摘要
针对世界汽车保有量与日俱增带来的能源短缺、环境污染等一系列问题,通过减轻车体自重实现汽车的轻量化是节约能源、降低油耗和减少排放污染的最有效途径。使用双相高强钢(Dual Phase Steel)为代表的轻量化材料,能够在不降低车身强度和刚度等各项性能指标的前提下,有效降低汽车重量,实现车身材料轻量化,目前车身普遍采用的低碳钢以及低合金钢材料正在越来越多地被双相高强钢材料所替代。电阻点焊由于其低成本、易于实现自动化作业的特性,仍然是车身制造中首选的连接工艺,然而,基于相变强化机制的双相钢由于其特有的微观组织结构,点焊过程中焊点内部冷却速度不均将产生裂纹、气孔等缺陷,导致焊点熔核区强度低于母材强度,当进行焊点力学性能测试时,会产生点焊熔核界面撕裂问题。发生熔核界面撕裂的焊点十字拉伸强度将下降约10%,而焊点低周疲劳强度将下降约25%,因此,研究双相钢焊点发生界面撕裂的机理、建立评价该失效模式的特征指标、优化焊接工艺参数来实现焊点界面撕裂的有效控制,是亟待解决的重要问题,本文正是在上述背景下开展的研究工作。
基于以上背景,本文主要以1.4mm双相钢(DP600)材料为研究对象,首先通过静载拉剪实验、动态疲劳实验研究双相钢焊点界面撕裂对点焊熔核质量的影响规律,采用微观金相和SEM实验探讨双相钢点焊熔核界面撕裂的形成机理;通过建立双相钢点焊接头力学模型,获得评价双相钢焊点界面撕裂的临界熔核直径指标;研究双相钢点焊熔核界面撕裂的微观组织演变规律,分析保压时间、后处理、多脉冲、锻压力等工艺参数对点焊熔核微观组织和硬度的影响;最后基于响应面方法对焊接工艺参数进行优化,以实现双相钢焊点界面撕裂的有效控制。论文旨在揭示双相钢焊点界面撕裂形成机理的同时,提高双相钢焊接质量,确定最优焊接工艺参数,开展的主要研究工作如下:
(1)双相钢点焊熔核界面撕裂特征及成因分析
双相钢电阻点焊快速冷却过程中形成的淬硬马氏体会增加点焊接头脆性,产生熔核界面撕裂问题,降低了焊接质量。本章首先分析双相钢焊点的不同失效模式并定义其失效级别:0级为合格焊点的熔核剥离模式,1～4级分别为焊点界面撕裂模式,且级数越高,界面撕裂程度越大。然后采用静态拉剪实验及动态疲劳实验研究各级焊点界面撕裂对焊点质量的影响规律,结果表明,2级以上的界面撕裂对焊点静载拉剪强度、动态疲劳强度均有较大的影响,尤其对疲劳强度的影响更为显著;并且界面撕裂级数越高,影响程度越大。在此基础上,结合微观金相和SEM试验,研究双相钢焊点界面撕裂的机理。结果表明,焊点力学性能和微观组织变化是引起双相钢焊点界面撕裂的主要原因,从而为后续研究提供了理论依据。
(2)双相钢焊点界面撕裂力学评价模型建立
针对传统熔核直径经验公式4 t难以有效评价双相钢焊点质量的问题,本章通过理论计算建立双相钢点焊熔核在拉剪方式下的界面撕裂评价模型:综合考虑板厚、压痕、熔核维氏硬度等参数,对双相钢焊点受载过程进行力学建模,研究界面撕裂和熔核剥离两种模式下焊点承受的不同应力分布规律,应用极限应力理论和Tresca失效准则,计算两种失效模式下焊点所能承受的最大载荷,由此获得了界面撕裂失效模式时的临界熔核直径d Cr;最后针对1.4mm及1.8mm双相钢DP600焊点拉剪失效模式进行验证。通过与经验公式的对比表明,本章所推导的临界熔核直径公式能够实现双相钢点焊熔核界面撕裂模式的准确评价。
(3)双相钢点焊熔核界面撕裂的微观组织分析
考虑点焊微观组织是影响其熔核界面撕裂的内在因素,本章采用理论结合实验的方法研究了点焊工艺参数对焊点微观组织的作用规律:通过分析Fe-C合金连续冷却相变图(CCT)和点焊冷却过程,得出点焊过程中主要发生马氏体相变;基于点焊熔核马氏体含量和维氏硬度测定结果,获得了不同焊点失效模式下的马氏体含量和硬度变化规律;最后研究保持时间、后处理工艺、多脉冲及锻压力等单个焊接工艺参数对点焊熔核微观组织及硬度的影响,为控制双相钢焊点界面撕裂的多参数优化提供研究基础。
(4)控制双相钢焊点界面撕裂的焊接工艺参数优化
针对双相钢点焊熔核界面撕裂的工艺参数优化问题,本章采用响应面方法进行了参数设计、分析和优化:以实际熔核直径d与临界熔核直径d Cr的比值η作为设计目标,采用均匀实验设计法,建立了二次响应面回归模型,分析各参数对η的影响,结果表明:焊接电流、时间及锻压力是影响焊点失效模式的最主要因素,焊接力、保持时间、后处理时间为次要因素;然后采用多目标函数的序列二次规划(SQP)优化算法,综合考虑熔核直径比η及其对焊接工艺参数的敏感度ψ,获得了最优的焊接参数,既增加了熔核直径比,又降低了其对焊接参数的敏感度,从而实现了双相钢点焊工艺参数的优化,控制了双相钢焊点的界面撕裂问题。
通过以上工作,本文对双相钢焊点界面撕裂有了一个比较全面和系统的分析。从它的特征、机理、评价、控制等方面都进行了较为深入的探讨,对于双相钢点焊质量的评价与工艺参数控制,均提供了理论和实践依据,为双相钢在车身制造中的进一步推广,奠定了坚实的基础。
The energy and environment problems caused by the increase of automobile are becoming more and more urgent. Weight reduction of automobile is the most effective way to solve these problems. Dual-phase steel (DP) is such a kind of material used in automobile production to realize automobiles’weight reduction without decrease strength and stiffness. Therefore, the traditional low carbon and low alloy steel used in automobile industry are gradually replaced by high strength steels such as DP. Resistance spot welding process is still the major joining method in automobile industry due to its advantages such as low cost and easy automation. However, the special microstructure of DP will cause asymmetric cooling speed and result in crack and shrinkage voids during resistance spot welding process. The strength of nugget will lower than base material and interfacial fracture failure mode will appear during mechanical test. This failure mode will reduce the cross tension strength by 10% and low cycle fatigue strength by 25%. As a result, studying the mechanism, setting up evaluation model and optimizing welding process parameters to avoid the interfacial fracture mode of DP are becoming more and more important. This dissertation is conducted under this situation.
