Aluminum Vacuum Brazing: The Essentials

Aluminum is one of the most commonly used metals in modern manufacturing. Its corrosion resistance, low weight, and ability to transfer heat efficiently make aluminum useful in everything from aerospace systems to EV batteries. 

But joining aluminum components for these applications can be more challenging than many other metals. 

Aluminum’s biggest challenge is how quickly it oxidizes. When exposed to air, aluminum forms an oxide layer almost immediately. That oxide essentially forms a barrier on the surface of the metal that makes joining it difficult. 

The metal also has a relatively low melting point. Joining aluminum components requires precise control over both the temperature and the furnace conditions.

Aluminum vacuum brazing allows engineers to remove oxygen and other contaminants from the vacuum furnace, while precisely controlling the temperature to prevent the parent materials from melting.

For assemblies that require clean, leak-tight, and thermally conductive aluminum components, aluminum vacuum brazing is the process that modern manufacturers turn to.

What is Aluminum Vacuum Brazing?

Traditional joining methods like welding require the parent materials to be melted in order to create the joint. In applications where components have thin walls, hundreds of intricate internal channels, or tight dimensional requirements, that can create distortion or weakness in the assembly.

The aluminum vacuum brazing process eliminates the need to melt the parent materials by melting a filler material instead, known as a braze alloy. 

As the filler material melts, it flows into the joint through capillary action while the parent materials stay intact. The assembly is cooled, allowing the filler material to harden into a leak-tight, thermally conductive joint. 

This approach offers another major advantage over other types of metal joining: cleanliness. Vacuum brazing produces clean, flux-free joints with minimal oxidation compared to other joining methods.

The Challenges of Aluminum Brazing

On the surface, vacuum brazing sounds simple: heat the assembly, melt the filler, form the joint. 

In reality, there is very little room for error.

Narrow Temperature Control Windows

Aluminum-silicon braze alloys melt at around 600ºC (1,112ºF). By comparison, the melting point of aluminum is roughly 660ºC (1220ºF). 

That leaves an extremely narrow temperature window to melt the braze alloy without melting the aluminum components themselves. When brazing other metals, there is usually a more easily manageable temperature gap. Aluminum isn’t so forgiving. 

If the heat is too low, the braze alloy won’t melt properly and flow into the joint. Apply too much heat, and the parent materials risk softening and distorting.

Vacuum Furnaces Heat Through Radiation

Another challenge of the aluminum vacuum brazing process is the way vacuum brazing furnaces transfer heat. 

Inside a vacuum brazing furnace, radiant heating elements are used to heat the components. As the heating elements heat up and glow, they emit infrared energy that transfers heat to the load. 

The problem is that radiant heating doesn’t always distribute heat evenly across complex assemblies inside a vacuum furnace. 

When heating is uneven, it can create thermal gradients that cause thin sections of aluminum to heat faster than thicker ones. That can create uneven flow of the filler material, and produce a less durable joint.

To overcome this challenge, engineers must carefully design fixtures and load configurations to ensure even heating.

Aluminum Oxide Development

When aluminum is exposed to air, the metal quickly oxidizes. Chemically, it creates a ceramic-like layer on the metal that prevents the filler material from properly wetting the surface. 

The vacuum environment helps to remove the majority of oxygen and other atmospheric contaminants, but magnesium can also be introduced into the vacuum furnace to react with any residual oxygen before it can react with the aluminum. This helps prevent additional oxide formation during the brazing process.

Filler Materials and Clad Aluminum

In most aluminum brazing applications, the filler used is typically an aluminum-silicon alloy. The inclusion of silicon keeps the alloy’s melting point below that of the aluminum parent materials. But not every application will require the same filler materials. 

Sometimes, clients know they want to use aluminum parent materials, but aren’t sure what filler material will work best for their needs. An experienced aluminum brazing services provider like VPE can help determine what filler alloys are most compatible, as well as what filler form is best suited for the application, including foils, powders, pastes, and wires.

Some aerospace applications that require high-performance thermal management use clad aluminum as their foundational material, rather than separately applied braze alloys. This is common in heat exchangers and liquid cooling plates. 

Clad aluminum is essentially an aluminum sheet that has been pre-prepared for brazing. The aluminum core accounts for roughly 90-95% of the sheet’s thickness. The filler is typically a thin layer of aluminum-silicon that has already been applied and bonded to the surface, and accounts for 5-10% of the remaining sheet’s thickness.

Because the filler has already been evenly distributed across the surface of the clad aluminum sheet, engineers do not need to manually place the filler wires, pastes or foils into each joint. That cuts down on the prep work before brazing begins.

Common Applications for Aluminum Vacuum Brazing

Applications where both low weight and thermal management are critical are where aluminum vacuum brazing shines most.

Heat Exchangers

Heat exchangers are used to transfer heat between two or more fluids, and are commonly used in aerospace, military, and industrial applications.

Vacuum brazing makes it possible for manufacturers to create heat exchangers with hundreds or even thousands of internal flow paths. These flow paths allow the heat exchanger to efficiently transfer heat from hydraulic systems, electronics, and power systems, in compact spaces.

Cold Plates

Cold plates are similar to heat exchangers, but use a single fluid path to cool a metal plate. Cold plates are particularly useful in EV battery systems, and high-power electronics where excess heat can reduce component performance and shorten the overall lifespan.

These devices feature intricate internal channels, which are made possible by aluminum vacuum brazing to create sealed cooling assemblies.

Electronics and IGBT Cooling

Another high-demand application of aluminum vacuum brazing services is the cooling of insulated gate bipolar transistors (IGBT). These devices are used inside machines that need to control or change large amounts of electrical power, and generate considerable amounts of heat.

Aluminum brazed cold plates keep these systems cool. Their lightweight construction and high thermal conductivity keep AC/DC conversion systems, industrial drives and renewable energy equipment from overheating.

Aerospace and Defense Systems

Engineers in the aerospace and defense industries often favor aluminum brazed components for their lightweight construction and thermal performance. These properties are extremely useful in heat exchangers, avionics cooling systems, radar cooling assemblies, and cooling systems for military vehicles.

VPE: Industry Leader in Aluminum Vacuum Brazing Services

Whether it’s heat exchangers for aerospace applications or cold plates for power electronics, aluminum vacuum brazing is a vital process in industries where lightweight thermal management systems are needed.

These applications require complex aluminum assemblies that are both lightweight and leak-tight. Meeting these requirements is extremely difficult using traditional joining methods, which is why aluminum vacuum brazing has become more widely used.

However, achieving consistent, reliable results in this process demands engineers overcome unique technical challenges. 

Precise temperature control through a narrow window is necessary to prevent parent materials from melting. At the same time, radiant heating of vacuum furnaces can make it difficult to ensure that filler metals are melted evenly. Overcoming these challenges calls for significant process control experience. 

With over 50 years of experience providing metallurgical and materials joining services, VPE has the skill and experience to help you create aluminum vacuum brazed components used in aerospace, defense, and power electronics applications. 

To learn more about our aluminum vacuum brazing services, contact us today.