Titanium turnings are an excellent “gettering” agent for use in vacuum brazing furnaces. Because titanium is highly reactive with oxygen, it will readily react with any available oxygen in the vacuum furnace to form tenacious titanium oxides during furnace heating, thus effectively reducing the amount of free oxygen moving about in the furnace chamber during any brazing process. A photo of some titanium turnings from a titanium machining operation is shown in Fig. 1.
Webster’s Dictionary defines a getter as: ”any substance introduced into a partial vacuum to combine chemically with the residual gas in order to increase the vacuum.” Titanium is an excellent material 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 prevents that oxygen from being able to cause any problems with any brazing processes going on in that vacuum chamber.
The larger the amount of surface area of titanium exposed to the furnace atmosphere, the greater its oxygen-scavenging capability. Thus, titanium turnings are highly recommended for use in gettering since they present a very large surface-area-to-weight (mass) ratio – far better than heavier, more massive titanium pellets or solid discs.
When should titanium getters be used?
In my opinion, titanium getters should be used as part of your shop’s furnace maintenance schedule during a vacuum furnace clean-up cycle rather than waiting to use it only during a brazing run. In our job shops, we always conducted the titanium gettering during our vacuum furnace clean-up runs prior to committing 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 alongside of the parts being brazed. That’s fine, and it 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-temperature vacuum furnace clean-up (burnout) cycles so as to obtain the cleanest possible vacuum chamber prior to loading parts into the chamber for subsequent brazing.
Be Sure the Ti turnings are Clean!
Be sure the Ti turnings are clean – no oils or lubricants remaining on them from their prior machining operations. This can be done by washing them in appropriate solvents to remove those particular types of lubricants and then a final rinse in acetone or alcohol. Once thoroughly cleaned and dried, the turnings can be placed on thin metal trays and then into the vacuum furnace. Depending on the degree of furnace contamination, the chips will darken and may have to be discarded after their first use if the darkening is severe (almost black).
In our shops, we would then repeat the process with fresh turnings until the furnace was clean enough to allow us to place a thin sheet of titanium foil on top of the turnings during that last run. If that titanium sheet, when removed from the furnace, could be folded back on itself without breaking, we knew we then had a very clean vacuum chamber.