2. Combination of eddy current inspection and imaging analysis

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1 11th European Conference on Non-Destructive Testing (ECNDT 2014), October 6-10, 2014, Prague, Czech Republic Innovative Eddy Current Inspection on Lightweight Components More Info at Open Access Database Gregor GRZONKOWSKI, Gerald SCHNEIBEL Rohmann GmbH; Frankenthal, Germany Phone: ; Abstract Due to the specification of a maximum weight savings in today's automotive industry more and more lightweight materials are used. Above all, aluminium finds wide application in this case, not only in the body area, but also in engine and transmission components. Due to the electromagnetic properties of aluminium, it is out-standing for testing using the eddy current method. Thus, multiple approaches revealed for testing complex aluminium components. In the present paper the potential of this non-destructive inspection technique in connection with classical image processing will be shown by two examples. Keywords: Eddy current, automotive, c-scan, automatic inspection, imaging analysis, aluminium 1. Introduction Using standard eddy current technology in combination with a mechanical scanning device, images of the inspected components can be created. This images represent the geometrical outline as well as material properties and local in-homogeneities and can much easier be interpreted then the pure impedance signal. Figure 1. C-Scan image of an aluminium casting component with holes (one hole with cracks) As it can be seen in the above C-Scan image, the hole with the edge defects in different orientations is clearly visible. Besides this, the background noise level (represented in the slight grey shadows of the image) gives an idea of the homogeneity of the internal microstructure of the aluminium. Normally this noise level can t be interpreted in the impedance plane evaluation but often is carrying some microstructure or machining process information. 2. Combination of eddy current inspection and imaging analysis Based on the long experience in c-scan imaging the idea was born to combine the images with standard image processing algorithms to get high sophisticated evaluation results even if important interfering contours are present on the components to be tested. In the present paper two examples for the state of the art inspection of lightweight components should be presented: Inspection of aluminium crankcases Inspection of die casted transmission components

2 In both applications the different evaluation approaches are realized which lead to a full production inspection of the different components. 2.1 Example: Inspection of aluminium crankcases For a few years weight reduction had been an important issue also for engines. Therefore different developments were taken and one of them leaded to light metal crankcases with aluminium. The eddy current testing, in addition to approaching the detection of production related defects such as voids, cracks and grooves, is able to detect microstructure inhomogeneity and can classified it by image analysis. This becomes necessary when the aluminium base material is merged with other substances to increase wear resistance. For each cylinder bore a rotating eddy current head was used supporting 2 probe elements. This probe elements scan the inner bore surface at 1000 RPM with a resolution of about 0,25 mm²/sample. Figure 2. 2-channel rotor head Figure 3. 6-cylinder engine The results of each probe element were combined, so an overall image of each cylinder can be created. This image is the basis for the following image processing. Figure 4. Combination of the 4 individual scans to a summary scan In a first step interesting indications were identified using histogram functionality. Once the indications were separated, analyzing algorithms determine the form, the size and the distribution of the indications. Finally they are classified and based on an evaluation grid an index per image segment is calculated which finally leads to an overall index for each cylinder bore.

3 Figure 5. Image processing result The overall test result of a complete crankcase is finally transferred to the process monitoring software and is stored for each individual crankcase for later quality reviews. For using this technology in production, a machine was designed were crankcases for up to 6 cylinders can be inspected using 6 parallel driven rotary heads. Figure 6. High throughput inspection mechanics for 6-cylinder crankcases This solution reaches an inspection time of 10s per cylinder which leads to an overall cycle time of 25s per crankcase. It has been in production now for more than 5 years in 3 shift production. 2.2 Inspection of die casted transmission components In the development of automatic transmissions an optimum of comfort for the passenger in combination with a minimum of fuel consumption is demanded. A new development results in an 8 shift transmission. To realize this transmission in the available space, completely new approaches were implemented. As the weight of the moving parts is a major factor, many of them are based on a lightweight aluminium alloy. The component in this example has to transfer up to several hundred Newton of torque at even high rotation speeds and is therefore object of closer investigation for material defects. In this case a single probe element is used which scans the rotating component. A C-Scan mapping of the result is generated as basis for the further interpretation of the results.

4 Figure 7. C-Scan result of a die casted transmission pot with different test notches The above picture shows, that besides the holes (for weight reduction) also the die casting flow lines appear and give an idea of the homogeneity of the aluminium alloy. Two different tasks had to be covered by the interpretation of the inspection results: Detection of cracks, even in the edges of the holes Detection and classification of pores against the customers specification For the crack detection, the main problem was that the strong signals caused by the holes themselves had to be suppressed. Also the holes, due to the manufacturing process, were not perfectly round but may be deformed to an elliptical outline. Additionally there might be additional bore holes resulting in the dynamic balancing of the components. Finally there is no really fixed zero position to which the component is positioned in the inspection machine. All this was resolved using a special algorithm, which first detects the holes using a histogram function and then verifies the outline of the holes for discontinuities. The algorithm is resistant to certain grade of deformations as well as varying diameters and positions of the holes. Figure 8. C-Scan of a component with reference defects and the resulting evaluation result Basis for the evaluation for pores was the customer s specification. In which, different pore classes were defined which are acceptable or not. In general pores with bigger diameters (> 1 to 4 mm) as well as a number of smaller pores (> 0,75 2 mm) in a certain area lead to a rejected part. The evaluation was implemented in a parallel algorithm which runs on the same data as the crack detection.

5 Figure 9. Complete automatic system, dedicated probe and used eddy current instrument Two different machines were built up, one with a linear axis system for the sensor positioning and one using a KUKA robot for the sensor positioning. With both machines a short inspection time of about 15-20s per component had been realized which qualified the systems for integration into a production line with 100% inspection. 3. Conclusion In conclusion it can be said that the eddy current method in the field of lightweight materials, especially for aluminum alloys, has an enormous potential which can be substantially increased by using appropriate image analysis algorithms. This makes it applicable to many upcoming inspection tasks.