Converting fabrication to casting solves host of problems

Citation

(2003), "Converting fabrication to casting solves host of problems", Aircraft Engineering and Aerospace Technology, Vol. 75 No. 4. https://doi.org/10.1108/aeat.2003.12775daf.007

Converting fabrication to casting solves host of problems

Converting fabrication to casting solves host of problems

Keywords: Aircraft, Manufacturing

The problem

An airframe manufacturer seeking ways to reduce production costs identified a complex aluminium structural component for improvement. The structure selected was a rudder assembly – a large, asymmetrical component that was then being manufactured from numerous detail parts. These details included three separate aluminium castings, multiple sheet metal assemblies and aluminium hog-outs, as well as associated aircraft fastening hardware. A key production element targeted for reduction or elimination was the 50h required to assemble and finish the myriad detail parts of the piece prior to integration into the airframe.

In addition to focusing on assembly-time elimination, the airframer examined a number of production factors associated with the process in order to identify other areas for improvement. The search criteria included finding opportunities to reduce component- to-component variability, part count, raw material input, component weight, scrap rates, and machining requirements. The manufacturer also expected to benefit by freeing up expensive floor space in the factory, having many fewer parts to inspect and fewer drawings to administer.

The solution

The airframer decided to convert the fabricated assembly into a single-piece, monolithic investment casting. Alcoa's Howmet Bethlehem Casting operation, located in Bethlehem, Pennsylvania, USA, was selected to produce the part. After 3-months of concurrent engineering, the final casting configuration was optimized. The final part shape was characterized by a unique "bicycle frame" design. The new, unitized rudder component proved to be approximately 10 per cent lighter in weight than the fabrication it replaced. This was due to the reduced wall thickness of the new design, which is 0.095in., as well as the reduction in hardware needed to assemble the component. Due to the component's large, asymmetrical, open shape (Plate 2), Howmet devised special Plexiglas wax pattern holding fixtures to ensure that the large, open, thin- walled part remained dimensionally correct during installation of the risering system. Wax-injection tooling for a rudder part of this size required a well-thought-out design that took into account such production elements as wax-injection cycle time, injection pressures and tooling transportability. To reduce costs, the wax-injection tooling was also redesigned. Adopting an unconventional design approach to tooling reduced raw material requirements, resulting in a tool that was approximately 185 pounds lighter than a conventional die configuration would have been. This weight reduction made it significantly easier to transport the tooling within the wax department and injection press areas.

Plate 2 Howmet Aluminium Casting "Rudder" component for airframe A357 monolithic investment casting nicknames "The bicycle frame"

The redesigned rudder component also included provisions for advanced, cast-in, datum features. The designers' intent was to create a sophisticated inspection fixture that could serve both as a check during in-process manufacturing and as a quality-assurance gauge during final dimensional inspection. Designers envisioned that such a fixture would help shorten the manufacturing cycle time as well as have a positive impact on rejection rates.

The combination of pattern-assembly fixtures and the device to measure dimensional accuracy at multiple process stages solved one of the manufacturer's biggest problems – article-to-article variability. The flexibility demonstrated by the investment casting process in accommodating complex designs helped engineers achieve the manufacturing repeatability goals they had targeted. Achieving improved repeatability has greatly contributed to reducing variation and rejection rates.

The redesigned part measures 36–24–14 in. It is cast in A357 with mechanical properties of 41,000T, 31,000Y, and 2.5 per cent E. The mechanical properties are derived from five different locations on the casting (Table IV).

Summary of advances

Consolidating the multiple detail parts of the previously fabricated assembly into a single- piece casting eliminated 50h of assembly time. Capitalising on the design flexibility offered by the investment casting process, engineers were able to produce near-net shapes that lessened raw material input and machining requirements. Moreover, cast-in features such as datum pads, accelerated set-up and execution of numerous finish machining, and also in-process and final inspections.

Improvements in investment casting over recent years have resulted in advances in process control. Greater process control enables the casting of large parts in a variety of large and asymmetrical configurations. Achieving higher levels of casting precision has driven continuous repeatability and yield improvements, which in turn have expanded the investment casting process's applications capabilities. These expanded capabilities, in turn, are increasing the pool of parts that make good candidates for conversion from other costly and time-consuming metalworking processes to the investment casting process.

Nowadays, producing the rudder component as a one-piece casting has provided consistent, dimensionally accurate iterations that are reducing inspection requirements and rejection rates compared to the previous production route. Lastly, the monolithic "bicycle frame" casting limits procurement and a host of other administrative costs to a single part rather than the multitude of parts required for the previous assembly.

Monolithic rudder component
Configuration	One-piece investment casting
Material	A357
Dimensions	36 x 24 x 14 in.
Tensile strength	41,000 T
Yield strength	31,000 Y
Elongation	2.5 per cent E
Weight reduction	10 per cent

Table IV

Supplier profile

Alcoa Howmet Castings, which recently relocated its headquarters to Cleveland, Ohio, from Darien, Connecticut, USA, serves casting buyers globally. The company produces structural and airfoil components of superalloy, aluminium and titanium, primarily for jet aircraft and industrial gas turbine engine original equipment manufacturers. Howmet also provides hot isostatic pressing, precision machining and protective coating services. In addition, Howmet is a supplier of superalloy metals, titanium ingots, ceramic products and advanced tooling. The company conducts extensive research to aid in the development of its material, product and process technologies. Howmet and affiliates operate 27 manufacturing facilities in the United Kingdom, Canada, France, Japan and the United States.

Details available from: Howmet Castings, Tel: +1 610 266 0270; Fax: +1 610 266 1990.