【作者】 滕海涛；

【导师】 李廷举；

【作者基本信息】 大连理工大学 ， 材料加工工程， 2009， 博士

【摘要】 镁合金作为最轻的商用金属结构材料,具有高的比强度、比刚度,优良的阻尼性能和电磁屏蔽性能,良好的铸造性能及丰富的资源优势等特点,在汽车、通讯、电子及航空航天等领域具有重要的应用价值和广阔的应用前景。传统铸造工艺得到的镁合金产品的显微组织及析出相比较粗大且分布不均衡,致使其强度、塑性变形性能、热稳定性能、抗氧化性能及耐腐蚀性能不够理想,难以满足高性能结构材料的需求,因而镁的现有使用情况远没有充分发挥镁合金材料的潜在优势。亚快速凝固具有较高的凝固冷却速率,能够显著细化晶粒和第二相,增加组织和成分的均匀性,扩展合金元素的固溶度和形成新的亚稳相等,从而可大幅度提高镁合金的力学性能、加工性能和耐蚀性能。因此,通过亚快速凝固使镁合金获得不同于常规凝固方式的组织结构,是提高镁合金性能,扩展其应用前景的有效途径。本文利用自行开发设计的几种不同的工艺装置实现了镁合金的亚快速凝固,研究了不同工艺及冷却速率下镁合金的凝固组织特征及形成机理,分析了显微组织与铸件力学性能、耐腐蚀性能的关系,探讨了不同凝固条件下镁合金铸件的强化机制和腐蚀行为。主要研究内容和结果如下:设计了真空吸铸急冷模法制备镁合金小尺寸薄片铸件工艺装置,实现了镁合金的亚快速凝固。研究了亚快速凝固条件下镁合金凝固组织的特点,并与常规凝固镁合金铸件的显微组织进行了比较分析。结果表明:常规凝固条件下,镁合金的凝固组织晶粒粗大且不均匀:主要由初生α-Mg及晶间大量分布的β-Mg₁₇Al₁₂（包括离异共晶组织β相及二次析出β”相）两相组成;溶质元素Al、Zn在凝固过程中富集、偏析于晶界处形成金属间化合物β-Mg₁₇Al₁₂相,而只有少量的合金元素固溶于晶粒（初生α-Mg相）内,元素的微观偏析严重。亚快速凝固镁合金薄片铸件等轴晶区的晶粒明显细化,晶粒度达到几个微米,且随着冷却速率的增加,晶粒度不断减小;较高的冷却速率抑制了溶质元素的扩散,形成了以过饱和α-Mg相为主的凝固组织,晶界处析出的β-Mg₁₇Al₁₂相极少或者消失,增加了铸件成分及组织的均匀性;溶质元素大量固溶于基体Mg中,Al原子以置换的形式取代晶体结构中的Mg原子,导致了晶体Mg的晶格常数减小,其XRD衍射峰向右偏移。测量了亚快速凝固铸件显微组织中二次枝晶臂间距,从数量级上估算了亚快速凝固的冷却速率,结果表明,本实验镁合金薄片铸件的冷却速率达到了10²～10⁴ K·s^-1,其凝固过程属于亚快速凝固范畴。探讨了镁合金不同凝固过程中的溶质分配原理、组织演化及形成机理,分析表明:亚快速凝固条件下,极快的冷却速率使得熔体凝固过程中固/液界面推移的速率很快、局部凝固时间很短,合金元素来不及完全扩散就已经被高速移动的界面“淹没”而凝固,使其很大程度上固溶于基体α-Mg中。较快的冷却速率下,形核在以晶核为中心沿六个方向生长过程中,绝大部分还没有来得及长大,就彼此交集生成细小的等轴晶粒;另一部分晶核不能完全发育长大或者由于受到其它形核生长的阻挡,生长成近似花瓣状（花瓣实质为一次枝晶组织）结构,而只有少部分形核最终能够长大成具有细小的二次枝晶臂的枝晶结构组织。通过对AZ61A镁合金不同铸件的力学拉伸性能和腐蚀性能的测试,研究了亚快速凝固组织对镁合金性能的影响,探讨了亚快速凝固镁合金的强化机制和腐蚀行为。结果表明:与常规凝固镁合金铸件相比,亚快速凝固镁合金的拉伸性能及耐腐蚀性能都有了不同程度的提高;亚快速凝固铸件强度及塑性的提高是细晶强化、固溶强化和位错强化综合作用的结果,而耐蚀性的改善则主要归因于细小而均匀分布的晶粒组织,抑制了晶界处不连续分布的β相的析出,以及大量的溶质元素Al固溶于α-Mg基体中。设计了真空吸铸-阻尼冷却管法实验装置,实现了AZ91D镁合金半固态浆料的制备与铸件流变成形的一体化,得到了AZ91D镁合金半固态浆液的亚快速凝固组织,观察显示:镁合金半固态浆液具有触变性及更高的黏度,以平稳、层流的充型方式完成充型,能够有效改善成形件的质量。其凝固过程可分为两部分:“第一次凝固”—近液相线温度的金属液流经阻尼冷却通道时的凝固,产生了大量初生α-Mg固相颗粒及晶核并裹入存活于熔体而形成了半固态浆料;“第二次凝固”—半固态浆液高速射入模具型腔后,在紫铜模具的激冷作用下,残留液相亚快速凝固生成细小的二次α-Mg晶粒和β-Mg₁₇Al₁₂相,初生α-Mg固相颗粒弥散分布在由非常细小的二次α-Mg相及β相构成基体组织中,初生α-Mg固相颗粒的尺寸主要在15μm～35μm,二次α-Mg相晶粒尺寸约为6μm。在对亚快速凝固镁合金小尺寸薄片铸件凝固组织特点、凝固行为及性能研究的基础上,利用冷模离心铸造工艺制备大口径镁合金薄壁管,以此实现较大尺寸规格的镁合金铸件的亚快速凝固。制备得到了形状完整、壁厚均匀的AZ61A镁合金薄壁管,铸管显微组织晶粒得到了显著细化,形成以过饱和初生相α-Mg为主相的凝固组织,合金元素的固溶度大大提高,微观偏析及成分起伏明显减小,整体分布均衡;拉伸力学性能得到提高,塑性得到了明显改善,断口形貌存在大量拉伸韧窝,具有准解理断裂特性。更多还原

【Abstract】 Magnesium and its alloys,with a number of desirable features including low density,the highest strength-to-weight ratio of any of commonly used non-ferrous and ferrous metallic materials,better damping characteristics and shielding properties,well castability,and abundant resources,are thus very attractive for applications in the automotive,electronic and aeronautical industries.However,the magnesium alloys produced by conventional casting process exhibit the drawbacks of less than desirable strength,inferior formability,low thermal stability,inadequate creep resistance,poor oxidation resistance and inferior corrosion. Consequently,the extensive use of the magnesium-base alloys is restricted.In general,the grain size and microstructure of cast metal are directly influenced by the cooling rate. Different degrees of structural refinement and transformation can be obtained by the sub-rapid solidification technique,which presently provide high cooling rates up to 10³ K·s^-1.The microstructure changes,which involve microcrystalline structures,phases with large and non-equilibrium solid solubility,new metastable crystalline phases,can improve the mechanical properties,processability and corrosion resistance.Therefore,sub-rapid solidification process may be an effective technology to achieve better microstructures and obtain Mg alloy castings with the maximum combination of mechanical properties and corrosion resistance.In the present paper,sub-rapidly solidified Mg alloy