How Diffusion Bonding Expands the Design Options for Joining Dissimilar and Reactive Metals

Imagine you’re designing a component for a high-performance aerospace application. The requirements call for properties that no single material can match: the thermal conductivity of copper, and the corrosion resistance of stainless steel. 

However, due to their very different metallurgical and thermal properties, joining those two materials together through welding is problematic. Brazing is an option, but the introduction of a braze alloy isn’t always desirable. 

The challenge is reliably joining these metals in a way that preserves the properties that make each one ideal for your application. Diffusion bonding is a solid-state joining process that gives engineers another method for joining dissimilar metals, capable of combining the characteristics of each material so they can each perform a specific function.

With 50 years of experience joining dissimilar and reactive metals using diffusion bonding, VPE can help you build precision assemblies using challenging material combinations. 

Why Dissimilar and Reactive Metals Are Difficult to Join

Metals have many different properties that can be useful across a wide range of engineering applications. 

Stainless steel provides corrosion resistance and durability, copper is an excellent thermal and electric conductor, titanium has a high strength-to-weight ratio, and ceramics can provide properties that metals often can’t such as electrical insulation. 

Instead of compromising and choosing only one material, engineers may want to combine them, so that each can perform the function it is best suited for. However, traditional joining methods can make the joining of dissimilar metals impractical. 

Take welding for example. Dissimilar metals, like copper and stainless steel, have different melting temperatures, thermal expansion rates, and other metallurgical properties. Because conventional fusion welding requires the joining surfaces to be melted, some material combinations are not strong candidates for welding. The different melting temperatures and expansion rates of each material can produce undesired results like cracking or distortion. 

Brazing is another common joining option that offers more versatility than welding because the parent materials are not required to melt. However, a filler material is melted instead, which introduces another material into the assembly. The filler can introduce other considerations, such as altering the chemistry or corrosion resistance of the joint, or compromising its hermeticity. 

As a solid-state joining method, diffusion bonding is a valuable option for many dissimilar metal combinations because the process can create a joint without melting the parent materials or using a filler alloy. With proper dissimilar metal joint design, a diffusion bonded joint retains properties approaching those of the parent materials. 

How Diffusion Bonding Joins Dissimilar Materials

Diffusion bonding works by bringing two materials together into intimate contact, joining them together in a controlled environment using heat, pressure, and time. At an elevated temperature, the atoms of the mating surfaces of each material diffuse across the joint interface. 

Because diffusion bonding is a type of solid-state metal joining, the surfaces of the materials are not melted, as they would be during welding, and no filler alloy is used, as brazing would require. The pressure applied flattens microscopic irregularities, and over time, the original interfaces effectively merge together and form a metallurgical bond. 

Depending on the materials being bonded, diffusion bonding design engineers determine the appropriate combination of heat, pressure, and time needed to create the joint.

1. The mating surface of the two metals are cleaned and prepared, then placed into the diffusion bonding furnace. 
2. Heat and pressure are applied in a controlled process designed for the specific materials and joint application.
3. The atmosphere is also highly controlled, in many cases using a vacuum to reduce the amount of elements in the furnace that could cause unwanted effects. 
4. Under the appropriate combination of temperature, time, and pressure, atoms at the mating surface diffuse across the interface
5. As diffusion progresses, the original interface becomes a metallurgical bond between the materials.

In some cases, particularly when joining reactive metals through diffusion bonding, the process isn’t as simple as joining metal A + metal B. In these cases, a very thin interlayer of a third material is needed. This can act as a diffusion barrier or moderator, or as a transition between incompatible materials. Depending on the application, the interlayer can be introduced as a thin foil or deposited or coated material.

The Challenges of Dissimilar Metal Joint Design Using Diffusion Bonding

Diffusion bonding dissimilar metals allows for many material combinations that would be much more difficult to accomplish using traditional joining techniques, but it doesn’t mean it is without its own set of challenges. 

One of the biggest issues engineers face is when materials have mismatch in their coefficient of thermal expansion (CTE). When a diffusion bonding material combination has different CTEs, the metals will expand at different rates. That also means they contract at different rates. That can cause the materials to be mismatched after cooling, which can result in unwanted deformation. These differences in CTEs need to be accounted for.

Another challenge is that different elements diffuse at different rates. For example, when titanium is diffusion bonded with stainless steel, different elements can diffuse across the interface at different rates. That imbalance can cause small vacancies to form, called Kirkendall voids. They can weaken the joint and potentially lead to tearing or separation. This is where using an interlayer can be an appropriate solution. 

