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The Interdependence model is now widely used to analyze the results of grain refinement studies. Although the model was developed to predict the grain size of an alloy cast under the assumptions of near equilibrium solidification and the presence of potent nucleant particles, it has been found to be applicable to a wide variety of alloys, casting methods, and cooling conditions. However, the strength of the Interdependence model is when it is used as a diagnostic tool that can reveal the mechanisms influencing the refinement of alloys under particular solidification conditions. This paper presents an introduction to the Interdependence model, its recent validation by experiment, and examples of how it can be applied to the solidification of alloys during additive manufacturing. For example, the model explains the difficulties in promoting a transition from columnar to equiaxed grains during additive manufacturing while also providing insights into how a fully equiaxed grain structure can be achieved.

Multi-material 3D printing has created a wide range of applications because of its high-resolution and multi-functional capabilities. It is important to understand the interaction of the materials both macroscopically and microscopically. This article investigates the mechanical responses of rigid-rubbery polymeric material fabricated using the PolyJet technique as an individual constituent and as an integrated composite unit cell. A series of experiments were conducted to obtain the mechanical responses for individual VeroMagentaV (rigid) and Agilus30 (flexible) polymers with different shore-hardness levels. Tensile results show that the interface of the dual material is strong enough to withstand the stretching during the tensile experiment. The interfacial hardness and local elastic modulus in dual-material parts investigated using nano-indentation and visual inspections showed distinct transition properties. The introduction of different types of rigid reinforcement particles of the 3D-printed composite has been demonstrated and quantified. A numerical model is developed, and the results show good agreement with the compression experiments.

We present our latest results on linking the process–structure–properties–performance (PSPP) chain for metal additive manufacturing (AM), using a multi-scale and multi-physics integrated computational materials engineering (ICME) approach. The abundance of design parameters and the complex relationship between those and the performance of AM parts have so far impeded the widespread adoption of metal AM technologies for structurally critical load-bearing components. To unfold the full potential of metal AM, establishing a full quantitative PSPP linkage is essential. It will not only help in understanding the underlying physics but will also serve as a powerful and effective tool for optimal computational design. In this work, we illustrate an example of ICME-based PSPP linkage in metal AM, along with a hybrid physics-based data-driven strategy for its application in the optimal design of a component. Finally, we discuss our outlook for the improvement of each part in the computational linking of the PSPP chain.

Tantalum is a refractory metal with a melting point of 2996°C but it offers outstanding biocompatibility for bone implant applications. In this study, the selective electron beam melting (SEBM) process was used for the first time to fabricate both dense and fine lattice tantalum structures. The use of 90-ppm-oxygen Ta powder for SEBM ensured excellent ductility of the as-printed fine Ta lattice implants with strut diameter of just 350 μm. The as-printed dense Ta samples (99.90%) achieved tensile ductility of 45% compared with the minimum requirement of 25% by ISO 13782 and the reported 2% fabricated by SLM using 1800-ppm-oxygen Ta powder. Since 2016, 27 clinical applications have been achieved in China using the custom-designed and SEBM-printed Ta implants by the authors of this study. All these Ta implants (mostly Ta lattice structures) have performed satisfactorily in patients’ bodies so far. Three selected clinical applications, Ta lattice hip, fibula and femur implants, are briefly discussed in this article.

Using gas flow to reduce laser plume attenuation is critical in the process control of laser powder bed fusion (LPBF) of metal powders. First, this work investigated Hastelloy X (HX) samples built at different gas flow speeds. Higher porosity with lack of fusion defects was found in the samples built at lower gas flow speeds, which indicates a significant influence of laser plume attenuation. Then, particle pickup experiments were conducted to investigate the limit of further increasing the gas flow speed without disturbing the powder bed. Eight different powders of four alloys (Al, Ti, steel, and Ni) with mean sizes ranging from 25 µm to 118 µm were studied. A model was introduced to predict the pickup speeds of different powders. Lastly, a method based on porosity and particle pickup speed was proposed for the reference of setting the lower and upper limits of gas flow speed in LPBF.

