October 5, 2017

DEFINITION OF BRAZING AND FIRST FUNDAMENTAL RULES

THE BASIC CONCEPTS AND ASPECTS OF THE BRAZING PROCESS

The first topic that we will address in our blog covers the definition of Brazing.
Without entering too much into technical matters, we will initially concentrate on a basic explanation, that will however give us some precious information that will enable us to identify clearly the basic aspects of the process and the first fundamental rules that one should always keep in mind in order to get best practical brazing results.
Brazing is a method of joining together two same or different materials by means of supplying heat and by using a suitable filler material that is distributed between the closely fitted surfaces of the pieces to be joined by capillary attraction forces.
The following are distinct features of brazing:
– The process takes place by heating up the pieces to be joined at a temperature equal or above 450 °C (if the maximum process temperature is below 450 °C, the process is instead called “Soldering“).
– The temperature reached during heating is such that only the filler material will melt, while the base materials to be joined will remain solid.
– The pieces to be joined have to be closely fitted and assembled together in a way that athin capillary gap (that is: max. dimension of a few tens of millimeter or less) will exist between them.
– The filler material, in the molten state, must adhere to and “wet” the solid surface of the pieces to be joined.
– Under these conditions, the molten filler material will be drawn inside of the capillary gap by virtue of capillary forces and will, after cooling and solidifying, form a permanent jointbetween the pieces.
Brazing is therefore a joining process based on the phenomenon of adhesion of a liquid metal (the molten filler material) on a solid surface (the pieces to be joined) and on the phenomenon of capillary flow (*).
A more detailed explanation of some concepts such as capillary gap and capillary forces, as well as adhesion and wetting of a surface by a liquid, will be addressed in subsequent blogs.
For the moment it is enough to know that, if a liquid adheres to the inner surfaces of a thin, capillary tube (that is: if the liquid “wets” the capillary tube surfaces), then the liquid level will raise inside of the tube, and that this phenomenon is called capillarity.
In brazing, in order to get the liquid filler material to adhere and wet to the solid surfaces of the pieces to be joined, it is necessary that both the filler material and the pieces to be joined are clean and with no superficial oxidation.
The molten filler material will not adhere and wet on dirty (grease, oil, dust, lubrificants, etc.) or oxidized materials, and therefore, in such circumstances brazing will not occur.
From the above, it is now possible to set the first fundamental rules of brazing:
– Because brazing is a capillary flow process, it is therefore necessary that the joint is designed and the pieces assembled in a way that creates a proper capillary gap and path.
 All filler and base materials have to be thoroughly cleaned and degreased before brazing.
– All filler and base materials have to be free from oxidation and protected from oxidation.
In particular, as the oxidation rate of metals increases with temperature, and as brazing requires a strong supply of heat that raises the temperature of the pieces, as above explained, well over 450 °C, it is necessary to protect the assembly, during the whole heating stage, by means of appropriate de-oxidant fluxes.
We have so far given a first definition of brazing, from which we have also gained some important consequences and set some rules.
In the next blogs we will go deeper in the explanation of the fundamentals of brazing.
For the time being it is enough to keep in mind these basic concepts:

– The joint has to be a capillary joint.
– Filler and base materials have to be clean.
– Filler and base materials have to be free from and protected from oxidation.

Following these basic rules is fundamental in order to obtain proper brazing practice.

(*) Note: in some brazing instances the concepts of capillarity and capillary flow are not applicable and therefore the above definitions are not appropriate.
The present discussion, even with these limitations, is however representative of the majority of the day-by-day practical brazing instances and is especially useful for the understanding of the fundamental concepts of the process.