ALUMINIUM ALLOYS

The primary administrative body that regulates typical compositions of aluminum alloys is ANSI (American National Standards Institute), analogous to AISI (American Iron and Steel Institute) which functions similarly for steel. In this way, most aluminum alloys are named "ANSI XXXX" where each "X" is a numerical variable.

The first "X" in each 4-digit series indicates the family to which they belong. The second "X", if not 0, signifies a variation of the specific standard grade within the series. The last two variables "XX" are purely arbitrary and merely designate the alloy's numerical order, without value regarding its composition.

All of this has been done to facilitate product access. Standardization norms are important as they designate the most suitable grades for one application or another. From this, it can be said that thanks to ANSI, the "standards" for each specific use have been established.

ANSI does not manage all aluminum alloys, by any means. Nor does AISI for steel. There are other organizations, such as SAE, DIN, etcetera, but personally, I have always preferred both because I consider them more accurate, they have more complete catalogs, and in some ways show greater commitment by providing more data and specifications. Furthermore, they are easily accessible to the public.

The classification of aluminum alloys according to the ANSI system is organized into eight main series, identified by the "thousands" order in a four-digit sequence, where the first digit indicates the compositional family. This structure allows for rapid identification of the general properties of each alloy, facilitating its selection according to the technical requirements of each application. 

The 1000 series corresponds to commercially pure aluminum, characterized by high electrical conductivity and excellent corrosion resistance. These properties make it ideal for applications in the electrical industry and in environments where exposure to corrosive agents is constant. However, its limited mechanical properties—low hardness and poor structural strength—restrict its use in components subjected to stress. Additionally, these alloys are not heat-treatable, which further limits their versatility. 

The 2000 series, whose main alloying element is copper, resembles the original duralumin and stands out for its high stiffness and hardness. Most of these alloys are heat-treatable, allowing their performance to be improved through hot hardening processes. Nevertheless, the presence of copper significantly reduces corrosion resistance, necessitating surface treatments or protective coatings in aggressive environments. Its use is widespread in the aeronautical and aerospace industries, where the weight-to-strength ratio is critical. 

The 3000 series incorporates manganese as the primary alloying element, which imparts moderate stiffness and hardness to these alloys, sufficient for light structural applications. They are used in the manufacturing of door and window frames, cookware, buckets, and, in the industrial sector, in heat exchangers. Like the 1000 series, these alloys are not heat-treatable, which limits their capacity for mechanical improvement compared to other families. 

The 4000 series is based on silicon as an alloying element, which reduces aluminum's melting point, increases its hardness, and decreases its coefficient of thermal expansion. Although they exhibit good corrosion resistance, their mechanical properties are poor, restricting their use to specific applications such as decorative castings or models where aluminum acts as a "white metal," analogous to tin, indium, or lead. 

The 5000 series, with magnesium as the primary alloying element, offers an excellent combination of low density and good mechanical properties. These alloys are especially valued in the nautical industry for their high tolerance to corrosion in saline environments. They are also used in the manufacturing of ultralight components for luxury automobiles, high-end bicycles, and aerospace structures. However, when magnesium content exceeds 3%, some alloys can weaken at high temperatures, limiting their use in aeronautics. These alloys are not heat-treatable. 

The 6000 series combines silicon and magnesium, achieving a synergy that allows for heat treatment and improves heat tolerance. They exhibit good structural properties and corrosion resistance, making them suitable for architectural, automotive, and general mechanical applications. 

The 7000 series, with zinc as the primary alloying element, is the strongest of all. These alloys can be thermally hardened and achieve levels of toughness that rival certain steel grades. Notable examples like ANSI 7034 and ANSI 7068 illustrate the potential of this family in high mechanical demand applications. Zinc, in proportions close to 7.5%, is the element that contributes most to the hardening of aluminum, allowing its use in structural components of moderate responsibility. 

The 8000 series, the last in the classification, incorporates elemental iron in proportions rarely exceeding 10%. These alloys are distinguished by their resistance to high temperatures, making them useful in thermally demanding environments. 

The "strong" aluminum alloys primarily belong to the 2000 and 7000 series, where copper and zinc, respectively, act as primary alloying elements. Their capacity for heat treatment allows for the optimization of their mechanical properties, albeit at the cost of lower corrosion resistance, especially in the case of the 2000 series. On the other hand, combinations with magnesium, manganese, and silicon offer a balance between mechanical strength and thermal tolerance, making them suitable for manufacturing engines and components subjected to prolonged thermal stress.

