Aluminum brazing continues to grow. As it does, I receive questions from people asking me: "Can I use available furnace time in my high-temp aerospace vacuum furnace to do some aluminum brazing?"

It is very fortunate for them that they asked BEFORE they actually tried to do it, since companies should NEVER use their regular high-temperature vacuum furnaces to braze aluminum base metals! Two of the strongest reasons for not doing this are the very tight temperature tolerance control needed for aluminum brazing and the contamination issues presented by outgassing of aluminum brazing products. Let’s begin by looking at the reasons for the needed tight temperature-tolerance control. In a future blog, we’ll address the contamination issues.

Temperature Control

Figure 1 is a typical high-temperature aerospace vacuum furnace with its front door open, showing a number of the internal components. Notice that there are six 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 aerospace-type vacuum furnaces can typically operate as high as about 2400°F (1300°C), and temperature uniformity in the chamber can be controlled by varying the amount of power sent to each of those heating elements. 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 2400°F max, control of the actual temperature in the hot zone may be limited to about +/- 25°F (about +/- 15°C), which means a maximum temperature differential of up to about 50°F (30°C) within that work zone.

Take a look at Figure 2, which compares some aluminum base metals on the left side of the chart with the aluminum brazing filler metals (BFMs) on the right side. Notice that on the left side of Fig. 2 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.

We will continue this discussion in Part 2.