Fig. 1 — Ti-turnings have much greater surface area than larger sheet stock.
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.
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: