Copper and Copper Alloys Casting Problems

Pure copper is extremely difficult to cast as well as being prone to surface cracking, porosity problems, and to the formation of internal cavities. The casting characteristics of copper can be improved by the addition of small amounts of elements including beryllium, silicon, nickel, tin, zinc, chromium and silver.

Copper alloys in cast form (designated in UNS numbering system as C80000 to C99999) are specified when factors such as tensile and compressive strength, wear qualities when subjected to metal-to-metal contact, machinability, thermal and electrical conductivity, appearance, and corrosion resistance are considerations for maximizing product performance. Cast copper alloys are used for applications such as bearings, bushings, gears, fittings, valve bodies, and miscellaneous components for the chemical processing industry. These alloys are poured into many types of castings such as sand, shell, investment, permanent mold, chemical sand, centrifugal, and die casting.

The copper-base casting alloy family can be subdivided into three groups according to solidification (freezing range). Unlike pure metals, alloys solidify over a range of temperatures. Solidification begins when the temperature drops below the liquidus; it is completed when the temperature reaches the solidus. The liquidus is the temperature at which the metal begins to freeze, and the solidus is the temperature at which the metal is completely frozen.

Group I alloys

Group I alloys are alloys that have a narrow freezing range (about 50^oC), that is, a range of 50^oC between the liquidus and solidus temperature. Group I alloys includes: copper (UNS No. C81100), chromium copper (C81500), yellow brass (C85200, C85400, C85700, C85800, C87900), manganese bronze (C86200, C86300, C86400, C86500, C86700, C86800), aluminum bronze (C95200, C95300, C95400, C95410, C95500, C95600, C95700, C95800) nickel bronze (C97300, C97600, C97800), white brass (C99700, C99750).

Pure Copper and Chromium Copper. Commercially pure copper and high copper alloys are very difficult to melt and are very susceptible to gassing. In the case of chromium copper, oxidation loss of chromium during melting is a problem. Copper and chromium copper should be melted under a floating flux cover to prevent both oxidation and the pickup of hydrogen from moisture in the atmosphere. In the case of copper, crushed graphite should cover the melt. With chromium copper, the cover should be a proprietary flux made for this alloy. When the molten metal reaches 1260^oC, either calcium boride or lithium should be plunged into the molten bath to deoxidize the melt. The metal should then be poured without removing the floating cover.

Yellow Brasses. These alloys flare, or lose zinc, due to vaporization at temperatures relatively close to the melting point. For this reason, aluminum is added to increase fluidity and keep zinc vaporization to a minimum. The proper amount of aluminum to be retained in the brass is 0.15 to 0.35%. Above this amount, shrinkage takes place during freezing, and the use of risers becomes necessary. After the addition of aluminum, the melting of yellow brass is very simple, and no fluxing is necessary. Zinc should be added before pouring to compensate the zinc lost in melting.

Manganese Bronzes. These alloys are carefully compounded yellow brasses with measured quantities of iron, manganese, and aluminum. The metal should be melted and heated to the flare temperature or to the point at which zinc oxide vapor can be detected. At this point, the metal should be removed from the furnace and poured. No fluxing is required with these alloys. The only addition required with these alloys is zinc. The amount required is that which is eeded to bring the zinc content back to the original analysis. This varies from very little, if any, when an all-ingot heat is being poured, to several percent if the heat contains a high percentage of remelt.

Aluminum Bronzes. These alloys must be melted carefully under an oxidizing atmosphere and heated to the proper furnace temperature. If needed, degasifiers can be stirred into the melt as the furnace is being tapped. By pouring a blind sprue before tapping and examining the metal after freezing, it is possible to tell whether it shrank or exuded gas. If the sample purged or overflowed the blind sprue during solidification, degassing is necessary. Degasifiers remove hydrogen and oxygen. Also available are fluxes that convert the molten bath. These are in powder form and are usually fluorides. They aid in the elimination of oxides, which normally form on top of the melt during melting and superheating.

Nickel Bronzes. These alloys, also known as nickel silver, are difficult to melt. They gas readily if not melted properly because the presence of nickel increases the hydrogen solubility. Then, too, the higher pouring temperatures aggravate hydrogen pickup. These alloys must be melted under an oxidizing atmosphere and quickly superheated to the proper furnace temperature to allow for temperature losses during fluxing and handling. Proprietary fluxes are available and should be stirred into the melt after tapping the furnace. These fluxes contain manganese, calcium, silicon, magnesium, and phosphorus and do an excellent job in removing hydrogen and oxygen.