Based on the background mentioned above, 1.4mm thick DP600 is selected as the research material. Firstly, tensile-shear test and fatigue test are conducted to study the influence of interfacial fracture mode DP600 weld on the weld quality to investigate the mechanism of DP’s interfacial fracture mode. Secondly, critical weld diameter is determined from the builded module of weld’s mechanical test to evaluate the failure modes of DP. Thirdly, transformation rules of martensite during resistance spot welding are investigated. Holding time, post-heat, multi-pulse and forging force are studied to find out their influence on micro-hardeness of spot welds. At last, welding process parameters are optimized using the response surface methodology (RSM) to avoid interfacial fracture failure mode of DP. The purpose of this dissertation is to study the mechanism, improve weld quality and optimize welding parameters for DP welds. The main tasks are listed as follows.
(1) Characteristic and mechanism study on DP weld’s interfacial fracture mode
The martensite transformed during the quench stage of resistance spot welding process will make the weld joint brittle and cause interfacial fracture failure mode. It will greatly decrease DP weld’s quality. This section analyzes the different failure modes of DP welds and defines their fracture degrees. 0 degree represents good weld quality with button pull-out mode while 1~4 degree represents interfacial fracture mode. The fracture magnitude will increase as the degree increases. Tensile-shear and fatigue test are conducted to study the influence of different fracture degrees on weld quality. Results show that interfacial fracture higher than 2 degree will greatly affect the weld quality especially on the fatigue life. The effect will be greater as the increase of fracture degree. Based on these studies, metallographic and SEM experiments are used to investigate the mechanism. It can be concluded that the load capacity of weld, microstructure of martensite are the source of interfacial failure mode of DP weld. The conclusions will provide theoretical support to the following investigation.
(2) Study of mechanical evaluation module for DP weld’s interfacial fracture mode
For the traditional nugget formula 4 t can not evaluate the high strength steel weld quality effectively, this chapter calculate the critical nugget formula based on the different failure mode of DP weld under tensile-shear test. Mechanical module of weld nugget is builded considering the thickness, indentation and micro-hardeness of weld material. According to the different stress distribution under interfacial fracture mode and button pull-out mode, the maximum force for these two kinds of failure modes are determined based on Tresca criteria and limit stress theory. Then critical weld diameter is calculated. 1.4mm and 1.8mm thick DP600 steel are used to validate the critical weld diameter. Compared to the experiential formula, the critical formula can evaluate interfacial fracture mode of DP weld effectively.
(3) Microstructure analysis of DP weld’s interfacial fracture mode
Considering that the weld’s microstructure is the source of DP’s interfacial fracture mode, influence of welding process parameters on microstructure transformation is investigated by theoretical and experimental methods in this chapter. According to the continous cooling transformation (CCT) diagram of Fe-C alloy and cooling stage of spot welding process, martensite transformation is the dominate process of microstructure transformation. Content of martensite and Vickers hardenss are measured for different fracture modes to see the influence of martensite performance on fracture modes. At last, the influence of welding parameters, such as holding time, post-heat conditions, multi-pulse welding, forging force experiments are conducted to do find out their single effect on the microstructure. The results can provide foundation for the parameter optimization in the next stage.
(4) Control of DP weld’s interfacial fracture mode by optimizing welding process parameters
Focus on welding parameters optimization to avoid DP’s interfacial fracture failure mode, response surface methodology (RSM) is used to conduct welding parameters design, analysis and optimization. Nugget size ratioηof real nugget diameter d versus critical nugget diameter dCr is used in this section to evaluate interfacial fracture degree of DP welds. Quadradic equation module is concluded from the RSM and uniform experimental design matrix to show that welding current, time and forging force are the most important factors that influence the nugget size ratio while welding force, hold time and post-heat time are the unimportant factors. Considering the maximum of nugget size ratioηand minimum insensitive to welding parametersψ, the optimum welding process parameters are concluded using the SQP method. As a result, control of DP steel weld’s interfacial failure mode by optimizing welding parameters is realized.
Based on the contents mentioned above, a general and systemic analysis of DP weld’s interfacial fracture failure mode can be got. The characteristic, mechanism, evaluation and control of DP weld’s interfacial fracture failure mode are studied. It can provide theoretical and experimental support for welding DP steels. Besides, it will help the implementation of DP steel in automobile production.
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