Permanent Mold Cast Copper Alloys Provide Opportunities for Conversion Success

Permanent mold casting of copper alloys dates to ancient times when copper was poured into molds chipped from stone to form bronze axes, arrowheads and other weapons. Although the concern at the time wasn’t with grain structures and microhardness, strength and appearance were prevalent factors.

Within the last 50 years, the practice of permanent mold casting of copper-based alloys has grown in the United Kingdom, Germany, France, Italy and the former Soviet Union for the casting of yellow brasses, aluminum bronzes and high conductivity copper. These countries are producing automotive parts, plumbing components and fittings, and parts for sprinklers, electric motors and door locks.

During this growth in Europe, only a few American companies have embraced the process. Although permanent mold has become a widely-used option for aluminum, sand casting continues to dominate casting production for copper-rich alloys.

To American foundries, permanent mold casting of copper alloys was a European process, with European equipment and European technical literature. The resources weren't available in the U.S. or in the English language. In addition, the few American foundries that embraced the technology weren't willing to share their experience and successes. These attitudes, however, have begun to change.

Today, for example, 14 U.S. foundries produce copper-based alloy castings via permanent mold, compared to only five foundries 15 years ago. In 1997, 8000 ton of copper alloy permanent mold castings were shipped, compared to only 2500 in 1970.

Two main factors can be attributed to this slow but upward trend in the U.S. market. First, stringent federal environmental regulations limiting emissions and the disposal of spent foundry sands have shifted copper foundries toward permanent mold. Second, customer demands on the foundry industry have forced cost reductions and quality improvements, therefore the excellent surface finish and higher dimensional accuracy obtained via permanent mold casting reduces the secondary operations required after casting, allowing copper alloy permanent molders to deliver a finished, engineered component.

Why Permanent Mold?

An essential feature of permanent mold casting is its outstanding capacity to produce thin-walled components that display excellent surface finish and metal soundness. In addition, the permanent mold cast copper alloys meet high requirements on hardness, strength, wear resistance, and thermal and electrical conductivity.

Permanent mold castings can be produced at a lower cost than traditionally fabricated components by modifying and redesigning shapes to closely meet design requirements, rather then simply trying to duplicate shapes. Permanent mold casting is commonly viewed as a competitor of sand casting and shell molding in mass production and small parts. Although true to a limited extent, permanent mold casting also is directly competitive with various methods of fabrication, like deep drawing, forging, welding assemblies and machined components. Many parts currently machined from solid stock, assembled and welded from stampings, or purchased as rough sand castings could be candidates for conversion to permanent mold castings

When considering an identical part to be made in either sand or permanent mold, a sand cast part will be less expensive. For permanent mold casting to be a successful option, the thought process should be geared toward making a value-added, finished component. Because of its chill characteristics, a permanent mold casting has a fine grain structure to provide higher strength. The permanent mold process is capable of thinner sections, thereby saving metal content without compromising strength. The machining of the part can be reduced by the near net shape of permanent mold castings, including small holes, bosses and gear teeth.

Other advantages include :

• rapid solidification that results in a cleaner, finer grain structure and enhanced mechanical properties;
• smoother surface finishes (150-200 min. RMS) compared to sand casting (150- 500 min. RMS);
• dimensional tolerances of 0.01 in.;
• thinner cross sections of 0.06-0.08 in., allowing weight reduction of the casting;
• minimized defects like microshrinkage by using short and medium freezing range copper alloys;
• fewer process variables than conventional sand castings, which makes parts more uniform from part-to-part and lot-to-lot;
• fewer scrap losses due to sand inclusions, scabs, shifts and other common sand casting defects;
• environmental friendliness, as the process produces minimal waste and exhaust;
• the ability to remelt all scrap and excess metal from gates and runners.

In addition, copper alloys are frequently cast onto or around other metals, a process known as insert casting. The insert, usually made of steel, is positioned in the die so that the two metals become physically bonded as the copper alloy solidifies. Insert casting enables permanent mold castings to replace more expensive fabricated parts. Examples of insert castings include shifter forks with a pre-machined steel shaft and door handles with a threaded steel insert.