White Manganese Bronze. There are two alloys in this family; both of them are copper-zinc alloys containing a large amount of manganese and, in one case, nickel. They are manganese bronze type alloys; they are simple to melt, and can be poured at low temperatures because they are very fluid. They should not be overheated, as this serves no purpose. If the alloys are unduly superheated, zinc is vaporized and the chemistry of the alloy is changed. Normally, no fluxes are used with these alloys.

Group II alloys

Group II alloys are those that have an intermediate freezing range, that is, a freezing range of 50 to 110^oC between the liquidus and the solidus. Group II alloys are: beryllium copper (C81400, C82000, C82200, C82400, C82500, C82600, C82800), silicon brass (C87500), silicon bronze (C87300, C87600, C87610, C87800), copper-nickel (C96200, C96400).

Beryllium Coppers. These alloys are very toxic and dangerous if beryllium fumes are not captured and exhausted by proper ventilating equipment. They should be melted quickly under a slightly oxidizing atmosphere to minimize beryllium losses. They can be melted and poured successfully at relatively low temperatures. They are very fluid and pour well.

Silicon Bronzes and Brasses. The alloys known as silicon bronzes, UNS alloys C87300, C87600, and 87610, are relatively easy to melt and should be poured at the proper pouring temperatures. If overheated, they can pick up hydrogen. While degassing is seldom required, if necessary, one of the proprietary degasifiers used with aluminum bronze can be successfully used. Normally no cover fluxes are used here. The silicon brasses (UNS alloys C87500 and C87800) have excellent fluidity and can be poured slightly above their freezing range. Nothing is gained by excessive heating, and in some cases, heats can be gassed if this occurs. Here again, no cover fluxes are required.

Copper-Nickel Alloys. These alloys (90Cu-10Ni, UNS C96200 and 70Cu-30Ni, UNS C96400) must be melted carefully because the presence of nickel in high percentages raises not only the melting point but also the susceptibility to hydrogen pickup. In virtually all foundries, these alloys are melted in coreless electric induction furnaces, because the melting rate is much faster than it is with a fuel-fired furnace. When ingot is melted in this manner, the metal should be quickly heated to a temperature slightly above the pouring temperature and deoxidized either by the use of one of the proprietary degasifiers used with nickel bronzes or, better yet, by plunging 0.1% Mg stick to the bottom of the ladle. The purpose of this is to remove all the oxygen to prevent any possibility of steam-reaction porosity from occurring. Normally there is little need to use cover fluxes if the gates and risers are cleaned by shot blasting prior to melting.

Group III alloys

Group III alloys have a wide freezing range. These alloys have a freezing range of well over 110^oC, even up to 170^oC. Group III alloys are: leaded red brass (C83450, C83600, C83800), leaded semi-red brasses (C8400, C84800), tin bronze (C90300, C90500, C90700, C91100, C91300), leaded tin bronze (C92200, C92300, C92600, C92700), high-leaded tin bronze (C92900, C93200, C93400, C93500, C93700, C93800, C94300).

These alloys, namely leaded red and semi-red brasses, tin and leaded tin bronzes, and high-leaded tin bronzes, are treated the same in regard to melting and fluxing and thus can be discussed together. Because of the long freezing ranges involved, it has been found that chilling, or the creation of a steep thermal gradient, is far better than using only feeders or risers. Chills and risers should be used in conjunction with each other for these alloys. For this reason, the best pouring temperature is the lowest one that will pour the molds without having misruns or cold shuts. In a well-operated foundry, each pattern should have a pouring temperature, which is maintained by use of an immersion pyrometer.

Fluxing. In regard to fluxing, these alloys should be melted from charges comprised of ingot and clean, sand free gates and risers. The melting should be done quickly in a slightly oxidizing atmosphere. When handled at the proper furnace temperature and cooled to the proper pouring temperature, the crucible is removed or the metal is tapped into a ladle. At this point, a deoxidizer (15% phosphor copper) is added. The phosphorus is a reducing agent (deoxidizer). This product must be carefully measured so that enough oxygen is removed, yet a small amount remains to improve fluidity. This residual level of phosphorus must be closely controlled by chemical analysis to a range between 0.010 and 0.020% P. If more is present, internal porosity may occur and cause leakage if castings are machined and pressure tested.

In addition to phosphor copper, pure zinc should be added at the point at which skimming and temperature testing take place prior to pouring. This replaces the zinc lost by vaporization during melting and superheating. With these alloys, cover fluxes are seldom used. In some foundries in which combustion cannot be properly controlled, oxidizing fluxes are added during melting, followed by final deoxidation by phosphor copper.