Additive Manufacturing of Metals: Applications of Solidification Fundamentals: In Situ Characterization
Sponsored by: TMS Materials Processing and Manufacturing Division, TMS: Additive Manufacturing Committee, TMS: Solidification Committee
Program Organizers: Alex Plotkowski, Oak Ridge National Laboratory; Lang Yuan, University of South Carolina; Kevin Chaput, Northrop Grumman; Mohsen Asle Zaeem, Colorado School of Mines; Wenda Tan, The University of Michigan; Lianyi Chen, University of Wisconsin-Madison

Tuesday 8:30 AM
March 16, 2021
Room: RM 4
Location: TMS2021 Virtual

Session Chair: Mohsen Aisle Zaeem, Colorado School of Mines; Lang Yuan, University of South Carolina


8:30 AM  Invited
Characterization of Material Solidification Behaviors in Laser Powder Bed Fusion Using Operando Synchrotron X-ray Imaging: Tao Sun1; Lianyi Chen2; 1University of Virginia; 2University of Wisconsin-Madison
    In laser powder bed fusion (LPBF), a high-power laser scans across the powder layer with high velocities. This creates extreme thermal conditions (i.e. high temperature, fast cooling rate) and complex multi-phase dynamics (i.e. strong melt flow, high recoil and vapor dynamic pressures). As a result, LPBF-processed materials possess unique microstructures and thereby different mechanical properties than casted or wrought samples. Understanding the solidification behaviors of materials in LPBF requires the quantitative characterization of the solidification velocity and thermal gradient. At the Advanced Photon Source, we applied high-speed x-ray imaging to probe the dynamics of the keyhole and melt pool with micrometer spatial resolution and single x-ray pulse temporal resolution (i.e. ~100 ps). The result reveals that the solidification line (i.e. rear melt pool boundary) exhibits interesting morphological oscillation, driven by the keyhole fluctuation and melt flow, which leads to different solidification velocities at different depth of the melt pool.

9:00 AM  
In-situ High-speed X-ray Diffraction Study of Phase Transformation in a Laser-Processed 420 Stainless Steel: Xuan Zhang1; Andrew Chihpin Chuang1; Meimei Li1; 1Argonne National Laboratory
    Steels that are martensitic in conventional forms are often found to contain a certain amount of austenite when manufactured by laser additive manufacturing (AM) processes. The amount of retained austenite is known to vary with the processing parameters, but in-depth understanding is lacking. In this study, we use high-speed synchrotron x-ray diffraction to record the melting and solidification processes during the laser scanning of a 420 martensitic stainless steel under different laser parameters. The transformations have been observed to happen in multiple stages: in the ultra-high temperature regime, the phase transformation follows primarily the equilibrium phase diagram; when cooling down to the martensite start temperature, the martensitic transformation happens in a continuous or step-wise manner depending on the laser parameters. This study provides deep insights into the AM of martensitic steels.

9:20 AM  
In-situ Observation of Ferritic vs Austenitic Solidification Mode Competition in 316L Laser Powder Bed Fusion Welds with Synchrotron X-ray Diffraction: Joseph Aroh1; Seunghee Oh1; Rachel Lim1; Benjamin Gould2; Andrew Chuang2; P. Chris Pistorius1; Anthony Rollett1; 1Carnegie Mellon University; 2Argonne National Laboratory
    It is generally accepted that, at high solidification rates, austenite becomes the primary solidification phase in stainless steels with low Cr/Ni equivalent ratios (e.g. 316L) because of dendrite growth kinetics. Recent developments at the Advanced Photon Source allowed us to critically examine this notion through in-situ synchrotron x-ray diffraction studies of the phase transformation kinetics that occur in Laser Powder Bed Fusion (LPBF) single tracks of 316L. A 2D high-speed detector was employed to capture several complete Bragg rings during the relevant phase evolution with acquisition rates of up to 500 Hz allowing solidification rates to range from 1 to 100 mm/s. The experimental results from the time-resolved XRD were compared to both the predictions from a dendrite growth kinetics model and post-mortem characterization of the solidification microstructure. Invariably, ferrite always appears during solidification of the single track but is fully transformed to austenite upon further cooling.

