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The Intricacies of Brass and Aluminum Die Casting

Die casting is a versatile metal fabrication process used to produce precisely dimensioned, sharply detailed metal parts and products. Two of the most commonly die cast metals are brass and aluminum. While both brass die casting and aluminum die casting involve the same basic process, there are some key differences between casting these two non-ferrous alloys.

What is Die Casting?

Die casting is a metal casting process characterized by forcing molten metal under high pressure into a steel mold or die. The mold cavities are created using two hardened tool steel dies which have been machined into shape and work similarly to an injection mold during the process. The dies come together and are locked in place while the molten metal is injected into the mold at high pressures, then opened after solidification to eject the casting part.

Die casting provides an efficient, high-volume manufacturing means to produce precise, repeatable metal parts with a very fine surface finish, tight tolerances, and thinner walls than possible with other casting methods. The high pressures involved result in a more uniform part with minimal porosity, ideal for applications requiring high strength and durability.

Brass Die Casting

Brass is a copper-zinc alloy known for its strength, corrosion resistance, machinability, attractive appearance, and excellent acoustic properties. It has long been used in applications from musical instruments to valves and plumbing fixtures. Brass die casting is commonly used to produce parts for industrial machinery, hardware, hydraulic components, costume jewelry, musical instruments, and more.

Brass die casting alloys contain between 15-40% zinc, with higher zinc content increasing strength and hardness while decreasing ductility and machining qualities. Common brass alloys used for die casting include Alloy 360 (60% Cu, 39.25% Zn, 0.75% Pb), Alloy 377 (63% Cu, 37% Zn), High Strength 380 (60% Cu, 39% Zn, 0.75% Pb, 0.07% Sn), and Eco Brass (62% Cu, 36% Zn, 2% Pb).

Brass parts can be produced with excellent mechanical properties and a surface finish of 0.4 μm (15 microinches) or better. Brass casts well at die temperatures around 1000°F (540°C) and can be cast thinner, closer to final dimensions than many other alloys. Brass die castings provide excellent corrosion resistance and antiseptic qualities along with strength, ductility and impact resistance.

Aluminum Die Casting

Aluminum is lightweight, corrosion resistant, highly conductive, and easily machined or cast. Aluminum die casting allows high-volume production of complex, detailed aluminum parts with high dimensional precision and repeatability at a relatively low per-unit cost.

Aluminum die casting alloys contain between 92-99% aluminum. Popular aluminum die casting alloys include Alloy 380 (92% Al, 8% Si), Alloy 413 (86% Al, 10% Si, 4% Cu), Alloy 360 (99% Al, 1% Si, 0.6% Fe, 0.5% Cu), and Aural-3 (93.5% Al, 4% Cu, 1% Fe, 1% Mg, 0.5% Ni).

Aluminum die castings meet tight tolerances and can be produced with smooth surface finishes of 0.4 μm (15 microinches) or less. Aluminum offers excellent corrosion resistance, electrical and thermal conductivity, reflectivity, strength-to-weight ratio, and recyclability. However, aluminum has a higher melting point than some alloys so die temperatures can approach 1150°F (620°C).

Aluminum is lightweight (1/3 the density of brass) but strong enough for most applications. It can readily be cast into complex shapes with thinner walls and intricacies than possible with brass casting. Aluminum parts also resist tarnishing and wear. Primary applications include automotive components, electronics and electrical housings, machinery parts, and medical equipment.

The Die Casting Process

While brass and aluminum have distinct properties influencing the final die cast part, both alloys go through the same basic die casting process:

1. Molten metal is melted in a holding furnace and maintained at a set temperature and viscosity. Brass is melted between 900-1050°F. Aluminum melts at 1220°F. A holding furnace keeps the metal molten for delivery into the die.

2. The two die halves are sprayed with lubricant and closed. They are aligned and locked in place on a die casting machine.

3. The molten metal is ladled or sucked via vacuum into a gooseneck holding chamber and delivered under pressure through sprues and runners that lead into the die cavities.

4. Extremely high hydraulic pressures (up to 25,000 psi) force the metal into every crevice of the die cavity, forming the part in precise detail.

5. The metal rapidly cools inside the die and solidifies. Brass solidifies faster than aluminum due to its lower melting point.

6. After solidification, the die halves are opened and the casting is ejected from the die by action of the ejector pins.

7. The casting is then trimmed of extra flash and runners and subjected to post-casting treatments like heat treatment or surface finishing, if required.

8. The castings are then ready for inspection, testing, and use.

Design Considerations for Brass vs. Aluminum

When designing parts for brass versus aluminum die casting, engineers should consider:

- Brass has a higher melting point so it can better fill thinner cross-sections and finer details. Aluminum may struggle to fill sections less than 0.04 in (1 mm) thick.

- Brass is heavier than aluminum (8.5 g/cm3 vs 2.7 g/cm3 density). This can affect functional performance.

- Brass is stronger than aluminum so it may be preferred for parts needing durability. Heat treatable aluminum alloys can help close this gap.

- Aluminum is more reactive than brass so may adhere to die surfaces during casting. Special coatings help performance.

- Brass is more expensive currently than aluminum ($2.5-3.5 per lb vs $1.6-2 per lb for aluminum).

- Brass offers better wear properties while aluminum provides better thermal conductivity.

The choice depends on factors like: dimensional requirements, strength needs, appearance, weight limits, conductivity, and cost. Consulting with knowledgeable die casters is important to selecting the best alloy for each application.

Benefits of Die Casting Brass and Aluminum

- High productivity, high volumes. Die casting is highly efficient, ideal for mass production. Casting rates average 15-50 cycles per hour.

- Excellent dimensional consistency and repeatability. Dimensions are precise and repeatable within +/- 0.005 in (for the first inch) over long production runs of thousands or tens of thousands of castings.

- Cost-effectiveness. Once dies are made, per-unit costs for die cast parts are very low in medium-high production volumes. Automation further lowers costs.

- Complex, detailed parts. Die casting can produce complex shapes and micro-sized features not practical with other methods. Intricate shapes, hollow sections, pockets and bosses are all possible.

- Superior mechanical properties. High pressure injection results in dense, porosity-free castings with strength near that of wrought alloys.

- Wide range of alloys. Most major copper, aluminum, zinc, and magnesium alloys can be die cast successfully. Even exotic and custom alloys are possible for specialized needs.

- Decorative finishes. Brass and aluminum can be cast to a smooth finish and buffed/polished with little effort. Plating like chrome or anodizing aluminum can further enhance appearances.

In summary, brass and aluminum die casting each provide benefits for producing high-quality, precise, cost-effective metal components and products. With their distinct properties, both alloys will continue seeing widespread use across many demanding industrial and commercial applications. CNC Milling CNC Machining