【作者】 沈洁；

【导师】 来新民； 张延松；

【作者基本信息】 上海交通大学 ， 车辆工程， 2011， 博士

【摘要】 安全、节能和环保是汽车技术发展的永恒主题,汽车轻量化是满足上述要求的有效手段和方法。双相钢等先进高强钢材料以其良好的成形性及耐撞性等优点正在轻量化车身制造中得到越来越广泛的应用。例如:在车身前纵梁、B柱等关键部位低碳钢内外板与双相钢加强件的差强差厚三层钢板匹配工况显著增加,应用比例达33%。而采用传统电阻点焊工艺连接差强差厚多层钢板时常常会带来焊接飞溅严重、工艺窗口狭窄和电极磨损剧烈等问题。为此,汽车制造商试图采用韧性结构胶接技术部分替代电阻点焊工艺,但受环境温度、湿度和胶粘剂老化等因素影响,胶接接头强度波动大,难以满足车身承载能力要求。因此,电阻点焊与胶接的复合连接工艺(简称胶焊技术),因其综合了上述两种连接工艺的技术优点,是目前多层钢板连接的首选工艺方法。然而,在差强差厚多层钢板胶焊过程中,受胶层和钢板材料属性的影响,熔核形成规律及质量控制复杂。例如:胶层的高粘稠性使钢板间接触不充分,引起通电生热异常,甚至难以焊接。差强钢板材料电阻率、导热率等物理属性的差异,使胶焊熔核区温度场分布不均匀,带来熔核偏移问题,熔核尺寸难以保证。因此,迫切需要开展差强差厚多层钢板胶焊熔核形成机理及质量控制方法的研究。为解决上述问题,本文以差厚差强三层钢板胶焊过程为研究对象,从胶层入手,分析了胶焊预压过程中胶层耦合作用下的钢板间接触电阻变化规律、焊接通电过程中钢板-胶层间的传热行为及其对胶焊动态电阻的影响,建立了三层钢板胶焊熔核形成过程的有限元模型,揭示了差强差厚三层钢板胶焊熔核形成机理及熔核偏移的内在原因。基于上述理论研究结果,提出了用于改善熔核偏移的不对称电场输入方法,形成了三层钢板胶焊工艺规范,推动了多层钢板胶焊技术在汽车车身制造中的应用。全文开展的主要研究工作如下:1)胶焊预压过程中胶层耦合作用下的钢板间接触电阻变化规律针对韧性胶层的高粘稠性导致的钢板间接触不充分问题,建立了考虑胶层膜电阻的钢板间接触电阻模型,获得了预压阶段中钢板间接触电阻变化规律,确定了保证钢板间充分接触的胶焊临界预压力。研究结果显示,预压后残留于钢板表面的胶粘剂形成的胶层膜电阻会使得钢板间接触电阻增大。高粘稠度的胶层种类引起的钢板间接触电阻上升更明显;而胶层厚度(0~1.4 mm)对于钢板间接触电阻影响不显著。对于厚度为0.8 mm的DC04, 1.4 mm的DP600和1.8 mm的DP780钢板,使其充分接触的临界预压电极力分别约为1.0 kN, 3.0 kN和4.5 kN。2)胶焊通电过程中钢板-胶层间的传热行为及动态电阻变化规律针对胶焊过程生热异常、飞溅严重问题,通过分析胶焊通电过程中钢板与胶层间的传热行为建立了胶焊动态电阻方程。研究发现,相同条件下胶焊过程中的电阻热由于钢板间接触电阻的增大而增多,而散热量却由于钢板表面通过空气向外散热的途径受到胶层阻碍而减少,这使得胶焊动态电阻比传统电阻点焊有所上升:粘稠度较大的胶层引起的动态电阻上升量更大;而胶层厚度并无显著影响;且同时涂有两层胶粘剂的三层钢板胶焊接头具有最大的动态电阻。3)差强差厚三层钢板胶焊熔核形成规律针对差强差厚三层钢板胶焊熔核偏移问题,建立了考虑胶层作用下界面接触状态的三层钢板胶焊过程有限元模型,分析了三层钢板胶焊熔核形成过程,揭示了差厚差强三层钢板熔核形成机理及熔核偏移的内在原因。研究发现,差厚差强三层钢板胶焊熔核在两个钢板间接触面上的形成时刻不同是造成熔核偏移的关键原因。对于厚度为0.8 mm DC04+1.4 mm DP600 + 1.8 mm DP780的三层钢板胶焊,熔核在两个钢板间接触面上的形成时间差约80 ms,胶层的存在使得相同电流下三层钢板胶焊熔核的形成时刻比无胶层存在时提前约40 ms。粘稠度较大的胶层能获得较大的熔核尺寸;而胶层厚度对于熔核尺寸的影响并不显著。4)差强差厚三层钢板胶焊工艺优化基于上述研究结果,分析了胶层涂覆位置、板材厚度匹配、板材匹配顺序等工艺因素对于差强差厚三层钢板胶焊熔核偏移影响规律。研究结果发现,只在薄板侧涂覆胶层或薄板侧钢板选用高强钢能使该侧熔核增大,有利于减小熔核偏移。在差强差厚三层钢板接头中,上下层钢板的厚度比不应超过1:3。为更好地提升胶焊质量,提出了采用4+6 mm电极的不对称电场输入工艺优化方法,此方法不仅可获得较大的熔核尺寸,还可拓宽三层钢板胶焊工艺窗口50%。综上所述,本文对于差强差厚三层钢板胶焊的熔核形成过程及其质量问题的成因进行了系统地研究,并提出了三层钢板胶焊工艺优化方法,为推动三层钢板胶焊工艺在汽车车身制造中的应用奠定了基础。更多还原

【Abstract】 Automotive lightweight is an effective way to realize the vehicle development for safety, energy conservation and environmental protection. Advanced high strength steels, such as dual phase steel, are steadily applied more and more widely in light-weighting auto-body manufacturing in the advantages of good forming and crash performance. For example, in side rail and B pillar assemblies, there are many joints made with three steel sheets including low carbon steels and dual phase steels. The application of multiple steels joint reaches about 33% of a joint on a common vehicle. However, when resistance spot welding, as the main joining method in production, applied to multiple sheet stackups, issues such as weld expulsion, narrow weld lobe and interfacial failure have developed. To deal with these issues, some vehicle manufacturers try to partly replace resistance spot welding with adhesive bonding technology. But the bond quality is significantly affected by the production environmental, like temperature, humidity, etc., and consequently the adhesive bonding is hard to meet the requirements of the vehicle body. As a result, the combination of adhesive bonding and resistance spot welding, called as weld-bonding, is recognized as the preferred method to join three steel sheets. However, weld-bonding of three steel sheets is affected by both