Diffusion Bonding: The Equipment (part 2)
The Doctor has always been intrigued by the innovation and design capability of engineers. Nowhere is this better illustrated than in equipment used for diffusion-bonding applications. These designs demonstrate ingenuity as well as the ability to adapt conventional vacuum units into a technology with proven performance. Let’s learn more.
Multiple stacked component layers (Fig. 1) are typical in vacuum hot-press applications (Fig. 2), which utilize diffusion bonding. Since many metals are easily oxidized, the process is commonly run in specialized vacuum furnaces operating in the range of 10-4 to 10-6 Torr or in a hydrogen partial-pressure atmosphere. Hot-press furnace systems operate at temperatures from 400-1230°C (760-2250°F) with up to 30 tons (60,000 pounds) of compacting force or higher. Specialized systems are capable of extending the maximum operating temperature, pressure and vacuum levels.
What we know from our discussion about the process (Industrial Heating, April 2017) is that diffusion bonding can unintentionally occur anytime parts are held in intimate contact with one another at a sufficiently high temperature such that interatomic diffusion occurs between the surfaces. In vacuum hot-press diffusion bonding, however, this process is intentional and aided by the application of both temperature and pressure.
In general, vacuum levels can be reduced near to, or just below, the boiling point of any of the major alloy constituents present in the materials being processed. This can be predicted from Ellingham diagrams (Industrial Heating, April 2011). The mass transport rate for the highest vapor-pressure alloy constituent is used to assist in determining bonding rates and controlling the quality of the bond.
Many diffusion-bonding operations are conducted using simple dead weights placed on the top of the workload, which is then placed inside the furnace on a base plate. This ensures that all surfaces of the bond interface are in intimate contact. For thicker parts or stacks where residual stress may be present from prior manufacturing operations, however, a natural stress relief occurs upon heating and often results in uncontrolled distortion. In these instances additional force is required to maintain flatness, making the use of dead-weight loads impractical.
Over the years, the need for larger stacks has resulted in the routine use of tooling methods that make use of either a differential expansion mismatch or some type of active-loading system, such as pressure bellows or pneumatic/hydraulic ram systems that can be used to apply the necessary amount of force.
With bolted differential expansion or so-called caged-tooling designs, the parts almost always suffer from the fact that the maximum load is applied before reaching the final bond temperature. This can result in part distortion (due to strain). Since the parts are also under significant stress during heat-up, surface degradation and bending/bowing (often referred to as “potato chipping”) may result. This is due to differential expansion (shear stress working against the outwardly growing surface layers) during heating and cooling. One indication that this is happening is galling tracks on the part surfaces.
The need for better control of vacuum level and applied load stress has led to the development of more advanced vacuum hot-press systems. These are conventional vacuum furnaces with the addition of external rams (Fig. 3) and a hot platen inside the furnace. The arrangement is similar to an arbor press. Flat platens distribute the load uniformly onto all part surfaces. The advantages of uniform force are significant from both a mechanical and metallurgical perspective.
A typical small hot press (Fig. 4) would have features such as:
- 300 x 300 mm (11.8 x 11.8 inches) refractory-metal pressing plates
- 150 kN (33,720 pound-force) pressing force
- 1350°C (2460°F) maximum temperature
- Mid-10-6 Torr (10-6 mbar) ultimate vacuum
Recent advances in the diffusion-bonding industry have led to the development of vacuum hot-press systems that include gas-quench cooling capabilities with internal heat exchangers so that the parts may be diffusion bonded and then quenched to optimize the post-bond heat treatment (solution annealing, aging or hardening) and resulting material properties. Typical features of these specialized systems might include:
- 2,275-kg (5,000-pound) load capacity
- 914 x 1,220 mm (36 x 48 inches) pressing platen footprint
- 890 kN (200,000 pound-force) pressing force
- 1455°C (2650°F) maximum temperature
- Mid-10-6 Torr (1x10-6 mbar) ultimate vacuum
- Partial-pressure control for hydrogen
- 2-bar gas-quench cooling
Many different tooling and bond joint-preparation methods can be used in vacuum diffusion bonding. Ultimately, the end user must make the decision on the best method(s) to employ to minimize part production cost. This decision is often driven by such factors as part geometry, allowable strain, resultant microstructure or simply the total number of parts that must be produced.
Vacuum diffusion bonding has made great strides in recent years and overcome many of the problems that plagued the technology in the past (e.g., dirty materials, uneven contact surfaces, equipment design limitations and equipment design flaws). As such, it has become a robust technology and one worthy of consideration by the design engineer.
- Herring, Daniel H., Vacuum Heat Treatment, Volume II, BNP Media, 2016
- Norm Hubele, Refrac Systems (www.refrac.com), technical and editorial contributions and private correspondence
- Tom Hart, SECO/VACUUM TECHNOLOGIES, LLC (www.secovacusa.com), technical contributions and private correspondence
- Wolfgang Rein, PVA Industrial Vacuum Systems GmbH (http://www.pvatepla.com), technical contributions and private correspondence