High Strength Copper – Beryllium Alloys

Resumé:

Beryllium additions, up to about 2 wt%, produce dramatic effects in several base metals. In copper and nickel, this alloying addition promotes strengthening through precipitation hardening. In aluminum alloys, a small addition improves oxidation resistance, castability and workability. Other advantages are produced in magnesium, gold, zinc, and other base metals.

Beryllium additions, up to about 2 wt%, produce dramatic effects in several base metals. In copper and nickel, this alloying addition promotes strengthening through precipitation hardening. In aluminum alloys, a small addition improves oxidation resistance, castability and workability. Other advantages are produced in magnesium, gold, zinc, and other base metals.

The most widely used beryllium-containing alloys by far are the wrought beryllium-coppers. They rank high among copper alloys in attainable strength while retaining useful levels of electrical and thermal conductivity. Applications for these alloys include:

  • Electronic components, where the strength, formability, and favorable elastic modulus of these alloys make them well suited for use as electronic connector contacts
  • Electrical equipment, where their fatigue strength, conductivity, and stress relaxation resistance lead to their use as switch and relay blades
  • Control hearings, where anti-galling features are important
  • Housings for magnetic sensing devices, where low magnetic susceptibility is critical
  • Resistance welding systems, where hot hardness and conductivity are important in structural and consumable welding components.

Commercial copper-beryllium alloys are classified as high-copper alloys. Wrought produce fall in the nominal range 0.2 to 2.00 wt% Be, 0.2 to 2.7 wt% Co (or up to 2.2 wt% Ni), with the balance consisting essentially of copper. Casting alloys are somewhat richer, with up to 2.85 wt% Be. Within this compositional band, two distinct classes of commercial materials have been developed, the high-strength alloys and the high-conductivity alloys.

The wrought high-strength alloys (C 17000 and C 17200) contain 1.60 to 2.00 wt% Be and nominal 0.25 wt% Co. A free-machining version of C 17200 which is modified with a small lead addition and available only as rod and wire, is designated C 17300. The traditional wrought high-conductivity alloys (C l7500 and C 17510) contain 0.2 to 0.7 wt% Be and nominal 2.5 wt% Co (or 2 wt% Ni). The leanest and most recently developed high-conductivity alloy is C 17410, which contains somewhat less than 0.4 wt% Be and 0.6 wt% Co.

The high-strength casting alloys (C 82400, C 82500, C 82600, and C 82800) contain 1.60 to 2.85 wt% Be, nominal 0.5 wt% Co, and a small silicon addition. Grain refinement in these foundry products is achieved by a minor titanium addition to the casting ingot or by increased cobalt content (up to a nominal content of 1 wt% Co) as in C 82510. The high-conductivity casting alloys (C 82000, C 82100 and C 82200) contain up to 0.8 wt% Be.

Copper-beryllium alloys are available in all common commercial mill forms, including strip, wire, rod, bar, tube, plate, casting ingot, and cast billet. Free-machining copper-beryllium is offered as rod.

Copper-beryllium alloys respond readily to conventional forming, plating, and joining processes. Depending on mill form and condition (temper), the wrought materials can be stamped, cold formed by a variety of conventional processes, or machined. Cast billet can be hot forged, extruded, or machined, and castings can be produced by a variety of foundry techniques.

Finished components can be conventionally plated with tin, nickel, semiprecious metals, or precious metals. Alternatively, strip can be clad or inlayed with other metals. Surfaces can also be modified by various techniques to enhance performance or appearance. Beryllium-copper alloys are solderable with standard fluxes and, if care is taken to preserve the properties achieved by heat treatment can be joined by nominal brazing and many fusion welding processes.

Heat Treatment

Solution annealing is performed by heating the alloy to a temperature slightly below the solidus to dissolve a maximum amount of beryllium, then rapidly quenching the material to room temperature to retain the beryllium in a supersaturated solid solution. Users of copper-beryllium alloys are seldom required to perform solution annealing, this operation is almost always done by the supplier.

Typical annealing temperature ranges are 760 to 800oC for the high-strength alloys and 900 to 955oC (1650 to 1750oF) for the high-conductivity alloys. Temperatures below the minimum can result in incomplete recrystallization. Too low a temperature can also result in the dissolution of an insufficient amount of beryllium for satisfactory age hardening. Annealing at temperatures above the maximum can cause excessive grain growth or induce incipient melting.

Age hardening involves reheating the solution-annealed material to a temperature below the equilibrium solvus for a time sufficient to nucleate and grow the beryllium-rich precipitates responsible for hardening. For the high-strength alloys, age hardening is typically performed at temperatures of 260 to 400oC for 0.1 to 4h. The high-conductivity alloys are age hardened at 425 to 565oC for 0.5 to 8h.

Within limits, cold working the alloy between solution annealing and age hardening increases both the rate and the magnitude of the age-hardening response in wrought products. As cold work increases to about a 40% reduction in area, the maximum peak-age hardness increases. Further cold work beyond this point is nonproductive and results in decreased hardness alter age hardening and diminished ductility in the unaged condition. Commercial alloys intended for user age hardening are therefore limited to a maximum of about 37% cold work in strip (H temper). For wire, the maximum amount of cold work is commonly somewhat greater.

