Applications in metallurgy of X-ray computed tomography with variable focal spot-size and infrared thermography / Impieghi della tomografia computerizzata rX a fuoco variabile e della termografia attiva ad infrarossi in settori di interesse metallurgico

Industrial X-ray tomography and active infrared thermography are very promising imaging techniques. Computed tomography (CT) is performed scanning samples through a X-ray beam (fig. 1); after the acquisition of the transmitted radiation (fig. 2), a suitable mathematical algorithm returns the scanned volume in digital form (fig. 3). Currently there is a wide range of CT equipments for industrial applications, ranging from linear accelerators [1], that cross steel walls with thicknesses up to 300 mm, to optical systems for the study of microelectronic components [2]. CT is used in the field of non destructive testing [3], to study porous materials such as metal foams [4] and in applications of reverse engineering [5]. Active infrared thermography, recognized as a valuable tool to identify cracks and other types [6], is based on the thermal transient generated by an external source [7]: defects and discontinuities modify the thermal flux induced inside the material, causing anomalies in the surface distribution of temperatures that are measured by an infrared camera [8]. In previous works CT [9] and active infrared thermography [10] applications have been experimented. This paper reports results of the two techniques in areas of interest for metallurgy. The working geometry of a tomographic system is showed in fig. 4. The three-dimensional return of the object is obtained through algorithms [11] developed from the early studies of J. Radon [12]. The observable object size decreases strongly with the increasing of the resolving power, as outlined in figure 5. The tomographic system utilized (figures 6 and 7) works at an acceleration voltage up to a maximum of 225 kV, in the range microfocus-macrofocus, changing the spot size from 250 to 800 μm, in order to give priority to spatial resolution (between 30 and 200 um), penetration capacity in materials with high absorption (up to 10 mm in the case of ferrous alloys) or object under examination size (diameter 180 mm, height 250 mm). Active infrared thermography performs radiometric measurements of the temperature distribution generated by halogen lamps, flash or jets of heated air (optical methods) [10], or ultrasounds (acoustic methods) [13] . In the ultrasonic technique experimented in this work (fig. 8), a piezo-ceramic transducer, coupled to the surface of the object under examination, generates acoustic waves (vibrations) at high frequency, that are locally converted into heat by dissipative phenomena at internal discontinuities. The excitation signal consists of an ultrasonic wave (carrier signal, with a frequency ranging between 15 and 25 kHz), modulated in amplitude by a low frequency signal (signal of lock-in, with a frequency between 0.01 and 2 Hz). The correct time synchronization between the excitation and the acquire signal is an essential aspect of the lock-in technology. The emitted radiation acquisition is performed by means of an infrared camera with high spatial resolution (640 × 512 pixels), equipped with a focal plane array of indium antimonide. The detector operates in the spectral range MWIR (3 to 5 mm) and has a temperature resolution of about 20 mK at room temperature. The data acquisition system works at an integration time in the range between 10 ms and 5 ms, with a frame frequency up to 100 Hz at full resolution. The images in figure 9 give an application example of CT: they show a joint, butt welded by laser beam, between two carbon steel plates clad by hot rolling with a Ni alloy. The defects, at the resolving power limit of the system and not detectable with thermographic technique, have the typical aspect of porosity. Due to their cellular nature, thermographic technique is not applicable to the study of metal foams, while CT is particularly useful to evaluate shape and size of cells. Figure 10 shows the three-dimensional reconstruction of an aluminum alloy foam with closed cells and the image of a section. The study of cells evolution during compression can be performed by tomography without having to cut samples. Figure 11 shows some sections of a steel tube filled with an aluminum alloy foam: adhesion problems between wall and foam filler are highlighted. Thermography and tomography offer the possibility to perform dimensional investigation of industrial products and defects monitoring. Specifically, it was considered a cast iron collector for an automotive turbine produced by casting. Figure 12 shows a phase image of this component obtained by the ultrasonic active thermography: the presence of a darker area is indicative of an internal defect. To assess shape and size of the defect, tomographic sections at parallel planes spaced apart by 0.2 mm are considered (fig. 13). In Figure 14, close to the photographic image of a cast iron sample for tensile test, thermographic and tomographic surveys are reported for comparison. The two techniques show defects of small sizes. In particular, CT allows to analyze defects by means of longitudinal and transverse sections and identify both spatial position and morphology (figs. 14 and 15).