get•ter (gĕt′ər) n. Any substance introduced into a partial vacuum to combine chemically with the residual gas in order to increase the vacuum. Webster’s College Dictionary, © 2010 Copyright 2005 by Random House, Inc. All rights reserved.
Vacuum brazing is a growing industry, with more and more companies entering it each year, due primarily to the bright, clean, as-brazed component surfaces resulting from brazing in a vacuum environment, which, when conducted properly, allows brazed components to be used immediately, with no additional cleaning operations needed after brazing.
Of course, that assumes that the vacuum furnace is clean and tight, with a minimal leak-up rate. Leak-up rate? What? Do vacuum furnaces leak? Yes, every vacuum furnace, unfortunately, is leaky! There are many fittings, connections, seals, etc., on each vacuum furnace, and it is very important that all such seals and connections be as leak-tight as possible. Otherwise, air will leak into the furnace through any of those potential leak-paths and the pressure inside the furnace will start to go back up toward atmospheric. This “leak-up” rate must be measured for each vacuum-brazing furnace, and that information made available to brazing personnel prior to starting any vacuum brazing cycle. A vacuum-furnace’s leak-up rate tells the brazing personnel how rapidly oxygen is leaking into the furnace. This is very important because hot metal surfaces can readily oxidize in the presence of any oxygen present in the vacuum chamber, and — understand this — brazing filler metal (BFM) does not like to bond to, or flow over, oxidized metal surfaces. Thus, it is the job of all brazing personnel to be sure that the amount of oxygen in the vacuum-brazing chamber be kept low enough so that BFM-flow into and through a brazed-joint is not adversely affected by the amount of oxygen present in the furnace chamber during brazing.
|Fig. 1 — Ti-turnings have much greater surface area than larger sheet stock.|
How can a low level of oxygen be achieved? There are a few ways, all of which should be tried. First of all, be sure that all furnace leaks are found and eliminated! One of my articles last year “Helium leak detection in vacuum brazing” described the use of helium-mass-spectrometer leak-testing machines which can help you locate the source of any leaks in your vacuum furnace. Once any such leaks are found and eliminated (sealed), your vacuum brazing furnace should be able to give you excellent brazing results, even if you are brazing highly sensitive base metals in your furnace.
Another method that brazing personnel can use to get rid of oxygen in their furnaces is to place titanium “getters” in their vacuum furnace chamber, such as the clean machined-turnings shown in Fig. 1, doing so either before or during a brazing run, in order to allow these titanium-turnings to “getter” (i.e., to adsorb) any oxygen present in the chamber.
Excellent articles have already been written about “gettering”, one of which can be found on VacAero’s website in a monthly column written by Dan Herring, and is entitled “Getter Materials“. Another discussion of gettering is included in chapter 12 (entitled: “Gettering”) in Dan Herring’s book: “Vacuum Heat Treatment” (BNP Media, copyright 2012). Both articles show why titanium is good for this purpose of gettering.
Let me now add to what has already been discussed in the articles just mentioned.
You will note that at the start of this present article I showed one of the definitions for the word “getter”, namely that a getter is ”any substance introduced into a partial vacuum to combine chemically with the residual gas in order to increase the vacuum.” Titanium happens to be an excellent substance to introduce into a vacuum chamber for this purpose, because it readily and actively reacts with any oxygen present in the vacuum chamber, holds onto it, and thus prevents that oxygen from being able to cause any problems with any brazing processes going on in that vacuum chamber.
Let’s look again at a chart of metal-oxides curves that I wrote about in an earlier article “Reducing Metal-Oxides in Brazing – Part 1“, which chart is shown again here:
|Fig. 2 Metal/metal-oxides curves, including titanium.|
Note that this chart is laid out in such a way that the temperature of the vacuum furnace is shown along the horizontal axis, and the amount of vacuum in the furnace is indicated along the vertical axis on the right side of the chart. Each of the curved lines in the chart represents a particular kind of metal-oxide, such as the chrome-oxide (Cr2O3) line near the center of the chart. What this chart indicates is that we can get rid of chromium-oxide if we are positioned to the right side of the Cr2O3 curve. Thus, if we were operating our vacuum furnace at about 2000°F (1100°C), and at a vacuum level of about 10-1, we could “thermodynamically-reduce” (get rid of) the oxide of chromium, leaving the Chromium (Cr) free to alloy with a BFM without any interfering oxide of chromium being present.
