Behind the scenes with Alcoa & the bearing bracket challenge

After receiving an overwhelming number of entries for our GrabCAD Challenge, Alcoa Fastening Systems & Rings (AFSR) quickly set forth evaluating each design using a systematic approach. PH15-5 material properties were applied to the parts to determine their mass. We then placed each design within the given part envelope to ensure the requirement was met. The entries that did not meet the part envelope were not considered further. As a next step, we evaluated the stress under both load conditions using FEA.

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Since the given loading conditions were to be applied down the centerline of each part, the parts were sectioned in half, and symmetry boundary conditions were utilized to reduce computational time. To ensure that the simulation would be representative of the application, we introduced multiple bodies including bolts, bearing, and base structure with the bracket bolted to the structure.

A simulation template was created such that each design could easily be evaluated with identical supports and boundary conditions. The model included bolt preload to accurately model the behavior of the bolted joint between the bracket and the base support. Bolted joint analysis is a competency at AFSR, but there are also many good references online which provide guidance to bolted joint design using FEA methods.

 

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Over 150 designs were simulated before we narrowed down on our top ten finalists. As you can imagine, this was a computing intensive exercise, but a cluster was used which significantly increased the speed of the simulations. Ten semi-finalists were identified, with the five lightest designs being selected for actual fabrication on our Direct Metal Laser Sintering (DMLS) machine.

To allow for testing in the two main loading conditions (horizontal and vertical), we printed two samples of each design. These ten parts barely fit on our 10” x 10” build plate and due to their relatively tall height, the build took a little over 96 hours to complete. Once fabricated, the parts were removed from the build plate via wire electrical discharge machining, then the bearing holes had to be machined in preparation for mechanical testing.

 

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After the parts were printed, we utilized blue light interferometry technology to dimensionally inspect them. We saw that the parts deformed slightly, presumably due to the thermal stresses caused by the DMLS process. We then verified this measurement using a more traditional optical comparator, and saw the same phenomenon.

 

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Material characterization of the parts was carried out using an optical microscope and a Scanning Electron Microscope. Typical fully lath martensitic structure of 15-5PH was observed in the as-deposited material.

 

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While the parts were being produced, we designed and fabricated a modular test fixture to hold the brackets in the necessary positions and load the parts in the specified horizontal (left image) and vertical (right image) static loading conditions.

 

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Each part was first loaded to 1,250 lbf in the horizontal configuration, and the deflection was measured. Then, the duplicate parts were loaded in the vertical configuration until failure. The peak force sustained by the part before failure was recorded and used in conjunction with the part’s weight to determine the design’s strength-to-weight ratio. While FEA analysis was used for screening the entries, ultimately what mattered most was the measured strength of the parts. Therefore the three winning designs were selected based on highest strength-to-weight values.
In the near term the AFSR team will further refine the winning design consistent with our additive manufacturing process and with the knowledge of the mechanical test results.

 

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The objective is to develop a fully industrialized design with all the information required for flight certification.  This effort includes fabricating a significant number of samples that will be used for endurance testing. As metal additive manufacturing processes evolve and additive-specific materials emerge, we expect that both the fabrication cost and the mechanical properties will significantly improve. Alcoa has a significant project aimed at developing novel metal additive-specific materials.  This challenge points the way to new development approach combining best in class design with state of the art material and process technologies.

Thank you

We sincerely thank all of the participants. This has been a rewarding learning experience and we appreciate the comments and feedback you have provided throughout the challenge. The AFSR team was awestruck by the diversity of thought and creativity that was evident in the entrants that motivated the team to provide timely, relevant feedback. We were especially impressed with the level of detail and commitment displayed by the entrants.

Special thanks to GrabCAD for their effective management of the challenge. Congratulations to the winners – your work is truly inspirational!


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