Brazing is a highly versatile joining technique that can be used to join many different types of metals, and can even be used to permanently bond engineered-ceramic materials, such as alumina, to a variety of metals. It is being done everyday in industry.

Alumina, which consists of aluminum-oxide powder granules imbedded in a glassy matrix binder system of calcium-oxide and silicon-dioxide (among others), can be joined to ceramic or metal structures primarily by two different methods, as shown in Fig. 1. By Dan Kay

Wide Gap Brazing when Parts don’t Fit Together well for Brazing – A common occurrence (unfortunately) in the brazing world is the need to join two parts together by brazing in which the brazing gap is too large, i.e., in the range of 0.010-inches (0.25 mm) or larger. Ideal brazing clearances should be in the area of 0.000-inches to 0.005-inches (0.00mm to 0.125mm) maximum for most brazing filler metals (BFMs).
Brazing depends on capillary action to draw the liquid BFM into the brazing joint, and tight clearances are needed for best brazing to occur. If the BFM is pre-placed in the brazing joint prior to assembly of the parts then capillary action is not a major factor since the BFM will melt in-situ and join the two members without the need for flowing any distance through tight capillary spaces. By Dan Kay

Why does Brazing require Temperatures above 450C (840F)? Brazing, when performed correctly, is a joining process that produces a permanent bond between two or more materials by heating them to a temperature above 450C (840F), but lower than the melting-temperature of any of the materials being joined, and a permanent, metallurgical bond between these materials is produced when capillary action draws a molten brazing filler metal (BFM) through the clean, closely fitted faying surfaces of the joint.

The filler metal is not supposed to become fully liquid (i.e., have a “liquidus”) until the brazing temperature reaches at least 450C (840F). If the liquidus of the filler metal is below 450C (840F) then that filler metal would commonly be called a “soldering alloy”. People often wonder about the temperatures used to differentiate brazing from soldering. Why 450C (840F)? Is there some significance to these “exact” numbers? By Dan Kay

Over the years I’ve helped many brazing shops resolve common brazing problems (such as leakers, non-wetting surfaces, etc.). In evaluating these situations, it is not uncommon to discover that sub-components (such as brackets, or fittings, etc.) from outside suppliers can actually be the trouble-makers!

Often the brazing shop is not aware of how some of their suppliers are making the sub-components that will be subsequently brazed. Then when there is a problem brazing some of the assemblies containing these sub-components, the brazing shop may try to solve the problem by trying to find out what is wrong with their own in-house brazing operations, getting very frustrated when an in-house cause for the problem can’t be found. Many suppliers are not aware that their own manufacturing processes can have a negative impact on brazing. Unless you have talked with them extensively about how certain processes will, or will not, hurt brazing, they will continue to do what works best for them in supplying a nice looking product for you in as cost-effective a manner for themselves as possible. By Dan Kay

Even with built-in “holds” when heating low-carbon steel parts up to brazing temp, some heat treaters are getting a high percentage of the tubular brazements “pulling apart” somewhere during the cycle, i.e. the smaller-diameter tubing pulls away from the larger-diameter tubing, even snapping the welded clips off one of the tubes so that they are not brazed together along their length. What’s happening, and how can they “fix” this problem? By Dan Kay

As aluminum brazing continues to grow, some folks are asking: “Can I use available furnace time in my aerospace vacuum furnace to perhaps do some aluminum brazing?” There are a number of reasons why that is a VERY bad idea, but I will briefly review just two of the more obvious ones below.

Temperature control – Shown below is a photograph of a typical hi-temp aerospace vacuum furnace metal hot zone. Notice that there are six (6) heating elements connected around the OD of the hot-zone, as well as heating elements on the back wall of the furnace. By Dan Kay

As the name of this product-type indicates, a brazing “stop-off” is supposed to be something that “stops” molten brazing filler metal (BFM) from flowing into areas where it is not supposed to be, thus keeping it “off” surfaces that are supposed to remain clean and free from the presence of any BFM.

First of all, brazing filler metals (BFMs) do not like to bond to (or flow over) dirts, greases, or oxides.  The presence of any of these contaminants on the surface of parts to be brazed can literally prevent the BFM from flowing over surfaces on which any of these contaminants are located. By Dan Kay

Next month:  As aluminum-brazing continues to grow, some folks are asking: “Can I use available furnace time in my aerospace vacuum furnace to perhaps do some aluminum brazing?”  Let’s look at why that is a VERY bad idea!

This is Part 2 of the article Shelf-Life and Expiration-Dates for Brazing Paste

Brazing paste is not difficult to make yourself. All you need is some brazing filler metal powder, a gel-binder, and a paint-shaker. Sound easy enough? Let’s see…

First, procure the desired brazing filler metal (BFM) in powder form from one of the BFM manufacturers.· I show a listing of such manufacturers on my website at http://www.kaybrazing.com/sources.htm, and each company’s name is a “hotlink” to that company’s webpage. By Dan Kay

Next Month: In next month’s article, we’ll consider brazing stop-offs, and how they are correctly and incorrectly used in many brazing shops today!