| In this paper, the wrought magnesium alloys of low cost, high strength and high elongation were developed as the experimental materials. Selecting the Mg-6Zn-0.5Y-2Sn magnesium alloy which has more second phases and the AZ80-4Sn magnesium alloy which has a high melting point and a larger size of the second phase as the base alloys, the ultrasonic treatment was conducted to the base alloy melt, to discuss its influence on the microstructure and mechanical properties of as-cast and extruded alloy. With the help of OM, SEM, EDS, XRD and mechanical property tests, the influence mechanism was further investigated, in order to design and develop the magnesium alloys of high strength which the research group needed.The results show that, the appropriate ultrasonic treatment can not only improve the microstructure of the as-cast and extruded experimental alloys, but also significantly improve their mechanical properties.The microstructure of the as-cast Mg-6Zn-0.5Y-2Sn alloy is mainly consisted of ?-Mg phase, MgZn2 phase, MgSnY phase, Mg2 Sn phase and I-phase(Mg3Zn6Y, with small amount). With the application of ultrasonic treatment, the small amount I-phase and the existed coarse dendrites microstructure nearly disappear, and with the increase of the ultrasonic treatment power, the coarse dendrites microstructure is gradually changed into roundish equiaxed grains. And the morphologies of MgZn2 phase, MgSnY phase and Mg2 Sn phase are transformed from coarse, semicontinuous and non-uniform to fine, discontinuous, uniform and dispersive. When the ultrasonic treatment power is 700 W, the best ultimate tensile strength(245.6MPa) and elongation(20.2%) of Mg-6Zn-0.5Y-2Sn alloy are obtained, the best 0.2% tensile yield strength(83.0MPa) is achieved when the power is 500 W. Compared with the untreated alloy, the best 0.2% tensile yield strength, ultimate tensile strength and elongation are improved by 43.1%, 30.2% and 66.9%, respectively.The microstructure of the as-extruded Mg-6Zn-0.5Y-2Sn alloy is mainly consisted of ?-Mg phase, MgSnY phase and Mg2 Sn phase. After 700 W ultrasonic treatment, the microstructure of the as-extruded Mg-6Zn-0.5Y-2Sn alloy is mainly consisted of ?-Mg phase and MgSnY phase. The MgSnY phase is fine, dispersive and uniform. The Mg2 Sn phase has been broken into fine and dispersive particles and dissolved into ?-Mg matrix significantly. The average recrystal grain size reduces from about 21?m to 15?m. When the ultrasonic treatment power is 300 W, the best 0.2% tensile yield strength(178.8MPa) is achieved. When the ultrasonic treatment power is 500 W, the best elongation(19.6%) is achieved. When the ultrasonic treatment power is 700 W, the best tensile strength(327.7MPa) is achieved. Compared with the untreated alloy, the best 0.2% tensile yield strength, ultimate tensile strength and elongation are improved by 7.2%, 12.3% and 74.3%, respectively.The microstructure of the as-cast AZ80-4Sn alloy is mainly consisted of a-Mg matrix, Mg17Al12 phase and Mg2 Sn phase. With the application of ultrasonic treatment, the coarse dendrites microstructure of the as-cast experimental alloy has been blown out, become fracture and separation obviously. After 600 W ultrasonic treatment, the continuous nets Mg17Al12 phase changed into discontinuous broken nets, the blocky Mg2 Sn phase broke and solid solution into ?-Mg matrix partly, and the average grain size reduce from about 112?m to 95?m. With the application of 600 W ultrasonic treatment, the tensile properties and elongation of the as-cast AZ80-4Sn alloy are significantly increased, which are 150.1MPa and 2.4%, respectively. Compared to the untreated alloy, the ultimate tensile strength and elongation are improved by 48.3% and 20.0%, respectively.The microstructure of the as-extruded AZ80-4Sn alloy is mainly consisted of a-Mg matrix, Mg17Al12 phase and Mg2 Sn phase. The grains of the extruded AZ80-4Sn alloy without ultrasonic treatment are larger and distribute nonunifoum, the Mg17Al12 phase and Mg2 Sn phase, which are distributed at grain boundaries, is blocky or banding. After 600 W ultrasonic treatment on AZ80-4Sn alloy, the grain size become very uniform, and the average grain size reduce from about 15?m to 7?m. Meantime, the blocky or banding Mg17Al12 phase disappeared, the Mg17Al12 phase and Mg2 Sn phase turn into fine, dispersive, and uniform in the matrix, which can provide strong bonding with the matrix. With the application of 600 W ultrasonic treatment, the tensile properties, the 0.2% tensile yield strength and elongation of the extruded AZ80-4Sn alloy attained to the maximum, which are 360.0MPa, 281.2MPa and 9.0%, respectively. Compared to the untreated alloy, the ultimate tensile strength and 0.2% tensile yield strength improved significantly, whicn improved by 24.1% and 9.6%, respectively, especially the elongation improved very obviously, improved by 172.7%. |
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