【作者】 沈琦；

【导师】 林忠钦； 李永兵；

【作者基本信息】 上海交通大学 ， 车辆工程， 2012， 博士

【摘要】 节能、环保、安全是当今汽车工业的发展主旨。为了在实现车身轻量化的同时满足乘员安全性要求，以双相钢为典型代表的先进高强钢材料在白车身结构中的使用比例正日益上升。目前，电阻点焊技术仍是车身制造中的主要连接手段，据统计，平均每辆白车身上约有3000～5000个焊点。然而，高强钢的大量使用，却给电阻点焊工艺带来了一系列挑战。区别于传统低碳钢，高强钢中普遍含有较高的碳当量与较多的合金元素，在快速加热、冷却的焊接过程中，极易形成大量淬硬、粗大的板条状马氏体组织，导致接头韧性、疲劳寿命降低，容易发生接头界面断裂失效。传统基于工艺参数调整或焊后热处理的质量控制方法，能够快速增大熔核直径或部分弱化接头组织的淬硬性，但往往以能耗的大幅攀升或生产效率的降低为代价。作为一项节能、高效的工艺方法，电磁搅拌控制（简称磁控）技术通过外加磁场对熔化金属的搅拌作用，影响并改变工件内部的温度场分布与冷却结晶过程。迄今，磁控技术已成功应用于连续铸造、电弧焊等领域。基于上述背景，本文提出将磁控技术与传统电阻点焊过程相结合的磁控电阻点焊工艺方法，旨在通过外加磁场与焊接电流的交互作用产生洛仑兹力，改变熔核内的流动传热模式。针对这一研究目标，本文从磁场施加模式入手，探讨了外部磁场的最优施加方向与分布模式，在此基础上搭建了磁控电阻点焊原型实验平台；并进一步建立了综合描述力、电、热、磁、流五个物理场变化的磁控电阻点焊过程数值模型，从“电磁搅拌改变熔化金属流动模式”的本质行为揭示了外加磁场对电阻点焊过程传质传热行为的控制机理；通过对点焊接头的宏观、微观形貌分析、以及静态、动态力学性能测试，明确了磁控技术对高强钢电阻点焊焊接质量的改善效果，并基于对磁控电阻点焊熔核形成规律的研究建立了质量评价准则；最后对典型车用超高强钢的电阻点焊接头进行了磁控效果实验验证，并从工程应用角度出发，进一步开发完善了磁控实验系统。本文为磁控电阻点焊工艺方法在车身制造中的推广应用奠定了理论基础。全文的主要研究内容如下：1)外部磁场施加模式与磁控电阻点焊原型实验平台理论分析了多种外部磁场作用模式下，电阻点焊过程的电、磁场分布特性、熔化金属流动模式、以及形核状态，从而明确了磁控电阻点焊方法所施加的外部磁场需满足三方面条件：平行于工件/工件接触面、围绕电极臂中心线呈中心轴对称形式分布、以工件/工件接触面呈镜面对称形式分布。在此基础上，建立了以一对轴向充磁环形钕铁硼永磁体为外部磁场源的磁控电阻点焊原型实验平台。2)磁控电阻点焊过程多物理场数值建模与传质传热行为演化机理基于ANSYS11.0/Multiphysics软件平台，通过二次开发与耦合策略制定，实现了结构场、电场、热场、磁场、流场之间的交互载荷传递与数据转换，建立了综合描述磁控电阻点焊物理过程的数值模型。从“电磁搅拌改变熔化金属流动模式”的本质揭示了外加磁场对电阻点焊过程传质传热行为的控制机理。结果表明，在磁控电阻点焊过程中，熔化金属不仅在通过电极中心轴的径向平面内流动，同时也在外加周向磁场力的驱使下表现出强烈的冲刷熔核径向边缘的离心运动趋势；且随着焊接时间的增加，该周向离心运动模式逐渐占据主导。与传统点焊相比，磁控电阻点焊过程中的磁流体流速更快；熔核内高温带呈长条状沿直径方向均匀分布；最终形成的熔核形状呈现“两头厚、中间薄”的花生壳状。3)磁控技术对高强钢电阻点焊焊接质量问题的改善机理通过对DP590、DP780的实验研究发现，与传统电阻点焊的椭圆体状熔核相比，磁控电阻点焊的花生壳状熔核呈现“更宽、更扁”的变化趋势；点焊接头的微观组织更均匀，淬硬性降低。对于1.25mm DP780钢板，在外加磁场作用下，接头的拉剪力增幅约12.0%，失效时刻所对应的最大位移增幅约15.9%，接头的高周疲劳性能有所提升。当焊接电流处于发生界面断裂与母材撕裂的临界值附近时，外加磁场能够有效改善电阻点焊接头的失效模式，且失效断口表面存在更多吸能性强的韧窝组织。4)磁控电阻点焊熔核尺寸评价准则与典型工程应用针对磁控电阻点焊的熔核形状特点，以接头拉剪强度为衡量标准，建立了熔核尺寸评价准则：当焊透率大于25%时，熔核直径越长，接头强度越高；当焊透率处于15%～25%时，熔核直径与焊透率同时成为衡量接头强度的主要尺寸参数；当焊透率小于15%时，焊接质量均不合格。基于此评价准则，通过进一步实验研究发现：为了获得相同质量的点焊接头，采用磁控技术能够将所需的焊接电流降低约300～500A，并适用于DP1000等超高强钢。此外，基于励磁线圈的电磁搅拌系统的开发在一定程度上提升了磁控电阻点焊技术在车身制造中的可应用性。综上所述，本文系统性地研究了外加磁场对电阻点焊过程的控制机理，形成了面向车用先进高强钢的磁控电阻点焊工艺方法，为磁控技术在电阻点焊质量控制领域的应用奠定了理论基础。更多还原

【Abstract】 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.更多还原