| Because of its good ductility and toughness, AM60 magnesium alloy is one of the magnesium alloys, which have been originally used in the industrial production. However, its lower strength, poor wear resistance, small modulus of elasticity and bad high temperature property, have greatly limited its application area, though it is considered as the lightest metal structural material. For example, its overall properties still can not reach the using requirements of automobile spare parts, such as the wheel hub. So, recently, how to change this situation is a major focus in research. And, the general way is to alloy it, so as to improve its ductility and strength.Through the preparation of AM60-based alloy by ordinary metal mold casting formation technology and changing the adding amount of alloying elements Sm and Ti, and by means of optical microscopy, X-ray diffraction, scanning electron microscopy, electronic universal testing, etc., this paper discusses the effect of Sm and the composite modification of Sm and Ti on the microstructure and the properties of AM60 magnesium alloy. Also it further discusses the strengthening and toughening mechanism of alloy, and the effect of the alloyed fine grain which is achieved by the optimal ratio of alloy. Meanwhile, it tries to explain the improvement of the mechanical properties of AM60 magnesium alloy via the heat treatment. The results show that:(1) When an appropriate amount of rare earth element Sm is added into it, the Sm can significantly refine the grain. So the semi-continuous network structure ofβ-phase disappears and the granular is formed, which disperses in a-Mg matrix. Then the new granular phase of Al2Sm is formed in the boundary and the inside of the grain, so the organization of the alloy microstructure is significantly improved. However, with the increasing of the amount, there occurs the needle-like phase of Al3Sm, which plays a negative impact on the substrate.(2) With the adding of Sm, the tensile strength and elongation of AM60 alloy firstly increases, then decreases. And when the amount of Sm is 1%, they reach to the peak of 210Mpa and 6.9% respectively, which are increased by 25% and 43.8% over the alloy without Sm. Also, when the temperature reaches to 200℃, the tensile strength and elongation are 144Mpa and 10.1% respectively, which are increased by 30.3% and 77.2% respectively over the alloy without Sm. Moreover, the adding of Sm can slightly increase the hardness of AM60. When the amount of Sm reaches to 2.0%, the hardness of alloy is 54.8HB, which is increased by 15.6% over the master alloy. The improvement of the strength of alloy is mainly caused by the joint function of the solid solution strengthening of the Sm element, grain refinement and dispersion strengthening of the Al2Sm phase.(3) With the adding of Sm, the fracture mechanism of AM60 magnesium alloy transforms from the cleavage fracture to the quasi-cleavage fracture and local ductile fracture. And there is no obvious necking when it is tested by the tensile at high-temperature.(4) By the insulation of 410℃and the solution heat treatment of 24h, the coarse mesh ofβphase distributed along the grain boundaries in the cast is completely dissolved and it forms a single-phase supersaturatedα-Mg of the solid solution. Under this situation, the rare earth phase still exists, and its thermal stability is high. After the T6 treatment of the insulation of 410℃and the solution heat treatment of 24h, pluses the insulation of 200℃and the solution heat treatment of 12h, a large number of small dispersedβ-phase is separated from the base body. And theβ-phase distributes in the continuous or discontinuous forms. Meanwhile, the mechanical properties of AM60 magnesium alloy are increased.(5) Adding Sm and Ti into AM60 alloy can obviously refine the grain. The semi-continuous network structure ofβ-Mg17Al12 phase turns into granular and pebble-like form, which disperses inα-Mg matrix. So the microstructure is significantly improved. And when Ti is added into AM60 alloy, a small amount of Al3Ti is generated. However, When Sm and Ti are added together into AM60 alloy, the granular form of Al2Sm phase, the granular form of Mg41Sm5 phase, and the halo-like compounds of Al18Ti2Mg3, are generated. The appropriate treatment of T6 makes the rod phase in the alloy passive or disappear, and the coarse particles round and small.(6) The adding of Sm and Ti improves the mechanical properties of the alloy. And the mixed addition has better effect than the single addition. When the 1.0% Sm and 0.4% Ti are added into it, the tensile strength and elongation of the cast alloy are 226MPa and 6.7% respectively, which are increased by 34.5% and 39.6% respectively over the master alloy. And when the temperature is 200℃high, the tensile strength and elongation are 170MPa and 10.4% respectively, which is increased by 53.2% and 82.5% respectively over the master alloy. The T6 treatment can further improve the mechanical properties of the alloy.
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