Additive Manufacturing Fatigue and Fracture: Effects of Surface Roughness, Residual Stress, and Environment: Session V Joint Session with Fatigue in Materials Symposium - Microstructure-based Fatigue Studies on Additive-Manufactured Materials
Sponsored by: TMS Structural Materials Division, TMS: Additive Manufacturing Committee, TMS: Mechanical Behavior of Materials Committee
Program Organizers: Nik Hrabe, National Institute of Standards and Technology; John Lewandowski, Case Western Reserve University; Nima Shamsaei, Auburn University; Steve Daniewicz, University of Alabama; Mohsen Seifi, ASTM International/Case Western Reserve University
Wednesday 2:00 PM
March 22, 2023
Room: 22
Location: SDCC
Session Chair: Jean-Charles Stinville, University of Illinois at Urbana-Champaign
2:00 PM Invited
Predicting the Fatigue Performance of AM Ti-6Al-4V Components: Derek Warner1; Terrence Moran1; Peipei Li1; 1Cornell University
Traditional approaches to using additively manufactured components in load bearing applications involve the fabrication and testing of many identical components. This negates one of the greatest benefits of additive manufacturing, i.e. the ability to quickly and cheaply fabricate one-off components. To address this challenge predictive modeling must be harnessed, particularly for high cycle fatigue performance, where mechanical testing requires significant resources and produces stochastic results. High cycle fatigue failure is difficult to predict because it can depend nonlinearly on many parameters and their history. This has motivated a succession of fatigue models with ever increasing mechanistic fidelity, raising the question of: what level of mechanistic detail is required to sufficiently predict the performance of AM Ti-6Al-4V components? In this talk, I will give my perspective on this question, building from a decade of AM Ti-6Al-4V fatigue modeling and experimentation across scales.
2:30 PM
Experiments to Enable Machine Learning of Fatigue Performance in DMLM Ti-6Al-4V with Respect to Microstructure: Samuel Present1; Monica Soare2; Johan Reimann2; Laura Dial2; Kevin Hemker1; 1Johns Hopkins University; 2General Electric Research Laboratory
Understanding and predicting fatigue performance is paramount for aerospace applications, and rapid qualification and certification of metal alloys for use in cyclic loading environments is necessary for widespread adoption of additively manufactured components. Fatigue studies of additively manufactured metals and alloys have elucidated the fact that surface roughness and microstructural features can profoundly affect fatigue life. In the current study, four-point bending fatigue experiments were employed to identify the number of cycles to, and specific location for, crack nucleation in direct metal laser melted (DMLM) Ti-6Al-4V samples. Cross-correlation with EBSD maps of the underlying microstructure facilitated identification of critical nucleation sites. These experimental results are being used to underpin finite element simulations and to create training sets for expert-informed machine learning protocols, to enable rapid simulation of thin-wall fatigue performance.
2:50 PM
In-situ Microscopy of Crack Initiation and Growth in Laser Powder Bed Additively-manufactured Ti-6Al-4V: Matthew Krug1; JoAnn Ballor2; Lewis Forman1; Michael Velez1; Molly Walters1; Sushant Jha3; Carl Boehlert2; 1Air Force Research Laboratory; 2Michigan State University; 3University of Dayton Research Institute
Laser powder-bed fusion (LPBF) of Ti-6Al-4V has become widely adopted in the aerospace and medical equipment industries, which both trade in high-value near-net shaped parts that are cyclically-loaded and have low tolerance for failures. Fabrication defects inherited from the LPBF process can act as fatigue crack initiation sites. In this study, fatigue specimens of LPBF Ti-6Al-4V were tested in-situ using SEM imaging and EBSD orientation measurements. Crack initiation at surface defects and subsequent crack growth were observed as they occurred during cyclic loading. Defect size and shape, and the local phase and orientation distributions were characterized to understand their role in site selection for crack nucleation and growth. The effects of load orientation relative to build direction, and of a hot isostatic pressing heat treatment will be presented and discussed.
3:10 PM
Improving the Low Cycle Fatigue Life of Additively Manufactured High-manganese Steels by Tailoring the Crystallographic Textures: Efthymios Polatidis1; Miroslav Smid2; Jan Capek1; Michal Jambor2; Daniel Koutny3; Christian Haase4; 1Paul Scherrer Institute; 2Czech Academy of Sciences; 3Brno University of Technology; 4RWTH Aachen
By carefully selecting the processing parameters of Laser Powder Bed Fusion (LPBF) it is possible to engineer the dominant crystallographic texture with respect to the loading axis. Consequently, the transformation-induced plasticity (TRIP) effect in metastable steels, which is orientation-dependent, can be hindered or enhanced. The present study focuses on high-manganese steels processed by LPBF with <001>, < 011>, <111> and random crystallographic textures along the loading direction, subjected to symmetrical cyclic loading. The results show significant differences in the cyclic hardening, hysteresis loop asymmetry and mean stresses. In order to follow the TRIP effect, in-situ neutron diffraction under low cycle fatigue tests were carried out. In combination with comprehensive microstructural characterization by electron microscopy, we were able to identify distinct differences in the deformation behavior between the different textures. It is concluded that texture manipulation is a promising route for achieving prolonged fatigue life and superior hardening behavior in metastable austenitic steels.
