Additive Manufacturing Fatigue and Fracture: Developing Predictive Capabilities: On-Demand Oral Presentations
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; Mohsen Seifi, ASTM International/Case Western Reserve University; Steve Daniewicz, University of Alabama

Monday 8:00 AM
March 14, 2022
Room: Additive Technologies
Location: On-Demand Room

On the Fatigue Properties of Additively Manufactured Materials Processed at Non-optimal Conditions: Thomas Niendorf1; 1Universitaet Kassel
    Powder bed additive manufacturing (AM) is used for fabrication of many high-performance parts. Assessment of structural integrity of such parts is of utmost importance and any kind of process induced imperfection has crucially to be avoided. Thus, only a part built in a perfect AM scenario, i.e., established without any process instability, is considered for application. This strict limitation is not to be transferred to every envisaged application. Using the two most common AM processes, i.e., electron beam- and laser-based powder bed fusion, different scenarios were assessed. In case of both AM processes metal powders were employed not being perfectly spherical in shape. Furthermore, AM processes were stopped and continued afterwards. Finally, AM parts of different build orientation were joined. Results show that non-optimal conditions are not an insuperable roadblock to several envisaged applications such that the processing windows for many parts are much larger than expected so far.

Probabilistic Methods for Additive Manufacturing – How to Understand and Manage the Uncertainty: Mohsen Seifi1; Martin White2; Mahdi Jamshidinia2; Doug Wells3; 1ASTM International/Case Western Reserve University; 2ASTM International; 3NASA
    The AM process exhibits variability throughout, from controlling the feedstock to the effects of defects and subsequent influence on fatigue and fracture behavior. In particular, some of the flaws are related to process escapes (e.g., powder short-feed or recoater issues, residual stress cracking, etc.) and can be considered as rogue. As such, there is an enormous opportunity to deploy probabilistic methods to help quantify and manage the uncertainty in the process. The ASTM International AM CoE is currently working with NASA to build a probabilistic framework that can be used to manage uncertainty and support future certification methods. This presentation will provide an overview of these probabilistic approaches for both printing of the material, as well as the evaluation in terms of structural integrity lifing models. A key topic will consider the likelihood of rogue defect occurrence, as well as the probability of defect detection using available Non-Destructive Testing methods.

Effect of Surface Roughness on Fatigue Behavior of 316L Stainless Steel Produced by Binder Jetting Process: Wei-Jen Lai1; Avinesh Ojha1; Zhenxuan Luo2; 1Ford Motor Company; 2Shanghai Jiao Tong University
    Tension-compression fatigue tests were performed on 316L stainless steel produced by binder jetting (BJ) process with the as-built and polished surface conditions to understand the effect of surface roughness on fatigue performance. The porosity was analyzed using X-ray CT scan to study the distribution and the correlation to fatigue crack initiation. The fatigue results indicate that BJ 316L shows a much lower fatigue strength compared to the ones produced by laser powder bed fusion (L-PBF) process and wrought 316L. Both ultimate tensile and yield strengths are much lower than L-PBF and wrought 316L. Porosity does not have a significant effect on fatigue crack initiation in the studied BJ 316L. The as-built surface roughness lowers the fatigue strength by roughly 21%. The BJ 316 yield strength is lower than the fatigue strength. Cyclic plastic shakedown was observed for samples tested near the fatigue strength. Cyclic ratcheting towards the negative strain was also observed. Pores in the sample show a distribution pattern related to the layer-by-layer deposition process.

