Let’s join materials! What's Brazing?
Among various joining processes, brazing is a particular variant that allows joining two different components without melting either of the materials (base materials). The brazing process is characterized by the creation of a liquid that is made to flow into and fill the spaces between the components to be joint and then allowed to solidify.
The liquid, appropriately termed as “the filler” since it fills the space between the joint, has a comparatively lower melting point than the materials being joined. Hence, no melting of the substrate occurs. Consider this as being a liquid glue to join two materials together. The only difference is that the glue has to be melted before it can be activated for joining.
The filler material has to be of a different composition than the material of the components to be joined. The choice of the filler material largely depends on the materials that are to be joined together. Braze fillers can be ceramic for joining ceramics to ceramics, or metallic for joining metals to ceramics or themselves. The process is clean, efficient and easily reproducible. The brazing process is particularly effective since it involves a substantial degree of inter-diffusion between the filler and the substrate. You can consider it as atoms moving from the filler material to the base material.
Capillary action, wetting and possible chemical reactions during the process ensure a powerful metallurgical bond due to bonding at the atomic level. This is why brazing is often the first choice in joining simple and complex structures ranging from the radiator cores in your automobiles to honeycomb sandwich cores in the aerospace industry.
What’s so special about brazing
For starters, brazing maintains the compositional integrity of the base materials that are joined. This is because only the filler material is melted during the brazing process. This ensures that the base material characteristics are preserved. The heating, for the melting of the filler material during the process, is highly localized. Since brazing temperature can be kept relatively low compared to the substrates’ melting temperatures, the process can be an ideal choice where base material melting cannot or should not occur. Thermal distortions, heat-induced stresses are thus largely minimized.
As no melting of the base materials occur, brazing is frequently the choice for joining different material types. The limited atomic intermixing ensures that the different material types are properly wetted and a sound joint assembly can be formed. The brazing process has the ability to join the cast, wrought, or powder-processed metals, dissimilar metals, oxide and non-oxide ceramics, metals to ceramics, carbonaceous materials (e.g., graphite), and fiber- or dispersion-strengthened composites with metallic, ceramic, inter-metallics, or carbonaceous matrices.
Brazing provides a simple means for bonding large joint areas or long joint lengths thus enabling even distribution of stresses over a large area. Due to its versatility, a large variety of thickness can be congregated together especially assemblies that are composed of thin-to-thin and thin-to-thick structural components.
It’s all about wetting
If you are wondering how one gets a good braze, the answer is unambiguous. It is due to the ascendant physical principle of capillary flow, which occurs due to a liquid (molten filler) lowering the surface free energy of a solid-vapor interface by wetting the solid (materials to be joined) according to Young’s equation:
ϒνs = ϒΙs + ϒvl cosΘ
Where ϒvs, ϒls, and ϒvl are the surface free energies for the vapour-solid, liquid-solid, and vapour-liquid interfaces, respectively, and is the angle of contact or ‘‘wetting angle’’ for the liquid on the solid surface.
You can consider the principle of wetting as being equivalent to soaking the materials entirely with the liquid till the maximum possible surface on the substrate is covered. The better the coverage of the liquid, the better the wetting, the better the capillary flow and the stronger the brazement. For a good wetting, low angles of Θ are desirable as they facilitate a good capillary flow. The better the capillary flow, the more effectively the molten filler liquid covers the substrates. One can hence consider that the process of brazing is predominantly controlled by surface conditions of the base materials that are to be joined together.
How to braze?
The process of brazing is simplistic. There are 4 critical steps involved:
I. The surfaces to be joined must be cleaned to remove all contaminants.
II. The surface of the base materials should be coated with a material called flux. Flux is employed to get rid of residual contaminants on the surfaces to be joined and for the prevention of oxidation.
III. The joint area must be heated to melt the flux. The molten flux’s chemical reactivity cleans the substrate and prevents further oxidation by remaining as a layer of liquid flux.
IV. The filler metal is melted between the surfaces of the substrate, displacing the flux, wetting the surfaces and forming the braze joint.
That’s it. That’s how a braze joint is made. The process is easily automated and can be carried out in minutes.
That’s one of the benefits of brazing. It’s quick and reproducible.
Since brazing is a capillary force driven process, the flux and filler material has to be melted. The heating can be done to locally heat the joint or the whole assembly undergoes generalized heating.
Brazing processes themselves are often classified according to the sources of heating. Chemical heat sources for melting include Torch Brazing, Furnace Brazing, Exothermic Brazing and Vapor- Phase Brazing. Electrical sources are often widely used in an automated industrial setup and include Chemical Dip Brazing, Molten Melt Brazing, Induction Brazing, Infrared Brazing, Electron Beam Brazing and Diffusion Brazing.
These examples are not exhaustive or complete, but they do give an idea about the various heating sources, for melting the filler material and flux, that are prevalent in the industry.
Which braze filler alloy do I use?
This is often the most crucial decision to make. What filler should one use for a particular brazement?
The filler material should possess distinctive characteristics to be used for brazing. It should have the required mechanical, chemical and physical properties that can be imparted to the braze joint. Most importantly, the filler material should have a coefficient of thermal expansion (CTE) close to that of the base materials. This ensures that no thermal stresses are generated after solidification.
The melting point of the filler material should be lower than the one of the base materials and the molten liquid formed during the melting process should have the appropriate fluidity which can enable it to flow and distribute properly into the joints via the capillary action. Also, the composition of the filler material must be homogeneous and stable so as to inhibit the separation of the constituents.
So there you have it. These are some general principles of which are considered while selecting the appropriate filler material for the brazing process.
Brazing is a versatile process for producing permanent, mechanically stable and leak-proof joints without melting the base material. It is characterized by the use of a filler material that melts above 450°C but below the melting temperature of the base material. It is necessary for the molten filler material to properly wet the surface of the joint area to facilitate capillary flow and proper distribution.
By following simple guidelines for choosing the appropriate melting source and the filler material, a desirable braze joint can be produced.
 Philip Roberts, Industrial Brazing Practice, Second Edition, CRC Press, 2013
 J.J. Stephens, K.S. Weil, Brazing and Soldering – Proceedings of 3rd International Conference, ASM International, 2006
 Derek Pritchard, Soldering, Brazing and Welding – A Manual of Techniques, 2000
 Dušan Sekulić, Advances in Brazing, Science, Technology and Applications – First Edition, Woodhead Publishing, 2013
 Mel M. Schwartz, Brazing, Second Edition, ASM International, 2003
 D.L. Olson, T.A. Siewert, S. Liu, G.R. Edwards, ASM Handbook Volume 6: Welding,Brazing and Soldering, ASM International, 1993
 Umicore KG&A, BrazeTec Unit, Hanau, Germany
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