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

brazing-paste wsI am often asked about the differences between brazing and soldering.  Perhaps this is a good time to describe the two processes in more detail, so that readers can understand the significant differences between them.

There are some similarities between soldering and brazing, but many significant metallurgical differences.  They are both used to join metals together to form a bond between the metals being joined, but the bonding mechanisms are very different.  Let’s take a look at these two processes, and see how they compare. by Dan Kay

 

Bare-Hand wsA common concern in the brazing field is the cleanliness of parts that are to be brazed.  Some people think that cleanliness can be achieved by merely heating the parts in a furnace or via a torch-flame, and those high-heat conditions will effectively “burn off” any surface contaminants and render the parts sufficiently brazeable. NOT TRUE!

Parts that are going to be brazed need to be thoroughly cleaned prior to assembly for brazing, which usually involves degreasing of parts and thorough drying. Then, once the parts have been cleaned, they need to be handled and assembled with clean hands in order to maintain that surface cleanliness. So one important question to answer is:  “Can I get my hands clean enough to adequately handle surfaces that will be brazed? by Dan Kay

 

iron-carbon-diagram wsMany brazing shops use graphite fixtures on which to set parts that are to be brazed. Graphite fixtures have excellent thermal stability, enabling them to be used again and again through many brazing cycles. This high thermal stability is often coupled with low cost compared with some metals used for making fixtures (such as the high nickel/chromium or moly- alloy type fixtures).

If you are using graphite fixtures, or intend to consider doing so, it is VERY important to remember that graphite is carbon, and pure carbon likes to react chemically/metallurgically with metals containing iron (such as steel), to form low-melting eutectic compositions at temperatures just under 2100°F/1150°C, as shown in the iron-carbon (Fe-C) phase diagram illustrated in Fig. 1. This temperature is often lower than some of the brazing temperatures being used in many brazing shops today! by Dan Kay

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