Casting of the Year: Front Subframe Engine Cradle by Magna
Automotive demand for structural aluminum parts continues to grow, and the size of the parts OEMs want is also increasing. A single casting provides benefits over a multi-part steel weldment, including weight reduction and improved dimensional stability. Magna’s Cosma Casting Michigan (CCMi) out of Battle Creek, Michigan, pushed its capabilities and design engineering expertise to produce a 1,159 x 1,070 x 199-mm front subframe on the largest commercially available diecasting equipment at the time—a 4,400-ton press. The 38.5-lb. (17.46 kg) diecasting resulted in a 29% weight savings over a 17-part stamped and welded steel assembly and helped the 2021 Acura TLX achieve “2021 Top Safety Pick+” status, which is the highest safety rating from the Insurance Institute for Highway Safety (IIHS).
For its innovation, improved strength and safety, and weight reduction on a large-scale structural aluminum casting, the subframe developed by CCMi and Acura was named the 2021 Casting of the Year. The benefits achieved with this part will have a lasting effect on the casting industry. As one judge noted: “More conversions and other sizes will follow, and existing methods of other products will be reconsidered for this.”
Location, Location, Location
Today, the 6,200-ton Giga Press has made headlines for being the largest diecasting machine in the world. But back in 2016, Idra Group’s 4,400 ton press had that distinction, and it was the largest machine available when CCMi and Acura sought to design an aluminum cast front subframe for the 2021 Acura TLX. Producing subframes is nothing new for Magna’s body and chassis business unit, Cosma, which has been making the product for years—first in steel and then expanded to aluminum castings in 2012. But the dimensions of this single aluminum casting would test the limits of the existing machinery and inspire innovative gating techniques and thermal controls to produce defect-free castings.
“Usually we cast a smaller K-frame architecture; it’s a smaller casting so it is easier to do,” said Randy Beals, global engineering specialist for Cosma Engineering. “This is the first one to encompass the perimeter architecture and that is where it became a challenge with how big it actually got. The size is quite a chore.”
Conventionally, Magna places the gating at the outer edges of the subframes, but the new Acura subframe, which is 9,918 sq. cm, would require a machine with much greater tonnage capacity. Roughly 1.5 to 2 times larger than a typical K frame, the 2021 Acura TLX perimeter-type subframe supports the engine, transmission, front suspension and steering system. At this size, Magna’s 4,400 ton diecasting machine would not be able to handle the part with the gating system located on the outer borders. Instead, Magna engineers designed a central gating system, with shorter distances to feed the metal, to meet the OEM’s requirements.
“With thin-walled castings, you are always interested in minimizing flow length because you lose temperature of the metal as it fills the cavity,” said Chuck Barnes, senior engineer for Cosma Casting Technology Group. “You want to keep that metal as warm as possible while you are filling. Filling from the center rather than from the edges minimizes that flow length.”
Diecasting a subframe at this size helped Acura achieve critical weight savings for the TLX. The super-integrated one-piece casting significantly reduced part count and trimmed 7.1 kg. Attachment points for the front lower control arm, steering gear, stabilizer bar and torque strut were all incorporated into the cast design. These locations are machined after casting to meet the dimensional requirements.
Process Controls for Casting Integrity
When Acura first began the project with CCMi to convert the steel fabrication to an aluminum casting, the automotive company provided a design tailored for the sand-casting process.
“We started with a typical sand cast design, and using analysis software, optimized the rib thicknesses and pattern to make it more feasible for diecasting, while meeting Acura’s performance and packaging targets,” said Amar Jadhav, product engineer for Cosma Engineering.
Initial solidification simulation work was performed by Cosma’s engineering team in Germany (Cosma Casting Technology) to kick the project off. Prototyping and trials were then performed in the U.S.
“After the initial simulation, we built the prototype tool here in North America. From our prototype runs we took the process information and input that into new simulations to refine our model,” Barnes said.
Following simulations, Cosma engineers added ribs where needed for strength. This allowed the stiffness to be increased—improving the ride performance and vibration characteristics to meet the targets for the 2021 Acura TLX. They also implemented process changes to thermally manage solidification—avoiding shrinkage and other defects.
Dimensional control was a main concern because the decision was made not to perform straightening or any other dimensional correction process before or after heat treating.
“In a larger casting, especially with this thickness in the gating system, you can expect to have some dimensional distortion coming from the diecasting, heat treating or quenching,” said Nandha Mariapillai, engineering manager at CCMi. “We had to perform several development studies at each stage of the process. And then we would attack the issue and optimize the process.”
A few key creative solutions resulted from the product development phase. Along with the central gating system, CCMi incorporated squeezer cores and jet cooling.
“When we have specific areas where we see shrinkage is a concern on simulation, we apply pressure by activating a core movement in the die with a hydraulically actuated cylinder,” Mariapillai said.
The pressure is applied when the casting is in the semi-solid condition. The squeeze pin engagement time is kept between 1 and 3.5 seconds. This moves the shrinkage from the critical location while maintaining an optimal cycle time for production efficiency. A jet cooling pin helps further reduce porosity. The pin has a small hole that reaches almost the tip of the core pin; it serves as a cooling channel to use a mix of air and water at high pressure directed at a specific location of concern.
“Lessons learned from these tooling upgrades to the Acura program were applied to other programs.” Mariapillai said.
Besides reducing weight, producing the part in the high-pressure vacuum diecasting process led to other benefits. Reduced part count meant fewer resources required for assembly. Fewer assembly joints resulted in a safer, stronger, and better performing subframe. According to Acura, the cast aluminum subframe reduces the vertical and fore/aft forces felt in the front end, and contributes to better weight distribution in the car, which improves handling and fuel efficiency.
The success of the 2021 Acura TLX front subframe highlights the opportunity for further market expansion for large structural aluminum die castings.