the adhesive layer and steel properties. The viscous adhesive would make the steel sheets hard to contact enough. Dissimilar steel grades and thicknesses would lead to the unbalance distribution of the electric and temperature field in weld-bonding. The combination of these effects resulted in the weld nugget shift, and consequently it is hard to control the weld quality in weld-bonding of three sheet stackup. Thereforce, it is imperative to study the mechanism of weld-bonding of three sheet stackup to improve the weld quality.In this dissertation, the weld-bonding of three sheet stackup is focused on by starting from the adhesive layer. Theoretical analysis and experimental methods have been employed to study the contact resistance between the steel sheets with the adhesive during the squeezing stage, and the heat transfer and dynamic resistance during the welding stage. Based on these two aspects and considering the different properties of dissimilar steel sheets, a finite element model was developed to model the welding process and reveal the mechanism of weld-bonding of three steel sheets with various strengths and thicknesses. Furthermore, a new method using asymmetric electric field input was brought up to deal with the weld nugget shifting phenomenon and improve the weld quality in weld-bonding of with three steel sheets. Process specifications for weld-bonding of three steel sheets were established to widen the application in the auto-body. The main content in this dissertation contains four parts:1) Contact resistance between the steel sheets with the adhesive during the squeeze cycleSince the viscous adhesive is used, the steel sheets could have a poor contact under a given electrode force, and consequently result in the weld expulsion. To deal with this issue, a theoretical analysis model containing the influence of the adhesive has been brought up to study the contact regulations between the steel sheets with adhesive layer. The results showed that the remaining adhesive on the surface of the steel sheet increases the film resistance, and consequently the total contact resistance between steel sheets rises. The larger the adhesive viscosity is, the more the contact resistance would increase. However, the thickness of the adhesive layer takes little influence. It has also been found that a suitable electrode force is required to obtain the proper contact between the steel sheets in weld-bonding. The minimum electrode force to ensure the contact state in weld-bonding of 0.8 mm thick DC04, 1.4 mm thick DP600 and 1.8 mm thick DP780 is about 1.0 kN, 3.0 kN and 4.5 kN, respectively.2) Heat transfer and dynamic resistance between the steel sheets with the adhesive during the welding cycleDuring the welding cycle of weld-bonding, the adhesive change in heat is complicatedly coupled with the joule heat generation, which makes the process of weld-bonding obviously different from the traditional spot welding. To solve this problem, an equation for the balance between the heat generation and cooling in weld-bonding was established. On this basis, it was found that the adhesive not only increased the heat generation during welding due to the great contact resistance, but also minimized the heat loss on the steel surface. As a result, the dynamic resistance during welding for