castings were prepared by using several different methods with self-designed devices.The microstructure characteristics and formation mechanism of Mg alloy castings produced by different processes were studied.In addition,according to the investigation of the performances of the Mg alloy castings,it could be clearly seen that the final microstructures had a close relationship with mechanical properties and corrosion resistance.And then strengthening mechanisms and corrosion behavior of Mg alloy castings produced by different technique were investigated.Main research details and results are as follows:The small size AZ-series Mg alloys sheets were prepared by using sub-rapid solidification technique with the self-designed vacuum suction casting（VSC） setup.The microstructure characteristics of sub-rapidly solidified Mg alloy sheets,including grain size, phase composition and the microdistribution of alloying elements,were studied.For comparison,conventionally solidified castings were also produced and investigated.The results show that the typical micrographs of conventional castings consist of well-developed primaryα-Mg dendrites withβ-Mg₁₇Al₁₂（including divorced eutecticβphase and secondary precipitatedβ" phase） along theα-Mg grain boundaries in the form of network;In addition, grain size is quite large and its distribution is non-uniform.The alloying elements Al,Zn elements mainly distribute on grain boundaries and only a very small amount exist in grains （primaryα-Mg phase）,and thus,the microsegregation of alloying elements is severe in the conventionally solidified microstructure.Whereas,the grains of sub-rapidly solidified sheets are obviously refined,and mean grain size is only a few micrometers.The higher cooling rate there is,the smaller grain size become.The elements segregation and enrichment inherent for conventional castings are suppressed to a great extent due to high cooling rates and thus,the microstructures of sub-rapidly solidified sheets dominantly consist of supersaturatedα-Mg solid solution,which may lead to the change of lattice constant of Mg.Therefore,it can be seen that a little migration of diffraction peaks in the XRD patterns of sub-rapidly solidified sheets.The cooling rate during sub-rapid solidification process was estimated in order of magnitude by measuring the secondary dendrite arm spacing.The results show that the cooling rate of the Mg alloys sheets produced by sub-rapid solidification technology is up to about 10²～10⁴ K·s^-1.Solute redistribution mechanism,microstmcture evolution and formation mechanism under different solidification conditions are discussed systematically.The analysis shows that in sub-rapid solidification process,the movement velocity of solid/liquid interface is very fast and the solidification time is very short due to the extremely high cooling rate. Consequently,there is no time for the solute atoms near interface to enrich and diffuse sufficiently to the far distance of the melt and thus,they are captured by the high-speed moving solid/liquid interface and solubilized in the matrixα-Mg phase.In addition,under sub-rapid solidification condition,the vast majority of nuclei have no time to grow up and thus,they generate large numbers of fine equiaxed grains.A portion of nuclei can grow up and generate the petal shaped microstructures,which are essentially the primary dendrite arms. Only a small quantity of nuclei eventually generates the dendrite structure with extremely fine secondary arms showing sixfold symmetry.The mechanical properties and corrosion resistance of AZ61A Mg alloy castings produced by different technique were studied in detail