Plastic flow and microstructure evolution during thermomechanical processing of laser-deposited Ti-6Al-4V preforms
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INTRODUCTION
FORGINGS of alpha/beta titanium alloys for the aerospace industry are typically manufactured via closed-die forging of wrought mill product. The mill product is usually made via an ingot-metallurgy (IM) route utilizing doubleor-triple melted vacuum-arc-remelted ingots. During such processing, ingots are subjected to a series of hot-working and heat-treatment steps, first in the high-temperature beta field and then at lower temperatures in the alpha-beta field to break down the transformed-beta microstructure.[1] By this means, a uniform, fine equiaxed microstructure, which provides good forgeability and which can be final heat treated to obtain a range of other microstructures, is produced. Fabrication of forged shapes usually comprises deformation in a series of dies (e.g., preblocker, blocker, and finisher), which represents a very large capital investment. Furthermore, depending on part complexity and sonic-inspection requirements, the finish forging may be considerably oversized and require substantial rough and finish machining. For example, the “buy-to-fly” ratio is often on the order of S.L. SEMIATIN, Senior Scientist, Materials Processing/Processing Science, and P.A. KOBRYN, Materials Engineer, are with the Air Force Research Laboratory, Materials and Manufacturing Directorate, AFRL/ MLLM, Wright-Patterson Air Force Base, OH 45433-7817. E.D. ROUSH, Undergraduate Student, is with the Department of Mechanical and Materials Engineering, Wright State University, Dayton, OH 45435. D.J. FURRER, Manager, Advanced Materials and Process Technology, is with Ladish Company, Cudahy, WI 53110. T.E. HOWSON, Chief Technologist-Forgings, is with the Wyman-Gordon Division, PCC, North Grafton, MA 01536. R.R. BOYER, Technical Fellow, is with Boeing Commercial Airplane Group, Seattle, WA 98124. D.J. CHELLMAN, Technical Fellow, Advanced Structures and Materials Division, is with Lockheed Martin Aeronautical Systems, Marietta, GA 30063. Manuscript submitted July 19, 2000. METALLURGICAL AND MATERIALS TRANSACTIONS A
5:1 for “simple” axisymmetric shapes (e.g., jet engine disks), but may approach or exceed 20:1 for large and complex airframe structural components (e.g., bulkheads). Thus, there are considerable cost drivers to reduce both the number of forging stages and the input-material requirements. Solid freeform fabrication (SFF) comprises a broad class of processes developed to reduce manufacturing costs through the elimination of dies and molds. Originally conceived as a means of making prototypes to establish form and fit, SFF techniques are now evolving into methods to make load-bearing parts for structural applications. They are especially attractive for small lot sizes for which the cost of tooling is difficult to justify. One SFF process that is receiving much attention from the aerospace industry is that based on laser deposition (LD).[2,3,4] During LD, prealloyed (or, less frequently, elemental) powder is fed, laser-melted, and solidified onto a substrate. By rastering the substrate (or laser/powder feed)
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