All metals want to react with oxygen as the metals are heated. The higher the temperature, the greater the thermodynamic driving force to have those oxides form.
This is true for all metals, even though the oxides of some metals are not as stable as the oxides of other metals. Gold and nickel are examples of metals whose oxides are not stable at any temperature we would encounter in our daily activities, and thus, do not concern us at all. Copper oxides and iron oxides are examples of metals whose oxides are not stable at higher temperatures, in that those oxides are easily and quickly dissociated at elevated temperatures. Chromium-oxide, however, is an example of a fairly stable oxide (up to about 1850F/1000C before dissociating in a typical brazing atmosphere furnace), whereas aluminum-oxide will be extremely stable in a brazing furnace, and is beyond the capability of any standard brazing atmosphere to reduce that oxide. Titanium-oxides behave in a very similar fashion to aluminum oxides in typical brazing furnace atmospheres.
Brazing filler metals (BFMs) do not like to bond to, or flow over, dirt, oils, or surface oxides, and consequently, if any such surface contaminants are present on the faying surfaces inside a braze-joint, any externally applied BFM will find it hard, if not impossible, to flow by capillary action into that braze-joint. Instead, it will remain outside the joint. If the BFM is pre-placed inside the faying surfaces of a joint that is still contaminated with dirt, oxides, etc.,, the BFM may merely “ball-up” and not flow through the joint.
Cleanliness of both base metal and BFM is essential for a successful braze, since, as just mentioned, BFM will not “wet” surfaces that are dirty or contaminated with oxides. Yet, in spite of this, I often hear brazing-shop personnel say: “Don’t worry about that, the furnace will clean it up”, which is, unfortunately, a grave mistake, since I have rarely seen that happen! Yes, a typical furnace atmosphere (gaseous, or vacuum) may be able to “reduce”, i.e., eliminate, surface oxidation on the outside surfaces of components being brazed, but I defy anyone to truly be able to deep-clean surface contamination from the faying surfaces inside a joint once the parts have been assembled and the BFM applied to the joint. I have seen many such assemblies fail to braze properly, because the entrapped oxides, dirt, etc., inside the joint prevent the BFM from wetting those surfaces and properly flowing through a braze-joint.
RULE OF BRAZING: ALL surfaces to be brazed must be cleaned PRIOR to brazing, and then KEPT CLEAN by assembling with clean gloves!
Once assembled and placed in a brazing furnace, the furnace atmosphere will keep the parts clean, and allow brazing to take place.
Oxygen is present in ALL furnace atmospheres, including vacuum! Gaseous atmospheres, such as hydrogen, nitrogen, or argon, all have some moisture present in them as they are being delivered to the brazing furnaces from their storage tanks. The amount of this moisture content is measured by the dewpoint of the gas when it reaches the brazing furnace. A typical gaseous atmosphere might have a dewpoint of -60F/-50C when it reaches the furnace. At one standard atmosphere, the dewpoint of a gas is the temperature to which that gas must be cooled to get the first drop of moisture to condense (much like at night when the warmer ground of the earth causes “dew” to form when the cool night air touches the warmer ground).
A typical vacuum brazing furnace will operate at a pressure of approximately one-tenth to one-hundredth of one millitorr (see chart in Figure 1.)
At such a vacuum level, the amount of moisture present in the “atmosphere” remaining in the vacuum chamber is extremely small, creating a level of “dryness” far better than that which can ever be achieved in any gaseous-atmosphere brazing furnace available. Thus, your everyday, average vacuum brazing furnace (assuming of course that it is well maintained) will easily be able to achieve and operate with “low-oxygen atmosphere” levels that cannot be obtained in any kind of standard gaseous brazing atmosphere.
This then is the major reason that more and more people are using vacuum furnaces for brazing! It’s interesting to note that a reasonably “dry” (low-oxygen/low-moisture) atmosphere level can be obtained in a vacuum furnace with just the mechanical pumps alone (2-stage blower/mechanical-pump), even before a diffusion pump begins to operate! It is typical in many vacuum furnaces that there will be an automatic switch-over to the diffusion pump (or cryopump) when the pressure in the furnace gets down to about one millitorr or less. For some applications, brazing at about one millitorr is adequate to achieve excellent results (e.g., copper brazing of carbon steel parts). If the secondary diffusion pump is needed, it can further reduce the pressure to whatever level of vacuum the diffusion pump is sized to achieve.
As the chart in Figure 1 shows, reduced atmosphere is still popularly measured by terms such as “Torr” and “Micron”(millitorr). A “Torr” (named after Evangelista Torricelli who is credited with first quantifying barometric pressure) is approximately 1/760th of one standard atmosphere, since one standard atmosphere will suspend a column of mercury (symbol: Hg) to a height of 760-mm in a glass tube. Thus, one Torr is equal to the pressure required to lift the column of Hg to height of 1-mm, etc.
Many people today still like to use some of the older vacuum terminology, such as “soft vacuum”, “rough vacuum”, “hard vacuum”, etc., as shown in Figure 1.
NOTE: Good brazing does not necessarily require a very hard vacuum!
How “hard” a vacuum is necessary for good brazing? Just “hard” enough to reduce the amount of oxygen present in the chamber to the level that the number of oxygen atoms remaining is not sufficient to cause damaging surface oxidation on the faying surfaces of the metals being brazed. Thus, it is not uncommon for people to hear me say: “Braze with the weakest vacuum you can get away with!”, meaning that they should reduce the pressure in their vacuum furnace only to the level that there isn’t enough oxygen to cause oxidation that will interfere with brazing. To achieve this, it may be necessary to back-fill the furnace with enough gaseous atmosphere to build up a partial-pressure in the vacuum chamber. This will be discussed further in next month’s article.
In June’s article, we’ll look at how creating “partial-pressures” in vacuum furnaces by back-filling vacuum furnaces with an inert atmosphere is sometimes necessary to achieve successful brazements!
Dan Kay operates his own brazing consulting/training company, and has been involved full-time in brazing for 40-years. Dan regularly consults in areas of vacuum and atmosphere brazing, as well as in torch (flame) and induction brazing. His brazing seminars, held a number of times each year help people learn how to apply the fundamentals of brazing to improve their productivity and lower their costs. Dan can be reached via e-mail at [email protected], and his website can be visited at: http://www.kaybrazing.com/
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