Fluxing of Copper Alloys
Fluxing practice in copper alloy melting and casting encompasses a variety of
different fluxing materials and functions. Fluxes are specifically used to remove
gas or prevent its absorption into the melt, to reduce metal loss, to remove specific
impurities and nonmetallic inclusions, to refine metallic constituents, or to lubricate
and control surface structure in the semicontinuous casting of mill alloys. The last
item is included because even these fluxes fall under the definition of inorganic
chemical compounds used to treat molten metal.
Fluxes for copper alloys fall into five basic categories: oxidizing fluxes, neutral
cover fluxes, reducing fluxes (usually graphite or charcoal), refining fluxes, and
semi-continuous casting mold fluxes.
Oxidizing fluxes are used in the oxidation-deoxidation process. The
principal function here is control of hydrogen gas content. This technique is still
practiced in melting copper alloys in fuel-fired crucible furnaces, where the products
of combustion are usually incompletely reacted and thus lead to hydrogen absorption and
potential steam reaction. The oxidizing fluxes usually include cupric oxide or manganese
dioxide (MnO2), which decompose at copper alloy melting temperatures to
generate the oxygen required.
Neutral cover fluxes are used to reduce metal loss by providing a fluid
cover. Fluxes of this type are usually based on borax, boric acid, or glass, which melts
at copper alloy melting temperatures to provide a fluid slag cover. Borax melts at
approximately 740°C (1365°F). Such glassy fluxes are especially effective when
used with zinc-containing alloys, preventing zinc flaring and reducing subsequent zinc
loss by 3 to 10%. The glassy fluid cover fluxes also agglomerate and absorb nonmetallic
impurities from the charge (oxides, molding sand, machining lubricants, and so on).
Oxide films in aluminum and silicon bronzes also reduce fluidity and mechanical
properties. Fluxes containing fluorides, chlorides, silica, and borax provide both
covering and cleaning, along with the ability to dissolve and collect these
objectionable oxide skins. Chromium and beryllium-copper alloys oxidize readily
when molten; therefore, glassy cover fluxes and fluoride salt components are useful
here in controlling melt loss and achieving good separation of oxides from the
Reducing fluxes containing carbonaceous materials such as charcoal or graphite are
used on higher-copper lower-zinc alloys. Their principal advantage lies in reducing
oxygen absorption of the copper and reducing melt loss. Low-sulfur, dry, carbonaceous
flux materials should always be used with copper alloys to avoid gaseous reactions with
sulfur or with hydrogen from contained moisture. However, carbonaceous materials will
not agglomerate non-metallic residues or provide any cleaning action when melting fine
or dirty scrap.
The need to refine specific metallic impurities is highly dependent on and variable
with the specific alloy system being refined. An alloying element in one family of
copper alloys may be an impurity in another, and vice versa. In red brass
(Cu-5Zn-5Pb-5Sn; UNS C83600), the elements lead, tin, and zinc are used for
alloying, while aluminum, iron, and silicon are impurities. In aluminum bronzes, on
the other hand, lead, tin, and zinc become contaminants, while aluminum and iron are
Fire refining (oxidation) can be used to remove impurities from copper-base melts
roughly in the following order: aluminum, manganese, silicon, phosphorus, iron, zinc,
tin, and lead. Nickel, a deliberate alloying element in certain alloys but an impurity
in others, is not readily removed by fire refining, but nickel oxide can be reduced
at such operating temperatures. Mechanical mixing or agitation during fire refining
improves the removal capability by increasing the reaction kinetics. Removal is
limited, however, and in dilute amounts (< 0.05 to 0.10%) many impurities cannot be
Oxygen-bearing fluxes can be effective in removing certain impurities, although they
are less efficient than direct air or oxygen injection.
Lead has been removed from copper alloy melts by the application of silicate fluxes or
slags. The addition of phosphor copper or the use of a phosphate or borate slag flux
cover and thorough stirring improves the rate of lead removal.
Sulfur, arsenic, selenium, antimony, bismuth, and tellurium can occur as impurities
in copper alloy scrap, foundry ingot, and prime metal through incomplete refining of
metal from the ore, electronic scrap, other scrap materials, or cutting lubricant.
These impurities can largely be controlled by application and thorough contacting with
fluxes containing sodium carbonate (Na2CO3) or other basic flux
additives such as potassium carbonate (K2CO3).
Sulfur is a harmful impurity in copper-nickel or nickel silver alloys. It can be
removed from these materials by an addition of manganese metal or magnesium.
Aluminum is often a contaminant in copper alloy systems, particularly the leaded tin
bronzes and red brasses. Porosity and lack of pressure tightness result when the
aluminum content is as little as 0.01%. Aluminum can be removed by a flux containing
oxidizing agents to oxidize the aluminum, and fluoride salts to divert the
Al2O3 from the melt and render it removable. Silicon can also
be removed, but only after the aluminum has reacted.
Fluxing of Zinc Alloys
In the melting of clean, pure zinc and zinc alloys, there is little need for cover
fluxes to protect the melt because zinc does not oxidize appreciably or absorb
hydrogen at normal melting temperatures. However, chloride-containing fluxes that
form fluid slag covers can be used to minimize melt loss if they are carefully
skimmed from the melt before pouring. When melting dirty metal, scrap returns, and
the like, both a cover flux and a reactive flux are advantageously used to separate
entrapped metal from oxides.
The principal alloying elements present in zinc die casting alloys include aluminum,
magnesium, and copper. Manganese and silicon may be present as impurities in the
remelt; chromium and nickel are often seen when plated scrap is remelted.
In most cases, zinc die casting alloys can tolerate these impurity amounts up to
the limit of their solubility during solidification (generally < 0.02%). The
quantities of these elements present in excess of solubility limits will form oxides
and/or complex intermetallic compounds, especially when iron is present (from melting
or casting vessels). It is possible to collect some of these impurity constituents,
particularly oxides, in the dross phase; therefore, a fluid slag or flux cover is
beneficial. At typical zinc melt operating temperatures, various mixtures of zinc
chloride (ZnCl2), KCl, and NaCl form the basis of such fluxes. These fluxes will
also agglomerate nonmetallic residues and contaminants from dirty scrap and will
help cleanse charge materials during remelting The zinc alloy drosses that form
during melting, holding, turbulent transfer, and remelt operations can contain as
much as 90% entrapped finely divided free metallic zinc, in addition to oxides,
intermetallic compounds, and other debris.
An exothermic dressing flux, usually containing nitrate salts and silicofluoride double
salts, can be used to recover much of this entrapped zinc. The exothermic reaction,
assisted by rabbling or raking the flux into intimate contact with the dross phase,
serves to raise the local temperature, increase fluidity, decrease the surface
tension of the oxide skin, and chemically reduce ZnO.
Flux injection is a relatively new process in which fluxing compounds are introduced
into the molten metal by a mechanical device using an inert gas carrier. Appropriate
nozzle mixing technology and the use of special lance materials and insulated
construction have been developed by the manufacturers of flux injection equipment
to overcome clogging and premature melting or reaction of flux materials within
Current manual application of fluxing compounds usually involves surface application
and subsequent manual raking or rabbling of the flux to achieve good mixing and
reactivity. Essentially, the manual process is labor intensive and is often merely a
surface or near-surface treatment limited to the dross and the molten metal immediately
adjacent to the dross layer. Flux injection permits better reactivity and mixing through
the bubbling action of the inert gas carrier, and it also permits refining of the full
molten metal bath because it submerges the fluxing materials to substantial depths
within the melt.