Selection of Brazing Flux

Brazing is a very old process used to join two similar or dissimilar metals by heating them in the presence of filler metal. Joints made by brazing usually show greater strength as compared to soldered or welded joints. Flux play a very significant role in all these process. The selection of a proper flux is very much important for the strength of joints. Below are some of the important tips which helps you in the selection of proper flux :


Fluxing Agent: Selection Criteria
Selection of flux is very important to understand the different types of brazing. During evaluation, various operating parameters are required. It should be kept in mind that they are not listed in any order of priority.

• Safety
  While considering the operating parameters, safety is the most important aspect. The person involved with the handling and use of fluxes should be educated, trained and instructed to follow the operating and handling guidelines.
  • Plant Safety
    Plant safety is important to minimize the potential risks that might be encountered during brazing. The flux material should be inert to the various materials with which it might come into contact during use. In order to prevent the possibility of explosions in the event of overheating, the rate of decomposition should be very slow.
    
    
  • Personal Safety
    While using flux you should be sure that the necessary safety equipment and materials are provided or not and they are working properly or not. The brazing area should be properly ventilated and safe.
    
    
• Temperature & Time
  Temperature and time are the other important factors in the selection of fluxes. The amount of heat is utilized to decrease the amount of time required to execute the brazing application. The faster the brazing takes place, the less time there will be for any excessive heat to travel.
  • Temperature
    Flux temperature should match the overall temperature range needed for the entire brazing process. Brazing alloys, methods and equipments, which are used to perform the brazing process determines the temperature range.
  • Time
    Fluxes with shortest brazing time are preferred in almost all applications. It becomes more important where there are chances of exposing heat sensitive components to longer brazing times. The flux selected for the application should quickly affect the brazing system.
  
  
  
• Corrosion
  All fluxing agents are corrosive, some to lesser extent while some to higher. The corrosion must be controlled to weaken the brazed joints. However, if the corrosion of a flux is not checked then it becomes important to ensure that its byproducts are properly removed after the brazing is over.
• Cost
  The total operating cost does not only include the initial price of the flux, but also also the cost of cleaning materials and other expenses. Cheaper flux might create problems during cleaning. The efficient but expensive flux is a good option to save money over time providing increased reliability.

What Fluxes Do
At first it is important to know the working of fluxes. What they do and how they do it is essential to know.

• As heat is applied to the flux, it dissolves surface oxides and protect the surfaces from further oxidation. It remove the oxidation products, thus allowing the filler metal to contact and wet the base materials. It transfers the heat from the source to the joint.
  
  
• Brazing fluxes fuse at temperatures below the melting point of the filler metals. Whether they are in paste or powder form, they must be in close contact with the joint surfaces. At brazing temperature they change in liquid or gaseous state. The important point is that they remove surface oxides and tarnish only. The other contaminants like oil, grease, lubricants, lacquer, and paint must be removed prior to brazing either mechanically or chemically.

Safety Precautions
You should take care a lot while dealing with the fluxes. They should be irritating and dangerous when come in contact with the skin. Occasional contact isn't so much dangerous however long term use can be harmful. If there are any cuts or breaks in the skin, then it should be covered with dressing. Fluxes, which contain chlorides or fluorides may delay the healing of wood. Fluxes when heated above their maximum defined temperatures produce fumes, which are harmful for human beings. So the brazing areas should be properly ventilated and shouldn't be done in closed spaces.