Aluminum Fluxless Brazing: A Step-by-Step Guide

Aluminum is an extremely versatile metal used in a variety of industrial applications. Its high corrosion resistance, high thermal conductivity, and high strength-to-weight ratio make it ideal for applications such as heat exchangers for ICE cooling (radiators), electronics cooling (cold plates), and structural components for aerospace. 

While aluminum has many benefits, there are challenges when joining aluminum to itself or to other metals. Aluminum’s affinity for oxygen causes a thin layer of oxide to rapidly form on its  surface when left in the open. It’s this oxide and general component cleanliness that can impede brazing or soldering processes unless properly addressed.

Through over 50 years of experience, VPE has found that fluxless brazing techniques offer the most effective, highest integrity solution for joining aluminum.

When executed correctly, brazing produces strong metallurgical bonds and leak-tight assemblies with large surface areas. Brazing also allows component counts into the hundreds for a single brazement, such as with radiators.

In this aluminum brazing guide, we’ll walk through the aluminum brazing process step by step.

What is Aluminum Brazing?

Aluminum brazing techniques are among the most reliable methods for joining aluminum.

Brazing joins two or more components together by melting and flowing a brazing filler metal (BFM) between them. The BFM may be preplaced in the joint or it may be placed outside of the surfaces to be brazed.

Brazing occurs at temperatures above  450°C (840°F). For successful brazing, the BFM alloy must have a melting point above 450°C, yet below that of the parent surfaces. Once heated to its melting point, the BFM will flow by capillary action into the joint at brazing temperature.

As it cools and solidifies, the braze alloy forms an extremely durable bond between the two parent materials. With proper joint design and process control, aluminum brazing is also capable of producing leak-tight hermetic joints.

Aluminum-silicon alloys are often used as the filler metal in the aluminum brazing process; they are inexpensive, corrosion resistant, and flow well at brazing temperature.

Aluminum’s affinity for oxygen causes oxides (Al₂O₃) to form on the surfaces that inhibit heat and electrical conduction. These oxides must be removed by either mechanical or chemical cleaning before brazing.

Key Factors in the Aluminum Brazing Process

Aluminum brazing techniques require precise process control in 5 specific areas:

1. The joining surfaces must be cleaned to prevent oxides from inhibiting wetting and bonding.
2. The ambient environment surrounding the braze joint must prevent oxygen from reacting with the aluminum at high temperature.
3. The gap between the parent materials must be tightly controlled, allowing the braze alloy to flow uniformly between the two surfaces being joined.
4. The temperature must be closely controlled so that it exceeds the melting point of the braze alloy, but not that of the parent materials. 
5. The parent material’s form must be supported (with tooling) so that it does not deform as the temperature rises close to the BFM melting point.

How is the Aluminum Brazing Process Different From Other Joining Methods?

Welding, soldering, and brazing are common metal joining methods. Yet, distinct differences between the three processes make each better suited for some applications than others.

Welding is perhaps the most common aluminum metal joining process. It uses high heat to melt parent surfaces, sometimes with the use of a filler material. Typically, components that require butt joints, like tubing, are joined by welding. While capable of creating extremely strong bonds, welding may distort the parent materials. The weld joint may not be hermetic due to small defects and cracking.

Soldering is similar to brazing. It works by melting a filler material into the joint, typically using a corrosive flux. The difference is that soldering occurs at temperatures below 450°C (840°F), while brazing occurs above that threshold.

In practice, soldering does not provide the same strength as a brazed or welded bond, but it is ideal for joining electronic components and small parts, where high temperatures cannot be tolerated. 

Aluminum Brazing Applications

Aluminum brazing techniques are employed in many applications because the process is highly reliable and cost effective. VPE specializes in fluxless brazing of aluminum which produces more reliable joints than techniques that rely on corrosive fluxes.

At VPE, our most common applications are chip coolers, radiators, cold plates, compact microchannel heat exchangers, and flanged tubing assemblies. The fluxless technique is extremely useful because a leak-tight hermetic seal can be created that keeps the liquids restricted to internal liquid cooling channels. Use of other joining methods for these products could result in leak-prone joints or joints that do not meet overall assembly strength requirements.

At VPE, we’ve perfected our cost-effective aluminum brazing manufacturing process for production of high-quality, high reliability, and often intricate-featured assemblies.

Aluminum Brazing Step by Step

1. Surface Cleaning

The aluminum brazing process begins by thoroughly cleaning the parent metal surfaces to remove contaminants, including oils, grease, machine fluids, and even fingerprints. Our extensive ultrasonic cleaning process uses alkaline cleaning solutions and organic solvents for removing residue from machining operations.

2. Oxide Removal

After the initial cleaning, the next step is to ensure that BFM flow-inhibiting heavy oxides are removed from the parent surfaces.

Chemical oxide removal is done using an ultrasonically agitated caustic solution, followed by thorough rinsing and drying of the surface. Following these steps, all components are handled with clean, lint-free gloves.

3. Assembly

After cleaning, the parent materials and BFM are assembled and tooled for brazing. In some assemblies, the BFM is clad to either one or both parent metal surfaces.

The contact and applied load at the surfaces are carefully controlled to enable good flow of the BFM at brazing temperature. If the gap is too tight, the molten braze alloy could be precluded from filling the joint. Too loose, and the joint will have voids. 

4. Vacuum Furnace Brazing

Once the materials are assembled, they are positioned inside the furnace hot zone. Vacuum pumps evacuate the brazing chamber to high vacuum, removing oxygen from the atmosphere. Magnesium is also used as a getter in the furnaces to further reduce residual oxygen before it can react with the aluminum.

The temperature inside the brazing furnace is ramped up and held to just below the brazing temperature. This “pause” allows all components and tooling to reach a uniform temperature.

After the pre-braze temperature “soak,” the furnace temperature is increased and the braze alloy flows within the gap by capillary action.

After a short soak at the braze temperature, the furnace load is cooled to room temperature. The cooling rate itself depends on the specific braze alloy used, as well as the “thermal mass” of the tooling and parts combined. 

Depending on customer requirements, the brazed assemblies may be heat treated to develop mechanical properties and corrosion resistance.

5. Inspection and Testing

After aluminum brazing is completed, VPE conducts visual inspection of the joints. Additional testing is done based on customer needs. Some common testing methods include helium mass spectrometer leak testing (particularly with heat exchangers), strength testing, and dimensional tolerance checks.

Why Choose VPE for Fluxless Aluminum Brazing?

VPE excels at providing detailed application engineering, with a team of industry experts that sets us apart from our competitors.

Our team includes brazing engineers, highly-skilled technicians, and an in-house “Thermal Processes Group” staffed by materials science PhDs and other materials specialists. Our TPG has developed techniques to diffusion bond aluminum and braze thin-walled tubing, fins, and intricate assemblies. 

Key benefits of working with VPE for your aluminum brazing needs include:

• Thorough cleaning of aluminum and parent material surfaces prior to brazing
• Magnesium-based atmosphere scrubbing to remove residual oxygen during processing
• Multiple well-instrumented vacuum furnaces for precise temperature and process control
• In-house expertise in tooling and fixturing design to maintain dimensional integrity
• Post-process heat treatment, machining, and welding available as required

Conclusion

With decades of research and development in fluxless aluminum brazing and aluminum diffusion bonding techniques, VPE has the right technical and design support resources to meet your unique materials joining needs.

Contact VPE for more information on how we can help you solve your aluminum joining challenges with high-quality, durable flux-free aluminum-brazed components.