Preliminary study of the heating mechanism of magnetic nanoparticles for induction welding

Abstract

The aerospace industry has increasingly prioritized sustainability by adopting lightweight materials, shifting from metal alloys to polymers and polymer composites. Thermoplastic polymers, in particular, are gaining attention due to their recyclability and ease of reshaping. As the demand for high-performance structures grows, advanced joining techniques are becoming essential. Traditional joining methods are expected to be replaced by fusion bonding, with induction welding standing out for its non-contact nature and continuous processing capability. This technique typically relies on susceptors (e.g., metal meshes) to convert electromagnetic energy into heat. However, these materials can cause uneven heating and weaken mechanical properties. Magnetic nanoparticles present a promising alternative as minimally invasive conductive susceptors.
Originally studied for biomedical applications, magnetic nanoparticles generate heat under an alternating magnetic field through hysteresis losses. Researchers have demonstrated their ability to heat polymers above their melting points for induction welding. However, a major challenge is preventing excessive heating, which can degrade thermoplastics. To address this,it was proposed to select ferromagnetic susceptors with Curie temperatures that match thermoplastic processing temperatures. This ensures effective heating while preventing overheating, offering a self-regulating thermal solution for induction welding.

This thesis aimed to gain knowledge on thermal and magnetic capabilities of selected nanoparticles: magnetite (Fe3O4), Nickel (Ni), Nickel-Zinc ferrite (NiZnFe4O4) and Chromium Dioxide (CrO2). These materials were tested via MagneTherm equipment to identify the optimal induction heating parameters, in terms of strength and frequency of the applied magnetic field. Magnetite showed the best heating capabilities among all particle types, however due to the high detected Curie temperature it does not represent a good candidate for further research. A good alternative was found while testing Nickel and Ni-Zn ferrite nanoparticles. These materials have shown slightly lower heat generation, compensated by Curie temperatures within the range of processing temperatures of multiple thermoplastics, and consequent good potentiality for this application.
Further characterization of the particles was performed to gain better understanding on their magnetic and morphological properties and to find correlations with their heating capabilities. In this second part, more traditional techniques such as SQUID, XRD, and TGA were employed.

This study confirms that MNPs are viable candidates for non-contact induction welding in aerospace applications, presenting an efficient alternative to conventional heating methods. Induction heating experiments confirmed the correlation between alternating magnetic field parameters (amplitude, frequency) and power generation. However, further optimization of nanoparticle dispersion, morphology, composition and scalability is required for industrial implementation. Future research should explore alternative nanoparticle compositions, embedding techniques, and evaluate their performance in thermoplastic induction welding prototypes.