Abstract

The contribution gives a short overview of IZFP investigations towards the process integrated characterization of microstructure and evaluation of mechanical properties. The propagation velocities of ultrasonic waves allow the evaluation of the elastic constants of the alloy and good correlations are found between the acousto-elastic constant of different alloys and their yield strengths. Using the times-of-flight of different ultrasonic waves, the residual stress states in plates and welded parts are evaluated. The texture in Al-sheets is determined and the earing parameter is characterized using measurements of ultrasonic times-of-flight as function of the ultrasonic propagation direction. Ultrasonic backscattering technique is applied to visualize the distribution of ceramic reinforcements in automotive pistons and conrods made of Al-based metal matrix composites. The volume content on reinforcement is evaluated from the ultrasonic velocities. Backscattering and time-of-flight measurements are used to visualize and to characterize the burrs in Al- hollow castings.

Table of contents
1. Elastic Constants and Material Strength
2. Stress States in Plates an Welded Sheets
3. Texture and Earing Parameter
4. Local Distribution and Volume Concentration of Reinforcements in Al-Matrix Composites
5. Localization and Characterization of Burrs in Hollow Pressure Castings
6. References

1. Elastic Constants and Material Strength

It is common practice to evaluate the elastic constants Young`s- and shear moduli using the material density and the propagation velocities of ultrasonic longitudinal and shear waves. The evaluated constants are generally found in a good agreement with those determined by tensile testing. Ultrasonic velocity meaurements can be successfully used to detect areas of a component with microstructural variations caused e.g. by pores, damages or plastic deformations. Since the velocities can be determined with an accuracy better than 0.1%, those changes of the microstructure which are influencing the elastic behaviour in that order of magnitude can be detected and characterized nondestructively (1,2).

The above mentioned elastic constants describe the elastic behaviour in first approximation. A more precise description of the real elastic properties of a material is possible if third order elastic constants are used. Acousto-elastic constants finally are combinations of the elastic constants Young`s, shear moduli and the third order constants. As expected, these acousto-elastic constants are more sensitive to elastic properties than the moduli. In experimental investigations good correlations are found between the acousto-elastic constants and the percentages on solid solution phase of three work hardening and two heat treatable Al-alloys. Furthermore, linear correlations are found between the acousto-elastic constants and the yield strengths of the mentioned alloys as to be seen in Figure 1 (3). Similar investigations dealt with the characterization of the elastic properties of Al-based MMC`s with different types and volume percentages of reinforcements (4). The influence of fatigue damages on the third order elastic constants and on the acousto-elastic constants is shown in (5). Besides the elastic constants also their temperature dependencies are found to be suitable quantities to characterize microstructural states (6). A two frequency eddy current device has been developed for the in process evaluation of the tensile strenght of Al air bag components and for the simultaneous detection of surface defects (7). In Figure 2 the nondestructively evaluated strength values are shown in comparison with the tensile test results.

Figure 1: The acoustoelastic constant AEC33 of a longitudinal wave propagating perpendicular to the load for work hardening and precipitation hardening Al-Alloys with different content on solid solution phase (left) and with different yield strengths (right).

Figure 2: Nondestructively evaluated strength values in comparision with the tensile test results.

2. Stress States in Plates an Welded Sheets

Complementary to the established techniques to evaluate stress states, ultrasonic techniques permit the evaluation of stresses in surface layers and in the bulk of components. One advantage of ultrasonic techniques is the possibility of a fast evaluation of stress states enabling a continuous analysis along traces to get information about the stress distribution and stress inhomogeneities. Different ultrasonic techniques have been developed to meet the requirements concerning the local resolution, the evaluation of one, two or three axial stress states, and the environmental conditions. Set-ups for the automated data aquisition and evaluation are available as well as sensors of different types and sizes. The stress states in plates and sheets cause sometimes difficulties during the sawing and cutting process. Hence, the stress state and the stress distribution is of interest in order to prevent the difficulties as well as unnecessary stress relief treatments. Ultrasonic stress analysis was applied to casted Al-plates. Using the skimming longitudinal wave, the surface stresses have been evaluated and using linearly polarized shear waves, the volume stress states have been characterized. The results, displayed in three-dimensional figures clearly show stress inhomogeneities (8,9). It is known that the residual stress states due to welding strongly influence the dynamic and static behavior of a welded component. Hence, it is of significant importance to know the residual stress states after welding and after post-welding treatments. But in order to take advantage of the locus continuous ultrasonic techniques, some experimental investigations are needed because the quantitative evaluation of stress states using ultrasonic techniques presupposes the knowledge of the acousto-elastic constants. These constants have been evaluated for two Al-alloys frequently used by automotive industry. Using the evaluated acousto-elastic constants, the stress states in and around weldments in Al-sheets were mapped. The Figures 3 and 4 show the profiles of the two principal stresses acting parallel and perpendicular to the weld seam. Comparisons with the results of an established technique demonstrate the reliable applicability of ultrasonic techniques (10).

