Vacuum Brazing – A Way to Manufacture Heat Exchangers for the 21st Century

Andreas Engelhardt and John Broadbent,
Thermacore Europe Ltd.

Liquid cold plates can look deceptively simple, but what are the benefits of one technology over another and what are the respective manufacturing processes?  How would one select a manufacturing partner for these products? 

Why vacuum brazing?
Vacuum brazing is a way of producing liquid cold plates and other complex heat exchanger products, offering significant benefits over other joining technologies:

  • Flux free process leads to very clean parts
  • Highly repeatable and controllable batch process
  • Excellent thermal properties and the ability to deploy highly enhanced surface structures inside heat exchangers.
  • Joints can be produced to near parent metal strength, leading to leak and void free parts with high proof pressures.
  • Ability to produce internal joints, even in complex structures, increasing the part’s overall strength.
  • Ability to join large surfaces together
  • Uniform material properties during and after brazing.
  • High temperature resistance of joints

Vacuum brazing as part of the product development process for thermal solutions
In order to produce high performance thermal solutions, Thermacore design engineers begin with the use of industry standard CFD (Computational Fluid Dynamics) software, as well as proprietary spread sheets to determine thermal requirements, such as component temperatures, coolant temperature rise, pressure drops and flow rate optimisations.

Figure 1: Temperature Distribution across liquid cold plate surface

But, as these results alone do not lead towards manufacturable products, engineers need to apply mechanical design rules, which have been developed through thousands of hours of design and manufacturing experience, to each individual product design. This reservoir of experience should be equally applied to new design solutions, as well as on a consultancy service basis to “build to print” scenarios, where potential design flaws are highlighted and possible improvements are suggested.

Figure 2: CAD design of cold plate completed

Once the design is completed, the component can follow the vertically integrated manufacturing route, with machining, vacuum brazing, heat treatment, test and stringent quality control, all under one roof.

Vacuum brazing is one of the key manufacturing processes, and part of the brazing process family, where a filler material is taken above its melting temperature, which exceeds 450°C. This filler material is then fed between close fitting components and distributed by capillary action, or in the case of vacuum brazing is already in place before the heating process begins. Once all the filler material is molten, the part is cooled down and the filler material forms a joint through atomic attraction and diffusion. During vacuum brazing, the parts are placed in a vacuum furnace, which provides the required heat and at the same time removes the need for flux and the risk of oxidisation during the process. Therefore the components will be clean and bright after brazing.

Figure 3: Vacuum Brazing Furnace

Depending on the requirements of the part and its complexity, a Helium leak check can be part of the standard procedure at this stage. Once leak tightness has been ensured and when required, the parts will undergo heat treatment to either a T4 or T6 hardness stage. This will allow machining at similar feeds and speeds as those for untreated Aluminium. Depending on individual part geometries, additional processes such as straightening may allow more effective ways of machining the finished components when required.

Typical Products
Products which are typically vacuum brazed are liquid cold plates, flat tube heat exchangers and plate fin heat exchangers, but certainly not limited to these components. Generally vacuum brazing is employed as a process, where lightweight, high performance assemblies with increased internal surface areas are required. Additionally, vacuum brazing can be used to produce larger assemblies with complex geometries, benefiting from the low distortion introduced by the process itself. These structures can range from plank type assemblies with internal channels to electronic enclosures, with and without internal features such as cold walls.

Figure 4: Cold Plate modified for demonstration purposes

All of the afore mentioned products benefit from the high internal cleanliness due to the fluxless process, and the lack of other substances involved in the process.  Wave guides are amongst the non-thermal products, that are also regularly produced by vacuum brazing.

With any of the products mentioned above, early involvement of Thermacore engineers not only reduces the time to market, it may also significantly reduce the number of design iterations required to achieve the desired product quality. Thermacore carefully controls all its manufacturing processes in order to achieve and maintain a 98% plus yield on proven out designs and products. The application of proven design rules leads towards an improved product from the onset of a project, a service, which is rarely found with normal contract brazing houses.

Markets
The market segments that vacuum brazed assemblies can be found in are quite diverse. They can range from automotive products, such as oil cooler type assemblies, at the lower cost end of the market, towards highly complex assemblies, with reduced weights, for the aerospace or defence industries.

Recently, the power market and renewable energy technologies as well as the emerging market of electrical and hybrid electric vehicles have shown a significant interest in vacuum brazed assemblies to cool their components. These include components such as batteries and dc-dc converters but are not limited to these. Generally, vacuum brazed assemblies can be found in markets, where good quality joints, lightweight assemblies and high thermal and mechanical performances are required.

Summary
Vacuum brazing is an attractive manufacturing method to produce lightweight and reliable assemblies and thermal products, such as heat exchangers and liquid cold plates. Vacuum brazed products are of a superior quality in comparison to other joining technologies such as conventional salt dip brazing and TIG welding. Additionally, there are no further requirements for internal cleaning of the assemblies and contamination risk of the system due to salts and others are non-existing. There is less distortion and a cleaner exterior due to the fluxless process. Superior thermal performance can be achieved through the ability of producing complex internal geometries and significantly increased heat transfer surfaces.

For more information please visit www.thermacore.com

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