Dan Thoma

• Doctor of Philosophy
• University of Wisconsin–Madison

With the advent of additive manufacturing, manipulation of typical microstructural elements such as grain size, texture, and defect densities is now possible at a faster time scale. While the processing–structure–property relationship in additive manufactured metals has been well studied over the past decade, little work has been done in understand...

This paper studies design criteria and performance improvement of a Gyroid-based triply periodic minimal surface heat exchanger using data-driven modeling coupled with genetic algorithm optimization and additive manufacturing. Key design parameters of a Gyroid structure as a unit-cell heat exchanger were optimized through data-driven modeling. With...

Synchronous reluctance (SynR) machines are promising rare-earth material-free alternatives to permanent magnet machines. However, structural challenges limit their operating speed and power density. This paper proposes and investigates multi-material additive manufacturing (MMAM) as a key-enabler to realize power-dense and high-speed SynR machines....

The oxidation of a titanium (Ti)-modified Mo-Si-B alloy designed for aerospace applications was investigated. Test samples were produced using arc melting and laser powder bed fusion (LPBF) additive manufacturing methods. To address high-temperature oxidation, a three-step coating strategy was employed, comprising a Mo precoat, Si and B co-depositi...

Generating reduced-order, synthetic grain structure datasets that accurately represent the measured grain structure of a material is important for reducing the cost and increasing the accuracy of computational crystal plasticity efforts. This study introduces a machine-learning-based approach, termed texture adaptive clustering and sampling (TACS),...

This paper presents a set of high-performing heat sinks that exhibit twice the thermohydraulic performance in terms of conductance compared to conventional rectangular fin heat sinks. The heat sinks presented here are designed through three-dimensional topology optimization (TO), manufactured using additive manufacturing (AM), and their performance...

Synchronous reluctance (SynR) motor technology is promising to realize rare-earth material-free electric machines. However, structural challenges limit operation speed and subsequently power density compared to machines with rare-earth permanent magnets. This paper proposes and investigates multi-material additive manufacturing (MMAM) as a key-enab...

To address the challenges of processing ultrahigh-temperature refractory metal alloys, a novel reactive synthesis-based additive manufacturing technique has been developed to fabricate chemically uniform and dense alloys. The present study demonstrates the reactive additive manufacturing of Mo-4Si-6B (at.%), a high-temperature refractory alloy, usi...

Bulk metallic glasses can exhibit novel material properties for engineering scale components, but the experimental discovery of new alloy compositions is time intensive and thwarts the rate of discovery. This study presents an experimental, high-throughput methodology to increase the speed of discovery for potential bulk metallic glass alloys. A we...

We use a random forest model to predict the critical cooling rate (RC) for glass formation of various alloys from features of their constituent elements. The random forest model was trained on a database that integrates multiple sources of direct and indirect RC data for metallic glasses to expand the directly measured RC database of less than 100...

One of the main challenges in the laser powder bed fusion (LPBF) process is making dense and defect-free components. These porosity defects are dependent upon the melt pool geometry and the processing conditions. Power-velocity (PV) processing maps can aid in visualizing the effects of LPBF processing variables and mapping different defect regimes...

ASME Sec(III) Div(5) code-certified structural alloys for Molten (halide) Salt Reactors (MSRs) such as 800H, SS316, and IN617 have significant Cr content. This makes them readily-degradable in molten halide salts due to the thermodynamic favorability of soluble chromium halide formation. It is therefore imperative to discover alloys with corrosion...

In laser powder bed fusion (LPBF)-based metal additive manufacturing, support structures play a crucial role in ensuring part-printability. However, support structures often consume significant amount of material, print-time and post-processing time. Furthermore, the optimal design of these support structures is challenging due to the transient nat...

One of the main factors keeping additive manufactured metal parts from being used in industry is the relatively low fatigue life of the as-printed parts when compared to their conventionally manufactured counterparts. In addition, certain areas on additively manufactured parts need finishing operations in order to make them functional. While laser...

