Burnout Cycles for Vacuum Aluminum Brazing (part 2)

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. 

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 

Important Note 

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. 

Fig. 1. Metal/metal-oxide curves for metals (published in the AWS Brazing Handbook, Fifth Edition, 2007, pp. 120) // Credit: Dan Kay