Prevorovsky, Zdenek; Institute of Thermomechanics of the Czech Academy of Sciences; Czech Republic
Prevorovsky, Z.; Institute of Thermomechanics AS; Czech Republic
Krofta, J.; Institute of Thermomechanics AS; Czech Republic
Kober, J.; Institute of Thermomechanics AS; Czech Republic
Chlada, M.; Institute of Thermomechanics AS; Czech Republic
Kirchner, A.; Fraunhofer Institute for Manufacturing Technology and Advanced Materials IFAM; Germany
Session: Nonlinear Ultrasonics 3
Time: 10:00 - 10:20
Current metal additive manufacturing (AM) technologies imply complicated processes with many variable parameters determining their performance and quality of resulting product. The process optimization needs a feedback from nondestructive evaluation (NDE) of manufactured parts. The presence of complex internal structure, shape and geometry limits applicability of standard, preferably online NDT/NDE techniques. In this contribution we tested chances of nonlinear elastic wave spectroscopy (NEWS) methods to classify two types of pre-determined defects in Ti-6Al-4V prismatic samples fabricated by EBM system. Two series of samples (60x15x10 mm) with variable internal defects were tested: a) samples with circular defects (gaps of thickness 200 to 500 m), and b) samples with gradually growing stochastic porosity. Each group of samples contained also completely dense samples for a comparison. Prior to NEWS application, the micro-computer-tomography was used to characterize defects. Five ultrasonic NEWS procedures were applied to classify defects variability: Nonlinear resonant spectroscopy (NRUS), harmonics growth, scaling subtraction method (SSM), nonlinear wave modulation spectroscopy (NWMS), and nonlinear time reversal mirrors (NLTRM). Relatively simple experimental facility was used to interrogate samples and record their response. Two piezoelectric transducers of frequencies tens to hundreds kHz were glued to sample ends, one for wave excitation and second for signal detection.
The best results were obtained by NWMS procedures. Samples (a) were successfully classified by two mixed excitation frequencies (62 and 116 kHz). Slopes of their amplitude dependent inter-modulation sidebands clearly distinguished thicknesses of circular defects. In group (b) samples was mixed chirp signal 50-300 kHz with constant frequency 191 kHz. Direct porosity quantification was successful as selected spectral features exhibited double exponential growth with growing porosity, which defined reliable classification criterion. Results of NEWS tests indicate that properly selected procedures could be used also for online AM process monitoring.