Additive Manufacturing Fatigue and Fracture: Effects of Surface Roughness, Residual Stress, and Environment: Session IV
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 8:30 AM
March 22, 2023
Room: 22
Location: SDCC
Session Chair: John Lewandowski, Case Western Reserve University
8:30 AM Invited
Fatigue Crack Initiation in Additively Manufactured Alloys: Synergistic Effect of Microstructure and Volumetric Defects: Shuai Shao1; Nima Shamsaei1; 1Auburn University
The fatigue behavior of additively manufactured alloys has different sensitivity to the presence of volumetric defects. For instance, fatigue crack initiation in IN718 is often from persistent slip bands while that in Ti-6Al-4V and 17-4 PH stainless steels is almost always from volumetric defects. This presentation focuses on synergistic effects of volumetric defects and microstructure on fatigue behavior of laser beam powder bed fusion fabricated alloys. This synergy is central to address the compromised and/or hard-to-predict structural integrity (such as fatigue properties) of these alloys due to the presence and variability of volumetric defects and the surrounding microstructure. To provide understanding on this synergy, a simulation guided, experimentally validated research effort has been undertaken. The simulations include both linear elastic finite element analysis of cracks surrounding volumetric defects and crystal plasticity modeling of the fatigue crack initiation to assess the influence from microstructure.
9:00 AM
Fatigue Crack Propagation in Additively Manufactured Titanium Alloy with Lamellar and Bi-lamellar Microstructures: Zhiying Liu1; Yu Zou1; 1University of Toronto
Enhancing fatigue crack growth resistance (FCGR) is crucial for engineering applications of additively manufactured titanium alloys. However, the local plastic deformations that govern the fatigue crack growth in additively manufactured titanium alloys with lamellar microstructure are still unclear. Here, we investigate the fatigue crack growth in additively manufactured Ti-6Al-2Zr-Mo-V alloy with lamellar and bi-lamellar α+β microstructures, respectively. The plastic deformation behaviours at the front of cracks include well-known slip banding (slip nucleation and transfer) and unwell-known local shearing in both samples. We reveal a dual role of local shearing in fatigue crack growth: the local shearing direction along and perpendicular to fatigue crack can promote and block its propagation, respectively. The formations of slip activities and local shearing and their influences on the fatigue crack growth in the lamellar and bi-lamellar microstructures are also discussed. This study enhances the understanding of fatigue crack growth in additively manufactured titanium alloys.
9:20 AM
Hot Isostatic Pressing to Increase Isotropic Behavior of Wire DED Ti-6Al-4V: Larico Treadwell1; Jonathan Pegues1; Shaun Whetten1; Tyler Chilson1; 1Sandia National Laboratories
Hot Isostatic Pressing (HIP) can be a useful tool to increase the density of metal Additively Manufactured (AM) parts. This post-process route can have significant impacts on the microstructure and material properties such as strength, ductility, and fatigue resistance. Many metal AM materials, especially those fabricated using wire based directed energy deposition (W-DED) process, have a high degree of anisotropic behavior resulting from the columnar prior microstructure. Titanium based alloys can be highly sensitive to these massive grains as the prior boundaries are primary nucleation sites for grains promoting the formation of continuous across the length of the boundaries. This talk discusses preliminary efforts to utilize HIP as a means to reduce variability in mechanical properties and its effects on the anisotropic behavior of W-DED Ti-6Al-4V products.
9:40 AM
Characterizing Surface Roughness and Linking to Process Parameters in Powder Bed Fusion AM: Srujana Rao Yarasi1; Elizabeth Holm1; Anthony Rollett1; 1Carnegie Mellon University
Additively manufactured surfaces have an inherent surface roughness which may be undesirable. In some applications, it can result in fatigue crack initiation sites while in others it serves to increase surface area available. There are multiple factors that affect surface roughness, including powder size and processing parameters. The measurement of surface roughness is a data rich problem that can benefit from characterization with machine learning techniques. AM rough surface characterization and subsequently, the relationships connecting the processing parameters to it are explored in this study. Build orientation is seen to affect the surface roughness and a Convolutional Neural Network (CNN) is used to distinguish between the differently oriented surfaces. The ability to control surface roughness using process parameters reduces the need for post-processing and furthers our understanding of its impact on mechanical properties.
