Additive Manufacturing: Solid-State Phase Transformations and Microstructural Evolution: In Situ Characterisation and Material Response to Build Processes
Sponsored by: TMS Materials Processing and Manufacturing Division, TMS: Additive Manufacturing Committee, TMS: High Temperature Alloys Committee, TMS: Phase Transformations Committee
Program Organizers: Bij-Na Kim; Andrew Wessman, University of Arizona; Chantal Sudbrack, National Energy Technology Laboratory; Eric Lass, University of Tennessee-Knoxville; Katerina Christofidou, University of Sheffield; Peeyush Nandwana, Oak Ridge National Laboratory; Rajarshi Banerjee, University of North Texas; Whitney Poling, General Motors Corporation; Yousub Lee, Oak Ridge National Laboratory
Thursday 2:00 PM
March 18, 2021
Room: RM 5
Location: TMS2021 Virtual
Session Chair: Katerina Christofidou, The University of Sheffield; Bij-Na Kim, Carpenter Additive
2:00 PM
In Situ Synchrotron Observation of Directed Energy Deposition Additive Manufacturing Process: Yunhui Chen1; Samuel Clark1; David Collins2; Sebastian Marussi1; Thomas Connolley3; Robert Atwood3; Oxana Magdysyuk3; Gavin Baxter4; Martyn Jones4; Chu Lun Alex Leung1; Peter Lee1; 1University College London; 2The University of Birmingham; 3Diamond Light Source; 4Rolls-Royce plc
The mechanical performance of Directed Energy Deposition Additive Manufactured (DED-AM) components are highly process condition dependent due to rapid laser induced heating and cooling rates. However, the fast non-equilibrium solidification process during DED-AM is not easily measured with conventional methods. Through combined in situ synchrotron imaging and diffraction using a unique DED-AM process replicator implemented at a synchrotron beamline, we experimentally reveal the underlying mechanical and phase transformation behaviour which controls the build quality in IN718, a nickel based super-alloy widely used for safety critical components. Quantification of the solidification sequence was performed to reveal the residual stress development in a high temperature region is thermal dominant. The observed transformation behaviour during DED-AM newly relates the AM processing conditions to the eventual microstructures and properties. The results presented in this work provide an enhanced fundamental understanding of the DED-AM process with relevance to microstructure control in AM fabricated components.
2:20 PM
In-situ TEM Solid-state Thermal Cycling of a Stainless Steel Fabricated via AM: Manas Upadhyay1; Lluis Yedra-Cardona2; Eva Héripré3; Simon Hallais1; Alexandre Tanguy1; 1LMS, CNRS, Ecole Polytechnique, Institut Polytechnique de Paris; 2MSSMat and SPMS, CNRS, CentraleSupeléc, Université Paris-Saclay; 3MSSMat, CNRS, CentraleSupeléc, Université Paris-Saclay
Currently, most experimental/modeling efforts are aimed at studying the role of melt-pool dynamics and rapid solidification on microstructure formation during additive manufacturing (AM). However, we are interested in studying the solid-state microstructural evolution occurring after solidification, i.e. during solid-state thermal cycling (SSTC) due to the continuation of the building process; large thermo-mechanical driving forces are generated during SSTC which can result in important microstructural changes. In this talk, some novel results from a series of in-situ TEM SSTC experiments performed on an AM steel will be presented. The motivation behind these tests comes from the intractability of following in-situ microstructural changes during AM within the processing chamber. Instead, we reproduce the same SSTC, that the material would experience during AM, inside a TEM to facilitate in-situ observations. The in-situ TEM experiments provide novel insight on the changes induced by SSTC to the submicron-sized precipitates and dislocation structures.
2:40 PM
Time-resolved Synchrotron X-ray Diffraction Studies of Phase Evolution in Ni alloy 718 during Laser Melting: Seunghee Oh1; Rachel Lim1; Joseph Aroh1; Joseph Pauza1; Andrew Chuang2; Benjamin Gould2; Joel Bernier3; Tao Sun4; Robert Suter1; Anthony Rollett1; 1Carnegie Mellon University; 2Argonne National Laboratory; 3Lawrence Livermore National Laboratory; 4University of Virginia
Laser-based additive manufacturing is accompanied by rapid changes in microstructure, phase, and temperature due to its extreme heating and cooling conditions. In this study, in-situ synchrotron X-ray diffraction was combined with a 520W fiber laser allowing for time-resolved observations of phase evolution in Inconel 718 upon cooling. Due to the high temporal and spatial resolution in the measurement, the rapid evolution of the melted region beneath the surface of the sample can be captured. By analyzing asymmetric peak based on an assumption of overlapped peaks from γ, γ’ and γ”, the relative phase fractions can be quantified. The γ’ and γ” lattice parameters show diverging behavior upon cooling, but they are similar to γ at high temperatures. The evolution of secondary phases is characterized, despite their weak diffraction intensity, because of the relatively small phase amount. Moreover, the thermal history profile was approximated from thermal expansion.
