Melting and Casting of Copper and Aluminum Alloys

Aluminum and copper are second and third most produced metal worldwide after iron (steel). But in melt treatment technologies they are probably the number one. Although copper is the metal that is longest used technologically by mankind and aluminum one of the shortest, in both industries a variety of melt treatments and sometimes similar technologies were investigated over time. But as for most industries a look into the technology of a non competing neighbor often is missing.

The major difference between aluminum and copper is their affinity to oxygen. While aluminum is a very un-noble element and its melt forms insoluble oxides rapidly, copper is considered a half noble metal but with a high solubility for oxygen in the liquid state. The major similarity is the outstanding heat and electrical conductivity of both metals. Although copper has a ˜50 % better conductivity than aluminum, the conductivity to density ratio is in favor of aluminum. This is especially of interest in mobile applications as for example heat exchangers in automobiles. In comparison, copper heat exchangers are preferably used in stationary and elevated temperature applications.

For both metals partly similar melt treatment techniques were developed. Impurities like dissolved gases and solid inclusions are battled with the same principles, whereas dissolved metallic impurities have to treat differently. Impurities in aluminum melts can be divided into "solid inclusions" and "dissolved impurities".

Solid impurities in aluminum have different sources. The exogenous inclusions may come from the melt environment as the refractory linings of furnaces, ladles, reactors or launders etc. Mainly these are simple oxides as Al₂O₃ and MgO, K-, Ca- and AI- silicates, Na-, Ca- and Mg- aluminates, spinels like AI₂O, MgO or TiB₂ cluster originating from grain refining. The endogenous inclusions for e.g. Al₃C₄, AIN or AIB₂ are formed in the melt during production, e.g. in the electrolysis cell, at the melt treatment operations esp. during gas purging, or during storage and cooling down steps of the melts. Depending on the material produced the most important inclusions are Al₂O₃, MgO and A₄C₃.

Dissolved impurities may be foreign metals and dissolved gas. Foreign metals are Na, Li, and Ca coming from the electrolyte. Remelted metal may contain Fe, Si, and Cu as impurities. These metals are not removed industrially and must be diluted by the addition of pure aluminum or corresponding alloys in the casting furnace. The only dissolved gas in aluminum melts is hydrogen, because it does not form compounds with aluminum as other gases (e.g. nitrogen forms AIN, oxygen forms AI₂O₃).

Compared with iron and copper, aluminum has a rather low solubility for hydrogen (at 660°C liquid aluminum dissolves 0.69 ppm H and solid aluminum only 0.039 ppm H). Hydrogen has to be removed, because bubbles originating during solidification lead to unacceptable gas pores in the produced material. Due to the rather small solubility of hydrogen in aluminum melts its removing is a demanding task.

The issue of impurities in copper can be separated in two parts: impurities in primary copper remaining or collected after refining electrolysis and impurities in secondary not electro refined copper scrap.

The refining electrolysis produces cathodes with min. 99.995 wt.% Cu; the major remaining impurities are silver, sulfur, nickel and iron. But the contents are usually so small that they are not detrimental to the properties of Copper. The more critical elements in this sense namely hydrogen and oxygen as well as inclusions enter the primary Copper usually during the remelting and casting process.

In secondary materials the impurity matter is more complicated. Remelting of copper scrap makes ecological and economical sense because the material does not have to be lead back into the energy intensive primary electrolysis. There are two types of scrap, the sorted and mostly clean production scrap which is easily reusable and end of lifecycle scrap ("old scrap") consisting often of a mixture of different alloys or even compounds with other metals and materials. In the process of producing a clean and specified alloy the undesired elements either have to be removed or diluted. They can e.g. form intermetallic phases in the copper matrix and as a result decrease the mechanical properties like the ultimate yield strength and the ductility.

Due to the noble character of copper, most elements like silicon, aluminum and iron can easily be removed from a copper melt by selective oxidation at least down to a very low concentration (activity). Physical and chemical more similar elements like nickel, cobalt, tin and lead have to be treated with more attention. Dissolved metallic impurities in small amounts mostly are not detrimental to the properties of copper, but some elements as for example Lead and Arsenic precipitate at the grain boundaries of the copper materials and lead to embrittlement of the material.