Figure 3: Residual Stress parallel (l) to the weld seam in a welded aluminium sheet. Figure 4: Residual Stress prependicular (q) to the weld seam in a welded aluminium sheet.

3. Texture and Earing Parameter

Regarding the aluminium beverage can production, the economical significance of the right texture becomes obvious: The two predominant crystallographic textures that exist in the aluminium sheet need to be balanced. One kind of texture arises during annealing of the alloy after the strip is hot-rolled from the ingots. It causes four ears to appear every 90 degrees. The second kind results from cold rolling the sheet producing four ears also. Proper control of annealing and rolling can lead to a combination of the two textures such that ears caused by one fill the valleys caused by the other process. The result is eight very low ears and a reduction of the amount of aluminum needed. Up to now, the texture and/or the earing behavior is checked by destructive testing. Samples of the production are usually cut from the head and the tail of the strip. Slight changes of the processing parameters can cause changes of the texture in the head and tail part of a roll and hence, earing values are evaluated which are not representative for the quality of the strip.

The interdependencies between the crystallographic texture and the propagation velocities of ultrasonic waves are known and have been widely used and published as are the interdependencies between the texture and the earing behavior. The ultrasonic SH-wave was found to be the most suitable ultrasonic mode to characterize the texture. The major advantage of using a SH-wave is the possibility to keep a distance between the ultrasonic sensor and the surface of the strip in order to prevent any surface damage. The distance between the ultrasonic transmitter and receiver is usually kept constant. Hence, the ultrasonic time-of-flight is the only measuring quantity. Using our prototype system, the reproducibility of the time-of-flight measurement is better than 0.3 ‰. Measurements were performed at the positions 50%, 75% and 95% of the width of the strip in order to check on the homogeneity of texture. The absolute time-of-flight data are taken at stepwise changes of the ultrasonic propagation direction with respect to the rolling direction. As example, the relative change of the ultrasonic time-of-flight with the propagation direction of the SH-wave is displayed in Figure 5. The difference of the extreme values of the relative time-of-flight data is found to be in a very satisfying correlation with the earing parameters as shown in Figure 6. The earing parameters were evaluated by the manufacturer of the strips using the established technique 11. A similar ultrasonic system for on line texture analysis and characterization of deep drawability of ferritic steel strips is already in industrial use (12).

	Figure 5: Relative change of the ultrasonic time-of-flight versus the angle between the sound propagation and the rolling direction of hot rolled aluminium sheets.
	Figure 6: Correlation between the ultrasonic quantity and the earing parameter of hot rolled aluminium sheets.

4. Local Distribution and Volume Concentration of Reinforcements in Al-Matrix Composites

Compared with the established techniques to characterize the local distribution of reinforcement, to evaluate elastic constants and to inspect the fiber alignments, the ultrasonic techniques have the advantage of a fast and nondestructive information, and hence they are also regarded as of considerable help for the optimization of manufacturing parameters. Based on a high level of physical understanding and measuring techniques, the elastic interactions of ultrasonic waves with the reinforcement and the matrix are used to develop an element of the MMC quality assurance concept.

In order to visualize the distribution of the density-Al₂O₃ short fibres in a AlSi1₂CuMgNi-matrix, a longitudinal ultrasonic sensor with a centre frequency of 50 or 80 MHz is manipulated across the surface of the part under test. Ultrasonic scattering occurs at the matrix / fiber interfaces and after significant amplifications, the peaks of the backscattered ultrasonic pulse amplitudes are detected. The amplitudes are digitized and colour coded, corresponding to their values. High values, corresponding with lighter colours in the so called C-scans indicate areas where the reinforcement in the matrix increases the ultrasonic backreflection and backscattering. In Figure 7 the inhomogeneous distribution of the reinforcement of a piston crown is shown. Using advanced computational possibilities, the visualization of the distribution of the reinforcement along the thickness direction of the component is also possible.

Figure 7 : Distribution of Al2O3 short fibers in an automotiv piston crown with combustion bowl.

The density of MMC components can be calculated from the densities of the two constituents and their volume or weight percentage using the law of mixture. The Young's- and shear moduli do not follow the law of mixture. Calculations using the Voigt or Reuss model result in the extreme values; the average of both (approximately the Hill model) usually fits with experimental results. The change of the elastic constants with the volume percentage of short fibers are calculated for the above mentioned MMC. Using the calculated dependencies of the Young's and shear moduli on the volume percentage of short fiber reinforcement and the corresponding dependency of the density, the ultrasonic longitudinal and shear wave velocities in MMC`s are calculated as function of the volume percentage of Al₂O₃-fibers. The relative changes of the longitudinal and shear wave velocities with the volume percentages are found to be different: 1% change of volume percentage causes about 4.3‰ change of the longitudinal velocity and a change of about 7.8‰ of the shear wave velocity. Applying linearly polarized shear waves, the change of the volume percentage on Al₂O₃ short fibers in a piston crown is evaluated along a diametral trace. The vibration of the wave was parallel to the trace and perpendicular to it. As to be seen in Figure 8, there is no difference of the elastic properties of the material found along the directions of wave vibration. Hence, the piston is planar isotropic. Using the earlier mentioned calculations, the variation on the volume percentage of Al₂O₃ short fibers across the diameter of the piston is evaluated. 13.