Lastly, joining reactive metals with diffusion bonding, such as titanium, can present additional challenges. Titanium has excellent strength-to-weight characteristics and corrosion resistance, making it highly desirable in aerospace and medical applications. However, it is also highly reactive and its metallurgy can interact unfavorably with other elements during processing. 

When working with titanium or titanium alloy diffusion bonding, a vacuum environment can help minimize unwanted reactions with atmospheric gases. However, the specific chemistries of the interfaces still matter greatly. 

Working with a team with extensive experience diffusion bonding dissimilar material combinations can help overcome many of these challenges. 

Common Diffusion Bonding Material Combinations

Diffusion bonding is a highly valuable process because it can allow engineers to create many useful and unique combinations of metals that would be difficult to combine with other joining methods. However, diffusion bonding shouldn’t be used just for the sake of it alone. The more important question to ask is what benefit can we achieve by diffusion bonding these materials together?

Some of the most useful diffusion bonding material combinations include:

Copper + Stainless Steel

Copper features excellent conductivity, but is prone to oxidation. Meanwhile, stainless steel is corrosion resistant. Diffusion bonding can allow engineers to use copper where its conductivity is needed, while incorporating stainless steel as an oxidation-resistant interface or surface where greater environmental resistance is required. 

Titanium + Stainless Steel/Titanium Alloys

An extremely useful combination is titanium diffusion bonded with stainless steel. This combination takes advantage of titanium’s low weight, strength, and corrosion resistance, and pairs it with stainless steel’s durability and other beneficial properties.

Aluminum Combinations

Aluminum’s combination of thermal conductivity and low weight make it useful for cold plates and thermal-management components. Aluminum can also be joined to other materials when additional strength, corrosion resistance or temperature capability is needed in a specific area of an assembly.

Nickel and superalloys

Nickel-based superalloys, like Inconel and Haynes alloys, offer a valuable combination of mechanical strength and corrosion resistance, but they can be cost-prohibitive for an entire component. Instead, diffusion bonding allows these high-performance materials to be strategically joined with other metals, using them where their temperature or environmental resistance is most useful.

Metal + Ceramic

Ceramics offer properties that most metals cannot match, such as high-temperature stability, electrical insulation, chemical resistance, wear resistance, and hardness. However, ceramics cannot be conventionally welded like metals. 

Brazing can also be difficult because it typically requires additional processing to overcome the CTE mismatch and low wettability of the ceramic surfaces. Diffusion bonding is an alternative to brazing ceramic-to-metal interfaces; however, additional development is required, as the process has been demonstrated on only a limited basis.  

While diffusion bonding can technically bond many different materials together, the right combination of materials for the job depends on the specific application, and the desired effects.

Applications Where Diffusion-Bonded Material Combinations Expand Design Options

Diffusion bonding is most valuable when an application requires complex internal structures, dissimilar materials, high-temperature performance, or a filler-free joint. Some applications where diffusion bonding has proven particularly useful include:

Microchannel Heat Exchangers and Cold Plates

These devices require intricate channel networks for transferring heat through fluids, which can be difficult to machine from a single piece of material. Diffusion bonding allows for many layers of material to be joined into a solid assembly, while preserving the intricate channels needed for efficient heat transfer and fluid flow. 

Aerospace

Diffusion bonding can enable the construction of lightweight, high-strength components and complex assemblies used in demanding aerospace applications. 

Medical Devices

Some precision medical components require expensive specialty materials. Diffusion bonding allows them to be used in combination with more cost effective materials, only where their properties are needed.

Corrosion-resistant Manifolds

Dissimilar materials can be combined through diffusion bonding to provide corrosion resistance at critical interfaces, while using other materials where their properties are more advantageous. 

Microfluidic and Precision Devices

Diffusion bonding is often used to create complex, yet compact assemblies used in applications like fuel cells, reactors, reformers, and other fluid-handling systems. 

For design engineers, diffusion bonding isn’t just another way to join materials. It offers the ability to create useful combinations of materials and intricate geometries that offer significant advantages that would be difficult or even impossible to achieve with conventional joining methods.

Expand Your Design Options With VPE’s Diffusion Bonding Capabilities

Successfully joining dissimilar and reactive metals must take into account their differences in thermal expansion, diffusion behavior, surface chemistry, and the conditions within which the end component will operate. To determine whether an application is a good candidate for diffusion bonding takes specialized expertise. 

With 50 years of experience in diffusion bonding and complex materials joining, VPE’s engineers can help you evaluate the materials, joint design, and process requirements for today’s most demanding applications. 

We have the extensive facilities and expertise to join complex assemblies and precision components using diffusion-bonded solutions for a wide-range of industries. 

If you have an application that requires joining dissimilar or reactive metals, contact VPE to discuss whether diffusion bonding can help make your design possible.