pany was able to determine that the increased shrinkage of the metal would not make a pattern change necessary in this case. “[Solidifi cation] modeling showed exactly where there was a shrink on the alternate no-lead alloy, but it happened to be in an area that didn’t cause any leakage or void after machining,” said Leigh Omer, foundry manager. “T e modeling showed shrinkage porosity exactly where it showed in the casting after destructive testing.”
The Alternatives to Lead
T e two most common alloys used to cast parts for the plumbing industry, C84400 and C83600, contain higher
lead concentra- tions than are allowed by law in some states, including Fresno Valves’ home state of Califor- nia. Researchers estimate that all leaded coppers eventually will be replaced by six to eight diff er- ent alloys. While the
Fig. 1. Copper alloy C87500 exhibits more than 3% more shrink than C83600, making the transition to the no-lead alloy tricky.
Fresno Valves boiler head transi- tion concerned a switch to a silicon- based copper alloy,
the most well-known alternative to leaded copper is copper- bismuth. T e greatest challenge in developing bismuth as an alternative to lead is its supply—the material is scarce, and little is known about the global supply—and resulting cost. “Silicon alloys as a rule are cheaper than the bismuth alloys,” Omer said. “But there’s also a trade-off : yield and machin-
LEAD IN COPPER CASTINGS
Lead is commonly added to many cast copper alloys. Because of the low solubility of lead in copper, true alloying does not occur to any measurable degree. During the solidifi cation of castings, some constituents in a given alloy form crystals at higher temperatures relative to others, resulting in dendrites. The small spaces between the dendrites can interconnect to form micropores. This micro porosity is a consequence of the solidifi cation process. The role of lead is to seal these intradendritic pores. This results in a pressure- tight casting, which is important for fl uid handling applications.
Lead also allows the machining of castings to be performed at higher speeds without the aid of coolants because it acts as a lubricant for cutting tool edges and promotes the formation of small, discontinu- ous chips that easily can be cleared. This results in improved machined surface fi nishes. Lead also plays a role in providing lubricity during service, as in cast copper bearings and bushings. Lead does not have an adverse effect on strength unless present in high concentrations, but it does reduce ductility. Although lead-containing copper alloys can be soldered and brazed, they cannot be welded.
—Sidebar adapted from an article by the Copper Development Association Inc., New York
42 | MODERN CASTING November 2011
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