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Casting In Aluminum Knocks Weight Down Greatly

Jiten Shah

This developmental engine block casting is for an advanced 2-stroke, opposed-piston, gasoline, compression-ignition engine. The first stage of the project used an iron block––this next evolution of the project was to design the block to be cast in aluminum, which significantly reduces weight to further improve fuel economy. 

This aluminum block saw a greater-than 60% mass reduction versus the cast iron Gen I block. This advanced design incorporates integrated intake and exhaust ports, cooling and oil passages. The customers’ design team and the foundry, AFS Corporate Member TEI (recently acquired by GM) worked through multiple design iterations to reach the final product, which was cast with additively manufactured cores. 

The 3D printed sand additive manufacturing process is toolingless, hence all the tooling-related design constraints, such as drafts and feature orientation are eliminated, giving the design engineer a lot more freedom with feature placements and much higher complexity than conventional sand casting process. 3D sand printing allows an opportunity for part consolidation and lightweighting with mass optimization, reduced lead time, and savings of several hundred thousand dollars in tooling cost than with conventional sand castings.

Multiple consolidated core configurations [1] comprised of the intake and exhaust ports, cooling and oil passages were designed, consolidating multiple complex features, not manufacturable with a conventional tooling approach. 

• 3D printed sand process allows any complex core shapes to be made irrespective of their location and orientation or any drafting constraints typically experienced with conventional sand-casting process.

• Complex cored passages lighten up the structure by creating a thin-wall, hollow, lightweight design for optimum performance at high temperature. Core consolidation eliminates the potential for any internal flash, which are difficult to remove and may interfere with the engine performance.

• Low pressure casting process coupled with 3D sand printing additive manufacturing allows thin-wall, lightweight, complex designs to turn into reality with feature and part consolidations and tighter dimensional tolerances and accuracy. Low pressure precision sand casting allows thin-walled features to be filled with liquid metal against the gravity as experienced with conventional sand-casting process, thereby allowing to make sound castings with desired solidification pattern, dimensional accuracy and repeatability with proper process control and sound rigging system.
3DPS provided freedom for the placement of features such as [2] lightening holes or pockets, [3] cored holes, and [4] strengthening stiffener—no drafting required to achieve uniform wall thickness. Drafting constraints are eliminated. 

• Keeping generic castability and fluidity, design engineers have full freedom with the placement of features including ribs, cored holes, draft, thin to thick transition, along with risering and gating design with the 3DPS process. This results in a robust optimum design with lower weights.

• Carefully designed gating and risering for these complex, thin wall, large structural castings needs to be validated using advance casting simulation tools to ensure the liquid metal fills properly and solidified to get a sound casting with desired properties and quality.

• Complex sand castings require generous [5] fillet, radii, and transitions for a better cast component.

• With the exception of the mold-making method, 3DPS is no different than conventional sand casting. The melting, pouring, cleaning, and post-processing remain identical to the conventional sand-casting process. Smooth transitions and generous fillet and radii offer better flow of liquid metal. Smoother transitions allow reduced turbulence of liquid metal while filling into the mold cavity.

• Complexity make the rigging design challenging to produce 3DPS molded sand castings with desired quality; however, casting process modeling is very valuable to validate the rigging scenarios and process conditions, and to predict the casting quality before printing the mold with a higher degree of confidence, which is very crucial for the toolingless process. The lack of tooling takes away the conventional trial-and-error methodology options.

• Casting orientation, with respect to the gravity or pressurization and gating design, determines the location and amount of machining stock. The hole diameter, depth, positional tolerance, and location determine whether to make it as-cast/cored or cast solid and machined or drilled afterwards. 

• Occasionally, cored holes are desired design features to place and hold the core in position and make it robust during filling and solidification, preventing the core from floating or breaking.