For a long time, it was assumed that engines had to be made of steel due to the extreme mechanical demands they face. However, aluminum engines have been in use for over half a century, and their lighter weight has allowed for the design of blocks with larger displacement and volume, resulting in superior performance. This evolution has led to large engines in vehicles like American muscle cars—for example, the Dodge Challenger—which combine immense power with a surprisingly lightweight structure, an aesthetic that I personally find fascinating for its excessive character.

It is worth noting that most aluminum alloys do not contain a single alloying element but combine several elements to achieve specific properties. Occasionally, less common components are incorporated, such as lithium, tin, titanium—though its processing is costly—, lanthanides like erbium, and thermal stabilizers like yttrium oxide (Y₂O₃), also known as yttria, which improves structural stability at high temperatures.

Although now considered an obsolete alloy in terms of mechanical performance, duralumin represents a historical milestone in the evolution of structural materials. Its standard composition is based on aluminum with approximately 4% copper by mass, accompanied by small quantities of magnesium. This formula was developed by the German engineer Alfred Wilm in 1903, though it would not be until 1909 that the alloy would enter large-scale production. At the time, duralumin represented a true technological revolution, offering a much more resistant alternative than pure aluminum, a metal too soft for structural applications. The emergence of this alloy marked the beginning of a new era in material engineering, paving the way for families of alloys that would transform aluminum into an essential component of modern life.

Despite the existence of alloys with superior properties today, the term "duralumin" is still used colloquially to refer to any strong aluminum alloy, even when it does not contain copper. This linguistic persistence reflects the profound impact it left on the history of metallurgy.

The impact of duralumin was especially notable in the aeronautical field. Its low density — steel weighs more than three times as much as aluminum — combined with a surprising capacity to withstand high loads, made it the ideal material to replace wood in structures requiring lightness without sacrificing strength. In an era dominated by iron and wood, duralumin offered a malleable, ductile, and easy-to-forge alternative, without the need for protective coatings to prevent corrosion. Its use in early airships, such as the Zeppelin, was decisive: it acted as the metallic skeleton that supported the gas envelope, allowing for large structures with a remarkable reduction in weight.

Following the tragic accident of the Hindenburg airship — named in honor of the Prussian military leader Paul von Hindenburg — the use of duralumin in aeronautics saw a recession, but it strongly re-emerged in the manufacturing of the first metal aircraft. Until then, airplanes were built with fine woods, a practice that is difficult to conceive today. Duralumin met humanity's need to conquer the sky, and although current processes are more sophisticated, with precise thermal controls and advanced alloying techniques, the spirit of Wilm's original formula remains relevant.

The general characteristics of duralumin and its variants define it as a light alloy — with an approximate density of 2.5 g/cm³ — strong, tough, malleable, and ductile. It can be forged, machined, extruded, and molded with ease. Its behavior at high temperatures is notable, as it retains a good portion of its mechanical strength. Furthermore, it is completely recyclable, non-toxic, non-polluting, and abundant in the Earth's crust. Its manufacturing does not present great technical complications, which contributed to its popularization in the 20th century. 

Regarding corrosion resistance, duralumin exhibits acceptable behavior, although inferior to that of pure aluminum. The addition of copper — as occurs with most hardening elements, with exceptions like scandium or zirconium — alters the electrode potential of the alloy, reducing its resistance to corrosive agents. Even so, duralumin resists well in air and fresh water environments and can be anodized to improve its surface protection. In seawater, its resistance is moderate, so its direct use without treatment is not recommended. It does not form sulfates at room temperature, but it reacts with acids and bases, increasing its reactivity as the temperature rises. 

The current applications of original duralumin are mainly concentrated in the aeronautical industry, in components not exposed to extreme temperatures. For decades, aircraft panels — regardless of their size — were manufactured with this alloy. It was also used in bushings, pistons, cylinders, and other mechanical parts. Although today the alloying agents have changed in type and proportion, aluminum remains the basis of these compositions, and the legacy of duralumin remains a reference. 

It is fundamental to clarify that not all aluminum alloys are duralumin. This term refers exclusively to alloys of the 2000 series, where copper is the main alloying agent. Compositions with zinc, magnesium, or other elements do not belong to this family. The confusion is due to the fact that, for a large part of the 20th century — from 1909 until after World War II — duralumin was practically the only relevant aluminum alloy. Today, the landscape is much more complex, with an extensive database and technical classification. Just as with steels and stainless steels regulated by the AISI, aluminum alloys are organized into ANSI grades under the supervision of The Aluminum Association, a non-profit entity based in Virginia, United States.