In the present study, a high throughput (HT) methodology is applied to rapidly assess the surface characteristics of 83 additively manufactured 316L stainless steel. The variation in surface roughness (Sa) showed a good correlation with a dimensionless number (π) and provided direct linkages to the internal porosity defects. Lack-of-fusion regime r...

This paper reports on spall damage mechanisms in laser powder bed fusion (LPBF) fabricated stainless steel 316L (SS316L) subjected to uniaxial, strain plate-impact loading and explores failure evolution with increasing impact velocity and peak pressure. Analysis of velocimetry profiles reveals a heterogeneous failure response with several different...

The objective of this work was to assess the energy consumption of a novel remanufacturing method that utilizes mechanically-generated feedstock for directed energy deposition. Gas-atomization is the state-of-the-art production process for directed energy deposition feedstock, and for this work was incorporated into a remanufacturing process path t...

The availability of additive manufacturing enables the fabrication of cellular bone tissue engineering scaffolds with a wide range of structural and architectural possibilities. The purpose of bone tissue engineering scaffolds is to repair critical size bone defects due to extreme traumas, tumors, or infections. This research study presented the ex...

Insufficient availability of molten salt corrosion‐resistant alloys severely limits the fruition of a variety of promising molten salt technologies that could otherwise have significant societal impacts. To accelerate alloy development for molten salt applications and develop fundamental understanding of corrosion in these environments, here an int...

Bone-related defects that cannot heal without significant surgical intervention represent a significant challenge in the orthopedic field. The use of implants for these critical-sized bone defects is being explored to address the limitations of autograft and allograft options. Three-dimensional cellular structures, or bone scaffolds, provide mechan...

Acoustic topological systems explore topological behaviors of phononic crystals. Currently, most of the experimentally demonstrated acoustic topological systems are for airborne acoustic waves and work at or below the kHz frequency range. Here, we report an underwater acoustic topological waveguide that works at the MHz frequency range. The 2D topo...

The use of supercritical carbon dioxide (sCO2) as a working fluid is garnering interest in next generation power production systems due to the possibility of increased operational efficiencies and lower associated costs. Its implementation requires alloys with excellent high temperature strength and corrosion resistance, which makes Haynes 282® (H2...

The aim of this work is to provide firsthand knowledge of 316 L stainless steel (316 L) and Hastelloy X (HX) multi-material processing via laser powder bed fusion (PBF). Specifically, microstructure of the interface is studied using scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), and X-ray diffraction (XRD). Surface...

The present study demonstrates the development of a dimensionless number to predict the build height in the additive manufacturing technique of directed energy deposition (DED). The build height also can be used to estimate the dendrite arm spacing, and thus the cooling rate, in the fabrication of samples. A baseline sample, 316L stainless steel, w...

This paper demonstrates how an analytical and experimental method can be used to rapidly define the additive manufacturing settings for a new alloy where the process parameters were previously unknown. A nickel-based superalloy, Haynes 282, was chosen for the analysis. An experimental matrix of focused processing parameters was predicted with a dim...

Additive manufacturing (AM) is a promising means of production of austenitic stainless steel (SS) parts for hydrogen service. The hydrogen embrittlement resistance of SS 316 L parts manufactured by powder-bed-fed selective laser melting (SLM) and directed energy deposition (DED) was examined using slow strain rate tensile testing. The influence of...

Insufficient availability of molten salt corrosion-resistant alloys severely limits the fruition of a variety of promising molten salt technologies that could otherwise have significant societal impacts. To accelerate alloy development for molten salt applications and develop fundamental understanding of corrosion in these environments, here we pre...

While metal additive manufacturing has seen significant growth in recent years, the surfaces produced often need post-processing to improve surface finish, mitigate residual stresses, and remove surface-connected porosity. Laser polishing, by means of remelting a thin layer of the surface, is one post-processing method being investigated for surfac...

Several advanced nuclear reactor designs promise efficiency and safety improvements over the current reactor fleet but are limited by the current set of ASME code-qualified materials. Novel alloys including high-entropy alloys (HEAs), and more broadly compositionally complex alloys (CCAs), have shown promising irradiation-tolerance. However, the va...

