Additive Manufacturing Fatigue and Fracture: Effects of Surface Roughness, Residual Stress, and Environment: Session II
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
Tuesday 8:00 AM
March 21, 2023
Room: 22
Location: SDCC
Session Chair: Nima Shamsaei, Auburn University
8:00 AM Invited
Effects of Process Conditions and Flaws/Surface Roughness on Fracture and Fatigue of AM-processed Alloys: John Lewandowski1; 1Case Western Reserve University
It is desirable to use various AM techniques to fabricate replacement parts that are flight critical. In order to continue to gain confidence in AM, it remains important to understand the effects of rogue flaws/surface roughness on properties. The performance of nominal (defect-free) material with machined surfaces is being compared to material with readily detectable surface/sub-surface flaws as well as to material with difficult to detect flaws and/or surface roughness. Ongoing research with collaborators is investigating the effects of purposely introduced defects (e.g. LoF, Keyhole, etc.) on mechanical properties by systematically varying process parameters, including surface roughness. Samples have been fabricated with selective placement of flaws of various types and sizes in addition to nominally defect-free material and tested in either machined or as-deposited conditions. The fracture and fatigue behavior will be reviewed along with quantification of fracture surface details in the context of a defect-based Kitagawa-Murakami-type approach.
8:30 AM
Effects of Scan Strategy Induced Microstructural Differences on Thin-Wall SLM IN718 Fatigue Performance: Connor Varney1; Imran Noor1; Paul Rottmann1; 1University of Kentucky
As-printed microstructures of additively manufactured parts is a function of many variables spanning from scan strategy to part geometry. Elucidation of the complex relationship between geometry, scan strategy, and resultant microstructure is necessary to enable the widespread adoption of AM parts in life-limiting and load bearing applications. In this study a series of IN718 compact tension fatigue samples were printed via SLM using a contour/hatching based scan strategy with both rotated and aligned beam directions, with half having the crack oriented vertically, and half horizontally. Half of each sample type were printed with an internal (~0.5mm) pore to simulate the effects of porosity near the contour/hatching boundary. These samples were tested in LCF, and the tests were monitored through digital image correlation to identify the initiation and propagation of fatigue cracks. After combining with EBSD datasets this will allow for a greater understanding of the processing-properties-performance relationship of AM IN718.
8:50 AM
Effect of Defect Variability in Aluminum Alloys on Ultrasonic Fatigue Performance Across Additive Manufacturing Platforms: Harsha Phukan1; Rob Rhein1; Niloofar Sanaei1; Eric Johnson1; Jason Carroll1; 1Eaton Corporation
The optimization of process parameters is an important step towards realizing component builds across multiple additive manufacturing (AM) platforms. The present work is focused on the quantification of defect variability and fatigue performance of builds across multiple laser powder bed fusion (LPBF) equipment. The study of defect variability is predicated upon the hypothesis that, given process variables yielding a similar population/ type of porosity based on process maps for different LPBF machines, the defect size distribution across the multiple AM platforms should be similar. Aluminum alloy (AlSi10Mg) specimens of cylindrical geometry built on three different LPBF machines are characterized using computed tomography (CT). The distributions of defect size are compared using Kolmogorov-Smirnov (K-S) tests. Fatigue properties of the builds from different LPBF platforms are quantified using ultrasonic fatigue (USF) under fully reversed loading conditions. Finally, fatigue performance is correlated to the defect distribution in builds across the three platforms.
9:10 AM Invited
The Influence of Surface Finish and Build Orientation on the Low Cycle Fatigue Behaviour of Laser Powder Bed Fused Stainless Steel 316L: Robert Lancaster1; William Beard2; Thomas Jones2; Nicholas Barnard1; 1Swansea University; 2Rolls-Royce
Additive manufacturing (AM) is under consideration for marine based components due to the numerous benefits the techniques offer. However, there are multiple challenges associated with the introduction of AM-parts into safety critical applications related to their mechanical behaviour. One of the main factors influencing a component’s cyclic performance is the surface finish. As-built AM-parts exhibit a high surface roughness, owing to the layer-by-layer nature of the AM-process and the presence of partially-bonded powder particles and sharp rooted surface features that can vary in severity, promoting surface-breaking stress-raising sites. This behaviour is further influenced by the build orientation of the component, with alternative orientations providing a different surface profile due to the nature of scaffolding, alongside a contrasting microstructural morphology. This research will explore the LCF behaviour of LPBF stainless-steel 316L built in different build orientations and subjected to several post-manufacture finishing procedures to identify the optimal condition for mechanical performance.
