Aluminum extrusions form lighter, stiffer engine cradle

Milwaukee--Automotive engineers have a rule of thumb: Strip a pound of weight from a vehicle, and you'll save a gallon of gas over its useful life. So how would automotive engineers react if they learned that they could strip substantial weight from a vehicle, stiffen it, and cut cost all at the same time?

An extruded aluminum engine cradle designed by engineers at A.O. Smith Corp. will deliver all these benefits. Incorporated in a Chrysler LH sedan on a test basis, the aluminum engine cradle weighs only 39 lbs. In contrast, a steel cradle used on the same vehicle weighs 58 lbs. What's more, the new engine cradle dramatically improves the vehicle's lateral stiffness, which translates into better handling characteristics.

The extruded aluminum cradle consists of 20 separate pieces, including one specially designed cross member. Nineteen of the pieces were made from 12 different part profiles, then assembled with tongue-and-groove connections to attain the final configuration.

Key to the design was the use of an aluminum extrusion process. A.O. Smith engineers considered several other options, including stampings and castings. Ultimately, however, several considerations caused them to settle on extrusions. Primary among them was that the fact that extrusions allowed them to vary the thicknesses of the parts, enabling engineers to place material exactly where they wanted it. "Using extrusions, it's very easy to create a structure like this one, because you can place material in the places where you need it the most," notes Peter Fritz, engineering manager of advanced structures and analysis for A.O. Smith Corp. "A steel product would have used the same gauge all the way around."

Extrusions also enabled them to cut costs. "Our tooling bill was about $75,000," Fritz explains. "With steel, we would have started at one million dollars."

A.O. Smith engineers configured the engine cradle using PTI Mechanica software from the Rasna Corp., San Jose, CA. They employed 13 different loading situations, also used previously in the design of a steel engine cradle for the same vehicle. The situations represented various loading scenarios, including crash test, wide-open throttle, and engine inertia, among others. By configuring the system in FEA software, instead of in a solid model, they were able to quickly reshape the members as "shell models."

In all, they performed more than 250 iterations before finally settling on the 19 pieces and 12 part profiles. Once they settled on the basic configuration, the engineering team members designed the parts using Pro/Engineer CAD software. Kaiser Aluminum, Sherman, TX, and London, Ontario, then extruded the 12 part profiles.

Attachments, such as suspension control arms, were joined to the structure using a friction-stir-welded connection. Stir welding, a relatively new process for automotive applications, eliminates the need to melt metal in the weld zone. Consequently, distortion is reduced and the heat-affected zone minimized.

A.O. Smith engineers say they were initially surprised by the strength of the aluminum parts. They employed a 6061-T6 aluminum with a yield strength of 43.8 ksi and an ultimate strength of 45.7 ksi. This material choice resulted in a bending capacity roughly equivalent to that of steel. In lateral stiffness, however, the aluminum engine cradle proved vastly superior to steel. Lateral stiffness measurements revealed that the aluminum cradle is about 80% stiffer than the steel version. Stiffness for the aluminum cradle is 100,000 lbs/inch, while the steel design came in at 55,000 lbs/inch.

Fritz says A.O. Smith launched the aluminum cradle project in 1994 after aluminum prices dropped dramatically, from about $0.90 per pound to $0.49 per pound, making it more competitive with steel.

By employing the aluminum engine cradle, A.O. Smith engineers say they improved the performance characteristics of the vehicle. It is stiffer and more nimble than versions with steel cradles, enabling test drivers to change lanes and perform emergency maneuvers more quickly, Fritz says. "It's obviously a safer vehicle with the aluminum cradle," Fritz contends. "The reason is that we could put material where we wanted it. So even though we were using a material with a lower modulus, the vehicle became significantly stiffer."

Additional details...Contact Peter Fritz, A.O. Smith Corporate Technology Center, 12100 West Park Place, Box 23990, Milwaukee, WI 53223, (414) 359-4270.

Other Applications

Applications for tongue-and groove extruded aluminum parts include: