The acoustic emission characteristics of solidification and formation of casting defects in aluminum alloy 319.

Abstract

Acoustic emission activity during solidification of a restrained bar test casting configuration for aluminum alloy 319 has been investigated. Extending the length of the restrained bar from 11 inches to 14 inches resulted in a range of severities for hot tearing and hot cracking defects. Variation of molten metal parameters resulted in assorted levels of gas porosity defects. The defect levels and severities were verified through computational techniques, and microstructural examination and analysis. The acoustic emission activity was monitored during the solidification process for castings poured in both green and dry sand molds. Upon subsequent analysis of acoustic emission signals generated during solidification, trends in acoustic emission activity were observed. The relative size, duration times and energy of signals were investigated as to their statistical significance to defects levels and severities. These results were characterized with respect to specific solidification and defect events in the form of numerical relationships. Pattern recognition techniques were applied to the frequency characteristics of signals. A number of source classes of these signals were identified and analyzed with respect to solidification and defect event generation, and the condition of the molding sand. Linear discriminant functions were developed based on the statistical differences in the spectral characteristics of these signals and a number of trends and signatures were observed. The limitations of the analysis were noted and the errors were attributed to data scatter and the simultaneous generation of acoustic emission signals for a number of these sources. The successful characterization of a number of trends regarding acoustic emission activity, and specific solidification and defect events permits identification of the events based on their resulting signal characteristics. The research should assist in the development of a successful on-line nondestructive monitoring technique in the production of castings.