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  Laser-ultrasonic detection of subsurface defects in processed metalsUSPTO Application #: 20070234809Title: Laser-ultrasonic detection of subsurface defects in processed metals Abstract: Subsurface defects in a processed metal are detected by a laser-ultrasonic method involving generation of a surface acoustic wave at one location on the processed metal surface, and detection of a scattered acoustic wave at another location on the processed metal surface. The method can be used in-line to provide real time monitoring of laser cladding and other metal processing operations. (end of abstract)
Agent: D. Morgan Tench - Camarillo, CA, US Inventors: Marvin Klein, Todd Sienicki, Jerome Eichenbergeer USPTO Applicaton #: 20070234809 - Class: 073602000 (USPTO) Related Patent Categories: Measuring And Testing, Vibration, By Mechanical Waves, Beamed, With Signal Analyzing Or Mathematical Processing The Patent Description & Claims data below is from USPTO Patent Application 20070234809. Brief Patent Description - Full Patent Description - Patent Application Claims
BACKGROUND OF THE INVENTION [0002] 1. Field of the Invention [0003] The present invention is generally related to processing of metals and alloys, and is more specifically concerned with detection of defects generated near the surface of a workpiece during processing. [0004] 2. Description of the Related Art [0005] Advanced metal processing methods are continuously being developed to enable economical manufacture and repair of parts with improved physical properties and often complicated shapes. For example, laser cladding (also called laser powder deposition) is being developed for build-up of stainless steel, titanium and other metals (from metallic powders) to enable near net shape manufacturing and repair of critical parts. Advanced joining methods include laser welding and friction stir welding. The friction stir approach, which involves passing a rotating tool through a solid metallic material, may also be used for friction stir processing (FSP) to locally create a fine-grain microstructure providing improved mechanical properties [F. D. Nicholas, Advanced Materials Processes 6/99, 69 (1999)]. [0006] Advanced metal processing typically occurs at high speeds and often involves expensive workpieces and materials so that rapid feedback on the quality of the processed region is critical to controlling scrap rates and costs. Defects that may occur within processed regions of metals include voids, pores, bondlines (incompletely formed bonds), disbonds and cracks. Ideally, such defects would be detected in-line during metal processing so as to minimize scrap and improve product quality via timely corrective action, which could include adjusting processing parameters and/or interrupting the process. Metal processing defects often occur below the surface of the processed region where they cannot be detected by optical, spectroscopic or laser profilometer techniques. Conventional ultrasonic detection methods are sensitive to such subsurface defects but require that the inspected workpiece be in contact with a fluid, which is not practical for in-line use. Inspection methods requiring physical contact between the workpiece and a probe are generally impractical for in-line defect detection. In addition, surface irregularity and roughness typical of processed metal surfaces tend to produce noise signals that interfere with ultrasonic detection based on piezoelectric or EMAT transducers, as well as other conventional methods. [0007] Subsurface defect detection is also a critical requirement for the inspection of cast and forged metals, including ingots and railway rails, for example. Typical defects in ingots and castings include pores and inclusions. Typical defects in railway rails include cracks, which need to be detected in-service. [0008] Laser ultrasonic methods have been developed for non-contact detection of defects in solid parts. Since the "bottom" surface of a part is often inaccessible during machining or processing operations, the most useful laser ultrasonic methods involve both generation and detection on the "top" surface of the part. In this case, a pulsed generation laser beam incident on the part surface at a predetermined generation spot generates ultrasonic waves that propagate within and along the surface of the part. The propagated ultrasonic waves, including those reflected from defects and the bottom surface of the part, are detected via a detection laser beam incident on the part surface at a predetermined detection spot. The propagated ultrasonic waves produce a temporal displacement of the part surface at the detection spot, which is measured via an interferometer that analyzes a portion of the detection laser beam reflected from the part surface. Ultrasonic waves reflected from defects may be distinguished from other reflected ultrasonic waves from the difference in time of arrival of the waves at the detection spot. [0009] Laser ultrasonic methods involving generation and detection on the same surface have been applied to detection of various defects, including voids and cracks, in parts of varied shapes and comprising various materials. These methods have typically involved generation and detection of bulk ultrasonic waves, namely compressional waves, which tend to travel along the surface normal, and shear waves, which tend to travel at angles to the surface normal. Laser ablation produces strong compressional and shear waves, whereas compressional waves produced thermoelastically are relatively weak. Bulk ultrasonic waves are well-suited for detecting defects that are relatively distant from the generation-detection surface. However, bulk ultrasonic waves are not well-suited for detecting near-surface (i.e., subsurface) defects for which the delay time for waves reflected from defects is very short, making ultrasonic measurements difficult. [0010] In addition, application of prior art laser ultrasonic methods has generally been limited to smooth and relatively even surfaces to avoid speckle noise associated with surface roughness and unevenness. In contrast, metallic surfaces processed by laser cladding, friction stirring or other methods tend to be uneven and relatively rough. Consequently, prior art laser ultrasonic inspection methods cannot be directly applied to detection of defects in processed metallic workpieces. [0011] The limitations of prior art laser ultrasonic methods are particularly acute for defect detection during laser cladding. The cladding is typically applied in thin layers and each new layer needs to be inspected for defects before it is buried under subsequently applied layers. This requires detection of subsurface defects that are very near the top surface, which cannot be accomplished using the bulk ultrasonic waves generally employed in the prior art. [0012] The present invention utilizes Rayleigh waves (surface acoustic waves) to detect subsurface defects in processed metallic surfaces. Rayleigh waves have been used in the prior art for characterization of near-surface material properties and for detection of surface-breaking cracks. U.S. Pat. No. 5,894,092 to Lindgren et al. describes use of transducers to generate and detect Rayleigh waves in order to determine near-surface material properties by measuring the Rayleigh