Quantitative fractographic analysis of variability in the tensile ductility of high-pressure die-cast AE44 Mg-alloy

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AbstractMethodologyMaterial ModelInput DataResultsAcknowledgmentsReferences

Abstract

Plot of true stress versus true strain result of tensile testing AE44 alloy. The alloy experiences a large variation of ductility behavior.

The correlation between sample porosity and the variation in ductility of AE44 magnesium alloy during tensile testing was observed. Two sets of tensile tests were performed at room temperature and 394 K. The set of tensile tests at 394 K showed a smaller variation of ductility compared to the room temperature set. After testing, microscopy samples of the fracture surface were created and observed in a scanning electron microscope. Quantitative measurements of the porosity at the fracture surface were determined. No quantitative relation was found between the variation of ductility and the volume fraction of porosity. However, at the fracture surface, a relationship between the tensile ductility and area fraction of porosity was observed to follow a simple power type equation.

Author(s): S. G. Lee, G. R. Patel, A. M. Gokhale, A. Sreeranganathan, M. F. Horstemeyer


Figure 1. Optical microscope image of fine-grained microstructure "skin" at cast surface. (click on the image to enlarge).
Chemical composition of AE44 alloy. (click on the image to enlarge).

Methodology

Cast AE44 magnesium alloy composition is provided in Table 1. The surface of these cast alloys contain a fine-grained microstructure referred to as "skin," shown in Figure 1. Although the behavior of this surface layer differs from the bulk microstructure, industrial applications of the alloy do not remove the layer after casting. The skin has shown to provide improved mechanical properties of the alloy. Therefore, 6 mm round tensile specimens of AE44 were cast using a high-pressure die-casting metallic mold, and the skin was not removed from the gauge sections. At least 10 specimens at room temperature and at 394 K were tested at an engineering strain rate of 8.3 x 10^-3 %/s. Fracture surfaces of the specimens were viewed with a scanning electron microscope and the shrinkage and gas porosities were quantified over the fracture area.

Material Model

N/A

Input Data

N/A

Results

The tensile tests results are provided in Table 2. The relationship between the porosity of the fracture surface and the % elongation was observed, as shown in Figure 3a. Figure 3b shows the power type relationship between the axis by plotting the natural log of both axis. The equation for the fitting line is given by Equation 1 with the two corresponding coefficients provided in Table 3.
Figure 1.
Figure 3a. Scatter plot of AE44 alloy tensile tests and fitting power equation.
Figure 3b. Modified axis shows linear relationship between porosity and elongation
Equation 1.
Table 3. Fitting parameters for Equation 1.

Acknowledgments

The authors thank Richard Osborne and Don Penrod for discussions, encouragement, and support, and Westmoreland Mechanical Testing and Research Laboratories for conducting the tensile tests. The authors thank Magnesium Competence Centre, Hydro Aluminum Research Centre, Porsgrunn, Norway for high-pressure die-casting of the AE44 alloy tensile test specimens. The research was supported by research grants from Structural Cast Magnesium Development (SCMD) program of USCAR Project, the U.S. National Science Foundation (Grant no. DMR-0404668), and American Foundry Society. The financial support is gratefully acknowledged. Any opinions, findings, and conclusions or recommendations expressed in the paper are those of the authors and do not necessarily reflect the views of the funding agencies.

References

S.G. Lee, G.R. Patel, A.M. Gokhale, A. Sreeranganathan, M.F. Horstemeyer. "Quantitative fractographic analysis of variability in the tensile ductility of high-pressure die-cast AE44 Mg-alloy," Materials Science and Engineering, A 427, 255-262 (2006).

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