| 其他摘要 | For exploiting tensile properties potency of low-cost Mg-Al-Zn casting magnesium alloys, the influences of antimony element, carbon inoculation by adding magnesite particles, and heat treatment on the microstructure and tensile properties of Mg-Al-Zn alloys with 10±2wt% aluminum and zinc contents were investigated.
The study discovered evolution rules of secondary phase type with Zn/Al mass ratio. Secondary phase changed from γ phase to Φ phase, τ phase and unknown Mg-Al-Zn ternary phase with the Zn/Al mass ratio increasing. By studying cooling curves acquired by thermal analysis technique, forming temperatures of secondary phases mentioned above were disclosed. The forming temperature of γ phase descended from AZ60 alloy’s 434~428℃ down to AZ66 alloy’s 371~365℃ with the increasing of solid solution content of zinc element in γ phase. The forming temperature range of Φ phase is around 360~350℃. The forming temperature range of τ phase is around 351~343℃. The forming temperature range of the unknown Mg-Al-Zn ternary phase is around 344~330℃. It was validated that 0.1~0.5 wt.% antimony addition refined the primary grains to some extent and turned lamellar secondary phases into massive secondary phases. The investigation also revealed that carbon inoculation by adding magnesite particles greatly refined primary grains and dispersed secondary phases, and it was discovered that aluminum content in this method should be greater than 4 wt.%. The heat treatments of these alloys were studied systemically. Referring to ending temperature of solidification, the condition of optimum solid solution (T4), which can make secondary phases dissolved into matrix thoroughly, were obtained. Optimum T6 heat treatment (i.e. optimum T4 solid solution plus age hardening) made the solid solved aluminum and zinc elements precipitated from matrix more dispersively than that in casting condition.
Experiments of tensile properties indicated that Y.T.S (Yield Tensile Strength) increased and ductility decreased with aluminum and/or zinc content increasing at casting condition and optimum T6 heat treatment condition. At casting condition, alloys with 10 wt.% aluminum and zinc content got the maximum U.T.S (Ultimate Tensile Strength). At optimum T6 heat treatment condition, alloys with 11 wt.% aluminum and zinc content got the maximum U.T.S. 0.1~0.5 wt.% antimony were propitious to enhancement of strength and ductility. Carbon inoculation enhanced Y.T.S, U.T.S and ductility greatly at the same time.
Four keys to improving strength and ductility of the medium Al/Zn content casting magnesium alloys were put forward as follows: matching of aluminum and zinc contents; carbon inoculation; micro-element modification; proper heat treatment. High strength and high ductility magnesium alloys were developed based on such idea. The results showed that AZ64-0.1Sb alloy at optimum T6 heat treatment condition (300℃, 2h; 320℃, 2h; 340℃, 2h; 350℃, 4h; 360℃, 4h; 370℃, 2h; 180℃, 20h) got excellent tensile properties, including Y.T.S of 160MPa, U.T.S of 313 MPa, and elongation of 6%. AZ64-0.5Sb alloy at such optimum T6 condition also got excellent tensile properties, including Y.T.S of 150MPa, U.T.S of 308 MPa, and elongation of 6.7%. These tensile properties are keeping ahead in the world among same alloys prepared by similar method.
In the interest of improving castbility of Mg-Al-Zn alloys with medium aluminum and zinc contents, according to principles of double thermocouple method, computer aided thermal analysis technique for characterizing alloys’ solidification was set up. According to principles of Couette type rheometer, continuous torque measurement technique for testing alloys’ mushy zone shearing strength was set up. The solidification pathway and sequence related to microstructure formation were investigated. Evolution rules of mushy zone temperature parameters, solid fraction, and mushy zone shearing strength were studied also. These results provided important physics metallurgical fundament for understanding and control relationship between microstructure and tensile properties.
New concept of Tn1 (first characteristic temperature of primary grain nucleation) and Tn2 (second characteristic temperature of primary grain nucleation) were put forward, and concepts of Tch (temperature of dendritic coherency) and Tpk (temperature of dendritic packing) were introduced. The investigation of the influence of aluminum and zinc contents revealed that Tn1, Tn2, and Tch decreased gradually, and temperature difference of (Tn1-Tn2) increased slowly, and temperature difference of (Tn2-Tch) almost remains constant while zinc and aluminum contents increased. The fs-ch (solid fraction of dendritic coherency) of medium Al/Zn content magnesium alloys varied from 0.27 to 0.37. Investigation on carbon inoculation by adding magnesite particles showed that heterogeneous nucleus heightened the value of Tn1. Studies also revealed that carbon inoculation by using magnesite particles and antimony addition, which both can refine primary grains, both heightened the valve of fs-ch and reduce temperature difference of (Tn1-Tn2).
According to the investigation on mushy zone shearing strength of these alloys, the relationships between mushy zone shearing strength and castability were disclosed. The results indicated thatτpk (shearing strength at dendritic packing point), the increasing speed of shearing strength with temperature or solid fraction are three main parameters. The study also showed that alloys having high hot-cracking susceptivity had high value ofτpk, and these alloys’ mushy zone shearing strength increased slowly with temperature or solid fraction commonly, which went against to rapid setting up of the mushy zone shearing strength. |
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