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Toolset for the Robust Design of Materials for Superplastic Forming Processes for Titanium Structural Components


OBJECTIVE: The objective of this project is to provide a suite of tools to model the sequence of unit operations, and materials response, in the processing of titanium alloy plate or sheet from primary ingot and/or sponge form. This would allow the primary metals suppliers to design robust processing sequences, trouble-shoot process problems, and refine processes to reduce costs without sacrificing materials quality. This project supports the goals of the Materials Genome Initiative (MGI) in the area of Integrated Computational Materials Engineering (ICME). DESCRIPTION: Superplastic Forming (SPF) is a well-established process for solid crystalline materials, such as titanium. In the superplastic condition, the flow stress of the material is low and the ductility is extremely high. Forming under these conditions allows for the production of large, very complex shapes. This allows for piece-part consolidation -- eliminating the need for multiple sub-parts, bolts that increases inventory, labor, and cost. It also produces a uniform final microstructure, which results in uniform properties with improved performance and reliability. The aerospace and automotive industries use this process to manufacture very complex geometries. It is most beneficial for these industries because it is critical to reduce the weight in order to achieve higher speed or fuel efficiency. Specific applications include aircraft frames and skins; window and doorframes, floor structures, and dashboards; and propulsion and ducting systems. Historically, there have been several challenges to using superplastic forming in production. One major challenge is the need for very consistent uniform microstructure and properties in the starting sheet or plate. In addition to lot-to-lot variation in materials, end-to-end variations in sheet properties may lead to defects and failures during superplastic forming. This could lead ultimately to high scrap rates, unacceptably high overall costs, and reduced part reliability. Material variability from a primary metals supplier stems from multiple causes, including melt-related and wrought-processing-related defects. The former include the presence of iron and other transition metals in the sponge, and high oxygen content from the melting processes. These can lead to macrosegregation in solidified ingots that subsequent remelting may not mitigate. Wrought processing, from ingot to billet to plate, and from plate to sheet, involves a number of unit operations. Mill suppliers also have a strong incentive to reduce the overall costs of their operations. They can accomplish this by reducing the number of processing steps, increasing product yield, et cetera. Any process modification, however, may affect final microstructure and subsequent superplastic properties and require careful consideration. We have good understanding of the physical processes for each of the operations, but not necessarily detailed models for material behavior and microstructure evolution during processing. This is the case especially for two-phase materials, such as the alpha-beta titanium alloys like Ti-64. Hence, models for these phenomena may not be fully adequate at present for process optimization and troubleshooting processing problems. In addition, the superplastic forming process involves several physical phenomena to allow large deformations without large net changes in the microstructure of the sheet. This does not mean that the microstructure is static, but that the microstructure evolves with deformation to maintain a stable state. Various metrics in microstructure are important in this. For the single-phase materials, a fine uniform equiaxed structure is desirable. For a two-phase material, the average grain size may not be the most appropriate microstructure metric, but some other measure such as the phase-boundary area, et cetera. It is important that modelling of the processes to produce input materials for superplastic forming also incorporate models for the superplastic forming process, in order to allow for optimization of the entire production operation from start to finish. PHASE I: In the phase I effort, the investigators should focus on the plate-to-sheet thermomechanical processing sequence to make superplastic sheet of a common commercial two-phase titanium such as Ti-64. These steps should include characterization of the starting condition/microstructure of the plate, preheating of the plate, the rolling operation(s), and intermediate/final heating/heat treatment steps. The investigators will show the ability to model each processing step using existing or new models and software to predict optimal processing parameters for the rolling/heating operations, and to track material history along the length of a sheet through all of the unit operations. The investigators need to define deficiencies in current physics-based understanding, and associated models, of microstructure evolution as well as the R&D required to alleviate such deficiencies. Full success in this phase will produce a protocol for tracking materials history across the processing operations, with a preliminary toolset for the optimization of the plate-to-sheet process. PHASE II: In the phase II effort, the investigators should complete the models for plate-to-sheet processing, to allow prediction of microstructure evolution and subsequent superplastic forming properties such as the constitutive behavior of the material as a function of the chosen processing steps and process variables. The team should pay particular attention quantifying variability in microstructure and constitutive response within a sheet and from sheet to sheet as result of the nature of the material synthesis operations and TMP steps. The investigators must verify and validate the applicability of the plate-to-sheet material behavior and process models by working with a primary metals supplier to model a typical production-scale process. They should also work with metal fabricators that use superplastic forming to ensure that the sheet properties are appropriate for the forming operations and useful to the customer shops. Finally, the models need to be useful for other alloy systems than the target for this project. PHASE III: The immediate application of the toolset is to primary metals suppliers. The investigators should connect with the various metals suppliers to provide either support services for process optimization, to sell the tool-sets to allow the suppliers to perform in-house optimizations. The investigators will need to define and assess the state-of-the art of models for the ingot-to-plate processes using the techniques that they developed for the plate-to-sheet models as appropriate. They should begin to model microstructure and properties throughout the plate as a function of ingot starting conditions and processing parameters in the multi-step process. They will also need to provide integration with existing ingot solidification models. Finally, with the current low-cost titanium efforts focused on powder production, the investigators will likely find it useful to explore the modelling of the direct consolidation of powder into plate product and possibly direct plate casting. PRIVATE SECTOR COMMERCIAL POTENTIAL/DUAL-USE APPLICATIONS: This toolset will allow the metals suppliers to optimize their plate and sheet processing operations for any applications, including for the oil and chemical industries where titanium is important for corrosion-resistant applications. The material-behavior and process models, and the protocols for retaining material history, should be relatively materials agnostic. This is of considerable interest to the aerospace and automotive industries.
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