If you opt to use aluminum for your next mold project, it’s important to select the appropriate aluminum alloy. Alloys are created when elements including copper, magnesium and zinc are added to the base aluminum. Each added element contributes individual beneficial properties that will improve the aluminum.

There are two main classifications for aluminum alloys: wrought and cast. Wrought alloys are initially cast as billets and then are mechanically worked through hot or cold methods such as rolling, forging, extrusion and forming to achieve the desired forms. Rolling is used to produce aluminum sheets, foil or plates; forging is used to produce complex shapes with superior properties, and extrusion is used to produce tubes or rods. Cast alloys are directly cast into the desired form, making them well-suited for applications involving complex shapes.

Wrought and cast alloys are further categorized as heat-treatable and non-heat-treatable. Heat-treatable alloys contain alloying elements that will increase the material’s strength and solubility through heat treatment, but the heat-affected zone (HAZ) typically will not be fully annealed, which can impact the strength of any weld. Non-heat-treatable alloys are strengthened through cold-working methods.

These classifications are important considerations in regards to the weldability and repair of the aluminum alloy mold. Certain materials are better-suited to particular welding processes, so the welding method must also be taken into account as the moldmaker decides on the optimal material.

Some of the most commonly used production aluminum alloys come from the 7000-series family. These alloys are attractive to molders because they generally possess high strength that is comparable to steel, and they are more durable than other series. Zinc is the primary alloying element in these aluminum alloys, and this, in combination with other elements such as magnesium and copper, contributes to making them among the strongest alloys at ambient temperatures. The addition of zinc also makes the aluminum heat-treatable, which allows for precipitation hardening, a heat technique used to increase yield strength. Through precipitation hardening, 7000 series alloys can reach tensile strength as high as 700 MPa (megapascals), the highest among all aluminum alloys. Heat treating is most effective after the alloying elements have been dispersed through rolling and forging. The combination of all three processes will build up the properties required of high-strength aluminum. Although 7000-series alloys offer good fatigue strength and machinability, they are less corrosion-resistant than other aluminum alloys, making them susceptible to stress-corrosion cracking and, in turn, difficult to weld.

Other popular choices for production mold applications are alloys from the 2000, 5000 and 6000 series. Those in the 2000 series, like the 7000 series, can be precipitation-hardened to strengths similar to steel through the addition of copper, but alloys in the 2000 series offer lower corrosion resistance, making them even more vulnerable to stress-corrosion cracking than those in the 7000 series. Many 2000-series alloys are considered non-weldable because of this vulnerability.

Alloys in the 5000 series feature the addition of magnesium, which provides for solid solution strengthening and improved strain-hardening, giving them the highest non-heat-treatable strength among aluminum alloys. Because of these qualities, however, 5000-series alloys are among the most difficult and expensive to extrude. They are mainly fabricated as sheets and plates, and only occasionally used as a molding option.

Aluminum in the 6000 series are alloyed with magnesium and silicon, creating heat-treatable, strong and easily extruded alloys with good corrosion resistance. While 6000-series alloys are among the most commonly used for general-purpose applications, they cannot reach the high strengths of those in the 2000 and 7000 series.

These other alloy families are typically produced by the same mills making billets for the 7000-series alloy families, however they are not rolled, forged and heat-treated for strength. Instead, these alloys are directly cast into their final form, allowing them to derive their properties, including strength and weldability, from their alloy composition rather than through the manufacturing process. Because cast alloys do not need to first be rolled or forged into plates or sheets, they can be more economically used for complex product shapes. They will not, however, possess the same level of strength as their rolled or forged counterparts. Cast alloys are a popular choice for prototype production because they cost about half of what a 7000-series alloy costs, and for a prototype, a weaker alloy is acceptable. A modified 2000-series alloy also is available in rolled or forged form, offering a good balance between strength and welding characteristics. Both forms of alloys are readily weldable as per their individual properties.

The desirable qualities inherent in each series of alloys will determine the production applications for which they are best suited. Because of their high strength, 7000-series alloys are most commonly used for high-performance applications such as aerospace, armored vehicles and sporting equipment. The comparably strong 2000-series alloys are most often used for aircraft or aerospace applications, while 5000-series alloys are more weldable and provide non-heat-treatable strength, making them useful for a variety of structural applications such as bridges, buildings, truck or train bodies, shipbuilding, and pressure vessels. Alloys in the 6000 series are the easiest and most economical to extrude, making them suitable for welding as well as for a wide range of extruded shapes.