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NiTi shape memory alloys (SMAs) are used in a broad range of biomedical applications because of their unique properties including biocompatibility and high corrosion and wear resistance as well as functional properties such as superelasticity and the shape memory effect. The combination of SMAs and additive manufacturing can lead to revolutionary changes to the uses of SMAs in the biomedical industry. This article discusses the potential biomedical applications of NiTi that benefit from the AM process. We share the lessons learned in processing NiTi alloys with a focus on the laser powder bed fusion (LPBF) technique. The manufacturability, build quality, stable phases and transformation temperatures, microstructure, thermomechanical properties, microstructure tailoring, and functional properties of NiTi alloys produced via AM processing are reviewed. Current challenges such as expanding the process window, controlling the chemistry, and the performance and property responses are discussed, and potential opportunities including alloy design are discussed.

As one of the lightest metallic materials, magnesium is considered as a promising replacement for aluminum, titanium, and steel. However, lightweight energy-saving magnesium and magnesium alloys suffer from poor ductility (5% to 8%) due to their hexagonal closed packed crystalline structure and a limited number of easily activated independent slip systems at room temperature. This creates some challenges for magnesium and its alloys for various industrial applications. One technique to improve both strength and ductility, without any tangible weight penalty, of magnesium and its alloys is to add nanosize (less than 100 nm) reinforcements (e.g. nanoparticles) to the matrix as creat so-called magnesium nanocomposites. Various nano-reinforcing materials including oxides (Al2O3, TiO2, Y2O3, ZnO, ZrO2), carbides (SiC, B4C, TiC), nitrides (BN, AlN, TiN), borides (TiB2, SiB6, ZrB2), and carbonaceous materials (carbon nanotube and carbon nanoplatelets). The volume fraction of these nano-size reinforcing structures are normally less than 3 vol.% to avoid the clustering (agglomeration) of the nano-reinforcements which can adversely affect strength and ductility. That is, depending on the matrix (magnesium and magnesium alloys) and the nanosize reinforcement, there is an optimum volume fraction beyond which the properties (strength and ductility) would decrease. Though various aspects of the microstructure/mechanical property/manufacturing process of the magnesium nanocomposites have been studied so far, the failure and, in particular, fracture of these materials have not been yet classified properly. Investigating the fracture of magnesium nanocomposites, including the type of fracture, fracture mechanism, the effect of the nano-reinforcements on the fracture of Mg nanocomposites, enables the researchers to assess the role of nanosized reinforcement on the modes of failure and mechanical properties. This paper aims at providing a review of the fractography of compressive and tensile tests of nanocomposites Mg, classifying fractography by all kinds of reinforcement and working conditions of the Mg nanocomposite. Also, the effect of test temperature, cyclic loading, and heat treatment on the fracture of magnesium -matrix nanocomposite is discussed thoroughly. Among various nanoparticles and according to the review of the available literature, Al, Y2O3, and NiTi can significantly improve the tensile strength of the Mg matrix while ZnO and Sm2O3 reinforced nanocomposites show higher compressive yield stress.

By advancing additive manufacturing (AM) techniques for nitinol (NiTi), a shape memory alloy, more researchers investigate this tool for a wide range of applications. AM-fabricated NiTi parts may contain micropore/voids and have a rough surface due to the nature of the fabrication. As a solution, we proposed a modification to the NiTi-selective laser melting/SLM approach that could possibly reduce the internal micropores/voids and improve the relative density and surface roughness of the parts. In this approach, here referred to as remelting, the laser rescans each layer followed by the main laser scan and prior to the powder deposition for the next layer. To this end, we fabricated AM NiTi coupons with combinations of power and scanning speed with the same hatch spacing for evaluating the effect of remelting. SEM analysis, density and surface roughness measurements, and X-ray CT, as well as DSC and compression tests were utilized for characterization. It was revealed that the remelting procedure in each layer homogenizes the surface, reduces the defects, and improves the powder bed quality of the next layer. It was found that using proper combinations of remelting process parameters, it is possible to fabricate NiTi components with increased density, fewer defects, and smoother top surfaces, which are important factors in fatigue life and functionality of the parts. Although the transformation temperatures and compression response were not significantly affected by the remelting procedure, the process is expected to improve the fatigue life of the SLM NiTi samples, which will be investigated in the future studies.