The aim of this work is to propose a methodology for rapidly predicting suitable process parameters for additive manufacturing of a 316L-Cu multi-material part with a compositional gradient by using machine learning. Specifically, an algorithm based on a multivariate Gaussian process is developed to predict part density and surface roughness for a...

Critical-sized bone defects represent a significant challenge in the orthopedic field. Limitations on autograft and allograft as treatment techniques led researchers to explore the implantation of artificial bone tissue scaffolds. Bone scaffolds are three-dimensional cellular structures that provide mechanical support and behave like a template for...

One of the main challenges facing selective laser melting processes is finding suitable process parameters to achieve maximum density (pore-free) parts. In this letter, two newly discovered dimensionless numbers are presented that correlate process parameters to a part's density allowing for an initial estimation of suitable process parameters with...

In this experiment, the origin of dislocation structures in AM stainless steels was systematically investigated by controlling the effect of thermal stress through geometric constraints for the first time. Stainless steel 316L parts were produced in the form of “1D” rods, “2D” walls, and "3D" rectangular prisms to evaluate the effect of constraints...

In the present study, a combination of high-throughput (HT) and low-throughput (LT) techniques was used to rapidly determine the processing window and generate processing maps for Selective Laser Melting (SLM) of 316L stainless steel. The HT method includes the fabrication of hundreds of hex nut-shaped specimens, each processed with a unique combin...

The objective of this work is to study the feasibility of mechanically-generated feedstock for use in directed energy deposition (DED) processing. Mechanically-generated powder was created by machining 316L stainless steel bar stock followed by comminution of the resulting chips through oscillation ball milling. This methodology's production yield...

Mechanically-generated powder could potentially provide a more sustainable and lower cost feedstock alternative for the additive manufacturing industry. This research compares properties of stainless steel parts printed on a commercially available directed energy deposition system using both gas-atomized and mechanically-generated feedstock. The ma...

The integration of topology optimization (TO) and additive manufacturing (AM) has the potential to revolutionize modern design and manufacturing. However, few instances of manufactured optimized designs are documented, and even fewer examples of experimentally-tested designs are available. The lack of validation combined with the influence of AM pr...

High-entropy alloys (HEAs) are a class of alloys that can exhibit promising properties including enhanced irradiation resistance, high-temperature strength, and corrosion resistance. However, they exist in a relatively unexplored region of quasi-limitless composition space. Thus, to enable the development of promising compositionally complex alloys...

Metal additive manufacturing offers potential advantages for producing structural materials, such asaustenitic stainless steels, in nuclear power systems. However, the microstructure developed duringmetal additive processing is notably different from the one developed in conventional processing, andthe influence of the microstructural differences o...

Cerebral aneurysm clips are biomedical implants applied by neurosurgeons to re-approximate arterial vessel walls and prevent catastrophic aneurysmal hemorrhages in patients. Current methods of aneurysm clip production are labor intensive and time-consuming, leading to high costs per implant and limited variability in clip morphology. Metal additive...

The mechanical properties of uranium-niobium alloys evolve with aging at relatively low temperatures due to subtle microstructural changes. In-situ neutron diffraction measurements during aging of a monoclinic U-6Nb alloy at temperatures to 573 K were performed to monitor these changes. Further, in-situ neutron diffraction studies during deformatio...

Historically, metals are cut up and polished to see the structure and to infer how processing influences the evolution. We can now peer into a metal during processing without destroying it using proton radiography. Understanding the link between processing and structure is important because structure profoundly affects the properties of engineering...

Supplementary Information

Supplementary Information Video S1

The transition from the acicular to banded structure in dilute U–Nb alloys (∼5 at.%Nb) has been investigated. The acicular morphology consists of large, single orientation laths with high densities of dislocations and deformation twins, while the banded morphology is comprised of fine transformation twins, often with complex, multiple twin orientat...

In situ characterization techniques are now affording direct interrogation of opaque materials during synthesis and processing. In this work, synchrotron X-ray radiography and tomography were performed at Argonne National Laboratory's Advanced Photon Source to monitor metallic alloys during melting and solidification. X-ray radiographs of microstru...

