双相钢电阻点焊熔核特征分析及质量检测方法研究
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摘要
节能和环保一直是现代汽车工业迫切要求突破的技术难题。在满足碰撞安全的前提下，降低车身重量是平衡车辆安全与节能环保的有效方法之一。其中，采用先进高强钢板替代传统的低碳钢板，对提高汽车结构强度及降低车身重量具有重要意义。在使用的先进高强钢板中，双相钢（DualPhase Steel）因其强度和塑性兼优的综合特性，在轻量化车身制造中的应用最为普遍。在采用的连接工艺方法中，传统电阻点焊由于低成本、高效率的优点，目前仍是双相钢材料的主要连接工艺。考虑到由铁素体和少量马氏体组成的双相钢，在电阻点焊过程中熔核区域易出现缩孔、裂纹等。当进行点焊接头拉剪测试时，易出现熔核界面撕裂，并存在钢板镀锌层与电极端面的合金化作用带来的电极表面不规则等问题，因此，如何对双相钢点焊熔核的内部和表面特征进行有效识别和评价，进而实现双相钢点焊接头质量的在线检测，是亟待解决的重要问题。
基于上述背景，本文以双相钢材料为研究对象，首先开展了点焊接头拉剪强度及熔核断裂的试验研究，提出表征双相钢点焊熔核的特征参数；针对熔核表面缺陷的检测问题，采用图像模式识别方法来评判焊核表面质量；遴选超声回波波形特征，确定点焊接头缺陷对应的超声回波特征值最优范围，建立基于支持向量机的双相钢点焊熔内部缺陷智能识别方法；利用伺服焊枪位置反馈特性，建立基于焊点压痕的点焊接头质量评价模型，实现双相钢点焊接头质量的在线检测。
本文主要开展的研究工作如下：
（1）双相钢点焊接头质量的试验研究及熔核特征分析
双相钢电阻点焊保持阶段由于冷却速度较快，易形成淬硬马氏体，将增加点焊接头脆性并产生熔核界面撕裂等问题。本章首先基于建立的伺服电阻点焊实验系统，获取双相钢在不同工艺参数下的工艺窗口，其次采用静态拉剪实验研究不同工艺参数的组合对焊点拉剪强度的影响规律，并利用多元线性回归给出了焊接工艺参数与焊点最大拉剪力的定量关系。最后分析了双相钢焊点熔核区及热影响区在点焊快速冷却过程的微观组织和硬度变化规律，确定了点焊接头不同断裂模式及点焊过程中易产生的缺陷，对其进行了归类和成因分析。研究结论表明，针对点焊缺陷引起的接头质量问题有必要进行熔核表面及内部缺陷的识别和检测。
（2）双相钢点焊熔核表面特征的数字图像模式识别方法
针对双相钢点焊熔核表面易出现的点蚀等特征，提出采用数字图像模式识别方法实现熔核表面质量的有效检测。根据熔核表面特征的图像特点与精度要求，设计了基于CCD的图像采集系统，在获得原始图像的基础上，利用修改软阈值的方法对获得的熔核表面图像进行降噪、缺陷特征的图像分割及基于面积几何特征的点蚀缺陷特征的定量分析。设计了基于Matlab的熔核表面缺陷特征图像模式识别程序，实现基于图像特征的熔核表面质量的评价。
（3）基于支持向量机的焊点熔核缺陷超声波检测方法
本章首先归纳了不同点焊接头缺陷的超声检测回波波形的规律和特点。基于点焊接头超声检测回波波形，结合力学性能试验和微观组织检验遴选回波信号中衰减率、底面回波波峰间隔、有效回波数、最小增益、及中间回波峰值等五个能反映点焊熔核内部缺陷的特征值参数；建立基于回波特征值和支持向量机的双相钢点焊熔核缺陷检测方法。实现对点焊接头缺陷的快速、准确地识别。试验结果为焊点的超声波检测在实际生产中的广泛应用提供了数据支持。
（4）基于动态电阻和压痕深度的点焊质量在线检测方法
本章提出了基于动态电阻与压痕深度的接头质量在线检测办法。分析了点焊过程中动态电阻的变化规律，阐明了动态电阻与焊接接头形成过程的映射关系，建立了基于动态电阻曲线的点焊熔核尺寸在线评价模型。基于伺服焊枪对电极位移的可控性，建立了焊点压痕的在线获取方法，分析了焊接工艺参数对焊点压痕的影响，建立了基于压痕的点焊可焊性范围及焊点压痕与点焊接头质量的对应关系。试验结果表明，利用焊点压痕都很好检测出焊点质量。
通过以上工作，本文对双相钢点焊熔核特征及接头质量检测有了一个比较全面和系统的分析。从它的特征、机理、检测及评价等方面都进行了较为深入的探讨，对于基于伺服焊枪的双相钢点焊质量检测与评价，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，提供了重要的理论参考。
The energy and environment crisis caused by the increase of automobileproduction are becoming more and more urgent. Weight reduction ofautomobile is considered as the one of most effective ways to overcome aboveproblems. Dual-phase steel (DP) is such a kind of material used in automobileproduction to realize automobiles’ weight reduction without decrease strengthand stiffness. Therefore, the traditional low carbon and low alloy steel used inautomobile industry are gradually replaced by high strength steels such as DP.Resistance spot welding process is still the major joining method inautomobile industry due to its advantages such as low cost and easyautomation. However, the special microstructure of DP will cause asymmetriccooling speed and result in crack and shrinkage voids during resistance spotwelding process. The strength of nugget will lower than base material andinterfacial fracture failure mode will appear during mechanical test. In addition,the weldability of the hot dip galvanized steel is poor and the effect of alloyingbetween zinc coating and electrode face result in change of character of nuggetface. How to recognize inside and outside weld nuggets defects and thenrealize on-line inspection is becoming urgent for automobile industries.
