The winning ablation-cast space frame nodes represent the design advances achieved through close collaboration among R&D, material and design engineers, and casting technology experts.

Shannon Wetzel, Senior Editor

(Click here to see the story as it appears in the April issue of Modern Casting.)

When Honda decided to relaunch a new generation of its Acura NSX sportscar, which originally debuted 25 years ago, it put an R&D team in charge of its development and announced global production would be in the U.S. The carmaker wanted to push itself to use advanced materials and processes in building the supercar. After three years of design, the production version of the NSX was unveiled this January, featuring a space-frame body built with advanced materials and processes—some for the first time in production. Key to the space frame were three cast aluminum connecting joints produced by Honda via the groundbreaking ablation casting process.    

“Just like the original, which was the first all-aluminum production car, the new NSX will feature ‘world’s-first materials’ and innovative construction,” said Ted Klaus, Chief Engineer and Global Development Leader, NSX, at the North American International Auto Show in January. “A multi-material chassis comprised of an aluminum intensive space frame, with strategic use of ultra-high-strength steel, carbon fiber and all-aluminum suspension.”

The space-frame concept primarily utilizes extruded aluminum lengths and cast aluminum nodes serving as joints. In the early design stages, Honda engineers had small cast aluminum joints in the space frame but outside of the crash zones. They saw an opportunity for additional part consolidation and space frame performance if they could design the castings within the crash zone. To do this, the cast parts would have to match the properties of the surrounding extrusions (265-295 MPa UTS, 175-205 MPa YS, > 12% elongation). Consistency of properties within the crash zone is critical in establishing a repeatable crash absorption mode, according to Philip Vais, Principal Engineer-Materials Research for Honda R&D America.

Honda was able to achieve the necessary properties and additional design requirements with ablation casting by partnering with Alotech Limited, Cleveland, Ohio. Ablation casting is a new technology invented by Alotech that combines the complete flexibility of traditional sand casting techniques with rapid cooling of the molten alloy through the use of a water-soluble binder.

The lower front body, upper front body and rear body nodes, currently cast at the Honda of America Anna Engine Plant in Ohio, showcases Honda’s ability to take external technlogy and work collaboratively with Alotech to integrate ablation casting into existing manufacturing requirements (a real-world technology transfer). For this reason, Honda and Alotech have earned the 2015 AFS/Metal Casting Design & Purchasing Casting of the Year Award for the Acura NSX space frame nodes.  

The materials group at Honda R&D America was aware of the ablation casting process for several years. It first heard about the process by word of mouth and a conference paper and was working with Alotech on developing the ablation process for potential use on future cars.

“The properties of ablation cast parts would match the properties of the surrounding extrusion members of the space-frame structure, allowing them to be located in the crash zone,” Vais said.

As the NSX designers started their second round of design work, they successfully increased the nodes’ size to consolidate several surrounding stampings and extrusions from the first-round design.

To be located within the crash zone, the castings must meet several important requirements:

1. The mechanical properties of the node castings must match the properties of the surrounding extrusions.
2. To crush in the proper mode, certain areas of the node castings must match the wall thickness of the extrusions (3-4mm). At the same time, the nodes must incorporate thick section bosses to rigidly mount to the front suspension arms.
3. The nodes must be hollow in order to allow the extrusions to be slotted inside and welded and to optimize weight reduction.
4. The nodes must keep a low dimensional tolerance to reduce problems within space-frame construction.
5. The nodes must have minimal porosity in order to achieve good welding quality.
6. The rear node casting must withstand 210 kN without breakage.

“No other process could provide the combination of hollow shapes, small wall thickness, welding performance and material properties to meet the design requirements,” Vais said. “The NSX body designers were amazed at the amount of design freedom they could get from ablation casting as well as the mechanical properties. Ablation casting allows the shape freedom of sand casting while getting the material properties of a premium die casting.”

The high solidification rates of the ablation process enables the formation of a fine microstructure (specifically, the size of the eutectic silicon phase). In the crush-critical areas of the nodes, the castings achieve an American Foundry Society (AFS) modification level between 5 and 6, indicating the aluminum’s microstructure has been fully modified for highest ductility and fracture strength. The fineness of the silicon phase largely benefited the ductility of the casting.

By consolidating additional components with the redesigned nodes, Honda realized cost savings due to the reduction of tooling investment. Additionally, the application of a casting in the space frame eliminated the need for MIG (metal inert gas) weld length on each side of the vehicle, thereby minimizing thermal distortion of the space frame during construction.

“Through the ablation casting process, Honda was able to achieve the mechanical property requirements for application in the crash zone, produce weldable and hollow node components with both thick and thin wall sections, and maintain an acceptable dimensional tolerance,” Vais said. “This new technology was a key element to achieving the final performance targets of the new NSX supercar body structure.”

With the January launch of the production version of the NSX, the nodes are one of the first public real-world applications of ablation casting and a solid example of the cutting-edge advantages ablation gives designers to make lightweight structural parts.

“The original Acura NSX debuted in America 25 years ago. NSX stood for ‘New Sports Experimental.’ But when it proved to be no experiment, the name stuck. And NSX redefined what it meant to be a supercar,” said Mike Accavitti,  Senior Vice President and General Manager, Acura Division, at the North American International Auto Show in January. “Supercar performance, delivered in a unique and challenging way, powered by new technologies and utilizing new materials, that is the heritage of NSX.”