F1-lead-in-pic wsIn 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

dew-point-transmitters wsOn 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

Mg-chips ws

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

ti-pellets wsVacuum 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

egr-cooler-fins-WSErosion is defined in the AWS Brazing Handbook (published by the American Welding Society in 2007) as follows: “Erosion is a condition caused by the dissolution of the base metal by the molten brazing filler metal, resulting in a reduction of the base metal thickness.”

Thus, the phrase “base-metal erosion by the brazing filler metal” is used to describe a process in which a molten brazing filler metal (BFM) which is highly soluble in a given base-metal (parent metal), is applied to the surface of that base metal, is heated to brazing temperature, and in so doing, actively diffuses into that base metal, alloying extensively with (dissolving) it. by Dan Kay

xray wsRadiography (X-ray) is a means of looking at the inside of a braze joint without actually having to cut it apart to see it, as long as the appropriate conditions are met to allow its use.  Radiography is heavily dependent on the thickness and mass of the part being radiographed. 

In order for x-ray to yield a useful image of any voids or inclusions in a brazed joint, the thickness of that void or inclusion should be at least 2% of the thickness of the metal through which those x-rays are being sent, in order for it to be visible in a radiograph, or, if using real-time radioscopy (RTR) on a TV screen or monitor. The “2% Rule” in brazing is a very important guideline to follow, since it can effectively rule out radiography as a method for inspecting components that are too thick to be able to see any of the imperfections inside a braze joint. by Dan Kay

 

CrackedJoint wsAlthough liquid-penetrant inspection, such as dye penetrant inspection (DPI) and fluorescent penetrant inspection (FPI), are useful tools for inspecting fusion-welds, they should NOT be used for inspecting brazed joints.  This is especially true for any parts on which subsequent braze-repair may be required, such as many aerospace components that are vacuum-brazed, then placed in service for long periods of time, and then come back for later repair or rebuilding and then sent back out for more field-service.

DPI and FPI have long been used in the welding industry, and should certainly continue to do so, since weld-cracks and surface imperfections can readily be seen by these techniques and subsequently repaired without difficulties. by Dan Kay

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