Additive Manufacturing Fatigue and Fracture: Effects of Surface Roughness, Residual Stress, and Environment: Session VI
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

Thursday 8:30 AM
March 23, 2023
Room: 22
Location: SDCC

Session Chair: Nicholas Derimow, National Institute of Standards and Technology


8:30 AM  Invited
Elevated Temperature Testing of LPBF Ti6Al4V: James Dobbs1; Cory Cunningham1; 1Boeing Company
    Titanium alloys are of interest for the aerospace industry because of the excellent strength to weight ratio. Ti6Al4V is the most common of the Ti alloys, and is used in many structural applications for aircraft airframes. This alloy is also used at moderately elevated temperatures, such as 450°C. This paper will present the results from fatigue and stress rupture testing at this selected temperature. Test data will be shown, as well as fracture surfaces and microstructures from the material before and after testing.

9:00 AM  
Unnotched Fatigue of Inconel 718 Produced by Laser Beam-Powder Bed Fusion at 25 and 600°C: Jayaraj Radhakrishnan1; Punit Kumar2; Shihao Li1; Yakai Zhao1; Upadrasta Ramamurty1; 1Nanyang Technological University; 2Lawrence Berkeley National Laboratory
    The microstructures and mechanical properties of the as-built Inconel 718 alloy, produced using laser beam powder bed fusion (LB-PBF) process, were investigated at room temperature (RT) and 600°C. Fatigue strength (σf) at 600 °C is 23% lower than that at RT due to the lower work hardening rate at 600°C, facilitating easy crack initiation at LOFs that are both favorably located and oriented. However, dynamic recrystallization at the crack tip of the specimens tested at 600°C retards the short fatigue cracks (SFCs) and leads to a substantially higher fatigue life as compared to that at RT. Postmortem analyses were utilized for understanding the initiation and growth mechanisms of SFCs and the roles of plasticity/oxidation induced crack closure on them. It shows that the LB-PBF induced microstructural characteristics such as solidification cells with high dislocation density are effective in resisting the growth of SFCs at 600°C, as compared to RT.

9:20 AM  
Fatigue Crack Growth of Laser Powder Bed Fusion Produced Alloy 718 at Room and Elevated Temperatures: Jamie Kruzic1; Halsey Ostergaard1; 1University of New South Wales (UNSW Sydney)
    Laser powder bed fusion (LPBF) produces unique microstructures (e.g., columnar grains, crystallographic texture, etc.) that influence the mechanical properties of alloys. In this work, the fatigue crack growth properties of LPBF produced alloy 718 were examined for two orthogonal cracking orientations relative to the build direction. Two post heat treatments were used: solution plus duplex ageing both with and without prior hot isostatic pressing (HIP). HIP was found to recrystallize much of the columnar microstructure, remove residual stresses, and mostly eliminate the weak anisotropy observed in the room temperature fatigue crack growth behavior for the non-HIP material. Post-HIP, room temperature fatigue thresholds reached values reported for wrought 718. At 650˚C, anisotropy in fatigue crack growth rates became significant and LPBF 718 performed better than wrought 718. Microstructure and residual stress analysis, fractography, and crack profiles were used to further understand the microstructure influence on the fatigue crack growth properties.

9:40 AM  
On the Fatigue Performance of Additively Manufactured Metamaterials: A Combined Experimental and Simulation Study: Daniel Barba Cancho1; Antonio Vazquez Prudencio1; Conrado Garrido1; Sergio Perosanz1; Massimiliano Casata1; Toby Wilkinson1; 1Universidad Politécnica de Madrid
     Architected metallic metamaterials fabricated by additive manufacturing are called to expand infinitely the variety of available properties observed in bulk alloys. However, the high surface-to-volume ratio of the architected metamaterials due to their intricate geometries and the surface inherited of the AM process is translated to a complex fatigue behaviour when compared with bulk conventional alloys. This is a serious concern in the use of this new class of architected AM materials in technological applications.In this work, this problematic is tackled by a systematic multiscale study of the metamaterial design – microstructure and defects – fatigue properties’ interconnection. Commercial aluminium and titanium alloys processed by selective laser melting are used as base material. By means of combined fatigue experimentation, computational modelling and machine learning approaches, the effect of processing conditions and design geometry on microstructural defects and surface quality is rationalised and connected with the fatigue life of metamaterials.

