alloy (Fig. 6[b]) provide reasonable strengthening. Such strengthening is further enhanced in the AM80 alloy (Fig. 6[c]) in which the β phase particles have largely covered the grain boundaries, while the ductility of the alloy is provided by the interconnected α-Mg matrix. However, when all of the grain boundaries are covered with the β phase, which forms a complete network as in the case of AM100 (Fig. 6[d]), the α-Mg matrix became isolated, and the ductility decreases greatly. The best combination of strength and ductility occurs for the AM80 alloy where the β phase frac- tion has increased, but the phase has not decorated the grain boundaries sufficiently to reduce its ductility.
Die Casting Samples
Fig. 7 shows the as-cast microstructure of AM80 alloy cast- ing (2 mm thickness) produced by SVDC process. It is clear that a “skin” phenomenon can be defined by the bimodal distribution of α-Mg and the intergranular β-Mg17
the eutectic mixture. Additionally, very few intergranular shrinkage pores are found in the SVDC casting, Fig. 7(c), since the gas porosity caused by en- trapped air has been eliminated in the SVDC process.12
structure also forms the matrix of the “core” section where large α-Mg grains are evenly distributed (Fig. 7[c]). Simi- lar to the GPMC microstructure (Fig. 6), dark intermetallic Al8
Al12 Mn5 particles are entrapped either in the α-Mg grains or
Figure 5. Effect of heat treatment (T5 and T6) on the tensile yield strength of AM alloys (GPMC).
surement was not done in this study, a previous publication,14
While porosity mea- using density
measurement, has shown that the use of vacuum can significantly reduce the gas pores in die casting, but it is not effective in reducing shrinkage-related porosity.
The exact mechanism of “skin” forma- tion in magnesium die casting is not clear. It is likely due to the fact that initial solidification occurs in the shot sleeve during cold chamber die cast- ing process, and up to 20% solid frac- tion can be formed in the shot sleeve in magnesium die casting.15
This is con-
sistent with the amount of large α-Mg grains in the SVDC AM80 microstruc- ture. The partially spheroidized mor- phology of these large α-Mg grains also suggests that they are likely nucle- ated and evolved via the initial solidifi-
International Journal of Metalcasting/Fall 10
across the section thickness. As shown in Figs. 7(a) and (b), a surface layer about 100 µm deep is essentially free of large α-Mg grains (typically 10-40 µm in size as measured using a liner inception method) which are largely and uniformly distributed in the “core” section of the casting. Instead, the “skin” layer is characterized by the very fine eutectic mixture of α-Mg and β-Mg17
Al12 particles (1-5 µm). Such eutectic
cation in the shot sleeve. The lower volume fraction of the large α-Mg grains in the “skin” layer is also supported by the solidification sequence in semi-solid die casting. Under the high shear stresses in the gating system, and the velocity gra- dients across the section of the die casting, it is expected that the solid particles would migrate towards the “core” section of the lowest shear and highest velocity.15
Thus, when the
metal has filled the cavity there is a higher volume fraction of liquid with higher aluminum content in the metal close to the die walls (than in the center), which forms very fine eutectic structure in the “skin” section. Subsequently, the remaining liquid (also rich in aluminum) in the center forms the fine eutectic microstructure surrounding the large α-Mg grains in the “core” section.
phase
Figure 6. Optical micrographs showing the as-cast microstructure of AM alloys (GPMC).
55
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