Forging Materials: Copper Alloys
Our series on forging materials concludes with an examination of copper alloys. A general description of forged copper alloys is given, followed by details of their chemistry and microstructures. Applications of forged copper components, forging issues and special considerations for forgers who handle copper alloys will also be discussed.
In its pure form, copper is very soft, so alloying elements are normally added to increase its strength. When zinc is added to copper it is commonly called brass. Bronze is another copper alloy in which tin is added. Cupronickels are, as their name implies, copper-nickel alloys. Alloys of copper with large amounts of nickel are called Monel. Aluminum bronzes are alloys of copper with aluminum and iron. In each case the alloy is much stronger than pure copper.
Copper is an excellent conductor of both heat and electricity. It also possesses outstanding corrosion properties. The vast majority of electrical wire and connectors are copper, which exhibits very low resistance relative to any other metal of a similar cost. While gold is a better conductor, the price is too high for most applications. Small electrical connectors are typically cold formed, with larger connectors frequently being forged, especially in high-current and power-transmission applications. Their high thermal conductivity makes copper alloys ideal for heat exchangers and heating components where thermal efficiency is critical.
Welding tips are almost always copper due to the requirement for thermal and electrical conductivity without corrosion. These are generally cold formed. Excellent corrosion resistance makes copper an ideal roofing material. Brass padlocks are used by utilities and refineries in outdoor applications. The shackle and lock bodies are forged (extruded). Copper pipe, tubing, valves, faucets and fittings are used in a wide range of water and steam systems, especially drinking water. Marine applications include seawater piping, fittings, valves and some structural applications, many of which are forged or extruded.
The “golden” color of copper alloys is often appealing to humans and, hence, many artistic items are made from copper alloys without the need to use costly gold. Copper roofs and statues made from copper alloys develop a blue-greenish color with time called a patina, which provides for good corrosion resistance.
In industrial settings, machinists are usually very happy when copper or one of its alloys is selected for the fabrication of a part, since cupric metals have very good machinability.
Chemistry and Grades
Copper alloys are strengthened by either solid solution or by precipitation hardening. For the alloys that have solid solution as the primary strengthening mechanism, copper-zinc (brasses) are most common. The single-phase alpha brasses are alloys of copper with up to 32% zinc. These alloys can also be strengthened by cold work. An unusual property of these alpha brasses is that, in some cases, you can have an alloy with additional zinc and it is both stronger and more ductile than a leaner alloy. The alpha-beta brasses are two-phase metals containing 32-40% zinc.
Bronzes with up to 10% tin are not normally forged – they are typically cast. The aluminum bronzes with up to 10% aluminum and 4% iron plus small additions of other elements (excluding tin) are fairly forgeable. Forgeable cupronickels can contain up to 30% nickel.
The highest-strength copper alloys are the copper-beryllium alloys, which can be precipitation strengthened. These alloys contain up to 2% beryllium and must be handled with care. They can reach strengths over 200 ksi and are often used in electrical contacts where high strength is required.
Figure 1 shows the microstructure of annealed brass. Note the fairly uniform grain size and the large number of annealing twins, which are the straight lines that run from one side of the grain to the other. The microstructure of a hot-forged alpha brass of 30% zinc would be a single-phase material that would also exhibit a number of annealing twins. The cold-formed microstructure of the same alloy would be dramatically different from the hot-formed and annealed material. The cold-formed alloy would show significant grain distortion and possess much higher strength than the annealed brass.
Forged copper and brass alloys are used in electrical components, decorative applications and corrosion-resistant components. Copper tubing and sheet are very common for heat-transfer applications. The versatility of forged copper is displayed in Figure 2, which shows a number of rather atypical applications. Industrial applications for forged copper components include a wide assortment of electrical connectors, fasteners, locks and numerous other components.
Forging of Copper Alloys
Copper and its alloys exhibit good ductility and are generally considered as easy to forge. When hot forging, the preheat temperatures are typically 1350-1700°F. Figure 3 illustrates the typical hot- and cold-forging ranges as well as the processing and application ranges for copper alloys.
The most forgeable (hot) copper alloy is one with 38% zinc and a small amount of lead. This alloy is a two-phase alpha-beta brass at room temperature, but the hot-forging temperatures take the alloy into the single-phase beta region where deformation can easily occur. Lubrication requirements are generally minimal because the copper oxide that forms on the surface is a natural lubricant.
As noted, copper and copper alloys can be cold forged. Cold forging is especially useful for small-sized components that can be formed to net shape with tight tolerances. Cold forging also adds cold work to the component and increases its strength, as shown in Figure 4. The caveat is that work hardening in copper alloys is more pronounced than it is in most other metals, with increasing flow stress and eventual fracture after excessive cold work.
When forging copper-beryllium alloys, they must be handled with care. Operators need to use appropriate safety equipment since beryllium is toxic and can cause severe lung problems called berylliosis or chronic beryllium disease.
Processing After Forging
The beryllium coppers can be heat treated in a fashion similar to the precipitation-hardenable aluminum alloys. They are heated to a high temperature (called solution treatment) to dissolve all the alloying elements and form a single-phase structure. They are then quenched to room temperature, which locks in the single-phase microstructure. In the final step, they are given another thermal treatment (lower than the solution temperature) in which a very fine second-phase solid-state precipitation occurs, leading to the increase in strength. This last heat treatment is called aging.
If the copper alloy is cold forged, its strength will be increased but its ductility may be too low for the intended application. The ductility of these alloys can be re-established by an annealing heat treatment. Be aware that ductility will increase during annealing, but the strength will decrease.
It should be noted that pure copper is easy to forge. The alpha-beta brasses are also easy to forge, especially in the beta-phase temperature region. The alpha brasses can be hot forged but are more difficult. The aluminum bronzes are also forgeable but challenging. The copper-nickel alloys have higher forging temperatures as compared to the other alloys.
An interesting side note about copper alloys is that in ancient times many of the copper ores that were smelted contained arsenic. Consequently, the copper alloys were high in this poisonous metal. The early sign of arsenic poisoning is the loss of muscle control, giving the appearance of craziness. It is likely that the Greek god of metal working, Hephaestus, who is often portrayed with a limp and drool coming down from his mouth, is indicative of early Greek metalsmiths who had been poisoned in pursuit of their craft.
Copper alloys have a number of good properties that favor them in a variety of applications. The most common copper alloy is brass, which is a blend of copper and zinc. Copper alloys can be hot or cold forged. These alloys are easily forgeable in many cases and can be machined to final specifications.
This installment brings to conclusion our series on forging materials. We hope that you learned a few things from these articles on ferrous and nonferrous alloys.
The support for this work from the PRO-FAST Program is appreciated. The PRO-FAST Program is enabled by the dedicated team of professionals representing both the Department of Defense and industry. These teammates are determined to ensure the nation’s forging industry is positioned for challenges of the 21st century. Key team members include: R&D Enterprise Team (DLA J339), Logistics Research and Development Branch (DLS-DSCP) and the Forging Industry Association (FIA). This work was originally prepared for the FIA Theory & Applications of Forging and Die Design course by Scientific Forming Technologies Corporation.
Co-author Dr. Chet Van Tyne is FIERF Professor, Department of Metallurgical Engineering, Colorado School of Mines, Golden, Colo. He may be reached at (303) 273-3793 or email@example.com. Co-author John Walters is vice president of Scientific Forming Technologies Corporation, Columbus, Ohio. He may be reached at (614) 451-8330 or firstname.lastname@example.org.