19Apr

The Difference Between Aluminum, Magnesium and Zinc Alloys

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From the creation of scale replica cars and airplanes to the durable mechanical parts housed in their real life counterparts, die casting is one of the most cost-effective and versatile manufacturing processes in the world.

Die casting is a precision process that involves injecting molten metal under high pressure into a die, or mold, of a desired shape. Dies are usually constructed with long-lasting, quality steel. Upon solidification and cooling of the material, the design, which can support a range of complex geometries and intricate details, is ejected for quenching, machining or finishing.

This process can be repeated again and again with incredible dimensional accuracy, making it one of the best ways for manufacturing a high volume of castings.

In addition, some castings can be completely finished when they are ejected from the die, which can eliminate the need for machining and additional finishing work. By offering a high-speed production process, precision and high-quality castings, die casting has become a valuable manufacturing method since its invention in 1838.

One of the most important choices to make when setting out to die cast a design is determining the type of alloy best suited to its specific application. Aluminum, magnesium, zinc, and zinc-aluminum (ZA) are the most common types of metallic alloys used in the die casting process.

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Alloys and the Die Casting Process

Each alloy has its own physical and mechanical properties, which are important factors in determining the longevity, strength, and overall functionality of the finished product. Unlike copper or ferrous alloys, the most commonly used alloys have lower melting temperatures, which influences castability. The complexity of the part, the minimum wall thickness and the required precision of the part will also influence how castable it is.

Depending on the application of your design and the metal alloy used, there are several different methods of die casting which offer more flexibility in manufacturing. These include the hot chamber, cold chamber and squeeze casting processes.

Hot chamber die casting is ideal for metals with lower melting temperatures like zinc and magnesium, and cold chamber processes are used for metallic alloys like aluminum that have higher melting temperatures. Squeeze casting is a process that requires no gas entrapment and produces high-quality components.

Different Alloy Properties

Aluminum, magnesium, zinc and zinc-aluminum are the four most commonly used alloys in die casting processes across North America. Your design’s application, material density, tensile strength, yield strength, melting temperature and modulus of elasticity are all important factors in selecting the alloy best suited to your design needs.

Aluminum Alloys

While it is not considered as castable as zinc due to a higher melting temperature, aluminum is the most commonly used alloy in the die casting industry, and it remains one of the most economical materials used in high-volume casting projects. Die cast aluminum parts can be found in everything from the components used to run your car to the electronics and power tools millions of Americans utilize every day.

With a specific gravity of 2.7, aluminum is considered to be a lightweight, structural material, but it is rarely cast in its purest form.

Because of the risk of hot cracking and shrinkage, aluminum is often alloyed with other materials including silicon, magnesium and copper. Silicon increases fluidity, pressure tightness and modulus of elasticity in the alloy. In addition, silicon also reduces the metal’s specific gravity, thermal expansion and shrinkage. It also improves corrosion resistance.

Copper is used to increase corrosion resistance, tensile strength and the hardness of the alloy, providing it with superior mechanical properties.

There are now several aluminum alloys available to designers which are practical for a wide range of applications because of their varying physical and mechanical properties.

In addition to being lightweight, aluminum alloys are also resistant to corrosion, maintain strength at high temperatures provide high dimensional stability when casting intricate geometries and areas with thin wall thickness. They also possess high thermal and electrical conductivity. Machining aluminum is easier as well when compared to other materials including iron, steel and titanium.

Seven of the eight most commonly used aluminum alloys are based on a system of eutectic composition, where 11.7 percent silicon is used as a reference point in grouping them.

Commonly Used Aluminum Alloys and Their Benefits

Alloy 380 is used in a variety of products including home appliances, furniture, power tools, automotive parts, electronics and lawnmower housings because it offers the best range of properties for both the finished product and casting. With a combination of 8.5 percent silicon, and 3.5 percent copper, alloy 380 also offers a tensile strength of 324 megapascal, or 47 kilopounds per square inch.

Alternatives to alloy 380 include alloys 383 and 384, which are more resistant to hot cracking, and they offer improved die filling characteristics during the casting of complex components.

When compared to other aluminum alloys, alloy 360 offers greater strength during exposure to higher temperatures. It also offers better resistance to corrosion.

Alloy 443 offers the highest ductility, or the ability to stretch under tensile strength, of all aluminum alloys.

While zinc is ideal for casting components with thin walls, aluminum alloy 413 is useful for casting a variety of intricate parts. When manufacturing hydraulic cylinders, or other pressure machinery, it also offers better pressure tightness compared to its counterparts.

Alloy 390 provides better resistance to abrasion and wear, and it was originally developed for automotive engine blocks. However, alloy 390 offers the lowest ductility of aluminum alloys with elongation less than 1 percent.

Higher ductility comes from Alloy 518, which also possesses excellent resistance to corrosion when compared to other alloys used in casting.

Magnesium Alloys

Magnesium is another lightweight structural material commonly used in die casting, but like aluminum, it is alloyed with other metals to provide greater stability and better mechanical properties. In combination with metals like silicon, manganese, aluminum and zinc, magnesium alloys have become a great asset to the die casting industry. With a specific gravity of 1.7, magnesium is the lightest of the commonly used alloys available.

Alloy AZ91D is about 9 percent aluminum and 1 percent zinc, and it is the most widely used available magnesium alloy. AZ91D offers excellent strength, corrosion resistance and better castability compared to other magnesium alloys. By putting limits on impurities such as iron, copper and nickel, better corrosion resistance is obtained, making AZ91D one of the best choices when casting components that need to withstand wear.

Alloys AM60B, AM50A and AM20 are also commonly used in die casting, but they offer better ductility, while still maintaining corrosion resistance and strength. For applications that require enhanced strength when exposed to higher temperatures, alloys AS41B and AE42 are often the best option.

