The pursuit of enhancing NiTi superelasticity through laser powder bed fusion (L-PBF) and [001] texture creation poses a challenge due to increased susceptibility to hot cracking in the resulting microstructure with columnar grains. This limitation restricts NiTi's application and contributes to material waste. To overcome this, we introduce a pioneering approach: utilising spark plasma sintering (SPS) to heal directional cracks in [001] textured L-PBF NiTi shape memory alloy. Diffusion bonding and oxygen utilisation for Ti₂NiO_x formation was found to successfully heal the cracks. SPS enhances mechanical properties, superelasticity at higher temperatures, and two-way shape memory strain during thermomechanical cycling. This work provides an alternative solution for healing cracks in L-PBF parts, enabling the sustainable reuse of cracked materials. By implementing SPS, this approach effectively addresses hot cracking limitations, expanding the application potential of L-PBF NiTi parts while improving their functional and mechanical properties.

Additive manufacturing of NiTi shape memory alloys has attracted attention in recent years, due to design flexibility and feasibility to achieve four-dimensional (4D) function response. To obtain customized 4D functional responses in NiTi structures, tailorable phase transformation temperatures and stress windows as well as one-way or two-way shape memory properties are required. To achieve this goal, various heat treatments, including direct aging, annealing and annealing followed by aging, were optimized for the Ti-rich NiTi (Ni49.6Ti (at. %)) fabricated by laser powder bed fusion (L-PBF). Microstructural evolution, phase transformation, precipitation and shape memory behaviour were systematically investigated by multiscale correlative microstructural, differential scanning calorimetry analysis and thermomechanical analysis. Based on optimized heat treatments, ∼25 K phase transformation temperature windows and ∼90 MPa stress windows were achieved for the one-way shape memory effect. Solutionized annealing was found to be the most effective way to improve one-way shape memory degradation resistance, due to the reduction of defects and solid solution strengthening. One of the main findings of this study is that the heterogonous microstructures between hard intergranular Ti₂NiO_x and soft NiTi matrix, induced by solutionized annealing with subsequent aging, result in strain partitioning and enclosing the internal stress state, which was found to promote a pronounced two-way shape memory effect response. The results of this work provide in-depth knowledge on tailoring and designing functional shape memory characteristics via heat treatments, which contributes to expanding L-PBF NiTi application fields, such as biomedical implants, aerospace components, and other advanced engineering applications.

Superelastic metallic materials possessing large recoverable strains are widely used in automotive, aerospace and energy conversion industries. Superelastic materials working at high temperatures and with a wide temperature range are increasingly required for demanding applications. Until recently, high-temperature superelasticity has only been achievable with multicomponent alloys fabricated by complex processes. In this study, a novel framework of multi-scale models enabling texture and microstructure design is proposed for high-performance NiTi fabrication via laser powder bed fusion. Based on the developed framework, a Ni-lean Ni(49.4 at.%)-Ti alloy is, for the first time, endowed with a 4% high-temperature compressive superelasticity. A 001 texture, unfavorable for plastic slip, is created to realize enhanced functionality. The unprecedented superelasticity can be maintained up to 453 K, which is comparable with but has a wider superelastic temperature range (∼110 K) than rare earth alloyed NiTi alloys, previously only realizable with grain refinement, and other complicated post-processing operations. At the same time, its shape memory stability is also improved due to existing textured 100 martensite and intergranular precipitation of Ti₂NiOx. This discovery reframes the way that we design superior performance NiTi based alloys through directly tailoring crystallographic orientations during additive manufacturing.

The paper investigates the effect of hot isostatic pressing of single-crystal nickel-based alloy manufactured by selective laser melting (SLM) with a high-temperature substrate preheating. A study of the structure and phase composition of the material before and after treatment has been carried out. It was found that as a result of such treatment, the ratio and proportion of the strengthening phases change; however, due to slow cooling after treatment, the optimal ratio and shape of the inclusions are not fixed. In addition, the hardening particles are precipitated.

Laser powder bed fusion (L-PBF), categorized as additive manufacturing technique, has a capability to fabricate NiTi (Nitinol) shape memory alloys with tailorable functional properties and complex geometries. An important processing parameter, hatch distance (h), is often related to macroscale structural defects; however, its role on controlling the microstructure and functional properties is usually underestimated in L-PBF of NiTi. In this work, equiatomic NiTi (50.0 at% Ni) parts were fabricated with various hatch distances to tailor the microstructure and their shape memory characteristics. Contrary to what is observed in Ni-rich NiTi alloys, in this work, we demonstrate that phase transformation temperatures of L-PBF equiatomic NiTi do not decrease proportionally with hatch distance but rather relate to a critical hatch distance value. This critical value (120 μm) is derived from the synergistic effect of thermal stress and in situ reheating. Below this value, epitaxial grain growth and in situ recrystallization are enhanced, while above, irregular grains are formed and dislocations induced by thermal stresses decrease. However, the critical value found herein is characterized by high dislocation density and fine grain size, resulting in a superior thermal cyclic stability. The proposed finite element model is proven to be an effective tool to understand and predict the effect of hatch distance on grain morphology and dislocation density evolutions in L-PBF NiTi SMAs. In the present study, we provide a comprehensive understanding for in situ controlling L-PBF NiTi microstructure and functional characteristics, which contributes to designing 4-dimensional shape memory alloys.