High-Strength Wrought Alloys. When age hardened at 315 to 335oC strength increases to a plateau in about 3h for annealed material, or about 2h for cold-worked material and remains essentially constant thereafter. At lower age-hardening temperatures, longer aging times are required to reach an aging response plateau.

High-Conductivity Wrought Alloys. Aging at 450 to 480oC for 2 to 3h is commonly recommended. Overaging is less pronounced than in the high-strength alloys and can be employed to advantage because the appreciable cobalt or nickel content of these alloys increases the thermal stability of the age-hardening precipitates.

Underage, Peak-Age, and Overage Treatments. Material that has been aged for an insufficient amount of time to attain the maximum possible hardness at a particular temperature is said to be underaged. Material aged at time-temperature combinations resulting in maximum attainable hardness is said to be peak aged.

Mechanical Properties

Wrought products are supplied in a range of both heat-treatable and mill-hardened conditions. The heat-treatable conditions include the solution-annealed temper (commercial designation A, or ASTM designation TB00) and a range of annealed and cold-worked tempers (1/4 H through H, or TD01 through TD04) that must be age hardened by the user after forming. Increasing cold work, within limits, increases the strength obtained during age hardening.

Heat-treatable tempers are the softest and generally most ductile materials in the as-shipped condition, and they can be formed into components of varying complexity depending upon the level of cold work. Age hardening these heat-treatable tempers develops strength levels that range higher than those in any other copper-base alloys. After age hardening by the user, the solution-annealed material is redesignated AT, or TF00, and the annealed and cold-worked tempers are redesignated 1/4 HT through HT, or TH01 through TH04.

Mill-hardened tempers, designated AM through XHMS, or TM00 through TM08, receive proprietary cold-working and age-hardening treatments from the supplier prior to shipment, and they do not require heat treatment by the user after forming. Mill-hardened tempers exhibit intermediate-to-high strength and good-to-moderate ductility; these property levels satisfy many component fabrication requirements.

Strip. Wrought high-strength copper-beryllium alloy C 17200 strip attains ultimate tensile strengths as high as 1520 MPa in the peak-age-hardened HT (TH04) condition; the corresponding electrical conductivity is on the order of 20% IACS. Because of its slightly lower beryllium content, alloy C 17000 achieves maximum age-hardened strengths slightly lower than those of C 17200. Mill-hardened C 17200 strip is supplied in a range of tempers that have ultimate tensile strengths from 680 to 1320 MPa.

Ductility varies inversely with strength. It decreases with increasing cold work in the heat-treatable tempers and with increasing strength in the mill-hardened tempers. Beryllium-copper C 17500 and C 17510 strip can be age hardened to tensile strengths up to 940 MPa and electrical conductivities in excess of 45% IACS. Mill-hardened tempers of these high-conductivity alloys span the tensile strength range of 510 to 1040 MPa and include one specially processed temper with a minimum electrical conductivity of 60% IACS.

Other Wrought Products. Plate, bar, wire, rod and tube also are available in the solution-annealed temper, the annealed and cold-worked heat-treatable temper, and the mill-hardened temper. Strength and ductility combinations in wire are similar to those of corresponding alloys in strip form. Age-hardened strengths of plate, bar, and tube products range somewhat lower than those of strip or wire and, to a minor degree, vary inversely with section thickness. In addition to these traditional heavy-section product properties unique property combinations often can be developed by proprietary mill-hardening treatments in response to the changing requirements of emerging applications.

Forgings and hot-finished extruded products are available in the solution-annealed temper and the annealed and age-hardened temper. Cold work is not imparted prior to age hardening.

Cast Products. Regarding typical mechanical properties, four conditions exist for castings:

  • As-cast (C temper or ASTM M01 through M07: the ASTM temper designation depends upon the casting practice, such as sand, permanent mold, investment, continuous casting and so on)
  • As-cast plus age hardened (CT temper, no ASTM designation)
  • As-cast plus solution annealed (A temper, or ASTM TB00)
  • As-cast plus solution annealed and age hardened (AT temper, or ASTM TF00).

The solution-annealing temperature range for the high-strength casting alloys C 82400 through C 82800 is 760 to 790oC, these alloys are age hardened at 340oC. The high-conductivity casting alloys C 82000 and C 82200 are annealed at 870 to 900oC and age hardened at 480oC. Annealing times of 1h per inch of casting section thickness are recommended, with a minimum soak of 3h for the high-strength alloys to ensure maximum property uniformity. An age-hardening time of 3h is recommended for the temperatures indicated.

Maximum strength is obtained from the casting alloys in the AT (TF00) temper. These alloys reach strength levels slightly lower than those of the corresponding wrought AT temper copper-beryllium. The CT temper produces strengths slightly lower than those of the AT temper: however, the lower strength is offset by reduced processing costs. In addition, CT temper components experience less shrinkage and age-hardening distortion than the AT temper castings.

The slower solidification and cooling rates associated with sand or ceramic molds or heavy sections can result in lower CT temper strength. Castings in the solution-annealed and age-hardened (AT) temper are less susceptible to the effects of a slow cooling rate or variable section size. Water quenching of annealed temper castings with a large cast grain size may cause cracking. Slowing the cooling rate during quenching is recommended in such cases; however, this will reduce the AT temper aging response of the materials.

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