If you’re planning to extrude parts or components, it is vital to have a clear knowledge of aluminum alloys. A mistake designers can make in alloy specification stems from concerns over strength. While high-strength alloys may at first glance appear to be appropriate for a new design, there may be hidden drawbacks associated with these specialized alloys which disqualify them from being used for your design.
For example, 7075 is an aluminum alloy with zinc as the primary alloying element that has strength comparable to many steels. It also has good fatigue strength and average machinability. But, the alloy is considerably more difficult to extrude intricate shapes, is not optimum for welding (versus other aluminum alloys), and considerably more expensive than other alloys. For an aircraft wing spar, 7075 may be an excellent choice, but for a truck frame it will likely be cost prohibitive.
It is also important to understand that higher material strength doesn’t necessarily boost part rigidity. A stronger aluminum alloy may help only if peak or cyclic loading conditions make it imperative to use a higher strength material. Fortunately, common extrusion techniques are enabling certain alloys to be further strengthened, making them usable for a greater number of applications.
Benefits of Heating, Cooling and Aging Treatments
The majority of aluminum extrusions are made from 6000 (Al-Mg-Si) series alloys, with 6061 being the most popular alloy in the series. The reason for its popularity is that the 6000 series alloys don’t work-harden quickly. This means the alloy can be more easily extruded and profiles more cost effectively manufactured.
While the alloys themselves do offer certain levels of strength, based on the secondary mineral or compound added to the aluminum, that alone should not be the basis for selecting one over another. Depending on the composition of your alloy, aluminum will be further strengthened and hardened using quenching (cooling), heat treatment, and/or cold working techniques. This is called the temper and appears as a hyphenated suffix to the basic alloy number. For example, 6060-T4 indicates the use of 6060 alloy and a T4 temper (extrusion is heated treated after being extruded and then naturally aged).
“Solution heat treatment” usually refers to a three-stage process in which the alloy is first heated to a desired temperature and held at that temperature — “soaking” it — for the necessary amount of time to dissolve alloying elements. The solution heat treating temperature must be selected and carefully controlled for each aluminum alloy, because the range between the solution temperature and the melting point may be quite narrow.
Next, the alloy is extruded then quenched (cooled) using air, water, or a combination. The goal of air and water quenching is to drop the temperature of the extrusion within a specific time frame to achieve specific hardness requirements and minimize distortions. In recent years, there have been significant advancements in quenching technology. New quenching machines can be programmed to apply any number of cooling variations, including water and air temperature, force, and delivery method, to an extrusion to produce specific strength, performance, and surface requirements.
Immediately after quenching, when alloying elements are still somewhat malleable, straightening/stretching operations are often performed.
Additionally, for some alloys a natural aging process, whereby an extrusion sits at room temperature for a certain period of time, will continue to bring out strength properties. Many other alloys, however, harden slowly at room temperature, continuing to strengthen appreciably for years. Those alloys are artificially aged by holding them for a limited time at a moderately raised temperature. Even alloys which naturally age rapidly may be artificially aged using an aging oven to quicken the aging process and bring out other alloy properties.
The benefits of quenching, aging, and heat treatment techniques for certain alloys gives designers cost effective options when designing for aluminum extrusions, as well as opens up new opportunities to replace prior components made from steel because they needed to meet certain strength requirements. In addition to being more cost effective to manufacture, aluminum’s lighter weight makes it easier to handle and less expensive to ship—reasons that have made aluminum an attractive material for use by many industries, including aerospace, construction, and automotive.
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Disclaimer: This content is provided solely for the purposes of providing information about the aluminum extrusion process and use of aluminum alloys. Vitex does not make any guarantee or promise about how an alloy may perform under the extrusion techniques noted.