Titanium aluminide alloys are considered to be attractive materials for aerospace and automotive high-temperature applications due to their high specific strength, creep resistance, and heat resistance. However, their brittleness makes it difficult to manufacture complex-shaped components of these alloys using conventional processes making additive manufacturing a promising way to produce titanium aluminide parts. Additive manufacturing of titanium aluminide alloys involves high preheating temperatures affecting their microstructure; hence a proper heat treatment is needed to obtain desired properties. In this paper, a titanium aluminide alloy produced using Selective Laser Melting process with a high-temperature preheating was subjected to various heat treatments to study their effects on microstructure and properties of the alloy.

In this paper, the effect of microstructural anisotropy on the fatigue crack growth behaviour of the functionally graded Inconel 718 fabricated through laser powder bed fusion (L-PBF) is investigated. Different manufacturing parameters, including low and high laser powers, were used to produce a variety of non-graded (NG) and functionally graded (G) specimens in two build directions, vertical and horizontal. In addition, a group of heat treated wrought samples was tested as a reference. It was observed that the different manufacturing parameters result in various grain size, crystallographic textures, precipitates and Laves phases, porosity, and un-melted particles. Three-point bending fatigue tests were conducted to measure the threshold stress intensity factor (ΔK_th) and fatigue crack growth rate (FCGR),da/dN. Only the lower laser power L-BPF Inconel material was found to have comparable to the wrought heat treated material fatigue crack growth behaviour. Furthermore, a new approach of automatically controlling ΔK as a function of the crack length was employed for graded specimens to investigate the crack growth rate as a function of local microstructure. The FCGR value of the vertical L-PBF samples, in which the crack direction was perpendicular to the build direction, remained constant. In contrast, the da/dN value of the horizontal samples with the crack direction parallel to the build direction increased constantly with the increase of the crack length. This behaviour is in good agreement with the hardness profile of the graded materials. Melt pool boundaries, graded interface boundaries, and grain orientations close to 〈001〉 were found to deflect the crack path. Additionally, it was found that L-PBF material is more affected (at a low stress ratio of R = 0.1) by the roughness-induced crack closure than the wrought counterparts. This study has successfully demonstrated the feasibility of using an additive manufacturing process to fabricate functionally graded materials featuring tailorable fatigue response of the local microstructures.

The aim of this study was to determine the influence of heat treatment on the microstructure of single-crystal nickel-based alloy manufactured by selective laser melting with a high-temperature substrate preheating. The change of gamma/gamma prime -phases ratio was established as a result of various heat treatment modes.

Intermetallic titanium aluminide (TiAl) alloys are considered attractive materials for high-temperature applications in aerospace, automotive, and energy industries. Additive manufacturing is a promising way of producing complex TiAl-alloy parts; however, it remains challenging due to brittleness of this alloy. While high-temperature preheating can mitigate cracking during selective laser melting, the microstructure of TiAl-alloys still needs to be optimized to achieve better mechanical performance. In this work, multiple laser exposures were used during selective laser melting of TiAl-based alloy to tailor its microstructure. Applying additional laser exposure of up to 20 times per layer induced an in situ heat treatment, which allowed to modify volume fraction and size of different phases. Microstructure, phase and chemical composition, and hardness of TiAl-alloys were investigsated with regards to several laser exposures during the selective laser melting process.

Nitinol (nickel-titanium or Ni-Ti) is one of the most utilized alloys exhibiting the Shape Memory Effect, which makes it possible to use it in many applications, such as aerospace, automotive, biomedical and others. The development of Additive Manufacturing technologies allows creating the latest time-dependent 4D products from nitinol that can realize the Shape Memory Effect after 3D-printing, which in the future will further increase the use of nitinol. However, processing nitinol is highly sensitive to compositional and thermal changes, affecting the final phase structure and thus temperature of the martensitic transformation. This paper presents a review of various effects of composition and 3D-printing process parameters on changes in transformation characteristics, as well as possible methods for their prevention and post-process heat treatments. It is reported that only by precise process and temperature control it is possible to create 4D products with the ability to realize the multi-stage shape memory effect. Finally, the paper discusses the various application of 3D-printed Nitinol and its advantages as compared to conventional processing routes.

Direct laser deposition (DLD), implemented on the basis of a 5-axis robot manipulator, is a very flexible technique with many adjustable process parameters affecting the quality of the final product. Objects manufactured by the DLD method may contain defects significantly reducing mechanical and functional properties. This article provides an overview of possible DLD process-induced defects that were identified by increasing the density of the produced material. The following defects were identified and discussed in the current work: Lack of fusion, porosity, cracking, oxide inclusions, and metal influx. It is shown that by changing such process parameters as laser power, powder feed rate, head lift and hatch distance, it is possible to improve the quality of the cladding and substantially reduce the process-induced defects.

