Chapter 7: Assembly and Fusion (or Joining) Processes

A. Soldering and Brazing

Soldering and brazing are closely related methods for joining separate components. In both cases, a filler metal that melts at a lower temperature than the melting or maximum exposure temperature of the pieces to be joined, "wets" the surfaces to be joined and, when it solidifies, provides a solid mechanical or metallurgical bond between the pieces. In soldering, the filler metal has a liquidus (melting point) below 800 F (425 C). Common solders are alloys of tin and lead. Antimony and silver are also included in some solders in lesser percentages. Relative solderability of base metals in descending order is as follows: tin, gold, silver, copper, brass or bronze, lead, nickel, zinc, iron, steel, stainless steel, chromium, and aluminum. Solder joints are made to provide an electrical connection (the prime current application), to provide a seal, to provide a mechanical joint between parts (although the strength of soldered joints is usually inferior to those that are brazed or welded), or to aid in heat transfer between the parts being joined.

In brazing, the liquidus of the filler metal is above 800 F (425 C). Common brazing alloys utilize silver or copper as the major element. Phosphorus, silicon and aluminum are other alloying ingredients. Brazed joints are made for the same purposes as soldered joints but the prime application is to provide a strong mechanical assembly of separate pieces. Brazing may be an economical method for fabricating complex or bulky components including those composed of parts made with different processes or...