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

vsmr151 wsAs mentioned in last months article, all vacuum furnaces will leak air into their vacuum chambers over time, causing the pressure in the vacuum chamber to go back up. Thus the term “leak-up rate” is used to determine just how fast that vacuum chamber is allowing air back in.

Of course, air represents the presence of oxygen entering the furnace, which is not good for brazing. The air will also contain moisture, which then becomes another source of oxygen. Moisture itself can become an adherent problem on the metal surfaces being brazed, with adsorbed moisture often being very difficult to drive off the metal surface (can be quite a problem in aluminum vacuum brazing). by Dan kay

high-vac wsAll vacuum furnaces are leaky! That’s right, all vacuum furnaces can leak outside-air into the vacuum chamber, which could potentially cause some problems with your brazing, since an air-leak represents an influx of oxygen into the furnace.

Fortunately, the problem is usually quite controllable, and regular furnace inspections can usually keep those leaks completely under control. Leaks most typically occur through some of the sealing-surfaces in the furnace, the most common leak-source being the O-ring seal-surface in the furnace door itself. As the door is opened and closed everyday, the light coating of vacuum-grease on the door seal and on the O-ring can pick up dust and dirt, which, if not properly removed regularly, might begin to initiate small holes/cracks in the O-ring seal, which can eventually open up enough to allow air to start to leak into the furnace during furnace operation. by Dan kay

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