Are Recessed Braze-Fillets Okay?

This question comes up frequently and needs to be addressed again. As shown in Figure 1, the brazing filler metal (BFM) has filled the inside of the tubular joint but has a slight recess at the top edge of the joint. There is no large external fillet (or “meniscus”) of BFM on the outside of the brazed joint. Notice in the photo how the recessed material has a concave shape to it. The “meniscus” of any liquid is the curved shape of the surface of that liquid caused by surface tension. A meniscus can be either concave (desirable in brazed joints) or convex. In Fig. 1, there is a concave recessed meniscus to the BFM at the top edge of the joint. Is this okay?

Many people who see such a joint may incorrectly think that any fillet, whether it is for welding or for brazing, must extend beyond (outside) the joint in order to be acceptable and that any joint that has a recessed-meniscus, as shown in Fig. 1, has to be rejected as being “incomplete”. This is what I call “weld-think”, and has resulted in many such joints being re-brazed (unnecessarily) in order to add more BFM to the joint until the resulting joint shows a large external fillet. This is erroneous thinking that can actually hurt the brazed assembly.

Last Updated on Thursday, 21 March 2019 01:08

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Revisiting Refrigeration of Brazing Paste. Is Refrigeration Really Necessary?

Revisiting Refrigeration of Brazing Paste. Is Refrigeration Really Necessary?A number of years ago I wrote an article about the question of refrigerating brazing paste, something that has caused real problems for a number of people in the brazing industry, and which is still doing so today. A recent question about this topic was sent to me and indicated to me that it’s time to once again discuss this topic.

Many people in brazing shops today are still receiving brazing filler metal (BFM) pastes in containers (both large and small) indicating on the label that the BFM-paste must be refrigerated prior to use. Without proper explanation, a simple statement such as “Must be refrigerated prior to use” can lead to significant misunderstandings about what is meant by such a phrase, and has caused a lot of difficulties for brazing personnel who have erroneously believed that brazing paste, according to that warning, has to be “cold” when it is being used in the shop. Thus, before they use the brazing paste, they place it in a small refrigerator, such as that shown in Fig. 1, and then remove it the next day for use in their shop. That is completely wrong.

Last Updated on Thursday, 12 November 2020 22:38

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Vacuum brazing of EGR Coolers

Vacuum brazing of EGR CoolersExhaust Gas Recirculation (EGR) coolers have been used extensively for many years on diesel truck engines, and are now also being used on gasoline engines. EGR coolers are specialized heat-exchanger assemblies that make extensive use of furnace-brazing (usually vacuum brazing) to create strong, leak-tight brazements that are capable of handling the very high temperatures involved in engine operations. EGR coolers used in diesel-engine applications help to reduce the formation of various nitrogen-oxides, such as N-O (nitrogen monoxide) or N2O (nitrogen dioxide), since such emissions are considered atmosphere pollutants, and are formed within a narrow temperature band in the combustion cycle. By recirculating some of the engine’s exhaust gases back to the engine through the EGR cooler, this cooled recirculated-gas gets mixed in with the incoming air entering the engine-cylinders and helps to reduce the combustion temperatures just enough so that less of these pollutants are formed.

Last Updated on Friday, 13 November 2020 00:12

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Don’t ever use a Brazing Paste-flux when Vacuum Brazing!

A number of years ago I wrote an article for this website about the use of a brazing flux in a vacuum furnace, in answer to an inquiry I had received about the advantages or disadvantages of using a paste flux to enhance the brazeability of components in a vacuum brazing furnace. It is time to once again bring up that subject for today’s brazing community, in order to be sure that everyone understands that you should NEVER put a brazing flux in a vacuum furnace!

The brazing fluxes that people usually refer to when asking about the use of “a flux” in a vacuum furnace are the same fluxes that they use when hand-brazing with a torch or induction coil out in the open air. Such fluxes, as shown in Fig. 1, are thick paste products, with the consistency of thick mayonnaise, and the paste is either white or black in color.

Last Updated on Thursday, 12 November 2020 22:27

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Torricelli’s Importance in Vacuum Brazing History

Vacuum brazing involves the removal of all gaseous atmosphere from the sealed chamber of a “furnace” used for high-temperature brazing, a “furnace” being defined as any enclosure that can be heated to a high enough temperature to accomplish a specific task, such as heating a home, or heating an atmosphere to a temperature that can melt particular substances, such as a solder or a brazing filler metal (BFM), etc. By removing the atmosphere from a furnace during brazing, oxidation risks are eliminated (or greatly reduced), and brazing success is enhanced greatly. Thus, brazing in a vacuum furnace continues to gain popularity in the brazing world every day, because of its ability to “create a vacuum”, i.e., significantly reduce the amount of atmosphere (thus, oxygen), inside the brazing furnace. But — where did our understanding of vacuum come from? One of the first practical experiments with vacuum was conducted by Evangelista Torricelli, an Italian scientist, back in 1643. Torricelli was a great thinker and put into action many of his theories related to both physics and mathematics

Last Updated on Thursday, 12 November 2020 22:40

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Reducing Metal-Oxides in Brazing

Reducing Metal-Oxides in BrazingLet me make two important statements right at the start: 1.  Surface-oxidation of metals will prevent effective brazing. 2.  Brazing filler metals (BFMs) do not like to bond to or flow over, oils, dirt, greases, or oxides on metal surfaces. Thus, if any of the surface contaminants just mentioned are present on the metal surfaces to be brazed, effective brazing will not occur.  Effective brazing requires the BFM to be able to alloy with (i.e., diffuse into) the base-metal being joined in order to form a strong, leak-tight metallurgical bond.  The amount of alloying required is not large, e.g., copper BFM on steel actually alloys/diffuses much less than 5% and yet forms very strong, leak-tight brazed joints on steel.

Surface-oxidation is a common source of problems in commercial brazing, especially in those shops where production personnel say: “Don’t worry about that oxidation; the furnace will take care of that!”   Wishful thinking, and highly impractical, since furnace atmospheres may be able to “clean up” the outside surface of the assembly, but will NOT be able to effectively clean deep down inside a braze-joint if any of those inside surfaces (faying surfaces) were oxidized or contaminated prior to assembly.  Parts to be brazed must be cleaned BEFORE assembling the parts for brazing, and then must be kept clean during the brazing process.

Last Updated on Thursday, 12 November 2020 22:38

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Centerline Eutectics in Ni-brazing

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.

Last Updated on Thursday, 12 November 2020 22:22

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