and subsequently,the influence of sub-rapid solidification microstructure on these properties was investigated.Moreover,the strengthening mechanisms and corrosion behavior were discussed.The results show that the mechanical properties and corrosion resistance of sub-rapidly solidified sheets are improved in varying degrees compared with those of the conventionally solidified Mg alloy castings. The improved mechanical properties of Mg alloy castings can be ascribed to the conjoint and mutually interactive influences of the fine-grain strengthening,solution strengthening and dislocations strengthening.The improvement of corrosion resistance is mainly attributed to the refining and homogeneous distribution of grains,the disappearance of the discontinuously distributedβphase on grain boundaries,and the supersaturated matrixα-Mg solid solution containing abundant Al element.A novel semi-solid processing technique,called vacuum suction casting—damper cooling tube method（VSC-DCT method）,was used to manufacture high quality components of AZ91D Mg alloy directly from a liquid metal.The outstanding feature of the VSC-DCT process is attributed to the fact that it combines the semi-solid slurry making and component forming operation into one step and thus,the sub-rapid solidification of Mg alloy semi-solid metal（SSM） are achieved.The resulting apparent morphologies and microstructures are characterized in detail,and linked to the corresponding mold-filling behavior and subsequent solidification behavior.It is revealed that the SSM with higher viscosity can be caused to fill the mold with "solid-front fill",as compared with the liquid metal "spraying" in the conventional vacuum suction casting（CVSC） process,and the surface quality of the sheets fabricated by the VSC-DCT method is improved significantly.Solidification in the VSC-DCT process takes place in two distinctive stages;one is primary solidification in the shear-cooling system under high intensity of turbulence,and the other one is secondary solidification inside the mold cavity with high cooling rate.The final microstructures of the sheets via VSC-DCT method exhibit that the "preexisting" primary solid particles formed in the primary solidification,with the morphology of near-globules or rosettes,are surrounded by the eutectic mixture of fine secondaryα-Mg grains and fine precipitates ofβ-Mg₁₇Al₁₂ intermetallics,resulting from the rapid solidification of the remaining liquid of SSM in the secondary solidification step.The final primary solid size is within a range of 15μm～35μm as examined by quantitative metallography,and the average size of the fine secondaryα-Mg grains is about 6μm in diameter.On the basis of the experimental results and sub-rapid solidification principle presented above,a thin-walled AZ61A Mg alloy tube was fabricated by using centrifugal casting technique,and the sub-rapid solidification of large-sized Mg alloy casting was achieved.The microstructure and mechanical properties of the centrifugal casting tube were presented and discussed.The results show that the grain size of the tube is significantly refined.The eutectic transformation L→α-Mg+β-Mg₁₇Al₁₂ and microsegregation are suppressed to a great extent. As a consequence,the alloying elements Al,Zn show much higher solid solubility and the microstructure of the tube dominantly consists of supersaturatedα-Mg solid solution. Mechanical properties and corresponding fractography of the centrifugal tube are significantly improved compared with those of the conventionally solidified Mg alloy casting.The fracture surfaces of the centrifugal tube exhibit ductile fracture characteristics.更多还原