Advanced Metal Joining: Diffusion Bonding vs Brazing

In high-performance manufacturing work, engineers are quick to consider performance. And rightfully so. They often need their components to be strong, thermally efficient, and in many cases leak-tight.

These are important characteristics in aerospace, thermal systems, and medical components. But engineers don’t always consider how components will be joined when they approach suppliers.

Through 50 years of advanced materials joining experience at VPE, we’ve had our share of clients approach us to produce components that have never considered the joining method. In reality, the joining method plays a crucial part in what the part can actually be in the first place.

For many high-performance joining applications, the choice often comes down to one of two methods: diffusion bonding vs brazing. Which method is right for a project depends on part geometries, material selection, internal flow channels, and how loads move through the structure.

When the joint design is an afterthought, it can mean a design that looks complete in CAD actually needs serious re-evaluation before manufacturing can begin.

Both diffusion bonding and vacuum brazing can solve many of these joining challenges, but the way they form a joint and the way they constrain design are inherently different.

Understanding the difference from the beginning can prevent countless hours of redesign and iteration later.

The Core Four Principles of Metal Joining

Before we dive into the differences between diffusion bonding and brazing processes, it’s helpful to understand the fundamentals of metal joining that apply to virtually every method used in modern manufacturing. 

Every type of metal joining requires control over four process parameters: heat, force, time, and environment. 

Heat is the first key. The temperature of the metal being worked must be regulated to control deformation and keep consistent quality during formation and joining. 

Depending on the joining method, force may be used to bring the surfaces into sustained contact, control the rate of deformation, or to simply keep materials fixed in place. 

The timing of heating, deformation, and cooling must be precisely controlled during the joining process. 

A controlled environment is needed to prevent oxidation and contamination so the end product can meet quality requirements. 

All advanced joining methods depend on controlling these four variables. Even small changes in these areas can materially alter the strength of a joint.

In addition, all methods of joining metal require:

• Clean surfaces to remove oxides and contaminants
• Precise alignment of parts
• Oxygen-free vacuum or inert environment 
• Controlled heating and mechanical restraint

No matter whether you intend to weld, braze, or diffusion bond components, it is virtually impossible to produce a structurally sound joint without control over these areas. 

What is Brazing?

Brazing is a method of joining two or more materials together by melting a filler material between their surfaces. 

The brazing process occurs at temperatures over 450ºC (842ºF), which is enough to melt the filler material without melting the base materials. It’s a process similar to soldering, which occurs at temperatures lower than 450ºC. As the braze material melts, it flows by capillary action between the closely fitted surfaces of the parent materials.

Joint design is crucial to the brazing process. Properly designed joints must be aligned, precisely spaced, and held in place to ensure that the molten filler flows precisely where it is needed, and not into other passageways. 

To create a strong joint, the filler material must wet the base metal. Careful preparation and environment control are crucial to the brazing process. Oxides and contaminants must be removed from the material’s surfaces before the brazing process begins.

Brazing is often done inside a vacuum furnace, which removes much of the oxygen and contaminants from the atmosphere. Inside, the furnace is heated to a temperature high enough to melt the filler alloy, typically over 450ºC. The melted filler wets the base metals, where it eventually cools and solidifies into the joint.

What is Diffusion Bonding?

Another commonly used method for joining metals is diffusion bonding, however it differs fundamentally from the brazing process.

Diffusion bonding process is a solid state joining process which does not require any materials to be melted to create a joint. 

Instead, diffusion bonding uses a precise combination of heat, time, and force to merge two surfaces into each other, effectively creating one solid piece of metal.

The diffusion bonding process uses heat in a controlled furnace environment to raise the temperature to just below the parent metal’s melting point. The surfaces become soft enough that when sustained pressure is applied over time, atoms begin to diffuse from one surface into the other. 

It sounds relatively simple, but in practice the challenge lies in applying equal force throughout the part. 

