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Aluminum Extrusion FAQ
This FAQ section has been compiled from a recent Q&A session during a webinar covering automotive applications for aluminum extrusions, however many of the questions are of a general nature that may be of interest to a wider range of readers. Click on a question to read the answer linked below.
"Infinite" is a high hurdle. Any material, steel or aluminum, can be bent to the point where the part will fracture.
6063 would be a good starting point
Depending on the profile and other structural requirements, other alloys are even better heat sinks (1000 series and 3000 series), although they may not offer the threaded retention or other requirements.
6005A was created in part to address fatigue resistance of 6005. The re-alloying gives very good fatigue resistance
The latest Aluminum Standards and Data do designate T6 for 6005A. Previous versions did not.
Other than alloys within those families designed for machinability, lead is present in trace amounts only.
I believe this to be a primarily rolled product.
6061 is not available in T3. It typically requires water quench and is listed as available in T4, T5, and T6, as well as F, as extruded.
Typical performance for elongation and ultimate yield is similar. The actual aging procedures utilized will dictate the actual mechanical properties vs. the standard.
Diameter is a function of press size. The chart in the presentation shows the availability of different diameters. Generally, the larger the diameter, the larger the minimum wall thickness - please go to "Find an Extruder" and locate possible extrusion suppliers for your needs. Work with them to understand their particular limitations. Smaller diameters can have fairly thin wall thicknesses (.040" for 2" tubular shapes, .045" for 3" tubular shapes, etc. again - specific extruders may be able to achieve better than this, or not quite this, depending on their equipment and process capabilities)
Yes - given you account for the moments of inertia for the frame. The ability to vary wall thickness helps make extrusion cost effective.
Yes, but not recip. components. For example, a unique "spiral" extrusion is used as an impeller in superchargers found on a number of Audi and Corvette models in lieu of a cast part. Read more about the Supercharger Impeller here.
Design features can play a significant role; however, bending process methods are extremely important to the end result.
Yes. Several are in production and more to come.
In both, objectives are similar - low weight with critical strength requirements. The alloys used may differ, with 6000 series more typical in autos and 7000 more likely for aircraft - due to differing ultimate performance requirements. Note also that 7000 is often more costly to produce due to low extrudability
Fatigue life can be another factor in design. Aluminum typically does not have an infinite life in cyclic loading applications. What alloys or tempers provide better performance when considering fatigue?
There is a broad range of alloys tailored to meet specific design criteria for better performance when considering fatigue.
Specific alloy and processes can be developed to improve fatigue life.
Based on design criteria, several alloys and processes offer excellent energy absorption.
Typically a surface treatment is used prior to paint - liquid or powder- to seal the surface and promote adhesion
5000 and 6000 alloys would be best because they anodize the best. Chemical resistance involves special sealing techniques as well as adequate anodic coating thickness. A 13.5 pH means you have a strong alkaline. Experience shows that the suppliers sealing additives only work on mild alkaline (not strong). For example, the cleaning liquids in an automatic car wash uses a pH around 9 to protect the car trim, because an anodic coating is normally resistant up to pH 8. There are sealing systems on basic Silicate chemicals which give a resistance >pH 13. But then the anodic coating is so alkaline resistant that it is hard to remove the anodic layer in hot alkaline solutions. The disadvantage is that the silicate sealed layers have minor acid resistance. You can read more about this silicate sealing systems in the book "The Surface Treatment & Finishing of Aluminum & Its Alloys" by P.G. Sheasby & R. Pinner. Refer to the chapter on sealing methods. (Answer provided by The Aluminum Anodizers Council's Technical Panel; www.anodizing.org)
You can read more about silicate sealing systems in the book "The Surface Treatment & Finishing of Aluminum & Its Alloys" by P.G. Sheasby & R. Pinner. Refer to the chapter on sealing methods. (Answer provided by The Aluminum Anodizers Council's Technical Panel; www.anodizing.org)
In general, custom extrusions will be more costly than profiles from an extruders "standards" catalog, due to tooling costs, production quantities, etc. However, extrusion tooling is relatively inexpensive, so the difference is often not that great; and when the ability to design-in specific desired features is considered, a custom part may be less than a machined standard.
Inspection is similar to that with other metal forming. CMM, video measuring, hand tools. Also a variety of metallurgical tests are employed - tensile, fracture/toughness, etc. Standards are defined by the Aluminum Standards and Data manual published by the Aluminum Association, though often extruders perform to higher levels.
Often the longitudinal (die) weld is NOT a limitation at all - extruded hollow parts are used successfully in severe structural applications regularly. Depending on the application, special attention may be required to ensure that the billet-to-billet weld (which also extends along the initial length of the extrusion) is minimized or removed. This can be done through die design and through proper process planning
Yes. Dependent on the requirements and specification
Many extruders have a "standards" shape list, but frankly, for many applications the low cost of tooling an extrusion die suggests that a part customized to the exact use, putting material exactly where needed, is a better choice. The exception to this may be for "one-off" applications.
There are over 125 extruders in North America, operating about 500 presses in nearly 200 locations. Of course, not all of them are active in the automotive market, though an increasing number are. You will find similar robust supply networks in Europe and Asia, with smaller - but still highly competent - supply bases in South America and MENA. A small number of extruders are global in scope.
At temps approaching 300 C the tensile strength may drop as much as 90%.
