Friday, February 24, 2012

Alloys Used Die Casting

As a fabrication process, die casting is notable for its capacity to manufacture parts with a high degree of uniformity, close design accuracy, and quality surface finishes. In many cases, die casting can reduce or eliminate the need for post-production machining, raising the cost-efficiency of the process and shortening fabrication time. While it may be difficult to die cast sturdier metals, such as certain grades of steel, there are many other types of alloys well-suited for die casting methods.

Die casting molds are usually constructed from hardened steel and they are often the most expensive component in a die casting machine. These molds can handle a range of different alloy families with varying results, but die casting is generally most effective on metals with low fusing temperatures. For this reason, the common die casting alloys fall into a handful of categories based on their composition and material properties.

Zinc Alloys

Zinc-based materials are relatively easy to die cast, and respond well to the die molding process. These materials are comprised of multiple metals in specific ratios. For example, a typical zinc-based die casting workpiece consists of 86 percent zinc, 4 to 7 percent copper, and 7 to 10 percent tin. Slightly higher proportions of tin make the workpiece more flexible, while increased copper levels improve rigidity. Zinc alloys have a melting point in the range of 700 to 800 degreees Fahrenheit.

Zinc alloys are often used in place of cast iron or brass, but tend to have lower tensile strength than their sturdier counterparts. Unless it is specially reinforced during the alloying process, zinc-based material cannot exceed approximately 17,000 pounds per square inch of force. As a result, die cast zinc products are generally not used in applications involving high mechanical loads. Zinc castings can also be corroded by alkaline substances or salt-water, and are often plated to preserve their luster despite atmospheric conditions.

Tin Alloys

Alloys composed with a significant amount of tin as a base metal are most often used in applications requiring corrosion resistance, such as those involving the food industry or internal and external bearings. While the proportion of metals in these alloys can vary widely, a typical tin alloy consists of 90 percent tin, 6 percent antimony, and 4 percent copper, which is added to strengthen the material’s durability. Tin alloy die castings generally weigh under ten pounds and rarely exceed 1/32 of an inch in thickness. They are valued for their resistance to alkaline, acids, and water, but feature a comparatively low tensile strength rating of below 8,000 pounds per square inch. Bronze and Brass Alloys Most bronze and brass materials can be die cast as effectively as zinc-based alloys, although small holes can only be drilled into the workpiece after casting, rather than during the casting process. Bronze and brass are commonly used to create washers, camshaft components, and decorative products (due to their distinctive coloring and potential for surface finishes). A typical brass alloy consists of 60 percent copper, 40 percent zinc, and 2 percent aluminum, but there are many variations on this mixture. Die casting bronze and brass is capable of yielding products with a durable surface and highly accurate interior specifications.

Some brasses have difficulty tolerating shrinkage from high temperature processes, but despite these challenges, most of these alloys can be used for products weighing up to fifteen pounds and with thicknesses at or under 1/32 of an inch. They are generally suitable for applications requiring tensile strength of less than 8000 pounds per square inch.

Aluminum Alloys

Die cast aluminum alloys are often found in automobile parts and gears, and have been used to create surgical instruments in the past. They are generally stronger and lighter than most zinc-based materials, but tend to be more expensive to create. Using aluminum alloys can reduce the need for finishing treatments, such as plating, and a common grade is composed of 92 percent aluminum mixed with 8 percent copper. Magnesium may be added to this alloy to improve its tensile strength from around 21,000 pounds per square inch to approximately 32,000 per square inch, while nickel can be included to increase rigidity and provide a higher surface finish. The melting point for an aluminum alloy is around 1150 degrees Fahrenheit.

Lead Alloys

Like tin alloys, lead-based materials tend to be used for their corrosion resistance and in applications requiring no more than 8000 pounds of tensile strength per square inch. Common applications include fire-safety equipment, bearings, and various decorative metal goods. They are relatively inexpensive for producing castings under 15 pounds, but lead alloys cannot be used for products that will be in contact with food. A typical lead alloy might be 90 percent lead and 10 percent antimony, with tin being a common addition as well. The melting point is usually around 600 degrees Fahrenheit, and product thickness rarely exceeds 1/32 of an inch.

For insight on the history of die casting procedures and the scientific principles underpinning them, please see Die-Casting, by Edmund Francis Lake and Chester L. Lucas.

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