10 Things to Know About Casting Copper Alloys
Cast copper is a versatile material with a variety of alloys. The metal is used in plumbing fixtures, ship propellers, power plant water impellers and bushing and bearing sleeves because it is easy to cast, has a long history of successful use, is readily available from a multitude of sources, can achieve a range of physical and mechanical properties and is easily machined, brazed, soldered, polished or plated. In the U.S., copper accounts for approximately 2.8% of total casting production, according to the 2018 World Census of Casting Production reported by Modern Casting magazine. Following are 10 qualities of cast copper alloys design engineers should know.
1. Almost all copper alloys retain their mechanical properties at low temperatures.
Typical mechanical properties of copper include good corrosion resistance, impact toughness, superior thermal and electrical conductivity, and the ability to inhibit marine organism growth.
2. All copper alloys can be produced via sand casting.
Other casting methods conducive to copper alloys include centrifugal, continuous, permanent mold, investment and diecasting. The choice of alloy and casting method determines the mechanical and physical properties, section size, wall thickness and surface finish that can be achieved.
3. Leaded copper alloys still have several industrial applications.
While leaded alloys are no longer used in potable water applications, they are still useful for other instances where low friction and wear rates are desired. For instance, high-leaded tin bronzes are cast into sleeve bearings and exhibit lower wear rates than steel.
4. 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).
Leaded copper-base alloys are the easiest to machine. 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.
5. Post-casting processing can further enhance cast copper parts’ appeal.
Secondary steps such as polishing, plating, soldering, brazing and welding can be performed on cast copper alloys for improved surface finish and high tolerance control.
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.
6. Cast copper comes in a wide range of alloy choices, making it a suitable candidate for many applications, depending on design loads and environment corrosivity.
7. Designing for cast copper alloys requires careful planning for thick and thin sections.
Using both should be avoided, but when both are necessary, the thicker section should always be blended or tapered gradually into the thinner one. 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.
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.
8. Whenever possible, L, T and X intersections should be avoided.
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.
9. Costs are comparable to other metals due to high yield, low machining costs and little requirement for surface coatings, such as paint. CS
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