What is the first choice of filler metal for brazing work?

 Select filler metal for a brazed joint depends on five main points: 

properties of the filler metal 

joint design 

service requirements 

base metal composition

cost and availability

1.Properties. Melting point or melting range determines whether the material flows all at once or over a range of temperatures. Fluidity, poor or good fit up of the braze joint determines if the job needs sluggish or rapid-flowing filler. Knowing the fluidity of the filler metal lets the technician prescribe proper joint clearance. Joint remelt temperature. Filler material alloys with the base material at rates depending on the compositions of both materials and temperature. This interaction determines the composition and the melting temperature of the brazed joint, the temperature at which it will fail by fusion. Vapor pressure. In vacuum brazing the filler metal should have a vapor pressure lower than the vacuum pressure to prevent out gassing of filler metal elements. Such out gassing results in a braze of poor quality.

2.Joint design. For the reasons listed previously, the technician needs to know joint geometry and clearance in order to choose the filler metal with the right flow properties. 

3.Service requirements. These include joint strength, ductility, and heat and corrosion resistance. The chemistry, flow characteristics, and mechanical properties of the final braze metal determine the choose of filler. Depending on the application, the alloy may be in the form of a stick, paste or preform. A pre-formed brazing alloy is normally the best choice when even distribution and repeatability are paramount considerations.

4.Base metal composition. The base metal alloys with the braze filler. The properties of the alloyed braze metal ultimately determine the properties of the joint. 

5. Cost and availability. Cost of raw materials in today’s market have a significant impact on the cost of a brazed assembly. Silver, copper and aluminum alloys are commonly-used filler metals; silver is frequently chosen because it has a relatively low melting point. Copper braze has a higher melting point but is generally more economical. Volatile and unpredictable fluctuations in both precious metals and strategic metals markets greatly affect the cost of brazing. The effect of this unpredictability can be controlled to a degree by engineering and designing parts that utilize brazing filler metals that are less sensitive to fluctuations in the metals markets.