Magnesium is lightweight, and it has a durable structure for certain die cast components. Substituting magnesium components in place of heavier aluminum ones may contribute to lower fuel costs in vehicles, which has car manufacturers developing new technology to utilize magnesium’s unique properties.

For engine components that are susceptible to higher temperatures and corrosion, alloys AS41B and AE42 are an excellent choice. All magnesium alloys exhibit a high tensile yield strength and modulus of elasticity.

Aluminum vs. Magnesium Alloys

Like aluminum, magnesium alloys are utilized in the casting of automotive parts and provide their own unique mechanical and physical properties. While there have been experiments conducted to substitute magnesium for aluminum, it is still softer, less stable, more expensive and tends to bend easier when under stress.

While aluminum alloys take longer to solidify compared to magnesium alloys, its alloys provide a longer die life. In addition, aluminum does not require as much finishing work as magnesium. Special treatments and coatings are needed when determining a magnesium casting’s application.

When compared to the cost of melting aluminum, new technology has lowered the expenses required to melt magnesium alloys, but casting requires a higher injection speed. Conversely, magnesium has a quicker ejection time over aluminum castings. Magnesium is also better at casting components with thinner walls and tighter tolerances than aluminum.

However, even with the many advantages of magnesium, aluminum remains a less expensive alternative for die casting.

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Zinc vs. Magnesium alloys

One of the main differences between magnesium and zinc alloys is that zinc and zinc-aluminum alloys require lower pressure and temperatures for casting. Because of a lower casting temperature, zinc provides a much longer die life than magnesium, which can help reduce production costs.

Unlike magnesium, which requires special treatments and coatings for corrosion resistance and finishing, zinc alloys also offer excellent corrosion resistance and a better surface finish when they are ejected from the die.

Zinc and Zinc-Aluminum Alloys

When it comes to casting components with tight tolerances and areas with thinner wall sections, no other alloys compare to the zinc and zinc-aluminum alloys. The element has a specific gravity of 7.0, making it one of the heaviest materials commonly used in die casting. Zinc is perfect for miniature die casting parts in high volume through a special hot-chamber injection process.

Like magnesium and aluminum, zinc is alloyed with other metals to provide better corrosion resistance, stability, dimensional strength and impact strength. Several of the available zinc alloys are referred to as ZAMAK, an acronym for zinc, aluminum, magnesium and copper.

Zinc 3 is the most commonly used zinc alloy, and it is sometimes exclusively used by die casters because of its tendency to be priced lower. In addition, the alloy provides a better surface finish during higher production rates, and it’s still capable of casting stable, intricate designs and complex components.

Another alternative to Zinc 3 is Zinc 5, which is used for its increased tensile strength, hardness and lower ductility. When it comes to producing a higher volume of components with thin walls, Zinc 7 can be used as it has a higher fluidity than other zinc alloys, which could increase production rates.

When looking for the highest strength and creep resistance, or deformation under mechanical stress, the Zinc-Aluminum alloy, ZA-8, is the best choice. With 8.4 percent aluminum and 1 percent copper content, ZA-8 provides lower densities with higher resistance to wear. Zinc alloys also provide better impact strength when compared to other commonly cast alloys.

Aluminum vs. Zinc Alloys

Just like with magnesium, one of the main differences between aluminum and zinc alloys is that zinc has a lower melting temperature and requires lower pressures for casting. Zinc is considered to be the most castable of all of the commonly used alloys.

By having a lower melting temperature, this enables a zinc die to last much longer than an aluminum die. Dies are expensive, and keeping them in service longer will provide a more cost-effective approach to large volume casting projects.

In addition, zinc’s lower melting temperature allows for hot chamber casting, which is less expensive than cold chamber processes. By using the hot chamber die casting method, production speed will also be increased. Zinc is one of the toughest alloys around, and it surpasses even aluminum in its ability withstand impact.

Another advantage to using a zinc alloy over an aluminum one is that zinc is great for castings with very intricate details and thin walls. When utilizing zinc, very little machining, trimming or finishing work is needed as the lower pressure and melting point reduce the thermal shock it experiences during casting. When casting in zinc, it maintains a smoother surface when a component is ejected from a die.

Determining Alloy and Casting Costs

Aluminum remains the most inexpensive alloy per cubic inch of all the commonly used alloys in die casting, but market value still fluctuates. Depending on the application and size of your design, costs may be lower based on the amount of material needed and the weight of the material.

Magnesium and aluminum are lightweight materials that offer excellent stability, but they have higher melting temperatures than zinc, which can contribute to higher casting costs. Hot chamber casting, which can be utilized with zinc and magnesium alloys, is less expensive than cold chamber processes, and it can run at a faster rate.

Die manufacturing is also an expensive process, and while the costs remain steady for the actual production of the die, determining the overall expected life a die is a good way to reduce expenses. For alloys like zinc, die life lasts much longer. Magnesium’s melting temperature is lower than aluminum and will allow for a longer die life as well.

When considering what alloy best suits your design needs, machining and finishing operations should also be taken into account. Zinc and zinc-aluminum alloys offer superior results over aluminum and magnesium when it comes to finishing, and they can drastically reduce the workload needed to produce a final product.

The most important thing to consider when selecting an alloy is the unique mechanical and physical properties they provide and how those will impact the performance and longevity of your design’s application.

For projects involving exposure to higher temperatures, aluminum offers better strength than other alloys. For designs that require a high yield strength and modulus of elasticity, zinc alloys are the best choice. Magnesium also offers many benefits due to its lighter weight, and it can be substituted for aluminum for specific components.

To find out more about aluminum alloys and how they and other materials might work for your die casting project, contact Premier Die Casting online or by phone at 800.394.3006.

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