Nindestructive Evaluation of Metal Matrix Composite Products with Implanted Defects
The Westinghouse Science and Technology Center has undertaken a program to develop nondestructive evaluation (NDE) techniques for characterizing the internal structure of SiC particle-reinforced aluminum matrix composites at critical stages during fabrication [1–5]. Because of the large number of processing variables in the manufacture of metal matrix composites (MMC), the likelihood of having detrimental discontinuities is high. The detection of potential defects early in the processing cycle would increase the overall system yield, lower costs, and enhance final product quality . The aim of this investigation was to develop and conduct NDE at various stages of MMC fabrication, correlate the results with microstructural characterization, and establish qualified product quality assurance processes. A large-scale billet was fabricated specially using powder metallurgy techniques to facilitate this objective. The billet contained implanted silicon-carbide particle and aluminum powder clusters as inspection targets. The billet was subsequently extruded into a primary cylindrical extrusion, and finally into a flat plate. The NDE objectives included evaluating the detectability and mapping the implanted defects through each of the processing steps. Comprehensive evaluation of MMC structures requires the use of multiple NDE techniques, including ultrasonic, eddy current, and radiographic testing. This paper concentrates on the results of the ultrasonic investigations. Our experimental approach was: (1) fabricate a MMC billet with intentionally placed inhomogeneities; (2) develop and implement NDE techniques to characterize the MMC internal structure; (3) extend the NDE techniques to intermediate processing and final product forms; and (4) correlate the NDE data with microstructural characterization and mechanical testing results.