Mechanism-based Characterization of the Mechanical Behavior of PBF-EB Manufactured IN718 Lattice Structures: Daniel Kotzem1; Tizian Arold2; Thomas Niendorf2; Frank Walther1; 1TU Dortmund University; 2University of Kassel
     Based on the layer-by-layer manufacturing, additive manufacturing offers various possibilities for the design of new lightweight constructions. On the way to a reliable application within safety-relevant components, the deformation and damage behavior, especially under cyclic loading, have to be understood in detail. Within this work, the mechanical behavior of electron beam powder bed fusion (PBF-EB) Inconel 718 (IN718) lattice structures is characterized. Computer tomographic scans are conducted in order to quantify the present defect density, the process-induced surface roughness and the nominal cross section. The material’s fatigue behavior is investigated by means of coupled multiple and constant amplitude tests at fully-reversed loading using various measuring techniques such as digital image correlation (DIC), infrared camera, and direct current potential drop system (DCPD). Based on the results, a good correlation between the measurement techniques could be highlighted which can be linked to the specific material reactions.

High Strain Rate Deformation of EBM-Ti-6Al-4V: Microstructure, Texture, Mechanical Properties, Fracture Surface, Deformation Mechanism, and Constitute Modeling: Reza Alaghmandfard1; Mohsen Mohammadi1; 1University of New Brunswick
    In this study, cylindrical rods of Ti-6Al-4V were additively fabricated through the electron beam melting technique in horizontal and vertical directions. High strain rate compression tests using Split-Hopkinson pressure bar machine were performed from the strain rate of 150s-1 to 2350s-1 on both horizontal and vertical samples. Mechanical properties improvements along with microstructural and texture evolutions were evaluated. Deformation mechanisms including active slip and twin systems were comprehensively characterized. High strain rate induced phenomena such as adiabatic shear bands were defined, and utilizing in-depth microscopy techniques, physical metallurgy phenomena within these regions were investigated. Fracture surfaces were also studied, and texture evolution at the surface close to the fracture surface was discussed. The predictions of constitutive models used in this study were in good agreement with experimental observations. Generally, horizontally built samples revealed higher total strain at a specific strain rate, while higher strength was achieved in vertical samples.

Methodology of Low Cycle Fatigue Testing on Thin-walled Stainless Steel 316L Manufactured by Laser Powder Bed Fusion: Cheng-Han Yu1; Johan Moverare1; Ru Peng1; Alexander Leicht2; 1Linköping University; 2Chalmers University of Technology
    The process of laser powder bed fusion leads to characteristic microstructure of the as-built component, such as high surface roughness, cell structure and high surface residual stress. To examine the microstructural effects on the low cycle fatigue properties when the printed component is close to the dimensional limitation, tubular fatigue specimens with different wall thicknesses, 1mm and 2mm, were tested at room temperature. The advantage of using tubular specimens is to avoid possible buckling when the fully revered fatigue test is carried out. The comprehensive effects of surface roughness and heterogeneous microstructure at different applied strain range are quantified by estimating the fatigue notch factor, Kf, which the influence becomes severe when approaching lower applied strain range. Furthermore, the same fatigue tests have also been applied to a reference group of wrought stainless steel 316L, and a suppressed martensitic phase transformation is observed in the additively manufactured specimens.

Prediction of Fatigue Life Based on the Microstructure and Porosity Distribution Using the Novel Computationally Efficient Multiscale Modeling: Mohamed Elkhateeb1; Yung Shin2; 1Mansoura University; 2Purdue University
    Additive manufacturing is mainly challenged by the existence of porosity and inhomogeneous microstructure in the fabricated parts. Prediction of the effects of such defects on the fatigue life requires very time-consuming and expensive experiments and multiscale mechanics models, which can be computationally cost-prohibitive for large structures. In this work, the novel scheme called extended mechanics of structure genome (XMSG) is used as a computationally efficient multi-scale homogenization scheme to predict the bulk behavior and fatigue life with considering porosity and microstructural details. In the XMSG, homogenization is conducted on a structure genome, which represents the smallest unit cells containing the basic microstructural and porosity details, and does need updates with porosity growth and coalescence. The predicted results showed a very good agreement with the experimental results with a drastic 5 orders of magnitude reduction in the computational time compared with the corresponding heterogeneous models-based finite element simulations.

