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Joint clearances must be tight for effective Ni-brazing. 1. Nickel-based brazing filler metals (BFM) can leave a hard, non-ductile eutectic phase in the middle of a brazed joint.

The hard, non-ductile metallurgical phase-structures that form upon solidification of Ni-brazed joints must be carefully controlled, or else they can, and will, result in cracks inside the joint in stressful mechanical or thermal-cycling service.

The last phases to solidify when brazing with nickel-based brazing filler metals (BFMs) will be those phases that are the lowest-melting, i.e., those phases rich in the temperature-lowering, eutectic-forming, elements (meaning those that are rich in boron, silicon, or phosphorus). Remember, “eutectic” refers to the composition of an alloy that is the lowest melting point portion of the BFM. Thus, eutectic phases will not only be the first composition to start melting during heating of the BFM but also will be the last to solidify during cooling. Thus, during cooling these eutectic-phases will “migrate” towards the center of the joint as the “solidification-front” of the BFM moves from the base-metal/BFM interface toward the center of the joint, and will be forced to solidify right at the center of the joint.

Unfortunately, all of these temperature-lowering, eutectic-forming, elements in nickel-based BFMs are also hardeners, that is, the phase-structures resulting from solidification of these elements have virtually zero ductility! Thus, the last phases to solidify (in the center of the joint) will be hard, and non-ductile. If the joint is thicker than only about 0.004” (0.10mm) max., these hard centerline eutectics can actually form a continuous line down the center of the joint, and cause the joint to become very prone to cracking under any kind of thermal or mechanical stress or strain in service.

Two years ago I wrote a series of articles for this column about the important steps that must be followed in order to ensure good brazing, including the need for proper cleaning of all surfaces to be brazed prior to assembly and actual brazing. Misconceptions still exist in the brazing world about the best way to clean surfaces, and, as shown in Fig. 1, people still ask me if it is necessary to both decrease and then pickle the metal surfaces, or if they can merely use a pickling-acid to both degrease and remove oxides at the same time.

I strongly believe in the expression “Cleanliness is next to Godliness” when I talk or teach about brazing preparation. The faying surfaces MUST be cleaned thoroughly prior to their being assembled for brazing because:

RULE OF BRAZING: The brazing filler metal (BFM)
will NOT bond to, or flow over, oils, dirt, grease,
lubricants, or oxides.

Proper temperature control within each furnace brazing cycle is essential.  It will not only ensure proper brazing filler metal (BFM) flow but can also prevent part distortion.  To accomplish this, multiple thermocouples (or “TC’s” as they are often called) need to be placed in strategic positions within each furnace load.

In this article on TC’s and their use in brazing, I’d like to briefly look at what TC’s are, and the types commonly available for use in brazing furnaces today (much more exhaustive discussions about TC’s can be found on the websites of a number of thermocouple manufacturers and suppliers). I’ll also look at the correct placement of TC’s in furnace brazing loads, and how, together with correct furnace heating/cooling rates, they can help to maximize uniformity of temperature throughout each brazing load and minimize any distortion of components that are being brazed together.

Figure 1 shows a cross-sectional drawing of an actual tubular joint made from 6061-aluminum, furnace brazed using 4047-aluminum brazing filler metal (BFM). Notice how the brazing filler metal (BFM) has formed a nice concave-shaped fillet on the outside of the joint (right side of drawing), but also displays a nice fillet at the far left (bottom) of the joint. But there are also a lot of trapped-air voids in the middle of the joint. How then did the BFM get all the way down to the bottom of that long braze-joint? Is that even possible?

We were told that a ring of 4047-aluminum was used on the outside of the joint, but the rest of the joint was a bit of a mystery that required investigation.

QUESTION FOR READERS: How did the BFM get all the way down to the bottom of that long braze-joint? Is that even possible for that externally applied 4047-ring to do that? Aren’t we supposed to be operating with the RULE OF BRAZING that states: “Feed the BFM from one end of the joint, and inspect the other end of the joint to verify that the molten BFM has pulled all the way through”?

In this article, I will explore the effect that thermal expansion has on joint clearance, and thus, on brazed joint strength and quality. It’s an important concept, and although it is well known in the brazing world, many folks today still do not take this topic seriously enough when designing brazed assemblies. This article is based on one I wrote many years ago for an in-house brazing publication at a brazing filler metal supplier. Included in this article I will look more closely at polymorphic metals, such as carbon steels, and will attempt to explain why they exhibit their very strange thermal expansion curves.

Please note that ALL metals expand (grow) when they are heated, and contract (shrink) when they are cooled. This fact has been thoroughly explored over the years, and data-tables have been published showing how fast each metal expands as temperature increases. This important information about the expansion characteristics of each metal should always be used in developing braze procedures when different kinds of base metals are to be brazed to each other. The success or failure of a braze procedure may very well depend on it!

All metals, when heated up to brazing temperature, will want to react with any oxygen present in the atmosphere around the part, which will cause an oxide layer to form on its surface. Unfortunately, brazing filler metals (BFMs) do not like to bond to oxides. Therefore oxides must be prevented from forming during the brazing process. This is especially true for base metals that contain small amounts of titanium or aluminum, such as Inconel 738 for example, since such oxides, once formed, cannot be “reduced”, i.e., removed or dissociated during any brazing process. Wow — what to do?

The best “safety procedure” to use is to prevent those strong oxides from forming at all! This can be accomplished by nickel-plating the surfaces prior to brazing. Nickel-plating is compatible with most metals, and BFMs will readily bond to, and alloy with, the applied nickel-plating.

1. Two types of nickel-plating.

a. Electrolytic nickel-plating. This is pure nickel and melts at about 2600°F (1300°C). This is high enough in temp so that it will provide a tough, adherent surface to which to braze, and it will prevent any of the aluminum or titanium in the base metal from oxidizing; thus the plated surface should be quite brazeable!

b. Electroless nickel-plating. This is an alloy of nickel, not pure nickel. The primary added element is phosphorus (in varying percentages), but boron is sometimes used as well. In either case, electroless nickel-plating tends to melt down around 1600°F (850°C), which is often well below that needed for successful brazing. If electroless nickel plating is accidentally used, the plating may melt and flow away long before you have reached brazing temp, thus allowing the underlying aluminum and titanium components of the base metal to become exposed on the surface, oxidize, and thus prevent brazing altogether.