In this review, we propose a comprehensive overview of additive manufacturing (AM) technologies and design possibilities in manufacturing metamaterials for various applications in the biomedical field, of which many are inspired by nature itself. It describes how new AM technologies (e.g. continuous liquid interface production and multiphoton polymerization, etc) and recent developments in more mature AM technologies (e.g. powder bed fusion, stereolithography, and extrusion-based bioprinting (EBB), etc) lead to more precise, efficient, and personalized biomedical components. EBB is a revolutionary topic creating intricate models with remarkable mechanical compatibility of metamaterials, for instance, stress elimination for tissue engineering and regenerative medicine, negative or zero Poisson's ratio. By exploiting the designs of porous structures (e.g. truss, triply periodic minimal surface, plant/animal-inspired, and functionally graded lattices, etc), AM-made bioactive bone implants, artificial tissues, and organs are made for tissue replacement. The material palette of the AM metamaterials has high diversity nowadays, ranging from alloys and metals (e.g. cobalt-chromium alloys and titanium, etc) to polymers (e.g. biodegradable polycaprolactone and polymethyl methacrylate, etc), which could be even integrated within bioactive ceramics. These advancements are driving the progress of the biomedical field, improving human health and quality of life.

This study investigates the feasibility of using electron beam melting (EBM) to manufacture windings for electric motors from pure copper. A comprehensive parameter study is conducted to refine the process parameters which finally achieved over 99% of bulk density, 91.7±1.8 % of the electrical conductivity of standardized pure copper. Down-face sintering strategies are also employed to create sintered powder cake capable of supporting the overhangs. Additionally, the integration of cooling features into copper windings and the development of innovative continuous winding designs have been validated. This study demonstrates that EBM is a promising technique for manufacturing high-performance copper motor windings. Moreover, it effectively releases design limitations associated with copper motor windings, thereby enabling the integration of more advanced and functional design features.

This paper presents a novel design approach to soft robotic fingers based on the Fin Ray effect, which mimics the flexible yet adaptive behavior of fish fins. These soft, versatile grippers are ideal for applications requiring gentle manipulation and environmental adaptability. The study focuses on integrating multi-stage stiffening mechanisms into 3D-printed thermoplastic polyurethane (TPU) fingers. These mechanisms, featuring sequentially engageable inner structures, allow a multi-stage grasping load via increased deformation. By combining experiments with simulations, this work demonstrates how these inner structures improve the gripper's functionality and extend its application potential in soft robotics.

Herein, the quality and accuracy to manufacture delicate parts from NiTi powder using electron beam powder bed fusion (EB-PBF) technology is investigated. Therefore, benchmarks with thin cylinders and thin walls are designed and fabricated using two distinct scan strategies of EB-PBF manufacturing (i.e., continuous melting and spot melting) with different process parameter sets. After these optimizations, four different lattice structures (i.e., octahedron, cell gyroid, sheet gyroid, and channel) are manufactured and characterized. It is shown both continuous melting and spot melting modes are able to manufacture lattices with relative densities over 97%. And as-built lattice structures exhibit an excellent pseudoelasticity up to 8% depending on the design of the structure, e.g., the channel structure shows more deformation recoverability than the cell gyroid. This is attributed to the integrity of geometry as well as compressive mode of the mechanical loading. Of course, the compressive strength and ultimate compressive strength also increase with the increasing volume fraction. Moreover, the spot melting can be used as an engineering tool to customize a delicate beam-shaped structure with a superior pseudoelasticity.

This work investigates the influence of a single tensile preload, applied prior to fatigue testing, on the fatigue strength of 316L stainless steel parts manufactured using laser-based powder bed fusion (PBF-LB) with a porosity of up to 4 %. The specimens were produced in both the horizontal and vertical build directions and were optionally preloaded to 85 % and 110 % of the yield strength before conducting the fatigue tests. The results indicate a clear tendency of improved fatigue life and fatigue limit with increasing overload in both cases. The fatigue limits increased by 25.8 % and 24.6 % for the horizontally and vertically built specimens, respectively. Extensive modelling and experiments confirmed that there was no significant alteration in the shape and size of the porosity before and after preloading. Therefore, the observed enhancement in fatigue performance was primarily attributed to the imposed local compressive residual stresses around the defects.

