Published on 6/12/2019
Categories: Automotive

Automotive sector: vacuum brazing furnace applications

Automotive sector: vacuum brazing furnace applications

Before we examine just how useful vacuum brazing furnaces can be for car, truck and motorcycle manufacturers, it is probably best to define exactly what this process is for those who are not yet experts in this particular field.
Essentially, brazing makes two metals bond with one another by applying a metallic filler. For a good join to be made, this filler must possess a lower melting point than the temperature than either of the two metals that are being joined would liquefy at. When the filler is melted it must make physical contact with either side of the join, forming a solid bond once it solidifies.

Obtaining a Strong Joint

In most cases, this is achieved by vehicle manufacturers by using the right sort of filler metal for the particular join. In fact, there are plenty of options to choose between - the filler could come in the form of a wire, a thin plate or even a paste, for instance. It simply depends on the sort of application that the process is being conducted for.

Of course, in order to achieve a high-quality bond - both for physical properties of stress resistance and the ability to withstand corrosion - it is preferable to have a highly controlled environment. This is because grime and even airborne particles could contaminate the brazing process, something that the leading players in the automotive sector wish to avoid at all costs.

This is where vacuum brazing comes in which is what we will discuss next.
Later, you will also discover how the leading vacuum brazing furnace technologies work to ensure that the stringent quality control measures required of all metal joins in the automotive sector are maintained.

Automotive Heat Treatments and Vacuum Furnace Technology

For an effective brazing heat treatment, the cleanest level of atmosphere should be obtained which, for a highly precise engineering sector like the automotive industry, really means generating a vacuum.
Because a vacuum furnace can minimise the contamination of any impurities in the local environment to virtually zero, the joined metal will be high mechanically resistant and long-lasting. This is especially important for car makers where complicated parts must be connected to one another, often with non-standard shapes.

Given that the automotive sector also relies on many joining points in a relatively small space, vacuum furnace brazing also makes a great deal of sense. As such, the industry will often use this heat treatments for applications such as water heat exchangers joints. In the automotive sector, the use of aluminium as a principle element for top-notch heat exchangers is becoming ever more popular.
Both AA 3xxx and 4xxx aluminium alloys are commonly used materials in the sector, for example. The main reason why these particular alloys are so useful is that they afford a very low specific weight whilst still offering a high level of thermal conductivity.

As outlined above, one of the cleanest environments for precision brazing applications in the automotive sector is a vacuum. Therefore, readers with an interest in automotive heat treatments and aluminium alloys, in particular, should read on for a more in-depth analysis of the individual properties of semi-automatic vacuum brazing furnaces which makes them so suited to the common applications required of the automotive industry.

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Three Station Design for Vacuum Brazing Furnaces

The brazing furnaces made by TAV VACUUM FURNACES are ideal for the automotive industry because their design is composed of three distinct stations. These are:

  • A precision heating furnace
  • A functional loading station
  • A useful cooling station

To understand how these three stations function in ways which suit the vehicle manufacturing sector, we will deal with each in turn.

Heating Furnace Technology

Furnace Vessels

The vessel is an essential part of a furnace’s world-class heating process technology. This is because it makes a separate enclosure that isolates the internal section of the furnace.
Essentially, it contains the hot chamber and prevents the outside environment from interfering with the brazing process.
In terms of its design, the furnace has an extended horizontal axis, an elliptical shape, ideal for many automotive heat treatments.
There are a pair of flat doors, one at the front and one at the rear, for ease of access. These can be operated manually. In addition, TAV's vacuum brazing vessels have a front door which operates automatically.
This is used for both loading and unloading the vessel.

Hot Chambers

The hot chamber in a heating furnace is constructed from high-quality panels made of stainless steel with resistors fashioned from nickel-chrome. The hot chamber comes with dimensions of 180cm by 45cm by 365cm and inside there are in excess of 20 independently controlled heating zones.
Together, these make the hot chamber extremely controllable with the ability to fine tune the temperature, just as required. In fact, maintaining the uniformity of heat in the chamber is extremely important for automotive vacuum furnace applications.
Therefore, vacuum furnaces should never vary by more than three degrees from the set temperature in either direction. As this survey graph adequately demonstrates, the desired ±3°C temperature uniformity is achieved even with repeated use.

TUS

TUS example at a specific temperature with 12 TLC

Vacuum Systems

The final part of the vacuum furnace technology that should be discussed is the vacuum system itself.
After all, it is this which allows the hot chamber and the furnace vessel to do such a good job.
There are three pumping groups to ensure a good vacuum is maintained while the furnace is in use.
Two groups are made up of pair of rotary piston pumps. One is a positive displacement lobe pump, known as a Roots pump, and the other is an oil diffusion pump.
The third pumping group comprises a conventional mechanical pump, another Roots pump as well as a cryo-trap. These are deployed to remove unwanted humidity in addition to any impurities that might be released into the chamber during a treatment.
Without a load, the vacuum level that can be achieved with this vacuum system is an astonishing 10-6 mbar (range).

Loading and Cooling Stations

The other two stations needed to conduct vacuum brazing in a precise manner are loading and cooling stations.

Expertly designed steel shelves fixtures hold the required parts. They help to keep them together along with the filler metal that will be used for the brazing process itself.
Any load from 900 kilos up to 2,200 kilos can be processed simultaneously with these loading stations.

When a brazing heat treatment has been completed, the load is automatically transferred to a cooling station chamber. This is where the processed metal joins can be cooled down in a controlled manner with recirculated air.

Key Heat Treatment Considerations

Although you will, by now, have gained some useful insights into expert automotive heat treatments in vacuum environments, it is also worth considering a few other factors.
For example, prior to the required brazing temperature being reached, it is essential that the load should be kept at a lower temperature until all of the working oil plates from the heat exchangers has been removed.

Brazing cycle

Typical brazing cycle. Line yellow is the setpoint, line orange is the temperature TC, line blue is the high vacuum level and purple line is the partial pressure in mbar detected.

Another factor comes after the brazing process itself.
After the brazing step, the furnace reaches high nitrogen static partial pressure starting the cooling phase, as indicated in the following graph. After completion, furnace's front door opens automatically and loading station can be removed from within.

Conclusion

There are many applications that the automotive sector can put vacuum brazing technology to. However, it is only reliable when high-performance vacuum furnaces are used.
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