9:40 AM Break
10:00 AM
Isolated Influence of Upward and Downward Facing Surface Roughness on the Fatigue Life of Laser Powder Bed Fusion Ti-6Al-4V
: Jason Rogers1; Martin Leary1; Ma Qian1; Chris Wallbrink2; Joe Elambasseril1; 1RMIT University; 2Defence Science Technology
A fundamental relationship between surface roughness, inclination angle and fatigue strength has not been established for laser powder bed fusion (L-PBF) Ti-6Al-4V. Not all L-PBF components can be post-processed to address the inherently rough surface finish, including lattice and hollow structures. However, surface roughness is understood to be detrimental to the fatigue life of L-PBF components. Therefore, a thorough understanding of the impact of inclination angle on surface roughness and consequently, fatigue life is required for the design of fatigue-limited components with as-manufactured surfaces. This work aims to predict the fatigue strength of a component based on the manufactured inclination angle and expected surface roughness. This is derived from the experimental fatigue data of L-PBF Ti-6Al-4V samples manufactured at six different inclination angles with an as-manufactured surface finish. In each fatigue test, either the upward or downward facing surface was polished to isolate the influence of corresponding roughness.
10:20 AM
Effect of Microstructure and Surface Roughness on Fatigue Behavior of PBF-LB 316L Stainless Steel: Garrison Hommer1; Jorge Ramirez Lujan1; Simon Richardsen1; Jonah Klemm-Toole1; Joy Gockel1; 1Colorado School of Mines
Understanding the effects of geometric defects and microstructure on the fatigue of 316L stainless steel manufactured by laser powder bed fusion (PBF-LB) additive manufacturing (AM) is critical for structural applications. Surface treatments are expensive, time consuming and not all printed surfaces are easily accessible after PBF-LB. However, surface features are often the dominating location of fatigue crack initiation. Prior studies investigating surface roughness effects consider only total fatigue life, which is influenced by both surface roughness and microstructure. Thus, the actual effect of surface roughness is convoluted with microstructural influence. Fatigue results will be presented where specimens were tested in both as-built and heat-treated microstructure conditions with as-printed surfaces. The influence of surface roughness and microstructure on decoupled fatigue crack initiation and growth is determined. An understanding of the separated influence of surface roughness and microstructure provides guidance towards parameter development, monitoring needs and qualification requirements for critical AM components.
10:40 AM
Post-Processing Surface Finishing of Additively Manufactured Aluminum and Titanium Alloys for Optimal Mechanical and Corrosion-Resistance Performance: Joshua Boykin1; Agustin Diaz1; Patrick McFadden1; Justin Michaud1; 1REM Surface Engineering
Metal additive manufacturing (AM) is revolutionizing many industries, promising production time/cost reductions, complex design/parts, and advanced lightweight materials, such as aluminum- and titanium-based alloys. However, as-built metal AM components have considerable surface and near-surface defects: such as roughness, waviness, porosity, partially sintered powders, etc. These defects reduce component mechanical performance, leading to early failure and use risks. The work presented herein adapts technology developed for specific use on various aluminum and titanium alloys. This surface finishing technology is a combination of chemical polishing and chemical-mechanical polishing able to remediate all surface-related defects and impart significant mechanical and corrosion-resistance improvements. The developed process can uniformly remove up to 500 μm, achieving a defect-free surface with a submicron surface roughness Ra/Rq. Furthermore, with exceptional outcomes, chemical polishing has been applied for surface finishing internal channels and non-line-of-sight surfaces. Part of this work was funded by USAF (SBIR-FA864921P0815 and SBIR-FA864920P0930).
11:00 AM
Effect of a Build Stop on the Fatigue Behavior of Laser Powder Bed Fusion 316L Stainless Steel with As-printed Surfaces: Simon Richardsen1; Jorge Ramirez Lujan1; Charles Smith1; Garrison Hommer1; Jonah Klemm-Toole1; Joy Gockel1; 1Colorado School of Mines
Build stops can occur during metal additive manufacturing (AM) with laser power bed fusion (PBF-LB) for a variety of reasons such as power outage, insufficient gas flow, or sensor failure. It is economically desirable to continue a build after the issue is resolved. However, the effect on part quality, such as microstructure, porosity, surface roughness and geometric features is not well understood. This study considers parts fabricated with a 2-hour build stop. Quantitative comparisons of the dimensions, as-printed surface features, microstructure, porosity, and fatigue performance are determined. Based on comparison to prior work, build stop effects are highly dependent on the geometry and part layout on the build platform. Geometric deviation from part shift during the build stop location is significant, however other anomalies inherent to AM can still dominate failure. Understanding the effect of material changes from a build stop can help reduce scrap from unintended build interruptions.