SCR技术半固态制浆及组织形成机理

采用单辊搅拌冷却技术(Shearing-Cooling-Rolling,简称SCR)制备A2017合金半固态浆料,对SCR工艺参数对半固态浆料组织的影响以及组织形成机理进行了研究.结果表明,熔体浇注温度和辊靴型腔高度对半固态浆料组织的影响显著,降低熔体浇注温度或减小辊靴型腔高度,半固态浆料组织从粗大的枝晶和菊花晶转变为细小的等轴晶或球形晶.组织形成机理分析表明,熔体首先在工作辊和靴子表面形核生长,在液流冲击及剪切搅拌的作用下晶核脱落游离进入残余液相区长大成菊花晶,在强烈剪切搅拌作用下,菊花晶断裂破碎最终形成非枝晶组织.     

A2017半固态合金的半固态扩展成形

用单辊搅拌(SRS)实验机进行了A2017合金的半固态凝固实验,研究了A2017半固态合金组织的形成机理。在轧辊剪切力的作用下在轧辊粗糙表面生长初始枝晶,破碎后形成自由晶,自由晶自由运动和生长形成细小均匀的A2107合金球形组织。在710～750℃内浇注,可制备出具有细小、均匀球形组织的A2017半固态合金。使用在辊-靴型腔出口处的扩展成形模可实现A2017合金半固态连续扩展成形,使半固态成形与扩展挤压相结合,通过控制浇注温度得到了表面状态良好的断面尺寸为14mm×25mm的A2017扁型材料。用半固态扩展挤压方法得到的材料断裂强度为280MPa(比国标值提高100MPa);延伸率为49％(比标准值提高29％)。     

Influence of Solid Fraction Casting on Microstructure of Aluminum Alloy 6061

Semi-solid processing of AA6061 alloy near liquidus temperature with the addition of micro- and nanoparticles of same alloy may be highly attractive for small intricate shapes due to excellent mechanical properties. The present work utilizes semi-solid behavior of AA6061 alloy, which reduces macro- and nanosegregation of particles, porosity, and forming forces during the shaping process. The experiment utilizes the semi-solid slurry of different solid fractions mixed with a melt at pouring temperature range of 400 ? 640°C. The potential of solid fraction to produce semi-solid slurry has been investigated with the help of microstructure analysis, which is a crucial need for aluminum industries. The result shows that during the stirring, every dendrite modified itself to fine solid grains and dispersion of these grains takes place inside the molten metal. Alloy cooled directly from semi-solid state results in higher relative density with respect to conventional casting. With increase of solid fraction to 25%, the un-melted solid structure inside the pores and cavities in the direction of elongated grains results in the subsequent enhancement in the impact strength, hardness, and compressive strength as 19 kJ/m², 93 BHN, and 550 MPa, respectively, without any alteration in the basic metal matrix composite.     

电磁搅拌对半固态AZ91D镁合金组织的影响

研究了电磁搅拌参数对连续冷却条件下AZ91D镁合金组织的影响.结果表明:当电磁搅拌的频率达到或高于50 Hz时,半固态AZ91D镁合金浆料或坯料组织中的球状初生固相越来越多,越来越圆整;在电磁搅拌频率为200 Hz和冷却速率较低的条件下,搅拌的功率越大,半固态AZ91D镁合金组织中的球状初生固相越多,晶粒也越细小.在电磁搅拌条件下,AZ91D镁合金熔体的激烈流动导致较为均匀的温度场和溶质场、更加剧烈的温度起伏,促进了半固态AZ91D镁合金球状组织的形成.半固态重熔加热使半固态AZ91D镁合金坯料初生固相的形态发生进一步的球化.     

