Copper and Copper Alloys Casting Problems

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Challenges in Copper and Copper Alloy Casting

Pure copper is extremely difficult to cast and is prone to surface cracking, porosity issues, and the formation of internal cavities. The casting characteristics of copper can be improved by adding small amounts of elements such as beryllium, silicon, nickel, tin, zinc, chromium, and silver. Cast copper alloys are widely used in applications including bearings, bushings, gears, fittings, valve bodies, and various components for the chemical processing industry. These alloys are poured into many types of castings, including sand, shell, investment, permanent mold, chemical sand, centrifugal, and die casting.

Improving Copper Casting with Alloying Elements

Copper alloys in cast form (designated in the UNS numbering system as C80000 to C99999) are specified for applications where tensile strength, wear resistance, machinability, thermal and electrical conductivity, appearance, and corrosion resistance are important. These alloys offer optimal performance when the casting process is carefully controlled and tailored to meet specific demands.

Categories of Copper Alloys Based on Freezing Range

The copper-base casting alloy family can be subdivided into three groups according to their solidification (freezing) range. Unlike pure metals, alloys solidify over a range of temperatures. Solidification begins when the temperature drops below the liquidus and is completed when the temperature reaches the solidus. The liquidus is the temperature at which the metal begins to solidify, and the solidus is the temperature at which the metal is fully solidified.

Group I Alloys

Group I alloys have a narrow freezing range (approximately 50°C). These include:

• Pure Copper (C81100)
• Chromium Copper (C81500)
• Yellow Brass (C85200, C85400, C85700, C85800, C87900)
• Manganese Bronze (C86200, C86300, C86400, C86500, C86700, C86800)
• Aluminum Bronze (C95200, C95300, C95400, C95410, C95500, C95600, C95700, C95800)
• Nickel Bronze (C97300, C97600, C97800)
• White Brass (C99700, C99750)

Pure Copper and Chromium Copper: These are very difficult to melt and are highly susceptible to gassing. Oxidation loss of chromium during melting is a challenge for chromium copper. Both should be melted under a floating flux cover to prevent oxidation and the absorption of hydrogen from atmospheric moisture. Crushed graphite should cover the copper melt, while chromium copper should be covered with a proprietary flux.

Yellow Brasses: These alloys tend to lose zinc due to vaporization at temperatures close to the melting point. Aluminum is added to reduce zinc loss, with the ideal content between 0.15% and 0.35%. Any aluminum content above this range causes shrinkage during freezing, which necessitates the use of risers. Zinc should be added just before pouring to compensate for zinc loss during melting.

Manganese Bronzes: These alloys are compounded yellow brasses with iron, manganese, and aluminum. The metal should be melted and heated to the flare temperature, at which point zinc oxide vapor can be detected. After this, the metal should be removed from the furnace and poured, with zinc added to replace any zinc lost during melting.

Aluminum Bronzes: These alloys should be melted under an oxidizing atmosphere and heated to the correct furnace temperature. If necessary, degasifiers can be added to the melt as the furnace is being tapped. A blind sprue can be poured first to check for shrinkage or gas exudation during solidification. If gas is observed, degassing is required.

Nickel Bronzes (Nickel Silver): These alloys, which contain high amounts of nickel, are difficult to melt and gas easily. The presence of nickel increases hydrogen solubility, which exacerbates hydrogen pickup at higher pouring temperatures. These alloys must be melted under an oxidizing atmosphere and quickly superheated to the required temperature to mitigate hydrogen pickup. Proprietary fluxes containing manganese, calcium, silicon, magnesium, and phosphorus should be used to remove hydrogen and oxygen.

White Manganese Bronze: These copper-zinc alloys contain a significant amount of manganese, and sometimes nickel. They are easy to melt and pour at relatively low temperatures. However, they should not be overheated, as this causes zinc vaporization, altering the alloy's chemistry. No fluxes are typically needed during the melting process.

Group II Alloys

Group II alloys have an intermediate freezing range (50–110°C) and include:

• Beryllium Copper (C81400, C82000, C82200, C82400, C82500, C82600, C82800)
• Silicon Brass (C87500)
• Silicon Bronze (C87300, C87600, C87610, C87800)
• Copper-Nickel (C96200, C96400)

Beryllium Coppers: These alloys are highly toxic due to beryllium fumes. They should be melted quickly under a slightly oxidizing atmosphere to minimize beryllium losses. They are highly fluid and pour well.

Silicon Bronzes and Brasses: Silicon bronzes (C87300, C87600, C87610) are relatively easy to melt. Overheating can result in hydrogen absorption, though degassing is rarely required. Silicon brasses (C87500, C87800) have excellent fluidity and can be poured slightly above their freezing range. Excessive heating may cause hydrogen pickup, so careful temperature control is necessary.

Copper-Nickel Alloys: These alloys, such as 90Cu-10Ni (C96200) and 70Cu-30Ni (C96400), require careful melting due to the nickel content, which raises both the melting point and susceptibility to hydrogen pickup. They should be melted in induction furnaces and deoxidized using magnesium sticks to remove oxygen and prevent steam-reaction porosity.

Group III Alloys

Group III alloys have a wide freezing range (over 110°C) and include:

• Leaded Red Brass (C83450, C83600, C83800)
• Leaded Semi-Red Brasses (C8400, C84800)
• Tin Bronze (C90300, C90500, C90700, C91100, C91300)
• Leaded Tin Bronze (C92200, C92300, C92600, C92700)
• High-Leaded Tin Bronze (C92900, C93200, C93400, C93500, C93700, C93800, C94300)

These alloys have a long freezing range, making them ideal candidates for chilling or creating steep thermal gradients. A combination of chills and risers should be used for optimal results. The best pouring temperature is the lowest that allows the molds to be filled without misruns or cold shuts.

For fluxing, alloys should be melted from charges that consist of ingot and clean, sand-free gates and risers. The melting should occur quickly in a slightly oxidizing atmosphere. After tapping the furnace, a deoxidizer (typically 15% phosphor copper) is added. This deoxidizer removes oxygen while maintaining fluidity.

Best Practices for Melting and Pouring Copper Alloys

Effective melting and pouring of copper alloys require controlling temperature and atmosphere to prevent oxidation and hydrogen absorption. Fluxing and degassing processes play a significant role in maintaining metal quality. For alloys with wide freezing ranges, controlling thermal gradients with the help of chills and risers is essential. Proper fluxing, deoxidization, and the use of proprietary fluxes ensure the removal of hydrogen and oxygen and contribute to the quality of the cast product.

Applications of Copper and Copper Alloys in Industry

Copper alloys are used extensively in various industries, including:

• Bearings and Bushings
• Gears and Fittings
• Valve Bodies and Miscellaneous Components
• Chemical Processing Equipment

These alloys offer exceptional wear resistance, machinability, corrosion resistance, and thermal and electrical conductivity, making them indispensable in manufacturing industries that demand high performance and reliability.