Analysis on Internal Defect of 319S Aluminum Alloy Impeller by Semi-Solid Thioxcasting

Abstract

In the present investigation a batch of impellers made of 319S aluminum alloy with about 300 pieces and produced by semi-solid thixocasting process were subjected the ultrasonic testing (UT). The experimental results revealed that the equivalent size of the defects in the impellers was not more than φ0.4mm FBH+12dB. And there were two main types of defects in 12 mm depth area from the machined surface, where defect was apt to form. One type was single defect and the other was intensive defect. Moreover, other nondestructive testing (NDT) was used to test some impellers containing typical defect, and the result suggests that UT was more sensitive than x-ray testing. The defects existed in the impellers were investigated by Optical Microscope (OM) and Scanning Electron Microscope (SEM) equipped with Energy Dispersive Spectrometer (EDS). The observation showed that the both types of defects existed in eutectic zone. The single defect derived from billet and the intensive defect came from die casting process.