Diffusion bonding has been utilized to join high strength and refractory metals — those that are either difficult or impossible to weld by other means — for many years. The process — which involves applying high temperature and pressure to similar or dissimilar metals mated together in a hot press — causes the atoms on solid metallic surfaces to intersperse and bond. Unlike traditional brazing techniques, the resulting bond exhibits the strength and temperature-resistance of the base metals. It also eliminates the need for filler material that affects the final weight and dimensions of the mated metals.

Despite its benefits, diffusion bonding and its use in process applications has been limited by more practical considerations. These include the size limitation of the furnace chamber as well as limits to the amount, and uniformity, of the pressure applied across the entire surface area of the part. Run times also are long — often lasting a full day. All that may change, however, as advances in high vacuum hot presses used for diffusion bonding can eliminate many of those constraints.

The new equipment designs provide features such as:

• Pressure control.
• Feedback from embedded pressure transducers.
• Physical ink tests that show variations in pressure across the surface.
• Rapid cooling systems to improve the bond, increase yields and significantly increase cycle time.

These developments may influence a number of industries. Diffusion bonding already is used to create intricate forms for the electronics, aerospace and nuclear industries for items such as fuselages, actuator fittings, landing gear trunnions, nacelle frames and nuclear control rods. With the upgrades in equipment capabilities, diffusion bonding is increasingly being utilized for new applications. Products range from turbine blades to medical devices, heat exchangers and even lithium batteries.

Typical materials utilized in products welded via diffusion bonding include stainless steel, titanium, zirconium, beryllium, high alloyed aluminum, Inconel and tungsten. The process also is used to weld dissimilar metals like copper to titanium, copper to aluminum, copper to tungsten and even molybdenum to aluminum.

In addition to traditional diffusion bonding, the process also is being utilized for an additive-manufacturing process called laminated-object manufacturing (LOM). In this approach, thin sheets of metal (approximately 1 to 2 mm thick) are bonded in what is essentially an additive process. The layered sheets can be laser cut so that when combined together, they create cooling channels, for example, that can be used to dissipate heat. The final layered material, with all its layers, also can be machined using traditional CNC turning and milling equipment if needed.

Multi-cylinder diffusion bonding technology can be used to join high strength and refractory metals that are either difficult or impossible to weld by other means. The process involves applying high temperature and pressure to similar and different metals mated together in a hot press, causing the solid metallic surfaces to mix and bond.

Heat Treatment Furnaces with Integrated Presses

Because diffusion bonding is a product of heat and pressure, the heating elements and integrated hydraulic press play key roles in the quality of the final bonded material.

For the atoms of two solid, metallic surfaces to intersperse, they typically must be at approximately 50 to 70 percent of the absolute melting temperature of the materials. To achieve these temperatures, the surfaces are heated either in a furnace or by electrical resistance to temperatures as high as 2552°F (1400°C).

The pressure is applied by a hydraulic press or dead weights. Because the two mating pieces must be in intimate contact with each other, fixtures often are used. Once clamped, pressure and heat are applied to the components, usually for many hours.

Because oxidation also can affect bonding, most heat treatment furnaces operate under a high vacuum.

More Precise Controls

While heat and pressure — and the equipment to achieve the appropriate levels of heat and pressure — are common elements of the process, the missing piece has been precise control of each.

In the case of the pressure applied, for example, integrated single-cylinder hydraulic presses can apply a consistent, measurable amount of force, but this provides little control over large parts with more complex geometries.

To compensate, thick graphite pressing plates (approximately 10 to 15” in height) must be used to mate the layers of metal together at a more consistent pressure. Unfortunately, this takes up furnace space while adding to the time to heat up the surfaces of the metals.

To address these limitations, one manufacturer developed multi-cylinder systems with large pressing plates that can accommodate a range of parts. The largest can process parts as large as approximately three feet by four feet, which is quite large for diffusion bonding. The pressing force is 4,000 kN. By controlling each cylinder independently, the integrated press provides consistent pressure across the entire surface. The multi-cylinder diffusion bonding system includes built-in pressure transducers along the bottom of the pressing plate. Based on the readings, the individual hydraulic cylinders can be adjusted to achieve uniformity even over large areas. In addition, a physical ink-test method can be performed to identify areas on the part where uneven pressure is being applied.

To address temperature uniformity concerns, the multi-cylinder diffusion bonding system utilizes six heaters. The heaters are spaced to help ensure good temperature uniformity within the chamber. Cooling technology quickly reduces press temperatures so parts can be removed more quickly from the press without risk of cracking or other damage. The design features of the multi-cylinder diffusion bonding system also allow thinner fixturing plates (less than 3”) to be used. This frees up space in the furnace and allows for increased cycle times due to faster heating of the surfaces to the desired temperatures.

Because diffusion bonding is a product of heat and pressure, the heating elements and integrated hydraulic press play key roles in the quality of the final bonded material.

Additive Manufacturing Using Diffusion Bonding

As mentioned previously, the diffusion bonding process is being used for additive technology via laminated-object manufacturing. A key application for this technology is conformal cooling.

Parts are designed using traditional 3D-CAD modeling programs, then divided into two-layer layer sections that equal the thickness of each sheet of metal. The processing time is similar to 3D-printing (with a similar investment cost). Without restriction in regard to materials, however, larger parts can be produced.

An application related to conformal cooling is for plastic injection molds made in two-layer designs of tool steel and material such as stainless steel. Conformal cooling channels are cooling passageways that follow the shape or profile of the mold core or cavity to perform a rapid uniform cooling process for injection- and blow-molding processes.

With the multi-layer LOM design, more complex cooling channel designs can be incorporated into injection molds, allowing for higher pressures to be used. This decreases cycle times.

An example of the additive process of diffusion bonding is for heat exchangers, which are usually made from aluminum. Blend circuit heat exchangers typically are made of stainless steel or even titanium and titanium alloys. With LOM, the concept is to bond layers of sheet metal that contain machined micro-channel structures. When combined, the channels can provide for cooling or heat dissipation.

In conclusion, whether applied in layers or simply to bond two parts, the diffusion bonding process is a suitable process for joining refractory and other high strength alloyed materials together without the need for brazing. Although it has been around for decades, with more precise control of temperature and the uniformity of pressure across large parts, diffusion bonding opens up tremendous possibilities for a variety of next-generation products.