【作者】 马玉涛；

【导师】 张兴国；

【作者基本信息】 大连理工大学 ， 材料加工工程， 2009， 博士

【摘要】 本论文根据镁合金研究的现状及存在的不足,以高性能变形镁合金材料作为研究方向,将新型铸造技术——电磁搅拌、悬浮铸造应用到高性能变形镁合金的制备中,形成一种高效、无污染的镁合金熔体处理、组织细化技术,以提高合金的力学性能,从而促进镁合金在工业中的应用。自行设计了电磁搅拌—悬浮铸造装置,以悬浮剂和添加合金元素种类及剂量的变化对变形镁合金影响为主要研究内容,以具有代表性的Mg-Al-Zn系AZ31、AZ61合金为基,对变形镁合金材料的合金成分—组织结构—性能变化—材料应用之间的相互关系和作用规律进行了系统的研究。电磁搅拌—悬浮铸造实验结果表明:电磁搅拌—悬浮铸造技术可以细化金属镁及镁合金的显微组织,减小合金的晶粒尺寸;提高合金的抗拉强度和伸长率;减少合金中的显微缩松和气孔,提高合金的致密度,这些结果有利于改善镁合金的后续加工性能。CaC₂颗粒悬浮剂的加入显著的改变了金属镁的晶粒尺寸,晶粒大小从纯镁的2350μm减小到CaC₂加入量为0.26%时的125μm。CaC₂添加到AZ31、AZ61镁合金中,改善了合金的显微组织,晶粒细化效果明显,同时合金的抗拉强度和伸长率都得到提高。当CaC₂添加量为0.15%时,AZ31合金的晶粒最细小,抗拉强度和伸长率达到最大值（σb:195.3MPa,δ:12.9%）,分别比金属型铸造AZ31镁合金提高了18.72%和48.28%;当CaC₂添加量为0.36%时,AZ61合金的晶粒最细小,抗拉强度和伸长率达到最大值（σb:211.36MPa,δ:8.5%）,分别比金属型铸造AZ61合金提高了约24.2%和46.6%。Zr在含Al镁合金中会引起元素“中毒”现象的发生而使合金的性能急剧下降,但Zr是无Al镁合金的通用细化剂,Zr与Mg有共格关系（都是密排六方结构,有相似的晶格常数）,可以作为镁合金的异质形核核心。考虑到以上因素,采用电磁搅拌—悬浮铸造技术,成功制备出Zr颗粒悬浮剂细化AZ31、AZ61变形镁合金。适量的Zr颗粒悬浮剂在镁合金中主要以固溶原子和颗粒相存在,颗粒相尺寸在10μm以下,合金凝固时作为α-Mg晶粒的形核衬底,细化了含Al镁合金的显微组织,提高其力学性能。但是过量的Zr将与合金中的Al反应生成Al₃Zr沉淀相,消耗了合金中的Al元素,减弱了Al对镁合金的细化、强化作用,而且由于Al₃Zr与镁基体没有共格关系,不能起到细化晶粒的作用,合金组织粗化。因此,控制合金中Zr悬浮剂的加入量是十分重要的,Zr的添加量为0.07wt.%左右为宜。利用电磁搅拌技术,制备了稀土Er增强AZ31、AZ61变形镁合金,研究了稀土Er对AZ系变形镁合金组织和性能的影响,结果表明:少量的稀土Er对镁合金有显著的细化效果,合金组织均匀,等轴晶趋势明显,晶粒尺寸减小,力学性能提高;过量的稀土Er会使AZ系变形镁合金的铸态树枝晶急剧长大,晶粒粗化十分严重,导致合金力学性能下降。观察到少量的稀土Er在镁合金中以固溶原子的形式存在,过量的Er与合金中的Al反应的生成Al₂Er相,消耗了合金中的Al,削弱了Al对镁合金的强化作用。并且,Al₂Er是面心立方结构,晶体结构与Mg不同,既不能起到异质形核核心作用,又不能阻碍镁合金晶粒的长大,且由于Al₂Er和Mg基体的凝固收缩率不同而促进裂纹的萌发,导致镁合金力学性能的下降。研究分析结果表明,合金中Er含量为0.03—0.05wt.%是适宜的。采用动态墩粗极限变形试验,研究了电磁连铸镁合金的铸造、挤压、挤压+锻造+轧制状态和电磁搅拌—悬浮铸造及微合金化镁合金的极限变形程度和热变形行为,为镁合金热加工成形工艺的制定、推广提供依据。镁合金在动态墩粗变形过程中,由于应变速率高,发生了以孪生变形为基础的动态再结晶,动态再结晶晶粒以孪晶界为界形核、长大。墩粗变形量的增加,引起再结晶晶粒形核数量的上升,合金中的孪晶组织和大晶粒减少,再结晶晶粒数量增多,合金的晶粒逐渐细化。动态墩粗极限变形试验结果表明,电磁连铸铸态下AZ31镁合金在380—430℃时具有较好的塑性成型性能,锻造最大变形量为40%,挤压工艺可以使合金的锻造温度范围扩大到350℃—450℃之间,使合金在较低的温度下就可以得到较大的变形;电磁连铸AZ61镁合金锻造最大变形量为30%时,锻造温度范围为315℃—370℃之间,挤压工艺可以使合金的最大变形量加大到40%,使合金在锻造温度基本不变的情况下,得到了较大的变形。金属型铸造AZ31镁合金的最佳锻造温度在380℃—410℃之间,锻造最大变形量小于40%。金属型铸造AZ61镁合金的最佳锻造温度在300℃—350℃,最大变形量为30%。电磁搅拌—悬浮铸造技术可以改善合金的塑性变形性能,提高合金的最大变形量,扩大变形温度。悬浮剂Zr和稀土Er的加入可以提高AZ31、AZ61合金的锻造变形量,并且使变形温度范围扩大到300℃—410℃。更多还原

