ening effect, indicating that a solution treatment is not nec- essary to obtain the age-hardening response in Mg-Al based alloys. This will eliminate the process time, energy consump- tion as well as the risk of distortion during quenching. The generally poor heat treatment response in Mg-Al binary alloys is due to the lack of metastable stages in the precipitation of β- Mg17
phase and the α-Mg matrix.13 Al12 phase and the no full coherency between β-Mg17 Al12 Microstructure
The above results suggests that the alloy AM80 (Mg-8%Al- 0.3%Mn) provides a more balanced combination of tensile properties (higher strength than AM60 and higher ductility than AZ91), in addition to a favorable response to heat treat- ment in the GPMC samples.
Die Casting Samples
Table 3 lists the tensile properties of AM80 tophat castings in the as-cast (F temper) and T5 conditions compared to those of AM60 and AZ91 baseline alloys. It is evident that the AM80 alloy offers about 47% improvement in ductil- ity compared to commercial AZ91 alloy with similar yield and ultimate tensile strength. On the other hand, the AM80 alloy has about 18% higher yield strength compared to the AM60 alloy, but at the expense of about 30% reduction in ductility. In summary, the die casting results confirm that AM80 alloy offers a more balanced combination of strength and ductility than the commercial high-strength AZ91 and high-ductility AM60 alloys, and compares very favorably with the workhorse aluminum die casting alloy A380 (Table 3). It is noted that the aluminum A380 alloy properties are measured from the separately cast specimens and tend to be higher than what tested from the actual castings, such as this case for the magnesium alloys. However, Table 3 also shows that the tensile properties of AM80 magnesium alloy is still below those of premium high vacuum cast aluminum alloy Aural 2-T6, indicating the need of further magnesium alloy development with more age-hardening response.
Gravity Casting Samples
Fig. 6 shows the as-cast microstructure of AM alloys in the GPMC condition. For AM20 alloy, Fig. 6(a), the mi- crostructure consists of primary α-Mg grains and a small amount of dark intermetallic Al8 boundary β-Mg17
taining 5%Al (AM50), and is more clearly observed in the higher Al-containing alloys like AM80 and AM100 alloys, Fig. 6(b-c). It is well known that although β phase is a strengthening phase, it reduces the ductility of magnesium alloys due to its brittleness, which is consistent with the me- chanical properties of the AM alloys in this study (Fig. 1). It is apparent that the isolated β phase particles in the AM50
Al12 Table 3. Tensile Properties of SVDC Samples phase becomes evident in alloys con- Mn5 particles. The grain
Table 3 also shows that the T5 heat treatment has essentially no beneficial effect on the mechanical properties of SVDC specimens, i.e., similar strength but reduced ductility com- pared to the as-cast properties of AM60 samples. This is consistent with the results of AZ91 and AM60 SVDC sam- ples with and without heat treatment reported earlier.10
*
Data from Ref. 16 (test results from separately cast specimens);
** GM internal test results from casting plates of 4 mm.
(a)
(b)
Figure 4. Tensile properties of Mg-Al-Mn alloys (GPMC) in (a) T5 (5 hours at 232C [450F]); and (b) T6 (solutionized, water-quenched 5 hours at 232C [450F]) conditions.
54 International Journal of Metalcasting/Fall 10
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