Based on the background mentioned above, DP steel is selected as theresearch material in this paper. Firstly, tensile-shear are conducted to study thechange law of strength of nuggets and electrode wear; Secondly, in order toevaluate the weld quality by the data of characters of nugget face, A method ofimage pattern recognition is used to evaluate the quality of nuggets face;Thirdly, optimum range of the characteristic values of ultrasonic echo ofnuggets are determined. Then a pattern recognition method based on supportvector machine is developed. Lastly, a model of quality evaluation based ondynamic resistance and the weld’s indentation by character of feedback ofservo-gun is developed to realize on-line inspecting the spot quality of DP.
The main tasks are listed as follows:
(1) Experimental research on spot quality of DP and analysis ofnugget’s characters
It is easy to produce martensite during the holding due to quick cooling inresistance spot welding DP steel. The high hardness will increase theoccurrence of brittleness and interfacial fracture in weld nugget zone. First, theexperiment system of spot welding based on servo-gun is developed to achievethe welding parameters and acquire the weld lobe curve under differentprocess parameters. Second, the effect of process parameters on tensile-shearstrength would be analyzed. The quantitative relation between welding processparameters and maximum tensile-shear force using multiple linear regressionmethods would be achieved. Moreover, the law of change of microstructure inthe nugget zone and HAZ is analyzed. The different fracture models of spotwelding are defined basic on the characteristics of martensite and the defectsof spot welding are classified. The results show that defects can affect thequality of spot welding obviously.
(2) The characteristic recognition of weld nugget face by imageprocessing
The characteristics of weld nugget face, such as electrode pitting&crack,were complex, irregular and hard to measure accurately for the dual phaseresistance spot welds. The above characteristics would have an evident effecton joint quality. Therefore, an image recognition method was applied tomeasure the characteristics of pit. Firstly, an image collecting system wasdesigned based on CCD according to the requirement of image precision. Andthen an image processing procedure was developed to decrease noise of surfaceimages of nugget and to segment the image of defects. Then the quantitativeanalysis was done based on area algebra characteristic. Lastly, the procedure ofimage of nugget surface pattern recognition was developed to evaluate thequality of surface of nugget.
(3) Inspecting defects of nugget by pattern recognition based onsupport vector machine and ultrasonic echo.
In this section it selects five characteristic of ultrasonic echoes, such asattenuation, peak distance of bottom echo, valid numbers of echo, minimumgain and peak of middle echo based on different regular of ultrasonic echo corresponding to different defects of nugget. According to the differencebetween ultrasonic echo and different material and thickness of spot weld, onthe base of experiment of mechanical properties and metallography experimentthe optimum range of characteristic of ultrasonic echo can be made. Theintelligent method of inspecting defects of DP’s nugget based on SVM isdeveloped to realize recognizing the different quality of spot welds quicklyand accurately.
(4) On-line inspection of spot welds quality based on weld indentationof spot weld.
