2023 Annual International Solid Freeform Fabrication Symposium (SFF Symp 2023): Materials: Metals-Creep and Fatigue
Program Organizers: Joseph Beaman, University of Texas at Austin
Wednesday 8:00 AM
August 16, 2023
Room: 615 AB
Location: Hilton Austin
Session Chair: Joy Gockel, Colorado School Of Mines
8:00 AM
Evolution of Creep Damage of 316L Produced by Laser Powder Bed Fusion: Alexander Ulbricht1; 1Bundesanstalt fuer Materialforschung und -pruefung
Laser powder bed fusion (PBF-LB/M) tends to produce unique microstructures arising from rapid, directional cooling. These microstructures impact the damage mechanisms of metallics components manufactured by PBF-LB/M compared to conventionally manufactured variants of the same alloys. In particular, we present results of a study of the evolution of creep damage in stainless steel 316L specimens produced by PBF-LB/M. We used X-ray computed tomography to unravel the influence of the process-specific microstructure from the influence of the initial void distribution on creep damage mechanisms. To that end, the void distribution specimens tested at 600 °C and 650 °C was analyzed before the creep test, after an interruption, and after rupture. We conclude that the formation of damage is not connected to the initial void distribution. Instead, an accumulation of damage at grain boundaries resulting from extensive intergranular cracking is observed. We compared intergranular damage of PBF-LB/M/316L to hot rolled 316L.
8:20 AM
Effects of Surface Roughness, Process Parameters, and Heat Treatments on the Fatigue Properties of Laser Powder Bed Fusion of 316L Stainless Steel: Jorge Ramirez Lujan1; Joy Gockel1; 1Colorado School of Mines
This presentation will discuss how surface roughness, process parameters, and heat treatments affect the fatigue properties of 316L stainless steel produced by Laser Powder Bed Fusion (PBF-LB). The experimental methods and results will be highlighted, including microstructure analysis, surface roughness measurements, and fatigue testing. Previous studies have only considered total fatigue life, which is influenced by both surface roughness and microstructure, making the actual effect of surface roughness convoluted with microstructural influence. The presentation will cover fatigue results of specimens tested in both as-built and heat-treated microstructure conditions with as-printed and treated surfaces. The findings suggest that these factors significantly affect the fatigue properties of 316L stainless steel produced by PBF-LB. Understanding the separated influence of surface roughness and microstructure provides guidance toward processing development and qualification requirements for critical additive manufacturing components.
8:40 AM
Effect of Heat Treatment on High-cycle Fatigue of Additively Manufactured 17% Chromium 4% Nickel Precipitation Hardened Stainless Steel: Julianna Posey1; 1University of Maryland
Precipitation hardenable stainless steels have been identified for applications that require increased corrosion resistance and high strength capabilities. Alloyed 17% Chromium - 4% Nickel (17-4 PH) rectangular block structures were manufactured using an EOS M290 powder bed fusion AM system. Fatigue specimens are selectively harvested in horizontal- and vertical-to-build-direction orientations. Typically, this alloy undergoes a standard solutionizing and annealing heat treatment prior to industrial implementation. In this case, the melt pool and grain boundary interactions through lower-than-standard temperature heat treatments are documented. Resulting fatigue behavior in the high-cycle regime is examined. Microstructural characteristics in heat-treated AM steel are explored from the lens of fatigue behavior prediction and the unique fracture mechanics inherent in AM components.
9:00 AM Cancelled
Fatigue Strength Prediction of Additively Manufactured 17-4 PH Stainless Steel Specimens with Various Geometries: Jade Welsh1; Indrajit Nandi2; Jutima Simsiriwong1; Shuai Shao2; Nima Shamsaei2; 1University of North Florida; 2Auburn University
It is known that the shape and size of additively manufactured parts can significantly affect their volumetric defect population/structure, which subsequently influences their fatigue resistance. In this study, a defect-based model developed on a fracture mechanics concept is utilized to predict the fatigue strength of 17-4 precipitation-hardened (PH) stainless steel (SS), manufactured via laser-powder bed fusion (L-PBF). Uniaxial fully-reversed fatigue experiments are conducted on L-PBF and wrought 17-4 PH SS with various specimen geometries using conventional and ultrasonic fatigue test setups. Based on the fractography analysis, crack initiation mechanisms, as well as the size and location of the volumetric defects responsible for crack initiation are obtained, to explain the fatigue behavior and utilize in the defect-based fatigue model. The applicability of the utilized model is validated by comparing the predicted fatigue strengths with the ones obtained experimentally.
9:20 AM Cancelled
A Vibration Fatigue Life Prediction Model for Additively Manufactured Metal Components: Lei Yan1; Wei Gao1; Nagaraja Iyyer2; Meng Jia1; 1Nanjing University of Aeronautics and Astronautics; 2Technical Data Analysis, Inc.
Fatigue life prediction accuracy has been an issue that hinders the adoption of laser powder bed fusion (LPBF) processed metal components to fatigue critical applications. In the present work, a vibration fatigue life model considering internal defects and microstructure is proposed and validated with LPBF-processed SS316 double-notch beams. A few AA7075 and AA2024 conventional CNC double notch beams were also included to check out the vibration fatigue performance and narrow down the critical factors that downgrade LPBF components' fatigue performance. The developed model brings new ideas to the LPBF components qualification and certification research.