10:00 AM Break

10:20 AM  Invited
Microstructure-Driven Differences in Fatigue Crack Growth Behavior of Laser Powder Bed Fused Low Alloy Steel Parts for Automotive Applications: Whitney Poling1; Jake Benzing2; Tyson Brown1; Nik Hrabe2; 1General Motors, Global Research & Development; 2National Institute of Standards and Technology
    Additive manufacturing enables low volume part production and flexible manufacturing for the automotive industry. Metal alloys of known, repeatable properties that are resistant to fracture are of importance for vehicle and tooling applications, but additively manufactured parts often contain internal porosity and heterogeneities in the grain structure which reduce mechanical properties and reliability. Post-processing steps are limited to minimize cost leading to potential porosity and microstructure heterogeneity in the final parts. In this work, low alloy steel parts with 0.1% target porosity were manufactured by laser powder bed fusion and subjected to either a stress relief or a quench and temper heat treatment. The effects of these treatments on microstructure (optical microscopy and electron backscattered diffraction with x-ray computed tomography for porosity) and fatigue crack growth behavior (compact tension experiments) will be presented with an emphasis on discussing mechanisms that drive the measured differences in the Paris Law regime.

10:50 AM  
Effects of Heat Treatment on Microstructure of Nickel Silicide Beads Additively Manufactured Using Direct Energy Deposition Technique: Ibrahim Mohammad1; Geir Grasmo1; Ragnhild Aune2; Foysal Kabir Tareq1; 1University of Agder; 2Norwegian University of Science and Technology
    Nickel silicides are well known for their attractive chemical and physical properties, but inadequate room temperature ductility makes them challenging to cast/forge. As Additive Manufacturing (AM) techniques are rapidly evolving to meet the demands of various industries, it is hoped that they can take advantage of the unrivalled opportunities that nickel silicides offer. However, the thermal history of parts produced by AM is quite complex and can lead to micro-cracking due to rapid solidification along with repeated heating and cooling cycles. Based on this, the present study has sought to investigate the effects of heat treatment on printed beads. The Direct Energy Deposition (DED) technique was used to print single beads of the NiSi16 alloy onto a steel substrate, followed by heat-treatments at 4000C, 7000C, and 10000C. The beads were analyzed using OM, SEM, EDS, XRD, and EPMA, and the results discussed in view of the presence of micro-cracking.

11:10 AM  
Micromechanical and Microstructural Characterization of Filigree Additively Manufactured NiTi Structures: Thomas Straub1; Mario Schleyer1; Bernhard Mueller2; Sarah Fischer3; 1Fraunhofer Institute for Mechanics of Materials (IWM); 2Fraunhofer Institute for Machine Tools and Forming Technology (IWU); 3Fraunhofer Institute for Nondestructive Testing (IZFP)
    Additive manufacturing of intricate structures has high requirements concerning manufacturing defects, especially Laser Powder Bed Fusion (LPBF) process results in structures with complex microstructure. While many studies are based on macroscopic samples in the range of several millimeters, mechanical performance of filigree structures have not been studied in great detail, especially for NiTi. This is extremely important as basis for accurate material models due to their complexity. As the mechanisms are based on high strains, the use of linear elastic models yield to wrong property predictions. In this talk, the mechanical properties of cylindrical NiTi structures with diameters in the range of 180 µm to 350 µm manufactured with LPBF will be presented. We report on our findings on micromechanical testing, non-destructive characterization (µ-CT) as well as EBSD, to analyze the effect of feature size and writing strategy on the quasi-static mechanical properties of the specimen.

11:30 AM  
Tensile Properties and Fracture-related Findings of Two NIST AM Benchmark 2022 Challenges: Continuum and Sub-continuum Scales: Orion Kafka1; Nikolas Hrabe1; Jake Benzing1; Newell Moser1; Nicholas Derimow1; Li-Anne Liew1; Jordan Weaver1; Timothy Quinn1; Ross Rentz1; 1National Institute of Standards and Technology
    The NIST AM Benchmark series provide the community with high quality datasets in the form of “challenges” for modelers where selected data is temporarily withheld to enable the comparison of blind predictions. In the 2022 cycle, two tensile deformation challenges were posed: one to predict orientation and scan-strategy dependance of mechanical properties of interest such as Young’s modulus and stress at different points in strain in continuum-scale specimens, and the other to predict microscale (sub-continuum) deformation behavior and fracture location of a single thoroughly characterized specimen given EBSD of one surface and 3D X-ray CT data of the full specimen, and EBSD of a nearby perpendicular plane. This talk summarizes the experimental efforts, data created, and overall findings of the blinded prediction efforts (fully anonymized). We will also ask for audience feedback regarding the Benchmark challenges and discuss potential future challenges that could be developed.