Temperature Dependency on the Microscopic Mechanism in the Normal Direction of Wrought AZ31 Sheet under Dynamic Compressive Behavior

In order to analyze the competitive relationship of different deformation mechanisms in wrought AZ31 magnesium alloy, the dynamic compressive experiments were conducted by a Split Hopkinson Pressure Bar (SHPB) apparatus and a resistance-heated furnace in the range of temperature between 20 and 350 °C at the strain rate of 1000 s−1. With the help of Electron Backscattered Diffraction (EBSD) observation, theoretical calculated Schmid Factor (SF), Critical Resolved Shear Stress (CRSS), and critical equivalent stress (σ0.2), the dynamic compressive deformation behavior and corresponding mechanism of wrought AZ31 magnesium alloy along the normal direction (ND) were revealed in the current study. The results demonstrate that the c-axis of grains are gradually reoriented parallel to the normal direction of wrought AZ31-ND sheet with the temperature increasing, except the dynamic recrystallization (DRX) mechanism was activated or grains grew up. The non-basal slip and 101¯2 tension twinning are respectively the predominant deformation mechanisms at lower temperatures (≤250 °C) and higher temperatures (≥250 °C). The predominant type of DRX mechanism of wrought AZ31-ND sheet is rotational dynamic recrystallization (RDRX), which is regarded as an obstacle for the kernel misorientation concentration region enhancement.

Numerical/experimental investigation of bulge tests on a localized laser heat-treated magnesium alloy AZ31 sheet

Lightweight alloys can be considered among the most promising materials thanks to their capability to reduce the environmental impact, without affecting mechanical properties. In addition, when very complex shapes are required, a viable strategy could be represented by the adoption of non-conventional forming processes applied to tailored blanks that allow to obtain local variation of the material properties. In fact, referred to the Mg alloys, both grain size and temperature strongly influence the deformation behavior, as well as the mechanical properties. In this work, the effects of a selective Laser Heat Treatment (LHT) on a Mg AZ31B-H24 alloy sheet were investigated both numerically and experimentally. Experimental tests were performed, using a Diode laser source and keeping a square spot stationary in the center of the sample. The microstructure evolution was evaluated by means of light microscopy. Subsequently, the heat-treated samples were subjected to bulge tests under superplastic conditions (450°C) and using pressurized argon gas. The experimental microstructure distributions obtained were used for the numerical bulge tests analyses performed in the same conditions of the experimental trials. Experimental LHT results showed the capability to locally modify the microstructure when suitable temperatures and interaction times are selected. Regarding the bulge tests, the obtained results showed the possibility to effectively affect the thickness distribution of the final shapes.

Orthotropic Behaviour of Magnesium AZ31 Sheet during Strain Localization

It is known that metallic materials are characterized by anisotropy of their mechanical properties, with this being attributed to the conditions during the manufacturing process. For sheet metals, this anisotropy occurs symmetrically to the three orthogonal axes of the rolling, transverse and normal direction. This characteristic is referred to as orthotropic behaviour and manifests itself, for example, in earing during cupping tests. Therefore, orthotropic yield criteria are highly relevant for the numerical simulation of sheet metal forming processes. The Lankford coefficient, also known as the r-value, is a good experimental measure for characterizing orthotropic ductile behaviour of sheets, and can easily aid in parameter identification for yield criteria such as the Hill approaches. In the present investigations, Lankford coefficients were determined as a function of local strain in uniaxial tensile tests through high-resolution digital image correlation. The sample direction was varied between 0°, 45° and 90° to the rolling direction and the test temperature varied from RT to 350 °C at three different strain rates (0.01-1 s-1). By means of a novel backward analysis, the measuring range for the Lankford coefficients was positioned exactly in the necking area. An increase in temperatures showed a decrease in the initial Lankford coefficient. The results showed non-constant Lankford coefficients and commence the course of a natural exponential function depending on the local strain. Regardless of strain rate, the results revealed that the Lankford coefficients (r-values) at 150 °C, 250 °C and 350 °C approaches a steady-state of r = 1.14 with strains greater than 50 %.

Mechanical Anisotropy Induced by Strain Path Change for AZ31 Mg Alloy Sheet

The variation of strain paths induces anisotropy during practical sheet forming processes, which is very important for the subsequent processing technology of anisotropic Mg alloys. In this study, two-step loading tests (tension-tension) were performed to clarify the effect of strain path changes on the evolution of anisotropy on rolled AZ31 sheet. Specimens were preloaded with tension along the rolling direction (RD) with 9% of prestrain. Then, second tension was conducted along 0°, 30°, 45°, 60° and 90° from the RD. It was found that yield strength during the second loading increased along the same direction compared to uniaxial tension without prestraining. For the second loading, the yield strength and flow stress decreased with the increase of the angle from the RD. It was found that the strain path change resulted in stronger anisotropy than that induced by texture. Moreover, it was found that the main deformation modes were basal and prismatic slips during the second loading based on visco-plastic self-consistent (VPSC) modeling. The relative activities of basal and prismatic slips were affected by the second loading direction due to texture evolution. The mechanical anisotropy induced by strain path changes was ascribed to the coupling of the heterogeneous distribution of dislocations and texture evolution induced by prestraining.