Local Stiffness Tailoring of Magneto-Active Composites Produced by Laser Powder Bed Fusion

Mechanically soft sensors and actuators are beneficial when compliant and safe interaction with the human body is needed [1]. Magnetic actuation mechanisms allow fast response, wireless operation and the possibility to operate in enclosed confined spaces. With additive manufacturing, the production of magneto-active composites in complex and bioinspired shapes is possible. However, to completely transfer the functional properties of biologic systems to technology, the fabrication of composites with locally different mechanical properties is needed, since natural materials often have gradients in their mechanical properties [2]. In addition, in a soft robotics system like a locomotion assistance system the mechanical soft functional materials are often connected to rigid components, which results in the stiffness mismatch at the interface, which can lead to mechanical failure [3].<br/><br/>In this work, we present a method to locally tailor the stiffness of a magneto-active compound, consisting of hard magnetic Nd₂Fe₁₄B particles in a thermoplastic polyurethane (TPU) matrix with laser powder bed fusion. In this process, a laser beam selectively fuses powder layer by layer. By utilizing different laser parameters at different locations during the process, the mechanical properties of the composite are modified locally. For the process, the digital file of the parts must be divided into regions which should have different properties in the end. Hereby, the minimum layer height of these regions must be an integer multiplier of the layer height if the laser parameters are changed in the z-direction. If a gradient in x-y direction is needed, these regions should be the same size as the hatch distance.<br/>The range in which the mechanical properties can be tailored is investigated with compression and tensile tests of the composite produced with different laser parameters. The stiffness can be increased tenfold when the laser power is increased from low to high values. The stiffness gradient within one sample is verified by line scans of Vickers indentations with a nanoindentation system.<br/><br/>After magnetisation of the composites, the actuation performance is tested within an electromagnet. We present a shape that contracts when a magnetic field is applied. The shape consists of two platforms with beams in between the platforms to provide flexibility. The two platforms can then connect to other parts of a soft robotics system. With the presented method, the platforms can be produced with high stiffness and gradually reduced stiffness towards the centre of the shape, where more flexibility is needed for the actuation. A larger deformation can be reached when a magnetic field is applied to the gradient composite in comparison to a non-gradient counterpart. We show that the production of shapes with locally tailored stiffnesses with a single process is possible with laser powder bed fusion, which can be applied in bioinspired magneto active composites and help to integrate the soft functional components into robotic systems.<br/><br/>[1] Kim, Y., Parada, G. A., Liu, S., & Zhao, X. (2019). Ferromagnetic soft continuum robots. <i>Science Robotics</i>, <i>4</i>(33).<br/>[2] Liu, Zengqian, et al. "Functional gradients and heterogeneities in biological materials: Design principles, functions, and bioinspired applications." <i>Progress in Materials Science</i> 88 (2017): 467-498.<br/>[3] Rothemund, Philipp, et al. "Shaping the future of robotics through materials innovation." <i>Nature Materials</i> 20.12 (2021): 1582-1587.<br/><br/>This work was financially supported by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation), Project ID No. 405553726, TRR 270 and the RTG 2761 LokoAssist (Grant no. 450821862).