Someone recently suggested to their aluminum-brazing client that they should conduct a high-temperature burnout cycle in their vacuum furnace by “heating the furnace to 1600°F to ensure that all the oxides are removed.” Unfortunately, this statement is not correct since 1600°F (870°C) is not high enough to remove (dissociate) oxides of metals such as aluminum, magnesium and/or titanium from any vacuum-furnace walls or hot zone. Additionally, many vacuum furnaces used for aluminum brazing are simply not designed to be able to reach temperatures as high as 1600°F.
When using vacuum furnaces designed for brazing aluminum, any so-called “burnout” cycles (clean-up cycles) are not actually designed to remove oxides. Instead, they are supposed to remove any surface contaminants (oils, lubricants, fingerprints, etc.) that may have volatilized during a brazing cycle and then condensed onto the water-cooled furnace walls during normal brazing operations. To remove these kinds of surface contaminants, burnout cycles using very high temperatures are not required. However, implementing clean-up cycles using temperatures of only 1300-1400°F (700-760°C) should be more than adequate.
Figure 1 shows the well-known metal/metal-oxide (M/MO) chart. Each of the curves shown on the M/MO chart represents an oxide of a particular metal. Although the chart was originally created to be used for hydrogen atmosphere furnaces, the curves have been shown to be relatively accurate for a variety of atmospheres (argon, nitrogen and helium) and for vacuum.
To remove (dissociate) any particular oxide, the furnace conditions must be such as to allow the furnace to operate well to the right of that particular oxide curve by at least the diagonal of one of the little boxes that makes up the graphical section of the chart. Note that each of the oxide lines represents the peak of a bell curve, so to speak, and this “peak” can shift slightly for different atmospheres. Therefore, to take into account all the previously mentioned atmospheres, a furnace should be operated well to the right of any particular metal-oxide curve.
For example, when brazing 304L stainless steel in a hydrogen atmosphere furnace, a major concern would be the formation of chromium-oxide when heating because chromium reacts quickly with oxygen when heated to form stable chromium oxides, which gives 304 stainless its corrosion resistance when used in a variety of end-use service conditions.
Fig. 1. Metal/metal-oxide curves for metals (published in the AWS Brazing Handbook, Fifth Edition, 2007, pp. 120) // Credit: Dan Kay
However, brazing filler metals (BFMs) cannot effectively bond to oxides. Therefore, it is necessary to get rid of (i.e., reduce/dissociate) that Cr-oxide so that brazing can occur. As shown in Fig. 2, when brazing is performed in hydrogen at 2000°F (~1100°C) and the dew point of the hydrogen atmosphere is measured as it enters the furnace to be -60°F (-50°C) or drier, then the chart shows that the brazing process will indeed be operating at least one diagonal to the right of the Cr-oxide curve, allowing the Cr-oxide to dissociate. Good brazing should then be able to take place.
As seen in Fig. 1, there are many metal-oxides that can be dissociated (reduced) in vacuum at temperatures at or below 1600°F (850°C). This means that if the temperature/vacuum-level combo used is at least one diagonal to the right of any particular metal-oxide curve on that chart, that oxide should be able to be dissociated/reduced.
Many people who use Fig. 1 to determine appropriate dew-point/temperature combinations (or vacuum level/temperature combination) for their brazing operations make the mistake of assuming that they will eliminate (reduce) that oxide as long as they are slightly to the right of a particular oxide curve. They assume that the oxide line on the chart in Fig. 1 is a highly accurate, very thin and narrow target line that will, in fact, result in dissociation of that oxide when the furnace cycle operates ever so slightly to the right of that line.
Unfortunately, that is not at all true. The oxidation lines shown in Fig. 1 are approximations/guidelines only. To ensure that a given oxide is effectively dealt with (and reduced), the brazing cycle must be selected such that furnace operation is carried out well to the right of the oxide curve in question by a minimum of at least one diagonal or more.
Can I reduce aluminum oxides at 1600˚F?
At the start of this article, a question was raised about the ability to remove/dissociate aluminum oxides by heating a vacuum furnace used for aluminum brazing up to about 1600°F (870°C). Look at Fig. 1 once again and notice that the curve for aluminum oxide is at the far right on that chart, along with the oxide curves for titanium and magnesium.
According to Fig. 1, if someone was aluminum brazing in a vacuum furnace at approximately 10-5 torr and they desired to operate one diagonal to the right of the aluminum-oxide curve, they would need to heat their vacuum furnace up to about 2800°F (~1550°C), which is physically impossible. It must be understood that once aluminum oxide forms in a brazing operation, it cannot be removed by any type of thermal process in that furnace.
Fig. 2. Notice that the circle placed at the junction of lines drawn from -50°C dew point and 2000°F (~1100°C) is just over one diagonal to the right of the chrome-oxide line // Credit: Dan Kay
Guidelines for Furnace Burnout (Clean-up) Cycles
When considering furnace burnout/clean-up cycles, it is important to operate a brazing furnace at a temperature that is at least 100°F (50°C) above the highest processing temperature that is used in that furnace (whether that be for brazing, heat treating or another process) and to hold the furnace at that temperature for a minimum of one hour or longer in order to allow effective volatilization of any materials that may have condensed onto the inside walls during previous heating cycles. Such burnout cycles should also be able to remove/volatilize materials from the hot-zone cage (heating elements, connectors, etc.). Since the hot-zone cage often consists of a number of layers of material, it is important to hold the high-temperature cycle for at least one hour or more to be able to penetrate into those layers.
A furnace burnout cycle is not designed to remove (dissociate) oxides. Instead, it is designed to remove materials – such as oils, lubes, base-metal outgassing products, etc. – from the furnace walls that may have condensed on it over time.
Dan Kay operates his own brazing consulting practice in Connecticut (since 1996) and has been involved in brazing for almost 45 years. He received his BS in Metallurgical Engineering from Rensselaer Polytechnic Institute and his MBA from Michigan State University. Dan has been writing brazing blogs for www.industrialheating.com for 13 years.