Identifying microstructural features that control fracture in additive materials
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ORIGINAL PAPER
Identifying microstructural features that control fracture in additive materials T. Kobayashi · D. A. Shockey
· D. N. Wells
Received: 28 February 2020 / Accepted: 6 September 2020 / Published online: 26 September 2020 © Springer Nature B.V. 2020
Abstract A unique fractographic technique, FRASTA, was applied to additively manufactured IN718 J-R fracture test specimens to determine the microstructural failure mechanism. The results clearly showed that the material failed by nucleation, growth, and coalescence of microfailures in the highly stressed process zone in advance of the macrocrack front. Microfailure initiation sites were pinpointed on the fracture surfaces, enabling those weak spots to be examined metallographically and with EDX to identify and characterize the responsible defect or microstructural feature. The microfailure activity in a specimen processed differently was distinctly different, showing the effects and importance of processing conditions and providing an explanation of mechanical property variability in additively manufactured materials. The improved understanding of the microstructural failure process in additive materials achievable with FRASTA can guide development of more effective processing protocols and, ultimately, provide a route for achieving additive materials with improved and consistent fracture properties. Moreover, the microfailure evolution data provide a basis for computational model development and T. Kobayashi · D. A. Shockey (B) Formerly SRI International, 333 Ravenswood Ave., Menlo Park, CA 94025, USA e-mail: [email protected] D. N. Wells Marshall Space Flight Center, Martin Rd SW, Huntsville, AL 35808, USA
suggest a procedure for determining fracture toughness from fracture surfaces. Keywords Laser powder bed fusion · FRASTA · Microstructural failure mechanism · Fractography
1 Introduction Additive manufacturing technologies are hailed as the next industrial revolution (Lewandowski and Seifi 2016). However, the process of building a material layer by layer by depositing powder and melting it with a laser or electron beam can produce microstructural features such as voids, phase boundaries, directional grains, and poorly fused interfaces that may adversely affect mechanical properties, especially fatigue and fracture strength. As a result, materials with consistent mechanical properties cannot be produced with confidence and, consequently, additively manufactured components are not yet widely used in fracture-critical structures. Thus, a goal is to identify the microstructural features affecting fracture resistance, link these features to additive manufacturing process parameters, and establish process protocols that eliminate deleterious features. A unique method that analyzes the topography of conjugate fracture surfaces is described here and applied to replay microfailure activity that occurred in additively manufactured Inconel 718. The technique, called FRASTA (Kobayashi and Shockey 1991a, b),
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identifies microfailure initiation sites,
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