Versatile and cost-effective, copper and its variety of alloys provide a number of benefits to both casting designers and purchasers. 

An MCDP Staff Report

(Click here to see the story as it appears in the March/April issue of Metal Casting Design & Purchasing.)

In terms of overall metric tonnage, copper-based alloys accounted for 1.7% of global production and 2.5% of U.S. production, according to the 2013 Census of World Casting Production (MCDP Jan./Feb. 2015). While a relatively small piece of the industry, copper and its extensive varieties of alloys offer designers and purchasers unique properties that may prove beneficial.

For example, copper-based alloys offer designers excellent aqueous corrosion resistance and are known for their versatility. Common cast applications for copper alloys include plumbing hardware, bearings, electrical components, ship propellers, power plant water impellers, bushing and bearing sleeves, and architectural ornamentation. The alloys are easily cast, have a long history of successful use, are readily available from a multitude of sources, can achieve a range of physical and mechanical properties, and are easily machined, brazed, soldered, polished or plated.

Following is a list of physical and mechanical properties common to cast copper alloys (note: not every property is applicable to every alloy):
 

• Good corrosion resistance, which contributes to its durability in aggressive environments and long-term cost-effectiveness.
• Favorable mechanical properties ranging from pure copper, which is soft and ductile, to manganese-bronze, which rivals the mechanical properties of quenched and tempered steel. In addition, almost all copper alloys retain their mechanical properties at low temperatures.
• High thermal and electrical conductivity greater than any metal but silver. Although the conductivity of copper drops when alloyed, copper alloys with low conductivity still conduct both heat and electricity better than other corrosion-resistant materials. For example, the axial coils in Fig. 1 were cast in 98% pure copper to improve conductivity.
• Bio-fouling resistance because copper inhibits marine organism growth. Although this property (unique to copper) decreases upon alloying, it is retained at a useful level in many alloys.
• Low friction and wear rates, such as with the highly leaded tin-bronzes, which are cast into sleeve bearings and exhibit lower wear rates than steel.
• Good machinability, as leaded copper alloys are free-cutting at high machining speeds, and many unleaded alloys are readily machinable at recommended feeds and speeds with proper tooling.
• Ease of post-casting processing, because good surface finish and high tolerance control are readily achieved. In addition, many cast copper alloys are polished to a high luster, and plating, soldering, brazing and welding also are routinely performed.
• Large alloy choice; several alloys may be suitable candidates for any given application.
• Comparable costs to other metals due to their high yield, low machining costs and little requirement for surface coatings, such as paint.
• Good castability, as all copper alloys can be sand cast and many can be centrifugal, continuous, permanent mold or die cast.

Figure 2 shows a brass casting that was converted from a 10-piece fabrication. In this case, the permanent mold casting process produced better dimensional accuracy and reproducibility. Additionally, with the near-net-shape features, weight and secondary machining can be minimized.

Depending on the requirements of a particular cast component, copper-based alloys have a range of mechanical and physical properties. Table 1 shows the full listing of cast copper-base alloys, including properties and end-use applications.

Designing With Copper Alloys

The choice of alloy and casting method (sand, permanent mold, die or investment casting) determines the mechanical and physical properties (Table 1), section size, wall thickness and surface finish that can be achieved. Typical properties of common copper alloys are shown in Table 2.

The rules in design of thick and thin sections are simple: avoid designing a casting that contains both thick and thin sections. When such changes are necessary, the thicker section should always be blended or tapered gradually into the thinner one. Also, whenever possible, avoid L, T and X intersections. Radii inside and outside edges rather than designing a sharp L corner. When T sections cannot be avoided, adverse effects can be minimized by providing generous radii at corners and making the arms unequal in thickness. Additionally, “dimpling” (a small indent at the top of the T’s intersection) can help reduce the severity of hot spots. X intersections have particularly adverse effects in copper castings. They are almost always avoidable, though, by converting an X intersection into two offset T sections, for example.

Figure 3 shows a cast copper connector used in electrical power generation. The component’s design—which included 15 heat-dissipating fins, four cast-in elongated slots, a cast hole to accept mating parts, and three cast mounting surfaces—required an alloy that was capable of complexity while remaining conductive. The highly conductive C011 alloy met both specifications.

Thick to thin section design becomes an even larger problem for copper-base alloys with wide freezing ranges such as red brasses, tin bronzes and, to some extent, the medium freezing range alloys such as the yellow brasses. These alloys, which account for the highest level of casting production, do not solidify directionally. While proper risering helps combat this, it doesn’t have the same effect as directional solidification. With these alloys, shrinkage defects normally are internal and often found during machining, leading to scrapped castings.

To counteract the solidification issues with wide-freezing range copper alloys, metalcasters use chills and chromite and zircon sand cores to promote the proper solidification. Chilling these sections can be more effective than using a riser, though each of these tools increases the cost of a finished casting.

Copper Alloy Processing

Castings often require further processing after cleaning and finishing. Machining is the most common secondary operation. Welding is needed to repair minor defects or join several castings into a larger assembly. Surface treatments are commonly applied to decorative products, plaques and statues. Each of these processing steps contributes to the cost of the finished component. Therefore, the ease and efficiency with which an alloy can be processed influences its economic viability.

As a class, cast copper-based alloys are easy to machine (especially when compared to stainless steels and titanium, their main competitors for corrosion resistance). Easiest to machine are the leaded copper-base alloys. These alloys are free-cutting and form small, fragmented chips while generating little heat.

Next in order of machinability are moderate to high-strength alloys with second phases in their microstructures, such as unleaded yellow brasses, manganese bronzes and silicon brasses and bronzes. These alloys form short, brittle, tightly curled chips that tend to break into manageable segments. While the surface finish on these alloys will be good, the cutting speed will be lower and tool wear will increase.

The most difficult copper-base alloys for machining are the single-phase alloys such as high conductivity copper, chromium copper, beryllium copper, aluminum bronze and copper-nickel. Their general tendency during machining is to form long, stringy chips that interfere during high-speed machining operations. In addition, pure copper and high-nickel alloys tend to weld to the tool face, impairing surface finish.

Both gas-tungsten-arc and gas-metal-arc can produce X-ray quality welds when repairing minor defects in copper castings. Shielded-metal-arc welding also can be used, but the method is more difficult to control. Oxyacetylene welding mainly is used to join thin sections. Electron beam welding produces precise welds of high quality in both oxygen free and deoxidized copper.

In general, alloys containing appreciable amounts of lead cannot be welded, as the lead remains liquid after the weld solidifies, forming cracks in high stress fields. All cast copper alloys can be brazed and soldered to themselves and to steel, stainless steel and nickel-base alloys. Even leaded copper alloys can be brazed, but the conditions must be controlled.

Copper phosphorous alloys, silver-based brazing alloys and copper-zinc alloys are most often used as filler metals. Gold-based alloys are used for electrical applications, and tin-based solders are used for household plumbing.

The heat of brazing may cause some loss of strength in heat treated copper alloys, but special techniques have been developed to remedy the problem. When necessary, the entire brazed casting can be heat treated to produce a uniform structure. The corrosion resistance of copper-base alloys is not affected by brazing, except in special situations.   ■