Influence of Surface Preparation on Cracking Phenomena in TIG-Welded High and Medium Entropy Alloys

Multi-element systems with defined entropy (HEA—high entropy alloy or MEA—medium entropy alloy) are rather new material concepts that are becoming increasingly important in materials research and development. Some HEA systems show significantly improved properties or combinations of properties, e.g., the overcoming of the trade-off between high strength and ductility. Thus, the synthesis, the resulting microstructures, and properties of HEA have been primarily investigated so far. In addition, processing is crucial to achieve a transfer of potential HEA/MEA materials to real applications, e.g., highly stressed components. Since fusion welding is the most important joining process for metals, it is of vital importance to investigate the weldability of these materials. However, this has rarely been the subject of research to date. For that reason, in this work, the weldability depending on the surface preparation of a CoCrFeMnNi HEA and a CoCrNi MEA for TIG welding is investigated. The fusion welding of longer plates is described here for the first time for the CoCrNi alloy. The welds of both materials showed distinct formation of cracks in the heat affected zone (HAZ). Optical and scanning electron microscopy analysis clearly confirmed an intergranular fracture topography. However, based on the results, the crack mechanism cannot be conclusively identified as either a liquid metal embrittlement (LME) or hot cracking-like liquid film separation.

Tribology in Space Robotic Actuators: Experimental Method for Evaluation and Analysis of Gearboxes

Liquid lubricants are critical to enable long-life operation of high-performance machinery, such as geared actuators employed in robotics. In space applications, actuator gearboxes must operate in low temperatures, where liquid lubricants face inherent problems related to low temperature rheology. Heaters are relied upon to provide acceptable gearbox temperatures. Unfortunately, heating is energy-intense and does not scale well with increasing mechanism mass and performance. Effective boundary lubrication (BL), on the other hand, can minimize problems of low temperature rheology. BL relies on tribofilm formation over conventional fluid film separation. Effective space grade boundary lubricants can potentially allow for drastically reduced amounts of oil and the accompanying rheological problems. In this work, we describe the design of a methodology to evaluate and analyze tribology of actuator gearboxes operated under cryogenic oil-starved conditions in N2 atmosphere. The devised methodology enables research pertinent to space actuator tribology by accelerated testing and advanced analysis, as demonstrated by a lubricant candidate case study. Complementary microscopy techniques are discussed, and a novel methodology devised for gear internal microstructure analysis by X-ray microtomography (XMT) is presented.

Evolution of the Laser-Induced Spallation Technique in Film Adhesion Measurement

Laser-induced spallation is a process in which a stress wave generated from a rapid, high-energy laser pulse initiates the ejection of surface material opposite the surface of laser impingement. Through knowledge of the stress wave amplitude that causes film separation, the adhesion and interfacial properties of a film-on-substrate system are determined. Some advantages of the laser spallation technique are the non-contact loading, development of large stresses (on the order of GPa) and high strain rates, up to 108 /s. The applicability to both relatively thick films, tens of microns, and thin films, tens of nm, make it a unique technique for a wide range of materials and applications. This review combines the available knowledge and experience in laser spallation, as a state-of-the-art measurement tool, in a comprehensive pedagogical publication for the first time. An historical review of adhesion measurement by the laser-induced spallation technique, from its inception in the 1970s through the present day, is provided. An overview of the technique together with the physics governing the laser-induced spallation process, including functions of the absorbing and confining materials, are also discussed. Special attention is given to applications of laser spallation as an adhesion quantification technique in metals, polymers, composites, ceramics, and biological films. A compendium of available experimental parameters is provided that summarizes key laser spallation experiments across these thin film materials. This review concludes with a future outlook for the laser spallation technique, which approaches its semicentennial anniversary.

Resonant subwavelength control of the phase of spin waves reflected from a Gires–Tournois interferometer

Subwavelength resonant elements are essential building blocks of metamaterials and metasurfaces, which have revolutionized photonics. Despite similarities between different wave phenomena, other types of interactions can make subwavelength coupling significantly distinct; its investigation in their context is therefore of interest both from the physics and applications perspective. In this work, we demonstrate a fully magnonic Gires–Tournois interferometer based on a subwavelength resonator made of a narrow ferromagnetic stripe lying above the edge of a ferromagnetic film. The bilayer formed by the stripe and the film underneath supports two propagative spin-wave modes, one strongly coupled with spin waves propagating in the rest of the film and another almost completely reflected at the ends of the bilayer. When the Fabry–Perot resonance conditions for this mode are satisfied, the weak coupling between both modes is sufficient to achieve high sensitivity of the phase of waves reflected from the resonator to the stripe width and, more interestingly, also to the stripe-film separation. Such spin-wave phase manipulation capabilities are a prerequisite for the design of spin-wave metasurfaces and may stimulate development of magnonic logic devices and sensors detecting magnetic nanoparticles.

Hydrodynamic Lubricant Film Separation During Codirectional and Counter-Directional Rotations of Disengaged Wet Clutch Packs

The parasitic drag losses incurred by wet clutches, used in transmission systems, can significantly affect vehicular powertrain efficiency. This paper presents a novel implicit solution for hydrodynamic parasitic drag losses of disengaged clutches. These are generated by conjunctional friction, taking into account lubricant film separation during codirectional and counter-directional disk pair rotations. Lubricant film rupture is considered through application of incipient reverse flow boundary condition, which is representative of lubricant film separation. The results point to the operating conditions at which significant power losses occur. In particular, the time efficient model is able to represent the small losses incurred during codirectional rotation of disk pairs.

Oil film separation and drag torque in disengaged wet brakes

Multi-disc wet brake packs are commonly used for off-highway applications. Unfortunately, when disengaged, these components can generate significant levels of unwanted parasitic losses. This article presents a novel, validated model to predict the parasitic losses in disengaged multi-disc wet brake packs. The model employs pertinent boundary conditions to determine the position of the free boundary created by oil film separation. The experimental and analytical results demonstrate the significance of wet disengaged brake losses at a range of angular velocities, typically found in operation. The improved understanding accrued by the expounded time-efficient methodology presents significant opportunity to develop predictive tools for the whole driveline system efficiency calculations and component design optimisation for off-highway vehicles.