How to Handle Lead-Free Copper-Bismuth Alloys CAST TIP—Technical Information and Practice
AFS Copper Alloy Div. 3, Schaumburg, Illinois
ily, from pure copper to highly alloyed brasses and bronzes. Lead is a common element added
C
to sand cast copper alloys. It helps seal microporosity formed during so- lidification, resulting in better pressure tightness, and improves machinability, allowing machining at higher speeds and the production of good machined surface finishes without the aid of cool- ants. Among the most popular sand-cast alloys are leaded red and semi-red brasses (alloys C83600 and C84400), with lead content ranging between 4 and 8% (Table 1). New government and industry stan-
dards restricting the amount of lead in potable water components have in- creased the production of low-lead and lead-free alloys for potable water and food processing applications. Among the most popular no-lead options are alloys using bismuth or bismuth and selenium. In these alloys, bismuth replaces lead and contributes to their machinability and pressure tightness, which would otherwise be diminished. Selenium further enhances machin- ability. These alloys can be used as direct replacements for leaded red and yellow brasses in casting applications. Five common copper-bismuth alloys are (see Table 2 for details): • C89510 and C89520 (replace C83600
Common Leaded Brass Alloys* Copper Alloy No. C83600 C84400 C85800
Cu Cu Sn 1.5 Pb
opper-based alloys have been used for millennia for decora- tive, ornamental and com- mercial purposes. Tremendous variety exists in the alloy fam-
leaded red and C84400 semi-red brass);
• C89550 (a substitute for C85800 leaded yellow brass);
• C89833 (a substitute for C83600); • C89836 (a substitute for C83600).
Casting Copper-Bismuth Alloys Copper-bismuth alloys were devel-
oped to behave in the metalcasting process and have properties similar to existing leaded alloys. However, no-lead alloys can be more difficult to cast than their leaded counterparts, so metalcast- ing facilities may need to make some changes in their practices when using them. The following are some common issues encountered by metalcasters when using copper-bismuth alloys. Melting and Pouring—Lower fluidity
of copper-bismuth alloys may require pouring at slightly higher temperatures. This may lead to an increased affinity for hydrogen gas in the metal. When pouring heavy section castings (with a higher tendency for hydrogen gas porosity) or on high-humidity days, a degassing procedure may be needed. This is generally done with a graphite or steel lance and purge gas, such as nitrogen or argon. A test prior to pour- ing, such as pouring a blind sprue, can help determine potential gas levels. Leaking and Grain Structure—Some
casting facilities have reported bismuth- based alloys show a greater tendency for leaks. The individual experience and solutions may vary, but some success in reducing leaks has been reported by increasing zinc and/or nickel levels
Zn Fe
84.0-86.0 4.0-6.0 4.0-6.0 4.0-6.0 .30 78.0-82.0 2.3-3.5 6.0-8.0 7.0-10.0 .40 57.0min
1.5 31.0-41.0 .50
Common Non-Lead Alternatives* Copper Alloy No. C89510 C89520 C89550 C89833 C89836
Sn 1.2 Pb Zn Sb
.25 .25 .05
Fe Sb
86.0-88.0 4.0-6.0 .25 4.0-6.0 .20 .25 85.0-87.0 5.0-6.0 .25 4.0-6.0 .20 .25 58.0-64.0
.09 32.0-38.0 .50 .05
86.0-91.0 4.0-6.0 .09 2.0-6.0 .30 .25 87.0-91.0 4.0-7.0 .09 2.0-4.0 .35 .25
MODERN CASTING / July 2010
As -- --
.05
Ni S 1.0 .08 1.0 .08 1.0 .05
Ni
1.0 1.0 .50
P
or reducing pouring temperatures. Gating—Conventional gating and
risering systems employed for leaded alloys also can be used to pour copper- bismuth alloys. However, some facilities have found modifications in the systems may be needed to aid in the reduction of porosity and leaks. Bismuth-containing alloys may perform better than leaded alloys with respect to hot tearing, which could be used to improve the design of existing components. Machining—Copper-bismuth alloys
have machining characteristics similar to leaded brasses but may be less forgiv- ing. Metalcasters’ experiences machin- ing copper-bismuth alloys have shown that adjustments in feed and speed of cut and use of coolant or lubrication may be helpful for optimum machining performance. The alloys may produce fewer units cut per tool. Storage and Recycling—Lead-free al-
loys look similar to the leaded alloys, so clearly marked bins should be used to segregate scrap to avoid potential bismuth and selenium contamination. For example, unintentional mixing of C89510 and C89520 with their leaded counterparts (C83600 and C84400) would lead to a gradual increase in bis- muth and selenium content. Although this should have no adverse effect on properties at lower concentrations (up to about 0.3% bismuth in C83600 and 1.3% in C84400), selenium contamina- tion can have a deleterious effect on the ductility of both alloys and should be controlled below 0.1%.
MC S
.08 .08 .05
Al
.05 .005 .05 .005 .01 .10-.6
1.0 .08 .050 .005 .90 .08
.06 .005 P
.05 .02 .01
Si Al
.005 .005 .55
Bi Si
.005 .005 .25
Se
.005 .50-1.5 .35-.75 .005 1.6-2.2 .8-1.1 .25
.005 1.7-2.7 .005 1.5-3.5
*Copper alloys are designated by the Unified Numbering System and listed in ASTM Standard Designation for Wrought and Cast Copper and Copper Alloys. 41
.6-1.2 .01-.10 -- --
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