Copper Alloy Metallurgy Limitations
Dr. Bob on Copper Alloy Metallurgy Limitations
This article will cover limited aspects of copper alloy metallurgy as they apply to electronic connectors. The majority of connectors use a copper alloy strip for both plug and receptacle contacts. As mentioned in the previous article, this is because of the balance of strength, formability, and conductivity that can be realized with copper alloys. Now, let’s look at alloy types, strengthening mechanisms, and electrical conductivity.
Alloy types: Compositionally, the copper alloys used in connectors are predominantly copper, from 70 to 99+ percent copper, with the balance consisting of a relatively small number of alloying agents. The most common additions are tin (phosphor bronze), zinc (brass), and nickel (sometimes coupled with tin, zinc, or cobalt). Specialty alloying agents, usually in small percentages, include beryllium (beryllium-copper), silicon, cobalt, magnesium, iron, and zirconium.
There are two basic types of copper alloys: Solution hardened and precipitation/dispersion (P/D) hardened. Whether an alloy is solution- or P/D-hardenable depends on the alloying additions. All the alloys are also work-hardenable.
Alloys with tin and zinc are generally solution-hardened alloys. Solution hardening is a result of the solution of a few to several percent of the alloying agent into the base copper. For example, phosphor bronzes generally contain 4 to 8 percent tin, and the most commonly used brass alloy has the composition of copper (70 percent) and zinc (30 percent). These alloys retain the crystal structure of pure copper, face-centered cubic.
The specialty elements mentioned are often used to facilitate precipitation (beryllium) or dispersion (the others) hardening. The strengthening mechanisms of P/D alloys will be discussed in the next section.
In North America, copper alloys are identified by composition according to the Unified Numbering System (UNS) using CXXXXX, where C identifies copper and the Xs are the alloying system. The most common designations for the alloying agents mentioned above include C1XXXX (beryllium, silicon, magnesium, and zirconium), C2XXXX (zinc-brasses), C5XXXX (tin-phosphorous-bronzes), and C7XXXX (nickel, sometimes with cobalt, silicon, tin, and iron). For more information concerning other alloys and the UNS system, visit the Copper Development Association website, www.copper.org.
Strengthening Mechanisms: As mentioned, all copper alloys are work-hardenable, meaning, when they are deformed—or worked—they increase in hardness and strength. The strengthening mechanism is basically distortion and creation of defects within the crystalline copper lattice, which results in an increased resistance to further slip and distortion. It should be noted that work-hardening characteristics vary with alloy composition. If you recall, connectors are typically made from strip material. The processing of bulk ingots into strips consists of a series of rolling and annealing operations. The rolling work hardens the strip and the annealing “softens” it so that it can be subjected to additional rolling without cracking. The rolling/annealing schedules that produce strip for connectors in thicknesses of a millimeter to less than a tenth of a millimeter are proprietary to the copper alloy strip manufacturers. For this reason, different suppliers may have a different combination of yield strength and ductility/formability for a given material strength level or temper.
Next, consider phosphor-bronzes as an example of a solid solution-strengthened alloy system. As tin is incorporated into the base copper crystal lattice, substitutionally (on the copper lattice sites), the lattice is distorted locally due to the size difference between tin and copper atoms. This distortion increases the resistance of the lattice to slip mechanisms under applied stresses, in effect, strengthening the material. Solid solution strengthening depends on the alloying agent and the alloy composition. The strengthening effect of tin in copper, per percent addition, is higher than that of nickel or zinc, for example. But, zinc and nickel can be incorporated at higher percentages than tin, while maintaining the copper lattice structure.
Finally, we come to precipitation/dispersion hardening mechanisms. Precipitation- and dispersion-hardening mechanisms are similar in that they both rely on a fine distribution of particles of a second phase compound within the base copper lattice. P/D alloys of interest in this discussion are primarily in the C1XXXX UNS category, and are either coppers (minimum 99.3 percent copper) or high coppers (minimum 96 percent copper). Despite these low alloy concentrations, second-phase structures are nucleated and controlled to create the desired distribution of fine particles in the copper matrix.
The creation and functional role of the particles, however, is different for precipitation and dispersion hardening. Beryllium-copper, C17200, is an example of a precipitation-hardening, sometimes called age-hardening, alloy. C17200 contains two percent of alloying agents, typically 1.8 percent beryllium and 0.2 percent cobalt. Copper cannot hold 1.8 percent of beryllium in solution at room temperature. This fact is the key to precipitation hardening. Briefly, the Be-Cu alloy is heated to a solutionizing temperature to take all the beryllium into solution; it is then quenched to room temperature. The beryllium is retained in a metastable solution due to the rapid quenching. Next, the alloy is aged at a lower temperature to allow the beryllium to precipitate out as a controlled distribution of beryllide particles in the copper matrix. The distribution of beryllide particles provides an additional strengthening mechanism by preventing the motion of defects, called dislocations, under applied stress. This dislocation pinning mechanism is stable at elevated temperatures, where the work-hardening mechanism dissipates. Thus, Be-Cu alloys have superior stress relaxation performance compared to copper alloys that can only be work-hardened.
The manufacturing process for dispersion-strengthened alloys is more complex than that for beryllium-copper. For example, C19400, a high copper alloy containing 2.3 percent iron exceeds the solubility of iron in copper at room temperature and the particle dispersion is nucleated and controlled during annealing/rolling processing. The strengthening mechanism is also more complex than in Be-Cu. as dispersion-hardened alloys essentially enhance the response of the material to cold working processes, providing higher strength for lower amounts of cold working. As with Be-Cu, the dispersed phase provides enhanced thermal stability compared to standard cold worked materials and, therefore, improved stress relaxation resistance.
Conductivity: The original International Annealed Copper Standard (IACS) copper standard was defined over a century ago, and since then, copper processing technology has improved, so the IACS conductivity of C11000, pure copper, is now 101 percent (min) IACS instead of the “standard” 100 percent. For reference, some minimum IACS conductivities for copper alloys used in connectors follow: C26000 (brass), 28 percent; C51000 (phos-bronze), 15 percent; C17200 (Be-Cu), 18 percent; and C19400, 60 percent. The conductivity of copper alloys is strongly dependent on the alloying additions. As with solid solution strengthening, tin and nickel show stronger effects on conductivity than does zinc. Other factors affecting conductivity include grain size and work hardening, both of which have small effects compared to that of alloying additions.
Now that we’ve covered how copper alloys act, we’ll discuss some specific copper alloys and their areas of application in connectors in the next article in this series.
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