Correlation Between Ultrasonic Testing Results And Microstructure, Mechanical Properties Of Magnesium Alloy Die Castings

Magnesium alloys have a lot of excellent properties, such as lower density, gooddamping capacity and excellent castability, etc, which are widely used in automotive industry.Magnesium alloys are considered as new green engineering materials in the21st century inauto industry. With the improving requirements of lightweight, energy conservation andemissions reduction, Magnesium alloys have become the third type metal structure materialwhich can partially replace steel and aluminum alloy. In order to make magnesium alloy diecasting technology in our country maturing and provide the reliable experimental referenceand theoretical guidance for regulating quality of die casting magnesium alloy parts, a studyis performed on a high-pressure die cast (HPDC) AM50alloy sample using ultrasonic testing.Through microstructure observation method and industrial X-ray computed tomography (CT)method ultrasonic testing results are validated. The influence of defects in the specimens ontheir mechanical properties is analysed to improve accuracy of controlling quality ofmagnesium alloys die castings and give full play to the advantages of ultrasonic testing.In this article, the specimens are taken from vehicle seat frames and operated by manualultrasonic testing. Then two types of significant A-scan waveform are summarized from a lotof A-scan waveform in time domain signal. One is A-scan waveform in which defect wave isbetween the fundamental wave and bottom reflection wave, the other is A-scan waveform inwhich there is not bottom reflection wave except for defect wave.By confocal laser scanning microscope and scanning electron microscope defects indetection areas are observed and verified. The results showed that shrinkage porosity is foundin the detection area with the first kind manual test waveform signal, and it is consistent withthe ultrasonic testing results. In the detection area with the second kind manual test waveformsignal there is3mm long crack, and due to the crack orientation is not parallel with ultrasonictesting plane there is a certain difference from the ultrasonic testing results. The yield strength,ultimate tensile strength and elongation of the specimen are125.50MPa,139.83MPa and 2.04%respectively. Its fracture appears crystal and obvious cracks and pores. The specimenshows that ultrasonic testing results and mechanical properties results are highly unified. Andthere are more defects in the samples taken from the bottom of the car backrest, which makesthe mechanical properties lower. Besides industrial X-ray computed tomography (CT) is usedto verify the ultrasonic testing results, and the results are consistent with the ultrasonic testingresults.The boxplots of mechanical properties of the specimens as a function of samplinglocation indicates that, the tensile strength changes between yield strength and elongation,which shows that the influencing factors of yield strength and elongation can affect thetensile strength. The corresponding rate between ultrasonic testing results and tensile fracturelocation is nearly80%. The ratios are almost alike in different sampling position, except thatthe ratio of samples taken from the both sides of the car backrest is lower and the ratio ofsamples taken from the bottom of the car backrest is higher. The relationship betweenultrasonic testing results with the mechanical properties is obvious.