Impact of ultrasonic vibration frequency on the quality of produced NiTi and NiTi-20Hf powders for additive manufacturing applications

• High-frequency ultrasound refines NiTi powder shape and narrows size distribution. • Recycling via UPA retains alloy composition and suppresses elemental segregation. • Oxygen and carbon contamination increase with ultrasonic frequency in re-powdering. • Phase transformations shift with morphology changes in recycled NiTi and NiTiHf. • UPA offers a sustainable method for high-quality powder reuse in 3D printing. The rapid expansion of additive manufacturing (AM) technologies has intensified the demand for high-quality metal powders and prompted the need for effective recycling solutions to address waste generation during production. This study investigates the impact of ultrasonic vibration frequencies (20 kHz and 40 kHz) in an Ultrasonic-Plasma Atomization (UPA) system on recycling NiTi and NiTi-20Hf pre-alloyed powders. The objective is to evaluate the effects of frequency variation on key material properties, including impurity levels, powder characterization, and microstructural homogeneity, to determine the suitability of the recycled powder for AM applications. The findings reveal that both vibration frequencies significantly influence the properties of the recycled powders. At 40 kHz, carbon impurities increased by 39 % in NiTi and 89 % in NiTi-20Hf, while oxygen contamination rose by approximately 150 % and 60 %, respectively, compared to the virgin powders. Similarly, at 20 kHz, carbon and oxygen impurities exhibited notable increases, albeit to a lesser extent. These increases are attributed to process-induced surface area changes, thermal oxidation, and equipment-related contamination. Despite the increase in impurities, the microstructural analysis, particle size distribution, and shape uniformity of the recycled powders demonstrated consistent properties, aligning with the quality requirements typically specified for AM feedstock, such as those outlined in standards like ASTM F3049 or equivalent guidelines. By addressing the challenges of impurity management and ensuring physical consistency, this research highlights the viability of ultrasonic recycling processes as a sustainable strategy to enhance material reuse and reduce resource dependency in the AM industry.