But carbon-steels present a unique situation to designers and brazing companies, because when being heated all the way up to copper-brazing temperature, the metals will actually go through a temperature-range where the steel will actually be contracting (shrinking) while being heated, and then do just the opposite when cooling, thus potentially causing distortion and/or fracturing of brazements during a high-temp brazing cycle. Such a scenario is illustrated in Fig. 1 where an automotive fuel rail brazement failed to braze properly, because some unique CTE problems associated with carbon-steels was not properly taken into account during the furnace brazing cycle. by Dan Kay 

As mentioned in a previous blog-article, magnesium (Mg), often referred to simply as “mag”, is a highly effective “getter” that is used when vacuum-brazing aluminum. Because Mg is very effective at gettering (reacting with and removing) both oxygen and moisture that may be present in a vacuum-furnace atmosphere during aluminum-brazing operations, it can effectively prevent (or minimize) the reaction of these elements with aluminum, thus allowing aluminum-brazing to occur.

However, magnesium is a highly combustible metal, and when it condenses on the walls of a vacuum-furnace during aluminum brazing operations, extreme caution must be exercised in removing the condensed mag from the furnace walls during subsequent furnace clean-up, so that no sparks are generated which could cause rapid ignition of the condensed magnesium, resulting in explosive combustion, and even death. To prevent this, coating the walls of the vacuum furnace with a “non-stick” surface, may be highly effective. by Dan Kay 

Please note that a braze fillet is actually a casting along the outside of a braze joint that simply shows that the brazing filler metal (BFM) has melted and flowed along the edge of a braze joint. However, it doesn’t tell you if the BFM has adequately penetrated the joint. Caution is therefore strongly advised to anyone attempting to merely use the size of a braze-fillet as an inspection criteria for judging the overall quality of a braze joint. by Dan Kay 

A number of brazing shops today combine brazing and heat-treatment in their vacuum furnaces to join components together and then obtain certain desired base-metal properties in those brazed components via rapid cooling (quenching) immediately after brazing is done, and before the components are removed from the furnace. Thus, the same vacuum-furnace brazing cycle combines brazing and heat-treatment, yielding clean brazed components with special base-metal properties to meet unique end-use service conditions.

Vacuum furnaces today offer a number of options regarding the introduction and use of circulating gases in the furnace hot-zone during brazing processes. Gases typically introduced into the vacuum furnace are either argon or nitrogen, but a number of shops have found success with hydrogen and helium gases as well. Often gases are combined rather than just using one gas. Hydrogen/nitrogen and helium/nitrogen are typical combinations used in some brazing shops. by Dan Kay

Highly 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

by 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. 

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……. 

There 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

Over 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

A 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