Joining by Welding, Soldering, and Brazing
Welding Deoxidized copper will weld satisfactorily by the oxyacetylene
method. Sufficient heat input to overcome its high thermal conductivity
must be maintained by the use of torches considerably more
powerful than those customary for steel, and preferably by additional
preheating. The filler rod must be deoxidized. Gas-shielded arc welding
is preferred. Tough-pitch copper will not result in high-strength welds
because of embrittlement due to the oxygen content. Copper may be
arc-welded, using shielded metal arc, gas metal arc (MIG), or gas tungsten
arc (TIG) welding procedures using experienced operators. Filler
rods of phosphor bronze or silicon bronze will give strong welds more
consistently and are used where the presence of a weld of different
composition and corrosion-resistance characteristics is not harmful.
Brass may be welded by the oxyacetylene process but not by arc welding.
A filler rod of about the same composition is used, although silicon
is frequently added to prevent zinc fumes.
Copper-silicon alloys respond remarkably well to welding by all
methods. The conductivity is not too high, and the alloy is, to a large
extent, self-fluxing.
Applicable specifications for joining copper and copper alloys by
welding include
ANSI/AWS A5.6 Covered Copper and Copper Alloy Arc Welding
Electrodes
ANSI/AWS A5.7 Copper and Copper Alloy Bare Welding Rods
and Electrodes
ANSI/AWS A5.27 Copper and Copper Alloy Rods for Oxyfuel
Gas Welding
The fluxes used for brazing copper joints are water-based and function
by dissolving and removing residual oxides from the metal surface; they
protect the metal from reoxidation during heating and promote wetting
of the joined surfaces by the brazing filler metal.
Soldering Soldered joints with capillary fittings are used in plumbing
for water and sanitary drainage lines. Such joints depend on capillary
action drawing free-flowing molten solder into the gap between the
fitting and the tube.
Flux acts as a cleaning and wetting agent which permits uniform
spreading of the molten solder over the surfaces to be joined.
Selection of a solder depends primarily on the operating pressure and
temperature of the system. Lead-free solders required for joining copper tube and fittings in potable water systems are covered by ASTM B32
Solder Metal.
As in brazing, the functions of soldering flux are to remove residual
oxides, promote wetting, and protect the surfaces being soldered from
oxidation during heating.
Fluxes best suited for soldering copper and copper alloy tube should
meet the requirements of ASTM B813 Liquid and Paste Fluxes for
Soldering Applications of Copper and Copper Alloy Tube with the
joining accomplished per ASTM B828 Making Capillary Joints by Soldering
of Copper and Copper Alloy Tube and Fittings.

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