Synchrotron-radiation based microtomography of new materials for lightweight construction

Transcription

1 Synchrotron-radiation based microtomography of new materials for lightweight construction Felix Beckmann, Tilman Donath, Thomas Lippmann, Andreas Schreyer, Helmut Clemens: GKSS-Forschungszentrum, Geesthacht, Germany. Abstract X-ray computed microtomography using synchrotron radiation (SRµCT) is applied for nondestructive three-dimensional investigation of new materials for lightweight construction. Metal foams are a promising material for the automotive as well as for the aircraft industries. Several foam samples demonstrate the high spatial resolution, which can routinely be achieved in three dimensions with the microtomography setup. Furthermore first results of a study on the material transport in the friction stir welding process of aluminum are shown. Introduction The GKSS Forschungszentrum Geesthacht is planning and developing a new high energy X- ray synchrotron radiation beamline at the Hamburger Synchrotronstrahlungslabor HASYLAB of the Deutsches Elektronen-Synchrotron DESY. This beamline will enhance the capabilities of investigating large samples for materials research and industrial applications. The outstanding characteristics of this beamline will be the use of monochromatic high-energy X- rays with photon energies ranging from 25 up to 200 kev, which penetrate deeply into materials. The project is a collaboration of the HGF centers DESY Hamburg, GFZ Potsdam and GKSS Geesthacht. While DESY provides the source the two other HGF labs in the collaboration focus on research in the fields of engineering materials (GKSS) and geological materials (GFZ) respectively. The beamline will cover three different experiments. A high sample manipulator X-ray detector y z x monochromatic synchrotron radiation sample CCD camera lens fluorescent screen Fig. 1: Experimental setup used for µct at beamline W2 of HASYLAB at DESY. 299

2 pressure cell will be run by the GFZ Potsdam, a diffractometer together with the microtomography setup will be run by the GKSS. [1,2] Using the old wiggler HARWI at the beamline W2 of HASYLAB microtomography is used to investigate the microstructure of new materials for lightweight construction. Measurements performed on Ti-6Al-4V metal foam and friction stir welds of aluminum will demonstrate not only the high spatial resolution of the system but also the feasibility of applying microtomography using synchrotron radiation to large samples. Tomography setup The apparatus used for SRµCT was built to perform phase- and attenuation-contrast microtomography at photon energies in the range of 8 to 150 kev. [3] In recent years the experiment was setup at different beamlines at HASYLAB as an user experiment for attenuation-contrast µct with the following features: BW2 W2 BW5 Photon energy: 8-24 kev kev kev Sample height: 3.5 mm 4 mm 6 mm Sample width: 10 mm 15 mm 6 mm Spatial resolution: 2 µm 3 µm 10 µm The experimental setup shown in fig. 1 mainly consists of a sample manipulator and a 2-dim. X-ray detector. The monochromatic X-rays entering the camera are converted by a fluorescent screen into visible light which is projected onto a CCD-camera. The used components at beamline W2 are: CCD camera: Optical lens: KX2, Apogee Instruments, Inc.; 14 bit resolution at 1.25 MHz digitization rate, 1536x1024 pixel²; each 9x9 µm² Nikon Inc., 50 mm focal length, 35 mm focal length, respectively Fluorescent screen: CdWO 4 single crystal, thickness 500 µm The sample manipulator provides both for the rotation and for the x/z position of the specimen. It consists of a special manufactured goniometer (Huber Model 410, eccentricity < 2µm, wobble < ) and an x/y/z-positioning stage. The whole unit can be translated and precisely repositioned in the x-direction to perform reference measurements without the sample. 300

3 Structure determination of Ti-6Al-4V metal foam Metal foams are a promising material for future lightweight construction. Strong interest comes from the automotive as well as from the aircraft industry. Different foaming processes are being intensively studied, since the manufacturing process has strong influence on the resulting material properties. X-ray computed microtomography (µct) has been applied for the non-destructive investigation of a Ti-6Al-4V metal foam sample. The sample was produced starting from a powder and applying a hot isostatic pressing process under Argon inert gas atmosphere. After flattening, pore formation occurred when the material was diffusion annealed. The µct-measurements were carried out at beamline W2 at a photon energy of 33 kev on an approximately cylindrical sample with a diameter of 1 mm. Projection images were recorded at 720 angular positions, equally stepped over a 180 range. The tomography setup was set to an optical magnification factor of 6, resulting in a field of view of 2.3 x 1.5 mm² and an effective pixel size of 1.5 µm. Reconstruction of 1024 slices was performed on a 1300 x 1300 grid by the filtered back projection algorithm. Fig. 2 shows a reconstructed slice perpendicular to the sample axis. Pores with diameter of about 5 µm to 200 µm are clearly visible. The 3- dimensional arrangement of the porous structure in the total reconstructed volume data set (height 1.4 mm) is rendered in fig. 3 using the software VGStudioMax of Volume Graphics GmbH, Heidelberg, Germany. 18.0cm mm 1.4 mm 100um 0.0cm -1 Figure 2: Reconstructed slice of a TiAlV foam sample. Pores are visible in a size ranging from arround 5µm up to 200µm. Figure 3: Rendering of the full reconstructed volume data set. The foam sample has a diameter of 1.0 mm. 301

