Simulation-assisted Design and Characterization of Abrasive Magnetic Suspensions for High Precision Finishing
Synthesis and Properties of Functional Materials
Final Report Abstract
The DFG funded research project ChAMPion set out to explore and optimize a surface finishing process using magnetic abrasive suspensions. The process was analyzed in a twofold manner – by means of experimental studies in the laboratory on the one hand and by means of computer simulations on the other hand. The goal of a finishing process is to either remove unwanted structures from a work piece surface or to smooth a rough surface roughness in general. Both features are caused by the type of forming process which was used to obtain a given surface. A certain controlled surface roughness is often relevant for its functionality. An internal surface of a microfluidic channel should be sufficiently smooth in order to enable capillary effects and to avoid trapped air bubbles. Surface roughness of a micro lens can cause unwanted reflections and ray scattering. Suspension-based surface finishing processes make use of abrasive grains within a carrier fluid which is mechanically forced along a surface. The grains gain a certain momentum due to the drag force of the fluid and might rotate due to shear flow. Upon contact with the surface a part of the kinetic energy of a grain’s motion is converted into plastic deformation energy of the surface which – possibly not before several grain impacts – causes some material from the surface to be removed. A shortcoming of this sort of finishing process lies in the fact that the grain motion is closely linked to the fluid motion. If, for example, a fluid passes an obstacle like a step or a restriction in a channel, a dead flow region will form right after the obstacle. In this region, abrasive grains will experience only very small drag and shear forces and virtually no material removal will take place. A promising method to cause surface finishing even in regions with unfavorable flow conditions is to apply an additional force on the abrasive grains. Magnetism is a possible candidate to generate such a force. If the grains are made of a magnetizable material, a constant external magnetic field causes the grains to form chains while a magnetic field which becomes larger with decreasing distance from the surface will force the grains towards the surface. In addition, the grains will feel a torque if they have a fixed axis of magnetization which is not aligned with the external magnetic field. All of the aforementioned magnetic mechanisms were analyzed with respect to their usefulness to improve the material removal rate in surface finishing. It was found that the gradient of the external magnetic field, i.e. its variation in magnitude with respect to its location, is a delicate parameter for this purpose. If it causes a sufficient strong force on the magnetic abrasive particles their paths will deviate from the streamlines of the fluid and they will end up having a larger probability to hit the surface. However, if the magnetic gradient is too large, the force on the grain will cause them to form a layer which effectively protects the surface from material removal. As a consequence the gradient must be chosen to have just the right magnitude in order to improve the process. For angular – i.e. very edgy – grains it was discovered in simulations that a magnetization axis pointing towards a sharp corner of the grain can be beneficial. If the external magnetic field is tilted with respect to the surface the grains will preferably hit the surface with that sharp corner and thereby increase the material removal rate. Yet, care has to be taken in choosing the tilting direction as it was demonstrated experimentally that depending on the actual grain shape the frictional work between a magnetoabrasive suspension and a surface can either increase or decrease for a certain tilt angle. In order to study the magnetoabrasive process in the laboratory a novel lab-on-a-disk setup was developed which allows precise control of the suspension velocity by means of centrifugal forces. However, the necessary large magnetic field gradients predicted by the numerical simulations still pose an experimental challenge.
Publications
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Polymer Magnetic Composite Core Boosts Performance of 3D Micromachined Inductive Contactless Suspension. IEEE Magnetics Letters 7 (2016) 1-3
Poletkin, K. V., Lu, Z., Moazenzadeh, A., Mariappan, S. G., Korvink, J., Wallrabe, U., Badilita, V.
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A qualitative technique to study stability and dynamics of micro-machined inductive contactless suspensions. In: Proceedings of 19th International Conference on Transducers, Solid-State Sensors, Actuators and Microsystems (2017) 528-531
Poletkin, K. V., Lu, Z., Wallrabe, U., Korvink, J., Badilita, V.
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Stable Dynamics of Micromachined Inductive Contactless Suspensions. International Journal of Mechanical Sciences 131-132 (2017) 753-766
Poletkin, K. V., Lu, Z., Wallrabe, U., Korvink, J., Badilita, V.
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Levitating Micro- Actuators: A Review. Actuators 7(2) (2018) 17
Poletkin, K. V., Asadollahbaik, A., Kampmann, R., Korvink, J. G.
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Mechanical Thermal Noise in Micro-machined Levitated Two-Axis Rate Gyroscopes. IEEE Sensors Journal 18(4) (2018) 1390-1402
Poletkin, K. V., Korvink, J. G., Badilita, V.
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Modeling a Pull-In Instability in Micro Machined Hybrid Contactless Suspension. Actuators 7(1) (2018) 11
Poletkin, K. V., Korvink, J. G.
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Efficient calculation of the mutual inductance of arbitrarily oriented circular filaments via a generalisation of the Kalantarov-Zeitlin method. Journal of Magnetism and Magnetic Materials 483 (2019) 10-20
Poletkin, K. V., Korvink, J. G.
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Rheological behavior of Magnetic Colloids in the Borderline between Ferrofluids and Magnetorheological Fluids. Journal of Rheology 63(4) (2019) 547-55
Shahrivar, K., Morillas, J. R., Luengo, Y., Gavilan, H., Morales, P., Bierwisch, C., de Vicente, J.
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SPH Simulations of Magnetorheological Abrasive Flow Machining at a Microscopic Scale. In: Proceedings of PARTEC – International Congress on Particle Technology (2019)
Mohseni-Mofidi, S., Bierwisch, C.