Grade 23 Ti-6Al-4V additively manufactured by selective electron beam melting (SEBM) has found important clinical applications as bone implants since 2007. In general, an as-built rough surface is desirable for bone ingrowth, but at the expense of fatigue performance. This study assesses the relative influence of the surface condition and internal defects on the fatigue performance of SEBM Ti-6Al-4V. Chemical etching, standard machining, and precision machining are used to improve the as-built surface condition, while a significant two-step hot isostatic pressing (HIP) treatment is employed to heal internal defects. Detailed assessment of the fatigue performance of these samples with different surface and internal conditions leads to a range of informative observations. The fatigue results are superimposed on a well-established fatigue diagram for Ti-6Al-4V and further presented in a fatigue-processing condition diagram. It is shown that HIP is necessary only when the surface finish is sufficient and when there are no surface defects. Improving the surface condition is far more important than applying post-SEBM HIP. For example, as-built samples with machined surfaces of R a  = 0.05 µm and R z  = 0.45 µm without HIP exhibited much better fatigue performance than as-built samples with machined surfaces of R a  = 0.13 µm and R z  = 0.95 µm plus HIP. This study provides a quantitative basis for the design and application of SEBM Ti-6Al-4V as bone implants in terms of fatigue performance, as well as for other applications.

Optimization of metallic components fabricated from age hardenable alloys relies on the use of effective heat treatments. As additive manufacturing (AM) is taking its place as “another tool in the toolbox” for fabrication of metallic components, numerous processes are under development differing mainly by the starting feedstock and heat source. In parallel with these efforts is the optimization of the post-processing heat treatment to obtain the required properties. Due to differences in the various processes, it is expected that different microstructures will form in the AM specimens possibly affecting the mechanical behavior. To address optimization of heat treatments requires an understanding of how the starting microstructure responds to heat treatments. This study looks at inherent differences in the response of Inconel 718 to heat treatments developed for wrought 718. Three different AM processes were used to fabricate the samples in this study.

Additive manufacturing (AM) is a transformative technology that opens up many exciting opportunities. In metal AM processes, high-power heat sources are often used to locally fuse the metal feedstock to the previous layer. Extreme thermal conditions are involved and unique microstructures are developed in AM-processed materials. At the Advanced Photon Source, we applied high-speed x-ray imaging to probe the ultrafast dynamics of the vapor depression, melt flow and particle spattering, among other transient phenomena. Demonstrated by the scientific cases reviewed and cited here, high-speed x-ray imaging is a unique tool for metal AM research. It provides invaluable information that can help address the critical issues in AM associated with structural defects, high-fidelity models, build reliability and repeatability.

Metallic structural components made from dissimilar titanium alloys are playing important roles in high-end industries such as aerospace and aviation. In this paper, a Ti-5Al-2.5Sn/Ti-6Al-4V dissimilar titanium alloy material was additively manufactured by the selective laser melting (SLM) process. Interface characteristics, microstructure, and mechanical properties of the SLM-deposited samples before and after complete annealing treatment were researched to provide some technical basics for the integral fabrication of dissimilar titanium alloy components. The results demonstrated that a defect-free metallurgical bonded interface with a ~ 70-μm-wide element inter-diffusion region could be obtained between the Ti-5Al-2.5Sn and Ti-6Al-4V layers under both the as-deposited and the complete annealing states. The interfacial bonding strength between the dissimilar titanium alloys was always higher than the strength of the Ti-5Al-2.5Sn layers. Microstructure evolution and element inter-diffusion mechanisms of the SLM-produced Ti-5Al-2.5Sn/Ti-6Al-4V samples were also revealed and related to the change in mechanical properties.

Electron beam melting (EBM) is an established powder bed-based additive manufacturing process for the fabrication of complex-shaped metallic components. For metastable austenitic Cr-Mn-Ni TRIP steel, the formation of a homogeneous fine-grained microstructure and outstanding damage tolerance have been reported. However, depending on the process parameters, a certain fraction of Mn evaporates. This can have a significant impact on deformation mechanisms as well as kinetics, as was previously shown for as-cast material. Production of chemically graded and, thus, mechanically tailored parts can allow for further advances in terms of freedom of design. The current study presents results on the characterization of the deformation and strain-hardening behavior of chemically tailored Cr-Mn-Ni TRIP steel processed by EBM. Specimens were manufactured with distinct scan strategies, resulting in varying Mn contents, and subsequently tensile tested. Microstructure evolution has been thoroughly examined. Starting from one initial powder, an appropriate scan strategy can be applied to purposefully evaporate Mn and, therefore, adjust strain hardening as well as martensite formation kinetics and ultimate tensile strength.