By Dan Kay
Most brazing shops today will occasionally be given "rush" jobs by their good customers, and these brazing shops will usually do their best to respond as fast as they can, so that the customer will remain one of their best customers! No-one wants to do anything that would turn a good customer into an unhappy customer!
Let’s suppose that your vacuum brazing furnace has just completed a series of cycles that has left the vacuum furnace quite contaminated, and you are now preparing to have your maintenance and furnace crew thoroughly clean the furnace, a process that might involve both hand-cleaning and vacuuming-out of loose surface contaminants, as well as a high-temperature burn-out cycle, etc. You’re thus prepared to take that furnace out of service for the rest of that day, and perhaps even into the next day, depending on what the cleanup crew needs to do.
Your good customer suddenly shows up with a “hot-job” that "has to be done right away". The assembly to be brazed, let’s say, consists of components made from both 316L stainless, joined to a titanium alloy of some kind --- a combo that be difficult to braze in a clean furnace. But you’ve got a very dirty furnace to contend with.
So, what do you do? Do you attempt to braze it in your dirty vacuum furnace and "hope for the best"? Or, do you tell your good customer that you cannot do their "rush-job" that day? Or, do you “box” the components and braze them, knowing they’ll come out fine?
Figure 1. Making the base of the box to hold components for brazing
“Boxing” the components during the brazing run is a technique that more shops need to use. It’s really quite a simple procedure. Let’s look at it more closely.
Boxing of components for brazing in less-than-clean furnaces involves placing the components in a special foil-box that you can quickly construct by hand just for that item, so that the components are essentially being brazed in their own pristine mini-furnace chamber within the vacuum furnace chamber. The box is used only once, just for that particular assembly and for that one furnace run.
In my own brazing shops, a number of years ago, we used to keep rolls of titanium foil and stainless foil on hand for that purpose. When a customer had a very sensitive part that needed to be done "right away", we did not have to hold off brazing that component while waiting for our next furnace clean-up run to be completed. Instead, we placed the component on some thin, clean alumina sheet, which we then placed on top of some thin titanium (or stainless) foil. We then cut some short slits into the foil to allow the foil to be folded up to form a box around the component (not letting the component touch any of the foil). See Fig. 1 for a simple illustration of how to do this.
Once two walls are folded up, then the side-walls can be folded up into place, and the extra foil length folded around the box to insure no open seams to the outside. Everything is merely bent over, crimped, etc., using simple pliers, metal shears, etc., so that the box can be made quickly and easily by hand right there in the shop.
The walls of the box that we formed in this manner were tall enough so that the sides of the box were higher than the top of the part we needed to braze. We then took another piece of the metal foil, and made a loosely fitting cover which we laid on top of the box that we had just made. See Fig. 2 for an example of how this is done.
Figure 2. placing the cover onto the box to protect boxed-contents
We would fold down the edges of the cover so that there was absolutely no "line-of-sight" into the box. That cover was then merely laid on top of the box we had formed. That’s correct, the cover was NOT welded to the box, but merely laid on top. As just mentioned, the edges of the cover are folded down, so that there is no “line-of-sight” into the box. It’s as simple as that.
During the brazing cycle, the outside of the box will act as a "getter" in the furnace-chamber during the furnace cycle, and will catch all the outgas-products moving around in the larger furnace chamber that might normally want to condense on, or coat, the part being brazed. But the foil-box into which you placed the component can quite effectively keep all those contaminants from reaching that assembly housed inside the box. Then, when the cycle is over, and the box is carefully removed from the furnace, the outside of that box may appear to be very dark and discolored. However, when the lid is carefully lifted off the box, the components inside the box will be pristine clean, and will be seen to have brazed very nicely!
It works very well! Give it a try!
- Try to use the thinnest titanium (or stainless) foil you can buy. The thinner it is, the easier it will be to work with.
- Keep the foil clean! Store it in a place where it will not get dirty or oxidized!
- Wear clean gloves, and use clean tools when making the box and placing the components inside the box for brazing.
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© Copyright Dan Kay 2012
© Copyright Dan Kay 2012