The Tensile Behavior of Additively Manufactured 17-4 PH Stainless Steel with Different Heat Treatments: Saadi Habib1; Steven Mates2; Mark Stoudt2; Fan Zhang2; Olaf Borkiewicz3; 1National Institute of Standards and Technology; 2National Institute of Standards and Technology (NIST); 3Advanced Photon Source, Argonne National Laboratory
    Additive manufacturing (AM) of 17-4 PH martensitic stainless steel is of interest for its high strength and corrosion resistance for many engineering applications. However, as-printed AM 17 4 PH exhibits high residual stresses, a complex, heterogenous microstructure, and often unfavorable mechanical properties, particularly when processed in a nitrogen environment. Therefore, in this work, we investigate the influence of homogenization and aging heat-treatments on the tensile behavior of AM 17 4 PH. These heat treatments are designed to vary the strength, work hardening, and ductility by varying the amount of metastable austenite in the microstructure, which transforms to martensite during deformation. Electron microscopy and high-energy x ray diffraction (HEXRD) are used to quantify the microstructure for each heat treatment. Ex-situ HEXRD measurements are also used to quantify the austenite to martensite transformation as a function of strain. Finally, the fracture surfaces are analyzed to help explain the ductile fracture mechanisms.

High-throughput Characterization of the Fatigue Behavior of Additively Manufactured Metals toward Rapid Qualification: Adam Pilchak1; Pawan Vedanti1; Dan Satko1; Ayman Salem1; 1Materials Resources, LLC
    A high-throughput fatigue testing apparatus, dubbed the “Goodman Diagram Maker” (GDM), was developed to rapidly establish the safe operating condition of additively manufactured components. The GDM is an extensible test device that uses an electric motor to drive up to six Krouse-type flat-bending specimens. Each test arm can be run at a different mean stress and stress amplitude allowing one to quickly establish the fatigue strength of a material. GDM specimens are manufactured on the same build plate as the part they are intended to qualify and process modeling ensures that specimens destined for the GDM are clones of the critical region of the full-scale part built using process-parameters.

Load-dependent Degenerating Structures in Additively Manufactured Implants: Dennis Milaege1; Kay-Peter Hoyer1; Mirko Schaper1; 1Paderborn University
     If not absolutely necessary, implants should remain in the human body for a defined period of time and degenerate depending on time and/or load. Within the application phase, the implant should take over the entire support function and then successively transfer this to the surrounding tissue and bone. Therefore the implant should lose strength and stiffness and thus absorb less load compared to the reconnected bone structure. This has the advantage that a targeted growth-promoting load stimulation of the bone can take place and, in addition to stress shielding, explantation can be avoided. In previous approaches, it has been shown that a suitable structure and geometry selection can influencing the specific properties as well as the lifetime of additively manufactured structures by fracture mechanics phenomena.Thus different lifetime influencing measures are investigated, both eperimentally and numerically, by using the example of angle-stable plate osteosynthesis made of Ti-6Al-4V.

Effect of Post Heat Treatment on Fatigue Strength of AlSi10Mg Produced by Laser Powder Bed Fusion Process: Wei-Jen Lai1; Avinesh Ojha1; Ziang Li1; 1Ford Motor Company
    The effect of the post heat treatment on the fatigue properties was investigated for AlSi10Mg aluminum alloy produced by laser powder bed fusion process. The post heat treatments focused on in this study are stress relief, solutionization, T6, and T5. Quasi-static tensile tests and hardness tests were performed on samples with different post heat treatments and mechanical properties were compared. Fatigue tests were then performed on the same samples to understand the relation between static mechanical properties and fatigue strength. Results show that as-built samples have the highest UTS while T6 samples have the highest yield strength. The hardness has a positive correlation with UTS, not yield strength. The fatigue test results show that as-built samples have the highest fatigue strength and T6 heat treatment did not improve fatigue strength. Besides, stress relieved samples have a fatigue strength close to T6 samples. T5 aging curves show mild age hardening behavior.