Published on 5/11/2017
Categories: Aerospace

Vacuum diffusion bonding: a true success story

Vacuum diffusion bonding: a true success story

This is not just a technical article, but it is a story of passion, commitment, progress and challenge. The article provides competent information on diffusion bonding vacuum furnace, coming both from experts and other sources, along with their direct experience concerning the problem they face. Having said that, before discussing this matter, we must know the basics of diffusion bonding technology, in order to see how it works.

Diffusion bonding: understanding the basics

Diffusion bonding is the joining of two surfaces by the action of temperature and pressure. Hence, melting and melting related defects are avoided in the diffusion bonding process. Since the bonding pressure is well below the yield stresses of the material, bulk plastic deformation of the materials are completely avoided.

Vacuum diffusion bonding relies on temperature, pressure, time, and vacuum levels to facilitate atomic exchange across the interface between the materials. The process will work on similar or dissimilar materials so long as they are in intimate contact with one another.

The process produces high strength joints suitable for use by a variety of industries, including aerospace, automotive, shipbuilding, oil, petrochemical and process engineering. Joints are perfectly leak-proof and capable of withstanding high service temperatures and/or ultra low vacuum levels. Common applications include the production of shim assemblies for mini or micro-channel devices (used for manifolds, biomedical implants, nozzles, mixers, and other precision assemblies).

Diffusion bonding in a vacuum: the practical difficulties

More than twenty years ago, TAV formalized the first order of a vacuum furnace for the Titanium heat exchangers diffusion bonding.

The furnace dimensions request were big for a long, large and high charge. High vacuum, high gas (Ar) pressure and high temperature uniformity were requested. The vessel was with high body thickness and under pressure certification control. A very tough challenge: at that time the diffusion bonding in a vacuum was the first application of an innovative technology.

In vacuum, with the same insulation thickness overall on the heat chamber sides and because of the typical emissivity of a black body (as in the case of the black graphite panel), it would have been easy enough to obtain a good uniformity. On the contrary with the presence of gas we find a colder zone in the bottom and a hotter zone on the top. Indeed, the presence of gas in the hot zone changed completely the conditions to get a good uniformity. The requested uniformity was a difficult target in presence of convective flows in the hot zone and outside the heat chamber.

Also the power dissipation and heat transfer to the internal wall of the vessel were a problem.

Moreover, the use of fans, which is the most common solution to temperature uniformity problems with gas, proved not to be suitable for this specific application. In fact, at such high pressure, even the clearance around the fan shaft would be a too big gas leak, causing hot gas leaving the chamber and cold gas going in from outside.

Also, trying to contrast the natural convection flows with an even higher forced flow controlled through the fans would have further increased the thermal dissipation of the chamber.

This challenging task required a combination of experience and innovation to find a solution. How could we handle such concerns regarding the vacuum furnace for diffusion bonding?

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Vacuum diffusion bonding: structural features of the equipment

The intuition of TAV engineers, supported by several tests, was to try limiting the natural convective flows rather than increasing them with a forced convection.

The reason why there is a convective recirculation is that the hot gas, once it has reached the top of the heat chamber because of its lower density, dissipates heat through the isolation on the roof, so it becomes colder and returns to the bottom of the working volume. In order to prevent this, the heat chamber insulation made by board of graphite has required different thickness to uniform the heat losses in all internal surfaces. The insulating material biggest sections have been used on the higher levels of the hot chamber.

Limiting the convective flows helped to limit the dissipation and to create more reproducible process conditions with load of different geometries, but didn’t resolve by itself the uniformity problem. Therefore, we decided to also distribute the radiating heating elements with a not uniform disposition and with different heating power in the internal surface. This to avoid the overheat of the top zone.

Then, to protect the internal wall of the pressurized vessel from the high temperature, we decided to install an additional water cooled heat exchanger in direct contact with the vessel wall.

The calibration of all these different applied solutions has allowed to reach an easy the temperature uniformity result.

This has provided us with a lead in knowledge, technique and methodology which we have used to create a complete range of diffusion bonding vacuum furnaces.

This article is just a brief introduction to diffusion bonding. As the process is conducted under a vacuum, a diffusion-bonded joint has a minimum of impurity content, even in the case of highly reactive metals. The process has been used most extensively in the aerospace industries for joining materials and shapes that otherwise could not be made (such as honeycomb construction and multi-finned channels).

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