【作者】 罗爱辉；

【导师】 陈关龙；

【作者基本信息】 上海交通大学 ， 车辆工程， 2007， 博士

【摘要】 电阻点焊电极主要承载着导电、散热、施压三大功能。在电阻点焊过程中,电极的温度上升的很快,在高温高压作用下,电极很快就会磨损失效。电极磨损失效的快慢不仅会影响焊接质量的可靠性和健壮性,而且将直接影响到轿车装配焊接工艺的费用和生产效率。电极对流换热作用是电极散热的重要手段,电极冷却不佳,将直接导致电极过早磨损,增加生产成本,进而极大影响焊接装配的生产效率和生产质量。现有的点焊电极研究主要集中在电极材料的改进和替换方面,很少考虑电极的对流换热作用对电极磨损的影响,部分实验研究表明冷却流体流速,水冷管管径和水冷管高度是影响电极对流换热作用的主要因素。然而现有的研究中缺乏对电极对流换热性能的综合认识和评价的数学模型,无法最大限度的发挥冷却水的换热能力。为此,本文综合考虑电极对流换热的影响因素,开展基于热流场耦合的电阻点焊电极对流换热特性建模与优化研究,旨在提高电极对流换热性能和效果,延缓电极磨损失效的速度,降低轿车生产成本。针对电阻点焊电极对流换热的问题,首先详细的分析电极对流换热的过程和特性,明确电极换热流体流态和电极散热机理;采用有限元仿真和热流场耦合算法,分析确定电极对流换热的主要影响因素;引入电极对流换热性能评价指标-冷却率和排热率的概念,建立面向冷却率的电极对流换热性能评价的响应面模型;在此基础上,展开电极对流换热结构的改进设计,研究基于遗传算法的多目标并行优化的求解方法和结论;最后通过实际工艺条件下电阻点焊对流换热案例分析和优化试验,验证本课题研究方法和结论的有效性。本文的主要研究工作如下:1)电阻点焊电极对流换热问题分析在对电阻点焊工艺及电极水冷却系统结构构成介绍的基础上,将电极对流换热分为流和热两个过程来展开对问题的分析。着重分析明确电极冷却流体的流态和特性,剖析电阻点焊工艺的各个阶段电极冷却流体热交换的情况;基于对电阻点焊过程中热接触、热源和热阻的描述,分析电极传热、散热的途径和过程,重点研究电极热输入的机理和组成,建立电极热平衡方程。针对电极对流换热问题,引入冷却率和排热率的概念,建立电极对流换热特性和效果的评价指标,并对影响电极对流换热性能的具体因素进行了分析和归纳,为后续进行建模优化和实际案例分析的工作奠定基础。2)电极对流换热过程仿真在分析了电极对流换热性能影响因素的基础上,通过对电极热流场的直接耦合计算,实现电极对流换热过程的一次性求解。借助于编制Gambit和Fluent的自动求解程序,建立有限元分析环境下电极对流换热过程的数值仿真模型,并通过大量的计算,研究电极的冷却流体特性和对流换热特性,着重探讨各类影响因素与电极对流换热效果之间的关系,为后续进行实验建模的工作提供重要的依据和手段。3)电极对流换热性能评价建模在前述分析的基础上,确立以冷却流体流速,水冷管管径和水冷管高度为设计参数,以电极对流换热性能评价指标为设计目标,建立8二因子水平实验点,6轴点实验点和1中心实验点的实验设计方案。根据实验回归分析的响应面法和中心组合实验设计法的基本规则,建立面向冷却率的电极对流换热性能评价的响应面模型,并引入方差分析和F检验,验证模型的显著性。通过对响应面模型的曲面图和等高线图分析,明确电极对流换热性能与设计参数之间的变化规律。4)电极对流换热结构改进和参数优化电极水冷管末端形式和水冷却腔结构形式影响冷却流体的流场分布,导致冷却壁面处边界层的厚薄不同,进而影响到电极与流体之间的热交换效率。应用已建立的电极热流场耦合仿真模型和分析方法,研究电极结构形式对电极对流换热效果的影响关系,确定电极对流换热结构的改进设计方案。根据电极对流换热问题的实际需求,建立以电极冷却率,生产率和冷却能耗等为指标的多目标综合优化模型,借助遗传算法并行搜索和概率搜索的优异特点,实现不同权重下最优匹配变量的并行全局搜索,求得问题最优解。5)电阻点焊电极对流换热案例分析及实验验证在实际的电阻点焊工艺条件下,输入到电极的热量是瞬时变化的。根据电极热输入机理和组成的分析结论,采用理论公式计算的方法获取电极热电接触参数,并在已有点焊分析模型的基础上,确定和分析电阻点焊电极瞬时热量输入的大小和规律。应用本文提出的电极热流场耦合的分析方法,研究实际点焊工艺条件下电极温度特性和对流换热性能,分析结构改进和优化设计方案对实际电极对流换热的影响和作用。通过分析电极对流换热效果对电极表面状态的影响,确立电极对流换热实验目标,并根据实验测量数据验证电极对流换热结构改进和参数优化的结论。本文在充分吸收和借鉴前人研究成果的基础上,对电阻点焊电极热流耦合方法和对流换热特性进行了深入的研究和探讨,并具体应用于点焊电极的冷却优化实例。本文的工作不但对电极的磨损失效研究有重要的意义,而且对电焊机的设计和电阻焊的实际应用也提供了重要的指导依据。更多还原

