5-ball-tacked-assembly wsHighly complex assemblies, for a variety of end-use applications in such diverse fields as automotive, aerospace, medical, electronics, and tooling (just to name a few), can be effectively made via brazing.

For this to happen, however, each of the individual components in these complex assemblies must be able to be held together in proper alignment, have the appropriate brazing filler metal (BFM) applied to it, and this assembly then moved into a brazing-furnace, where it can be heated until the BFM melts and flows into the joints by capillary action, thereby permanently joining the components together to make a complex assembly. by Dan Kay

 

IMG 3419by Dan Kay

This is a question that arises once in a while, and needs to be taken seriously. Fortunately, for most of us, problems with Certificates of Conformance (or Certificate of Compliance) are very rare. Most manufacturers of brazing filler metals (BFMs) are reputable companies who pride themselves in being able to produce high-quality BFMs in such a way that the BFM product is homogeneous, and its chemistry is carefully controlled in a manner that can guarantee that it fully meets the requirements of the specification(s) to which it is being produced.

The Certificate of Conformance will show the BFM specification(s) to which it conforms, and the customer to whom the “cert” was sent should be able to fully rely on the accuracy and truthfulness of that document. Every once in a while, a customer, using a BFM product (paste, wire, preform, etc.) which they have purchased from their supplier, discovers that the BFM product does not perform the way it is supposed to, and many questions begin to surface about that product they have received. 

LN2-Tank-Big wsby Dan Kay

In AMS 2675G ("Brazing, using Nickel-Alloy Filler Metal"), paragraph 3.3 states that acceptable atmospheres for nickel-brazing are hydrogen, argon, or vacuum. No mention of nitrogen. People have asked me why nitrogen is apparently not allowed for furnace brazing with nickel-based brazing filler metals (BFMs), or for building up partial-pressures in a vacuum furnace for subsequent brazing.

ANSWER: Nitrogen, like hydrogen, can be reactive towards some of the metallic components in the base-metals and in the liquid BFM during brazing processes. As a safe-guard against any such problems, AMS 2675 excludes nitrogen from its list of acceptable furnace atmospheres for nickel-brazing. Will nitrogen ALWAYS be a problem in nickel-brazing in a furnace atmosphere? NO! Please be aware that the exclusion of nitrogen is a “general safety” recommendation (suggestion), and is not to be taken as a prohibition against nitrogen for any and all nickel-brazing. Let’s take a closer look……. 

 

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

 

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