Generally, oxygen and hydrogen pick-up can lead to very negative effects on mechanical and physical properties. The two gases have a high solubility in liquid Copper that decreases sharply during solidification. This can lead to bubble formation, i.e. porosity in the solid material. Oxygen can also form cuprous oxide (Cu₂O) above its solubility level that immediately reacts with the moisture of the air forming water vapor during annealing or welding, this phenomenon is called hydrogen illness. Dissolved hydrogen and oxygen (or Cu₂O) will react with water under extreme pressure in the lattice and will form cracks and lead to embrittlement.

Solid inclusions like intermetallics or oxides from alloying elements or the refractory material usually do not have a negative impact on copper and copper alloys. Because the density difference between the copper melt and the particles is very high the particles tend to float to the surface (e.g. the density of copper at 1100°C is 7.96 g/cm³ while iron oxide has a density of 5.25 g/cm³). However Stokes law predicts that even at high density differences very small particles tend to stay suspended.

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

Dissolved metallic impurities are generally not a problem as long as they are less noble than the target metal. But as aluminum is one of the least noble elements the variety and amount of more noble metals is much greater than in copper and aluminum is difficult to clean.

In copper main noble metals like Silver, Gold and PGMs can’t be removed from the melt, but also metals with low activities at low concentrations like lead or nickel, if very high purities are required. The high oxygen affinity of aluminum leads to a vast formation of oxides that can harm the products, In copper ceramic impurities are mainly from the refractory and from less noble alloying elements not being transferred to the slag.

Casting Cu-Base Alloys

Metal bath covering and purifying to provide oxidation protection, and gases absorption is accomplished by using charcoal, broken glass, graphite, salt ammoniac (NH₄Cl), natrium carbonate (soda) (Na₂CO₃), borax (NH₄)₃P(BO₄)₂, potash (NaNO₃). The process of purifying relates to solid particles conversion (inclusions, oxides) into liquid state whereto they being specifically lighter float out and turn into slag.

Degassing is conducted in order to remove gases from melt, first of all from hydrogen. Extreme hydrogen stability is considered as the main reason for porous casting appearance with copper and its alloys. Particularly harmful effect has simultaneous presence of hydrogen and oxygen.

Oxygen is present in Cu₂O form. Water vapor creation appears with reduction when, in the course of melting procedure, oxygen comes into contact with hydrogen.

(Cu₂O)+ {H₂} ↔ [2Cu] + {H₂O}

In this case, gaseous inclusions (bubbles) appear inside the melt, since vapor within copper and its alloys remain insoluble. The reaction is of reversible nature, enabling hydrogen recreation and its melt dissolution - "hydrogen copper illness". Practically, we can get simultaneously both hydrogen and oxygen in the melt.

One way to remove hydrogen is to melt it in vacuum. This is a very expensive procedure. Blowing hydrogen out with inert gases (N, Ar) represents other possibility and it is based upon Dalton's Law of partial pressures. Apart from degasification, blowing out enables an intensive melt mixing that promotes floating out of inclusions and oxides. This method does not ensure complete hydrogen elements based salts application and some other gases.

Deoxidization is carried out in the end of the melting procedure for the reason of oxygen removal. For this reason it is applied copper and phosphorous alloy Cu₃P with 15% P. It is customary practice to add 0.5% of melted metal. Beryllium can also be used for deoxidization since it exerts much stronger effect.

Pure copper casting temperature ranges between 1150-1180°C i.e. approximately 100°C over melting temperature.

Copper alloys are classified into the following groups:

1. Bronzes. They include copper alloys with Sn, AI, Ni, Mn, Si, Be, Cr etc.

• Tin bronzes
• Lead-tin based bronzes
• Red cast
• Lead bronze
• Aluminium bronze
• Beryllium bronze

2. Brasses. Cu-Zn alloys with up to 50% Zn. Impurities: Pb, Mn, AI, Fe, Sn, Sb, As, P.

• "Tombak" → Zn ≤ 33%
• Color: ranging from red → yellow brass color → Zn gray-white
• α Ms: Zn ≤ 39%
• β Ms: 39 < Zn < 45.5
• γ Ms: 45.5 < Zn < 50
• Special brass types

The best mechanical characteristics have Zn 30-40% alloys. Hardness increases with Zn% raise and it is accompanied with simultaneous plasticity reduction. With the temperature raise, it an abrupt strength decrease appears with these materials. Apart from Zn, they contain other intentionally added elements: Mn, AI, Fe, Sn, Ni, Co, Si, etc.