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The Ti–35Nb–7Zr–5Ta (wt%, TNZT) alloy was reinforced with TiB2 and synthesized by L-PBF. The relatively small TiB2 particles change the solidification structure from cellular to columnar-dendritic and lead to submicron TiB precipitation in the β matrix. This results in pronounced grain refinement and reduction of texture. However, the microstructure of the additively manufactured TNZT-TiB2 is still different from the as-cast, unreinforced TNZT, which contains equiaxed and randomly oriented grains. The β phase is less stable in the as-cast samples, leading to stress-induced martensitic transformation and recoverable strain of 1.5%. The TNZT with 1 wt% of TiB2 presents significantly higher compressive strength (σYS = 495 MPa) compared to unreinforced samples (σYS = 430 MPa), without sacrificing ductility or altering Young’s modulus (E ≈ 46 GPa). The addition of a small fraction of TiB2 to the TNZT alloy synthesized by L-PBF is a promising alternative for manufacturing sophisticated components for biomedical applications. Graphical abstract

The Ti–35Nb–7Zr–5Ta (TNZT) alloy is a promising alloy because of its biocompatibility, high specific strength, and low Young’s modulus. This work aimed at investigating the viability to process the TNZT alloy by selective laser melting (SLM) and at optimizing the processing parameters to obtain densified bulk samples. The single-track approach was first used, and the optimized laser parameters were determined to produce bulk samples with a relative density of 99.0% when an energy input of 58.3 J/mm3 was used. The effect of porosity on mechanical properties was studied, and the as-built SLM samples presented slightly lower compressive strength than samples produced by Cu-mould suction casting, as a result of a columnar grain structure in the SLMed samples. Prototypes were manufactured, proving the feasibility of manufacturing parts of the TNZT alloy with complex geometry by SLM.

Laser powder bed fusion (LPBF) can help to overcome two challenges occurring by casting of metastable Al alloys: (1) the high amount of casting defects and (2) the limited part size while maintaining rapid solidification of the whole cross-section. In this study, an Al92Mn6Ce2 alloy was processed crack-free without baseplate heating by LPBF. The high cooling rate during fabrication has a significant impact on the microstructure, which was characterized by SEM, TEM and XRD. The processing through LPBF causes a high amount and a strong refinement of the intermetallic Al20Mn2Ce precipitates. This leads, compared to suction-cast specimens, to a higher hardness (180 HV 5) and a higher tolerable compressive stress (>1200 MPa) associated with a pronounced plasticity without failure up to a strain of 40%. The extraordinary mechanical properties of additively manufactured Al92Mn6Ce2 can extend the possibilities of producing novel LPBF lightweight structures for potential applications under harsh conditions.

This study focuses on the influence of additive manufacturing process strategies on the specimen geometry, porosity, microstructure and mechanical properties as well as their impacts on the design of metamaterials. Filigree additively manufactured NiTi specimens with diameters between 180 and 350 µm and a nominal composition of Ni50.9Ti49.1 (at %) were processed by laser powder bed fusion in a first step. Secondly, they structures were characterized by optical and electron microscopy as well as micro tomography to investigate the interrelations between the process parameters, specimen diameters and microstructure. Each specimen was finally tested in a micro tensile machine to acquire the mechanical performance. The process strategy had, besides the resulting specimen diameter, an impact on the microstructure (grain size) without negatively influencing its quality (porosity). All specimens revealed a superelastic response while the critical martensitic phase transition stress decreased with the applied vector length. As a conclusion, and since the design of programmable metamaterials relies on the accuracy of FEM simulations, precise and resource-efficient testing of filigree and complex structures remains an important part of creating a new type of metamaterials with locally adjusted material behavior.

Dense and crack-free specimens of the shape memory alloy Cu71.6Al17Mn11.4 (at.%) were produced via laser powder bed fusion across a wide range of process parameters. The microstructure, viz. grain size, can be directly tailored within the process and with it the transformation temperatures (TTs) shifted to higher values by raising the energy input. The microstructure, and the superelastic behavior of additively manufactured samples were assessed by a detailed comparison with induction melted material. The precipitation of the α phase, which inhibit the martensitic transformation, were not observed in the additively manufactured samples owing to the high intrinsic cooling rates during the fabrication process. Fine columnar grains with a strong [001]-texture along the building direction lead to an enhanced yield strength compared to the coarse-grained cast samples. A maximum recoverable strain of 2.86% was observed after 5% compressive loading. The first results of our approach imply that laser powder bed fusion is a promising technique to directly produce individually designed Cu-Al-Mn shape memory parts with a pronounced superelasticity at room temperature.

