【作者】 高德明；

【导师】 翟启杰； 宋长江；

【作者基本信息】 上海大学 ， 钢铁冶金， 2009， 博士

【摘要】 镁合金具有高的比强度、比刚度、良好的减振性、导热性和可切削加工性,广泛地应用于汽车、电子、航空航天等领域。提高镁合金强度和塑性变形能力,以扩大其应用范围,近年来成为材料工作者努力的目标。晶粒细化是提高镁合金综合力学性能最有效的方法之一,因此研究镁合金晶粒细化技术具有十分重要的意义。本文通过正交实验,研究了Al、Zn、Mn三种合金元素对负压吸铸镁合金力学性能的影响规律,结果表明,Al含量是影响Mg-Al合金力学性能的关键因素,Zn、Mn含量变化的影响相对Al而言较弱。在本文研究条件下,Mg-6Al-0.6Zn-0.3Mn为负压吸铸的镁合金最佳成分。采用负压吸铸的方法,研究了Ca、Ce含量变化对Mg-6Al-0.6Zn-0.3Mn合金铸态组织影响规律。实验结果表明,Ca、Ce对该合金的铸态组织都有明显的细化效果:当Ca添加量在0～0.9%范围时,随着Ca含量的增加,晶粒尺寸逐渐减小;当Ce添加量在0～1.0%范围时,随着Ce量的增加,枝晶愈见发达,枝晶臂缩颈现象加剧,其中断开呈细小棒状的晶粒逐渐增多,晶界上的β-Mg17Al12相逐渐减少,但出现了新相Al4Ce;通过高温液淬实验和热分析技术,研究了Ca和Ce对Mg-6Al-0.6Zn-0.3Mn合金凝固过程的影响机理。结果表明,Ca促进了液相中的异质核心形核、抑制了α枝晶生长,从而使镁合金的等轴晶尺寸减小;Ce促使镁合金枝晶生长,从而抑制了镁合金枝晶前沿的形核。分析认为,Ca在凝固过程中富集于固液界面前沿,增大了界面前沿液相中的过冷度,从而促进了界面前沿液相中的异质形核,并抑制了先析晶粒的长大;Ce在凝固过程中吸附于固液界面,降低了界面能,从而促进了枝晶的生长,同时导致界面前沿液相中的过冷度减小,抑制了界面前沿液相中的异质形核,减小了α枝晶的二次枝晶间距。在Mg-9Al-0.8Zn-0.3Mn合金凝固过程中采用超声波、磁致振荡和脉冲电流等物理场进行处理,并与MgCO3变质处理的结果进行对比。实验结果表明,功率超声对合金的铸态组织有明显细化效果,随着功率的增加,细化效果更明显,当功率达到500W时,其细化效果比MgCO3变质处理的效果更好。磁致振荡对合金的铸态组织也有显著细化作用,且随着磁致振荡电压的增大,细化效果也越明显,当电压达到100jV以上时细化效果比MgCO3变质处理的效果更好。脉冲电流对合金的铸态组织同样有细化作用,随着脉冲电流电压的增大,细化效果增大,当电压达到250jV时细化效果与MgCO3变质处理的效果相当。分析不同物理场和MgCO3变质处理的冷却曲线表明,三种物理场处理对该合金的初始凝固温度影响较小,但是可以显著缩短合金的凝固时间;而MgCO3变质处理使得合金初始凝固温度明显提高,凝固时间略微缩短。分析认为,外物理场处理主要通过促进熔体散热和增加型壁晶核的脱落和漂移,从而促使其凝固组织细化;而MgCO3变质处理的主要通过增加异质形核质点。更多还原

【Abstract】 Magnesium alloys are widely used in the fields of automobile, electronic and aviation. Magnesium alloys possess many advantages, such as high specific strength, excellent damping capacity, better thermal conductivity, and good machinability. But low strength and plasticity restrict its applications to more fields. Structure refinement is an effective method to improve their mechanical properties. The aim of this work is to study structure refinement of Mg-Al alloys by addition of alloy elements and the external physical field.In this work, effects of Al, Zn and Mn on the mechanical properties and the structures of magnesium alloys were investigated by orthogonal experiment design in negative suction casting. For negative suction casting, Al content is the key factor to mechanical properties of Mg-Al alloys. At last, the determined optimal alloy compositions are Mg-6Al-0.6Zn-0.3Mn for negative suction casting, respectively.The influences of Ca and Ce elements on the structures of Mg-6Al-0.6Zn-0.3Mn alloys were systematically studied by negative suction casting. The results show that Ca and Ce elements can obviously refine the solidification structures. Increase of Ca content makes the grain size gradually decrease when it is below 0.9wt.%. Increasing Ce content (below 1.0wt.%) makes the number of the secondary dendrite increase, the link between secondary dendrite and first dendrite become thinner, even more secondary become new small crystals. Moreover, increase of Ce content makes content ofβ-Mg17Al12 phase decrease and a new phase (Al4Ce) appear.The effect mechanisms of Ca and Ce elements on the solidification structures of Mg-6Al-0.6Zn-0.3Mn alloys were systematically studied by liquid quenching. The results show that Ca element can promote nucleation in the melt ahead of dendrite tips and restrain crystal growth. But Ce element restrains the nucleation ofαphase and promotes crystal growth. Ca and Ce elements both can restrain the coarsing of the secondary dendrite and decrease the secondary dendrite spacing.The solidification structures of Mg-9Al-0.8Zn-0.3Mn alloy were examined when it solidified under physical fields including ultrasonic, magneto oscillation and pulsed electric current. The results show that the ultrasonic treatment can significantly refine its solidification structure. In the range of 300~700W, increasing the ultrasonic power makes the grain size gradually decrease. When ultrasonic power is over 500W, the grain size is smaller than that of the samples treated by MgCO3 modifier. The treatment of magneto oscillation also can clearly refine its solidification structures. In the range of 50~200jV, increasing the voltage makes the grain size gradually decrease. When the voltage is over 100jV, the grain size is smaller than that of the samples treated by MgCO3 modifier. The pulsed electric current also markedly refined its solidification structures. In the range of 100~250jV, increasing the voltage makes the grain size gradually decrease. When the voltage is 250jV, the grain size is the same as that of the samples treated by MgCO3 modifier.During solidification course of Mg-9Al-0.8Zn-0.3Mn alloy with different treatment method, the temperature was measured by thermal couple. The measured cooling lines show that treatment of three physical fields makes the solidification time obviously shorter, but have little effects on the initial solidification temperature. However, MgCO3 modification treatment makes its initial solidification temperature clearly increase, and makes its solidification time slightly shorter. Analysis suggests that refinement reason of the physical fields mainly is promoting heat elimination of melt and crystal multiplication, while MgCO3 modification treatment mainly increase the number of nucleation site.更多还原