inverted-tube wsThere are many vacuum-brazing shops out there that still believe it is necessary to try to use the strongest vacuum possible for brazing if they expect to get good results.

Such thinking is erroneous, and has led many shops to actually see “worse” results (increased void content of joints, increased discoloration on furnace walls, etc.) than they would have seen if they had merely used a “weaker” (less strong) vacuum. Many people today still like to use some of the older vacuum terminology, such as "soft vacuum", "rough vacuum", "hard vacuum", etc., and some of those same people still believe that a “very hard vacuum” is always necessary for effective brazing. IMPORTANT NOTE #1: Good brazing does not necessarily require a very hard vacuum! How "hard" a vacuum is necessary for good brazing? Just "hard" enough to reduce the amount of oxygen present in the chamber to the level that the number of oxygen atoms remaining in the hot-zone of the furnace is not sufficient to cause damaging surface oxidation on the faying surfaces of the metals being brazed. by Dan Kay

therm-chart wsOver the years, several different temperatures have been used to define the concept of brazing. When the American Welding Society (AWS) published its first Brazing manual back in 1955, brazing was officially defined using 800F as the liquidus temperature of a brazing filler metal (BFM), above which temperature a joining process using that BFM would be defined as “brazing” (see Fig. 1). If the liquidus temperature of the filler metal was lower than 800°F, a joining process using such a filler metal would be called “soldering”. 

First of all then, let’s define what we mean by the “liquidus” temperature of a BFM. When any BFM is heated, it will reach a temperature at which it will start to melt. Below that temperature the BFM will remain solid, but once it crosses that temperature it will start to melt. That temperature is called the “solidus” temperature of the BFM. Then, as heating continues and more and more of the BFM melts, it will finally reach a point where all the BFM has finally melted, and become completely liquid. It will be said to have crossed the “liquidus temperature” for that BFM. Technically, the “liquidus” temperature is determined by, and defined as, the temperature at which a molten BFM begins to solidify upon cooling from its fully-molten state. But for our purposes here in this article, I will merely assume that when a BFM crosses its liquidus temperature during heating, it will become fully liquid (molten). Liquation is not being considered. by Dan Kay

Prick-punch wsA number of people have inquired about how to keep tubing or piping centered in holes or fittings prior to brazing, thinking (erroneously) that if the tubing/piping does not remain centered in the joint, but instead touches one surface or another inside the joint (due to lack of centering) that the joint therefore may be weakened thereby, or that the molten brazing filler metal (BFM) will not be able to penetrate the area where the tubing/piping contacts one of the surfaces inside the joint.  That is incorrect thinking, because molten brazing filler metal (BFM) is able to penetrate extremely tight joints, even when there is metal-to-metal contact in some portions of the joint.  The microscopic surface roughness of the mating surfaces inside the joint will allow the liquid BFM to penetrate completely.

But, if you are in that group that feels that you must take steps to keep the tubing or piping centered in the joint to be brazed, and want to take steps to prevent any joint surfaces from touching, then there is a simple way by which to insure that the tubing/piping will remain centered in the joint throughout the braze-cycle. The simplest way is to "dimple" the OD surface of the tubing/piping using a prick-punch, a tool that is illustrated in Fig. 1. by Dan Kay

 

16-t-overalp-joint-wsIn 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

 

lap-joint-design wsA 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

SurfaceRoughness wsOver 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

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

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