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Prototype Redesigns Aluminum Forging as Ductile Iron Casting

Jiten Shah

Converting a front steering knuckle from aluminum forging and optimizing the design for casting resulted in 40% cost savings and a 20% stronger component in production, meeting the overall lightweighting objectives of the automotive customer. The optimized design achieved 30% weight reduction to the previous generation of ductile iron steering knuckle and provided secure mounting to 10 different suspension and brake sub-components without any interference with adjacent components. The design was optimized for extreme driving events using extensive CAE (computer aided engineering) coupled with advanced casting process simulations. The redesigned knuckle, produced by PROTERIAL, prevented brake noise and provided desired handling and ride. The new knuckle was validated by testing the prototype’s strength, durability, and stiffness.

Feature placements allow for off-set, vertically-parted green sand molding.

•    Casting design really starts off with the key decision making about the parting plane and orientation with respect to gravity as a first critical step in the whole development process. Off-set parting (1) is ideal for components where key features are not coplanar.

•    The metalcasting manufacturing process uniquely allows complex features placements freedom, cost effectively unlike aluminum forging. Modern casting process simulation tools are very effective to validate the casting and rigging design and process parameters to ensure the desired quality, microstructure and properties are achieved before the tooling is made, thereby eliminating the traditional pour and pray process.  

Accurate material properties are crucial for true design optimization.

•    The key to achieving realistic design optimization using CAE tools is the availability and use of accurate material properties, such as strain life fatigue. AFS has developed extensive engineering properties for various grades of ductile iron sand casting alloys, and they are available through a web portal: www.AFSCADS.com. These properties have pedigree information such as the section thickness, type of molding/casting process, typical chemical composition, and microstructure, which are backed with links to the reference research report available in the virtual library of AFS.

The size (diameter, height with reference to the wall section thickness) and location of holed features and their desired positional tolerance influence the design engineer’s decision whether to make them as-cast or cast solid and drill later (2) as a part of the machining.  

•    If the cored holes are too small, the sand will become fused, which will make the subsequent machining difficult.

 •    Sometimes, the decision is also influenced by the directional solidification intended to achieve the desired soundness.

Employing corner radii (3) and fillets (4) for smooth transitions gives better aesthetics and lower stress concentration.

•    Generous radii is required to extending features such as bosses and ribs as opposed to sharp edges and fillets at all the junctions—L, T, X, and Y. The overall effect is much reduced stress concentration associated with sharp changes in the cross sections for longer fatigue life, better flow pattern of the liquid metal, and overall cleaner castings.

•    From the strength of materials point of view, green sand castings allow smoother transitions across varying cross-sectional areas and eliminate or reduce the stress concentration effects, which in a lot of cases contribute to the premature failure of the structural components in service due to fatigue.  

Click here to view the column in the May/June Casting Source Digital Edition.