Preparing the parts for the diffusion bonding furnace requires clearly understanding the load path. The interfaces (surfaces) of the parent metals require heat and pressure to be brought together in intimate contact. Over enough time and pressure, the large bubble-like gaps known as pores begin to collapse. Eventually, this occurs at the atomic level and the joint essentially disappears, creating a continuous piece of metal.

The Key Differences Between Brazing and Diffusion Bonding

On the surface, the biggest difference between diffusion bonding and the brazing process is that brazing requires a molten filler material, while diffusion bonding creates a solid-state bond at the atomic level. 

But a more conceptual way to look at it is load vs. flow. 

To achieve diffusion bonding, a load path is required. This is how force is applied evenly across the surface of the parent materials. When the load distribution is uneven, you get areas that are fully bonded next to what amounts to bubbles or gaps in the joint. That is a problem. 

That’s why fixturing and part geometry matter so much in the diffusion bonding process. Joints must be designed so that load path can be evenly distributed across the surfaces of the metals, eliminating the voids over a long enough period of time. 

Compare that to a vacuum brazed joint. No load is required. Instead, the molten filler material relies on a controlled flow into the joint gap. The biggest risk with the brazing process is that the filler can flow into areas where it’s not supposed to. 

That brings us to another of the key differences between brazing and diffusion bonding: time. Both processes require time, but the time required to complete diffusion bonding is much longer than brazing. 

A vacuum brazing furnace can begin melting the filler material within seconds. Once it flows, the temperature can be reduced and the filler quickly hardens. The entire vacuum brazing process can take anywhere from a few minutes to perhaps half an hour at most. 

In contrast, the diffusion bonding process happens in several stages, with the first stage taking 30 minutes to several hours. Achieving diffusion requires sustained pressure over considerable time. The second and third stages of the diffusion bonding process can take several hours to a full day. 

The appearance of the joint itself is another major difference between the two processes. Brazing leaves a visible seam between the parent materials. Because diffusion bonding happens at the atomic level, it is nearly impossible to visibly see the joint line. For all intents and purposes, the two materials effectively become a single piece of metal.

When to Use Each Process

Engineers often design parts without putting much consideration into which joining method will be used. In practice, there are many applications that can realistically use either of these advanced joining methods, while some will specifically require one or the other. 

In some cases, trials can be done with each method to see what works best for a particular use case.

Throughout our 50 year history as vacuum brazing and diffusion bonding services suppliers, VPE has helped countless clients understand which joining process will serve their needs best.

Our experience has shown us that diffusion bonding is often the better choice when a design requires the maximum structural integrity possible. With the two surfaces effectively merging into a single piece of metal, diffusion bonding creates the strongest possible joint. This is often a priority in aerospace and defense applications where assemblies are often put under extreme conditions. Leak-tight joints can also be produced with this method, which is a critical requirement of applications like heat exchangers and cooling plates. 

On the other hand, vacuum brazing is a better choice when components have complex internal geometries. Diffusion bonding requires constant pressure over extended periods of time. When materials have more delicate structures, vacuum brazing is often preferred because it does not require pressure. 

The vacuum brazing process is also capable of producing multiple joints on an assembly simultaneously. With proper joint design, molten filler can flow through multiple gaps with a much faster production speed than diffusion bonding.

The final decision often comes down to factors like geometry constraints, timelines, and overall costs.

Need Help With Advanced Joining Methods? Contact VPE

Both the diffusion bonding and vacuum brazing processes are advanced, high-precision joining methods. The key difference is how they accomplish the task. 

Diffusion bonding creates the strongest joints, but requires high heat and constant pressure applied over significant time. Vacuum brazing uses molten filler materials to fill in the joint gap between parent materials. The process is much faster than diffusion bonding, and still produces a durable joint. 

Each process has its strengths and design constraints, but success depends heavily on considering which joining method fits your application best in the early stages of design. When joint design comes later, delays and redesigns become inevitable. 

The best results come when performance requirements are aligned with materials, geometry, and manufacturing constraints early on.

If you need help deciding which advanced joining method is best for your needs, VPE’s engineers and design consultants can help. 

Contact VPE today to schedule a consultation and let us help you choose the best method for your project.