Molten Light Metal Processing: Part Two

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 melt.

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 alloying elements.

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 removed economically.

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

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 the lance.

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.