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摘要: 在超声无损检测时，搭接激光窄焊缝接头尺寸较小，采用传统6 dB法对其界面处的熔宽判定存在较大的误差。为了提高检测精度，通过研究传统6 dB法的检测误差来源，采用有限元分析方法分析了激光焊接头内部入射超声波的传播规律和反射回波特性，构建了基于修正6 dB法的激光焊接头熔宽评估模型，并通过物理实验进行了验证。研究结果表明，上板底面的一次回波幅值可作为反映接头内部结构的特征值，当探头中心对应接头内部焊缝熔合线边缘位置时，一次回波幅值的衰减度随上板板厚而变化，据此可根据上板板厚选择衰减度值对传统6 dB法进行修正，从而定量计算接头内部板层接触面处的有效熔宽。实际激光焊接头的超声检测结果证实：采用修正6 dB法求解出的激光焊接头的熔宽与物理实验结果吻合良好，对实际生产中超声检测激光焊接头的精度提升提供了参考。Abstract: Due to the tiny dimensions of lap laser welding joints, there is significant error in weld width detection when using the traditional 6 dB method. In order to improve the method’s detection accuracy and study its source of error, the finite element analysis method is used to analyze the propagation law of incident ultrasonic waves and reflected ultrasonic echo characteristics inside a laser-welded joint. Based on a modified 6 dB method, a laser welding joint melt width evaluation model is constructed and verified through physical experiments. The experimental results show that the primary echo amplitude of the bottom surface of the upper plate can be used as a characteristic value that reflects the internal structure of the joint. When the center of the probe corresponds to the edge of the weld fusion line inside the joint, the attenuation of the primary echo amplitude varies with the thickness of the upper plate, and the traditional 6 dB method can be modified according to the attenuation degree which related to the upper plate’s thickness. Based on this, the effective weld width at the contact surface of the inner plate of the joint can be calculated quantitatively. The ultrasonic testing results of the actual laser welding joints confirmed that the melt width of the laser welding joints obtained by the modified 6 dB method agree with the results of the physical experiments, which means that this provides a very practical method for accurate ultrasonic testing of laser welding joints in real-world production.
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Key words:
- laser lap welding /
- ultrasonic testing /
- finite element analysis /
- modified 6 dB method
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图 3 激光焊接头的截面模型。（a）结构及尺寸；（b）求解域设置
Figure 3. Section model of the laser welding joint. (a) Structure and dimensions; (b) solution domain setting
图 6 母材区的超声场分布。（a） t=0.02 μs；（b） t=0.16 μs；（c） t=0.32 μs；（d） t=0.48 μs
Figure 6. Ultrasonic distributions at the base metal zone. (a) t=0.02 μs; (b) t=0.16 μs; (c) t=0.32 μs; (d) t=0.48 μs
图 7 过渡区的超声场分布。（a） t=0.02 μs；（b） t=0.16 μs；（c） t=0.32 μs；（d） t=0.48 μs
Figure 7. Ultrasonic distribution in the transitional zone. (a) t=0.02 μs; (b) t=0.16 μs; (c) t=0.32 μs; (d) t=0.48 μs
图 8 熔合区的超声场分布。（a） t=0.02 μs；（b） t=0.16 μs；（c） t=0.48 μs；（d） t=0.8 μs；（e） t=0.88 μs；（f） t=0.96 μs
Figure 8. Ultrasonic distribution in the fusion zone. (a) t=0.02 μs; (b) t=0.16 μs; (c) t=0.48 μs; (d) t=0.8 μs; (e) t=0.88 μs; (f) t=0.96 μs
图 9 接头不同区域的超声A扫描信号。（a）母材区；（b）过渡区；（c）熔合区
Figure 9. Ultrasonic A-scan echoes from different connection zones. (a) Base metal zone; (b) transitional zone; (c) fusion zone
图 10 不同尺寸接头超声A扫描信号一次回波变化规律。（a）不同熔宽尺寸接头U1幅值变化规律；（b）不同上板板厚U1衰减度变化规律
Figure 10. The change regularity of an ultrasonic A scan echoes at different joint dimensions. (a) U1 amplitude changes with different joint widths; (b) U1 attenuation degree changes with different upper plate thickness
图 11 激光焊接头各区域超声A扫描信号。（a）母材区；（b）母材侧过渡区；（c）焊缝侧过渡区；（d）焊缝连接区
Figure 11. Ultrasonic A-scan echoes from different zones of laser welding joints. (a) Base metal zone; (b) transition zone at the base metal side; (c) transition zone at the joint side; (d) fusion zone
图 12 激光焊接头连接状态表征曲线。 (a) 特征量选取；(b) 特征量变化规律
Figure 12. Characteristic curve of the connection status of laser welding joints. (a) Characteristic parameter selection; (b) change regulation of the characteristic parameter
图 13 激光焊接头熔宽测量结果。（a）修正6 dB法、传统6 dB法和金相测量结果对比；（b）修正6 dB法的误差分布
Figure 13. Results of joint width measured by different methods. (a) Comparison of detection results of the modified 6 dB method, traditional 6 dB method and metallographic section; (b) error distribution of modified 6 dB method
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