Empirical Approach to Understanding the Fatigue Behavior of Metals Made Using Additive Manufacturing
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INTRODUCTION
THE growth of additive manufacturing (AM) for metallic materials has stimulated wide-ranging research into the relationships linking processing, microstructures, and properties. Before the anticipated benefits of AM can be realized, the properties must be sufficiently characterized so that designers can rely on well-established or widely recognized mechanical properties data. For alloys used in aerospace applications, such a reference has been MIL-HDBK-5 (‘‘Metallic Materials and Elements for Aerospace Vehicle Structures’’)[1] and its successor Metallic Materials Properties Development and Standardization (MMPDS),[2] which provide statistically based mechanical properties, as well as numerous aerospace and industrial material standards that provide requirements for material properties and microstructures. As AM matures, it can be expected that best practices will be refined and post-build operations such as heat treatment will become standardized to such an extent that consensus material properties of AM-built alloys will emerge. Expensive ad-hoc property verification exercises will not be required for every combination of alloy and AM process or machine, and properties can be verified through standard metallurgical quality
DAVID B. WITKIN, Research Scientist, DHRUV N. PATEL, Member of the Technical Staff, and THOMAS V. ALBRIGHT, Technical Specialist, are with the Materials Science Department, The Aerospace Corporation, El Segundo, CA. Contact e-mail: david.b. [email protected] Manuscript submitted November 28, 2015. METALLURGICAL AND MATERIALS TRANSACTIONS A
assurance, such as tensile testing and microstructural verification of witness coupons.[3] At the current time, however, there is still active research exploring the fundamental relationships between process conditions and resulting material properties and microstructures. In the AM process, the influence of the layer-wise structure and rapid solidification have been identified by many investigators for their impact on the material properties. Each AM process will have its own influence on layer thickness and cooling rate. High-cycle fatigue (HCF) has long been recognized as a particular problem for AM-built metallic parts, because the surface roughness inherent to the process leads to reduced fatigue lifetimes or endurance limits. Two alloys of interest to the space and launch vehicle community are Ti-6Al-4V[4] and Inconelä 625.[5] The relationship between selective laser melting (SLM) processing conditions and surface roughness has been assessed for Ti-6Al-4V[6] and 625.[7] Numerous recent investigations have compared HCF behavior of as-built and machined surfaces in Ti-6Al-4V prepared using powder-bed fusion techniques such as SLM[8–13] and electron beam melting (EBM).[9,11,13,14] 625 alloy has been less studied, but it has been demonstrated that machining the surface of SLM 625 leads to HCF properties similar to wrought material.[15] Comparison of HCF data for as-built and machined HCF specimens provides convincing evidence that the decreased fatigue pr
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