The aim of this study was to determine the influence of selective laser melting (SLM) process parameters on the formation of cracks in nickel-based single crystal alloy. Several sets of laser scanning parameters, with varied laser power, scanning speed and hatch distance, thus resulting in different volumetric and linear energy densities, were investigated. The relationship between energy density, specimen densification, cracks formation and evolution of directional microstructure was shown. It was found that higher linear energy density results in fewer cracking and defects. Cracking in lower energy samples was attributed to the formation of thermal shrinkage pores in the interdendritic spaces and precipitation of Ta, Mo and Nb carbides along the dendrites boundaries.

Additive Manufacturing (AM) is an attractive way of producing parts of intermetallic titanium alloys. However, high brittleness of these alloys makes it challenging to produce crack-free intermetallic parts by AM. One way to overcome this problem is to use high-temperature powder-bed preheating. In this paper, Ti-48Al-2Cr-2Nb alloy was obtained by selective laser melting process with high-temperature preheating of 800-900 ºC. Crack-free specimens with a relative density of 99.9% were fabricated using an optimized process parameter set. Microstructure and phase composition were studied using scanning electron microscopy and X-Ray diffraction to reveal a fine microstructure consisting of lamellar a2/? colonies, equiaxed ? grains, and retained ß phase. Compressive tests and microhardness measurements showed that the produced alloy exhibited superior properties compared to the conventionally obtained TiAl-alloy.

16Cr-2Ni-Mn-Mo-xN (wt%) stainless steel powders were synthesized by mechanical alloying from elemental metal and chromium nitride (Cr2N) powders. Compaction of resulting powders was carried out using spark plasma sintering technology. The X-Ray diffraction and Scanning Electron Microscopy analysis showed that chemical homogeneity improves with increasing alloying time and decreasing nitrides powder content. Analysis of sintering samples showed the formation of a and ?-phase solid solution with nitrogen-containing samples containing undissolved nitrogen inclusions. The results of high-temperature tensile tests showed an increase of tensile stress for the sample containing 50% Cr2N. Furthermore, the microhardness was increased with an increase in the amount of Cr2N.

Selective laser melting (SLM) process has been lately extensively applied in manufacturing of Nickel-based super alloys, which compared to conventional manufacturing routes offers increased design flexibility and simplification of the manufacturing process. However, in order to make SLM process even more beneficial, its process time has to be reduced. One of the ways to tackle this problem is by tailoring process parameters through application of high laser power and base plate pre-heating. In this paper, a comparative study of optimum SLM fabrication conditions of Inconel 718 superalloy under high laser power and with and without plate pre-heating was conducted. Furthermore, the effect of layer thickness on melt pool characteristics, porosity and hardness were investigated to determine how laser power and pre-heating affect microstructure development.

Conventional manufacturing of titanium intermetallic alloys is associated with brittleness, hard machinability and, consequently, the high cost, which makes additive manufacturing a promising way of producing complex intermetallic parts. At the same time, γ-TiAl alloys exhibit good high temperature strength, fatigue, and oxidation resistance. In the present study the gamma-based alloy spherical powders were prepared by mechanical alloying from elemental powders followed by the plasma spheroidization process. Microstructure and phase composition of the produced powders were studied after different milling times in a planetary mill. The optimally milled powders were treated in the flow of thermal plasma to obtain powder particles with a high degree of sphericity. The produced spherical powders were used in Selective Laser Melting (SLM) process with high preheating temperatures to obtain crack-free intermetallic samples. The microstructure and phase composition of the SLM-ed TiAl-samples were investigated with regard to different process parameters.

In this paper, laser powder-bed fusion (L-PBF) additive manufacturing (AM) with a high-temperature inductive platform preheating was used to fabricate intermetallic TiAl-alloy samples. The gas atomized (GA) and mechanically alloyed plasma spheroidized (MAPS) powders of the Ti-48Al-2Cr-2Nb (at. %) alloy were used as the feedstock material. The effects of L-PBF process parameters-platform preheating temperature-on the relative density, microstructure, phase composition, andmechanicalproperties ofprintedmaterialwere evaluated. Crack-free intermetallic samples with a high relative density of 99.9% were fabricated using 900 °C preheating temperature. Scanning electron microscopy and X-Ray diffraction analyses revealed a very fine microstructure consisting of lamellar α₂/γ colonies, equiaxed γ grains, and retained β phase. Compressive tests showed superior properties of AM material as compared to the conventional TiAl-alloy. However, increased oxygen content was detected inMAPS powder compared to GA powder (~1.1 wt. % and ~0.1 wt. %, respectively), which resulted in lower compressive strength and strain, but higher microhardness compared to the samples produced from GA powder.

The paper presents the results of the composition, structure and mechanical characteristics of heat-resistant iron based alloy samples produced by laser cladding. The samples were manufactured using secondary raw material (chips), which were converted into powder by plasma spheroidization. The crystalline transformations for the as-processed as well as after several heat treatment conditions were analyzed. Phase, chemical composition and mechanical characteristics were investigated.