Spaceframe Propels Ford GT: permanent mold casting produced the key structural components for the aluminum spaceframe in Ford's new GT model.

The recently unveiled Ford GT super car is one of the fastest vehicles on the road today. With 500 hp and 500ft./lb. of torque available from a supercharged 5.4L V8 engine, the 3,500-lb. (1,588-kg) car rockets to 60 mph in 3.3 sec. and to a quarter mile in 11.8 sec. at 120 mph. It can exceed speeds of 200 mph.

In order to achieve the high performance goals of the GT while maintaining the styling and powertrain requirements established by the GT40 concept car, Ford Motor Co., Dearborn, Mich., chose an aluminum spaceframe as the basic structure of the vehicle. Although not the first aluminum spaceframe developed by Ford, the complexity and size of the vehicle required unique design and manufacturing techniques to achieve the low weight, high stiffness and package requirements.

The resulting design was a hybrid construction of castings, extrusions and stamped, flat and roll-bonded panels (Fig. 1). Castings were used at the suspension attachment locations as well as at the interface of the rear and central tunnel. Numerous extrusions with varying cross-sectional properties connected the castings and completed the structure. Panels were used for the floors, to close off the front and rear bulkheads, and for the central tunnel. All structural connections between the extrusions and the castings were fusion-welded with the exception of the front and rear crush structures, which were bolted to the castings for ease of repair.

[FIGURE 1 OMITTED]

"The use of extrusions and castings gave a clean and open design, allowing access to all trim and hardware and facilitating easy component change and part modifications during the pre-production build," said Adrian Elliott, advanced body construction group leader in the manufacturing and process department at Ford Research and Advanced Engineering.

As a sports car with an emphasis on performance, the frame needed to have considerable strength and stiffness, and the castings (which provide the majority of the interfaces and function) represented the key to the structure.

First Stages

During the initial prototype phase, the design for the spaceframe was created for specific areas of each function, with a vertical base wall that varied in thickness and location for each domain and large, heavy bosses to serve as the sites for suspension attachment.

The front and rear suspension attachment castings were produced via the green sand casting process in the initial prototype stage. But the size and shape of the castings, with the resultant variation in the cross-section, created flow problems in the mold, which resulted in shrinkage, porosity and dimensional instability and caused extended machining times. The robust nature of the design also delivered part weight that was beyond the frame target.

Change in Concept

After the first prototype design, the drive to reduce weight and improve casting capability called for a modified concept. The team of Ford and Ramsden Industries Ltd., London, Ontario, Canada, developed a tee cross section arranged in a truss-like system for the production castings (Figs. 2-3).

[FIGURES 2-3 OMITTED]

The typical section, with a vertical wall in the center, had several benefits to the casting process:

* improved material flow;

* reduced casting weight;

* improved ejection of the casting from the mold because of reduced rib depths;

* decreased tooling costs.

"Cross sections are tightly tailored to the applications, minimizing material waste and weight," Elliot said. "The complexity achieved in each individual casting allowed optimum joint and subassembly performance. All these characteristics were key to producing a low-volume vehicle that, although it competes in an expensive price class, must meet stringent cost and investment targets."

Load transfer for the spaceframe also was improved with the cross-section design because specific load paths could be created from the crush rails to the structure at both the front and rear ends.

Additionally, ribs were positioned to reinforce local suspension stiffness based on finite element analysis direction. By putting the rib in the center, it created a truss-like system to handle load inputs. The amount, size and location of the truss beams could be changed as needed by the design team.

The redesigned parts were cast via the permanent mold process, and the superior flow characteristics resulted in confirmation prototype parts of production intent with improved dimensional stability and reduced discontinuities. However, with parts at this level of complexity, completely removing all shrinkage and porosity defects would be difficult. The team believed that through the necessary casting process adjustments, quality parts still could be achieved.

Design for Casting

Using a design of experiments approach, process development was performed to optimize the manufacturing procedure for the front and rear suspension castings. Particular emphasis was placed on the production pouring and solidification process.

The resulting design of the front and rear spaceframe corner castings provided attachment for the upper and lower suspension control arms and shock absorbers, carried the resultant transmitted loads, located and secured the front and rear bumper assemblies, featured the durability to survive the force of impact through the crush rails and provided a place for attachment of body panels, such as the front fender inner panel and other miscellaneous hardware.

As an added benefit, the part met Ford's criteria for a functional design that was technically compatible with the overall theme of the super car. In addition, the single casting replaced what typically would be an assembly of 10-15 separate stampings, or in this case, numerous extrusions, which resulted in significant cost savings.

The application of permanent mold casting for these chassis components is unique to the automotive world. The process was chosen for its mechanical properties that met the stringent requirements of high strength and stiffness for the frame (minimum values for ultimate tensile strength of 180 MPa and elongation of 5%), dimensional accuracy, and suitability for the size of the castings and medium volume production.

Cast in A356 aluminum alloy, the components are extremely large, with the front suspension measuring 22 x 25 x 10 in. (560 x 635 x 254 mm) and weighing 35 lbs. (16 kg) and the rear suspension measuring 33 x 29 x 10 in. (850 x 736 x 254 mm) and weighing 40 lbs. (18 kg). They also have complex geometries and tight dimensional tolerances of +/- 1.5 mm for the castings and +/- 0.25 mm for the finished machined frame attachment hard points, control of which was compounded by the need for T-61 heat treatment.

Several key manufacturing limitations were defined through the workhorse build, which also helped guide the production design. By designing the wall in the center, the cutting of tools, especially the ribs, could be achieved without EDM burning. This not only decreased the time to create the tool but also helped with mold release. Additionally, the central wall, which improved the flow of material with an even distribution throughout the casting, also helped balance the solidification.

The end result of the optimized design was a component that incorporated many of the interfaces in one part while lowering the overall weight.

On the Road

The Ford GT program had a deadline to meet, and production intent vehicles had to be built and ready to drive for the Ford Motor Co. centennial celebration in early June 2004. The permanent mold casting of the spaceframe kicked off in late December 2003. Twelve weeks were allotted to design and build the molds and run sample production parts. National Pattern Inc., Saginaw, Mich., met its timeline for the tool build, and the 12-week deadline was met with the necessary castings.

Ford has contracted to build 4,500 GTs over the life of the program, concluding in December 2006, at a rate of nine per day. Design aspects of the aluminum extrusions and castings in the GT are likely to be used in future automotive projects.

"With the cost of steel rising and demands for more fuel efficient vehicles increasing, aluminum will continue to play an important strategic role in reducing vehicle weight," Elliot said. "Extrusions and castings will remain dominant in the low-volume arena due to low tooling investment. At higher volumes, selective use of extrusions and castings may be integrated into conventional unibody architectures to save weight and improve specific performance."

For More Information

"Selecting Casting Technology: Permanent Mold's Perspective," by J. Busch, MODERN CASTING, Feb. 2003, p. 43-45.

"Designing Cost-Effective Permanent Mold Tooling," by S. Robison, Engineered Casting Solutions, Spring 2001, p. 32-34.

Don Ramsden is the vice president of manufacturing at Ramsden Industries Ltd., London, Ontario, Canada.