wave velocity as a function of frequency. U.S. Pat. No. 4,274,288 to Tittmann et al. describes use of transducers to generate and detect Rayleigh waves in order to determine the depth of a surface-breaking crack through analysis of the ultrasonic frequencies contained in the detected ultrasonic wave. The transducer-based approach described in both of these prior art patents is unsuitable for use on processed metal surfaces, which tend to be relatively rough and uneven, and cannot be used for in-line monitoring during metal processing. In addition, the Rayleigh wave velocity measurements used by Lindgren are relatively insensitive to metal defects, and do not provide the directional information needed for detection of localized defects. Likewise, the ultrasonic frequency analysis used by Tittmann does not provide the directional information needed to locate subsurface defects. [0013] In contrast, the present invention is based on detection and analysis of scattered Rayleigh waves to detect subsurface defects. The prior art provides no suggestion that scattered Rayleigh waves might be useful for defect detection. Another important aspect of the present invention is the use of laser generation and detection of Rayleigh waves so that the invention can be applied to relatively rough and uneven processed metal surfaces, and may be used for in-line monitoring during metal processing. SUMMARY OF THE INVENTION [0014] The present invention provides a laser-ultrasonic method and device that are useful for detection of defects within a processed region of a metallic workpiece. The method involves a pitch-catch approach whereby a surface acoustic wave (Rayleigh wave) is laser-generated at a first location on the workpiece surface, and a scattered portion of the generated Rayleigh wave is detected at a second location on the workpiece surface, along with the direct-arriving (unscattered) Rayleigh wave. The invention is particularly useful for detecting voids in laser cladded metallic layers and friction stir processed layers. The method of the invention may be used for in-line monitoring of laser cladding and friction stir processes, which generally result in a line or bead of processed metal having an appreciable width. The line of processed metal may be curved or straight. [0015] In the method of the invention, a probe Rayleigh wave is generated in the workpiece by directing a generation laser beam of small dimensions to a predetermined generation area within the processed region of the workpiece. When the probe Rayleigh wave is scattered by a subsurface defect, the scattered Rayleigh wave is detected via the temporal displacement of the workpiece surface produced by the scattered Rayleigh wave. This surface displacement is measured using an interferometer and a detection laser beam of small diameter that impinges (interrogates) the workpiece at a detection spot within the processed region of the workpiece. A predetermined spatial relationship is maintained between the generation area and the detection spot. High sensitivity and resolution are attained via use of very small laser beam dimensions and a close spacing between the generation area and the detection laser spot. Sensitivity is typically highest when the detection laser spot overlaps at least a portion of the cross-sectional area of the defect when viewed along a line perpendicular to the surface of the metallic workpiece within the processed region. Further improvement in sensitivity may be provided via wavelet analysis of the detection signal. [0016] In a preferred embodiment, the invention is used to detect defects in a line or bead of processed metal. In this case, the laser generation area preferably has the shape of a rectangle with the long sides of the rectangle substantially perpendicular to the line of processed metal. The laser generation area preferably spans the width of the line of the processed metal. In this case, the entire width of the processed metal line may be continuously monitored for defects in real time during the metal processing operation. [0017] The method of the invention may also be used to provide an image of subsurface defects within the processed region of a metallic workpiece. In this case, measurements of acoustic waveforms (surface displacement magnitude vs. time) are made at regularly spaced locations along the processed workpiece surface, while a predetermined spatial relationship is maintained between the laser generation area and the detection laser spot. This may involve maintaining the generation and detection laser beams at stationary positions while the workpiece is moved so that the laser beams track along a line or bead of processed metal. Alternatively, the workpiece may be maintained in a stationary position while the laser beams are scanned along the surface of the processed metal. In either case, x-y raster scanning may also be employed. Preferably, the relative motion between the laser beams and the workpiece is such that both laser beams impinge the workpiece surface along the line of motion. [0018] In one embodiment, a single waveform corresponding to a defect-free location is chosen as a reference, and the overall amplitude of each waveform is normalized to the amplitude of the reference waveform. A computer program is preferably used to calculate the Mean Square Error (MSE) between the reference waveform and each of the other waveforms in the raster scan. A plot of MSE intensity versus x-y location provides an image of defects in the processed metal. [0019] In a preferred embodiment, the waveform acquired at each location on the workpiece surface within the processed metal region is analyzed using a wavelet transform. This analysis detects the characteristic changes in the waveform that are uniquely associated with scattering from subsurface defects. [0020] The device of the present invention for detecting a defect in a processed metal comprises a generation laser, a detection laser, an interferometer and an analyzer, and may further comprise a translation stage. [0021] The present invention provides significant advantages compared to prior art methods. A key advantage is that the laser-ultrasonic method and device of the invention can be used for in-line detection of metal processing defects, enabling 100% parts inspection and real-time process control. The invention may be applied to detection of defects in metals processed by a variety of methods, including laser cladding, laser welding, friction stir processing and friction stir welding. The invention permits each layer of a laser cladding process to be monitored for defects. [0022] Further features and advantages of the invention will be apparent to those skilled in the art from the following detailed description, taken together with the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS Continue reading... Full patent description for Laser-ultrasonic detection of subsurface defects in processed metals Brief Patent Description - Full Patent Description - Patent Application Claims Click on the above for other options relating to this Laser-ultrasonic detection of subsurface defects in processed metals patent application. ###  How KEYWORD MONITOR works... a FREE service from FreshPatents1. Sign up (takes 30 seconds). 2. Fill in the keywords to be monitored. 3. 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