In permanent mold casting, a full redesign of the component is essential to realize the total benefits of the process. Sand castings may be redesigned to give a lighter, stronger and sometimes easier-to-handle permanent mold part. Fabrications may be redesigned for the permanent mold process to make it an economical component.

In terms of metallurgical benefits, copper alloys have excellent engineering properties such as high strength, corrosion resistance, and high thermal and electrical conductivity. When cast in permanent molds, some copper alloys exhibit enhancement in their mechanical properties. Tensile and yield strengths can approach that of steel and other high-strength ferrous alloys. Also, corrosion resistance of aluminum bronze alloys is comparable to - or in some instances better than - corrosion resistant steel.

Copper-Based Alloys

The permanent mold process, due to the flowability and temperature requirements, does limit the copper-based alloys that can be cast. These alloys are grouped by yellow brasses, engineering alloys and high conductivity coppers.

The yellow brass (C85500, C85800 and B-2) group is the most widely used alloy in the U.S. and Europe for casting sanitary fittings and decorative faucets. It is a low temperature alloy that is conducive to polishing, buffing and plating, and it has a no-lead alternative.

The engineering alloy group includes aluminum bronzes, silicon brasses and nickel silvers. These alloys are the easiest to cast by permanent mold and have a fine finish, good physical properties and excellent corrosion resistance. These alloys are widely used to cast water pump impellers and cutout hardware.

The third group, high conductivity coppers, is used for electrical components such as brush holders and carriers, short bussbars and connectors. These alloys have very high melting points and are very susceptible to oxygen pickup, therefore they are the most difficult to cast.

Casting Copper Alloys

There are four processes for the permanent mold casting of copper-based alloys - bench pouring, hydraulic tilt pouring, counter-gravity low-pressure and counter-gravity vacuum casting.

The first two processes employ gravity pouring. Bench pouring is the most simple permanent molding operation as metal is hand-poured into a mold on a bench. The second gravity-pour process is the semi-automatic hydraulic tilt pouring machine. In a hydraulic tilt pouring machine, the liquid metal also can be hand poured or mechanically poured, but the die is mechanically rotated during pouring to improve flow and solidification. Both types of gravity-pour casting can accommodate any copper alloy.

The third process is counter-gravity low-pressure casting, which is used when the casting demand increases and further precision is necessary. In the counter-gravity low-pressure permanent mold process the liquid metal is pushed upward into the die via pressure. Low-pressure machines offer several advantages over gravity casting, including higher production, better process control and worker safety. Low-pressure die casting machines are predominantly used for yellow brass plumbing components and silicon brass alloy in mass production. With low-pressure, the alloy flexibility is compromised as a dedicated furnace is necessary.

The fourth type of permanent mold casting, counter-gravity vacuum casting, is used to gain complete control of the fill rate of the melt. In the process, the vacuum induces the metal to flow upward into the die. In the counter-gravity vacuum process, castings up to 150 lb and wall thickness of 0.06 in. have been produced, however, it is a specialized permanent mold process that requires a high level of engineering and technical skill to design parts. Although the type of copper alloy isn't limited with this process, for cost considerations the type of part is. An example of a part being cast is a water pump impellers from 3-14 in. in diameter.

Markets

The current market for permanent mold cast copper alloys is diverse. The electrical industry uses permanent mold castings for connectors, cutout hardware, fuse ends and brush holders. The pump industry uses permanent mold castings for impellers, bushings and small pump housings. Other part examples include shifter forks, door lock hardware, cooling fans, valves and valve bodies for marine and corrosive applications, dental and scientific lab equipment, railroad and dump truck components, and agricultural equipment for combines, harvesters and irrigation systems.

The future market for permanent mold cast copper alloys will depend upon conversion success. Although some existing permanent mold cast copper alloy markets - small, low-speed gears made in aluminum bronze and pure high-conductivity copper castings such as connectors, conductors, cryogenic parts and rotors for electric motors - will see an increase in shipments, the future of the market will follow the growth of fabrication conversion successes.

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