Classification and Properties of Copper Alloys

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Historical Context

Copper is the oldest metal used by humans, with its use dating back to prehistoric times. It has been mined for over 10,000 years, and a copper pendant discovered in present-day Iraq is dated to 8700 BC. By 5000 BC, copper was being smelted from simple copper oxides. The first high-strength copper alloy marked the beginning of the Bronze Age, which followed the Copper (Chalcolithic) Age over 4,000 years ago. Early bronzes primarily consisted of copper and tin, and the modern “phosphorus bronzes” used for electrical applications (containing around 5% tin and minimal phosphorus) are direct descendants of these primitive alloys. However, phosphorus bronzes cannot be classified as having high conductivity, as their conductivity is less than 10% that of pure copper.

Copper occurs as a native metal and in minerals such as cuprite, malachite, azurite, chalcopyrite, and bornite. It is also often a by-product of silver production, with sulfides, oxides, and carbonates being the most important ores.

Selection of Copper Alloys

The primary criteria for selecting copper and copper alloys include:

• Electrical Conductivity: Copper possesses the highest conductivity among engineering metals. Other elements may be added to enhance strength or softening resistance without significantly affecting conductivity.
• Thermal Conductivity: This property parallels electrical conductivity. Copper alloys may be chosen based on this characteristic, where good corrosion resistance can offset any loss of conductivity due to alloying.
• Color and Appearance: Many copper alloys exhibit distinctive colors, which may change as they weather. Most alloys can be easily prepared and maintained to a high standard, even in corrosive conditions. Alloys range in color from the salmon pink of copper to yellow, gold, green, and dark bronze. Atmospheric exposure can produce a green or bronze surface, and prepatinated alloys are available in some product forms.

Applications of Copper

Copper and its alloys are utilized in a wide array of applications, including:

• Power transmission lines
• Architectural applications
• Cooking utensils
• Spark plugs
• Electrical wiring, cables, and busbars
• High conductivity wires
• Electrodes
• Heat exchangers and refrigeration tubing
• Plumbing
• Water-cooled copper crucibles

The largest application of copper is in the building industry, where its use is extensive. Construction-related applications for copper include:

• Roofing
• Cladding
• Rainwater systems
• Heating systems
• Water pipes and fittings
• Oil and gas lines
• Electrical wiring

Types of Copper Alloys

Commercially pure coppers are very soft and ductile, containing up to about 0.7% total impurities. These materials are valued for their electrical and thermal conductivity, corrosion resistance, appearance, and ease of working. They have the highest conductivity of engineering metals, are highly ductile, and are easy to braze and generally weld. Typical applications include electrical wiring and fittings, busbars, heat exchangers, roofs, wall cladding, and tubes for water, air, and process equipment.

High Copper Alloys

High copper alloys contain small amounts of various alloying elements such as beryllium, chromium, zirconium, tin, silver, sulfur, or iron. These elements modify one or more fundamental properties of copper, such as strength, creep resistance, machinability, or weldability. The applications of these alloys are similar to those of pure copper, but the conditions of application are often more demanding.

Brasses

Brasses are copper-zinc alloys that contain up to about 45% zinc, with possible small additions of lead for machinability and tin for strength. Copper-zinc alloys remain single-phase up to about 37% zinc in the wrought condition, exhibiting excellent ductility. Alloys with more than about 37% zinc become dual-phase, possessing higher strength but limited ductility at room temperature compared to single-phase alloys. Dual-phase brasses are usually cast or hot worked.

Typical applications for brasses include architecture, drawn and spun containers, radiator cores and tanks, electrical terminals, plugs, lamp fittings, locks, door handles, nameplates, plumbing hardware, fasteners, and cartridge cases.

Brasses are categorized into two classes:

1. Alpha Alloys: Containing less than 37% zinc, these alloys are ductile and can be cold worked.
2. Alpha/Beta or Duplex Alloys: With 37-45% zinc, these alloys have limited cold ductility and are typically harder and stronger.

There are three main families of wrought alloy brasses:

• Copper-Zinc alloys
• Copper-Zinc-Lead alloys (Leaded brasses)
• Copper-Zinc-Tin alloys (Tin brasses)

Cast brass alloys can be divided into four main families:

• Copper-Tin-Zinc alloys (red, semi-red, and yellow brasses)
• Manganese Bronze alloys (high-strength yellow brasses) and Leaded Manganese Bronze alloys (leaded high-strength yellow brasses)
• Copper-Zinc-Silicon alloys (Silicon brasses and bronzes)
• Cast Copper-Bismuth and Copper-Bismuth-Selenium alloys.

Bronzes

Bronzes are alloys of copper with tin, along with at least one of phosphorus, aluminum, silicon, manganese, or nickel. These alloys can achieve high strengths combined with good corrosion resistance, making them suitable for springs, fixtures, metal forming dies, bearings, bushes, terminals, contacts, connectors, and architectural fittings. Cast bronze is commonly used for statuary.

Copper-Nickel Alloys

Copper-nickel alloys consist of copper with nickel, and may include small amounts of iron and other minor alloying additions such as chromium or tin. These alloys exhibit excellent corrosion resistance in marine environments and are extensively used in seawater applications, including heat exchangers, condensers, pumps, piping systems, and sheathing for boat hulls.

Nickel Silvers

Nickel silvers contain 55–65% copper alloyed with nickel and zinc, and sometimes an addition of lead to enhance machinability. Despite their name, these alloys do not contain any silver. They are used for jewelry, nameplates, and as a base for silver plating (EPNS), as well as springs, fasteners, coins, keys, and camera parts.

Properties of Copper Alloys

Corrosion Resistance

Copper alloys resist corrosion in freshwater and steam. In many rural, marine, and industrial atmospheres, they also demonstrate significant corrosion resistance. Copper is resistant to saline solutions, soils, non-oxidizing minerals, organic acids, and caustic solutions. However, moist ammonia, halogens, sulfides, and oxidizing acids like nitric acid can attack copper. Additionally, copper alloys have poor resistance to inorganic acids.

The corrosion resistance of copper alloys arises from the formation of adherent films on the material surface, which protect the base metal from further attack.

Ductility

Ductility can be restored by annealing, achieved through specific processes or incidental annealing during welding or brazing.

Strengthening Mechanisms

There are four common methods to harden (strengthen) copper:

1. Strain Hardening: Cold work (rolling or drawing) hardens copper and its alloys, increasing strength and hardness while reducing ductility. This method slightly decreases conductivity, but it can be restored through annealing.
2. Solid-Solution Hardening: Alloying elements that remain dissolved in solidified copper strengthen the lattice structure without forming secondary phases. However, this reduces electrical conductivity.
3. Precipitation Hardening: Some alloying elements exhibit higher solubility in hot copper than in cold. They can be dissolved at high temperatures and then precipitated at lower temperatures, producing fine precipitates that strengthen the matrix without harming conductivity.
4. Dispersion Strengthening: Insoluble particles are distributed within the copper matrix, enhancing strength without significantly affecting electrical conductivity.