The on-line weld quality inspection of spot welded joints based ondynamic resistance and indentation depth are analyzed in this section. First, thechanging law of dynamic resistance is used to analyze the relationship betweendynamic resistances and weld nugget formation. The model of on-lineevaluating the quality of spot welds based on dynamic resistance is developedduring the welding process. The session of the dissertation proposed theon-line method of acquiring the indentation depth based on the controllabilityof servo-gun. The influence of process parameters on indentation and weldlobe based on indentation is developed.
Based on above jobs, a comprehensive and systemic analysis oncharacteristics of DP’s nugget and quality of spot welds would be achieved.The dissertation shows the theoretical incomings and technical methods forinspecting and evaluating the joint quality. All these results would provide animportant theoretical reference for the wide use of dual-phase steel inauto-body manufacturing.
基于上述背景，本文以双相钢材料为研究对象，首先开展了点焊接头拉剪强度及熔核断裂的试验研究，提出表征双相钢点焊熔核的特征参数；针对熔核表面缺陷的检测问题，采用图像模式识别方法来评判焊核表面质量；遴选超声回波波形特征，确定点焊接头缺陷对应的超声回波特征值最优范围，建立基于支持向量机的双相钢点焊熔内部缺陷智能识别方法；利用伺服焊枪位置反馈特性，建立基于焊点压痕的点焊接头质量评价模型，实现双相钢点焊接头质量的在线检测。
本文主要开展的研究工作如下：
（1）双相钢点焊接头质量的试验研究及熔核特征分析
双相钢电阻点焊保持阶段由于冷却速度较快，易形成淬硬马氏体，将增加点焊接头脆性并产生熔核界面撕裂等问题。本章首先基于建立的伺服电阻点焊实验系统，获取双相钢在不同工艺参数下的工艺窗口，其次采用静态拉剪实验研究不同工艺参数的组合对焊点拉剪强度的影响规律，并利用多元线性回归给出了焊接工艺参数与焊点最大拉剪力的定量关系。最后分析了双相钢焊点熔核区及热影响区在点焊快速冷却过程的微观组织和硬度变化规律，确定了点焊接头不同断裂模式及点焊过程中易产生的缺陷，对其进行了归类和成因分析。研究结论表明，针对点焊缺陷引起的接头质量问题有必要进行熔核表面及内部缺陷的识别和检测。
（2）双相钢点焊熔核表面特征的数字图像模式识别方法
针对双相钢点焊熔核表面易出现的点蚀等特征，提出采用数字图像模式识别方法实现熔核表面质量的有效检测。根据熔核表面特征的图像特点与精度要求，设计了基于CCD的图像采集系统，在获得原始图像的基础上，利用修改软阈值的方法对获得的熔核表面图像进行降噪、缺陷特征的图像分割及基于面积几何特征的点蚀缺陷特征的定量分析。设计了基于Matlab的熔核表面缺陷特征图像模式识别程序，实现基于图像特征的熔核表面质量的评价。
（3）基于支持向量机的焊点熔核缺陷超声波检测方法
本章首先归纳了不同点焊接头缺陷的超声检测回波波形的规律和特点。基于点焊接头超声检测回波波形，结合力学性能试验和微观组织检验遴选回波信号中衰减率、底面回波波峰间隔、有效回波数、最小增益、及中间回波峰值等五个能反映点焊熔核内部缺陷的特征值参数；建立基于回波特征值和支持向量机的双相钢点焊熔核缺陷检测方法。实现对点焊接头缺陷的快速、准确地识别。试验结果为焊点的超声波检测在实际生产中的广泛应用提供了数据支持。
（4）基于动态电阻和压痕深度的点焊质量在线检测方法
本章提出了基于动态电阻与压痕深度的接头质量在线检测办法。分析了点焊过程中动态电阻的变化规律，阐明了动态电阻与焊接接头形成过程的映射关系，建立了基于动态电阻曲线的点焊熔核尺寸在线评价模型。基于伺服焊枪对电极位移的可控性，建立了焊点压痕的在线获取方法，分析了焊接工艺参数对焊点压痕的影响，建立了基于压痕的点焊可焊性范围及焊点压痕与点焊接头质量的对应关系。试验结果表明，利用焊点压痕都很好检测出焊点质量。
通过以上工作，本文对双相钢点焊熔核特征及接头质量检测有了一个比较全面和系统的分析。从它的特征、机理、检测及评价等方面都进行了较为深入的探讨，对于基于伺服焊枪的双相钢点焊质量检测与评价，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，提供了重要的理论参考。
The energy and environment crisis caused by the increase of automobileproduction are becoming more and more urgent. Weight reduction ofautomobile is considered as the one of most effective ways to overcome aboveproblems. Dual-phase steel (DP) is such a kind of material used in automobileproduction to realize automobiles’ weight reduction without decrease strengthand stiffness. Therefore, the traditional low carbon and low alloy steel used inautomobile industry are gradually replaced by high strength steels such as DP.Resistance spot welding process is still the major joining method inautomobile industry due to its advantages such as low cost and easyautomation. However, the special microstructure of DP will cause asymmetriccooling speed and result in crack and shrinkage voids during resistance spotwelding process. The strength of nugget will lower than base material andinterfacial fracture failure mode will appear during mechanical test. In addition,the weldability of the hot dip galvanized steel is poor and the effect of alloyingbetween zinc coating and electrode face result in change of character of nuggetface. How to recognize inside and outside weld nuggets defects and thenrealize on-line inspection is becoming urgent for automobile industries.
