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In my opinion, based on my experience, the amount of actual braze coverage in a joint is more important than the number of voids in that joint!.

As discussed in last month’s blog-article, a lap-joint with an overlap of “3T-to-6T” (where “T” is the thickness of the thinner of the two members being brazed) is all that is needed to provide full strength and hereticity in a properly designed brazed joint (1T-to-3T for aluminum alloys). By this I am saying that we need to look at the amount of GOOD braze coverage, rather than being overly concerned with trying to count the number of voids in a joint!  Counting voids is really the wrong way to approach the “goodness” of a brazed joint. by Dan Kay

A half century ago (back in the early 1960’s) a lot of research work was done by The American Welding Society (AWS) Committee on Brazing and Soldering to determine appropriate criteria for brazing lap joints (the preferred type of joint design for assemblies requiring the ability to withstand high pressure in service, such as gas bottles, etc.). The results were published in their committee report: AWS C3.1 in 1963, one of the recommendations of which was that joints should have an overlap of 3T or more, where “T” is the thickness of the thinner of the two sheet metal pieces being brazing together.

Here’s how that recommendation came about.  The AWS C3 committee arranged to conduct a series of round-robin testing in ten different laboratories around the country, using two different shear-type joint designs, four different base metals, and three different types of brazing filler metals (BFMs), for a total of about 1200 brazed shear test specimens.  Their intent was not only to find out what constituted a satisfactory joint overlap design for brazing, but also to develop an easily reproducible test specimen that was “realistic” to the real-life world of brazed components in industry and which could become a “standard” that everyone could (and would) use to evaluate joint strength. by Dan Kay

Over the years it has shown that the best surface for brazing, generally speaking, is the "as-received" (as-rolled, as-drawn, as-machined, etc.) surface roughness of the material coming into the brazing shop.  An illustration of what this surface roughness might look like, under high magnification, is shown in Fig. 1.

Surface roughness obviously increases the total surface area of each faying surface inside the joint, when compared to a flat, polished surface.  And, due to this “roughness”, it can be seen that there are many capillary paths for brazing filler metal (BFM) to follow between all the valleys and “peaks” on that roughened surface. by Dan Kay

In many vacuum brazing applications, it is deemed necessary to use an atmosphere gas inside the vacuum furnace, perhaps to quench components following a vacuum-brazing run, or to perhaps build up a partial-pressure atmosphere inside the furnace to prevent the outgassing/volatilization of higher vapor-pressure metals, or perhaps merely to allow gaseous conduction of heat from part to part being brazed.

Whenever a gas is introduced into a vacuum furnace for a brazing operation, I’m always very concerned about the dewpoint of that gas, since dewpoint represents moisture in the gas, and moisture represents the presence of oxygen.  In vacuum brazing of aluminum, moisture molecules present their own issues to the brazing process, in addition to their oxidizing characteristics. by Dan Kay

On a warm, moist day, our earth’s atmosphere will contain a significant amount of moisture in it.  During the night, when the sun has gone down, this atmosphere will become cooled, and will not be able to hold onto the amount of moisture (water) that it could when it was warm, and so, some of that moisture will condense out onto the grass in the form of “dew”.  Then, during the following day, when the sun heats the air up once again, the dew will evaporate from the ground.

It is well known that the warmer the gas, the greater will be the amount of moisture that gas can hold. At any given point in time, all gases will have what is called a “dewpoint”.   The “dewpoint” of any gas is the temperature to which that gas must be cooled to get the first droplet of moisture to condense out of that gas (assumed to be at one standard atmosphere of pressure).  The less the amount of moisture in that gas, the cooler must be the temperature to which that gas must be cooled in order to get the first condensation to occur.  Based on that fact then, it will be understood that the lower the dewpoint of a gas, the drier (lacking moisture) is that gas. by Dan Kay

In last month’s article, we looked at the use of titanium-“getters” when vacuum-brazing high-temperature base-metals that are very sensitive to oxidation.  In this month’s article, let’s look at how magnesium (Mg) is used as a “getter” when vacuum-brazing at temperatures of only about 1000-1100°F (540-600°C), as needed for joining aluminum base metals.

Magnesium (Mg), often referred to simply as “mag”, can be highly effective at gettering both oxygen and moisture that may be present in a vacuum-furnace atmosphere being used in aluminum-brazing operations. Aluminum (Al) reacts readily with oxygen to instantly form a tenacious Al-oxide layer on its surface.  This Al-oxide layer is very stable, and, if mechanically removed, will quickly re-form.  Thus, in real life, a layer of aluminum-oxide will constantly be present on the aluminum surface before, during, and after aluminum brazing.  Dealing with that oxide layer has proven to be a challenge to many brazing shops over the years. by Dan Kay

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. by Dan Kay