Melting and Casting of Copper and Aluminum Alloys

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Introduction to Copper and Aluminum Processing

Aluminum and copper rank as the second and third most produced metals worldwide after iron (steel). However, in melt treatment technologies, they likely hold the premier position. Although copper has been used technologically by mankind for millennia while aluminum's industrial history spans just over a century, both industries have developed various similar melt treatment techniques over time. Unfortunately, as in most industries, manufacturers often overlook potentially valuable technological approaches from non-competing sectors.

The fundamental difference between aluminum and copper lies in their affinity to oxygen. Aluminum, a highly reactive non-noble element, rapidly forms insoluble oxides when melted. In contrast, copper, considered a semi-noble metal, exhibits high oxygen solubility in its liquid state. Their major similarity is outstanding thermal and electrical conductivity. While copper offers approximately 50% better conductivity than aluminum, the conductivity-to-density ratio favors aluminum, making it particularly valuable for mobile applications such as automotive heat exchangers. Copper heat exchangers, meanwhile, are preferable in stationary and high-temperature applications.

Impurity Management in Aluminum Melts

For both metals, similar melt treatment techniques have been developed, though they address impurities differently. Impurities in aluminum melts can be categorized as either solid inclusions or dissolved impurities.

Solid impurities in aluminum originate from various sources. Exogenous inclusions may come from the melt environment, such as refractory linings of furnaces, ladles, reactors, or launders. These primarily consist of simple oxides like Al₂O₃ and MgO, potassium, calcium, and aluminum silicates, sodium, calcium, and magnesium aluminates, spinels like Al₂O or MgO, or TiB₂ clusters from grain refining. Endogenous inclusions such as Al₃C₄, AlN, or AlB₂ form within the melt during production processes, including electrolysis, gas purging, or during storage and cooling. The most critical inclusions are typically Al₂O₃, MgO, and Al₄C₃, depending on the material being produced.

Dissolved impurities include foreign metals and dissolved gases. Foreign metals such as Na, Li, and Ca originate from the electrolyte, while remelted metal may contain Fe, Si, and Cu as impurities. These metals cannot be removed industrially and must be diluted by adding pure aluminum or corresponding alloys in the casting furnace. Hydrogen is the only dissolved gas in aluminum melts, as other gases form compounds with aluminum (nitrogen forms AlN, oxygen forms Al₂O₃).

Compared to iron and copper, aluminum has relatively low hydrogen solubility (at 660°C, liquid aluminum dissolves 0.69 ppm H while solid aluminum dissolves only 0.039 ppm H). Hydrogen removal is essential because bubbles forming during solidification create unacceptable gas pores in the finished material. The limited solubility of hydrogen in aluminum melts makes its removal particularly challenging.

Impurity Challenges in Copper Processing

Impurities in copper can be divided into two categories: impurities in primary copper remaining after refining electrolysis and impurities in secondary, non-electro-refined copper scrap.

Refining electrolysis produces cathodes with minimum 99.995 wt.% Cu, with the primary remaining impurities being silver, sulfur, nickel, and iron. These content levels are usually so minimal that they don't significantly affect copper's properties. More critical elements—hydrogen, oxygen, and inclusions—typically enter primary copper during the remelting and casting processes.

Secondary materials present more complex impurity challenges. Remelting copper scrap is ecologically and economically advantageous because it avoids the energy-intensive primary electrolysis process. Two types of scrap exist: sorted and mostly clean production scrap, which is easily reusable, and end-of-lifecycle scrap ("old scrap"), often consisting of mixed alloys or compounds with other metals and materials. When producing a clean, specified alloy, undesired elements must either be removed or diluted. These can form intermetallic phases in the copper matrix, degrading mechanical properties like ultimate yield strength and ductility.

Due to copper's noble character, elements like silicon, aluminum, and iron can be removed from a copper melt through selective oxidation, at least to very low concentration levels. Elements more physically and chemically similar to copper—such as nickel, cobalt, tin, and lead—require more careful treatment. Dissolved metallic impurities in small amounts usually don't significantly impair copper's properties, though some elements like lead and arsenic precipitate at grain boundaries, causing material embrittlement.

Gas Management in Copper and Aluminum

Generally, oxygen and hydrogen absorption can severely impact mechanical and physical properties. Both gases have high solubility in liquid copper that decreases sharply during solidification, potentially causing bubble formation and porosity in the solid material. Oxygen can also form cuprous oxide (Cu₂O) above its solubility level, which immediately reacts with atmospheric moisture to form water vapor during annealing or welding—a phenomenon called "hydrogen illness." Dissolved hydrogen and oxygen (or Cu₂O) react with water under extreme pressure in the lattice, creating cracks and leading to embrittlement.

