading, internal pores, and micro leakage after pressure testing.

Unlike visible welding defects, vacuum brazing failures caused by moisture often occur inside the brazing joints, making them difficult to detect during production.

This article explains the mechanism behind summer humidity-related brazing defects and provides a complete prevention strategy covering workshop environment control, component preparation, vacuum brazing furnace maintenance, brazing temperature profiles, and assembly control.

Why High Humidity Causes Vacuum Brazing Defects in Aluminum Heat Exchangers

The surface of aluminum alloy naturally forms an Al₂O₃ oxide film. Under high humidity conditions, moisture is easily absorbed on aluminum surfaces, creating hydrated aluminum oxide layers.

During the vacuum brazing process:

1. When components are heated, hydrated oxides decompose and continuously release water vapor.
2. At high temperatures, water vapor reacts with aluminum and magnesium (Mg) elements in brazing filler materials, generating new dense oxide layers.
3. The oxide layer prevents Al-Si filler metal from properly wetting and spreading, resulting in:

• incomplete brazing joints
• brazing voids
• micro leakage channels

4. Moisture decomposition can also generate hydrogen gas. When trapped inside brazing joints, it forms internal pores that may cause leakage during pressure testing.

Typical Failure Patterns

• Large-area incomplete brazing across the whole furnace load: Usually related to high workshop humidity or moisture accumulation inside the furnace.
• Individual leakage points: Usually caused by insufficient cleaning, incomplete drying, or excessive exposure time after drying.

Five-Level Prevention System for Summer Vacuum Brazing Production

1. Control Workshop Humidity: The First Line of Defense

For aluminum heat exchanger manufacturing, controlling humidity in assembly and loading areas is essential.

Recommended targets:

• Assembly area and core stacking area relative humidity: ≤45%RH
• Maximum allowable humidity: ≤55%RH

Recommended improvements:

• Install industrial desiccant dehumidifiers in assembly and core stacking areas.
• Install air curtains near furnace doors to reduce humid air entering during loading and unloading.
• Avoid opening windows during summer high humidity periods.
• Record temperature and humidity data continuously for 24 hours.

When humidity exceeds 55%RH, production should be adjusted by increasing drying time or reducing exposure time.

Important Production Rule

After cleaning and drying, aluminum cores should not be exposed to open air for long periods:

• Normal humidity conditions: within 2 hours
• Humidity above 60%RH: reduce exposure time to less than 1 hour

Long exposure allows aluminum surfaces to continuously absorb moisture from the air.

2. Optimize Cleaning and Drying Before Vacuum Brazing

For vacuum aluminum brazing, insufficient drying means moisture is directly carried into the furnace.

Cleaning Process

Recommended process:

Ultrasonic degreasing → Multi-stage pure water rinsing

Final rinsing water:

• Conductivity: <5μS/cm

This helps prevent salt residues that can absorb moisture.

Summer Drying Parameters

Standard drying:

• Temperature: 110~130℃
• Holding time: ≥60 minutes

High humidity summer conditions:

• Temperature: 120~140℃
• Holding time: 90~120 minutes

A hot air circulation oven is recommended. Vacuum drying can provide even better moisture removal when available.

Transportation Protection

After drying and cooling:

• Assemble cores as soon as possible.
• Cover waiting products with PE film.
• Avoid leaving dried aluminum components exposed to humid air.

3. Vacuum Brazing Furnace Moisture Management and Maintenance

During summer production, furnace chambers, graphite fixtures, and insulation materials can absorb moisture. During continuous production, this moisture is gradually released and affects brazing quality.

Daily Furnace Dehumidification

Before production:

• Heat empty furnace to 450~500℃
• Hold for 60~90 minutes
• Maintain vacuum extraction during heating

This removes absorbed moisture before actual brazing.

Furnace Sealing Inspection

Check furnace pressure rise rate regularly.

Recommended value for aluminum vacuum brazing:

• Pressure rise rate: ≤0.5Pa/h

A higher value indicates possible sealing problems, allowing humid air to enter the furnace and increasing oxidation risks.

