Can I reduce aluminum oxides at 1600˚F? 

At the start of this blog series, 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. 1. Metal/metal-oxide curves for metals (published in the AWS Brazing Handbook, Fifth Edition, 2007, pp. 120) // 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.