In this study, depth-sensing indentation creep response of cast and additively manufactured (laser powder bed fusion) NiTi alloys in heat-treated conditions have been investigated at ambient temperature. Indentation creep tests were evaluated with the help of a dual-stage approach comprising a loading segment with a subsequent constant load-holding stage and an unloading phase afterward. The investigation was carried out at a maximum load of 50 mN along with a holding time of 600 s. Different creep parameters comprising indentation creep displacement, creep strain rate, creep stress exponent as well as the indentation size effect have been analyzed quantitatively for the employed materials. In addition, microstructural analysis has been performed to ascertain the processing–microstructure–creep property correlations. A substantial indentation size effect was seen for both cast and printed NiTi samples in heat-treated conditions. According to the creep stress exponent measurements, the dominant mechanism of rate-dependent plastic deformation for all NiTi samples at ambient temperature is attributed to the dislocation movement (i.e., glide/climb). The outcome of this investigation will act as a framework to understand the underlying mechanisms of ambient-temperature indentation creep of the cast and printed NiTi alloy in conjunction with heat-treated conditions.

NiTi (Nitinol) is a shape memory alloy with distinctive properties, such as shape memory, superelasticity, biocompatibility, and low density. All these unique properties make NiTi a great candidate in different applications. However, the conventional fabrication of NiTi encounters many challenges that significantly limits the practical applications of the alloy. As a solution, the Selective Laser Melting (SLM) which is an Additive Manufacturing (AM) technique has been recently used for the fabrication of NiTi parts. Although complex geometries can be fabricated directly from CAD files via SLM, different process parameters significantly affect the parts’ quality and must be optimized for each application. In most of the potential applications, NiTi components undergo cyclic loads and therefore its structural fatigue must be fully studied and considered in the design process. However, due to the nature of the SLM process, the fatigue behaviour of SLM fabricated NiTi is different from the conventional ones. In this work, as an initial step, the fatigue behaviour of the SLM fabricated NiTi in the horizontal direction is studied and the reasons for failure have been discussed. To this end, strain-controlled fatigue tests were performed on NiTi dog-bone samples, and fractography was used to analyze the different defects which could cause the failure or scatter in the results.

This study is the first study on the compressive and tensile stress-strain revealing the deformation anisotropy among laser powder bed fusion NiTi parts fabricated with the same process conditions. We investigated the effects of building orientation on the microstructure and the resulting shape memory properties. To this end, three orientations were selected, namely 0, 45, and 90-degree, measured from the build plate and fabricated with the same process parameters. A strong (001) texture was formed along the building direction for all of the samples; a different texture could however be observed along the loading direction (LD). Samples fabricated with 45-degree showed a texture of (110) along the LD, as confirmed through X-ray diffraction and backscattered diffraction while 0 and 90 samples still had the (001) texture along with the LD. These texture variations created anisotropic compression-tension behaviors with deformation patterns consistent with single crystals. The (001) -textured parts showed higher strength and lower transformation strain (2.87% @ 200 MPa in tension for 0 ) while the (110) samples showed higher transformation strain at lower stresses (5.31% @ 150 MPa in tension for 45 ).

Besides the possibility of fabricating complex geometries, a major advantage of additive manufacturing is tailoring the microstructure and consequently properties of the parts. By changing the process parameters (PPs). Functionally graded systems can be achieved without the need for post-processing or joining. In this study, fabrication of a functionally graded NiTi using selective laser melting was investigated. Using two different sets of PPs resulted in a part with two different functional properties. Several characterization methods were employed to study microstructural distribution. Thermomechanical full-field strain tests showed a novel behavior, a combination of superelastic and shape memory effects at room temperature.