Until the advent of third generation synchrotrons the ability to image the microstructure of metals during solidification was non-existent. Today's sources have sufficient energy and flux to perform real time radiographic imaging of solidification in thin samples with resolution sufficient to image dendrites, eutectic lamellae, and the density chan...

A new metallographic preparation technique has been developed for uranium–niobium (U–Nb) alloys. The technique uses a single methodology to optimally elucidate microstructural features without preparation artifacts and permits inclusion retention and identification. Specific examples of the applicability of this metallographic preparation technique...

of a paper presented at Microscopy and Microanalysis 2012 in Phoenix, Arizona, USA, July 29 – August 2, 2012.

We focus on the problem of segmenting scattered point data into multiple regions in a single segmentation pass. To solve this problem, we begin with a set of potential boundary points and use a Delaunay triangulation to complete the boundaries. We then use information from the triangulation and its dual Voronoi complex to determine for each face wh...

In-situ observation of the solidification phenomena in metals can lead to better understanding and control of microstructure evolution. Proton radiography offers the ability to image thick samples of high z material. Recently, in-situ proton radiography was used to directly observe dynamic processes during melting and solidification in bulk binary...

The single-crystal elastic constants of C15 NbCrâ have been computed by using a first-principles, self-consistent, full-potential total energy method. From these single-crystal elastic constants the isotropic elastic moduli are calculated using the Voigt and Reuss averages. The calculated values are in fair agreement with the experimental values. T...

The elastic properties, phase stability, and alloying behavior of NbCr2 and HfV2 C15 Laves phases have been investigated with a combined experimental and theoretical approach. Experimental results indicate that HfV2 has anomalous elastic properties as well as a structural instability, but NbCr2 does not. In addition, ternary phase fields of NbCr2 w...

Microstructural development in alloys bordering the NbCr2 Laves phase composition has been evaluated following solidification processing. With splat quenching of binary alloys, rapid solidification has yielded supersaturation of both Nb-rich and Cr-rich primary solid solutions as well as some solubility extension of the C15 phase during cooling as...

Many interesting segmentations take the form of cell complexes. We present a method to infer a 3D cell complex from of a series of 2D cross-sections. We restrict our attention to the class of complexes whose duals resemble triangulations. This class includes microstructures of polycrystalline materials, as well as other cellular structures found in...

Variable impedance actuation is characterized by the ability to independently set output force and output impedance for a robotic device. Adjusting the output impedance in real-time allows a device to better adapt to a variety of tasks, operate in human-like fashion, and support human safety. This paper focuses on a series clutch actuator based on...

Low temperature aging (<350 °C) of U–13 at.% Nb martensite results in increased strength levels accompanied by significant ductility loss. To determine the decomposition mechanism(s) responsible for these mechanical property changes, atom probe tomography was used to examine the niobium and impurity distributions after aging at 200 or 300 °C for ti...

Microcompression specimens, 10–15 μm in diameter by 20–30 μm in height, were produced from individual parent grains in a polycrystalline U–13 at.%Nb shape-memory alloy using the focused ion beam technique. The specimens were tested in a nanoindentation instrument with a flat diamond tip to investigate stress–strain behavior as a function of crystal...

of a paper presented at Microscopy and Microanalysis 2008 in Albuquerque, New Mexico, USA, August 3 – August 7, 2008

Particles formed during the reaction of cerium with hydrogen fractured into stacked plates with fully separated plate thicknesses averaging 100 nm and a finer partially separated thickness of 30 nm. The phonon density of states of these particles, measured using inelastic neutron scattering, showed a low-energy feature that could not be accounted f...

Detailed examinations of shape memory effect (SME) deformation structures in martensite of U–14at.% Nb were performed with electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM). An accommodation strain analysis, which has been previously used to predict SME deformation structures and texture evolution in polycrystalline...