9:40 AM  
In-situ X-ray Imaging of Melt Flow Dynamics in Laser Metal Additive Manufacturing: Qilin Guo1; Cang Zhao2; Minglei Qu1; Lianghua Xiong3; S. Mohammad H. Hojjatzadeh1; Luis I. Escano1; Niranjan D. Parab2; Kamel Fezzaa2; Tao Sun2; Lianyi Chen1; 1University of Wisconsin-Madison; 2Argonne National Laboratory; 3Missouri University of Science and Technology
    Melt flow plays a critical role in laser metal additive manufacturing, yet it is very challenging to characterize the melt flow behavior due to its opacity to most characterization techniques. Here, we use in-situ synchrotron X-ray imaging to characterization melt-flow dynamics during laser metal additive manufacturing by populous and uniformly dispersed micro-tracers. We revealed and quantified the location-specific flow patterns within the entire melt pool under different melting modes. The physical processes at different locations in the melt pool were discussed. This work opens the gate to study the detailed melt-flow dynamics under real additive manufacturing conditions. The results obtained provide crucial insights into laser additive manufacturing processes and are critical for developing reliable high-fidelity computational models.

10:00 AM  
In Situ Imaging of the Effect of Gas Flowrates on Directed Energy Deposition: Lorna Sinclair1; Yunhui Chen1; Samuel Clark1; Oliver Hatt2; Sebastian Marussi1; Saurabh Shah1; Robert Atwood3; Martyn Jones4; Gavin Baxter4; Chu Lun Alex Leung1; Iain Todd2; Peter Lee1; 1University College London; 2University of Sheffield; 3Diamond Light Source Ltd; 4Rolls-Royce plc
    Directed Energy Deposition (DED) is a fabrication technique for rapid part production, coatings, and repair applications. However, careful selection of processing conditions is essential for obtaining the desired mechanical properties. Gas flowrates can affect the powder delivery, powder capture efficiency, and oxidation of a component. In this study, different carrier gas (powder delivery) and shielding gas (creating an inert environment around the laser) flowrates have been investigated. A unique DED process replicator has been used to observe laser deposition in situ via high-speed synchrotron radiography, revealing gas flowrate impacts on melt pool size and shape. An industrial DED machine (BeAM Magic 800) was used to build tracks under analogous processing conditions for comparison. The results showed that low gas flowrates led to thicker deposition layers and track oxidation, whereas high shield gas led to flatter, wider melt pools, and thinner deposition layers than intended.

10:20 AM  
Microstructure Evolution and Nanoindentation Measurements after Laser Re-solidification of Hypo-eutectic Al-10 at %Cu: Mohammed Alamoudi1; Vishwanadh Bathula1; Jörg Wiezorek1; 1University of Pittsburgh
    After laser melting rapid solidification microstructures of multicomponent alloys exhibit morphological gradients, scale refinement, non-equilibrium phases and solute trapping, and associated property changes. We combined nano-indentation with microstructural analyses by SEM and TEM to study processing-microstructure-property relationships for laser-surface-melted hypo-eutectic Al-10at%Cu. A microstructure gradient develops as the solidification rate increases from the bottom to top of the melt pool. The central region typically comprised a micron-scale refined cellular structure of α-Al(Cu) and nano-scale intercellular θ-Al2Cu or eutectic. The cellular α-Al(Cu) consistently showed Cu-solute supersaturation and a 36% hardness increase relative to as-cast state. Considering the possible contributions from different strengthening mechanisms the observed hardness increase has been attributed to a combination of solute and grain size hardening. Additional studies are conducted to ascertain the effects of the increased solute content and more refined scales of the microstructure regions formed at the higher solidification rates towards the top of the melt-pool.