【Abstract】 According to the research status and insufficient of magnesium alloy,research works of this thesis aim at high performance wrought magnesium,and mainly focus on the effects of electromagnetic suspension casting（combines the virtues of the electromagnetic stirring and the suspension casting,ESC） and alloy strengthening on it.The formation of an efficient, pollution-free grain refinement of magnesium alloy melt processing techniques to improve the mechanical properties of the alloy,thereby contributing to magnesium alloy in the Industry.Electromagnetic Suspension Casting device is designed and set-up based on the principle of electromagnetic stirring and the suspension casting.The effects of suspension particles on typical wrought magnesium alloys,such as AZ31 and AZ61 alloy are investigated systematically for the relationship of their chemical composition,microstructures,mechanical properties and their applications.The results show that ESC technique has been efficient in refining the microstructure and intensify the tensile properties characterization of magnesium alloys.The increase of the relative density of alloys caused by the reduction of micro porosity and gas cavity by ESC has advantage in improvement magnesium alloy following workability.The results indicated that proper additions of CaC₂ particulates concentration were found to be efficient in refining the microstructure of magnesium materials.0.26wt.%CaC₂ addition reduced the mean grain size of pure magnesium 2350μm to 125μm。With the CaC₂ addition AZ31 and AZ61 alloy,the microstructures have significant improvement,the grain refinement and the ultimate tensile strength and relative elongation increase remarkable.The 0.15wt.%and 0.36wt.%CaC₂ addition refines the grains and improves tensile properties of AZ31 and AZ61 alloy most effective,respectively.The ultimate tensile strength is 195.3MPa, elongation is 12.9%of AZ31 alloy,which increased by 18.72%and 48.28%respectively compared to the die-casting AZ31 alloy.The ultimate tensile strength is 211.36MPa, elongation is 8.5%of AZ61 alloy,which increased by 24.2%and 46.6%respectively compared to the die-casting AZ61 alloy.Zr can be added to magnesium alloy not containing Al for grain refinement because of hypothesis relates to the lattice disregistry between Zr and Mg.Considering the Al element interferes with the grain refinement process by the Zr,the ESC technique has been used for casting.The Zr particles refined AZ31 and AZ61 magnesium alloy have been successfully fabricated by ESC technique.Appropriate amount of Zr are mainly dissolved inα-Mg or exist in the form of particles,and the size of Zr particles is less than 10μm.It is believed that Zr particles or elemental in solid solution may be a powerful nucleant of magnesium because of similarity in the lattice parameters,therefore providing efficient nucleation,improving mechnical properties.However,more amount of Zr can react with Al to Al3Zr,which weaken the strengthening effect of Al to magnesium alloy.Moreover,Al3Zr can not play the role of grain refinement due to no lattice match with magnesium.The grain of alloys coarsens with the more Zr addition.Consequently,it is allimportant of controlling the amount of Zr addition. The adding amount of Zr of about 0.07wt.%is appropriate.The effects of small amounts of rare-earth element Erbium on the microstructure and the mechanical properties of AZ31 and AZ61 magnesium alloy via the electromagnetic stirring（EMS） process have been studied.The results show that small amount of Er addition obviously refine the grain size,make alloy microstructure homogeneous,increase the rate of equiaxed grains,and improve the mechanical properties.More addition of Er obviously decrease mechanical properties of AZ31 based alloy because of the grain coarsening. Observed a small amount of rare earth Er in magnesium alloy to form solid solution atoms,an excessive amount of Er and alloys in the Al reaction generates Al₂Er phase,which weaken the strengthening effect of Al to magnesium alloy.Moreover,Al₂Er phase is face centered cubic structure,which is different with the structure of Mg（Hexagonal Close-Packed）.It cannot be the heterogeneous nuclei and have no effect on the grain growth.During deformation,the constituents with Er and Al will fracture first and act as the microcrack sources due to the stress concentration,which would descent the mechanical properties.Research results show that the Er content 0.03—0.05wt.%is appropriateThe elecrtromagnetic casting（EMC） and electromagnetic Suspension Casting（ESC） magnesium alloy have been deformed by dynamic upsetting.The deformation limits and behavior of thermal deformation have been studied to provide the basis for the establishment of magnesium alloy hot-working forming crafts formulation.Due to the high strain rate, magnesium alloy deformation occur the dynamic recrystallization based on twinning during dynamic upsetting process.The dynamic recrystallization grains take the twin boundary as in the place to nucleation and growth.Nucleation of recrystallized grains of alloy increase in the volume with the upsetting deformation increases,twin organizations and large grains decrease, recrystallized grains increased gradually refined grain alloy.The results of research shows that the appropriate foring deformation condition of EMC AZ31 alloy is temperature of 380℃—430℃,maximum deformation of 40%.Extrusion process can extend the EMC AZ31 alloy forging temperature range to 350℃—450℃,which make alloy get a larger deformation at lower temperatures.The appropriate foring deformation condition of as-cast EMC AZ61 is temperature of 315℃—370℃,maximum deformation of 30%.Extrusion process extend the EMC AZ61 alloy forging maximum deformation to 40%,which make the alloys have a relatively large deformation while the forging temperature is essentially the same circumstances.The appropriate deformation condition of permanent mould casting AZ31 and AZ61 alloy is temperature of 380℃—410℃and 300℃C—350℃,respectively,maximum deformation of 40%and 30%,respectively. ESC technology can improve the plastic properties of alloys to enhance the maximum deformation of alloy,expand deformation temperature range.The maximum deformation of can be improved and the forging deformation temperatures range can be extend to 300℃—410℃with the Zr and Er addition.更多还原