Graphite Fixture Management

• Remove aluminum particles and dust regularly.
• During humid seasons, preheat fixtures before use.
• Avoid loading cold and moisture-absorbed fixtures directly.

Vacuum Pump Maintenance

High summer temperatures may accelerate vacuum pump oil emulsification.

Recommended actions:

• Shorten vacuum oil replacement intervals.
• Check gas ballast valve regularly.
• Use gas ballast operation to remove moisture inside the pump.

A properly maintained vacuum brazing furnace is critical for stable aluminum heat exchanger production.

4. Optimize Brazing Temperature Profile for Humid Conditions

Using a standard brazing curve without adjustment during summer may cause moisture to remain inside components before entering the high-temperature brazing stage.

A recommended summer brazing profile:

Stage 1: Room Temperature → 180℃

Heating rate:

3~5℃/min

Stage 2: 180℃ Holding Stage

Holding time:

40~70 minutes

Purpose:

• Remove absorbed moisture
• Release crystal water
• Prevent moisture-related oxide regeneration at higher temperatures

Stage 3: 180℃ → 480℃

Heating rate:

5~7℃/min

Stage 4: 480℃ Holding Stage

Holding time:

30~50 minutes

Purpose:

• Further degassing
• Improve oxide removal through magnesium vapor reaction

Stage 5: 480℃ → Brazing Temperature

Heating rate:

6~8℃/min

Brazing temperature:

595~610℃

Stage 6: Brazing Holding Time

Compared with dry seasons:

Increase holding time by:

3~8 minutes

This compensates for reduced filler metal spreading ability under humid conditions.

Avoid Excessive Heating Speed

If heating is too fast, moisture cannot escape completely. Water vapor may become trapped by molten filler metal, creating internal pores and incomplete brazing.

5. Assembly Clearance and Filler Metal Control

Environmental control alone is not enough. Proper assembly quality is also essential.

Recommended Core Assembly Clearance

For plate-fin heat exchangers:

• Standard clearance: 0.02~0.06mm
• Clearance above 0.08mm increases the risk of incomplete brazing and leakage.

During high humidity periods, filler metal spreading ability decreases, so clearance control should be closer to the lower limit.

Additional controls:

• Check brazing sheet filler layer thickness.
• Ensure sufficient filler metal supply.
• Maintain uniform clamping force during stacking to prevent clearance expansion during heating.

Quick Diagnosis of Vacuum Brazing Leakage Problems

Problem Pattern	Possible Cause
Large-area incomplete brazing	High humidity, furnace moisture, poor drying
Random leakage points	Cleaning residue, transportation exposure
Internal pores	Fast heating, trapped moisture
Poor filler spreading	Oxide regeneration caused by water vapor

Emergency Improvement Priority

When summer brazing defects suddenly increase, follow this order:

1. Activate dehumidification equipment and keep assembly areas below 50%RH.
2. Perform daily furnace empty heating to remove moisture.
3. Extend drying time and reduce waiting time after cleaning.
4. Add or extend the 180℃ dehydration holding stage.
5. Check furnace pressure rise rate and replace aging seals if necessary.

Conclusion

In fluxless vacuum brazing of aluminum heat exchangers, magnesium vapor plays an important role in breaking aluminum oxide films. However, moisture acts as a magnesium consumer, reducing oxide removal capability.

The higher the humidity, the greater the risk of incomplete brazing and leakage.

A reliable production system requires a combination of:

• controlled workshop humidity
• optimized cleaning and drying processes
• regular vacuum brazing furnace maintenance
• adjusted brazing temperature profiles
• strict assembly clearance management

With professional vacuum brazing equipment, leak testing systems, and complete heat exchanger manufacturing solutions, manufacturers can maintain stable brazing quality even under challenging summer conditions.

SUNHOPE provides vacuum brazing furnaces, leak testing equipment, radiator manufacturing machines, and complete technical support for aluminum heat exchanger production.