Laser ultrasonics

Laser ultrasonics shows promise for in-process monitoring of laser weld integrity

Laser welding has become a critical joining process in a number of manufacturing industries, including automotive, medical, and aerospace. Other industries such as shipbuilding are beginning to introduce laser welding into their processes. Laser welding has many well-known benefits over former approaches, including reduced heat-affected zone, faster process speed, and improved access to recessed areas.

The measurement and control of weld integrity is highly desirable for a number of reasons. Specifically, such monitoring systems have the potential to:

Validate product conformance to customer specifications

Enable automated inspection and certification ofall parts

Improve productivity through increased process speed

• Enable better planning of equipment maintenance

• Provide feedback for process control.

Techniques for laser weld process monitoring and control have been under development for many years and some systems have been commercialized. The typical approach seeks to identify a correlation between measured process parameters and the integrity of the completed weld. The most common techniques are based on monitoring of the plasma plume and weld pool, as well as acoustic emission from the weld process. However, these techniques are indirect and require complex signal interpretation and processing to infer information about the actual condition of the weld after cooling. These techniques are also not well suited for predicting weld dimensions.

Other post-process inspection techniques have also been implemented. For example, laser profilometry has been used to record the shape of the weld bead. Such measurements can identify certain surface defects, but cannot provide direct information on the internal dimensions or defects inside the weld. Ultrasonic monitoring using electromagnetic acoustic transducers (EMATs) has also been implemented. However, EMATs are large and must be in near-contact with the surface. This limits the EMAT approach for use on flat or mildly curved surfaces only.

Laser ultrasonics


Figure 1. Schematic of laser ultrasonic detection of internal defect. In this configuration, a separate signal from the back wall appears at a later time and can be utilized for calibration.
Click here to enlarge image

One technology that shows great promise for measurement of weld integrity is laser-based ultrasonic inspection.1 As shown in Figure 1, laser ultrasonics is an extension of conventional transducer-based contact ultrasonic inspection. Instead of piezoelectric or EMATs, a pulsed laser is used to generate an ultrasonic wave and a separate continuous wave (CW) laser ultrasonic receiver is used for detection of the ultrasonic wave. Although the methods for generation and detection of the ultrasound are quite different from those using transducers, lasers can generate and detect the full complement of ultrasonic waves. Laser ultrasonic generation and detection provides significant advantages for in-process inspection of laser welds:

• No sensor contact with the workpiece is required for inspection.

• In-process measurements can be performed on parts at high temperature and/or moving at high speeds.

• High-frequency ultrasonic waves are generated and detected, greatly increasing spatial resolution and signal-to-noise ratio over contact ultrasonic methods.

• The small size of the interrogating laser beams allows measurements to be performed on curved or even recessed surfaces.


Figure 2. Laboratory version of Lasson’s third-generation AIR-532 laser ultrasonic receiver shown with a companion CW probe laser and measurement head.
Click here to enlarge image

The laser ultrasonic receiver shown in Figure 1 is almost always some type of optical interferometer that converts a phase change measured at the surface into a change in output signal amplitude. Adaptive receivers have been developed and marketed by Lasson Technologies (see Figure2).

Laser ultrasonics for weld inspection

Laser ultrasonics offers two pathways for improvement of laser weld productivity. First, it can be used for direct monitoring of a critical dimension of the weld, such as the penetration depth or the fusion width. This information can be fed back to the weld controller for maintenance of the desired dimension. Second, laser ultrasonics can be used for in-process detection and classification of defects. One example is the inspection of butt welds used in tailor-welded blanks, described later.

Laser ultrasonics has been extensively investigated for monitoring of the integrity of penetration lap welds used in the joining of automobile transmission components. 2, 3 In this case, the requirement was to determine the fusion width of the penetration lap weld at the interface between the two components. A complete offline system was designed and tested for this application. 2

Other automotive laser welds that can be inspected effectively using laser ultrasonics include those used for assembly of tailor-welded blanks as well as those now being introduced for body-in-white assembly. Tailor-welded blanks are custom-designed sheet metal panels for auto body manufacture. Different areas of the body have varying requirements for strength and corrosion resistance. Using a design-for-manufacturing approach, manufacturers are now producing large sheet metal panels or blanks made from smaller, individually engineered panels that are butt-welded together using a CO2 or Nd:YAG laser welding process. With this approach, panels with differing thickness, metallurgy, or surface treatment are joined to provide the desired attribute only in positions where it is required. The most critical weld condition that must be controlled is the integrity and shape of the weld nugget formed between adjacent base metal surfaces. Any single inspection approach must be able to detect deviations from the proper nugget profile, as well as cracks, porosity, pinholes, and lack of fusion.


Figure 3. Schematic drawing of laser ultrasonic system for in-line inspection of tailor-welded blanks.
Click here to enlarge image

Our inspection approach (see Figure 3) is to use laser-generated ultrasonic waves propagating from one panel through the weld nugget and into the second panel as a means of interrogating the weld. We then use signal processing for feature extraction and pattern recognition, thereby providing effective identification and classification of defective welds. 4

We have successfully used wavelet analysis to separate defect signatures from all other components of the detected signal. As shown in Figure 3, the panel is scanned while the weld is interrogated by the propagating ultrasonic wave. Continuity of signal properties across scans suggests an unflawed workpiece. The appearance of an abrupt discontinuity indicates the presence of a flaw.


Figure 4. Contour plot of detected signals after signal processing.
Click here to enlarge image

After processing to remove signals that contain no defect information, we obtain the plot given in Figure 4. The pinhole defect is easily observable at the 1.4-inch position.

Conclusion

Laser ultrasonics shows great promise in laser weld monitoring applications for a number of reasons. The use of laser beams enables laser ultrasonics to interrogate hot and/or moving parts, curved surfaces, or even recessed cavities with much higher spatial resolution and signal-to-noise ratio than traditional contact methods.

Laser ultrasonics can be used to directly monitor in real time critical dimensions of the weld such as penetration depth and fusion width. Also, and in real time, laser ultrasonics can be used for the detection of weld defects. In both cases, laser ultrasonics allows critical information to be sent through a feedback loop to the weld controller for process correction and optimization.

Laser ultrasonics is now being tested for inspection of welds for shipbuilding applications. Other critical applications are being sought where laser welding can be augmented by laser ultrasonics to speed processing and improve quality.

Marvin B. Klein and Tim Bodenhamer are with Lasson Technologies Inc., Culver City, California (www.lasson.com).

REFERENCES

1. Scruby, C.B. and L.E. Drain, “Laser Ultrasonics, Techniques and Applications,” Bristol: Adam Hilger, 1990.

2. Klein, M. B., G. J. Dunning, P. V. Mitchell, T. R. O’Meara, M. Chiao, Y. Owechko, and D.M. Pepper, “High-Volume Industrial Applications of Remote Laser-Based Ultrasound for Weld-Joint Inspection,” Proc. ICALEO, San Diego, CA, November, 1995.

3. Dunning, G. J., P. V. Mitchell, M. B. Klein, D.M. Pepper, T. R. O’Meara, and Y. Owechko, “Remote Laser-Based Ultrasonic Inspection of Weld Joints for High Volume Industrial Applications,” Review of Progress in Quantitative Nondestructive Evaluation, D. O. Thompson and D. E. Chimenti, Editors, Volume 15, p. 2257-2264, 1996.

4. Kercel, S.W., R.A. Kisner, M.B. Klein, G.D. Bacher, and B. Pouet, “In-process detection of weld defects using laser-based ultrasound,” Proc. SPIE 3852, p. 81-92 (1999).

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