3:30 PM Break
3:50 PM Invited
Efficient Computational Framework for Image-based Micromechanical Analysis of Additively Manufactured Ti-6Al-4V Alloys: Somnath Ghosh1; Maxwell Pinz1; Steven Storck2; 1Johns Hopkins University; 2JHU Applied Physics Laboratory
This paper develops an image-based statistically equivalent microstructural volume element (SEMVE) and associated crystal plasticity constitutive relations for efficient micromechanical simulation of additively manufactured Ti-6Al-4V alloys, characterized by complex Widmanstatten morphology with 12 HCP alpha lath variants. The models are built from microstructural characterization and mechanical testing data for this material. A major step towards significant efficiency is the creation of an effective crystal plasticity framework for polycrystalline ensembles of parent beta grains with a parametric representation of the size, shape, orientation, and crystallography statistics of encompassing alpha laths. A major contribution is the development of the self-consistent boundary conditions that is implemented through embedding the SEMVE in a homogenized exterior domain in a concurrent model. Finally, parametric studies void volume fraction, aspect ratios, and orientation are all important for determining the overall effect on the material response.
4:20 PM
Influence of Process Parameters on Fatigue Behavior and Defect Characteristics in LPBF Ti-6Al-4V: Austin Ngo1; David Scannapieco1; Francisco Medina2; Christian Gobert3; Anthony Rollett3; Jack Beuth3; John Lewandowski1; 1Case Western Reserve University; 2University of Texas at El Paso; 3Carnegie Mellon University
Four-point bending fatigue testing was conducted on both as-built and machined surface LPBF Ti-6Al-4V mechanical testing specimens built with varying process parameters. Optimized and sub-optimal process parameter sets were selected to fabricate baseline builds and defect-induced builds, respectively. S-N fatigue data was generated for each process parameter set, and specimen fracture surfaces were imaged using OM and SEM. Fractographic analyses consisted of quantifying all defects on the fracture surface and identifying ‘killer’ defects responsible for fatigue crack initiation. Different types of process defects (i.e. lack of fusion, keyhole) were found to be more prevalent based on particular process parameter sets, which further influenced S-N curves. Specimen surface roughness contributed to differences in fatigue crack initiation behavior. The effects of process parameters on defect characteristics and the resulting S-N fatigue behavior will be discussed in the context of a defect-based Kitagawa-Murakami-type approach.
4:40 PM
Globularization of Alpha Phase in Additively Manufactured Ti-6Al-4V Alloys and Effects on High-Cycle and Very-High-Cycle Fatigue: Reza T. Mousavian1; Anthony G. Spangenberger1; Austin Mann2; Cory Cunningham2; Diana A. Lados1; 1Worcester Polytechnic Institute; 2Boeing Research & Technology
The high cooling rates during the laser powder bed fusion (LPBF) fabrication of Ti-6Al-4V result in a martensitic alpha’ microstructure with acicular morphology, which causes undesirable ductility and toughness of the as-fabricated material. In this study, a novel heat-treatment process was developed for the LPBF material to create a bimodal microstructure, consisting of globular primary alpha grains and lamellar alpha+beta colonies. Optical and scanning electron microscopy studies, including electron backscatter diffraction and energy dispersive X-ray spectroscopy, were performed to characterize both the as-fabricated and heat-treated microstructures. Further, the fatigue behavior of the as-fabricated and heat-treated Ti-6Al-4V alloys was investigated in the high-cycle and very-high-cycle regimes using conventional and ultrasonic fatigue testing (R=-1). The effects of the primary alpha grains globularization on the fatigue response of the LPBF materials were systematically assessed, and the results were compared to those of a solution treated and overaged wrought Ti-6Al-4V with bimodal microstructure.
5:00 PM
Enhancing the Fatigue Performance of AM Metals via Conformal Coatings that Activate Uniform Surface Remelting and Smoothening
: Kendall Yetter1; Michael Sangid2; William LePage1; 1University of Tulsa; 2Purdue University
Mitigating surface roughness is a key to enhanced fatigue performance of AM metals. However, current surface treatment options are not sufficient for complex AM parts, such as internal channels or microlattices. Surface treatments for AM must be able to address beyond line-of-sight surfaces, as well as uniformly treat all component surfaces for predictable fatigue performance. This work introduces a new surface treatment strategy through a transient surface remelting process. Surface remelting is triggered during heat treatment with a uniform, highly conformal coating, such as films fabricated with atomic layer deposition or electroplating. During heat treatment, the coating triggers local melting by activating a eutectic reaction, resulting in smoother surfaces and better fatigue performance. This talk will describe this new class of surface treatments with the specific case of AM Ti-6Al-4V. By using conformal coatings of Cu, we have demonstrated pronounced surface smoothening that increases the fatigue strength by approximately 30%.