November 7, 2002
Editor's Note: This is the first installment of a two-part article. Part 1 covers the properties, characteristics, and applications of aluminum as a structural metal. Part II explores the use of structural aluminum in the design of the U.S. Botanic Garden's conservatory in Washington, D.C.
In appearance, aluminum sometimes is difficult to distinguish from other rust-free structural metals such as stainless steel, magnesium, and even galvanized steel. However, its properties are very different. Even different aluminum alloys can have dissimilar characteristics. This article discusses aluminum's unique properties as a structural metal for construction applications.
Today more than 3 billion pounds of aluminum are used annually in the U.S. in construction applications alone in everything from cryogenic vessels and piping to bridge guardrails and curtain walls used to clad office buildings.
Aluminum, however, is more costly than carbon steel—four to five times more by weight and about one and a half times more by volume. For this reason, it is important to understand aluminum's structural properties and efficient aluminum structural design.
The early metallurgists found aluminum to be very light in its pure state—0.1 lb. /in.3, about one-third the weight of an equal volume of steel. It also was formable and corrosion-resistant, because its oxide was very hard and blocked further oxidation once it formed on the metal's surface. However, it was not as strong as steel, only about 10,000 pounds per square inch (PSI). Perhaps even more significant, aluminum's modulus of elasticity—a measure of its stiffness and buckling strength when compressed—was also about one-third that of steel.
In an attempt to counter some of these shortcomings in the decades following discovery of the Hall-Heroult process (see sidebar), metallurgists discovered that alloying aluminum with small quantities of other elements (usually less than 5 percent by weight) could produce aluminum alloys with much improved strengths.
Around 1910 it was discovered that adding small amounts of copper to aluminum produced alloys with strengths greater than steel. These alloys, called duralumin, were used by the Germans in the Zeppelin airship frames. Unfortunately, the name duralumin turned out to be a bit of an overstatement because these alloys proved to be the least corrosion-resistant of the aluminum alloys. By the 1920s aluminum-magnesium and aluminum-magnesium silicide alloys were developed; however, they provided slightly less strength than the aluminum-copper alloys but suffered much less loss of corrosion resistance compared to pure aluminum than duralumin.
In 1954 The Aluminum Association, an industry association of U.S. aluminum producers, adopted a naming system for the numerous aluminum alloys that had been developed. This designation system has been accepted by most aluminum-producing nations of the world. The system is shown in Figure 1, along with some of the key characteristics of alloys from each series.
Aluminum, like other metals, also can be strengthened by cold-working, although this comes at the cost of ductility. Around the time metallurgists began alloying aluminum, they also found that some aluminum alloys could be strengthened by various heat treatments.The combination of alloying, cold-working, and heat treatment produces the various aluminum alloys and tempers used today. The temper designation (also developed by The Aluminum Association) is used as a suffix after the alloy designation and reveals how the metal has been cold-worked or heat-treated.
Examples of common aluminum alloys and their tempers are shown in Figure 2. Properties of all the common aluminum products can be found in the Aluminum Association's "Aluminum Standards and Data."
As with other metals, aluminum is sold in many different product forms, including castings, forgings, wire, rod, and bar. For construction applications, however, more than 90 percent of the aluminum materials used are flat-rolled sheet and plate and extrusions. Flat-rolled material thinner than 0.25 inch (6.3 millimeters) is called sheet; thicker material is called plate.
While flat-rolled material may be familiar to most metal fabricators, extrusions may not. Aluminum can be heated and pushed through an opening, with the same outline as the cross-sectional shape of the product. The resulting form is called an extrusion.While some other metals can be extruded, aluminum can be extruded in the greatest variety of shapes and alloys of any metal. Even hollow aluminum shapes can be extruded.
Even though aluminum's light weight and corrosion resistance often are cited, extrudability is actually the most important reason aluminum is used in construction applications. Extrusions may eliminate the need for many cost-intensive fabrication operations, such as welding and machining, that are required to make prismatic members with complex cross sections in other materials.
There are limitations, of course. Extrusions usually are limited in size to those shapes that fit within a circle of the diameter of the press that pushes the metal. Presses available for construction applications are limited to 18 in. (450 mm) in diameter, and extrusions made on smaller presses usually are less expensive.
Extrusion shapes can be custom-designed, or a shape can be produced from an open die (one that's available for anyone's use). Extruders provide catalogs of open die shapes, which include channels, angles, I beams, rectangular and round tubes, T's, and Z's. Custom-designed shapes are limited to those shapes that are practical to extrude. This means that in addition to fitting within the extrusion press diameter, the shape must not contain abrupt transitions in geometry, such as very thin parts connected to thick parts. A rule of thumb is to limit the ratio of thickness of thick-to-thin parts to 2-to-1.
While aluminum has many advantages, it can't do everything for everybody. The metals most commonly selected for structural applications are carbon steel, stainless steel, and aluminum. Common alloys of these metals are compared in Figure 3.
The cost index shown in Figure 3 uses carbon steel as a baseline. Aluminum's cost is about 2.5 times that of steel, although aluminum is priced at about five times as much as steel of equal weight, because aluminum structural components usually weigh about half as much as steel, considering density and strength differences.
What's interesting is that stainless steel costs nearly five times the cost of steel and twice the cost of aluminum. Generally, stainless steel is used because most fabricators are more comfortable welding stainless steel than aluminum.
Welding aluminum is tricky for several reasons. First, the tenacious aluminum oxide layer that rapidly forms on aluminum products has a much higher melting point than the base metal and must be carefully removed before welding. Welders who are more familiar with steel or who are in a hurry sometimes overlook this and get poor results.
The antidote for such problems is to weld in accordance with an established welding code, such as the American Welding Society's (AWS, www.aws.org) D1.2 "Structural Welding Code—Aluminum."
Second, many aluminum products rely on cold-working or heat treatments for their strength. The heat that is created during welding reverses these effects, reducing the metal to its annealed (and weakest) condition. Only aluminum alloys that principally derive their strength from their alloying elements (such as the 5xxx series) can avoid this fate. Designers must account for the weakened metal at the weld, called the heat-affected zone (HAZ).
After oxygen and silicon, aluminum is the most plentiful element in the earth's crust and the most common metal. Even so, aluminum was discovered much later than most other metals, including iron and copper, because in its natural state it is so tightly bonded to other elements.
In 1807 the English chemist Sir Humphry Davy named it aluminum after alumine, the metal of clay whose existence was theorized by the Romans. While Davy successfully isolated other previously undiscovered elements such as potassium, it wasn't until 1825 that the Danish chemist Hans Oersted became the first to isolate aluminum.
There was great interest in military applications for aluminum in France, where Sainte-Claire Deville renamed it aluminium and worked on less expensive methods to produce it. Although the metal's new name was adopted in Europe, the Americans stuck with aluminum.
Aluminum was still a precious metal when an American, Charles Hall, and the French chemist Paul Heroult, in a page out of Ripley's Believe It or Not®, independently discovered an electrolytic process for producing aluminum in 1886. Both received patents for this method, which required large amounts of power but was much less costly than earlier chemical reduction techniques. Through this discovery and improvements in the production of aluminum oxide from aluminum-bearing ore, the modern aluminum industry was born in the 1890s.
Part II of this article, which will appear in the December issue of The FABRICATOR®, explores the use of structural aluminum in the design of the U.S. Botanic Garden's conservatory in Washington, D.C. Topics such as tension, compression, bending, shear, and torsion are addressed.
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