dc.description.abstract | In this study, a series of experiments of tensile tests and gas blow forming was performed to explore the flow behaviors and rapid gas blow formability of the fine-grained AZ31B and AZ31B-H24 Mg alloy sheets. The AZ31B-O of material was supplied by POSCO Ltd, with a thickness of 0.6mm. The Mg alloy AZ31B-H24 sheet used in this work, with a thickness of 0.5mm in H24 temper condition, was provided by Magnesium Elektron North American Inc. (USA). Tensile tests were carried out on specimens in the rolling direction, using initial strain rates in the range of 4×10−3 to 1×10−1 s−1 at temperatures of 250, 300, 370 and 420 °C. The flow behaviors and associated microstructure changes of fined-grained wrought AZ31B-O and AZ31B-24 Mg alloy sheets deformed in tension were analyzed in this work. Free bulging tests were performed by deforming the sheet into a right cylindrical die cavity by compressed argon gas. Gas blow forming was carried out at temperatures of 200, 250, 300, 370 and 420 °C using various pressurization profiles. Decreasing the forming time in gas blow forming using stepwise pressurization profiles and constant gas blow forming, which were non-constant gas blow fotming, were conducted to investigate the rapid gas blow formability.
Results showed that variations in flow behavior under tension could be related to the changes in microstructure resulting from applied tensile conditions. During hot deformation, some metallurgical phenomena such as strain-hardening, dynamic recovery, and dynamic recrystallization (DRX) may occur simultaneously, resulting in the changes in the microstructure and mechanical properties. The stress-strain rate data showed that fine-grained AZ31B thin sheet on testing at higher temperatures exhibited strain rate sensitivity exponent values of approximately 0.27 in a strain rate ranging from 4×10‒3 and 2×10‒2 s‒1, indicating that the dislocation creep would be a possible deformation mechanism to reduce forming time in gas blow forming. The tensile test results of the AZ31B-H24 alloy sheet suggested that gas blow formability could be enhanced at lower temperatures. DRX should play an important role in enhancing formability. Thus, low temperature gas blow forming would be a possible process by using Mg alloy AZ31B in H24 condition at lower temperatures.
The AZ31B-O and AZ31B-h24 alloy sheets were successfully deformed into hemispherical domes at different temperatures under various pressurization profiles. A stepwise pressurization profile should be a suitable process at lower temperatures, whereas a constant or near constant pressure imposed during forming would be a better method at higher temperatures. The fine-grained AZ31B-O alloy sheet could be gas blow formed using Quick plastic forming (QPF) process. Whereas, the formability of AZ31B-H24 was better than that of fine-grained AZ31B-O during low temperature gas blow forming. Forming a shallow rectangular pan demonstrated the possibility of using QPF process. Cavitition level in Mg alloy AZ31B was lower than that in Al alloy 5083 during fast gas bloe forming. A significant reduction in forming time was achieved using traditional female die forming, in which a rectangular pan was formed with a height of 10 mm in less than 163 s. Fillet radius of the rectangular pan should be one of the key factors influencing forming time. An innovated male die gas forming with pre-deformation was also proposed in this study. It is feasible to form a rectangular pan with high surface quality in less than 160 s.
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