ability. Te yield is not as good on the silicon, and the bismuth machines much easier. With the silicon alloys, the melt temperature is lower, so your energy costs are lower…and it’s a little bit cleaner.” Leaded cop-
per long served the plumbing industry largely due to its unsurpassed machin- ability. Significant ductility and elonga- tion make the alloy capable of rapid machining without the use of coolant. No-lead alloys, when machined in a similar manner, tend to crack. Leaded alloys also are more castable than their no-lead counterparts. For silicon-based alternatives, this means more risering is necessary to aid in solidification. For bismuth alloys, which do not feed as well, new gating systems often have to be designed to make the transition. “You have a wider freezing range on [bismuth],” Omer said. “You can feed the silicon alloys with risers because of [their
The Fresno Valves boiler head casting was sound when modeled in a leaded alloy (left) but showed significant shrink porosity when the unleaded alloy was used.
freezing range]. Sometimes, you make major changes to the pattern.”
Fresno Goes Unleaded
Te no-lead alloy
C87500 (typically 82% copper, 14% zinc and 4% silicon) has a liquidus temperature of 916- 919F and a solidus temperature of 820- 821F. Te density of the material varies
with different solidification paths. While the alloy offers bet- ter feeding than many other alloy families, when feeding ends too early, its porosity levels are great. Terefore, casting the metal requires accurate density and fraction-solid profiles. Te purpose of the recent conversion from C83600 to C87500 at Fresno Valves was to examine the impact of direct alloy substitution for a 4-lb. boiler head. Te metal- casting company performed identical casting simulations of both alloys with identical process conditions (except modifi-
November 2011 MODERN CASTING | 43
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