【作者】 栾亦琳；

【导师】 刚铁；

【作者基本信息】 哈尔滨工业大学 ， 材料加工工程， 2008， 博士

【摘要】 扩散焊在航空航天、电子及核工业等领域得到了广泛应用,在许多新材料的连接方面,其应用范围也日益扩大。扩散焊界面的缺陷严重影响界面的完整性、结构的力学性能进而影响焊接结构的在役服务性能。因此,采用无损检测手段,掌握缺陷的类型、大小和位置,对保证焊接结构的性能和使用寿命具有十分重要的意义。由于扩散焊接头中缺陷多为微米级,常规超声波检测该类缺陷非常困难,尤其对于异种材料扩散焊质量的检测,因界面两侧材料声阻抗的差异,进一步增加了检测的难度。采用改变扩散焊温度和焊接压力的方式,制备了不同焊接质量的试样。应用超声波C扫描成像技术,获得了试样的界面C扫描图像和包含界面信号的数据。从界面显微结构、接头强度、断口形貌和界面超声信号等角度出发,阐述了扩散焊界面未焊合缺陷、弱接合缺陷、微孔缺陷和焊接良好区域的各自特征。分析了超声波在钛铝金属间化合物(TiAl)和40铬钢(40Cr)之间空气间隙上的反射特性,揭示了反射系数的幅度和相位与超声波频率和空气间隙厚度之间的关系。采用傅立叶变换的方法研究了超声波在TiAl和40Cr扩散焊界面三种缺陷和焊接良好界面的幅频和相频特性,分析了检测增益和探头中心频率对反射特性的影响。未焊合、弱接合和微孔缺陷的幅频和相频特性曲线与焊接良好界面信号的有明显差别,利用幅频和相频特征可区分开三种缺陷和焊接良好界面。当增益增加未引起信号饱和采样时,对信号幅频和相频特性没有影响。探头的中心频率增加,信号幅频和相频特性保持不变。采用连续小波变换研究了TiAl和40Cr扩散焊界面缺陷和焊接良好信号的时频特征,分析了四种复值小波表征信号时频特征的性能以及小波参数、尺度与步长对时频特征的影响。采用时频能量模图像和时频相位图像表征了未焊合缺陷、弱接合缺陷、微孔缺陷和焊接良好界面信号的特征。时频分析揭示了扩散焊界面每一时刻的信号均具有相同的反射特性。建立了扩散焊界面缺陷智能识别模型,系统分析了训练样本数量、核函数、参数优化方法和特征值对模型性能的影响。实现了扩散焊界面未焊合、弱接合和微孔缺陷的智能识别,消除了界面回波对缺陷判断的影响。在此基础上,给出了试样抗剪强度与焊合率的经验公式,实现了接头强度的预测,预测强度与实测强度具有较好地对应关系。更多还原

【Abstract】 Diffusion bonding has been widely used in aerospace, electronic and nuclear industry, especially in jointing of new materials. Diffusion bonding defects can significantly degrade integrity of interface, strength and service performance of bonding structures. Thus it is significant to evaluate type, size and location of the defects to guarantee the performance and life of the bonding structures. In general, the defects are only a few micrometers in size and very difficult to detect by conventional ultrasonic testing, especially in dissimilar bonding structures. Some ultrasonic energy is still reflected from perfectly bonded interface due to the effect of impedance mismatch between materials to be bonded.Diffusion bonding samples were prepared at various welding temperatures and press. Ultrasonic tests were performed by C-scan method to obtain C-scan images and signals of samples. Characteristics of unbonded, kissing bond, micropore and perfectly bonded area were represented on the points of view of microstructure, shear strength, fractography and ultrasonic interface signals.Characteristics of ultrasonic wave reflected from air gap between TiAl and 40Cr were analyzed. Relationships between reflection coefficient, ultrasonic frequency and air thickness were demonstrated. Amplitude-frequency and phase-frequency characteristics of three kinds of defects and the perfectly bonded interface of TiAl and 40Cr diffusion bonding were studied based on Fourier Transformation. Effects of gain and central frequency of transducers on the above characteristics were analyzed. Significant differences of the amplitude-frequency and phase-frequency characteristics are found between three kinds of defects and the perfectly bonded interface. Three kinds of defects and the perfectly bonded interface can be distinguished by the characteristics. The amplitude-frequency and phase-frequency characteristics aren’t influenced by the gain when the increasing of gain doesn’t result in the signal saturation. As the central frequency of the transducer increases, the amplitude-frequency and phase-frequency characteristics are still maintained.Time-frequency characteristics were studied based on Continuous Wavelets Transform. Effects of time-frequency characteristics of four complex wavelets, parameters of wavelets, scale and step were analyzed. The unbonded, the kissing bond, the micropore and the perfectly bonded interface can be characterized by time-frequency energy image and time-frequency phase image. Time-frequency analysis demonstrates that the signals have the same reflective characteristics at every second.Defects recognition models were established. Effects of numbers of train samples, kernel functions, parameter optimization methods and characteristics on the model performance were systematically analyzed. Defects of the unbonded, the kissing bond and the micropore are recognized successfully. Empirical formula of relationship between shear strength and bonding ratio is established to predict strength of samples. There is a good agreement between predicting strength and testing shear strength.更多还原