Differential Metal Expansion & Brazing Challenge Question

Differential Metal Expansion & Brazing Challenge QuestionIn 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!

Last Updated on Sunday, 03 June 2018 21:52

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Be sure to “Blister Test” your nickel-plating before brazing

Be sure to “Blister Test” your nickel-plating before brazingAll 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.

Last Updated on Friday, 18 May 2018 01:51

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One More Frequently Misused Brazing Term: “Brittle”

In my last two articles, we explored the definitions of the words “brazing”, “passivation” and “defect”. Each of these words has also been mentioned and discussed in this column in years past, but I am bringing them out again to help a new generation of brazing personnel to understand them correctly. Another word that needs to be explored once again, because of its misuse by many brazing personnel today, is the word “brittle”.

Brittle: Particularly when referring to parts that have been brazed with a nickel-based brazing filler metal (BFM), I still hear some people say that those nickel-brazed joints are “brittle” joints, and thus are probably not suitable for certain applications. Be careful! This is not true! Early in my metallurgical training (I am a graduate Metallurgical Engineer from Rensselaer Polytechnic Institute) I learned that “brittle” is not a word describing “a state of being”, but instead, is used to describe a mode of failure, as in the words: “…that joint failed in a brittle manner”.

Last Updated on Friday, 06 April 2018 20:38

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Two Frequently Misused Brazing Terms: Passivation, and Defect

In last month’s column we explored the definition of the word “brazing”, so that the meaning of that important term could be more fully understood and differentiated from the terms “soldering” and “welding”. In this month’s column, I would like to take a look at some additional words that are often misused in the world of brazing, namely, the words passivation and defect. These words have been mentioned and discussed in this column in years past, but it’s time to bring them out again for a new generation of brazing personnel. I still hear people use these metallurgical terms improperly when they are describing certain criteria related to braze-prep or braze-inspection; so let’s look at them again to understand them more correctly.

Passivation vs. Pickling: In a number of brazing shops I visit there still seems to be some confusion regarding the correct use of the term “passivation” (when “pickling” is actually meant) when it comes to preparing metal surfaces for brazing. The two terms, “passivation” and “pickling”, illustrated in Fig. 1, have completely opposite meanings, and thus, these two terms need to be clarified so that brazing personnel can use these two metallurgical terms correctly.

Last Updated on Friday, 06 April 2018 20:40

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“Brazing” Definition – Clarification of the Terminology

  As we start out this new year, I think it would be good to review the definition of the word “brazing”, since it is somewhat complex, and some aspects of the definition are still being misused by a number of people in the brazing industry, not only in their speech but also in their writing. So, it’s time to take a fresh look at the word, especially for those who are somewhat new to brazing, and perhaps for some older persons who never really understood what that definition meant in the first place!

The American Welding Society (AWS), in the Fifth Edition (2007) of their AWS Brazing Handbook, defines brazing as “a group of joining processes that produces coalescence of materials by heating them to the brazing temperature in the presence of a filler metal having a liquidus above 840F (450C) and below the solidus of the base metal. The filler metal is distributed between the closely fitted faying surfaces of the joint by capillary action.” The handbook goes on to state in its Preface that brazing needs to meet the following three criteria: “(1) The parts must be joined without melting the base metals. (2) The filler metal must have a liquidus temperature above 840F (450C). (3) The filler metal must wet the base metal surfaces and be drawn into or held in the joint by capillary action.”

Last Updated on Friday, 06 April 2018 20:28

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Essential Criteria for Brazing: Item 7 – Knowledgeable Inspection of Finished Brazed Assemblies.

Essential Criteria for Brazing: Item 7 – Knowledgeable Inspection of Finished Brazed Assemblies.Sherlock Holmes was a famous detective in some British spy stories written in the late 1800’s by British author Sir Arthur Conan Doyle. Sherlock was a great detective primarily because he was an expert observer. When he inspected a crime scene he saw things that others often overlooked. From these keen observations, and from his extensive experience, he was able to solve problems others seemingly could not.

In this month’s article, we wrap up our discussion of the seven (7) essential criteria for good brazing (see Table 1), by looking at what constitutes “knowledgeable inspection” of finished brazed assemblies. I often encourage people to “become a Sherlock Holmes” in their work when trying to solve brazing problems, and during final-inspection of brazed assemblies. Our goal should be to produce parts that have “zero-defects” (see Fig. 2), and to do this, each “inspector” needs to understand what a defect actually is, how to accurately find it, and what to do with the brazed assembly when an actual defect is found.

Carbon-fiber reinforced carbon (C/C) fixturing for brazing?

Carbon-fiber reinforced carbon (C/C) fixturing for brazing?This question is one being asked in many brazing shops today, and the answer is a resounding “Yes!” — carbon-fiber reinforced carbon (C/C) fixtures are being used more and more frequently in brazing shops around the world due to their lightweight, high strength, and dimensional stability over a wide temperature range as compared to similar fixtures made from steel.

As shown in Fig. 1, carbon-fibers are long filaments made of carbon, that when blended/woven in a matrix of carbon, creates a reinforced carbon structure that is very strong, lightweight, and has some flexibility. When woven into a carbon-fiber reinforced carbon structure, the material looks like that shown in Fig. 2, where the visible “weave” resulting from reinforcing carbon fibers is clearly evident. When such structures are stacked into a fixture for use in a brazing furnace, it would look similar to that shown in Fig. 3.

Last Updated on Wednesday, 04 April 2018 21:28

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