For many years, diffusion bonding has been utilized to join high strength and refractory metals that are either difficult or impossible to weld by other means. The process applies high temperature and pressure to similar or dissimilar metals mated together in a hot press, causing 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 and eliminates the need for filler material that affects the final weight and dimensions of the mated metals. Despite these benefits, the use of diffusion bonding has been limited by more practical considerations involving 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 a full day. However, advances in high vacuum hot presses used for diffusion bonding are eliminating many of those constraints.
Sophisticated equipment now provides superior pressure control, feedback from embedded pressure transducers, physical ink tests that show variations in pressure across the surface and rapid cooling systems to improve the bond, increase yields and significantly increase cycle time. This has ramifications for an increasing number of industries. Diffusion bonding is already used to create intricate forms for electronics, aerospace, and nuclear applications such as fuselages, actuator fittings, landing gear trunnions, nacelle frames and nuclear control rods. But now it is increasingly being utilized for new applications ranging from turbine blades to medical devices, heat exchangers and even lithium batteries. Typical materials utilized in these types of products that are welded by diffusion bonding include stainless steel, titanium, zirconium, beryllium, high alloyed aluminum, Inconel and tungsten. The process is also used to weld dissimilar metals such as copper to titanium, copper to aluminum, copper to tungsten and even Molybdenum to aluminum.
Because diffusion bonding is a product of heat and pressure, the heating elements and integrated hydraulic press play a key role in the quality of the final join. For the atoms of two solid metallic surfaces to intersperse, they typically must be at approximately 50 percent 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 1,400 deg C. Pressure is applied by a hydraulic press or dead weights. Because the two mating pieces must be in intimate contact with each other, fixtures are often used. Once clamped, pressure and heat are usually applied to the components for many hours. Because oxidation can also affect bonding, most heat treatment furnaces operate under a high vacuum. While these are common elements of the process, the missing piece to date has been precise control of each one.
Regarding the pressure being applied, for example, integrated single-cylinder hydraulic presses can apply a consistent, measurable amount of force but provide very little control over large parts with more complex geometries. To compensate, thick graphite pressing plates that are 10 in to 15 in tall 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 cycle time to heat up the surfaces of the metals. However, leading manufacturers such as PVA TePla America (Corona, CA) offer multi-cylinder systems with large pressing plates that can accommodate a variety of parts. The largest of these integrated press systems, the MOV 853 HP, uses 4,000 kN of pressing force to process parts as large as 900 mm (35.43 in) x 1,250 mm (49.21 in), which is quite large for diffusion bonding. By controlling each cylinder independently, this integrated press provides extremely consistent pressure across the entire surface.
The MOV comes with 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. PVA TePla has optimized a physical ink test method that can be performed to identify areas on the part where uneven pressure is being applied. For greater temperature uniformity, the MOV system utilizes six heaters for temperature uniformity within the chamber, instead of the usual one or two, for a maximum operating temperature up to 1,400 deg C. Rapid cooling quickly brings temperatures down so that parts can be removed in about half the time without risk of cracking or other damage. With superior temperature control and multiple hydraulic cylinders in the press, much thinner fixturing plates (less than 3 in) can be used to free up space in the furnace and allow for increased cycle times through faster heating of the surfaces to the desired temperatures.
That’s not all. Diffusion bonding is also being utilized for a revolutionary additive manufacturing technology called Laminated Objection Manufacturing (LOM), where thin 1 mm to 2 mm sheets of metal are bonded in what is essentially an additive process with tremendous potential applications for conformal cooling. 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 system, more complex cooling channel designs can be incorporated into injection molds, allowing for higher pressures to be used. This significantly decreases cycle times by as much as 40 percent while enhancing product quality.
Parts are designed using traditional 3D CAD modeling programs, then divided into two layer sections that equal the thickness of each sheet of metal. These layered sheets are laser cut and then combined together to create cooling channels used to dissipate heat. The final “sandwich” with all of its layers can then be machined to specification as needed using traditional CNC turning and milling equipment. The processing time is similar to 3D printing, with a similar investment cost. However, larger parts with no restriction in regard to materials can be produced. Heat exchangers, which are usually made from aluminum, are a prime application for using this additive process of diffusion bonding. Blend circuit heat exchangers are typically 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, these channels can provide for cooling or heat dissipation. The layers can be bonded up to a height of 600 mm in the MOV diffusion bonding press, retaining their strength as the parent materials.
Another application related to conformal cooling are plastic injection molds made in two layer designs of tool steel and material such as stainless steel (Stavax). Whether applied in layers or simply to bond two parts, the diffusion bonding process is an ideal process for joining refractory and other high strength alloyed materials together without the need for brazing. Although it has been around for decades, diffusion bonding with more precise control of temperature and the uniformity of pressure across large parts opens up tremendous possibilities for a variety of next generation applications.
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