Abstract Shape Memory Alloys (SMAs) are a popular class of actuators widely used in complex soft robotics applications due to their shape memory effect, high recoverable strain, and stress. However, most existing actuation models frequently fail to accurately capture hysteresis and dynamic loading behavior while remaining computationally efficient. Moreover, current control strategies often lack adaptability, robustness, and the ability to generalize to varying system dynamics. This paper presents a robust adaptive closed-loop controller for electro-thermally actuated Ni–Ti SMAs, developed based on a Finite State Machine framework to address these challenges. The proposed controller is designed to compensate for disturbances and uncertainties in the SMA behavior. Experimental validation and statistical analysis have demonstrated the effectiveness of the $${\mathcal {L}}_{1}$$ L 1 adaptive controller across various SMA configurations, enabling precise strain and stress target tracking. Finally, the controller is deployed to a case study involving a Ni–Ti SMA-powered assistive robotic device, where it successfully manages position tracking with enhanced performance.

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 Ni50.9Ti49.1 (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.