Cu-Al-Ni and Cu-Al-Ni-Mn shape memory alloys are deposited on Kapton polyimide sheets by a physical evaporation technique. Detailed investigations on the morphological, structural and thermal behaviour of the ternary and quaternary alloys are conducted. The developed samples exhibit an increase in martensite structures and decrease in transformation temperatures after Mn additions. Furthermore, the suppression of Cu3Al precipitates was evident from XRD analysis. The thermomechanical behaviour was analysed by actuating the bimorphs through Joule heating under varying mechanical loads. Fatigue analysis of the bimorphs reveals a superior performance for Cu-Al-Ni-Mn, exhibiting less than 30% loss in displacement after 15,000 cycles.

Selective laser melting (SLM) was used to manufacture fully martensitic (β′1) samples of the shape-memory alloy 81.95Cu-11.85Al-3.2Ni-3Mn (wt%). Crack-free specimens with a high relative density of about 98.9 ± 0.1% were produced. Immediate remelting of already processed layers during SLM enhances the relative density (99.5 ± 0.3%). Primarily by varying the scanning speed in the remelting step, the thickness of the remelted zone can be adjusted. Moreover, remelting alters the microstructure as well as the transformation temperatures, which tend to rise with the volumetric energy input. In this way, the shape-memory properties can be modified without compromising the relative density and the considerable plasticity of the samples. Thus, the remelting procedure proves to be an interesting tool for 81.95Cu-11.85Al-3.2Ni-3Mn and related alloys in order to optimize and tailor their performance already during SLM processing and without applying additional post-processing steps. Keywords: Additive manufacturing, Selective laser melting, Laser remelting, Shape-memory alloy, Microstructure, Mechanical properties

In this study the Ti-based alloy Ti-5Al-5V-5Mo-3Cr (wt%) was processed using selective laser melting (SLM) while keeping the substrate plate at a constant temperature of 500°C. Cubic samples were successfully fabricated with a relative density of about 99%. Their microstructures were analysed and correlated with the mechanical properties and compared with the fabrication state of non-heated selective laser melted Ti-5553. Investigations by XRD, SEM and TEM revealed the formation of a microstructure composed of a β-phase matrix and finely dispersed α-phase plates when heating was conducted. The microstructure critically depends on the thermal history and was found to have a pronounced influence on the mechanical behaviour. The mechanical properties of the heated specimens showed an enhanced compressive strength of about 1600MPa while the deformability remained relatively high. These results demonstrate that selective laser melting is capable of creating bulk parts with improved mechanical by precipitation hardening realized by applying an additional substrate heating during the process. Keywords: Selective laser melting, Additive manufacturing, Ti-based alloys, Ti-5553, In situ heat-treatment, Substrate heating

A NiTi shape memory alloy with the nominal composition Ni50.9Ti49.1 (at%) was processed by laser beam melting/laser powder bed fusion and the process parameters as well as the type of scanning strategy (point-like exposure) were optimized in a first step to obtain delicate lattice structures (strut diameters below 200 µm). In the second step, the lattice structures were analyzed by means of optical and electron microscopy as well as computer tomography to obtain the interrelation between the process parameters, strut diameter and the uniformity of the corresponding struts. The processing, especially the laser power and the type of point-like exposure, has a strong influence on the resulting strut diameter and, therefore, on the haptic stiffness of lattice structures and the mechanical properties (deformability, superelasticity). Unlike other approaches, our findings imply that filigree NiTi lattices with high uniformity can be manufactured on a standard industry laser powder bed fusion machine without modifying its hard- or software configuration.

