Most research to date in the field of L-PBF of nitinol has been with near equiatomic nickel-titanium binary pre-alloyed powders. Significant understanding over the last 10 years has been gained in relation to aspects such as microstructural evolution, control of elemental composition, phase transformation behaviour, control of defects and mechanical properties. Challenges with the use of pre-alloyed nitinol powders include expense and time constraints in producing new blends. Elemental blending with in-situ alloying of nickel, titanium and other constituents at the point of additive manufacture offers the opportunity to significantly accelerate the pace of research of nitinol material and part geometry design. Other potential advantages of elementally blended over pre-alloyed powders include reduced process costs, energy savings, and improved control over final part macroscopic properties as well as local microscopic composition and properties. The relationship between elemental and pre-alloyed powder characteristics, the nitinol L-PBF process parameters and the resulting melt homogeneity has not previously been examined. This paper addresses this gap by examining, for in-situ alloyed nitinol, the relationship between laser power, scanning speed, powder properties and the resulting solidification track characteristics, and comparing results to those from pre-alloyed powder.

The properties of nitinol, an alloy of nickel and titanium, include its good biocompatibility, corrosion resistance, damping capacity, fatigue strength, superelasticity, and shape memory characteristics. With other conventional methods, it has been challenging to achieve high precision and accuracy of the produced parts; however, laser powder bed fusion (L-PBF) has provided a useful new route for the processing of nitinol. While L-PBF offers many advantages, it also has drawbacks, including the potential for the formation of different phases and residual stress during rapid solidification. Post L-PBF heat treatment conditions aid in the generation of targeted stable phases. As reported by Lee et. al., the mechanical properties and transformation temperatures of the manufactured nitinol samples were largely influenced by the heat treatment. Fan et. al. showed an increase in the transformation temperatures by increasing the heat treatment temperatures after a solution heat treatment. Heat treatments that help in achieving the desired properties are two-step heat treatment processes. This study investigates the feasibility of applying a single-step solution heat treatment to Ni-rich nitinol and reports its effects on density, transformation temperatures, microstructures and microhardness for intended applications.

The processing of Ni 50 Ti 50 (at.%) alloy via powder bed fusion using a laser beam (PBF-LB) technique has been reported to significantly alter the atomic percentage of Ni in the final NiTi part when compared to the feedstock composition, and the ensuing variations in the phase characteristics and transformation temperatures. Residual stresses are also another challenge in the PBF-LB processing of NiTi. These PBF-LB challenges stem from the choice of laser scanning strategy, and process parameter selection typically defined by volumetric energy density (VED). Heat treatment also plays a crucial role in releasing residual stresses and effectively adjusting or tuning the final properties of as-built NiTi components.

Due to the unique characteristics of nitinol (NiTi) such as shape memory and super-elasticity, it is widely utilized in various industries, particularly in aerospace and biomedical applications. Additive manufacturing technology can provide the accurate dimensioning of NiTi components, even for those with intricate and complex geometries. However, the presence of residual stresses poses a significant concern from additive manufacturing. These residual stresses can adversely impact the structural integrity and performance of the manufactured parts, necessitating careful consideration and management. This study delves into a distinctive approach for predicting residual stresses within additive manufactured NiTi components, employing a simple, non-destructive, and cost-efficient method that integrates beam mechanics equations with finite element analysis (FEA). This work aimed to develop a rapid and accurate method for measuring the residual stress from additively manufactured nitinol beams which will be validated via established techniques such as hole-drilling, XRD and/or neutron diffraction.

Nickel-titanium offers a promising alternative to traditional actuation devices due to its shape memory effect (SME) large strain recovery, and damping capability. Shape memory alloy actuators, thereby, offer a low-shock alternative to traditional pyrotechnic release devices used in spacecraft and satellites as well as energy-absorption components such as spacecraft landing systems. Additive manufacturing (AM) allows the production of complex geometries benefiting from their lightweight and compact nature. A prime example is lattice structures, with a geometrically defined porous structures featuring a repeating unit cell pattern or patterns in space. Beyond being lightweight, these structures offer high specific strength, excellent shock absorption, heat dissipation, and biocompatibility. By examining different process parameters, AM presents the possibility of tailoring nitinol’s properties while potentially removing the additional step of post-processing heat treatments conventionally required for shaping shape memory alloys. This approach has the potential to save time, cost, and energy.

NiTi alloy stands out as a promising material, particularly in biomedical, aerospace, and heat pump applications, owing to its biocompatibility and distinctive properties, including exceptional shape memory (SME) and pseudoelasticity. These properties arise from the alloy's capacity to undergo phase transformations during heating/cooling and loading/unloading cycles. The specific phase transformation temperatures and pseudoelastic behavior are heavily influenced by the composition of the alloy. The introduction of transition elements such as V, Cr, Mn, Fe, and Co in place of Ni and Ti can lower the martensite start (Ms) temperature, while substitution with Hf, Zr, Ag, and Au for Ni and Sc, Y, Hf, and Zr for Ti can elevate the Ms temperature. Consequently, it is imperative to comprehensively comprehend the effects of third alloying elements on the properties of these alloys from an atomic perspective. Molecular dynamics (MD) simulations, operating at the atomic level, offer a valuable means to explore the impact of various compositions, including the addition of a third element, on the alloy properties, thereby enhancing their performance. However, a significant challenge in MD simulations lies in selecting reliable interatomic potentials between the elements. Developing new potentials poses challenges, prompting the evaluation of existing potentials for ternary systems in this study, which will be compared with experimental results. While several interatomic potentials have been proposed for binary NiTi SMAs, limitations arise when extending to tertiary alloys. Existing studies have reported ternary interatomic potentials for NiTiV, NiTiNb, and NiTiHf, but these are insufficient for capturing the phase transformation of these tertiary systems. To address this, a hybrid model will be employed, combining different types of interatomic potentials such as modified embedded atom method (MEAM), embedded atom method (EAM), Lennard-Jones (LJ), and Morse potentials. This approach aims to capture the phase transformation of ternary alloys doped with elements like Cu, Hf, Pd, Pt, Sc, Ta, Mn, Zr, Y, and Au, which can increase transformation temperatures such as the martensite start temperature (Ms), martensite finish temperature (Mf), austenite start temperature (As) and Austenite finish temperature (Af), during cooling and heating processes. These alloys, known as high-temperature shape memory alloys (HT-SMAs), hold significant potential for diverse applications, including actuators, owing to their unique properties and enhanced transformation behaviours.

Tantalum and silver are recognized for their outstanding biocompatibility and antibacterial ability, respectively. However, owing to their distinct chemical and physical properties, synthesizing alloys and composites by using Ta and Ag presents a considerable challenge. In this study, Ta-Ag composites, exhibiting good antibacterial ability, were successfully produced by using a solid-state cold spray technique. Notably, intriguing correlations were observed between Ag microstructure and antibacterial ability. To unravel this correlation, a comprehensive experimental and simulation analyze were conducted. It is found that the volume ratio of Ta to Ag in the feedstock powder result in different deformation histories for Ag during the cold spray process. This, in turn, leads to the formation of distinctive Ag microstructures within Ta-Ag composites. The varied Ag microstructures results in different Ag dissolution ability and the formation of an insoluble AgCl layer exhibiting varying morphologies, when Ag exposed in a high chorine ion environment, like in human body fluids. This consequently influences the concentration of Ag ion and ultimately determines antibacterial ability. The study demonstrates that Ag release rate and the related antibacterial properties could be alternatively controlled by changing Ag contains or by creating different deposition process by adjusting CS parameter.