weld-boding increased significantly comparing to that for spot welding. Furthermore, test results showed that the weld-bonding with a high viscous adhesive had greater dynamic resistance than that with a low viscous adhesive. The adhesive thickness affects the dynamic resistance little. Extensive tests were peformed to asses the effect of the adhesive placement on the weld-bonding of three sheet stackup and test results showed that two layers of the adhesive palced between the three sheet stackup produced the maximum dynamic resistance.3) Weld nugget formation in weld-bonding with three dissimilar steel sheetsThe weld nugget shifting phenomenon is common in weld-bonding of three steel sheets and it would degrade the weld quality. A finite element model considering the effect of the adhesive on the contact state between the steel sheets has been established to study on this issue. Model results showed the weld nugget formation process of weld-bonding with various types and thicknesses of steel sheets and different adhesive layers by structure, electric and temperature field to reveal the mechanism of weld-bonding with three steel sheets. It was found that the different time of nugget initiation on the two interfaces between steel sheets is the key reason for the weld nugget shifting. However, the nugget initial time is closely related to the properties of steel sheets and adhesive. In the weld-bonding with 0.8 mm thick DC04, 1.4 mm thick DP600 and 1.8 mm thick DP780, the initial time of weld nugget on the interface between 1.4 mm thick DP600 and 1.8 mm thick DP780 was about 80 ms earlier than that between 0.8 mm thick DC04 and 1.4 mm thick DP600. Additionally, the weld nugget in weld-bonding initiated earlier 40 ms than that of spot welding with the same welding parameters. Weld-bonding with a high viscous adhesive gets a big weld nugget; however, the adhesive thickness affects little.4) Weld nugget shifting and process optimization of weld-bonding with three dissimilar steel sheetsBased on the research result about the mechanism of weld-bonding of three dissimilar steel sheets, process specification for detail production condition was established including different placement of adhesive layer, sheet combinations with different thicknesses and orders. It was found that the weld-bonding with one adhesive layer on the thin steel side could get a better weld nugget only on that interface, so it could improve the weld nugget shifting obviously. In a weld-bonding of one low carbon steel and two dual phase steels, the maximum thickness ratio between the top and bottom steel sheets is about 1:3. To further improve the weld quality, a symmetric electric field input method was developed by using different electrode caps to adjust the weld nugget shifting. The results showed that a couple of electrode caps with 4 mm tip diameter as the top one on the thin sheet side and 6 mm tip diameter as the bottom one on the thick sheet side, simply called 4+6 electrodes, had the best efficiency to improve the weld quality.In summary, modeling and experimental methods have been used in this dissertation to study the mechanism of weld-bonding of three steel sheets. Process specifications and optimization are developed to improve the implementation of weld-bonding of three steel sheets in manufacturing.更多还原