【作者】 吴伟；

【导师】 胡壮麒；

【作者基本信息】 沈阳工业大学 ， 材料加工工程， 2010， 博士

【摘要】 镁合金是目前实际应用中最轻的金属结构材料,它具有密度小、高比强度、高比刚度、优良的导热性和导电性、良好的尺寸稳定性和易于回收等特性。但是,性能较差是阻碍镁合金广泛应用的主要原因之一,所以提高镁合金的力学性能是镁合金研究中要解决的首要问题。本文主要针对铸态和压铸态Mg-Al-Nd镁合金的显微组织、拉伸性能以及低周疲劳性能进行研究,以期为压铸镁合金的进一步开发和应用奠定必要的理论基础和提供可靠的理论依据；同时,对镁合金压铸件的压铸充型过程进行数值模拟,优化压铸工艺参数。实验结果表明：稀土元素Nd的添加可以有效地细化铸态和压铸态Mg-Al-Nd合金的显微组织,铸态含Nd量为1%时细化效果明显,而压铸态含Nd量为0.5%细化效果明显；稀土元素Nd的量为1%时铸态Mg-Al-Nd镁合金室温下的综合力学性能最高,而稀土元素Nd的量为0.5%时压铸态Mg-Al-Nd合金的力学性能最高；在本实验所采用的不同外加总应变幅下,压铸态Mg-Al-Nd合金在疲劳变形时均呈现明显的循环应变硬化；当外加总应变幅为0.3%,0.45%和1.2%时,Mg-8%A1-0.5%Nd镁合金呈现最长的疲劳寿命,当外加总应变幅为0.6%,0.8%和1.0%时,Mg-8%Al-1.0%Nd镁合金表现出最长的疲劳寿命；低周疲劳加载条件下,疲劳裂纹以穿晶方式萌生于疲劳试样表面的铸造缺陷处,并以穿晶方式扩展；计算机模拟仿真技术能够准确地再现镁合金发动机罩盖的充型凝固过程,根据正交实验得出优化的工艺参数为：压射速度6m/s、压铸型温度220℃和浇注温度700℃；压铸件发动机罩盖组织致密、晶粒细小,室温拉伸断口具有韧性断裂和解理断裂的混合断口特征,力学性能可以达到σb=241-244 MPa,σs=179-185 MPa,δ=5.5-6.7%。更多还原

【Abstract】 Magnesium alloys are the lightest structure materials for various application alloys, due to their low density, high specific strength, high specific stiffness, excellent heat conduction and electric conduction, good dimension stability and easy recovery. However, The use of magnesium alloys has been limited due to poor mechanical properties. So increasing mechanical properties of magnesium alloys is a key problem. Microstructure, tensile properties and low-cycle fatigue properties of as-cast and die-cast Mg-Al-Nd magnesium alloys were studied, in order to establish necessary theory foundation and provide reliable theory evidence for developing and applying magnesium alloys. Meantime, numerical simulation of die casting mold filling process for magnesium alloy die castings were studied, optimizing technological parameters of die-casting.Results show that the microstructure of as-cast and die-cast Mg-Al-Nd magnesium alloys can be refined for the addition of rare earth element Nd, the as-cast alloy with addition of 1% Nd was refined obviously, while the die-cast alloy with the addition of 0.5% Nd is refined obviously. The mechanical properties at room temperature of as-cast Mg-Al-Nd magnesium alloy is the highest with 1% rare earth element Nd, while with the addition of 0.5% Nd, the mechanical properties at room temperature of die-cast Mg-Al-Nd magnesium alloy is the highest. Under different imposed total strain amplitude, die-cast Mg-Al-Nd magnesium alloys exhibite obviously cyclic strain hardening during fatigue deformation, when the imposed total strain amplitudes are 0.3%,0.45% and 1.2%, Mg-8%Al-0.5%Nd magnesium alloys exhibite the longest fatigue life, when the imposed total strain amplitudes are 0.6%,0.8% and 1.0%, Mg-8%Al-1.0%Nd magnesium alloys exhibite the longest fatigue life. On the condition of low-cycle fatigue loading, fatigue cracks initiate transgranularly at casting defect which at the surface of fatigue samples, and propagate in a transgranular mode. Mold filling and solidification processes of magnesium alloy engine hood can be reproduced accurately by computer simulation technology, the optimal parameters are gained according to orthogonal test:injection velocity with 6m/s, die-cast mould temperature with 220℃and pouring temperature with 700℃. Structure of engine hood produced by die-cast is dense, fine grain size. Fractograph is mixture of ductile rupture and cleavage fracture. Mechanical property is that ab=241-244 MPa,σs=179～185 MPa,δ=5.5-6.7%.更多还原