Such “skin” layer has higher hardness than the “core” section due to the strengthening effect of the β phase. Therefore, the yield strength and ultimate tensile strength of HPDC AZ91 alloy increases substantially with decreasing section thickness. Similarly, the tensile properties of SVDC AZ91 and AM60 alloy castings (2 mm wall) in this study also show significant improve- ment to the thicker samples (5.5 mm wall) produced by the same process in an earlier study.10
The effect of “skin” layer on the mechanical proper- ties of magnesium die castings has been reported ear- lier.12
Fig. 8 shows the microstructure of the SVDC AM80 alloy casting in the T5 aged condition (5 hours at 232C [450 F]), which is very similar to the as-cast microstructure in Fig. 7. A closer comparison between Figs. 7(c) and 8(c) suggests that there is slight growth of the large α-Mg grains during heat treatment. This is consistent with a previous report on SVDC AZ91 and AM60 alloys. De-
aged specimens, the coarser α-Mg grains due to the grain growth during heat treatment may offset the precipitation- hardening effect.10
spite the precipitation of β-Mg17 Al12 Al12 phase particles.
phase particles in the Therefore, no strengthening effect is
observed in SVDC AM80 samples while the ductility is reduced (Table 3) due to the precipitation of brittle β- Mg17
It was reported 17 that β-phase in Mg-Al based alloys can
act either as a corrosion barrier or galvanic cathode ac- celerating corrosion. The dominating effect of the dual role in the corrosion process depends on the amount and distribution of the β-phase in the microstructure. Finely and continuously distributed β-phase can more effective- ly stop the development of corrosion in a Mg-Al alloy. Otherwise, the presence of β-phase can accelerate corro- sion. As shown in Fig. 7, the microstructure of AM80 die castings is characterized by the isolated α-Mg surrounded by fine and continuous β-Mg17
Al12 eutectic phase. Such
Figure 7. Optical micrographs showing the as-cast microstructure of AM80 alloy (SVDC): (a) overall section; (b) skin and core sections; and (c) core section.
56 International Journal of Metalcasting/Fall 10
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