Iron Castings - Sep 9, 2019
3-ways vehicles are reducing weightMCDP Staff Report | Metal Casting Design
Lightweighting vehicles is not a new concept—the Corporate Average Fuel Economy (CAFE) standards were passed in 1975—but automotive manufacturers continue to find ways to trim the pounds. While many of the weight reductions have come from replacing steel with aluminum, more creative solutions have been found to meet strict strength and stiffness requirements while helping improve the overall fuel economy of the vehicle.
Here are three examples of vehicle manufacturers working with their casting suppliers to engineer lightweight solutions through changes in material, innovative design or a different manufacturing process.
Stiffening Ribs Help Magnesium Replace Steel
The structural portion of automotive closure panels such as side doors, liftgates, hoods and trunk lids have been typically produced by joining several sheet steel stampings together by spot welding, clinching, adhesive bonding, and hemming. Replacement of steel stampings is an ongoing trend in vehicle manufacture, and General Motors identified panels like the side door could be candidates for redesign.
These assemblies are rangy pieces, but the fluidity of magnesium to enable thin sections, coupled with the long die life of magnesium casting, enabled GM supplier Meridian Lightweight Technologies to combine the multiple components into a single casting. Engineers incorporated strengthening ribs to increase local stiffness so that the final single magnesium casting met all product stiffness requirements while reducing the mass by 50%.
By diecasting the door, it was possible to vary wall thicknesses to reduce the material where strength and stiffness was not required. Ultimately, the magnesium door panel weighed 19.4 lbs. less than the steel construction.
Plus, reducing the number of parts meant that 105 spot welds were replaced by 13 mechanically fastened joints.
In areas where high stiffness was required in order to maintain the door-to-body seal when driving at high speeds, engineers included a ribbed open-section header to meet the requirements.
In the end, producing the door in magnesium via cold chamber high pressure diecasting reduced the number of parts from five to one and cut weight by 48%.
Ductile Iron Cuts Weight Over Aluminum
Reducing weight is not limited to changing the material to something lighter. Auto manufacturers and suppliers have been studying ways to cut the pounds by designing stronger parts using less material.
Such was the case for a lower control arm for Ford Motor Co.’s CD4 engine platform, which is incorporated in vehicles such as the Ford Fusion, Ford Taurus and the Lincoln Nautilus. In this instance, an aluminum part was converted to a ductile iron casting in order to achieve weight savings.
Engineers from Ford worked closely with suppliers AFS Corporate Member Waupaca Foundry and Hitachi Metals to produce the high-strength, lightweight component. Using design optimization software, an initial design concept was conceived, and it was then further refined for production feasibility. The design engineering team carefully detailed the mating part interfaces of the arm to enhance durability and ensure proper assembly function and shaped the surface profiles so the component fit into the packaging space.
Through collaboration among Ford, Hitachi Metals and Waupaca Foundry, the final design featured a hollow shape and weighed 25% less than it did at first concept. The thin-wall ductile iron component weighs 19.2 lbs. compared the 25.4-lb. aluminum version.
Similar Material, Different Process Trims Mass
An innovative manufacturing method helped General Motors further reduce the mass of a component it had already converted from a steel assembly to an aluminum assembly.
The five-piece aluminum stamped and welded side door mirror reinforcement bracket was a good start to cut weight, but GM worked with Cosma Casting Michigan (Battle Creek, Michigan) to engineer a casting solution that could further lighten the component. The bracket provides the attachment points and load carry for the large side door mirror and is crucial to providing stiffness to the side door window frame.
Cosma Casting opted to use high pressure vacuum diecasting, which creates a vacuum inside the die tooling for every machine cycle to create high integrity, structural castings. The die tooling was designed to produce four brackets per cycle for maximum overall equipment effectiveness (OEE). Cosma also modified its tooling to use water free lube for a faster cycle time over the conventional water spray die lube system. A casting is produced every 7 seconds with this system.
The new design represents an additional 20% mass savings of the aluminum stamped solution and 37% mass savings over the steel assembly.
Click here to see this story as it appears in the July/August 2019 issue of MCDP.