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Brass and Aluminum Die Casting - An Overview

Die casting is a manufacturing process that can produce geometrically complex metal parts by forcing molten metal under high pressure into a mold cavity. The two most common metals used in die casting are brass and aluminum. Brass is an alloy of copper and zinc, while aluminum is a lightweight metal known for its high strength-to-weight ratio. Both brass and aluminum are suitable for die casting due to their castability, machinability, and corrosion resistance.

Brass Die Casting

Brass is commonly used for die cast parts requiring high strength, hardness, wear resistance, and corrosion resistance. Some examples of brass die cast parts include automotive components like locks, plumbing fixtures like valves, and costume jewelry. Brass die casting alloys contain between 55-90% copper with the balance being zinc and other elements like lead and tin. The different combinations of alloys result in different material properties optimized for various applications.

The most common brass alloys used for die casting include:

- CDA 860 (85% Cu, 5% Sn, 5% Pb, 5% Zn): An economical alloy suitable for hardware components and decorative parts. It has good corrosion resistance and moderate strength. The addition of lead improves machinability.

- CDA 864 (81% Cu, 18% Zn, 1% Fe): A high strength brass alloy used for parts requiring thinner walls and complex shapes like gears. It has excellent wear resistance.

- CDA 870 (87% Cu, 8% Sn, 5% Zn): An alloy with very high hardness and strength. It maintains strength at elevated temperatures making it suitable for parts like valves and fittings. The tin enhances corrosion resistance.

- CDA 874 (83% Cu, 16% Zn, 1% Fe): A brass alloy with high ductility that can withstand high-stress applications. It also has excellent corrosion resistance.

The main advantages of brass die casting include:

- High dimensional accuracy and repeatability. Tolerances up to ±0.005 inches are possible.

- Ability to produce thin walls down to 0.025 inches. Complex geometries are possible.

- Excellent surface finish ranging from 63 to 125 Ra microinches.

- Mechanical properties like strength, ductility, hardness, and wear resistance can be tailored by alloy selection.

- Excellent corrosion resistance for long component life.

The main limitations are lower ductility vs other manufacturing processes and limitations on maximum part size, usually less than 17 inches. Brass parts may also need to be chrome or nickel plated to enhance corrosion protection.

Aluminum Die Casting

Aluminum is the most widely die cast metal. Some examples of aluminum die cast parts include automotive engine components, appliance housings, and hydraulic valve bodies. The aluminum alloys used for die casting contain over 92% aluminum with silicon and copper as the main alloying elements. Silicon enhances the castability while copper improves strength.

Some common aluminum alloys for die casting include:

- A380 (9% Si, 3% Cu): A popular general purpose alloy offering good corrosion resistance, ductility, and strength. It is one of the easiest alloys to cast and machine.

- A360 (10% Si, 0.6% Cu): This alloy has good ductility, toughness, and strength. It can withstand elevated temperatures so is used for parts like automotive engine components.

- A383 (10% Si, 1% Cu): Alloy 383 combines good castability with high strength and hardness. It is suitable for parts requiring thinner cross sections.

- A413 (12% Si, 1% Cu): Alloy 413 has excellent fluidity making it easy to fill thin wall sections and complex dies. It provides good strength and is heat treatable.

The key benefits of aluminum die casting include:

- Parts have a high strength-to-weight ratio. Aluminum is lightweight compared to other die casting metals.

- Superior dimensional stability and accuracy. Tolerances up to ±0.005 inches are possible.

- High production rates. Simple shaped parts can cycle in less than 30 seconds.

- Excellent corrosion resistance provided by the anodized aluminum surface.

- Lower melting point than brass allows faster cycle times and increased die life.

- Good machinability and mechanical properties. Alloy properties can be adjusted.

- Lower material costs than many alternate metals and manufacturing processes.

The main limitations are slightly lower wear resistance than brass and limitations on maximum part size, usually less than 45 inches.

The Die Casting Process

While both brass and aluminum are cast in dies, the specific die casting process used differs slightly between the two metals:

1. For brass, hot-chamber die casting is typically used. The brass alloy is melted in a crucible inside the die casting machine and the entire die assembly is submerged in the molten metal bath. When the dies are closed, molten metal is forced into the die under high speeds and pressure. Once the part solidifies, the dies open and the cast part is ejected. Cycle times can be very fast.

2. For aluminum, cold-chamber die casting is normally used. The molten aluminum starts outside the die casting machine. Each shot is ladled into an empty, unheated chamber. The chamber is closed and then the piston forces the metal into the die at high speed and pressure. Cycle times are a bit slower than hot-chamber but the process is more versatile for different alloys.

For both brass and aluminum, the molten metal must completely fill the die cavity and solidify rapidly for a fully formed cast part. Typical clamping pressures range from 5,000 to over 50,000 PSI depending on part size and geometry. The high pressures force the metal into every crevice of the die to ensure proper replication and uniform properties.

Die Design Considerations

To fully utilize the benefits of brass and aluminum die casting, the dies must be properly designed. Some key die design considerations include:

- Draft angles of at least 1-3 degrees must be added to allow for part ejection.

- Generous fillet radii help metal flow and prevent early solidification.

- Parting lines should be along natural splits in geometry.

- Internal tunnels and cored sections to reduce weight must allow for uniform metal flow.

- Non-fills due to trapped air must be vented through the dies.

- Cooling channels strategically placed to avoid hot spots and control directional solidification.

- Robust die materials like tool steel to withstand the high pressures.

When designed properly, the die casting dies facilitate complete fills and uniform material properties part-to-part. Life spans over 250,000 shots are possible for production dies.

Secondary Finishing Operations

Once ejected from the dies, most brass and aluminum die cast parts undergo additional secondary finishing operations to achieve the final part geometry, appearance, and performance. Typical secondary operations include:

- Shot blasting to remove surface imperfections and oxide layers from the high-temperature casting process.

- Machining operations like drilling and tapping to create holes, slots, threads, or meet tight tolerances.

- Deburring to break sharp edges and remove flashing.

- Polishing and buffing to enhance surface finishes.

- Plating and coating like chrome, nickel, or lacquer to improve corrosion and wear resistance or for decorative purposes.

- Heat treatments and age hardening to increase strength on some alloys.

- Leak testing parts like valves that must hold pressure.

The combination of precision die casting with skilled secondary machining allows for the high-volume production of complex, high-quality brass and aluminum components.

Die Casting Applications

The versatility of brass and aluminum die casting makes it ideal for a diverse range of applications across many industries including:

- Automotive: Engine components like manifolds, pistons, pumps; underhood parts like brackets; transmission components.

- Aerospace: Components like impellers, fittings, valves.

- Medical: Components for imaging equipment; surgical instrument handles.

- Industrial Machinery: Gears, pulleys, levers, nozzles.

- Plumbing: Valves, couplings, fittings.

- Telecommunications: Shielding, connectors, casings.

- Appliances: Housings, handles, hinges.

- Consumer Products: Sporting equipment, electronics, decorative items.

As demand grows for stronger, lighter, and more complex parts, brass and aluminum die casting will continue to thrive as an essential manufacturing process across many critical industries. The design freedom, precision, speed, and economical production volumes make die casting an ideal choice for both low and high-volume component production. CNC Milling