Additive manufacturing (AM) using powder bed fusion is becoming a mature technology that offers great possibilities and design freedom for manufacturing of near net shape components. However, for many gas turbine and aerospace applications, machining is still required, which motivates further research on the machinability and work piece integrity of additive-manufactured superalloys. In this work, turning tests have been performed on components made with both Powder Bed Fusion for Laser Beam (PBF-LB) and Electron Beam (PBF-EB) in as-built and heat-treated conditions. The two AM processes and the respective heat-treatments have generated different microstructural features that have a great impact on both the tool wear and the work piece surface integrity. The results show that the PBF-EB components have relatively lower geometrical accuracy, a rough surface topography, a coarse microstructure with hard precipitates and low residual stresses after printing. Turning of the PBF-EB material results in high cutting tool wear, which induces moderate tensile surface stresses that are balanced by deep compressive stresses and a superficial deformed surface that is greater for the heat-treated material. In comparison, the PBF-LB components have a higher geometrical accuracy, a relatively smooth topography and a fine microstructure, but with high tensile stresses after printing. Machining of PBF-LB material resulted in higher tool wear for the heat-treated material, increase of 49%, and significantly higher tensile surface stresses followed by shallower compressive stresses below the surface compared to the PBF-EB materials, but with no superficially deformed surface. It is further observed an 87% higher tool wear for PBF-EB in as-built condition and 43% in the heat-treated condition compared to the PBF-LB material. These results show that the selection of cutting tools and cutting settings are critical, which requires the development of suitable machining parameters that are designed for the microstructure of the material.

This paper develops a hybrid experimental/simulation method for the first time to assess the thermal stresses generated during electron beam melting (EBM) at high temperatures. The bending and rupture of trusses supporting Inconel 625 alloy panels at similar to 1050 degrees C are experimentally measured for various scanning strategies. The generated thermal stresses and strains are thereafter simulated using the Finite-Element Method (FEM). It is shown that the thermal stresses on the trusses may reach the material UTS without causing failure. Failure is only reached after the part experiences a certain magnitude of plastic strain (similar to 0.33 +/- 0.01 here). As the most influential factor, the plastic strain increases with the scanning length. In addition, it is shown that continuous scanning is necessary since the interrupted chessboard strategy induces cracking at the overlapping regions. Therefore, the associated thermal deformation is to be minimized using a proper layer rotation according to the part length. Although this is similar to the literature reported for selective laser melting (SLM), the effect of scanning pattern is found to differ, as no significant difference in thermal stresses/strains is observed between bidirectional and unidirectional patterns from EBM.

The electron beam powder bed fusion (PBF-EB) is limitedly used to manufacture complex structures such as delicate lattices. Nickel titanium (NiTi) has been chosen for fabricating the lattice structure due to its widely utilization in the biomedical sector. However, issues may arise when manufacturing angled trusses while the dimensional inaccuracy increased with the increasing of the angle between the truss member and the vertical build direction. Therefore, two different scan strategies: spot melting and linear melting were used to manufacture the lattice structures respectively to compare the dimensional accuracy of different structures. This investigation highlights that linear melting is prone to maintain the geometrical accuracy of line-based structure with a limited influence from the scan speed while the spot melting is more capable of manufacturing the point-based structure with a higher geometrical resolution.  

Electron beam powder bed fusion (PBF-EB) is used to manufacture dense nickel titanium parts using various parameter sets, including the beam current, scan speed and post cooling condition. The density of manufactured NiTi parts are investigated with relation to the linear energy input. The results implies the part density increases with increasing linear energy density to over 98% of the bulk density. With a constant energy input, a combination of low power and low scan speed leads to denser parts. This is attributed to lower electrostatic repulsive forces from lower number density of the impacting electrons. After manufacturing, densest parts with distinct parameter sets are categorized into three groups: i) high power with high scan speed and vacuum slow cooling, ii) low power with low scan speed and vacuum slow cooling and iii) low power with low scan speed and medium cooling rate in helium gas. Among these, a faster cooling rate suppresses phase transformation temperatures, while vacuum cooling combinations do not affect the phase transformation temperatures significantly. All the printed parts in this study exhibit almost 8% pseudoelasticity regardless of the process parameters, while the parts cooled in helium have a higher energy dissipation efficiency ( ), which implies faster damping of oscillations. 

Nickel titanium (NiTi) as one of the most utilized shape memory alloy has drawn significant interest due to its unique characteristics. However, NiTi is also considered a susceptible material to smoke during electron beam powder bed fusion (PBF-EB) process, which restricts the manufacturing possibility of the components. This work investigates processing windows for pre-heating and melting of NiTi powder to allow fabricating healthy parts. The smoke tests were carried out at different focus offsets and beam currents in relation to beam speeds. It is noted that a smaller EB spot can effectively prevents smoking while it may cause the strong powder bonding which can affect powder recycling negatively. Thus, a less focused beam (or larger EB spot) was selected to reach medium but efficient sintering. Moreover, it was observed that a negative defocused EB mitigates the smoke phenomenon compared to the positive defocused EB with a similar spot size. After that, parts having a relative density over 99% were successfully manufactured with PBF-EB. It is also found that with the same level of energy input, a set of low power with low scan speed leads to denser parts compared to a set of high power with high scan speed. This is attributed to less complexities in the melt dynamic which is related to lower density of impacting electrons. Besides, the combination of low power with low scan speed also improves geometrical accuracy of the parts attributed to the smaller spot size and smaller melt pool sizes.