Fig. 1 Vapor Pressure vs. temperature for some common elements. (From AWS Brazing Manual, Third Ed., 1975, p1
As mentioned in previous articles, more and more brazing shops are using vacuum furnaces. These furnaces are quite complex, offering more options for heating, cooling, partial pressure, or multi-bar pressure (pressure capabilities above one atmosphere) for high-speed cooling.
The overall effectiveness of the equipment, however, still lies with the people who program and run the furnaces. Figure 1 (below) shows a series of vapor pressure curves for a number of common metallic elements. Each curve shows the melting point of the element (indicated by a small circle along the curve) and, where the curve crosses the dotted line across the top of the chart, its approximate boiling point (where it wants to turn to a gas) at one standard atmosphere.
When the atmospheric pressure on a metal is reduced (as in a vacuum furnace), the temperature required to begin to vaporize it decreases, as easily noted by the fact that the curves bend downwards and to the left as the level of vacuum increases (the pressure in the furnace decreases). For example, the curve for copper (“Cu” near the top right of the chart) identifies its melting point (small circle on the curve) at 1981ºF (1083ºC), and its vaporization point (where it crosses the dotted line) at about 4700ºF (2600ºC).
By the way, when looking at the chart, note the units of measure for each of the vertical axes on the chart. Do you, the reader, recall what is a “Torr” and a “micron” from my previous article on this website? The chart shown in last month’s article explains these terms, and indicates that a Torr (named after Evangelista Torricelli) is a measure of vacuum equal to 1/760th of atmospheric pressure. Since one standard atmosphere will suspend a column of mercury (Hg) to a height of 760 mm (29.92 inches), a Torr is the name given to the amount of pressure required to suspend that column of Hg to a height of only 1-mm. Thus, the vertical axis on the left side of the chart in Fig. 1 shows pressure in a vacuum as measured in Torr (i.e., mm of Hg). On the right hand side the pressure is shown in microns (a micron is 1/1000th of a Torr).
As the vacuum pumps create a stronger and stronger vacuum in a vacuum furnace and the pressure continues to drop, a “harder” vacuum is created, to use one of the older words in vacuum terminology. However, as noted earlier in this article, this can create problems for a metal such as copper (Cu), which can more easily vaporize (outgas, volatilize) as the pressure in the vacuum furnace decreases.
Look at the curve for copper (Cu) again on the chart. Copper is a commonly used brazing filler metal (BFM) for brazing steel components, and copper brazing is typically performed at high temperatures of about 2025-2050ºF (1110-1120ºC). Note that the curve for Cu crosses the dotted line at the top of the chart around 4700ºF (2600ºC), indicating that copper would require such temperatures in order to begin vaporizing at full atmospheric pressure. But notice, that the “vaporization” curve for Cu drops quickly down and over to the left as the pressures decrease on the chart, indicating that less and less temperature is needed to cause Cu to outgas as the pressure gets less and less. At a pressure of only about 10-4 Torr, it appears that Cu would begin outgassing at only about 2000ºF (1100ºC) or so. Since it is necessary to heat the vacuum furnace to about 2050ºF (1120ºC) for the copper BFM to melt and flow properly, it can be seen that the temperature to properly braze with Cu is actually greater than the temperature where Cu starts outgassing at that level of vacuum! Thus, you could see vaporized copper condensing out on the furnace walls, on electrical connectors, and even on the insulators. Shorting out of some of the internal electrical connections could result over time. Additionally, the loss of copper can result in increased void-content in the BFM.
To solve this problem for Cu-brazing (or for any of the metals shown on the chart), first determine your desired brazing temperature, and then note where that temperature crosses the vaporization curve for that particular metal in question. Then move across to the right-hand axis to see the pressure (in microns) that that particular temperature represents. In order to prevent outgassing of the particular metal in question, the pressure in the vacuum furnace must be greater than that micron-level you just noted, since the level of vacuum you just found is that level needed to just begin the outgassing process for that particular metal. By keeping the pressure higher than that, you can effectively prevent the outgassing from occurring.
In the case of Cu, the suggested brazing temperature of 2050ºF (1120ºC) crosses the Cu-curve at a pressure of a little less than 1-micron. Therefore, it will be necessary to pressurize the vacuum chamber to more than 1-micron in order to keep the copper (Cu) from outgassing.
“Partial-pressure systems” are often built into vacuum furnaces to allow clean, inert gas to be introduced into the vacuum chamber during the brazing process in order to build up the pressure in the chamber to levels high enough to suppress the outgassing of a particular metal or group of metals. Such partial-pressure systems are quite effective. However, they may find it difficult to control pressures accurately at levels of only 1-to-10 microns or so, and it is therefore typical to use “back-fill pressures” (partial-pressures) of about 100-to-500 microns for this purpose.
The same situation occurs for people who try to use silver-based BFMs in vacuum brazing. As with copper, the silver would begin to vaporize well before you would be able to reach the desired silver brazing temperatures, and a partial pressure of gas is definitely needed to prevent “silver-plating” the insides of a vacuum furnace!
Important note: Never vacuum braze with any metal (base metal parts or brazing filler metals) that contains any zinc, lead, or cadmium, since these high-vapor-pressure metals will readily outgas upon heating irrespective of any attempts at using partial-pressures in the furnace to control them.