Long before President Obama announced the goal of achieving 54.5mpg by 2025, fuel economy has been a major concern of automakers. Reducing vehicle weight is one way to improve fuel efficiency. Automotive engineers routinely struggle to remove a few pounds of weight from a design. The use of lightweight materials is key to these efforts. Recent articles in Design News have covered such topics as aluminum metal matrix composite brake rotors, hybrid aluminum wheels with a polyurethane foam filling and a plastic outer shell, and carbon fiber composite structural components, among others.
Most of these developments have the disadvantage of adding significant cost. It’s rare to find a way to reduce weight that is cost-neutral, and rarer still to find one that actually reduces cost.
In a paper published in the January issue of the SAE International Journal of Materials and Manufacturing, and presented last year at the SAE Small Engine Technology Conference in Madison, Wis., Gouki Yotsuya and Ryo Yamauchi describe Suzuki’s successful efforts to do just that.
Suzuki developed an extruded aluminum lower control arm that weighs 50 percent less than a welded steel design, while costing roughly the same. It weighs 30 percent less than an aluminum die-cast control arm, and costs 22 percent less. The new extruded control arm is currently being used on the Suzuki Kizashi midsized sedan.
Yotsuya and Yamauchi’s paper is interesting not only because it describes a relatively new use for aluminum extrusions, but also because it provides a window into Suzuki's design process.
Suzuki already had experience using aluminum extrusions for motorcycle swing arms. For swing arms, extruded sections are typically welded to a cast cross-member. For the lower control arm, Suzuki decided to go with a single extrusion, eliminating the need for welding or multiple pieces.
In order to do this, a heat-treated extruded profile was cut to the desired length, then bent into the desired shape using a press. Lengthwise slots were cut on both ends of the extrusion (the bushing end and on the steering knuckle end), and on the top (for the coil spring). The material on the bushing end was then pressed (“shrunk”) to the desired width. Finally, holes were drilled for the steering knuckle and the bushing.
This process made it possible to form the control arm from a single extrusion, but the open section on the bushing end had a low torsional rigidity. Pressing in the bushing increased the stiffness. However, cyclic corrosion tests showed galvanic corrosion between the aluminum extrusion and the steel outer shell of the bushing. To avoid this problem, Suzuki engineers decided to make the outer shell of the bushing out of aluminum.
This eliminated the galvanic couple, but pressing aluminum into aluminum led to a new problem: galling. Reducing the interference fit to prevent galling did not leave enough press force to retain the bushing. The Suzuki engineers attempted to improve the retention by expanding the ends of the outer shell after installation, pressing on them to clinch the bushing in place. Unfortunately, the shell buckled before the ends expanded enough to provide an adequate clinch.
In order to solve this problem, the Suzuki engineers developed a special punch with four raised, curved features. Instead of pressing on the entire outside diameter of the bushing, the new punch pressed on the outer shell of the bushing into a four-leaf clover shape. This clinched the bushing into place without buckling the outer shell.
Normal manufacturing variation in the properties of the extruded aluminum was another challenge. If the material was too hard, it would crack in the forming process. If the material was too soft, it would break in endurance testing. Suzuki developed a special chemistry and heat treatment for the aluminum to avoid these extremes.
The result of all of these efforts was a savings in both cost and weight. Suzuki’s success in achieving both goals simultaneously is noteworthy. The behind-the-scenes look into the development process, as seen in Yotsuya and Yamauchi’s paper, provides valuable lessons. Their example may encourage other engineers to explore the potential of aluminum extrusions.