Note that the chart also indicates that operating at temperatures to the left of the chromium-oxide curves is “more oxidizing”, i.e., the combination of temperature and vacuum-level is not sufficient to remove oxides, but is high enough to cause significant formation of that metal-oxide. Thus, during heating in a vacuum furnace, oxides form as a metal gets hotter and hotter, the rate of that oxide formation slowing down as you approach the oxide line for that metal, and then as you move to the right side of that oxide line, only then can that particular oxide be “reduced” (eliminated).
|Fig. 3 — Pure titanium pellets. Photo courtesy of ESPI Metals.|
Notice far over to the right side of that chart the oxide-curve for titanium (shown on the chart as TiO). This chart tells me that as I heat up titanium in a vacuum furnace the metal will steadily oxidize, but for that oxide to become “reduced” (eliminated) I have to get to the right hand side of that curve. Well, if I am operating my vacuum furnace at about 10-4 Torr, I would need to operate above 2700°F (1500°C) to accomplish that. As you know, in regular vacuum brazing furnaces, that cannot be accomplished. Thus, when titanium oxidizes in such a furnace, it will hold onto that oxide layer strongly, since I cannot operate my furnace to the right of the TiO curve.
That, then, is the secret to effective “titanium gettering”! By placing titanium in the vacuum furnace chamber, it will steadily oxidize during heating, taking away much of the oxygen present in the vacuum chamber. Therefore, since that oxygen is now tied up as a titanium-oxide on the titanium metal surface, it cannot, therefore, be available to oxidize some other metals that I might be trying to braze in the furnace at the same time!
And, the larger the total surface area of the titanium exposed to the furnace atmosphere, the greater will be its oxygen scavenging capability. Thus, titanium-turnings (shown in Fig. 1) will have much greater total surface area exposed to a furnace atmosphere than the pellets shown in Fig. 3, or the titanium scrap shown in Fig. 4, or the discs shown in Fig. 5.
During many vacuum furnace cycles, some materials will outgas from parts being brazed. The outgassing materials may be brazing paste binders (organic materials), or perhaps even some of the alloyed metallic components in the base metals, or in the BFMs, being used in that furnace run. These outgassed materials can condense on the cold (water-cooled) inner walls of a vacuum furnace, and then, when heated again in subsequent furnace runs, may re-volatilize off those furnace walls, contaminating the surfaces of anything sitting in the hot-zone of that subsequent vacuum furnace run cycle.
|Fig. 4 — Titanium sheet cut up into small pieces for scrap.|
Because of the high reactivity of titanium to many of these outgassing contaminants, titanium can be a very effective “getter” of these materials when any of these contaminants come in contact with titanium in the furnace chamber. Although pure-titanium would be optimal, any titanium alloy can actually be used with great effectiveness for this purpose.
And, because the titanium-oxide curve is so far to the right side of that metal/metal-oxide chart, those titanium oxides can’t be eliminated in any regular vacuum brazing run. Thus, once scavenged by the titanium, those contaminants will adhere tightly to the titanium surface and remain there!
From this, you may correctly conclude that the greater the amount of titanium surface area exposed to the oxygen and contaminants the better. Machined turnings will probably, therefore, be much more effective at scavenging oxygen from a vacuum furnace chamber than large sheets of titanium, because of the significantly higher total surface area of the turnings as compared to the sheets.
It should be noted, too, that this “contamination” of the titanium surface will normally cause the darkened titanium getters to fall apart (crumble) when handled afterwards. This deeply discolored titanium material should not, in my opinion, be re-used, but should be discarded after use instead.
When should titanium-getters be used?
|Fig. 5 Titanium discs. Photo courtesy of ESPI Metals.|
In my opinion, they should be used as part of your shop’s furnace maintenance schedule during a furnace clean-up cycle, rather than waiting to use it during a brazing run. In our job-shops, we always conducted the titanium-gettering during our vacuum-furnace clean-up runs prior to commiting those furnaces for any subsequent critical vacuum brazing work, since we wanted to be sure that our vacuum furnaces were clean, tight (very low leak-up rates), and oxygen free BEFORE we began to braze.
Other brazing shops have stated that since vacuum furnaces do leak air into the brazing zone when heated, they prefer to also include titanium-getters inside their hot zone during the brazing run, along side of the parts being brazed. That’s fine, and is quite acceptable if you want to do that.
But, as mentioned earlier, I believe it is good practice to also use titanium-getters during high-temp vacuum furnace clean-up (burn-out) cycles so as to obtain the cleanest possible vacuum-chamber prior to loading parts into the chamber for subsequent brazing.
As I mentioned earlier, when we used titanium getters in our brazing shops, we preferred to use very clean titanium machined-turnings, which we placed on trays in the furnace during our furnace burn-out cycles. Following that cycle we threw out the dark titanium turnings and repeated the clean-up cycle a second time. This fresh load of titanium turnings would usually come out much lighter in color than the Ti-turnings in the prior run. If necessary (for a very dirty job-shop furnace) we might do a third clean-up cycle with more Ti-turnings, placing a thin sheet of titanium on top of the turnings during that run. If, when removed from the furnace, that titanium sheet could be folded back on itself without breaking, we knew we then had a very clean vacuum chamber.
In next month’s article we’ll look at the use of magnesium for gettering in aluminum-brazing operations in vacuum furnaces specially built for aluminum-brazing.
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Dan Kay – Tel: (860) 651-5595 – 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’s website can be visited at: http://www.kaybrazing.com/
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