Ultra-high temperature structural ceramics Mo5Si3 has been studied for materials processing and physical and mechanical properties. Single crystal Mo5Si3 has been synthesized by both optical floating zone and Czochralski methods. Thermal and elastic properties of Mo5Si3 single crystals were experimentally measured. Results show that Mo5Si3 is signi...

A soft-phonon feature associated with the shape-memory transition in NiTi is observed in the phonon density of states (DOS) of the B2 phase of both NiTi and Ni50Ti47Fe3 (with Fe substituted for Ti) using inelastic neutron scattering. In both alloys, the feature softens with decreasing temperature, but the softening occurs about 100 K lower in the F...

We present a method for extracting boundary surfaces from segmented cross-section image data. We use a constrained Potts model to interpolate an arbitrary number of region boundaries between segmented images. This produces a segmented volume from which we extract a triangulated boundary surface using well-known marching tetrahedra methods. This sur...

An aging study of compositionally homogeneous U–5.6Nb and U–7.7Nb (wt.%) was undertaken to improve the understanding of the decomposition of metastable martensitic phases in the U–Nb system. Artificially aging these depleted (238U) uranium alloys at 373, 473, 523 and 573K for times up to 70 days resulted in significant age hardening. However, the m...

Previously we presented Rietveld refinement results from laboratory X-ray powder diffraction data on artificially aged U-Nb alloys (1,2). The uranium- niobium alloy system is complex, exhibiting many different stable and metastable phases including martensite and shape-memory behavior. It is desirable to understand any long-term changes in the stru...

Recent experiments have indicated that the high-temperature properties of α-uranium may be strongly influenced by the formation of randomly distributed intrinsically localized vibrational modes, just a few atoms across in size. One observation was a loss of mechanical ductility that coincided with the formation of the intrinsically localized mode (...

of a paper presented at Microscopy and Microanalysis 2006 in Chicago, Illinois, USA, July 30 – August 3, 2006

Existing metallographic preparation techniques for uranium are limited to elucidating specific microstructural characteristics, and some of the techniques are regarded as being environmentally unacceptable. This paper describes a newly developed technique, which is not only more environmentally friendly, but reveals most microstructural features si...

The shape memory effect (SME) has been reported in the uranium–niobium alloy system in the region of the phase diagram surrounding U–6.5wt.% Nb. In this regime, the material may have either an α″ monoclinic (U–6wt.% Nb), or γ0 tetragonal structure (U–7wt.% Nb) and is two phase near 6.5wt.% niobium. In situ neutron diffraction studies during uniaxia...

The shape-memory alloy AuZn transforms martensitically (T-M = 65 K) from a cubic 132 (Pm3m) to a rhombohedral R-phase (P3) through softening of the TA(2)[110] phonon branch. We report elastic constants, specific heat, and thermal expansivity measurements through the transition. A large elastic anisotropy, A = 9 at T-M, associated with softening of...

Uranium is the only known element that features a charge-density wave (CDW) and superconductivity. We report a comparison of the specific heat of single-crystal and polycrystalline alpha-uranium. In the single crystal we find excess contributions to the heat capacity at 41 K, 38 K, and 23 K, with a Debye temperature ThetaD = 265 K. In the polycryst...

Uranium is the only known element that features a charge-density wave (CDW) and superconductivity. We report a comparison of the specific heat of single-crystal and polycrystalline $\alpha$-uranium. \red{Away from the the phase transition the specific heat of the polycrystal is larger than that of the single crystal, and the aim of this paper is to...

We report the work function Φ for single and polycrytalline α-uranium over the temperature range 173≤T≤873K. Photoemission spectroscopy (PES) measurements were made at energies 21 eV and 40 eV. Surfaces were prepared in situ by sputter/annealing cycles under UHV con-ditions. Like most metals the polycrystalline specimens showed little temperature d...

A thermodynamically complete equation of state for the compression and heating of near-equiatomic Ni–Ti alloy in the CsCl (B2) structure was predicted, based on quantum-mechanical calculations of the electron ground states and a Grüneisen lattice-thermal model. The quantum-mechanical calculations used ab initio pseudopotentials and the local-densit...