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As mentioned in previous articles, more and more brazing shops are using vacuum furnaces.  These furnaces are quite complex, offering more options for heating, cooling, partial pressure, or multi-bar pressure (pressure capabilities above one atmosphere) for high-speed cooling.

The overall effectiveness of the equipment, however, still lies with the people who program and run the furnaces. Figure 1 (below) shows a series of vapor pressure curves for a number of common metallic elements.  Each curve shows the melting point of the element (indicated by a small circle along the curve) and, where the curve crosses the dotted line across the top of the chart, its approximate boiling point (where it wants to turn to a gas) at one standard atmosphere. By Dan Kay

Which base-metal should I use for braze fixturing so that it will last the longest?

This question is not an uncommon one. Although I have never personally seen any kind of chart showing an "expected life" for fixture materials, it is important that people understand that there are a number of factors that will control the "life expectancy" of any fixturing material used in brazing, and all of these factors relate to the service conditions that the fixtures will encounter during the brazing process. By Dan Kay.

In August's article, I'll address the commonly used method of adding a lot of "dead weight" onto parts in an attempt to keep them flat during brazing!

We are pleased to announce the launch of a New Vacuum Brazing column in the April 2009 issue of VAC AERO’s What’s HOT! NEWSLETTER

The new column, written by brazing expert Dan Kay will be published monthly and will offer helpful vacuum brazing applications, tips and techniques to commercial and captive heat treaters alike. Dan Kay is an independent brazing consultant, who has consented to write articles on brazing for VAC AERO since many of our vacuum furnaces are used for brazing. However, his willingness to provide this service to VAC AERO readers does not imply his specific endorsement of our furnaces, but rather his desire to help VAC AERO furnace users to understand brazing better. Read Dan Kay's Biography 

Brazing has proven to be a highly versatile joining process for permanently joining many different kinds of metals and ceramic materials together in a variety of industries as diverse as aerospace and automotive.

Brazing has a long history of use, dating back to the time of the ancient Egyptians, and truly came into its own as a high-volume production technique during the twentieth-century, not only via torch-brazing, but especially with the development of well-controlled continuous and batch-type brazing furnaces.  In the last few decades, the steady increase in vacuum furnace technology made vacuum brazing a preferred method by the end of the twentieth century for many companies doing brazing. By Dan Kay

In my next article, I'll talk about vacuum atmospheres in brazing. Yes, vacuum is still an "atmosphere" in the technical sense of the word, since the gas inside the vacuum chamber has actually not been completely removed from the chamber, and those remaining molecules of gas must not be able to interact in a negative way with the faying surfaces of the brazement.

All metals want to react with oxygen as the metals are heated. The higher the temperature, the greater the thermodynamic driving force to have those oxides form. This is true for all metals, even though the oxides of some metals are not as stable as the oxides of other metals.  Gold and nickel are examples of metals whose oxides are not stable at any temperature we would encounter in our daily activities, and thus, do not concern us at all. Copper oxides and iron oxides are examples of metals whose oxides are not stable at higher temperatures, in that those oxides are easily and quickly dissociated at elevated temperatures. Chromium-oxide, however, is an example of a fairly stable oxide (up to about 1850F/1000C before dissociating in a typical brazing atmosphere furnace), whereas aluminum-oxide will be extremely stable in a brazing furnace, and is beyond the capability of any standard brazing atmosphere to reduce that oxide. Titanium-oxides behave in a very similar fashion to aluminum oxides in typical brazing furnace atmospheres. By Dan Kay

In June's article, we'll look at how creating "partial-pressures" in vacuum furnaces by back-filling vacuum furnaces with an inert atmosphere is sometimes necessary to achieve successful brazements!

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!. By Dan Kay