10:00 AM Break
10:20 AM Invited
Predicting Microstructure-sensitive Fracture Behavior in AM IN625 Using a Damage-enabled Elasto-viscoplastic FFT Framework: Ashley Spear1; Carter Cocke1; Brian Phung1; Laura Ziegler1; Elliott Marsden1; Vignesh Babu Rao1; 1University of Utah
In this work, we use a large-strain elasto-viscoplastic fast Fourier transform (LS-EVPFFT) code enhanced with a continuum damage mechanics model to predict failure response of a subcontinuum mesoscale tensile specimen in the context of the NIST AM Bench 2022 Challenge. In the Challenge, participants were provided with data from X-ray computed tomography and electron backscatter diffraction (EBSD) for an AM IN625 sample and asked to predict stress and strain response and locations of necking and fracture. To account for uncertainty in the subsurface microstructure, 10 semi-synthetic microstructures are instantiated using a Potts model in a modified version of the open-source software SPPARKS. While all 10 models maintain identical surface grain structure, surface roughness, and internal porosity, their subsurface grain structure varies due to randomness in the microstructure-generation procedure. Results from the blind predictions using the LS-EVPFFT framework are compared to the experimental results, and lessons learned will be discussed.
10:50 AM
Predicting the Influence of Inherent Pores on Mechanical Properties of Additive-Manufactured Ti6Al4V via an Empirical Model: Mu Gao1; 1Monash Centre of Additive Manufacturing
In this study, a novel “criticality” parameter was proposed, able to examine the pore state of an AM-ed part from its 2D surface and fracture surface via Optical and Scanning Electron Microscopy. From the porosity and aspect ratio measured and calculated, the mechanical properties can be predicted from simple empirical equation. Other than tensile properties, ductility and Charpy toughness can also be predicted directly from examining individual defects in 2D plane perpendicular to load, saving time and economical cost from μ-CT scan and complex modelling. The usage of criticality, concerning pore shape effect along with porosity, bridged the difference of samples built from different AM parameters in terms of mechanical behaviour due to presence of lack-of-fusion and keyhole pores inherent in AM-ed metal parts, to porosity up to 15%. This study of the relationship between the ubiquituous pores and mechanical properties would contribute to the confident application of AM-ed parts.
11:10 AM
Microstructural Origin of Fatigue Resistance in Additively Manufactured Steels: Punit Kumar1; Jayaraj Radhakrishnan2; Alexis Bryl3; James McKinnell3; Upadrasta Ramamurty2; 1Lawrence Berkeley National Laboratory; 2Nanyang Technological University; 3HP Inc.
The high cycle fatigue behavior of 304L, 316L, 17-4 PH stainless steels produced by laser powder bed fusion and binder jet printing processes were investigated. In conventionally manufactured parts, smaller and larger grains resist the crack initiation and propagation, respectively. However, in additively manufactured (AM) alloys, fatigue crack can easily initiate from pores. Considering this, we examine the effect of grain size on the fatigue resistance of AM steels with various porosity levels. Steels of distinct work hardening behaviors were produced by different AM processes, and hot isostatic pressing (HIP) was employed in certain conditions to reduce porosity. The small and equiaxed grains are beneficial for fatigue resistance; however, HIP is not always sufficient and necessary to improve the fatigue life. The implication of these results in terms of possible directions for designing AM steels with high fatigue resistance will be discussed in detail during the presentation.
11:30 AM Invited
Rapid Qualification of Additively Manufactured Fatigue-Limited Applications via Hybrid Experimental/Model Approach: Amber Andreaco1; Krzysztof Stopka2; Andrew Desrosiers1; Tyler Nicodemus1; Nicholas Krutz3; Michael Sangid2; 1GE Additive; 2Purdue University; 3Timet
Many industries are looking to qualify additively manufactured (AM) metallic materials for fatigue-limited applications. While the underlying sources of fatigue failure can be attributed to various factors, the existence of unintentional porosity is well-established as a leading cause for premature failures, including in AM materials. Historically, substantial experimental testing campaigns are required to characterize the fatigue behavior. The concern arises as to whether the fatigue design allowable encompasses both intentional variation and unintentional variation (e.g., undetectable, off-nominal porosity) to enable safe designs. This talk will summarize the concept of seeding defects, characterizing their morphology and distribution, as well as determining their impact on fatigue behavior, as the basis for model inputs to predict fatigue life. The work is intended to drive a standard practice for rapid qualification of fatigue-limited AM applications via a hybrid experimental/model approach.