The large reversible strain upon heating (shape memory effect) or unloading (superelasticity), high power-to-weight ratio, good functional stability, compact size, and lightweight make the rotary NiTi shape memory alloys actuators an interesting candidate for various engineering applications. Additive manufacturing (AM) provides a single step freeform manufacturing process that not only fabricates the complex geometries but also tailors the properties of the printed parts profoundly. In the selective laser melting (SLM) technique, the process parameters (PPs) and scanning strategy show the remarkable effect on the microstructure, the properties, and the size accuracy of as-built parts. Impurity pick-up during the AM process is an unintended incident altering the microstructure and thermomechanical properties of fabricated parts significantly. In this paper, slightly Ni-rich NiTi powder is utilized to fabricate the NiTi tubes with three different thicknesses via the SLM method. It is shown that the bidirectional scanning strategy results in the size deviation of thin-wall tubes. Transformation temperatures (TTs) of the as-fabricated samples are assessed and compared with those of the starting powder. A large shift in TTs is found between the powder and the SLM tubes. The x-ray diffraction pattern shows the martensite phase at room temperature for the starting powder, while the as-built tubes are in the austenite phase coexisting with a secondary phase of Ti-rich oxide. Scanning electron microscopy (SEM) confirms Ti-rich Ti4Ni2Ox precipitates form along the grain boundaries. The characterization of tubes under pure torsional loading shows the localized shear strain on the tube surface. The thermomechanical behavior of the as-fabricated tubes is investigated and shown to exhibit superelastic response with a stable transformation strain of 2.3% after 10 cycles.

Build orientation is one of the important parameters in the selective laser melting (SLM) process that could alter the mechanical and fatigue properties of the fabricated parts. In the first section of this paper, the monotonic tensile and low-cycle fatigue behavior of the SLM NiTi dog-bone samples fabricated in three different orientations are investigated. Based on the results, the highest fatigue life was for the samples fabricated in 45° with respect to the build plate, and the lowest fatigue life was for the samples fabricated on their edge. Furthermore, horizontally fabricated samples showed the highest scatter in fatigue life. It was revealed using SEM analysis that based on the build orientation, different types of defects, such as lack of fusion, entrapped gas porosities, and surface flaws could be generated inside the parts that could lead to a premature failure. In the second section of the paper, for the first time, the self-heating approach is utilized to investigate the high-cycle fatigue behavior of SLM NiTi samples fabricated in the horizontal direction. By considering the same origin for the fatigue damage and the one that generates the heat dissipation, self-heating approach can be used to predict the fatigue properties of the material. Using the self-heating approach, the fatigue limit for the virgin as well as pre-strained samples were evaluated. Similar to the conventional metals, the pre-strained sample showed a higher mean fatigue limit in comparison with the virgin samples.

The use of titanium bone fixation plates is considered the standard of care for skeletal reconstructive surgery. Highly stiff titanium bone fixation plates provide immobilization immediately after the surgery. However, after the bone healing stage, they may cause stress shielding and lead to bone resorption and failure of the surgery. Stiffness-modulated or stiffness-matched Nitinol bone fixation plates that are fabricated via additive manufacturing (AM) have been recently introduced by our group as a long-lasting solution for minimizing the stress shielding and the follow-on bone resorption. Up to this point, we have modeled the performance of Nitinol bone fixation plates in mandibular reconstruction surgery and investigated the possibility of fabricating these implants. In this study, for the first time the realistic design of stiffness-modulated Nitinol bone fixation plates is presented. Plates with different levels of stiffness were fabricated, mechanically tested, and used for verifying the design approach. Followed by the design verification, to achieve superelastic bone fixation plates we proposed the use of Ni-rich Nitinol powder for the AM process and updated the models based on that. Superelastic Nitinol bone fixation plates with the extreme level of porosity were fabricated, and a chemical polishing procedure used to remove the un-melted powder was developed using SEM analysis. Thermomechanical evaluation of the polished bone fixation plates verified the desired superelasticity based on finite element (FE) simulations, and the chemical analysis showed good agreement with the ASTM standard.