"The bimetallic engine cradle was design-validated for durability and corrosion at the joint level. We also performed durability and crash tests at full vehicle load conditions. The design met all of GM's requirements for those specific test conditions," said John Catterall, GM's Global Lead Engineer for Chassis Structures.

Although GM has not designated the cradle for a production vehicle application, the multimetal structure would translate to a 25 to 30% weight reduction vs. a traditional all-steel equivalent made with twice as many individual parts.

"It turns out the bimetallic cradle can almost reach the low mass of an all-aluminum cradle but at a lower cost," Catterall said.

During the casting process for the bimetallic cradle, aluminum is injected into the die containing steel crossmembers. The process produces a mold over the tube ends.

Explained Swamy Kotagiri, Executive Vice President of Engineering at Cosma International, "The differential in coefficient of thermal expansion between steel and aluminum creates a shrink fit at the joint."

Even though the shrink fit over the steel tubular structure is, according to Kotagiri, sufficient to manage engine cradle loads, an anti-rotation feature—essentially a mechanical lock—ensures that no separation can occur between the steel and aluminum.

"Cosma's patented bimetallic joint technology acts as an insurance for joint integrity," said Kotagiri.

Another key factor to producing the two-metal cradle is using a casting process that does not produce turbulent flow during the die-fill. In a traditional die-casting process when liquid aluminum is injected in an atomized state into the mold cavity, the resulting turbulent flow elicits high porosity and low mechanical properties.

"Cosma developed a high-integrity casting process that reduced the shot temperature and speed, giving rise to laminar flow into the die cavity. Laminar flow means a significant reduction in porosity, which correlates to improved mechanical properties. This casting process can achieve T6 heat-treat conditions with only T5 heat treatment, which saves the cost of additional heat treatment," explained Kotagiri.

GM does not have a bimetallic engine cradle in any of its current production vehicles, but the multimetal concept will be "re-evaluated whenever we have a vehicle that needs a new engine cradle. It has to earn its way into our vehicles based on the merits of mass, performance, integration considerations, and cost effectiveness," said Catterall.

Cosma is also developing, building, and testing bimetallic structures in materials other than aluminum and steel.

A complementary technology to bimetallic structures is next-generation hydroforming for the formation of tubular parts. According to Kotagiri, "Our goal was to enhance the capabilities of hydroforming to form high-strength materials and significantly reduce post operations like trimming and piercing if due care is taken during the design of components. Cosma has several patents on this new process that generates a sustainable pressure wave over long distances to form the part."

Cosma has produced a concept demonstration of the tubular components of a body side reinforcement using the advanced hydroforming process. The components, according to Cosma officials, would meet current and future government safety standards for roof and side impacts.

"The advanced hydroforming process is production ready, so Cosma is actively pursuing the right applications and vehicle platform for the technology," said Eric Wilds, Cosma's Executive Vice President of Sales and Marketing.