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Copper's Conductivity

Various cast copper-base alloys offer a range of conductivity levels that designers can match to their application. For many electrical components, only the highest level of conductivity will suffice. For these applications, pure copper is the material of choice. Other electrical applications may sacrifice some conductivity for added strength achieved through alloying. High coppers, such as chromium copper and copper-beryllium, exhibit varying degrees of balance between strength and conductivity to match an application’s requirements. Additionally, other copper-base alloys with lower percentages of copper still exhibit moderate conductivity that suffices for certain electrical components, such as terminal ends for electric cables.

Amped Up

Recently, the electrical industry has trended toward energy conservation. In order to dissipate as much electricity as possible, components made of materials with the highest levels of conductivity are increasingly in demand.

Although silver has the highest conductivity of any metal, it’s expensive and easily tarnished. Pure copper exhibits slightly less conductivity, but its relative affordability has made it the default material for these applications. Producing a high quality part that meets pure copper standards is difficult, but metalcasters have shown it’s possible—and common—to meet the requirements.

Metalcasting facilities with tight operations and good melting control can help electrical part designers benefit from pure copper’s conductivity and the casting processes’ geometrical flexibility.

“Any part can be made in various processes, but only one is optimal,” said Karl Schweisthal, president of permanent mold caster Piad Precision Castings, Greensburg, Pa. “We’ve converted copper parts that are brazed together into one-piece castings, as well converted parts machined from bar stock.”

A major electrical equipment manufacturer needed a new breaker design that was compact and cost efficient in order to compete with international firms. A copper line conductor used in a 1,200-amp breaker originally was machined from an extruded bus bar to which a smaller copper pad was brazed. The manufacturer worked with Piad to design a one-piece precision casting that eliminated all secondary operations except for the tapping of two holes. Additionally, the electrical performance of the line conductor was improved. The line conductor passed all heat rise and short circuit tests and exhibited a smooth surface finish adequate for electrical contact surfaces.

The part was cast in a pure copper alloy with 98% electrical conductivity. (The conductivity of copper is measured according to the International Annealed Copper Standard for conductivity.) Pure copper alloys have a minimum copper content of 99.3%; anything less causes a notable dip in electrical conductivity.

“Impurities in copper, such as traces of conventional metals that may get added, will ruin the conductivity,” said Dan Burnstein, president of Burnstein von Seelen, Abbeville, S.C., a permanent mold caster of copper parts. “It’s critical to keep the material clean.”

Pure copper also has a propensity for gas porosity. This often is counteracted through the addition of deoxidizing elements, such as phosphorous, lithium or beryllium, to the molten metal.

But the exact amount needed to de-oxidize the material cannot be determined, so a surplus of those elements remains in the metal after solidification. The extra elements detract from the copper’s conductivity.

Rather than adding a deoxidizing agent to the melt, controlled amounts of gas to disperse shrinkage in melting operations maintain the integrity of the copper’s conductivity, which allows copper castings to compete with copper fabrications.

Hardened Heart

Copper in its pure form is a soft material, so for many copper applications, alloying elements, such as nickel and copper, are added to promote strength and hardness. But beware: “Copper is very soft, but as soon as you alloy, you give up its conductivity,” Burnstein said.

Alloys like nickel and chrome can drop the conductivity percentage of the material to the mid to low 80s. Similarly, aluminum can be used in some electrical applications, but with 34% conductivity, parts may be required to be three times the size of a pure copper design.

However,  achieving the highest level of conductivity may be optimal, but in many cases, the strength of the component is just as, if not more, important.  For many applications, 80% conductivity is enough to meet their needs. High copper alloys contain more than 95% copper along with a percentage of strengthening alloys, such as beryllium, chromium or cobalt, which can be taken into the melt at high temperatures and precipitate once the casting cools. This precipitation increases the strength of the alloy, and because the alloying elements are no longer in solution, the copper matrix maintains high conductivity. For instance, chromium-copper exhibits up to twice the strength of pure copper with 80% of its electrical conductivity 80%.

However, one property often must be sacrificed to reach higher limits for another property. For instance, beryllium copper alloys either exhibit high conductivity with moderate strength, or moderate conductivity at high strength. While pure copper exhibits ultimate tensile and yield strengths of 28 and 4 ksi, beryllium copper can exhibit strengths of 80 and 40 ksi, but electrical conductivity is reduced to 20%.

Applications for high copper alloys include heavy-duty pole line and other electrical hardware. MetalcastingDesign.com