It is well-known that age hardening occurs in Mg-Al system alloys, when the alloy containing aluminum exceeds 6 mass%. This precipitation reaction depends on the aluminum content and aging temperature. The aging behavior in AZ91 magnesium alloy was investigated and it is the subject of this paper. However, for the Mg-Al system alloys, the influence of aluminum content on age hardening characteristics has not been researched in detail so far. In this study, continuous and discontinuous precipitations during aging in Mg-Al system alloys cast into sand and iron molds was investigated by means of hardness measurement and microstructure observation with optical microscope and transmission electron microscope. The microstructure of AM60 magnesium alloy with 6 mass% containing aluminum in the as cast condition was composed with a primary crystallized alpha magnesium phase and beta Mg 17 Al 12 phase. The hardness of cellular precipitates existing near the grain boundary in sand mold castings decreased with increasing the aging time. On the other hand, the hardness of intragranular precipitates was constant against the aging time. Variation of hardness with aging was found to be caused mainly by the discontinuous precipitation along the grain boundaries from the composite rule in hardness. In iron mold castings, it was found that the variation of hardness with aging was found to be caused mainly by the continuous precipitation inside the crystal grain.

Aging characteristics of AM60 magnesium alloy cast into sand mold were investigated by means of hardness measurement and microstructure observation with optical microscopy. The microstructure of this alloy in the as cast condition with the cooling rate of 5.2 K/s was composed with a primary crystallized alpha magnesium phase and eutectic phase consisting of alpha magnesium and beta Mg 17 Al 12 intermetallic compound. The eutectic phase was divided to none-equilibrium crystallized phase and precipitated phase. The hardness of cellular precipitation area in aged samples decreased with an increasing of the aging time. On the other hand, the hardness of intragranular precipitation area was constant against the aging time. Variation of hardness with aging was found to be caused mainly by the discontinuous precipitation along cell boundaries from the rule of mixture in hardness.

In this study, age-hardening behavior of Mg-Xmass%Al alloys (X=3, 4.5, 6, 7.5 and 9mass%) were investigated by Vickers hardness measurement and optical microscopic observation. Each alloy was solution-treated and then isothermal-aged at 473, 498 and 523K. In the case of aluminum content less than 6mass%, Mg-3mass%Al and Mg-4.5mass%Al alloys, occurred insignificant age hardening. In the case of aluminum content higher than 6mass%, Mg-6mass%Al, Mg-7.5mass%Al and Mg-9mass%Al alloys, occurred remarkable age hardening. For each aging temperature, with heighten aluminum content, increase the value of maximum hardness and shorten time to maximum hardness. Mean hardness of discontinuous precipitation during aging increased with lower aging temperature and higher aluminum content. Furthermore, over-aged microstructure of Mg-Al system alloys differed from aluminum content or aging temperature.

In recent years, Mg-Al-Zn system alloy has been used for the parts in the automobile for weight reductions. The age-hardening behavior of Mg-6mass%Al (-1mass%Zn)-0.3mass%Mn alloys sand mold castings were investigated by Vickers hardness measurement and optical microscopic observation. Both alloys were solution-treated and then isothermal-aged at 473, 498 and 523K. For each aging temperature, both alloys were indicated age-hardening obviously, and decreased the value of maximum hardness and shorten time to maximum hardness with heighten aging temperature. Age-hardening curves for both alloys approximately showed the same change of hardness. However, these optical micrographs after aging treatment are observed different microstructure. In case of AM60 magnesium alloy, discontinuous precipitation preferentially occurred in aged sample. The volume fraction of discontinuous precipitation was larger than that of continuous precipitation. On the other hand, in case of AZ61 magnesium alloy, the volume fraction of continuous precipitation was larger than that of discontinuous precipitation. Furthermore, over-aged sample of both alloys were consisted of discontinuous precipitation, continuous precipitation and pre-precipitation area.