Based on the background mentioned above, DP steel is selected as theresearch material in this paper. Firstly, tensile-shear are conducted to study thechange law of strength of nuggets and electrode wear; Secondly, in order toevaluate the weld quality by the data of characters of nugget face, A method ofimage pattern recognition is used to evaluate the quality of nuggets face;Thirdly, optimum range of the characteristic values of ultrasonic echo ofnuggets are determined. Then a pattern recognition method based on supportvector machine is developed. Lastly, a model of quality evaluation based ondynamic resistance and the weld’s indentation by character of feedback ofservo-gun is developed to realize on-line inspecting the spot quality of DP.
The main tasks are listed as follows:
(1) Experimental research on spot quality of DP and analysis ofnugget’s characters
It is easy to produce martensite during the holding due to quick cooling inresistance spot welding DP steel. The high hardness will increase theoccurrence of brittleness and interfacial fracture in weld nugget zone. First, theexperiment system of spot welding based on servo-gun is developed to achievethe welding parameters and acquire the weld lobe curve under differentprocess parameters. Second, the effect of process parameters on tensile-shearstrength would be analyzed. The quantitative relation between welding processparameters and maximum tensile-shear force using multiple linear regressionmethods would be achieved. Moreover, the law of change of microstructure inthe nugget zone and HAZ is analyzed. The different fracture models of spotwelding are defined basic on the characteristics of martensite and the defectsof spot welding are classified. The results show that defects can affect thequality of spot welding obviously.
(2) The characteristic recognition of weld nugget face by imageprocessing
The characteristics of weld nugget face, such as electrode pitting&crack,were complex, irregular and hard to measure accurately for the dual phaseresistance spot welds. The above characteristics would have an evident effecton joint quality. Therefore, an image recognition method was applied tomeasure the characteristics of pit. Firstly, an image collecting system wasdesigned based on CCD according to the requirement of image precision. Andthen an image processing procedure was developed to decrease noise of surfaceimages of nugget and to segment the image of defects. Then the quantitativeanalysis was done based on area algebra characteristic. Lastly, the procedure ofimage of nugget surface pattern recognition was developed to evaluate thequality of surface of nugget.
(3) Inspecting defects of nugget by pattern recognition based onsupport vector machine and ultrasonic echo.
In this section it selects five characteristic of ultrasonic echoes, such asattenuation, peak distance of bottom echo, valid numbers of echo, minimumgain and peak of middle echo based on different regular of ultrasonic echo corresponding to different defects of nugget. According to the differencebetween ultrasonic echo and different material and thickness of spot weld, onthe base of experiment of mechanical properties and metallography experimentthe optimum range of characteristic of ultrasonic echo can be made. Theintelligent method of inspecting defects of DP’s nugget based on SVM isdeveloped to realize recognizing the different quality of spot welds quicklyand accurately.
(4) On-line inspection of spot welds quality based on weld indentationof spot weld.
The on-line weld quality inspection of spot welded joints based ondynamic resistance and indentation depth are analyzed in this section. First, thechanging law of dynamic resistance is used to analyze the relationship betweendynamic resistances and weld nugget formation. The model of on-lineevaluating the quality of spot welds based on dynamic resistance is developedduring the welding process. The session of the dissertation proposed theon-line method of acquiring the indentation depth based on the controllabilityof servo-gun. The influence of process parameters on indentation and weldlobe based on indentation is developed.
Based on above jobs, a comprehensive and systemic analysis oncharacteristics of DP’s nugget and quality of spot welds would be achieved.The dissertation shows the theoretical incomings and technical methods forinspecting and evaluating the joint quality. All these results would provide animportant theoretical reference for the wide use of dual-phase steel inauto-body manufacturing.
引文
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