Copper and copper alloys offer a unique combination of material properties
that makes them advantageous for many manufacturing environments. They are
widely used because of their excellent electrical and thermal conductivities,
outstanding resistance to corrosion, ease of fabrication, and good strength
and fatigue resistance. Other useful characteristics include spark resistance,
metal-to-metal wear resistance, low-permeability properties, and
distinctive color.
Welding Processes
In manufacturing, copper is often joined by welding. The arc welding
processes are of primary concern. Arc welding can be performed using
shielded metal arc welding (SMAW), gas-tungsten arc welding (GTAW),
gas-metal arc welding (GMAW), plasma arc welding (PAW), and submerged
arc welding (SAW).
Arc Welding Processes. Copper and most copper alloys can be joined by
arc welding. Welding processes that use gas shielding are generally
preferred, although SMAW can be used for many noncritical applications.
Argon, helium, or mixtures of the two are used as shielding gases for
GTAW, PAW, and GMAW. Generally, argon is used when manually welding
material is less than 3 mm thick, has low thermal conductivity, or both.
Helium or a mixture of 75% helium and 25% argon is recommended for
machine welding of thin sections and for manual welding of thicker
sections of alloys that have high thermal conductivity. Small amounts
of nitrogen can be added to the argon shielding gas to increase the
effective heat input.
Shielded metal arc welding can be used to weld a wide range of thickness
of copper alloys. Covered electrodes for SMAW of copper alloys are
available in standard sizes ranging from 2.4 to 4.8 mm.
Gas-Tungsten Arc Welding. Gas-tungsten arc welding is well suited
for copper and copper alloys because of its intense arc, which produces
an extremely high temperature at the joint and a narrow heat-affected
zone (HAZ).
In welding copper and the more thermally conductive copper alloys, the
intensity of the arc is important in completing fusion with minimum
heating of the surrounding, highly conductive base metal. A narrow
HAZ is particularly desirable in the welding of copper alloys that
have been precipitation hardened.
Many of the standard tungsten or alloyed tungsten electrodes can be used
in GTAW of copper and copper alloys. The selection factors normally
considered for tungsten electrodes apply in general to the copper and
copper alloys. Except for the specific classes of copper alloys,
thoriated tungsten (usually EWTh-2) is preferred for its better
performance, longer life, and greater resistance to contamination.
Gas-Metal Arc Welding. Gas-metal arc welding is used to join
of the coppers and copper alloys for thickness less than 3 mm, while
GMAW is preferred for section thickness above 3 mm and for the joining
of aluminum bronzes, silicon bronzes and copper-nickel alloys.
Plasma Arc Welding. The welding of coppers and copper alloys
using PAW is comparable to GTAW of these alloys. Argon, helium, or mixtures
of the two are used for the welding of all alloys. Hydrogen gas should
never be used when welding coppers.
Plasma arc welding has two distinct advantages over GTAW: (1) the tungsten
is concealed and entirely shielded, which greatly reduces contamination
of the electrode, particularly for alloys with low-boiling-temperature
constituents such as brasses, bronzes, phosphor bronzes, and aluminum
bronzes, and (2) the constructed arc plume gives rise to higher arc
energies while minimizing the growth of the HAZ. As with GTAW, current
pulsation and current ramping may also be used. Plasma arc welding
equipment has been miniaturized for intricate work, known as microplasma
welding.
Plasma arc welding of coppers and copper alloys may be performed either
autogenously or with filler metal. Filler metal selection is identical
to that outlined for GTAW. Automation and mechanization of this process
is readily performed and is preferable to GTAW where contamination can
restrict production efficiencies. Welding positions for PAW are identical
to those for GTAW. However, the plasma keyhole mode has been evaluated
for thicker sections in a vertical-up position. Generally, all information
presented for GTAW is applicable to PAW.
Submerged Arc Welding. The welding of thick gage material, such as pipe
formed from heavy plate, can be achieved by continuous metal-arc operation
under a granular flux. Effective deoxidation and slag-metal reactions to
form the required weld-metal composition are critical and the SAW process
is still under development for copper-base materials, A variation on this,
process can be used for weld cladding or hardfacing. Commercially available
fluxes should be used for the copper-nickel alloys.
Alloy Metallurgy and Weldability
Many common metals are alloyed with copper to produce the various copper
alloys. The most common alloying elements are aluminum, nickel, silicon,
tin, and zinc. Other elements and metals are alloyed in small quantities
to improve certain material characteristics, such as corrosion resistance
or machinability.
Copper and its alloys are divided into nine major groups. These major
groups are:
- Coppers, which contain a minimum of 99.3% Cu
- High-copper alloys, which contain up to 5% alloying elements
- Copper-zinc alloys (brasses), which contain up to 40% Zn
- Copper-tin alloys (phosphor bronzes), which contain up
to 10% Sn and 0.2% P
- Copper-aluminum alloys (aluminum bronzes), which contain up
to 10% Al
- Copper-silicon alloys (silicon bronzes), which contain up to 3% Si
- Copper-nickel alloys, which contain up to 30% Ni
- Copper-zinc-nickel alloys (nickel silvers), which contain up
to 7% Zn and 18% Ni
- Special alloys, which contain alloying elements to enhance
a specific property or characteristic, for example machinability
Many copper alloys have common names, such as oxygen-free copper
(99.95% Cu min), beryllium copper (0.02 to 0.2% Be), Muntz metal (Cu40Zn),
Naval brass (Cu-39.5Zn-0.75Sn), and commercial bronze (Cu-10Zn).
