PAPERS: Process Planning Considerations for AM

Enhancement of Low-Cycle Fatigue Performance From Tailored Microstructures Enabled by Electron Beam Melting Additive Manufacturing Technology

[+] Author and Article Information
Philip A. Morton, Jorge Mireles, Heimdall Mendoza, Paola M. Cordero, Ryan B. Wicker

Department of Mechanical Engineering,
W.M. Keck Center for 3D Innovation,
University of Texas at El Paso,
El Paso, TX 79968-0579

Mark Benedict

Air Force Research Laboratory,
Wright-Patterson Air Force Base,
OH 45433-7801

1Corresponding author.

Contributed by the Design for Manufacturing Committee of ASME for publication in the JOURNAL OF MECHANICAL DESIGN. Manuscript received February 4, 2015; final manuscript received July 9, 2015; published online October 12, 2015. Assoc. Editor: Christopher Williams.This material is declared a work of the U.S. Government and is not subject to copyright protection in the United States. Approved for public release; distribution is unlimited.

J. Mech. Des 137(11), 111412 (Oct 12, 2015) (4 pages) Paper No: MD-15-1069; doi: 10.1115/1.4031057 History: Received February 04, 2015; Revised July 09, 2015

Electron beam melting (EBM) additive manufacturing (AM) technology has allowed the layerwise fabrication of parts from metal powder precursor materials that are selectively melted using an electron beam. An advantage of EBM technology over conventional manufacturing processes has been the capability to change processing variables (e.g., beam current, beam speed, and beam focus) throughout part fabrication, enabling the processing of a wide variety of materials. In this research, additional scans were implemented in an attempt to promote grain coarsening through the added thermal energy. It is hypothesized that the additional energy caused coarsening of Ti-6Al-4V microstructure that has been shown to increase mechanical properties of as-fabricated parts as well as improve surface characteristics (e.g., reduced porosity). Fatigue testing was performed on an L-bracket using a loading configuration designed to cause failure at the corner (i.e., intersection of the two members) of the bracket. Results showed 22% fatigue life improvement from L-brackets with as-fabricated conditions to L-brackets with a graded microstructure resulting from the selective addition of thermal energy in the expected failure region. Three L-brackets were fabricated and exposed to a triple melt cycle (compared to the standard single melt cycle) during fabrication, machined to specific dimensions, and tested. Results for fatigue performance were within ∼1% of wrought L-brackets. The work from this research shows that new design procedures can be implemented for AM technologies that involve evaluation of stress concentration sites using finite element analysis and implementation of scanning strategies during fabrication that help improve performance by spatially adjusting thermal energy at potential failure sites or high stress regions.

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Grahic Jump Location
Fig. 1

Test setup for fatigue testing

Grahic Jump Location
Fig. 2

Brackets used for fatigue testing; shaded box indicates the area where microstructure was studied (a) EBM-fabricated (horizontally oriented), (b) graded microstructure, (c) multiple beam scanning after CNC-machining, and (d) machined from wrought material

Grahic Jump Location
Fig. 3

Micrographs of (a) EBM-fabricated (horizontally oriented), (b) graded microstructure taken at the region with added thermal energy, (c) bracket exposed to three melt cycles, and (d) the CNC-machined bracket from wrought Ti-6Al-4V material

Grahic Jump Location
Fig. 4

Fractography of wrought bracket (left bracket) and Horizontal EBM (right bracket); individual micrographs show the fatigue stages, indicated by the marked regions for both brackets



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