10:40 AM  
Simultaneous, In-situ Synchrotron X-ray Radiography and Thermal Imaging of Liquid-to-solid Phase Transformation during Laser Fusion Processing of Ti- and Ni-alloys: Rakesh Kamath1; Ryan Heldt1; Logan White1; David Garcia2; Rongxuan Wang2; Zhenyu Kong2; Kamel Fezzaa3; Tao Sun4; Hahn Choo1; 1University of Tennessee Knoxville; 2Virginia Polytechnic Institute and State University; 3Argonne National Laboratory; 4University of Virginia
    Understanding the liquid-to-solid phase transformation is key in establishing process-structure correlations in fusion-based manufacturing processes such as metal additive manufacturing (MAM). The present in-situ studies used a laser-AM simulator (developed at beamline 32-ID-B, APS, Argonne National Laboratory) to mimic spot and line melt strategies used in MAM on Ti-6Al-4V and IN-625 alloy plates, along with varying relevant process parameters namely laser power, line scan speed, spot dwell time, and temporal and spatial intervals between two consecutive spot melts. In-situ dynamic synchrotron x-ray radiography was used in tandem with the laser-AM simulator to obtain the interface velocity (R) of the liquid-solid interface. Simultaneous thermal imaging was performed on the top surface of the melts during the laser melting experiments to obtain the thermal gradient (G) at the L-S interface. Post-mortem, ex-situ SEM-EBSD characterization of the melt pools were used to correlate the measured G and R evolutions to the resulting microstructure.

11:00 AM  
Ultrafast Dynamics of Solidification and Thermal Strain Evolution in Laser Powder Bed Additive Manufacturing Using High Energy X-ray Diffraction: Adrita Dass1; Chenxi Tian1; Shonak Bhattacharya1; Darren Pagan2; Atieh Moridi1; 1Cornell University; 2Cornell High Energy Synchrotron Source
    Understanding solidification behaviour during metal additive manufacturing (AM) is essential, as it directly influences final microstructures and residual strain states. Many efforts in this domain are based on numerical and computational approaches. Some studies focus on experimental determination of these parameters through in-situ monitoring including pyrometry, IR imaging or synchrotron x-ray imaging. However, these methods cannot directly measure the changes in lattice parameters in real-time, which is necessary for estimating transient parameters like thermal gradient and cooling rates. Here, we demonstrate a novel approach using in-situ synchrotron x-ray diffraction to accurately measure solidification parameters during laser powder bed AM of Inconel 625 and Stainless Steel 304. Using this approach, we calculate the melt pool and mushy zone dimensions, along with real-time thermal gradient, cooling rate and solidification front velocity. The effect of material properties and processing parameters on solidification parameters are discussed and compared with other approaches in the literature.

11:20 AM  
In-situ X-ray Imaging of Porosity Formation in Directed Energy Deposition: Sarah Wolff1; Benjamin Gould2; Aaron Greco2; Tao Sun3; 1Texas A&M University; 2Argonne National Laboratory; 3University of Virginia
    Directed energy deposition (DED) additive manufacturing (AM) is a key process for various applications, such as repair, remanufacturing, and fabrication of functionally graded structures. However, the laser-matter interactions and melt pool dynamics in laser DED with powder flow are still unclear, particularly in how pores form and flow inside the melt pool during the process. Understanding the porosity formation mechanisms is critical in the qualification, certification, and overall properties of a DED-AM part. Here, we reveal four types of pore formation mechanisms through in-situ and operando high-speed high-resolution X-ray imaging in the DED AM process that are unique to the laser-based, powder-blown DED process as a result of powder delivery, keyhole dynamics, melt pool dynamics, and shield gas. High-speed X-ray images provide direct evidence for pore formation mechanisms and show that the pores related to the interaction between the delivered powder and melt pool were the largest in size in laser-based powder-blown DED AM. These results will guide porosity mitigation, elimination, and control in DED AM.

11:40 AM  
Undercooling in Laser Powder Bed Fusion Metal Additive Manufacturing: Meelap Coday1; Minglei Qu1; Qilin Guo1; Lianyi Chen1; 1University of Wisconsin-Madison
    Undercooling is the most important parameter determining nucleation and growth of phases during solidification, yet the undercooling for metals under additive manufacturing (AM) conditions is still unclear. Here, we report the estimation of undercooling in the laser powder bed fusion (LPBF) AM process from the solidification rate measured by in-situ high-speed synchrotron x-ray imaging, based on the dendrite growth velocity model. The undercooling as a function of location and time and its dependence on material properties are revealed. The undercooling estimation method reported here provides a way to estimate the undercooling values of materials under additive manufacturing conditions. The temporal and spatial variation of the undercooling and the dependence of undercooling evolution trend on material properties revealed here are important for understanding microstructure evolution in fusion-based metal additive manufacturing processes.