You probably have heard about the aluminum truck, and maybe your first reaction had to do with crushing a beer can on your forehead. I can’t do anything about that instinct, but I can try to enlighten you about some of the different grades of
sheet aluminum and where you will find them.
Aluminum grades are designated by four numbers and are grouped into families that have the same major alloying additions and properties. The first digit in the grade number indicates the general family. When referring collectively to that specific family, the last three digits are shown as the letter X. For example, 5XXX indicates the 5000 series of aluminum alloys.
When numbers are the last three digits, a specific alloy is being described. The second digit indicates if the aluminum has alloy modifications (but not what they are), and the last two digits have no significance aside from distinguishing between alloys.
What this means is that only the first digit gives an indication of the composition and uses. The composition limits of specific rolled alloys are found in the Aluminum Association Teal Sheets.
The highest-purity aluminum grades are part of the 1XXX family, which are at least 99 percent aluminum. Since there isn’t much else in the structure, these are the softest, lowest-strength grades and have the highest electrical and thermal conductivity.
Cans
What about that beer can? State-of-the-art machines are capable of making over 3,000 beverage cans per minute, which contributes to a U.S. production of more than 100 billion cans per year. The chosen sheet metal needs to be soft enough to accommodate this manufacturing volume and speed, but strong enough to withstand filling, shipping, and stacking.
A beverage can body usually is made from AA3004. Alloys from the 3XXX series have about 1 percent manganese (Mn) added, but AA3004 also has 1 percent magnesium (Mg) for further strengthening that allows the finished can to maintain sufficient integrity with the thinnest possible wall. The incoming sheet aluminum starts at about 250 microns, or 0.01 inch. After forming with a draw and wall ironed (DWI) operation, beer can bodies have a wall thickness of 100 microns (0.004 in.) at their thinnest point.
The tops need to be stronger to open properly and consistently using the riveted pull tabs. To get the higher strength, it’s necessary to use a different alloy family—5XXX. AA5182 has 4.5 percent Mg and 0.3 percent Mn as the main alloying additions, which provide a balance between high strength and formability. The incoming aluminum stock is about the same thickness as the body metal (250 microns), but does not go through the same degree of work hardening as the can walls do during the DWI process. As such, the part strength and finished thickness of the lids are not significantly different from the incoming coil. To minimize the weight and increase the stiffness, the top of the can body is necked down so the lid does not have to be the same diameter as the majority of the body.
Incorporating these two alloys, today’s beverage cans weigh 13 grams, or less than half an ounce. Weight is important for several reasons. The most obvious one is that less metal needs to be purchased. Lighter cans also offer shipping and stacking efficiencies; significantly more can fit on a truck or on the shelf. Furthermore, the main alloying additions in the grades used for the body and the lid are Mn and Mg, which makes efficient and cost-effective recycling possible.
When a can is full of a carbonated beverage, its strength and stiffness are sufficient to support hundreds of pounds. Reportedly, four six-packs can support the weight of a 4,000-lb. vehicle. Without the internal pressure, the can buckles because of its low strength and light gauge.
Chart courtesy of
www.aluminum.org.
Cars
Automotive applications do not have the luxury of being pressurized, so panels need to be stamped from higher-strength alloys having a thickness typically greater than 1,000 microns (0.040 in.). AA5182 is used throughout the body structure, along with other grades from the 5XXX family. Although this grade provides the necessary strength, it cannot be used for skin panels because it is prone to stretcher-strain marks or Lüders lines, which print through a painted surface.
Products formed from the 6XXX series are not plagued by these visually unappealing features and have the added bonus of getting stronger when processed through a paint-curing cycle. This characteristic helps increase vehicle exterior denting resistance. Typical alloys from this family have additions of 0.75 percent Mg and 0.75 percent silicon (Si).
The Si addition is important to the properties of 6XXX alloys, but is an unwanted impurity in 5XXX alloys. This makes 5XXX with 6XXX grades incompatible for recycling together and leads to costly manufacturing scrap segregation. To get around this, some companies have chosen to use 6XXX series for both exposed and unexposed panels, even though they will probably pay a price premium compared with the 5XXX series.
High-volume vehicle manufacturers have the ability to negotiate restricted chemistry ranges to allow for easier recycling streams, which aids in their business case to switch to aluminum from steel in their vehicles’ body construction.
One of the things working in steel’s favor as a body material choice is that it has three times the elastic modulus of aluminum alloys. This means springback will be one-third that of aluminum for the same design at the same sheet metal thickness.
Steel stiffness also is greater by a factor of 3. To compensate for the lower stiffness, aluminum panels need to be thicker than a similar steel panel. This negates some of the weight advantage aluminum has over steel and further increases the panel cost.
When using a thinner
aluminum sheet, it is necessary to use a higher-strength grade. That’s where the 7XXX series is starting to make some inroads. The main alloying element in this series is zinc (Zn). This increased strength is the reason Apple is switching to 7XXX series aluminum from the 6XXX series used in the iPhone® models associated with “Bendgate.”
Historically, these grades were not used in automotive applications because they lacked sufficient ductility to be formed into complex shapes. Within the past year or two, some 7XXX alloys have been commercialized that have higher formability, and many automakers are evaluating their use in future models.
Nearly every military vehicle and airplane has contained aluminum from the 5XXX and 6XXX series for decades. These “military grades” are not significantly different from conventional grades used for many years in lower-volume, higher-end passenger cars. Recent regulations on fuel economy and emissions have positioned these grades for an increasing number of applications on a wider spectrum of mass-mark