9:40 AM Break
10:10 AM
Metal Additive Manufacturing of Rollers for Railroad Bearings: Rolling Contact Fatigue Performance: Joseph Turner1; Luz Sotelo2; Allen Fuller3; Cody Pratt1; Boone Gray1; Nathanial Matz1; Guru Madireddy1; Rakeshkumar Karunakaran2; Michael Sealy2; Timothy Liebe3; 1University Of Nebraska-Lincoln; 2Purdue University; 3Amsted Rail Brenco
We report on progress with respect to rolling contact fatigue (RCF) performance of railroad bearing rollers manufactured from 8620HC steel powder with laser powder bed fusion (LPBF). The print parameters were first studied using several measurements: material density, optical microscopy, ultrasound, and residual stress. Then a set of tapered cylindrical rollers was manufactured and integrated within a railroad tapered roller bearing. The bearings were subjected to accelerated fatigue and a simulated service life test with RCF typical of in-service bearings (~250,000 equivalent miles). After these tests, the AM rollers were removed, inspected, and compared with conventionally manufactured rollers subjected to the same loading conditions. The performance of the AM rollers was judged to be in-line with rollers manufactured using traditional methods. These results provide a clear foundation for additional AM roller designs that can exploit the unique capabilities of AM for challenging applications associated with fracture and fatigue performance.
10:30 AM
Uniaxial Fatigue Behavior and Life Prediction of Additively Manufactured Inconel 718 with Different Grain Morphology: Muztahid Muhammad1; Sajith Soman1; Nabeel Ahmad1; Douglas Wells2; Shuai Shao1; Nima Shamsaei1; 1Auburn University; 2NASA Marshall Space Flight Center
Understanding the fatigue behavior and failure mechanisms is essential for qualifying and standardizing additively manufactured metallic components. This study investigates the uniaxial fatigue behavior and failure mechanisms of laser powder bed fused (L-PBF) Inconel 718 (IN718) specimens with different grain sizes obtained by altering the process parameters and heat treatment. Uniaxial, fully-reversed, and strain-controlled fatigue tests were conducted on specimens with machined and polished surface conditions. Microstructural analysis and fractography using a scanning electron microscope were performed to measure the sizes of grains and facets. Fatigue cracks were initiated at the persistent slip bands near or at the surfaces rather than process-induced volumetric defects in all cases. The fatigue behavior of L-PBF IN718 specimens was correlated with the sizes of grains and facets. Fatigue life estimation incorporating the NASGRO equation and √area of the grain sizes was performed and shown to predict fatigue life within scatter bands of five.
10:50 AM
Corner Effects on Fatigue for As-printed Surfaces in Alloy 718: Rachel Tullis1; Joy Gockel2; Nathan Klingbeil1; Luke Sheridan3; 1Wright State Universtiy; 2Colorado School of Mines; 3Air Force Research Laboratory
Component geometry is an important factor in the life of parts subjected to repeated loadings. Prior studies have shown that cracks initiate at sharp corners in most rectangular fatigue specimens. This is likely due to the large stress intensity factors of flaws that are located on sharp corners, although the extent of this relationship has not yet been studied for additively manufactured metals with as-printed surfaces. Using four different cross-sectional geometries (squares, hexagons, octagons, circles), this work has investigated the relationships between sharp corners and fatigue life in as-printed additively manufactured nickel superalloy 718. Additionally, fracture surface analysis has been performed to determine the location of crack initiation for each test, allowing for further investigation of the fatigue crack mechanisms in each specimen. The results of this work highlight the effects of cross-sectional geometry in as-printed additively manufactured parts, providing guidelines for the design of components under fatigue loading conditions.
11:10 AM
The Effect of Near-surface Pores on Fatigue Life of Ti-6Al-4V Components: Christine Cummings1; Jayme Keist1; Edward Reutzel1; 1Pennsylvania State University
The assurance that parts will not fail prematurely is one of the largest obstacles to the widespread adoption of additive manufacturing for use in critical applications. Despite finely tuned processing parameters, pores can form stochastically during Laser Powder Bed Fusion (LPBF). A better understanding of the smallest pore that may initiate premature failure informs on the resolution required to ensure that failure-critical pores are identified during inspection. This work investigates the effect of pore size and shape on fatigue life of unHIP’d Ti-6Al-4V specimen manufactured via LPBF. Intentional near-surface pores with varying sizes and aspect ratios were included programmatically in fatigue specimen. The results quantify the effect of pores and begin to determine a maximum allowable pore size. Additionally, within a sub-selection of fatigue specimen, in-situ repair was attempted by increasing the laser power on the layer immediately following the pore. These results indicate how repair may impact fatigue properties.
11:30 AM
Comparing the Effects of Different Chemical Treatments on Surface Texture and Fatigue Behavior of U-notched Additive Manufactured AlSi10Mg: Nabeel Ahmad1; Erfan Maleki1; Shuai Shao1; Nima Shamsaei1; 1Auburn University
Manufactured parts by laser powder bed fusion (L-PBF) with complex geometries, such as notched ones, have high surface anomalies due to the presence of overhangs which can detrimentally affect the mechanical properties and fatigue behavior. Therefore, applying post-processing can play a crucial role in addressing these issues leading to improved mechanical properties. In this study, the effects of different chemical surface post-treatments of chemical polishing (CP), chemical brightening (CB), and electro-chemical polishing (ECP) as well as T6 heat treatment (HT) on the surface texture, microstructure, and rotating bending fatigue behavior U-notched L-PBF AlSi10Mg specimens were investigated comprehensively through various experiments. The obtained results indicated that chemical treatment was very influential on surface texture modification of the very narrow notched parts, resulting in considerable fatigue life improvement.