Synthetic Focusing for Imaging the Ultrasonic Backscattered Noise in Dual Phase Titanium Forged Parts

The inspection of titanium billets and forged parts with conventional immersion and laser-ultrasonic techniques was performed to localize macro-zones with strong texture. The microtextured regions in these dual phase titanium parts were detected by using backscattered noise measurements. A numerical focusing technique was developed for imaging the backscattered noise in parts with curved surfaces. This technique allows controlling the focusing of the ultrasonic beam to improve the contrast of backscattered noise images. Introduction: The ultrasonic inspection is the prime candidate for bulk non-destructive microstructural characterization of two-phase titanium-forged parts. Backscattered signal has been widely used to control or characterize two-phase titanium billets [1-4]. The two-phase titanium billet is formed of small grains grouped together within former large beta grains [5]. The small grains within a former large beta grain have very close crystallographic orientation. The “highly microtextured regions”, called “macrograins”, form macrostructures that influence greatly the elastic properties of the titanium-forged parts. The backscattered noise observed in this material should be sensitive to the shape and orientation of the macrograins. In this work, a procedure based on the SAFT (Synthetic Aperture Focusing Technique) is developed to map the backscattered noise. One advantage of this technique is the capability to control numerically the aperture of the acoustic beam. As a result, the acoustic beam can be focused at different depths inside the sample with the desired aperture. Note that, in the case of physical focusing and due to the refraction in titanium, a large acoustical beam aperture would require a focused transducer with a very large diameter. The results obtained clearly show noise bands in the titanium billet that originate from the thermo-mechanical processing. In the forged material, different high noise regions are detected and correlations of these regions with the flow lines are found. Noise regions with angular dependence that could be associated with the original micro-texture of the billet are also reported. Immersion technique: The immersion set up used for measurements is illustrated in Figure 1. The titanium billet is positioned onto a rotating table and the transducer is mounted on an arm that possesses degrees of freedom in translations and rotations to perform scans or adjustments. Line focused and unfocused transducers with 10 MHz center frequency are used in the measurements.