Solid inclusions such as intermetallics or oxides from alloying elements or refractory material typically don't negatively impact copper and copper alloys. The significant density difference between copper melt and these particles causes them to float to the surface (copper at 1100°C has a density of 7.96 g/cm³, while iron oxide has a density of 5.25 g/cm³). However, according to Stokes' law, even with high density differences, very small particles tend to remain suspended.

For both metals, hydrogen represents a major problem as a dissolved gas. Oxygen is insoluble in aluminum and immediately forms solid compounds. In copper melts, oxygen concentration can exceed 1 wt% and poses a second major problem due to its reaction with hydrogen to form water vapor or with carbon to form CO/CO₂.

Dissolved metallic impurities generally aren't problematic as long as they're less noble than the target metal. However, since aluminum is among the least noble elements, the variety and quantity of more noble metals present is much greater than in copper, making aluminum difficult to purify. In copper, noble metals like silver, gold, and platinum group metals cannot be removed from the melt, nor can metals with low activities at low concentrations, such as lead or nickel, when very high purities are required. Aluminum's high oxygen affinity leads to extensive oxide formation that can damage products. In copper, ceramic impurities mainly come from refractory materials and less noble alloying elements not transferred to slag.

Casting Copper-Base Alloys

Metal bath covering and purification provides oxidation protection and gas absorption through the use of charcoal, broken glass, graphite, ammonium chloride (NH₄Cl), sodium carbonate (Na₂CO₃), borax ((NH₄)₃P(BO₄)₂), and potash (NaNO₃). The purification process converts solid particles (inclusions, oxides) into a liquid state where, being specifically lighter, they float out and transform into slag.

Degassing is performed to remove gases from the melt, particularly hydrogen. Extreme hydrogen stability is considered the primary cause of porous casting in copper and its alloys. The simultaneous presence of hydrogen and oxygen is especially harmful.

Oxygen exists in the form of Cu₂O. Water vapor forms during reduction when oxygen contacts hydrogen during the melting process:

(Cu₂O) + {H₂} ↔ [2Cu] + {H₂O}

This reaction creates gaseous inclusions (bubbles) within the melt, as water vapor remains insoluble in copper and its alloys. The reaction is reversible, allowing hydrogen to reform and dissolve in the melt—the "hydrogen copper illness." In practice, both hydrogen and oxygen can simultaneously exist in the melt.

One hydrogen removal method is vacuum melting, though this is an expensive procedure. Alternatively, purging with inert gases (N, Ar) based on Dalton's Law of partial pressures can be effective. Beyond degasification, purging enables intensive melt mixing that promotes the flotation of inclusions and oxides. This method doesn't ensure complete hydrogen removal and may require the application of hydrogen element-based salts and other gases.

Deoxidization is performed at the end of the melting procedure to remove oxygen. For this purpose, a copper-phosphorus alloy (Cu₃P with 15% P) is commonly used, typically added at 0.5% of the melted metal weight. Beryllium can also be used for deoxidization, providing an even stronger effect.

Pure copper casting temperature ranges between 1150-1180°C, approximately 100°C above its melting temperature.

Classification of Copper Alloys

Copper alloys are classified into the following groups:

• Bronzes: These include copper alloys with Sn, Al, Ni, Mn, Si, Be, Cr, etc.
  • Tin bronzes
  • Lead-tin based bronzes
  • Red cast
  • Lead bronze
  • Aluminum bronze
  • Beryllium bronze

• Brasses: Cu-Zn alloys with up to 50% Zn. Common impurities include Pb, Mn, Al, Fe, Sn, Sb, As, and P.
  • "Tombak" → Zn ≤ 33%
  • Color ranges from red → yellow brass → gray-white with increasing Zn content
  • α Ms: Zn ≤ 39%
  • β Ms: 39 < Zn < 45.5
  • γ Ms: 45.5 < Zn < 50
  • Special brass types

The best mechanical characteristics are found in alloys with 30-40% Zn. Hardness increases with higher zinc content, accompanied by reduced plasticity. These materials experience abrupt strength decreases at elevated temperatures. Beyond zinc, they may contain other intentionally added elements: Mn, Al, Fe, Sn, Ni, Co, Si, etc.