Formability of stamping magnesium-alloy AZ31 sheets
Introduction
Due to its lightweight and high specific strength, magnesium-alloy has been widely used for structural components, notably in the automotive industry [1]. Although the principal manufacturing process has been die casting, the press forming has considerable potential because of its competitive productivity and performance. Among the fabrication processes of press forming, stamping of magnesium-alloy sheets is especially important for the production of thin-walled structural components. Since the magnesium-alloy usually exhibits limited ductility at the room temperature due to its hexagonal close-packed (HCP) structure, the mechanical properties of magnesium-alloy can be improved at elevated temperatures [2], [3], [4], [5], [6]. However, literature regarding stamping magnesium-alloy sheets is not profound, and the stamping process of magnesium-alloy is not well developed to date [7], [8], [9], [10], [11], [12]. A variety of magnesium-alloys have been applied to manufacture structural components. As for press forming, AZ31 is considered as the suitable magnesium-alloy for the stamping process at the present time.
In the present study, the formability of stamping magnesium-alloy AZ31 sheets was investigated using the experimental approach. The mechanical properties of magnesium-alloy AZ31 sheets at various temperatures ranging from room temperature to 400 °C were obtained from experimental results. In addition, the important forming characteristics of AZ31 sheets, such as forming limit, conical cup value (CCV), springback and minimum bending radius, were also examined by experiments.
Section snippets
Mechanical properties tests at elevated temperatures
The stress–strain relations are the basic information for the study of formability of a sheet metal. Since magnesium-alloy exhibits poor formability at room temperature, tensile tests at various temperatures ranging from room temperature to 400 °C were performed in the present study. The tensile test specimens made of AZ31 sheets of 1.2 mm thickness were prepared according to the ASTM standards. The specimens were cut along planes coinciding with the rolling direction (0°) and at angles of 45°
Stamping formability at elevated temperatures
In addition to the basic mechanical properties, the stamping formability of AZ31 sheets were also examined at elevated temperatures. In the present study, the forming limit tests, CCV tests, springback tests, and minimum bending radius tests were performed. These tests represent the stamping properties in a sheet-metal forming process.
Conclusions
The stress–strain relations indicate that AZ31 sheets have higher yield stress and smaller elongation at room temperature, but the yield stress drops significantly when the sheet is heated to a temperature higher than 200 °C. The work-hardening coefficient, n, also drops at elevated forming temperatures. From the point view of formability, AZ31 sheets exhibit good formability but greater possibility of local deformation at elevated forming temperatures. The forming limit tests also show the same
Acknowledgements
The authors would like to thank the National Science Council of the Republic of China for financially supporting this research under Contract No. NSC 89-2212-E-002-147, which makes the experimental work possible. They also would like to thank Mr. Kuan-Hua Chiu and Mr. Chi-Kuen Chang for their help in tooling design.
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