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Treatment of molten copper alloys

Dr. Dmitri Kopeliovich

Copper alloys are prepared for casting in Melting furnaces (mostly in either induction furnaces or fuel fired furnaces.

The techniques of melting copper alloys should take into account the following technological problems:

Metallic impurities. Most metals (tin, iron, nickel, aluminum, silicon, cobalt, arsenic) form intermetallic compounds with copper which reduce ductility and some other mechanical properties of the alloys. Lead does not form intermetallic compounds but its segregation at the copper grain boundaries also decreases ductility. Since copper is more noble metal than most of the metallic impurities, many of them may be removed from the molten alloy by selective oxidation.
Oxygen and hydrogen pick-up. Oxygen penetrates into the molten copper from the atmosphere or as a result of oxidizing action (oxygen injection or oxidizing fluxing). Hydrogen is picked up by the melt from the fuel fired atmosphere and together with hygroscopic materials (fluxes and scrap components). Solubilities of both Oxygen and Hydrogen in copper drop sharply during Solidification therefore they combine forming cracks and bubbles of water vapor at high pressure within the alloy structure. Residual oxygen form of cuprous oxide (Cu₂O) may react with atmospheric hydrogen during further heating operations of the solid alloy (e.g. welding) according to the steam reaction H₂ + Cu₂O = H₂O + Cu.
Non-metallic inclusions. Some oxide inclusions are formed as a result of the melt oxidation. Other solid inclusions (oxides, carbides) are sourced from the refractory materials and impure scrap. Since the densities of the non-metallic inclusions is significantly lower than the density of molten copper, the inclusions float up and most of them are absorbed by the flux. Small inclusions tend to stay in form of suspensions and produce adverse effect on the mechaniclal properties of the alloy.
Casting temperature. Copper alloys should be cast at a lowest possible pouring temperature in order to reduce the size of the copper grains.

The methods of treatment of molten copper alloys:

Oxidation

Oxidation of copper melt is the first stage of the oxidation-deoxidation teratment.

Oxidation is performed by an injection of Oxygen (from the furnace top or the bottom) combined with the action of oxidizing flux which prevents penetration of hydrogen from the atmosphere (particularly from the fuel fired furnaces). The fluxe also absorbs the oxide inclusions of copper and other metals forming during the oxidation stage.

Removal of hydrogen and metallic impurities (less noble than copper) are achieved in the oxidation treatment.

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Deoxidation

Deoxidation is the second stage of the oxidation-deoxidation treatment.

The deoxidation techniques:

Deoxidation with phosphorus is used for tratment of most copper alloys. Phosphorus is added into the melt in form of waffles or granules of master alloy 85%Cu-15%P which are either dumped into the melt and stirred or added on the ladle bottom before pouring. Phosphorus is added in excess providing its residual concentration 0.01-0.015%. Phosphorus reacts with copper and dissolved oxygen and forms cuprous phosphate slag Cu₂O*P₂O₅ which floats up and can be easily removed from the melt surface. Tin bronzes, red brasses and leaded bronzes are commonly treated by phosphorus. Because of its deterioration effect on electrical conductivity phosphorus is not used for deoxidation conductivity copper alloys.
Deoxidation with boron is similar to that with phosphorus. The deoxidation reaction product is copper borate Cu₂O*B₂O₃.
Deoxidation with lithium is an effective method for reduction not only oxygen but also hydrogen dissolved in copper. However lithium is more expensive than phosphorus. It is very reactive metal which is stored in oil or in sealed copper cartridges. High reactivity of lithium causes relatively fast erosion of the refractory materials.

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Degassing

Degassing is the technique of reducing the Hydrogen content in the molten copper.

Degassing is commonly performed by one of the two methods:

Inert gas purging. In theis method an inert gas (Nitrogen or Argon) is blown into the melt in form of numerous small bubbles. Hydrogen dissolved in the melt diffuses into the gas and leave the melt together with the bubbles. The larger the surface of the rising bubbles, the greater the degassing rate.
Degassing with fluxes utilizes solid substances liberating gas bubbles when heated in the melt. The bubbles collect hydrogen dissolved in the molten copper according to the same mechanism as in inert gas purging method.

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Grain refining

Fine Grain structure of cast Copper alloys may be achieved by the combination of casting at lowest possible pouring temperature with high cooling rate during Solidification. However small additives in the molten copper enhancing nucleation of the forming crystals may produce further decrease of the grains size.
Many copper alloys contain nucleating additives as major alloyng components (aluminum, zinc, etc.,)
For grain refining of copper and high copper alloys small additions of lithium, iron, lead or bismuth are used.
Boron and zirconium produce grain refining effect to tin bronzes.
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Filtration

The amount of non-metallic inclusions in a molten copper may be significantly reduced by passing the melt through a filtration media.
Plates made of ceramic foams mounted in the casting gates are used for filtering Copper alloys.
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