Selection of Filler Materials

Filler materials are made up of combination of different metals. They are used to join two similar or dissimilar metals. The properties of filler metals should resemble with the base metals to be joined. Understanding and evaluation of the three basic characteristics of filler metals are important, before selecting. They are available with different physical properties, melting behavior, melting range, and forms.

• Physical Properties
  The physical properties of filler materials depend upon their metallurgical composition. Their composition determines whether the filler metal is compatible for joining the metals or not. It also determines the capability of wetting the base metals and the flowing characteristics through the joint area without forming the damaging metallurgical compounds.
  Brazing of electronic components require filler materials with immense purity. Joints with resistivity against corrosion require the filler metals, which are corrosion resistant as well as compatible with the base metals. Vacuum brazing need filler materials free of any volatile elements such as zinc or cadmium.
  
  
• Melting Points
  Melting points of filler material is another such characteristics, which is also based on metallurgical composition. The melting range is determined by the alloy’s chemical composition. The melting range of a filler alloy is determined by the minimum temperature at which the alloy starts melting, ie, at solidus temperature, and the temperature at which the alloy is totally liquid, ie, at liquidus temperature.
  
  Filler alloys possessing wider range are used for filling larger gaps while alloys with narrow melting range are generally used for filling very narrow gaps. Generally the wide range alloys have a tendency to decompose into their basic constituents if they are heated too slowly. It is always beneficial to heat rapidly through the melting range to reach brazing temperature.
  
  
• Liquidus Temperature
  Liquidus temperature is the most important decisive factor in any brazing process. During brazing, the base metals should never melt. Hence, the filler metal should have its liquidus temperature lower than the solidus temperature of both of the base metals to be joined.
  
• Forms
  Filler alloys can be available in different forms like paste, wire, powder, rod, etc. Selection of proper form of filler material is very important for the strength of brazed joints. Brazing rod is best for some processes while for some other processes, paste powder is required.