Abstract Integration of circular economy principles into additive manufacturing (AM) has emerged as a critical strategy for addressing economic and environmental challenges associated with the high-cost, resource-intensive production of NiTi shape memory alloys (SMAs). This review presents a comprehensive analysis of the ultrasonic plasma atomization (UPA) technique as an advanced recycling approach for converting NiTi AM waste into high-quality feedstock. Furthermore, UPA demonstrates significant potential as a dual-function method, enabling both alloying and fine powder production within a single integrated process. AM processes, such as powder bed fusion-laser beam (PBF-LB), binder jetting, and direct energy deposition (DED), often result in substantial material losses, exacerbated by powder degradation phenomena including oxidation, particle morphology changes, and loss of flowability. While conventional atomization techniques, such as gas and plasma atomization, offer partial solutions for powder production, they are limited by significant energy inefficiencies, high capital investment requirements, and increased risks of contamination. Moreover, powders produced via gas atomization typically exhibit inferior quality compared to those generated by UPA, including lower sphericity and a higher prevalence of surface satellites. In contrast, the UPA system, a hybrid technique combining high-frequency ultrasonic vibrations and plasma melting, offers precise control over droplet formation through acoustic cavitation, Faraday wave instabilities, and rapid solidification, enabling the production of highly spherical, homogeneous, and contamination-minimized NiTi powders. Drawing on thermofluidic principles, vibrational mechanics, and metallurgical kinetics, this review systematically deconstructs the UPA mechanism, highlighting key phenomena such as resonance-induced cavitation dynamics, ultrasonic capillary wave collapse, and non-equilibrium solidification. Furthermore, it investigates contamination pathways specific to UPA and proposes strategies for impurity mitigation, including sonotrode design optimization, inert atmosphere refinement, and post-atomization conditioning. Experimental findings are analyzed to demonstrate the feasibility of achieving high sphericity (> 0.9), compositionally stable, and thermally responsive NiTi powders from recycled feedstock. Finally, the review outlines a roadmap for industrial-scale deployment, advocating the integration of machine learning and closed-loop recycling models to enhance process predictability, quality control, and resource efficiency. The insights presented herein position UPA as a transformative solution for enabling sustainable, high-performance NiTi powder regeneration within AM workflows, advancing the circular economy in advanced manufacturing.

Abstract In recent years, laser metal deposition (LMD) has received significant attention for its ability to fabricate large, complex structures with high productivity and reduced costs across various industrial sectors. This technology offers new opportunities to produce large-scale, intricately shaped components at faster deposition rates. LMD shows great potential for the fabrication of smart materials, such as shape memory alloys (SMAs), enabling the development of advanced structures for novel applications. This study investigates the functional performance of NiTi shape memory alloys fabricated using LMD technology. Commercially available NiTi wire was used as feedstock to produce fully dense samples. The thermo-mechanical behavior of the printed samples was evaluated under varying operational conditions, including different loads and temperatures. Strain recovery tests, conducted at applied loads across a temperature range of 0–200 °C, demonstrated a promising shape memory effect. Furthermore, high-temperature mechanical cycling tests revealed that the additively manufactured NiTi samples exhibited stable functional behavior without the need for post-processing heat treatment. These findings suggest that LMD-fabricated NiTi components are suitable candidates for actuator applications based on the shape memory effect within the 0–200 °C temperature range.

Abstract A binary Nickel-Titanium shape memory alloy was processed by Laser Powder Bed Fusion (LPBF) to manufacture lattice structures. The chemical composition of the powder was Ni 50.9 Ti 49.1 (at%) with the aim of obtaining pseudoelastic properties in the printed part. Adapted scanning strategies were chosen and used on a standard LPBF machine to achieve complex geometries with small, uniform feature sizes and a more precise modification of the local energy input. The as-built structures were analyzed with optical, thermal, and mechanical methods by means of optical microscopy, differential scanning calorimetry, and compression tests. The results demonstrate that LPBF combined with adapted scanning strategies can generate complex and homogeneous NiTi geometries (like metamaterials and programmable materials). Furthermore, the results show that scanning strategies have a significant influence on the thermal and therefore mechanical properties of the structures. We conclude that adapted scan strategies overcome the limitations of ordinary contour-hatch scan strategies and lead to shape memory properties which cannot be realized with conventional manufacturing techniques.

Abstract This study investigates the effect of carbon on the mechanical and functional properties of an additively manufactured Fe-32Mn-6Si-5Cr-0.5Nb-xC alloy. A base alloy was processed using laser powder bed fusion, and the carbon content was altered during manufacturing using a nanoparticle dispersion-based in situ re-alloying approach. This method enabled an increase in carbon content from 0.06 wt.-% to 0.15 wt.-% and 0.23 wt.-%, respectively. The three resulting alloys were analysed both in the as-built condition and after heat treatments, including direct artificial ageing, as well as solution annealing and subsequent artificial ageing. While the different heat treatment states led to distinct microstructures, the carbon content affected the microstructure only after the additional heat treatments. Here, a higher carbon content led to an increase in the volume fraction of precipitates. Mechanical and functional properties were analysed under compression loading and subsequent unconstrained thermal activation. Depending on the carbon content and, more significantly, the heat treatments, the mechanical and functional properties could be tuned in a wide range. The highest mechanical strength was achieved through direct ageing, while a carbon content of 0.06 wt.-% in combination with solution annealing and ageing led to the highest recovery strain of up to 80%.

Abstract Iron-based shape memory alloy FeMnAlNiTi that exhibits over 400 °C temperature window of superelasticity and near-zero temperature dependence of transformation stress has garnered significant attention in the scientific community. Presence of nanoprecipitates is crucial to the functionality of this SMA, and we present a direct link between the degree of ordering of the precipitates, spinodal modulation and superelastic functionality. Upon quenching from 1225 °C, the microstructure spontaneously decomposes into BCC matrix and DO3 precipitates, both of which exhibit spinodal modulation. Prolonged aging of up to 200 h at 200 °C increases the degree of order of the precipitates as the modulated domains are annealed. However, no changes were detected in the precipitates' size or area fraction, ruling out the possibility of precipitate coarsening or nucleation. Concomitantly, the composition of the BCC matrix remains unchanged and continues to exhibit spinodal modulation with a wavelength of 1.13 nm to 1.24 nm. Finally, it is demonstrated that ultrahigh transformation stress of about 1.4 GPa can be attained along with an exponential decrease in the specific damping capacity and a consequent increase in the functional fatigue resistance.