【Abstract】 Spot welding electrode takes three functions of conducting current, dissipating heat and applying pressure. During the process of spot welding, the temperature of electrode rises rapidly. Under high-temperature and pressure, spot welding electrode will quickly be wear and broken down. This failure process of electrode will not only reduce the reliability and robusty of weld quality, but also directly influences the cost and production efficiency of automobile welding and assembly. Heat convection is the main process of heat dissipation in electrode. Bad cooling condition on electrode will cause the premature wear and the increase of production cost. Furthermore, the production efficiency and quality of welding assembly will be influenced greatly. The existing researches of spot welding electrode are all focused on improving and replacing of electrode material without thinking about the result of heat convection in electrode. Also, there is absent of mathematic model on performance evaluation to electrode heat convection. It is incapable to make full use of the heat exchange potency of electrode cooling. Therefore, study on modeling and optimizing of heat convection for spot welding electrode based on heat and flow coupling analysis are performed detailedly and systematically. The aim is to improving the performance and effection of heat convection, reducing the speed of electrode wear and decreasing the production cost.Aiming at the problem of electrode heat convection, analysis about the process and characteristic of electrode heat convection are executed in details firstly. The flow pattern of cooling water is calculated out and the mechanism of heat dissipating is determined. By using finite element method and heat and flow coupling method, the relationship between the influencing factors and the heat convection performance is confirmed. According to the concept of cooling rate and heat removal rate which are the evaluating indicators for electrode heat convection, response surface model is built to evaluate the performance of electrode heat convection. Based on the above, improving design of electrode structure is implemented for better performance of heat convection and GA is utilized to study the multiple-object optimal method and result for electrode heat convection. Finally, the validity of the method and the conclusions is done by the case and experiment analysis of heat convection for spot welding electrode under actual technological conditions. To fulfill the above research objectives, the following five aspects of efforts are performed:1. Heat convection analysis of spot welding electrodeBased on the introduction of spot welding process and the composition of cooling system, the problem of electrode heat convection is separated into two procedures of heat and flow for analysis. Flow pattern and performance of cooling water are determined and the heat exchange of cooling water in each phase of spot welding is dissected. Based on the description of heat source and heat contact, the path and process of heat conducting, dissipating is analyzed and the emphasis is on the mechanism and composition of electrode heat inputting. In succession, heat balance equation is established for electrode. For convenience, the conceptions of heat cooling rate and heat removal rate are lead in to evaluate the performance and effection of electrode heat convection. At last, analysis on the influence factors of electrode heat convection is done as a base for the continued works.2. Simulation of the process of electrode heat convectionAccording to the analysis of influencing factors, one-time solution for the simulation of electrode heat convection is implemented by using direct coupling calculation of heat and flow fields. Based on the code Gambit and Fluent and its parametric language, numeric simulation mode for electrode heat convection in FE analysis environment is established. By numerous calculations, analysis of the characteristic of cooling flow and heat convection is done and the emphasis is on the relationship between the influencing factors and heat convection effect. This is the tool for the coming testing and modeling.3. Performance evaluating and modeling on electrode heat convectionBased on the front analysis and the reference research, flow velocity, pipe diameter and pipe height are selected to be defined as the design parameters and performance evaluating indicators of electrode heat convection are defined as the design object. At the same time, test design scheme with 8 two level factors test points, 6 axial test points and 1 center test point is carried out. According to the basic rules of response surface method and center utilized design method, response surface model for performance evaluating of electrode heat convection is built. Leading in the variance analysis and F test, validation of model significance is done. By analysis on the surface drawing and contour drawing of response surface model, the relationship between the design parameters and the performance of heat convection is distinct.4. Structure improving and parameters optimizing of electrode heat convectionThe type of pipe end and cooling cavity structure can influence the distribution of flow velocity which causes different thickness of boundary layer near cooling wall. Furthermore, the heat exchange efficiency between the flow and the electrode can be affected. By using the coupling analysis method of heat and flow fields, study on the relationship between the structure type and the heat convection is made to determine the improving scheme of electrode structure for heat convection. According to the actual request, the multiple-object optimal model is built with the cooling rate, the production efficiency and the energy consumption as the indexes. In the help of GA with the good features of parallel search and probability search, the parallel global searches for optimal match variables under different weighs are implemented. The optimized solutions are gained.5. Case analysis of spot welding electrode and experiment validationThe heat inputted into the electrode is changed with the time in practice. According to the mechanism and composition of heat inputting, the heat and electric contact parameters are gained by theory calculation and based on the exist spot welding model, the magnitude and rule of heat inputting are distinct. Applied the coupling method of heat and flow fields, study on the performance of electrode temperature and heat convection under actual spot welding conditions are carried out. Also, the schemes of structure improvement and optimal design are analyzed under the spot welding technical process. According to the analysis of the relationship between electrode wear and electrode heat convection, the experiment scheme is determined and the validation of the structure improvement and parameters optimization scheme by test data is accomplished.In this study, by absorbing and referring the exist achievements, the coupling method of heat and flow fields and the performance of heat convection for spot welding electrode are researched and analyzed. Furthermore, the study results are applied to the actual electrode cooling case. The works are not only benefit to the life research of electrode, but also to the design and application of welding machine practically.更多还原