Permanent magnets are used in electric motors, generators or actuators, as well as in diverse magnetic sensors, their chief purpose being to provide a constant magnetic flux [1, 2]. Ferromagnetic steels, ceramic hard magnetic ferrites as well as alloys such as AlNiCo or sintered SmCo and NdFeB are potential materials for magnetic devices [2]. The strength of the magnetic field the magnets can create, the magnetization M, and the resistance against opposing magnetic fields, the coercive field Hc determine the performance of permanent magnets, typically measured by the maximum energy density product (BH)max [3]. Rare earth permanent magnets such as SmCo or NdFeB are the most performant commercial permanent magnets. NdFeB type magnets provide the highest magnetization and SmCo type magnets can tolerate high temperature applications up to 350 °C and have higher coercive fields as well as corrosion resistance [4–6].
The time stability of the magnetic flux is a fundamental requirement, especially for magnetic sensors, to assure reliable measurements as small changes of intrinsic magnetic properties or the position of the magnet in the measurement setup affect the accuracy of the sensors [7–9].
Due to the brittleness of rare earth permanent magnets, handling and mounting are challenging [10]. Permanent magnets are typically mounted by adhesives, such as epoxy resins, and rarely by mechanical interlocking into diverse magnetic sensor devices [10–13]. The strength and durability of adhesive bonds are relatively low compared to other joining methods and, most importantly, the mechanical properties of the joints change during the lifetime of the device because of the viscosity of polymers [10, 14, 15].
Using adhesive joining techniques, applications at temperatures higher than 150 °C are also problematic as the joint strength decreases at elevated temperatures despite the use of high-performance polymers, such as Polyphenylene sulphide (PPS) [16].
Ultrasonic welding is one promising technique to realise hybrid joints between metallic substrates and brittle permanent magnets. So far, ultrasonic welding is mostly used in the packaging industry to reliably join polymers quickly and cost-effectively [17], or to join soft non-ferrous metals such as aluminium and copper for electrical applications [18]. Ultrasonic welding of hybrid material systems, such as metal/CFRP joints, has been a focus of research during the last decade. For example, aluminium/CFRP joints have been investigated [19–22]. In addition several multi-metal combinations like aluminium and magnesium, copper, steel or titanium have been ultrasonically welded [23–28].
Ultrasonic welding of brittle materials, such as glasses and ceramics with metals is challenging and has been investigated, partially using an air beared anvil and a ductile aluminium interlayer to ensure homogeneous stress distribution on the joining partners and to prevent damage to the brittle glass and ceramic parts [29, 30].
Regardless of the ultrasonic welding type (i.e. polymer or metal ultrasonic welding subdivided in spot-, torsional, or seam welding), the sonotrode must induce a sufficient amount of welding energy into the joining zone by oscillation in contact with the upper joining partner since the formation of joints by ultrasonic welding is caused by the relative motion between the joining partners. In the case of metal/metal joints, as investigated in this work, the peaks of the topography are flattened and plastically deformed and the surface structures of the joining partners assimilate in the first step. Afterwards, oxide layers on the surfaces are locally broken up, which allows a direct metal/metal contact between the joining partners additional to mechanical interlocking. [23, 25, 31–34]
Figure 1 shows the formation of a metal /metal joint by ultrasonic welding, schematically.
The aim of the presented investigations is the formation of high performance 316L/Nd2Fe14B joints for the first time to compete state-of-the-art adhesive joining featuring high joint strength as well as a speedy and reliable joining process. Hence, 316L flat rings, representing potential components of magnetic devices, are welded to thermally demagnetized Nd2Fe14B, one of the most common rare earth permanent magnetic materials [4–6], with the typical ~ 25 µm thick Ni/Cu/Ni coating against corrosion [35, 36]. To quantify the performance of the hybrid material systems, the joint strength and the magnetic flux density were determined and evaluated in relation to the flux of a single Nd2Fe14B, magnet in its initial state.