Many of the physical properties of copper alloys are important to the
welding processes, including melting temperature, coefficient of
thermal expansion, and electrical and thermal conductivity. Certain
alloying elements greatly decrease the electrical and thermal
conductivities of copper and copper alloys.
Several alloying elements have pronounced effects on the weldability of
copper and copper alloys. Small amounts of volatile, toxic alloying
elements are often present in copper and its alloys. As a result,
the requirement of an effective ventilation system to protect the
welder and/or the welding machine operator is more critical then
when welding ferrous metals.
Zinc reduces the weldability of all brasses in relative proportion to the
percent of zinc in the alloy. Zinc has a low boiling temperature, which
results in the production of toxic vapors when welding copper-zinc alloys.
Tin increases the hot-crack susceptibility during welding when
present in amounts from 1 to 10%. Tin, when compared with zinc, is far
less volatile and toxic. During the welding tin may preferentially
oxidize relative to copper. The results will be an oxide entrapment,
which may reduce the strength of the weldment.
Beryllium, aluminum, and nickel form tenacious oxides that must
be removed prior to welding. The formation of these oxides during the
welding process must be prevented by shielding gas or by fluxing, in
conjunction with the use of the appropriate welding current. The oxides
of nickel interfere with arc welding less than those beryllium or aluminum.
Consequently, the nickel silvers and copper-nickel alloys are less sensitive
to the type of welding current used during the process. Beryllium
containing alloys also produce toxic fumes during the welding.
Silicon has a beneficial effect on the weldability of copper-silicon alloys
because of its deoxidizing and fluxing actions.
Oxygen can cause porosity and reduce the strength of welds made
in certain copper alloys that do not contain sufficient quantities of
phosphorus or other deoxidizers. Oxygen may be found as a free gas or as
cuprous oxide. Most commonly welded copper alloys contain deoxidizing
element, usually phosphorus, silicon, aluminum, iron, or manganese.
Iron and manganese do not significantly affect the weldability
of the alloys that contain them. Iron is typically present in some
special brasses, aluminum bronzes, and copper-nickel alloys in amounts
of 1.4 to 3.5%. Manganese is commonly used in these same alloys, but at
lower concentrations than iron.
Free-Machining Additives. Lead, selenium, tellurium and sulfur are
added to copper alloys to improve machinability. Bismuth is beginning
to be used for this purpose as well when lead-free alloys are desired.
These minor alloying agents, while improving machinability, significantly
affect the weldability of copper alloys by rendering the alloys hot-crack
susceptible. The adverse effect on weldability begins to be evident at
about 0.05% of the additive and is more severe with larger concentrations.
Lead is the most harmful of the alloying agents with respect to hot-crack
susceptibility.
Factors Affecting Weldability
Besides the alloying elements that comprise a specific copper alloy,
several other factors affect weldability. These factors are the thermal
conductivity of the alloy being welded, the shielding gas, the type of
current used during welding, the joint design, the welding position,
and the surface condition and cleanliness.
Effect of Thermal Conductivity. The behavior of copper and copper
alloys during welding is strongly influenced by the thermal conductivity
of the alloy. When welding commercial coppers and lightly alloyed copper
materials with high thermal conductivities, the type of current and
shielding gas must be selected to provide maximum heat input to the joint.
This high heat input counteracts the rapid head dissipation away from the
localized weld zone. Depending on section thickness, preheating may be
required for copper alloys with lower thermal conductivities. The interpass
temperature should be the same as for preheating. Copper alloys are not
post-weld head treated as frequently as steels, but some alloys may
require controlled cooling rates to minimize residual stresses and hot
shortness.
Welding Position. Due to the highly fluid nature of copper and
its alloys, the flat position is used whenever possible for welding.
The horizontal position is used in some fillet welding of comer joints
and T-joints.
Precipitation-Hardenable Alloys. The most important
precipitation-hardening reactions are obtained with beryllium, chromium,
boron, nickel, silicon, and zirconium. Care must be taken when welding
precipitation-hardenable copper alloys to avoid oxidation and incomplete
fusion. Whenever possible, the components should be welded in the annealed
condition, and then the weldment should be given a precipitation-hardening
heat treatment.
Hot Cracking. Copper alloys, such as copper-tin and copper-nickel,
are susceptible to hot cracking at solidification temperatures. This
characteristic is exhibited in all copper alloys with a wide
liquidus-to-solidus temperature range. Severe shrinkage stresses
produce interdendritic separation during metal solidification. Hot cracking
can be minimized by reducing restraint during welding, preheating to slow
the cooling rate and reduce the magnitude of welding stresses, and reducing
the size of the root opening and increasing the size of the root pass.
Porosity. Certain elements (for example, zinc, cadmium, and
phosphorus) have low boiling points. Vaporization of these elements
during welding may result in porosity. When welding copper alloys
containing these elements, porosity can be minimized by higher weld
speeds and a filler metal low in these elements.
Surface Condition. Grease and oxide on work surfaces should be
removed before welding. Wire brushing or bright dipping can be used.
Miliscale on the surfaces of aluminum bronzes and silicon bronzes is
removed for a distance from the weld region of at least 13 mm, usually
by mechanical means. Grease, paint, crayon marks, shop dirt, and similar
contaminants on copper-nickel alloys may cause embrittlement and should
be removed before welding. Miliscale on copper-nickel alloys must be
removed by grinding or pickling; wire brushing is not effective.