Phase Transformations in Additively Manufactured Materials: Additive Manufacturing - Phase Transformations - In-situ and Ex-situ Experimental Work
Sponsored by: TMS: Phase Transformations Committee
Program Organizers: Antonio Ramirez, Ohio State University; Ashley Paz y Puente, University of Cincinnati; Matthew Steiner, University of Cincinnati; Vijay Vasudevan, University of North Texas; Bij-Na Kim, Lancaster University/LPW Technology; Eric Lass, National Institute of Standards and Technology
Monday 8:00 AM
November 2, 2020
Room: Virtual Meeting Room 7
Location: MS&T Virtual
Session Chair: Antonio Ramirez, The Ohio State University; Ashley Paz y Puente, University
8:00 AM Invited
Understanding the Effect of Thermal Gradients on Additively Manufactured (AM) Builds Using In Situ TEM: Sriram Vijayan1; Meiyue Shao1; Joerg Jinschek1; 1The Ohio State University
Complex additive manufacturing (AM) process conditions typically result in parts with anisotropic microstructures and sub-optimal mechanical performance. The cyclic deposition of new layers using electron beam powder bed fusion (EB PBF) exposes ‘previously deposited’ layers to directional reheating, large thermal gradients and rapid cooling rates. Previous studies of solid-solid phase transformations in Ti-6Al-4V (Ti64) have aided in optimizing processing routes to yield Ti64 with desirable mechanical properties. However, such transformations in Ti64 under AM process conditions are not yet well understood. Here, we use a micro-heater device to simulate AM-like process conditions (106 K/m and 103K/s) across a Ti64 sample inside the transmission electron microscope (TEM). In this study, heating experiments were performed to identify whether V diffusion is the governing process in the α’ → β → α’ + β transformation under large thermal gradients and rapid thermal cycling in Ti64 inside the TEM at high spatial resolution
8:40 AM
Laser Powder Bed Fusion of Grade 300 Maraging Steel for Tooling Applications: Peeyush Nandwana1; Rangasayee Kannan1; Andrew Nguyen1; Donovan Leonard1; Ryan Dehoff1; 1Oak Ridge National Laboratory
Maraging steels, owing to the presence of the softer martensite, high dimensional stability, and the ability to be precipitation hardened to high strengths make for excellent tooling materials via laser powder bed fusion (LPBF) for injection molding applications. The softer nickel enriched martensite prevents solid-state cracking resulting from the high cooling rates experienced during LPBF. In this study we discuss the as-fabricated microstructure of a Ti-free Grade 300 maraging steel and demonstrate that direct aging results in higher strength and higher elongation compared to the conventional solution treatment and aging. The underlying phase transformations will be discussed.
9:00 AM Cancelled
Nitrogen Effects in Additively Manufactured Martensitic Precipitation-hardenable Stainless Steels: Eric Lass1; 1University of Tennessee, Knoxville
The effects of nitrogen in microstructure in additively manufactured (AM) precipitation-hardenable stainless steels 17-4 and 15-5 is investigated. Residual nitrogen found in N2-atomized 17-4 powder feedstock dramatically affects phase stability and microstructural evolution. The as-built microstructure of N2-atomized AM 17-4 contains up to 90 % or more retained austenite. On the other hand, as-built Ar-atomized AM 17-4 can contain 90 % of more delta-ferrite and <5 % retained austenite; while N2-atomized 15-5 can exhibit a third distinct microstructure consisting of 95 % or more ferrite/martensite and a fine equiaxed grain structure. Differences between the AM solidification behavior of these alloys are discussed in terms of solidification phase selection maps. Computational thermodynamic modeling is employed to describe the observed effects of nitrogen on phase stability, and to identify optimized post-build thermal processing protocol to minimize the effects of residual nitrogen and identify maximum allowable nitrogen content in these materials.
9:20 AM
Process Optimization and Microstructure Analysis to Understand Laser Powder Bed Fusion of Stainless Steel 316L: Nathalia Diaz Vallejo1; Cameron Lucas1; Nicolas Ayers1; Holden Hyer1; Brandon McWilliams2; Kyu Cho2; Yongho Sohn1; 1University of Central Florida; 2US Army Research Laboratory
Microstructural development and mechanical behavior of 316L stainless steel was investigated over a wide range of laser powder bed fusion (LPBF) parameters (laser power, scan speed, hatch spacing, and slice thickness). The starting powder had a size distribution of D10 = 22 um; D50 = 36 um; D90 = 50 um. SLM 125HL was employed for LPBF. Use of energy density between 46 and 127 J/mm3 produced nearly fully dense (≥ 99.8 %) samples, and this included the best parameter set: power = 200 W; scan speed = 800 mm/sec; hatch spacing = 0.12 mm; slice thickness = 0.03; energy density = 69 J/mm3). Microstructural features including melt pool and cellular structure were quantified to better understand the LPBF process. Using the optimized LPBF parameters aforementioned, the as-built 316L had, on average, yield strength of 572 MPa, tensile strength of 710 MPa and 48% strain at failure.
9:40 AM
In-situ Characterization of Rapid Phase Evolution of AM Metals with High Energy Synchrotron X-ray Diffraction: Seunghee Oh1; Rachel Lim1; Joseph Aroh1; Joseph Pauza1; Runbo Jiang1; Venkata Satya Surya Amaranth Karra1; Sidi Feng1; Andrew Chuang2; Joel Bernier3; Benjamin Gould2; Robert Suter1; Anthony Rollett1; 1Carnegie Mellon University; 2Argonne National Laboratory; 3Lawrence Livermore National Laboratory
Laser-based processing in additive manufacturing (AM) is accompanied by rapid heating and cooling, resulting in abrupt changes in phase and microstructure. Using an in-situ high-speed high-energy synchrotron X-ray dynamic diffraction technique built with a 520W fiber laser system coupled with a scan head at 1-ID-E at the Advanced Photon Source provides key insights about AM processing. The fast acquisition rate, 250Hz, allows us to observe transient phenomena in phase formation, which were not previously accessible. A Pilatus area detector, which can capture whole diffraction rings, provides intuitive and quantitative information for quantifying the evolution of the phases during the AM process. Data processing was utilized to quantify several aspects, including phase fractions and microstrain. Thermal history was inferred from thermal expansion. Material in the fusion zone develops tensile residual strain, as expected. Unexpected precipitation behavior is observed, however. Time-resolved descriptions were used to represent the various trends.
10:00 AM
Effects of Laser Powder Bed Fusion Parameters and Heat Treatment
on Microstructure and Mechanical Behavior of Inconel 718 Alloy: Thinh Huynh1; Abhishek Mehta1; Sharon Park1; Holden Hyer1; Le Zhou1; Devin Imholte2; Nicolas Woolstenhulme2; Daniel Wachs2; Yongho Sohn1; 1University of Central Florida; 2Idaho National Laboratory
Excellent weldability and high temperature stability make IN718 one of the most desired alloys to be additively manufactured via laser powder bed fusion (LPBF). In this study, gas-atomized IN718 powders were used to fabricate IN718 samples by LPBF with independently varied laser power (125 - 350W), scan speed (200 - 2200 mm/s), and scan rotation (0 - 90°). In general, relative density greater than 99.5% was accomplished using the energy density range of 50-100 J/mm3; furthermore, a larger processing window was observed at higher laser power. Microstructural features including key holes, lack of fusion flaws, melt pool geometry and grain structure were documented and correlated to the LPBF parameters explored. Various heat treatments were carried out to observe the effects of solutionizing on the formation of δ-phase, the γ' and γ" precipitation, and mechanical properties of LPBF IN718 after the two-step aging process.