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%0 Thesis
%A Ukwungwu, David
%T Electromagnetic evaluation and quantification of welding process for packaging of electrical steel sheets; 1. Auflage
%V 63
%I RWTH Aachen University
%V Dissertation
%C Düren
%M RWTH-2025-06772
%@ 3819101381
%B Aachener Schriftenreihe zur Elektromagnetischen Energiewandlung
%P xi, 155 Seiten : 26 Illustrationen
%D 2025
%Z Dissertation, RWTH Aachen University, 2025
%X In order to ensure a mechanically stable magnetic core for electromagnetic energy converters such as electrical ac machines and transformers, it is essential that the electrical steel sheets are securely packaged together. The welding process is one of the most utilized packaging technologies nowadays. This is due to the ease of its integration into the manufacturing process. The complex interaction between the thermal degradation and the deterioration of the electromagnetic properties of welded cores necessitates the need to characterize the influence of weld-packaging on the electromagnetic properties of non-oriented (NO) electrical steel lamination with the view of developing a locally varying material model. The study on the impact of weld-packaging process parameters on the electromagnetic properties of packaged cores highlights the benefits of using low powered lasers to minimize the deterioration effect of weld-packaging. This is because, a general decrease in the magnetizability (relative permeability) and increase in specific iron loss is observed with increasing laser power due to decreases in temperature gradient. An increase in the laser focal position resulted in less deterioration, as it reduces the depth of thermal energy penetration (heat affected zone) through its impact on the resulting temperature gradient. Higher working pressure is observed to affect the electromagnetic property deterioration positively (reduction) through its impact on the reduction of the energy penetration depth. However, its overall influence on electromagnetic deterioration is dependent on the formation of weld defects resulting from the high energy densities. Furthermore, the analysis of the impact of different laser topologies revealed that high powered lasers with increased fibre diameter, leading to low power density, resulted in less deterioration of the electromagnetic properties of the weld-packaged cores in comparison to lasers with low fibre diameters. This is attributed in part to the reduced effective heat affected zone (penetration depth), resulting in low residual stress accruing from the low temperature gradient of the process, and partly due to decreased possibility of the weld defects formation because of the reduced thermal energy density. The quantification of the impact of applied stress using a series of electromagnetic characterization methods shows, that whereas, an improvement effect of low stress on the electromagnetic properties due to the reduced resistances to domain wall mobility resulting from the effect of stress on the free energy of the domain is seen, increased deterioration of the annealed samples is observed with increasing stress value. The deterioration behavior of the samples annealed at different temperatures is seen to be frequency dependent. This is because of the relationship between the increased average grain size and the different iron loss components. Conclusively, the determined advantage of using low laser power, increased (positive or negative) laser focal position and a determined optimal working pressure on the degradation effect of weld-packaging on the electromagnetic properties due to lower temperature gradient associated with the parameters validates the second hypothesis of this work, which states that a reduction in residual (internal) stresses and average grain size can be achieved with the modification of the laser welding process parameters. Characterization of the annealed samples shows that the effects of residual stress due to temperature changes are higher than the influences of increased grain size on the electromagnetic properties of the material. This is seen in the characterized deteriorations at low frequencies, which is due to Villari-reversal that leads to deteriorations. This validates the first hypothesis of this work, which states that the effects of micro-structural and residual stress changes on the magnetization and iron loss of electrical steel material can be quantified. In order to analyze and assess the impact of weld-packaging using simulation, a locally varying material model that accounts for the changes in the electromagnetic properties of the welded core due to the micro-structural degradation is developed. It maps the local variations in electromagnetic properties (macroscopic) caused by packaging effects with the changes in micro-structure and residual stress (microscopic). The simulated results show an overall decrease in the magnetizability and increase in specific iron loss of the core due to increased residual stress (reduced domain wall mobility) associated with weld-packaging. The reduced mobility is due to the obstructed domain wall movements resulting from the domain dislocations (increased residual stress) around the grain boundaries because of the domain restructuring aimed at minimizing the magnetic free energy. Although the overall impact of welding on excess loss components is generally positive due to increased domain wall smoothness, the increases in non-linear loss are due to increased domain shape deterioration, which hinders domain mobility at saturation results. The simulation results also verify the third working hypothesis, that a combination of experimental tests and simulations enables a separate consideration of the impacts of weld-packaging on the electromagnetic properties of the core.
%F PUB:(DE-HGF)11 ; PUB:(DE-HGF)3
%9 Dissertation / PhD ThesisBook
%U https://publications.rwth-aachen.de/record/1016145