4 3D-Visualisation of material flow in friction stir welding In late 1991 friction stir welding (FSW) was introduced and patented by The Welding Institute (TWI) in the United Kingdom [4]. FSW has been widely recognized for its ability to provide greatly improved weld properties over conventional fusion welds. It has found application in e.g. the welding of fuel tanks. Unlike conventional friction welding where one or both parts to be joined are rotated and then coupled under pressure, FSW employs a welding tool to facilitate the joining process. Heat resulting from the interaction of the tool with the workpiece causes material in this region to soften. The tool is then traversed along the join line where plasticized material is forced to flow around the rotating pin. The shape of the welding tool and interaction of the tool with the base material i.e. process parameters such as down force, travel and rotational speed influence the structure and the quality of the weld. For the prediction of weld properties from the welding parameters it is necessary to fully understand the welding process including material transport. Several publications have been investigating the material flow during FSW. In [5] small steel balls have been implanted as marker material into sheets of aluminum before welding. They were visualized by radiography after the welding had been stopped rapidly. A similar approach for the visualization of material flow is taken here. Ti-powder (particle size µm) is used as marker material and was implanted into 5 mm thick aluminum sheets. Welding was performed at a rotational speed of 800 rpm and a travel speed of 200 mm/min. Different sizes of Ti-powder were tested. Microtomography is applied for the visualization of the redistribution of marker. For each sample several tomographical scans at different positions relative to the pin were performed under varying tilt angles to avoid Figure 5: Radiographs of a friction stir weld at different sample heights. The sample is 12 mm wide and 5 mm thick. The welding pin is highly absorbing (black), the Ti marker distribution can be seen (dark grey). The grey scale corresponds to the absorption ranging from 0 to 2 in the frontal view on the left and from 0 to 4 in the side view on the right. 302

5 Figure 6: Rendering of a tomographic reconstruction. Here the weld was scanned with the welding pin of 5 mm diameter still inside the material. shadowing effects from the strongly absorbing pin. During each scan 720 images were acquired at a photon energy of 43.2 kev. The optical magnification was set to ~1, resulting in an effective pixel size of ~9 µm. Radiographs from the scans are shown in fig. 5. The weld pin has a diameter of 5mm while the full sample size is 12 x 12 x 5 mm 3. The redistribution of marker material is shown in the rendered image in fig. 6. The aluminum is partially transparent in this view and the redistribution of Ti-marker behind the pin is visible. Summary These first microtomography studies on friction stir welds reveal that it is possible to follow material flow with Ti-powder as tracer element. It is planned to run a series investigation on friction stir welds under varying process conditions with the final goal of setting up a simulation of the FSW process. The high quality volume data obtained for the TiAlV metal foam will be used for the further evaluation of structural properties. 303

6 Acknowledgement For providing the porous metal foam presented here we thank R. Willumeit (GKSS). We also thank J. dos Santos and R. Zettler (GKSS) for providing the friction stir weld samples. For assistance during measurement at W2 we thank W.-R. Dix and B. Reime (HASYLAB at DESY, Hamburg). References [1] M. Tischer, J. Pflüger, Upgrade Options for the HARWI Wiggler, HASYLAB annual report, pp , [2] A. Schreyer, T. Lippmann, F. Beckmann, J. Metge, K.-D. Liss, HARWI 2, a new synchrotron beamline for materials science,hasylab annual report, pp , [3] F. Beckmann, U. Bonse, T. Biermann, New developments in attenuation and phasecontrast microtomography using synchrotron radiation with low and high photon energies, SPIE Vol. 3772, pp , [4] W. M. Thomas, et al., Friction stir butt welding, International Patent Application No. PCT/GB , Great Britain Patent Application No ; [5] K., Colligan, Material flow behavior during friction stir welding of aluminum., Weld. J. July, pp , Contact Dr. Felix Beckmann, GKSS Forschungszentrum Geesthacht, Max-Planck-Straße, D Geesthacht, Germany felix.beckmann@gkss.de 304