Diffusion bonding for joining dissimilar metals

In aerospace, metal diffusion bonding is an essential joining method for achieving a high-purity interface when two similar metals require superior structural integrity. The process involves applying high temperature and pressure to metals mated together in a hot press, causing the atoms on solid metallic surfaces to intersperse and bond.

Hot press diffusion bonding produces consistently uniform results in materials joining for a range of aerospace applications. The applied pressure induced by hot-press equipment, combined with software and loop-back sensors for precise control with micrometer accuracy, can produce constant pressure over several square feet for component assembly. This has made the technology of interest to design engineers in the aerospace, semiconductor, and energy industries.

With its high degree of process control, diffusion bonding is increasingly used commercially to join titanium to iron-nickel alloys, titanium alloys to stainless steel, and even some aluminum to other metal applications. The process also enables coupling between different alloys in the same material group, such as mild steel, tool steel, and metal-matrix composites.

An understanding of the complexities of the interface and its effect on the chemical and thermo-mechanical properties of the bond is required to successfully use diffusion bonding. However, with the industry’s traditional focus on welding and brazing, there’s been minimal formal education on diffusion bonding, according to Thomas Palamides, senior product & sales manager – industrial furnaces, PVA TePla AG, a global manufacturer of industrial furnaces and PulsPlasma nitriding systems.

“Combining the beneficial properties of different metals is the main reason to explore diffusion bonding. However, when aerospace manufacturers reach out [to us about it], often the designers aren’t familiar with the best component design methods to optimize the bond uniformity or are unfamiliar with the best surface finish for the materials,” Palamides says.

Benefits of diffusion bonding

The importance of designing a dissimilar metal joint often lies in a desire to expose the correct metal surface to specific environmental conditions where a single alloy may not perform as well. Another reason is to introduce material systems lighter in weight or provide a level of corrosion resistance only achieved by packaging dissimilar metals.

Diffusion bonding is now a viable process for fabricating aerospace structural hardware or fluid and gas flow devices, including thermal management devices for defense weapons, liquid-fuel rocket engines, and millimeter-wave hybrid antenna horns for land and space surveillance.

The geometry of antennas for detecting polarized electronic radio waves is at the heart of this technology, and diffusion bonding provides for more complex designs for improved performance.

Additionally, diffusion bonding of titanium, steel, and copper alloys allows fabricating aerospace components with complex configurations. The method can be used successfully with blow forming to make near-net-shape aerospace components, including high-pressure tanks for attitude control of spacecraft, a combustion chamber with copper cooling channels, and lightweight structural panels.

Diffusion bonding also has tremendous potential applications for conformal cooling, or bonding layers of sheet metal that contain machined channel/microchannel structures. When combined, the channels can provide cooling or heat dissipation. The layers can be bonded up to a stack height of 600mm in the MOV [cold-wall furnace with] diffusion bonding press, retaining the same strength as the parent materials.

Another application related to conformal cooling is for plastic injection molds made in 2-layer designs of low alloyed tool steel with stainless steel such as STAVAX.

Exploring diffusion bonding

While ample research exists on the subject, design engineers can still find it challenging to convert the information into real- world manufacturing of a specific part. When this is the case, it can be helpful to partner with experts with an extensive database of successful processing parameters from previous applications and access to industrial-scale equipment.

“In most cases, we start talking with the aerospace manufacturer about introducing new designs, and consult on possible materials, designs, and also conduct pre-bonding runs as needed,” Palamides explains. PVA TePla provides support, including specific material combinations, processing times, and temperatures.

He notes proper design allows diffusion bonding of assemblies, whether an intimate interface or multiple interfaces that are planar parallel simultaneously. However, surfaces not perpendicular to the compressive force of the hydraulic ram won’t bond properly.

Palamides says the manufacturer begins by working with their mechanical, thermal, and modeling teams. Once a design is complete, the next step is to fabricate trial samples truthful to the characteristics of the final interface.

Advances in diffusion bonding equipment

Despite its benefits, the use of diffusion bonding in aerospace has been limited by more practical considerations until recently; specifically, the size limitation of the furnace chamber and limits to the amount and uniformity of the pressure applied across the entire surface area of the part. Run times are also long, often lasting an entire day.

Advances in high-vacuum hot presses now allow superior pressure control and rapid cooling systems to improve the bond, increase yields, and significantly decrease cycle time.

For pressure, integrated single-cylinder hydraulic presses can apply a consistent, measurable amount of force. However, this provides very little control over large parts with more complex geometries. To improve force distribution, thick graphite pressing plates (10" to 15" high) mate the metal layers together at a more consistent pressure. Unfortunately, this takes up furnace space while adding to the time to heat the metals’ surfaces.

Today, manufacturers such as PVA TePla offer multi-cylinder systems with large pressing plates that can accommodate various parts. The largest, the company’s MOV 853 HP, can process substrates as large as 900mm x 1250mm (35.43" x 49.21"); pressing force is 4,000kN.

By controlling each cylinder independently, the integrated press provides remarkably consistent pressure across the entire surface. The MOV also comes with built-in pressure transducers along the bottom of the pressing plate. The individual hydraulic cylinders can be adjusted by software to achieve uniformity even over large areas, based on the sensor feedback.

PVA TePla has optimized a physical ink test method identifying areas on the substrate where uneven pressure is applied.

“Today’s equipment provides detailed measurements of the material properties during bonding,” Palamides says. “This valuable feedback can show how the materials are compressing, if they’re being crushed, if a transient liquid layer is forming, and other key performance indicators of the procedure.”

Verifying bonding interface integrity

To ensure the quality of the interface, Palamides recommends analyzing samples through non-destructive inspection techniques, such as scanning acoustic microscopy (SAM) or scanning electron microscopy/energy dispersive X-ray spectroscopy (SEM-EDS). Subsequently, trial samples may be destructively analyzed and fabricated into standard mechanical test specimens to collect repeatable data.

While there’s growing interest in diffusion bonding, all applications require thorough research to optimize the joining process. Only a few global firms can work with manufacturers through the process to advise on commercial system adoption. Partnering with an expert in diffusion bonding will give manufacturers a competitive edge from part design through production ramp-up.