t-shaped-specimens wsMonitoring the actual quality of the brazing atmosphere inside a vacuum-furnace during brazing cycles is very important, and is not hard to do. When vacuum brazing, you have to wait until the brazed parts are removed from the furnace at the end of the brazing cycle in order to see if everything was actually okay during that brazing cycle. If, when opening the furnace after a brazing cycle, you see that the parts you were trying to braze are discolored or poorly brazed, then how can you determine exactly what went wrong during that cycle, and how can you know when the brazing problem actually occurred (did it happen during heating, or during cooling, etc.)? Also, how do you determine whether the poor braze results are caused by physical problems with the furnace itself, or if they might be related to the brazed-component’s base-metal (parent-metal) composition, or perhaps with the brazing filler metal (BFM)? by Dan Kay

strength-vs-gapOne of the most widely used charts in the field of brazing is the strength vs. clearance chart created from work done in the Handy & Harman laboratories in Fairfield, Connecticut back in the 1930's. This chart is shown below, in Fig. 1:

Notice that as the joint clearance gets tighter and tighter (moving from right to left along the bottom axis), the tensile strength (as shown on the vertical axis on the left-side of the chart) gets higher and higher. Although there is a lot of experience with this over the years, and general acceptance of this information is widespread, it must be pointed out that this chart is very specific only to the actual testing performed in making this particular chart, and may not be identical to tests performed by others using similar materials or conditions. But the general principal of increased joint strength with tighter gaps can be accepted. by Dan Kay

interstitial wsDuring vacuum brazing with nickel-based brazing filler metals (BFMs), it is possible to hold the brazed parts at brazing temperature long enough for the BFM to solidify completely while being held at brazing temperature! The key is “diffusion”, and involves tiny interstitial atoms in the BFM.

Isothermal solidification can be a very useful brazing process for some brazing filler metals (BFMs), and can result in a significant increase in the re-melt temperature of the BFM in that brazed joint. To better understand the process, let’s first examine the component parts of the phrase “isothermal solidification”. “Iso” essentially means “ equal, or the same”, and “thermal” of course refers to temperature. So we’re looking at a BFM solidification process in which that solidification takes place while the furnace is being held at the same, steady temperature! Although that may sound strange, there’s some real logic to it. Isothermal solidification (we’ll refer to it as ITS in this article) depends a lot on the diffusion capabilities of various components of the BFM while that BFM is being held at the brazing temperature. by Dan Kay

powder-mesh2 wsMost powder used in the manufacture of brazing filler metal (BFM), to be used in either its pure powder form, or blended to make a brazing paste, is initially produced by a gas-atomizing process.

This process begins with molten metal that is poured through an atomizing nozzle at the top of a tall atomizing tank, in which high-pressure/high-velocity inert gas hits the molten stream, blasting it into billions of droplets which then cool into individual tiny particles of powder as they fall down to the bottom of the tall atomizing tank, where the powder will then be collected for further processing. by Dan kay

furnace-chart-sm-3For a number of years I have been encouraging people to re-think the ramp-rates they use for their vacuum brazing cycles.  Many brazing shops using rather high ramp-rates during heating claim that "this is the way we've always done it".  Perhaps it’s time to re-think this. From a metallurgical point of view, too-rapid a heating rate can lead to stresses and strains in the metal assemblies being heated, which can often lead to distortion of parts during their heat-up, and can even lead to parts-failure (I've seen this too many times).

I have recommended to a number of brazing shops that they slow down their heating ramp rates (and I’ve seen excellent results), using the following guideline: Heat the parts at the fastest rate that will allow you to bring all the parts (assemblies) up to brazing temperature without the need for any holds (for temperature-equalization) on the way up. by Dan kay

Continuous centerline eutectic in nickel-brazed jointThe word “eutectic” is one that I use in each of my brazing seminars during our discussions about brazing filler metals (BFMs) as well as metallurgical phase-diagrams, but it is also a word used in describing some of the features of metallurgical-structures within a solidified brazed joint.

The word “eutectic” comes from the Greek word “eutektos” which means “easily melted”. Simply put, a eutectic-composition is an alloy of two or more metals, which, when heated to its melting point (solidus temp.), will completely change from solid to liquid at the same temperature (i.e., isothermally). Technically, theoreticians prefer to define a eutectic-reaction in reverse, proceeding from the molten state to the solid. However, I believe my description will help personnel in the brazing world grasp the general concept more easily. by Dan kay

vav4872 wsWe’ve been looking at the leak-up rate of vacuum furnaces in my last two articles, and this month’s article looks at another type of vacuum leak, that isn’t really a leak at all!

This kind of “leak that isn’t a real leak” is actually known as a “virtual leak”, and represents the outgassing of substances that have condensed onto the inner walls of the vacuum furnace during prior furnace runs. Then, when those walls get very hot during subsequent furnace runs, the condensed substances on the walls may volatilize and outgas once again into the vacuum chamber, the effects of which often appear to be very similar to an actual furnace leak. by Dan kay

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