【作者】 陈刚；

【导师】 彭晓东；

【作者基本信息】 重庆大学 ， 材料加工工程， 2005， 硕士

【摘要】 镁合金作为一种轻质金属结构材料,其明显的优点主要表现在密度小、比强度及比刚度高、阻尼性高、减振性好和电磁屏蔽能力强及易回收等,在大量使用压铸件的航空航天、交通工具、3C 产品等领域应用极其广泛,特别是在汽车行业发展尤为迅速。镁合金的综合力学性能与其晶粒大小有着极其密切的关系,细化镁合金晶粒,可以提高镁合金的综合力学性能。添加变质剂是其主要方法之一。目前,对Al-Sr、Mg-Sr 等中间合金型的变质剂研究甚少。本课题通过对比分析“对渗法”和“熔浸热还原法”两种方法制备Mg-Sr 中间合金的制备工艺和合金成分差异。在此基础上,着重研究工艺参数对合金组织的形态、成分及分布的影响,并对Sr/Al 比值对Mg-Sr-Al 三元合金的微观组织以及Mg-Sr 中间合金作为变质剂,对AZ91 镁合金的组织和力学性能的影响进行深入研究。同时,从热力学角度出发,分析了“熔浸热还原法”制备Mg-Sr 中间合金的可行性,在该过程中采用X 荧光分析、扫描电子显微镜(SEM)、X 射线衍射仪(XRD)、光学显微镜(OM)等对实验结果进行分析。研究结果表明,①镁锶中间合金组织呈枝晶网状组织,共晶组织主要呈鱼骨状分布在树枝晶枝杈和晶界上。②溶解温度过高或过低,保温时间过长或过短,都会降低合金中锶的实际含量,共晶组织比重以及晶粒的大小。Sr 含量对Mg-Sr中间合金晶粒的大小的影响可以用Sr 原子的晶界偏聚能力GRF 的大小来衡量。添加的锶愈多,锶原始浓度愈大,GRF 值愈大,锶对镁的细化能力愈强。③用氩气保护,制备出的合金中锶实际含量高于熔剂法制备的合金。④Mg-Sr-Al 三元合金组织成分与Sr/Al 比值有关,随着比值的增大,合金组织中第二相由Al4Sr 逐渐转变为Mg17Al12、Mg-Al-Sr 三元化合物和Al4Sr。⑤“熔浸热还原法”制备出的Mg-Sr中间合金锶实际含量很低,晶粒比较粗大,首次发现SrCO3 对镁有变质细化晶粒的作用。⑥Mg-Sr 中间合金可以改善AZ91 镁合金的组织,细化晶粒,由于晶界上存在Mg-Sr、Al-Sr 脆性相化合物,使其力学性能提高幅度不大。随着Mg-Sr 中间合金添加量的增加,Mg-Sr 中间合金对AZ91 镁合金晶粒的细化作用逐渐增强,抗拉强度略有提高。在0.6%左右达到最佳值,Mg-Sr 中间合金添加量进一步增加,会恶化AZ91 镁合金的力学性能。更多还原

【Abstract】 There are distinct advantages,such as lower density ,specific strength and specific stiffness, higher damping, strongly shielding ability of electromagnet,and easier recycling,to lightweight magnesium in powertrain components where large die-cast components are used in automotive applications ,industry of Aero-Space,and industry of electron, especially in automobile. The sizes of crystalline grains affect the mechanics properties of the magnesium alloys. It is one of the main methods of refining the grains that the additive of modification was added into magnesium alloys. Sr and Sr alloys have been applied to Mg or Al alloys as an additive of modification. In recent years, Al-Sr and Mg-Sr alloy is studied rarely,Mg-Sr alloy has not studied the report yet inland, research in this field is also very rarely abroad. In this papers, the preparation technologies of Mg-Sr alloys which were prepared by melt-leaching-reduction process and Mixture method respectively, and the difference of their composition were analyzed, on which of the basis, the effect of process parameters on morphology of the structure, composition ,and distribution were emphasly studied. at the same time ,the effect of the value of Sr/Al on the morphology of Mg-Al-Sr ternary alloys and that of magnesium-strontium alloys on the structure and the mechanics properties of AZ91 were also studied., in addition, the feasibility of Mg-Sr alloys prepared by melt-leaching-reduction process and the reason that is not to adopt SO₂ gas as protecting gas in the course of the preparation were studied according to thermodynamics theory. Moreover, the experimental result were studied by X fluorescence analysis, Scanning electron microscope （SEM） , X ray diffraction appearance （XRD ）,Optics microscope （OM ）,etc. The results are shown that: ①the morphology of magnesium-strontium alloy was network structure,and the laminary eutectic morphology distributed at the grain boundary or in the interdendritic regions. ②The holding temperature was too hight or too low ,the holding time was too long or too short ,which both reduced the amount of dissolved strontium ,affecting the percentage of eutectic structure and the size of grain .The effect of strontium on the grain size of Mg-Sr alloy was measured by the value of GRF（the ability of the grain boundary partial gathering） Which depended on the original concentration of strontium. the GRF value increased with the increasing original concentration of strontium,which contributed to improve the ability of refining grain.③Using the argon gas as protective gas ,the content of Sr in alloy was more than that of alloy made by the flux method. ④The organization and composition of Mg-Al-Srternary alloy were relation to the value of Sr/Al. as the value increasing,the second phase Al4Sr was gradually changed into Mg17Al12,Mg-Al-Sr ternary compound,and Al4Sr. ⑤the Mg-Sr alloy prepared by melt-leaching-reduction process,in which the actual content of Sr was too low ,and the grain of alloy was relative large. It is the first to find that SrCO3 , as an additive ,can refine the grain of magnesium or magnesium alloys during the research work. ⑥Mg-Sr alloy had the function of refining the grain of AZ91 alloy,but its effect on the mechanics properties of AZ91 was slightly, because Mg-Sr and Al-Sr brittle phases distributed at the grain boundary. the function of refining of magnesium-strontium alloy depended on the amount of added Sr. when the amount of Mg-Sr alloy was added to about 0.6%, The value of tensile strength and degree of hardness were maximum .if the mount of Mg-Sr alloy continue increased , the mechanics properties of AZ91 alloy would be deteriorated.更多还原