Effects Of The Contents Of Al And Sn On The As-cast Microstructures And Mechanical Properties Of Mg-Al-Sn Magnesium Alloys

At present, Magnesium is called as the lightest structural materials and the largest specific gravity metallic material in the engineering structural materials. Magnesium alloys were widely used in automotive automotive industry, aerospace industry and electronic communications because of many advantages, such as excellent thermal conductivity, high vibration resistance, good damping properties and so on.However, magnesium alloys can't achieve extensive application as a result of the lower absolute strength, worse corrosion resistance and worse plasticity and so on. The design and optimization of the magnesium alloys called as the basic technology may provide the theoretical and experimental evidence. Therefore, it's very necessary to design and optimize the magnesium alloys with the excellent synthetic mechanical properties.Recently, a novel magnesium alloys with higher superplasticity, AT33 and AT63 were designed. The AT33 show the better plasticity, the higher elongation (35%) and the better tensile strength (207-248MPa). And for AT36, the tensile strength and the elongation are 271-300MPa and 16-18% respectively.For systematical investigate, on the basis of Mg–Al–Sn alloy, we studied the change of the strength, plasticity and hardness along the different compositional ratio of the two alloying elements (Al and Sn). We analysed the cast microstructure of the 27 series of alloys with the different ratio of the alloying elements by OM, SEM and EDS and so on. We want to optimize a series of cast magnesium alloys, which has the excellent mechanical properties based on above study, and to provide the theoretical and experimental evidence for the research and development and the application of the new alloys.The results are mainly concluded as below: (1) The effect of the contents of Al and Sn on the cast microstructure of Mg-Al-Sn magnesium alloy have been given preliminarily: when the contents of the Sn are 1wt.%, 2wt.%, 3wt.% respectively, alloys is changed from the initial dendrite to the uniform equiaxed grain along with the contents of Al increased from 1wt.% to 9wt.%, and at the same time, the grain is going to be refined and the precipitated phase increased in grain boundary. Similarly, when the contents of the Sn are 1wt.%-9wt.%, the grain is going to be refined along with the increased contents of the Sn from 1wt.% to 3wt.%. We obtained the alloy with wider grain boundary (AT32 and AT33) and found some of compounds seed out in the grain boundary. And AT62 and AT63, in which more discontinuous compound phase seed out in the grain boundary. Numerous Net-like compound phase distribute in the grain boundary of AT81 and AT82, and the content of the Al with solution treatment in intracrystalline and grain boundary increased along with the increased content of Al.(2) The effect of the contents of Al and Sn on the mechanical properties of Mg-Al-Sn magnesium alloy have been given preliminarily: when the contents of the Sn are 1wt.%, 2wt.%, 3wt.% respectively, the ultimate tensile strength increased firstly and then decreased, the yield strength is increased, and the elongation-to-failure is also first increased and then decreased along with the contents of Al increased from 1wt.% to 9wt.%. When the contents of Al is 1wt.% and 9wt.%, the ultimate tensile strength, the yield strength are both increased and the elongation-to-failure increased firstly and then decreased along with the increased contents of Sn. In the alloys with the high content of Al, the the ultimate tensile strength, the yield strength and the elongation-to-failur are all decreased along with the increased contents of Sn. On the other hand, when the contents of Sn is 1wt.%, 2wt.% and 3wt.% respectively, the hardness is increased along with the increased contents of Al, but conversely, the hardness almost unchanged. Tensile rupture of the Mg–Al–Sn alloy is ductile rupture, and the mechanism is the mixing of the transgranular fracture and the intergranular fracture brought by second phase particle.(3) Several alloys with better mechanical properties have been optimized preliminarily: the alloys show the better superplasticity, the higher elongation-to-failure (39%) when the contents of the Al and Sn are 3wt.% and 2wt.% or 3wt.%, respectively. And when the contents of Al and Sn are 8wt.% and 1wt.% or 2wt.%, the alloys show the ultrahigh-tensile strength, the highest ultimate tensile strength is 280MPa. The alloys show the excellent synthetic mechanical properties when the contents of the Al and Sn are 6wt.% and 2wt.% or 3wt.%, the ultimate tensile strength is 260MPa, the maximal elongation-to-failure is 30%. When the contents of Al increased to 9wt.%, the alloys with the high contents of Al show the maximal hardness.