What is Brazing?
Definition: Brazing is defined as a process of joining two similar or dissimilar metals by using a fusible alloy called “spelter”.
Brazing is similar to the soldering process, but the only difference is that brazing uses spelter instead of solder.
Now you might have a question, What is spelter? Isn’t it?
Spelter refers to the alloy in which zinc is the main constituent. It is softer and has a lower melting point. Spelter also refers to zinc-copper alloy (i.e brass) which is used for brazing of metals.
Principle of brazing
In metal brazing, two or more metals are joined without melting them.
The extra filler material which is soft and has a lower melting point is used to join the metal parts.
In metal brazing, the filler material is heated above the temperature of 450 °C.
6 Types of Brazing
Different types of brazing methods used for heating the filler material are mentioned below.
- Torch brazing
- Furnace brazing
- Induction brazing
- Dip brazing
- Resistance brazing
- Laser brazing
#1) Torch brazing
In torch brazing, the heat required for melting the spelter (or filler material) is supplied through the torch. The fuel used for the torch can be acetylene, natural gas, butane or propane in combination with the atmospheric oxygen or air.
The flame of the torch heats up the filler material and melts it.
This molten metal (i.e molten filler material) gets diffused into the surface of the base metal and a strong joint is obtained on solidification.
The flux used in this method is in the form of power or paste.
#2) Furnace brazing
Furnace brazing is a high production process in which the heat required for melting the spelter is supplied by gas or electric heating coils.
The furnace used for brazing are either box type or continuous type.
In furnace brazing, the use of flux can be avoided if the inert atmosphere is maintained during the brazing operation.
This process is more suitable for higher production and fast brazing operations are required.
#3) Induction brazing
In induction brazing, the parts as well as filler material are heated by using a high frequency induction coil. The metal part which is to be brazed is placed inside this high frequency induction coil.
The heating of the parts is very rapid and the heat can be applied to the specific local area by properly shaping the induction coil.
In order to braze the metals using induction brazing method, the flux is necessary to apply on the brazing area to protect the brazing area from contamination. This flux can be applied in the form of paste.
#4) Dip brazing
Dip brazing is generally of 2 types.
- Chemical dip brazing: In chemical dip brazing, the metal parts with performed filler metal are placed into a molten bath of flux.
- Molten metal batch process: In this type of dip brazing, the assembled parts are first pre-fluxed and then they are immersed in a molten bath of filler metal.
The chemical dip brazing is more suitable for joining large parts while the second process is more suitable for small parts.
#5) Resistance brazing
Resistance brazing is similar to the resistance welding process, but the only difference is that in resistance brazing, the filler material is placed on the joint.
In resistance brazing, rapid heating takes place which minimizes oxidation and the HAZ is also less.
#6) Laser brazing
Laser brazing is a costly method and it is only used for very precise brazing work of high value.
Different types of brazed joints used for metal joining are mentioned below.
Lap joint and butt joint are generally used in sheet metal work while the scarf type of joints are used for brazing of rods or pipes.
How to clean metal surface before and after brazing?
In order to get the perfect brazed joint, it is necessary to clean the surface before starting the brazing of metals.
Before brazing iron or steel, the surface should be thoroughly cleaned either by filing process, grinding process, or a sand blasting process.
Bronze or brass parts can be cleaned by dipping them in a solution of one third nitride acid and two third is sulfuric acid.
This same solution can be used to remove the scale after brazing.
Advantages of brazing
Advantages of brazing are listed below.
- Dissimilar metals can be joined by brazing.
- Thin metals sheets can be joined easily by brazing.
- The brazed joints look clean and smooth.
- Thermal stress is very less and so the distortion of base metal does not occur.
Disadvantages of brazing
Disadvantages of brazing are mentioned below.
- Brazing is not suitable for large size workpieces.
- Skilled person is required to get the perfect brazed joints.
- Sometimes machining of the joint edges is necessary to get the desired fit. Such machining operations are sometimes costly.