| Nowadays, energy-saving, environmental protection, and safety have become thedevelopment theme of the automobile industry. In order to pursuit lightweight body aswell as meet occupant safety requirements, the proportion of advanced high strengthsteels, like dual phase steels, has been rising in body-in-white. By far, resistance spotwelding (RSW) is still the major joining method used in car body fabrication. It hasbeen reported that on average nearly3000to5000spot welds are made for each carbody. However, the traditional RSW technology has been faced with a lot ofchallenges due to the popularity of advanced high strength steels. Compared with thetraditional mild steels, advanced high strength steels usually contain higher carbonequivalent and more alloying elements, which lead to a large amount of coarse lathmartensites during the rapid heating and cooling of the RSW process. Those hard butbrittle microstructures will weaken the ductility and fatigue performance of the RSWjoints, and raise the probability of interfacial failure under external loads. It has beenpractically proven that process-parameters adjustment or post-weld heat treatment isable to increase the nugget diameter or partially reduce the brittleness. Nevertheless,these traditional quality control methods also raise energy consumption and reduceproduction efficiency. As an emerging technology featuring both low energyconsumption and high efficiency, magnetically-assisted (MA) control method base onthe electromagnetic stirring of molten metals has been successfully applied incontinues casting and arc welding by affecting the temperature distribution and thecrystallization during the heating and cooling process.In the present dissertation, a novel magnetically-assisted resistance spot weldingmethod (MA-RSW) has been proposed to affect the nugget formation process by theLorentz force generated from the interactions of an externally applied magnetic fieldand the conduction current. To achieve the research purpose, the mode of the externalmagnetic field was theoretically discussed, based on which an experimental platform of the MA-RSW process was established. Besides, a finite element model,considering the coupling of structural, electric, thermal, magnetic, and flow fieldsduring the MA-RSW process, has been proposed for the first time. The controlmechanism of the external magnetic field on the transport phenomena during theMA-RSW process were revealed by exploring the evolution of fluid flow under theelectromagnetic stirring effect. Moreover, the nugget shape, weld microstructures,joint mechanism performance in terms of quasi-static strength and fatigue life, wereall experimentally studied, based on which a nugget size evaluation criteria wasestablished for the MA-RSW joints. Furthermore, the quality improvement effect bythe MA-RSW method was experimentally validated on typical ultra-high strengthsteel used in car body manufacturing. In addition, the original MA-RSW experimentalplatform was further explored to improve its practicality. The present dissertationcontributes to the application of the MA-RSW method in automobile industrial. Thedissertation is composed of the following four parts:1) Mode of the external magnetic field and experimental setup of theMA-RSW processElectromagnetic field distributions, fluid flow patterns of the molten metal, andthe corresponding nugget shapes were theoretically analyzed and compared amongdifferent modes of the external magnetic field. The optimal mode was eventuallyconcluded as: parallel to the interface of sheet/sheet, axisymmetric about the centralaxis of the electrode arm, and symmetric about the interface of sheet/sheet. Base onthe theoretical analyses, an experimental platform of the MA-RSW process wasestablished with a pair of ring-shaped axially magnetized NdFeB permanent magnetsacting as the external magnetic source.2) Finite element modeling and the evolution mechanism of the transportphenomena during the MA-RSW processCoupling of the structural, electric, thermal, magnetic, and flow fields of theMA-RSW process were fulfilled by the proposed finite element model based onfurther development of the ANSYS11.0/Multiphysics platform. Control mechanismof the external magnetic field on the transport phenomena of the molten metal duringnugget formation of the MA-RSW process was revealed by exploring the evolution offluid flow under the electromagnetic stirring effect. Results showed that during theMA-RSW process, the molten metal would not only flow in the radial planes throughthe central axis of the electrode arm, but also show a strong trend of centrifugalmovement by rushing to the nugget edge along the diameter direction under the effectof the circumferential external magnetic force, which would become the dominantfluid flow mode in the middle-late welding stage. Compared with the traditional RSWprocess, the maximum flow speed of molten metal during the MA-RSW process was much higher. The band-shaped high-temperature zone within the MA-RSW nuggetuniformly distributed along the nugget diameter direction. The eventually formedMA-RSW nugget was peanut-shell-shaped with the two ends thicker than the center.3) Quality of the resistance spot welded advanced high strength steels underthe effect of the external magnetic fieldIt has been found from the experimental study on DP590and DP780thatcompared with the traditional ellipsoid RSW nugget, the peanut-shell-shapedMA-RSW nugget was wider and thinner. Besides, the weld microstructures were moreuniform with brittleness reduction. For spot welded1.25mm thick DP780steel sheetsunder the effect of the external magnetic field, the tensile-shear force and themaximum displacement at failure were increase by about12.0%and15.9%,respectively, and the fatigue performance was also improved, especially in high cycleconditions. Moreover, under the effect of the external magnetic field, the fracturemodes of the welding joints were improved when the welding current was close to thecritical value required to prevent interfacial fractures. Besides, more dimples, whichare capable of energy absorption, were found at the fracture surfaces.4) Size evaluation criteria for MA-RSW nuggets and typical engineeringapplications of the MA-RSW methodFor the shape characteristics of the MA-RSW nugget, a quality evaluationcriteria was established based on the relationship between nugget size andtensile-shear force as follows: for MA-RSW joints with the penetration more than25%, the wider the nugget diameter is, the higher that joint strength will be; forMA-RSW joints with the penetration between15%to25%, both the nugget diameterand the penetration are the major size attributes evaluating weld quality; forMA-RSW joints with the penetration less than15%, the strength will be tremendouslyreduced and the weld quality is poor. Further study based on the size evaluationcriteria found that under proper welding parameters the MA-RSW process could be analternative way to guarantee weld quality as well as reduce energy consumption bynearly300~500A welding current reduction. Moreover, the MA-RSW method wasexperimentally validated on ultra-high strength steel DP1000and the experimentalplatform of the MA-RSW system was further developed to improve the flexibility andcontrollability.In summary, the present dissertation has systematically studied the controlmechanism of the external magnetic field on RSW process with advanced highstrength steels, which has laid a firm foundation of the application of the MA-RSWmethod in the RSW field. |
PDF Full Text Request