Brazing is a popular form of welding. It is an excellent way to join two metals. Fillers are brought to high temperatures – usually above 800 degrees Fahrenheit – and join workpieces together by flowing into the spaces between them and cooling. A flux is often used to protect atmosphere around the work area. There are many techniques for brazing that a welder can choose from.
Torch brazing is the most common form of mechanized brazing. In some countries it is the method used for the majority of the brazing that is done. Specialized operations or small production volumes are often where this method is used. The three categories of torch brazing are machine, manual, and automatic.
Manual torch brazing has heat applied with a gas flame near or on the joint. It can be a hand held torch or held in a fixed position, depending on the method. Usually it is used where other methods are impossible or for small production volumes. Flux is required. Machine torch brazing is used when the operation is repetitive. It is a mix of manual and automated methods and uses flux, reduces the cost and works for small to medium production projects. Automatic torch brazing has a high production rate, reduced costs, and a uniform braze quality. A worker is needed just for unloading and loading the machine.
This method is semi-automatic and is used in industrial operations. It can produce large numbers of small parts, has a controlled heat cycle, and post braze cleaning is not needed. To prevent oxidation, inert, vacuum, or reducing atmospheres are used. It is cost efficient to run but uses a lot of power compared to other methods, is more difficult to design, and the equipment is expensive. The kinds of furnaces used are batch type, vacuum, retort with controlled atmosphere, and continuous.
Batch type furnaces have relatively low costs and heat each load separately. It can be turned on and off easily, has a large degree of flexibility, and is suited to medium or large productions. Flux or a controlled atmosphere can be used. Vacuum furnaces are fairly economical, work well with such oxides as aluminum or titanium, and is often used with refractory materials or alloys that can't be brazed in atmosphere furnaces. It is vital to clean because there is no protective atmosphere. Continuous type furnaces work by making a steady flow of parts go through the furnace on a conveyor. These are good for large productions. Retort-type furnaces have a sealed lining where the atmosphere can be completely changed inside and is best for semi-continuous or batch productions and alloys that resist oxidation.
This method uses silver alloy based filler for brazing. It is also known as hard soldering or silver soldering. The silver alloys have a lot of variety and different percentages of silver and other metals in them, such as cadmium, zinc, and copper. A special method of silver brazing is pin brazing (pinbrazing). It is used especially for cathodic protection installations or for connecting cables to railway track. It can be used in the tool industry to do such jobs as fasten hardmetal (like carbide) to such tools as saw blades.
This method uses a brass or bronze filler rod that is coated with flux in order to join steel workpieces. It requires more heat than basic brazing and acetylene or methylacetylene-propadiene (MPS) gas fuel is often used. The name comes from the fact that this method does not have capillary action. Dissimilar metals are able to be joined with this method, there is a reduced need for pre-heating, and minimal heat distortion. However, there is a loss of strength when the work is under high temperatures and it cannot withstand high stress.
Cast Iron "Welding"
Welding cast iron is actually a type of brazing. Filler rods that are mainly nickel are used but there are cast iron rods available. Cast iron is a difficult metal to work with and many find the skill extremely difficult to learn. It is used often in repairs.
In this technique the brazing process is done inside of a vacuum. There are many advantages such as flux-free joints that are very strong and have high integrity, are superior to other joins and are extremely clean. It can be an expensive process. Residual stresses are greatly reduced because of the slow heating and cooling cycles in this process. The material's thermal and mechanical properties are improved and things such as heat treating or age hardening can be done during the metal joining process. The process is done in a furnace and heat is transferred with radiation.
This particular technique is very suited for brazing aluminum because there is no air and therefore no oxidation. The brazing compound is usually applied in slurry form and the assembly dropped into a molten salt bath that will work as both a flux and heat transfer.