Closeup image of the existing 737 MAX fuel access door cover made from aluminum alloy A380. (Image Copyright: Boeing)

Super vacuum die casting to the rescue via improved quality, reduced costs

To large manufacturers, new technologies and variations on existing technologies demand bound conditions through specifications to determine standardized design values. Without this, no technology can make it beyond the R&D stage.

Although high-pressure die casting (HPDC) is a proven manufacturing process for casting aluminum and magnesium alloys, achieving ample mechanical properties, it is not a standalone process that merits its own designation and, in aerospace, does not entirely escape an association with poor quality and limited applications.

This lack of quality is a direct and persistent deterrent to combining lightweighting and die casting, and is caused primarily by air entrapment and general shrinkage porosity in the casting process. Alleviating these effects through installation of a vacuum pump, a technology known as super vacuum die casting (SVDC), produces a process that can have industrial recognition as standalone and significantly expand the design space by greatly improving the quality of die-cast parts.

Using this process, a designer is free to increase part complexity, unitize structure, and decrease wall thickness, which all contribute to decreasing weight, energy consumption, and cost. Traditional design that accounts for inherent processing defects has the primary upstream implication of thicker parts to achieve required strengths, which causes downstream implications of heavier parts, inability to perform heat-treatment due to blister formation, and higher risk for assembly and machining to reveal defects.

The advantages of the SVDC process allow for a compounding reduction in total part weight by improved properties as-cast as well as allowing for strengthening heat treatments to further improve properties. This compounding leads to as much as a 20% decrease in overall part weight and a 40% decrease in wall thickness. Combined, these decreases enable a reduction of energy use and cost with less material use, fuel consumption, and emissions, as well as the ability to heat treat parts to improve mechanical properties and expand the potential implementation space for parts.

The trade off to be considered is the increased facilities cost vs. the savings in material; for example, the Boeing 737 MAX fuel access door cover. This particular part is utilized repeatedly on the underside of the wing and across multiple commercial platforms, making it a prime example of aerospace die casting usage.

The method applied to evaluate the implications of SVDC is process-based cost modeling (PBCM). PBCM is composed of three interrelated and interdependent models: A technical process model, a production operations model, and financial accounting model.

In a technical process model, fundamental engineering principles are employed including materials, energy, labor, equipment, scrap, and mass balances. In an operations model, the key element modeled is time, which is essential in determining how the technical process is physically implemented and organized and how the factory manager plans to allocate capital equipment in a cost-effective, ease-of-operation way.

In a financial model, the resource requirements developed in the previous models are converted into economic costs. In this model, the production factors (such as energy, labor, material, and equipment) are weighted by their purchase price. One key element of these models is they are built based on physical and/or statistical relationships where they relate part characteristics (e.g., dimensions and mass) and processing decisions (e.g., temperatures and pressures) to key resource needs such as equipment specifications (e.g., pour capacity and die clamping force) and cycle times and then translate these requirements to specific cost.

The strategic question answered here was showing a break-even analysis between investing in this advanced manufacturing process compared to material and energy savings while considering production rates, part size, and application.

The attached graph shows a cost analysis for the fuel access door cover as well as two additional parts to evaluate the effects of part size variation on overall savings. While the red curve shows the unit cost given rate considering incumbent HPDC, the green curve shows the unit cost given rate when utilizing SVDC.

At very low production volumes for all three cases, the unit cost of SVDC is slightly higher than incumbent HPDC, but the cost converges and surpasses as production volume increases. The convergence points on the graph are represented with blue stars.

For the large-size part the break-even or convergence point is achieved at a production volume of 7180 and a unit cost of $118.14, whereas in case of medium-size part, the break-even point is achieved at a production volume of 8230 and a unit cost of $45.54. The large, medium, and small size parts considered in this analysis have a mass of 22 kg, 5.4 kg, and 0.4 kg.

While considering a production volume of 10,000 units per year for aerospace as a baseline scenario, the savings clearly outweighs investments for medium and large part sizes but for the small part scenario, the savings does not overcome the investment. This is due to a majority of the savings being reduced material and as the part size is very small, opportunity to save material is also very little.

However, it should be noted the negative cost for a small part size is on average only 1.2%, and it could break-even when coupled with larger parts in a business case or considering this technology’s implications on fuel consumption.

Furthermore, with many industries including aerospace desiring to increase performance, quality, reliability, and commonality of parts as well as reduce overall carbon footprint, a technology like SVDC is a prime example of improving the existing to meet ever more demanding industry needs.

This article was written for Aerospace Engineering by Muhammad Arsalan Farooq, Postdoctoral Associate, Materials Systems Laboratory, Massachusetts Institute of Technology, and Michael Zolnowski, Materials and Process Engineer, Boeing Research and Technology.

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