Ultrasonic Imaging Inspection of Projection Welds

Roman Gr. MAEV, Fedar SEVIARYN

The Institute for Diagnostic Imaging Research, Windsor, Canada Phone: 519 253 3000 ext2679, Fax: 519 971 3611 e-mail: maev@uwindsor.ca, seviazy@uwindsor.ca

Abstract: A high frequency ultrasound imaging technique was applied for quality control of several kinds of projection welds originating from automotive production. The projection welds were formed between V-shaped surfaces of solid components and zinc-coated steel sheets. Acoustic images were obtained both in high resolution and in express regime. Detailed three-dimensional data cubes were acquired using a Tessonics AM 1103 pulse-echo scanning acoustic microscope with a 50 Mhz spherically focused lens. The rapid 2D array technique's potential was demonstrated with a Tessonics RSWA (Resistance Spot Weld Analyzer). The data obtained were processed to determine the joint's basic parameters and to estimate its overall quality. The signal processing procedures were optimized for spatial resolution in conditions of multiple re-reflections. Images of C- -scans represent the welds nugget's geometry. The nugget's dimensions and crossection area were measured. Several defects (lack of fusion) were detected and classified. The results presented will be used for development of specialized instrumentation for in-line inspection of automotive body parts in an industrial environment. Keywords: Ultrasonic Testing (UT), spotweld, Acoustic microscopy, Ultrasonic Imaging 1. Introduction Ultrasonic inspection became one of the standard technologies for nondestructive evaluation of spot weld quality. Different variations of this technique are widely used in the automotive and aerospace industries due to its safety, simplicity and reliability. A corresponding variety of equipment is represented on the market – from simple gauges up to the high resolution imaging systems. However, the majority of those devices are aimed on the inspection of classical spot welds formed on the interface of two plane steel sheets. More sophisticated types of welds with pre-determined joint geometry do not have such attention. The goal of the present research work is to investigate the feasibility of acoustic imaging technology for nondestructive evaluation of projection welds in automotive production. 2. Samples The process of projection welding is similar to usual resistance spot welding, but the localization and geometry of the weld is determined by raised sections (projections) on one or both work pieces. Both heat and force are concentrated at the projections causing here significant plastic deformation of metal. As a result, a solid phase forge bond appears before melting and plays an important role in weld formation. The melted area may take only part of the total weld crossection or may be absent at all with insufficient welding current. Strength of the weld interface is different for melted and forged areas and the ratio between them affects overall weld performance. Our research includes two configurations of the projection welds. The first set of samples represents sheet joining with embossed dimple on one component (Figure 1). The strength and quality of the welds was intentionally varied along the set by gradually decreasing the welding current. The second type of the samples includes a nut with machined “V” projections welded to a metal sheet of galvanized steel (Figure 2). Proper geometry of the weld is formed by the shape of the contacting surfaces (three 600 arches surrounding the nut opening or full ring for several samples). The nuggets which form during the resistance

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welding process have corresponding shape. Weld supposed to be nominal if the melted area of the nugget is big enough.

Figure 1 Projection sheet welding and schematic crossection of the resulting spot weld.

Figure 2. Photo of projection nut and schematic crossection of the joint. 3. Scanning Acoustic Microscopy The high resolution acoustic images [1] of the welds were obtained with SONIX HS-1000 scanning acoustic microscope. The optimal resolution and image quality was obtained by using a 50 MHz spherically focused acoustic lens with focal distance of 12 mm. The scanning and measurement procedure was standard for the sheet welds [2]. Resulting C-sans show usual for the spot welds images with black color representing nugget, and light color representing areas with good sound reflection i.e. undisturbed sheet surface (Figure 3 a).In the case of nut welding area 20 x 20 mm centered on the nut opening was scanned from sheet side. Areas with the dark color reflect little or no ultrasound pulse and they correspond to the opening (central spot), area of beveled sheet edges (ring immediately following opening) and the weld nugget itself (outer dark ring, Figure 3 b and c).

Figure 3. Samples of projection welding: sheet welding (a), nut welded to the sheet by three

arc-shaped projections (b) nut with ring-shaped projection (c).