3:00 PM
The Effects of Scanning Strategy on Cracking and Grain Structure of the IN738LC Superalloy Produced by Selective Laser Melting: Marcus Lam1; 1Monash University
Selective laser melting (SLM) of high-strength nickel-base superalloys such as IN738LC can significantly shorten the development cycle and simplify the production process of gas turbine components. The high crack susceptibility of these alloys can however be challenging, especially on the surfaces of complex geometries where the cracks cannot be effectively closed or removed. In this work, the study of various scanning strategies revealed significant differences in their crack susceptibilities. More importantly, the cracking was found to be related to the grain structures produced during the solidification process. The result indicates a general trend in crack minimization by producing certain grain textures through better controlling the laser scanning paths. The practical limits in exploiting this microstructural trend was also demonstrated in terms of buildability. The findings from this research can lead to better scanning strategy design to mitigate the surface cracks and optimizing the grain structure.
3:20 PM Cancelled
Aging Effects on Phase Transformation and Microstructure Evolution in Selective Laser Melted NiTi Shape Memory Alloy: Madhavan Radhakrishnan1; Sayed Saghaian2; Mohammadreza Nematollahi3; Keyvan Safaei3; Osman Anderoglu1; Mohammad Elahinia3; Haluk Karaca2; 1University of New Mexico; 2University of Kentucky; 3University of Toledo
The study presents the effects of post processing on the microstructure and shape memory behavior of NiTi alloys fabricated via Selective Laser Melting (SLM) method. A detailed TEM study was conducted to understand and relate the microstructure (pore formation, martensite morphology, precipitation characteristics) to the shape memory behavior upon aging. Thermal cycling under constant stress and superelasticity tests were conducted to characterize the mechanical behavior of the aged alloys. It is found that aging increases the transformation temperatures, decreases the hysteresis and significantly improves the strength of the samples. Perfect superelastic behavior with fully recoverable strain of 7% and superelastic window of about 100ºC was observed for aged SLM samples. It is concluded that aging can be used to tailor the microstructure and properties SLM NiTi samples to have properties comparable or even better than to those fabricated by conventional methods.
3:40 PM
Study of the Role of Beam Scan Strategies on the Microstructure and Mechanical Properties of EBM Additively Manufactured Ti-6Al-4V Builds: Meiyue Shao1; Sriram Vijayan1; Sabina Kumar2; Sudarsanam Babu2; Joerg Jinschek1; 1The Ohio State University; 2University of Tennessee
In electron-beam-melting powder-bed fusion (EBM-PBF) process, variations in beam scan strategies allow localized control over the thermal signatures (thermal gradients and cooling rates) in the material during the build process, thereby controlling the local microstructure in additively manufactured (AM) builds. Here, we compare the effect of three different EBM scan strategies (raster scan, spot-ordered fill and random spot fill strategy) to build three identical 15 x 15 x 25 mm3 Ti-6Al-4V blocks. The microstructure and texture throughout the build were analyzed using scanning electron microscopy (SEM) and electron backscatter diffraction (EBSD). The analysis revealed statistically significant variations in alpha lath thickness, prior beta grain size and distribution of α/α grain boundaries (GBs). Site specific microhardness measurements were correlated with the microstructural features from the corresponding region. Furthermore, the structure and composition of the dominant α/α GBs observed in the three Ti-6Al-4V blocks were characterized using scanning/transmission electron microscopy (S/TEM).
4:00 PM
Microstructural Control and Refinement in DMLS Ti-6Al-4V: Matthew Vaughn1; Justin Unger1; Matthew Dunstan2; Andrew Gaynor2; Brandon Mcwilliams2; James Guest1; Kevin Hemker1; 1Johns Hopkins University; 2Army Research Laboratory
Metal additive manufacturing (AM) processes often generate heterogeneous and undesirable microstructures due to their far-from-equilibrium quench rates. However, microstructural control in Ti-6Al-4V has previously been demonstrated using a hydrogen mediated heat treatment as a method to improve mechanical properties. Thermo-Hydrogen refinement of microstructure (THRM) has enabled homogenized microstructures at lower processing temperatures than traditional methods, through a hydrogenation process that lowers the β transus temperature. Here we present a systematic study of post-processing heat treatments to determine the effect on the microstructure. AM bars were fabricated using direct metal laser sintering and were grouped into different post-processing conditions, including common processes like HIP and aging treatments, then subjected to mechanical testing and microstructural characterization. Our findings revealed that the THRM process was able to tailor the microstructure and improve mechanical performance over standard AM Ti-6Al-4V post-treatments.