1. Temperature Control

Shown above is a photograph of a typical hi-temp aerospace vacuum furnace, with its front door open, showing a number of the internal components. Notice that there are six (6) heating elements connected around the OD of the hot-zone, but there are none on the front door, or on the back wall of the furnace (the back wall does not open). These types of furnaces represent many of the standard aerospace vacuum furnaces being used today, but newer ones, such as those made by Vac Aero (photo below), often are supplied with heating-elements on the door and on the back wall.

These aerospace-type vacuum furnaces can typically operate as high as about 2400F (1300C), and temperature uniformity in the chamber can be controlled by varying the amount of power sent to each of those heating elements in the hot zone. In such a case, each heating element might then be called a separate heating “zone”. Sometimes more than one heating element might be controlled together, thus creating a multiple-element “zone” of heat control. Please note that the more heat “zones” a furnace can provide, the better should be the potential for refined temperature control and temperature uniformity in that furnace chamber.

For standard vacuum furnaces operating up to about 2400F (1300C) max, control of the actual temperature in the hot zone may be limited to about +/- 25F (i.e., about +/- 15C), which means a maximum temperature differential of up to about 50F (30C) within that work zone. However, take a look at the chart below, comparing some aluminum base metals on the left side of the chart with the aluminum brazing filler metals (BFMs) on the right side of the chart.

Fig. 1

Fig. 1

Notice that on the left side of Fig. 1 the lines for the approximate melting temperature (solidus temp) of the aluminum base metals are included with the suggested brazing range for those base metals. It is interesting to see that some of these aluminum materials need to be brazed at temperatures that are only about 10-50 F (5-30 C) below the melting points of those base metals! The reason for this is that, unfortunately, the aluminum-based BFMs available for this kind of work can only melt at temperatures very close to that of the base metals. (Note: research work continues in an effort to develop effective BFMs that operate at lower temperatures).

Since aluminum brazing in a vacuum furnace, therefore, can take place only a few degrees below the melting point of the base metals, furnaces that would be used for this kind of work must be capable of very tight temperature controls, much tighter than that which can be achieved in standard high-temp aerospace-type vacuum furnaces.

Horizontal aluminum vacuum furnace

A typical horizontal aluminum vacuum furnace with heating elements on the front and rear of the furnace (door and back wall of the chamber).

To achieve the very close temperature control needed for aluminum brazing, aluminum-brazing vacuum furnaces typically have many more controllable heating zones than a typical aerospace vacuum furnace. Not only might there be more heating elements around the wall of the furnace, each one separately controlled, but additional heating elements are typically added both on the front door of the furnace and on the back wall of the furnace (the back wall can often be opened as well). With all these closely controlled heating zones available, an aluminum vacuum brazing furnace can typically control temperatures to about +/- 1 degree in an empty condition, and about +/- 5-10F (i.e., about +/- 2-5C) when loaded with parts.

2. Furnace Contamination

Aluminum and magnesium can easily outgas and contaminate a vacuum furnace. This could be a major issue for aerospace furnaces used for stainless steels, etc., since any contamination from aluminum alloys can totally ruin future furnace runs for stainless steels and superalloys.

I have personally seen companies make the mistake of trying to braze with aluminum-based BFMs in regular aerospace furnaces, using as their reason for doing so the fact that they were merely trying to braze titanium components together using an aluminum BFM, and so, were not concerned by the lack of overall temperature control of the furnace, since the melting point of the aluminum BFM was well below the melting temperature of the titanium alloy being joined.

However, the outgassing contamination from the BFM, and from the magnesium getters they mistakenly added to the furnace to supposedly improve the furnace atmosphere for aluminum brazing, ruined the hot zone, aluminized the insulators, put a lot of mag-coating on the furnace walls that continually outgassed whenever the furnace was used, etc., thus rendering the aerospace furnace useless for regular aero-brazing.

Conclusion

If you desire to do any kind of aluminum brazing, whether its to join aluminum base metals together or other metals such as titanium, I strongly recommend that you NEVER use a regular aerospace-type vacuum furnace for that purpose, but instead, use only a vacuum furnace built specifically and only for aluminum brazing.

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