| Magnesium and magnesium alloys are attractive materials for practical applications due to their low density, high strength to weight ratio, good electromagnetic shielding characteristics and high elastic modulus, as well as other desirable properties. But due to their HCP crystal structure the mechanical properties and processing performances of magnesium alloys are still can not meet the needs of some important parts in vehicles and other application fields. Grain refinement is an effective way to improve the mechanical properties of magnesium alloys. The refined microstructure in as-cast components leads to an uniform distribution of solute elements and secondary phases on a fine scale. It also provides such advantages as superior extrudability and rollability, excellent resistance to hot tearing, good surface finish and considerable cost reduction in the production of wrought magnesium parts. As an effective refining element, strontium and its master alloys have been widely used in the industrial application of aluminum alloys while the investigation of their effects on magnesium alloys is very lack and scattered. As the most representative Mg-Al-Zn and Mg-Zn-Zr system alloys respectively, the AZ31 and ZK60 magnesium alloys are the most widely commercial applicated wrought magnesium alloys at present. Consequently, it is theoretically important and practically valuable to systematically investigate the influence of Sr-containing master alloys on the as-cast microstructures of AZ31 and ZK60 alloys.In the present work the effect of Sr amount, Sr addition type, Sr-containing master alloy state, melt holding temperature and holding time on the as-cast microstructures of AZ31 and ZK60 alloys and the refining mechanisms have been systematically investigated by means of Optical Microscopy(OM), Scanning Electron Microscopy(SEM), Energy Dispersive Spectrometry(EDS), X-ray Diffraction(XRD), Differential Scanning Calorimetry (DSC) and Thermo-Calco software etc.The research results show that the grain sizes and secondary dendrite arm spacings (SDAS) of the AZ31 alloys treated by Al-Sr and Mg-Sr master alloys and the ZK60 alloys treated by Mg-Sr master alloys are been greatly refined and the refinement efficiency increased with the increasing of the Sr content. The master alloy states and types, the melt holding temperatures and times are having great influence on the grain sizes, although the influence rules of those factors on the grain sizes and SDAS are similar but the influences on the SDAS are not great. The types of the Sr-containing master alloys have obvious influences on the grain refinement efficiency of Sr to the AZ31 alloys. In general, under the same condition the Mg-Sr master alloy has relatively higher grain refinement efficiency in the AZ31 alloys than the Al-Sr master alloy. Meanwhile, along with the change of the melt holding time the grain refinement efficiency of Al-Sr and Mg-Sr master alloys to AZ31 alloys have different change rules. For a given melt holding temperature, with the melt holding time increased the grain refinement efficiency of Al-Sr master alloys increases. As for the Mg-Sr master alloy, although it can obtain high refinement efficiency in a very short melt holding time, after that time the increase of the grain refinement efficiency is not very obvious. In addition, the melt holding temperature also has an obvious effect on the the grain refinement efficiency of the Al-Sr and Mg-Sr master alloys to the AZ31 alloy. Under the melt holding temperature of 740℃, the Mg-Sr master alloy has relatively higher grain refinement efficiency than the Al-Sr master alloy. However, under the melt holding temperature of 780℃, although with the melt holding time increased both the grain refinement efficiency of the Al-Sr and Mg-Sr master alloys first increases and then decreases, for a long melt holding time the Al-Sr master alloy seems to can obtain higher grain refinement efficiency than the Mg-Sr master alloy.The different original states of the master alloys have different efficiency on the grain refinement to the AZ31 and ZK60 alloys. For Al-Sr master alloys, although the commercial original, the heat-treated, the rolled and the remelting rapid cooled master alloys can effectively refine the grains of the AZ31 alloys, the master alloys with different states exhibit different grain refinement efficiency. The remelting rapid cooled Al-Sr master alloy has higher refinement efficiency than the commercial original, the heat-treated and the rolled, whose difference in the grain refinement efficiency is not obvious. For Mg-Sr master alloys, although the as-cast, the heat-treated, the rolled and the rapid cooled master alloys can effectively refine the grains of the AZ31 and ZK60 alloys, the master alloys with different states exhibit different grain refinement efficiency. The rolled Mg-Sr master alloy has the best grain refinement efficiency and followed respectively by the heat-treated and the as-cast. Oppositely, the grain refinement efficiency of the rapid cooled Mg-Sr master alloys are relatively poor.The melt holding temperature and time have great influence on the refinement efficiency. For the AZ31 alloy, under the conditions of adding Sr in the form of Al-Sr master alloy and the same melt holding temperature, an increase in the melt holding time from 20min to 120min causes the grain refinement efficiency of Sr in the AZ31 alloy to gradually increase. However, for a given melt holding time, an increase in the melt holding temperature from 700℃to 780℃causes the grain refinement efficiency of Sr in the AZ31 alloy to gradually increase. In addition, under the conditions of adding Sr in the form of Mg-Sr master alloy, the grain refinement efficiency of Sr in the AZ31 alloy treated at 740℃is relatively higher than that at 700℃and 780℃. For the ZK60 alloy, under the melt holding temperature of 740℃, an increase in the melt holding time from 20min to 120min causes the grain refinement efficiency of Sr in the ZK60 alloy to first increase and then decrease, and the melt holding time of 80min can obtain the best grain refinement efficiency.The secondary phases in the AZ31 alloy are mainly composed of sheet- or stick- like divorced eutecticβ-Mg17Al12, few lamellar eutecticα-Mg+β-Mg17Al12 phases and flower-like Al8Mn5 phases. While the Sr content is higher than 0.07(wt.)%, lamellar Al4Sr phase formed and Al4Sr phases were at the grain boundary. The secondary phases in the ZK60 alloy are mainly composed of lamellar- or bone- like MgZn phases, divorced eutectic MgZn2 phases and few ZnxMny phases. Being different from the AZ31 alloy, adding a small amount of Sr to the ZK60 alloy dose not cause the formation of any Sr-containing phases in the alloy.The mechanisms about the grain refinement of the AZ31 and ZK60 alloys with the addition of Sr are mainly related to the nucleation increase resulted from the supper-cooling temperature increase during solidification and that the grain growth of theα-Mg grains are restrained due to the decrease of the recalescence temperature variation. As for the difference in the grain refinement efficiency for the Al-Sr and Mg-Sr master alloys in the AZ31 alloy, the reason is possibly related to the different efficiency which the Al4Sr and Mg17Sr2 phases dissolution and/or melt in order to obtain the free Sr. Similarly, the above mentioned-mechanism is also suitable for the difference in the grain refinement efficiency for the Al-Sr and Mg-Sr master alloys with different states in the AZ31 and ZK60 alloys. In addition, the effects of the melt holding temperature and time on the grain refinement efficiency of Sr in the AZ31 alloy is possibly related to the efficiency which the Al4Sr and Mg17Sr2 phases dissolution and/or melt in order to obtain the free Sr.
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