Figure 8: Variation of shear wave velocity and evaluated volume percentage of Al2O3 short fibers along the diameter of an automotive piston crown.

5. Localization and Characterization of Burrs in Hollow Pressure Castings

Aluminium pressure castings are frequently used semifinished products of high functional versatility. The burrs caused by the joining process of different streams of the molten metal passing through the matrix can be areas of poor mechanical properties. Reduction in strength and toughness and a bad fatigue behavior are the major drawbacks of the material in and around bad burrs. Up to now the quality of the burr is checked by common destructive tests using samples cut from the product. The nondestructive ultrasonic approach to characterize the burr and the material properties takes advantage of the microstructural changes in the burr and in its vicinity. Changes of the grain size and shape as well as the presence of second phases like aluminiumoxide in the matrix material influence the ultrasonic scattering. Hence, ultrasonic scattering technique is applied to visualize the burr. The recrystallization process changes not only the sizes and shapes of the grains in the region of the burr but also their orientations. Hence, the direction dependencies of ultrasonic waves are applied to determine the texture in the area. Using samples cut at different length positions from a pressure casted hollow girder, the position of the burr and its shape is visualized as shown in Figure 9. Together with the results of the texture analysis, displayed in Figure 10, the part of the length of the girder is determined in which the burr is not perfectly developed. The comparison of the mechanical properties of samples cut from that area with the data of samples cut from the remainding part of the girder with a perfectly developed burr confirms the non-destructive result 14.

Figure 9: C-Scan of ultrasonic backscattering from an area of 100 x 100 mm2 around a badly (above) and a well developed burr (below). Figure 10: Texture caused direction dependence of ultrasonic shear wave velocitiy measured in a badly (above) and a well developed (below) burr.

6. References

1. E. Schneider, D. Bruche, R. Herzer, Qualitätssicherung durch Werkstoffprüfung, Berichtsband 34, DGZfP Berlin (1992) 134-139.
2. G. Dobmann, N. Meyendorf, E. Schneider, Bulletin du Cercle d´Etudes des Metaux, 10th International Symposium Assessment of Materials Aging and Damage Evolution by Nondestructive Evaluation Methods, St. Etienne, Tome XVI (1995) Kap. 3.
3. E. Schneider, S.L. Chu, K. Salama, Review of Progress in Quantitative Nondestructive Evaluation, D.O. Thompson, D.E. Chimenti (eds) Plenum Press New York London 4B (1985) 867-875.
4. D. Lee, K. Salama, E. Schneider, Nondestructive Characterization of Materials, P. Höller, V. Hauk, G. Dobmann, C. Ruud, R. Green (eds.), Springer Verlag Berlin Heidelberg (1989) 173-183.
5. J. Wahnschaffe, IZFP, Diplomarbeit, Universität des Saarlandes 1994.
6. H. Mohrbacher, IZFP, Diplomarbeit, Universität des Saarlandes 1991.
7. R. Becker, M. Disque, DGZfP Berichtsband 52.1, DGZfP Berlin (1996) 365-372
8. E. Schneider, A. Pierre, Colloque International des Progrès dans les Méthodes d'Investigation des Métaux et des Nouveaux Matériaux. Mesures des Contraintes Résiduelles, Saint-Étienne (1993) 17.1-17.15.
9. E. Schneider, W.A. Theiner, GMA-Bericht 29, VDI/VDE Düsseldorf (1996) 405-415.
10. E. Schneider, U. Arenz, IZFP Bericht 960165/TW (1996).
11. E. Schneider, L. Oesterlein, Nondestructive Characterization of Materials VII, A.L.Bartos, R.E.Green Jr., C.O. Ruud (eds.), Transtec Publications Ltd., Zürich, Part1(1996) 405-410.
12. M. Borsutzki, Ch. Thoma, W. Bleck, W. Theiner, Stahl u. Eisen 113 (1993) 93-99.
13. E. Schneider, L. Oesterlein, D. Bruche, 6th European Conference on Nondestructive Testing, Conference Proceedings Tome 2 ( 1994) 1013-1017.
14. E.Schneider, L. Oesterlein, D. Bruche, Nondestructive Characterization of Materials VII, A.L.Bartos, R.E.Green Jr., C.O. Ruud (eds.), Transtec Publications Ltd., Zürich, Part 1 (1996) 397-404.

Author

Eckhardt Schneider
Head of Department Process integrated Nondestructive Testing at
Fraunhofer Institut Zerstörungsfreie Prüfverfahren IZFP,
University Bldg 37, D-66123 Saarbrücken
Phone: +49 681 302 3840
Fax: +49 681 39580
Email: eschneider@izfp.fhg.de
Homepage http://mm.fhg.de/depts/izfp-e.html

The paper was presented on the DGZfP regional working Group meeting in Hamburg in April '97