Two shape-memory alloys with the nominal compositions (in wt.%) Cu–11.85Al–3.2Ni–3Mn and Cu–11.35Al–3.2Ni–3Mn–0.5Zr were prepared by selective laser melting (SLM). The parameters were optimised to identify the process window, in which almost fully dense samples can be obtained. Their microstructures were analysed and correlated with the shape-memory behaviour as well as the mechanical properties. Suction-cast specimens were also produced for comparison. Mainly, β 1′ martensite forms in all samples, but 0.5 wt.% of Zr stabilises the Y phase (Cu2AlZr), and its morphology depends on the thermal history and cooling rate. After annealing, the Y phase is primarily found at the grain boundaries hampering grain coarsening. Due to the relative high cooling rates applied here, Zr is mostly dissolved in the martensite in the as-prepared samples and it has a grain-refining effect only up to a critical cooling rate. The Zr-containing samples have increased transformation temperatures, and the Y phase seems to be responsible for the jerky martensite-to-austenite transformation. All the samples are relatively ductile because they mostly fracture in a transgranular manner, exhibiting the typical double yielding. Selective laser melting allows the adjustment of the transformation temperatures and the mechanical properties already during processing without the need of a subsequent heat treatment.

The Cu-based shape-memory alloy 81.95Cu-11.85Al-3.2Ni-3Mn (wt.%) was processed by selective laser melting (SLM) and the parameters were optimised to obtain compact, fully martensitic samples with a high relative density of up to 99%. Next to the energy dissipated in the material, the scanning strategy was varied and an additional remelting step was applied. The influence of the different processing parameters on the overall density, the defect distribution as well as the grain sizes was then analysed. The processing technique has a strong influence on the degree of porosity and the distribution of the pores, the grain size as well as the grain morphology. SLM thus allows the targeted manipulation of microstructures during manufacturing. Simultaneously, this work stresses the importance of establishing the proper processing parameters for producing high-quality samples.

Cu-based shape memory alloys (SMAs) show better thermal and electrical conductivity, lower cost and are easier to process than traditional Ti-based SMAs, but they exhibit a lower ductility and lower fatigue life. These properties can be improved by decreasing the grain size and reducing microstructural segregations, which may be obtained using laser surface remelting treatments. The aim of the present work was to produce and characterize laser remelted Cu-11.85Al-3.2Ni-3Mn SMA plates. Twelve plates with the dimensions of 50×10×1.5 mm were produced by suction casting in a first step. The surface of the plates was remelted afterwards with a laser beam power of 300 W, hatching of 50% and using three different scanning speeds: 100, 300 and 500 mm/s. The plates were characterized by optical and scanning electron microscopy, X-ray diffraction, differential scanning calorimetry as well as by tensile and microhardness tests. The remelted region showed a T morphology, with average thickness of 52, 29 and 23 µm for the plates remelted with scanning speeds of 100, 300 and 500 mm/s, respectively. In the plates remelted with 100 and 300 mm/s, some pores were found around the center of the track, due to the keyhole instability. The same phase formed in the as-cast sample was obtained in the laser remelted coatings: the monoclinic β′1 martensitic phase with zig-zag morphology. However, the laser treated samples exhibit lower transformation temperatures than the as-cast sample, due to grain refinement at the surface. They also show an improvement in the mechanical properties, with an increase of up to 162 MPa in fracture stress, up to 2.2% in ductility and up to 20.9 HV in microhardness when compared with the as-cast sample, which makes the laser surface remelting a promising method for improving the mechanical properties of Cu-based SMAs.

Selective laser melting (SLM) is an additive manufacturing process used to produce parts with complex geometries layer by layer. This rapid solidification method allows fabricating samples in a non-equilibrium state and with refined microstructure. In this work, this method is used to fabricate 3 mm diameter rods of a Cu-based shape memory alloy. The phase formation, thermal stability and mechanical properties were investigated and correlated. Samples with a relative density higher than 92% and without cracks were obtained. A single monoclinic martensitic phase was formed with average grain size ranging between 28 to 36 μm. The samples exhibit a reverse martensitic transformation temperature around 106 ± 2 °C and a large plasticity in compression (around 15±1%) with a typical “double-yielding” behaviour.

Shape memory alloys (SMAs) are a class of material that undergoes a reversible shape change after a plastic deformation. The recovery of the original shape is possible due to a structural transformation upon heating to a critical temperature. The shape memory effect is related to a martensitic-austenitic transformation from a phase with a low symmetry (martensite) to a high-temperature phase (parent phase) [1]. Cu-based shape memory alloys have the advantage of large thermal and electrical conductivities and the system Cu-Al-Ni alloys are quite attractive due to better stabilisation against aging phenomena [2].