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The obtained C-scans clearly show the outer boundary of the nugget. For most nut samples, the strip of unwelded area separates the nugget from the edge of the opening, but sometimes the inner boundary of the nugget is barely visible and is hard to detect. The overall weld quality can be estimated from the area of the nugget on the C-scans. For all samples, the outer diameter of the weld corresponds to the diameter of the nut (12.5 and 10.8 mm). Thickness of the nugget strip is about 1 mm. Small defects and thinner segments also are well detectable. 4. RSWA images Commercially produced by Tessonics Inc., the RSWA (Resistant Spot Weld Analyzer) device was developed for nondestructive evaluation of the nugget size of the resistant spot welds [3]. It provides a good performance for standard spot welds and precise measurement of a nugget diameter. Each element of 2D 8 x 8 array sends a short acoustical pulse towards the investigated sample and immediately switches into the receiving mode. Part of the initial wave is reflected from the front face of the sample and from interfaces inside it. The device measures amplitude of received signal at certain time intervals, corresponding to the depth of interface between welded parts. The set of data from all elements of array is used for the creation of the acoustic image of this interface. The native resolution of the RSWA device is relatively low due to the large element’s size (1.25 x 1.25 mm pitch), however, the image can be smoothened with corresponding processing. For each sample of sheet welding, the C-scan shows a clear, well-defined spot which ideally should represent the weld nugget (Figure 4). The size of this spot and corresponding RSWA indication is increasing with the increasing of the welding current (Figure 5) in almost linear dependence. These results are in good correspondence with data of acoustic microscopy measurements, but the peel test and the metallurgical crossection reveal a clear threshold for nugget formation. Low current welding (less than 300 Skt) produces a weld without a melted nugget, just by solid phase bonding process. Such welds are relatively easy to break along sheets contact. Proper strength can only be achieved with steel melting at a current above 300 Skt .

Figure 4. Comparison of the C-scans obtained with RSWA and scanning acoustic microscope Standard RSWA array covers a round area of 10 mm in diameter which is enough for the case of sheet welding, but less than the nugget size for nut samples. Therefore, in this case, the acoustic image of the whole projection weld was obtained by the overlapping of several individual scans. The final image was assembled from the recorded C-scans with their digital superposition. Figure 6 illustrates this operation. The bottom interface of the steel sheet reflects ultrasound and these areas are shown in red on the image. The areas shown in green represent regions where reflection from the bottom is absent. The inner round green region in

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the centre of the image is the opening. The next ring of red spots on Figure 7 represents the non-welded strip exposed on Figure 3c and the outer green ring shows the nugget. The nugget area merges with central hole at the images of some samples, but the outer diameter of the nugget is still available for measurement.

Figure 5. Dependence of RSWA readout from welding current for projection sheet welding. The imaging method reveals major defects in the welding nugget such as insufficient size, large bubbles and cracks. For example, Figure 7 b shows the interruption in the ring-shaped weld nugget. The real size of the defect is about 2 mm and is compatible with the native resolution of the RSWA device, but build-in advanced signal processing allow its detection.

Figure 6. Schematic representation of the array positioning on the projection weld.

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Figure 7. C-scans of nut joint obtained by assembling of RSWA indications.

5. Conclusion The scanning acoustic microscopy is suitable for detail inspection and precise measurement of the represented projection welds, however, it requires time and a laboratory environment. Device based on RSWA technology can be used for the fast on-the-floor estimation of the quality and nugget size. The applicability and performance of 2D array system in the case of large-spreaded projection welds can be improved by increasing the diameter of the array and, correspondingly, increasing the number of the elements. Additionally, to improve resolution in radial direction, the size of elements should be reduced. That led to significant increasing of the number of elements and, correspondingly, to the further increasing of overall system cost. In order to keep this number in reasonable limits, the special array design may exclude the non-informative elements in the center. Acknowledgements Authors would like to thank our partners from BMW AG and Keiper Company for the provided samples and valuable discussion of results. References 1. R. Gr. Maev ‘Acoustic microscopy: fundamentals and applications.’ John Willey, 2008, 291p. 2. A. M. Chertov, R. G. Maev, and F. M. Severin ‘Acoustic Microscopy of Internal Structure of Resistance Spot Welds’ IEEE Proceedings UFFC, Vol. 54, No. 8, pp1521 -1529, August 2007 3. A. A. Denisov, C. M. Shakarji, B. B. Lawford, R. G. Maev, and J. M. Paille, ‘Spot weld analysis with 2D ultrasonic array’ J. Res. Nat. Institute Standards Technol., Vol. 109, No. 2, pp 233– 244, Mar.–Apr. 2004.

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Defect: lack of fusion b a