Engineers and designers are simply NOT often exposed to aluminum extrusion in their formal educations. I graduated as an engineer from Penn State in 1980, and since my family business was the largest manufacturer of ladders in the world (Werner Co.), I desperately wanted to obtain an education in aluminum metallurgy, extrusion and fabrication. I had to rely on my uncles to provide me with technical information that I could use to craft self-study courses with my technical professors, as there simply were NO courses offered! In addition, on a per-pound basis, aluminum is more expensive than steel; thus unless weight reduction has value (as in significantly improving fuel economy) aluminum has been limited to niche applications due to its economics.
In one of the slides, light weighting from Al extrusions was shown to contribute only 15% towards meeting the 54.4 mpg, while transmission (I think) contributed 50%. Are there any applications of Al extrusions for the 50% share of the pie?
The 15% estimate was developed by Ducker Worldwide based on OEM's platform development programs. The 50% share of mileage improvement is expected to come from improvements to engines, transmissions (and we are already seeing 8-speed automatics), aerodynamics, friction reduction, etc. Extrusions may have a role in some of these areas - as components of supercharger or turbocharger systems for example, but those applications are typically not weight driven. The 15% contribution is primarily weight driven. FYI, the remaining 35% improvement is projected to be related to alternative power trains - hybrids, electrics, etc.
This is proprietary to individual extruders.
Many can. The "Find an Extruder" function on aec.org provides information about specific extruder capabilities, including fabrication
I cannot comment on the pricing for sheet or cast products, but European extrusion pricing has typically has been somewhat higher. Historically this has reflected market structure and energy and labor costs. Recently this gap has decreased.
Aluminum alloys can have a galvanic reaction with some dissimilar metals. The corrosion occurs because of direct connection between the metals with an electrolyte (water, etc.) From a high level viewpoint, small amounts of aluminum will sacrificially corrode in this situation (a small aluminum fastener in a large mass of steel, for example). Small steel parts do not have this same problem. Often, other materials can be substituted (some stainless steels do not have the same level of galvanic reaction as plain steel, galvanized steel, where the galvanized coating is intact, offers protection, etc..) The aluminum alloy and other material can also be separated by a bituminous material, or, very commonly used, by adhesives.
Individual adhesive manufacturers will suggest a pre-treat. Usually it calls for an acid or caustic pre-treat to remove the natural aluminum oxide surface, similar to preparing for paint.
I don't understand the "welding back together" of hollow extruded parts. Is it caused by the heat generated as the billet is forced through the die essentially melting them back together, or is there some other catastrophic mechanism that compels the grains to re-bond?
It is often thought that aluminum melts as it passes through the die. This is not the case. Rather, the high pressures involved cause a "solid state" weld, similar in concept to friction stir welding.
Two answers: For direct extrusion (MOST COMMON), the skin of the billet encounters friction as it is pushed through the container. The oxides and contaminants are contained in the skin. The flow of the material is such that the skin collects at the back of the billet, at the face of the dummy block. The extrusion process is stopped before this "butt" extends into the extrudate, and the butt is scrapped. For indirect extrusion (fairly uncommon), the billet is often "scalped" and/or scrubbed clean before extruding; in indirect extrusion, the surface of the material does extrude into the extrudate.
Extrusions emerge from the press at high temperatures (often 932 d.F. or higher), and are cooled by still or forced air, water mist, water spray or immersion in an water tank depending on the profile geometry and the alloy. Subsequent cooling on the "cooling table" can lead to gradual "bows" along the extruded length. Stretching is done to virtually ALL extrusions, where the profiles are gripped at each end and stretched in a controlled fashion (usually in the 2-4% range). This has VERY little effect on cross sectional dimensions (although channels can see their gaps altered) - in any case, the extrusions are designed so that the tolerances meet the customer requirements AFTER stretching, since this is part of virtually every extrusion process.
As extrudability goes down, issues with tolerances, straightness, etc. go up. These can be addressed given the shape, application and needed quench.
Extruded 3003 (p is a sheet spec) does not age harden.
Here you'll want to consult with your extruder. A lot depends on the specific shape and alloy. Try to design the profile so it doesn't have to be bent the "hard way". Also the bend tooling will need to be designed to keep key features from moving
Excellent surface finishes are possible with proper tooling consideration.
Beyond standard tolerance table, see individual extruders for specific capability.
Not necessarily. Either pre or post heat treat will work. The decision may be driven more by requirements for bending
Most extruders can easily extrude semi hollows with a tongue ratio of 3 or below - greater tongue ratios can also be extruded but may require higher billet temperatures and subsequently slower extrusion velocities, using more hours/ton of extrusions and increasing costs or with more die breakage (also increasing the expected cost). Tongue ratio is defined as the area of the void divided by the "base" of the steel tongue that would restrict the flow of material^2. For example, for a simple narrow C-channel with a 1/2" gap 3" high, the tongue ratio at the worst point (the full tongue in this case (refer back to the presentation to see a simple way to make the tongue less severe) would be (0.5*3) in^2/ (.5^2) = 6.0
Yes, co-extrusion of previously extruded aluminum profiles with polymers or other materials are commonly used in automotive applications.
Past practice has been to use steel, in part because steel is widely taught (in materials and engineering classes), in part because steel is cheaper on a per-pound basis. The current pressure to reduce weight to hit CAFE standards is motivating designers to reduce grams, hence more aluminum.
Hydroforming is used on particularly challenging bends, complex bends, tight radii, where features need to be controlled tightly and the higher cost of hydroforming (vs. stretch bending or other processes) is merited.