连续冷却搅拌A2017半固态合金流变行为研究

采用回转式熔体物性综合测试仪测定了A2017合金半固态浆料在连续冷却搅拌条件下的表观黏度变化,对其组织变化进行分析,研究了连续冷却搅拌条件下A2017合金半固态浆料的流变行为.实验结果表明,连续冷却搅拌条件下,A2017合金半固态浆料的表观黏度随固相率的增大而增大;在固相率一定的情况下,降低冷却速度或增大剪切速率,均导致表观黏度降低.显微组织观察表明,半固态浆料的流变行为与其微观组织结构有很强的关联,降低半固态浆料温度,浆料固相颗粒含量增加,表观黏度增大;降低冷却速度或增大剪切速率有利于半固态浆料固相颗粒形态向非枝晶形态的转变,因而半固态浆料表观黏度降低,流动性好.     

工艺参数对MIG焊铝铜合金焊缝组织的影响

研究了电磁搅拌工艺参数对2219铝铜合金脉冲MIG焊焊缝形状和组织的影响．结果表明,随着磁感应强度增加,焊缝枝晶数量减少,等轴晶数量增加,焊缝由指状熔深变为成形良好的椭圆形熔深．当磁感应强度和频率匹配合适时,焊缝成形良好,焊缝组织几乎为细小的等轴晶．但随着磁感应强度继续增加,焊缝成形变差,焊缝等轴晶数量减少,枝晶数量增多．电磁搅拌频率对焊缝成形影响不大,频率较低和较高时都会降低焊缝组织的细化效果．     

A2017合金半固态压缩的变形机制和成形性能

利用Gleeble-1500热学-力学模拟机,对A2017半固态合金进行半固态压缩变形实验,分析了应力-应变曲线和组织变化,研究了压缩变形机制及成形性能结果表明,用单辊搅拌冷却(SCR)技术制备的A2017半固态合金的组织为细小均匀的非枝晶等轴晶,二次加热后可转化为均匀的球形晶和共晶液相组成的半固态组织;A2017合金半固态压缩变形的塑性好、变形抗力低;随着变形温度的升高或者变形速率的降低,变形的抗力降低.在稳定的流动变形阶段,A2017合金的半固态变形机制主要由液相流动和固相颗粒的转动与滑动组成,触变性能稳定,最大半固态加工变形范围为60%左右.     

Dynamical Solidification Behaviors and Metal Flow during Continuous Semisolid Extrusion Process of AZ31 Alloy

In this paper, a novel near-net-shape forming process, continuous semisolid extrusion process (CSEP) of AZ31alloy was proposed, and the dynamical solidification behaviors and metal flow during the process were firstly investigated.During casting AZ31 alloy by this process, non-uniform microstructure distributions and nonequilibrium solidification region near the roll surface were found in the roll-shoe gap.Microstructural evolution from dendrite to rosette and spherical grains was observed during the casting by CSEP.Casting temperature,roll-shoe gap width and cooling ability have great effect on casting process and metal flow, so these factors should be carefully controlled, a proper casting temperature of 710-750℃ is suggested.The white a phases were strongly stretched during the processing, and the remnant liquids are correspondingly distributes along the solid phase boundaries and also show stripped lines.     

ZL201合金液相线铸造与二次加热的合金组织

为了研究半固态金属的成型规律,采用电子显微镜及图像分析仪,研究了近液相线半连续铸造ZL201合金的微观组织及其在二次加热过程中的变化.研究表明,在液相线温度下半连续铸造的ZL201合金组织为均匀、细小的蔷薇状组织,晶粒的平均等积圆直径为43.6μm,晶粒平均圆度为1.88.经二次加热后,铸造组织逐渐转变为等轴晶.在620℃下加热20min后,晶粒平均等积圆直径为111μm,晶粒平均圆度1.42.在640℃下加热20min后,晶粒平均等积圆直径为108μm,晶粒平均圆度1.58.研究表明,近液相线半连续铸造可以获得理想的ZL201合金半固态坯料.     

The fragmentation of primary dendrites during shearing in semisolid processing

Semisolid slurries with non-dendritic microstructure can be produced by solidification under forced convection. Dendrite fragmentation is a commonly accepted mechanism for shearing semisolid slurry. However, it requires from experimental observation to confirm the fragmentation of dendrite arms occurring during shearing process. Experiments are undertaken to investigate the dendrite fragmentation in Sn-15wt%Pb alloy under shearing by virtue of a twin-screw extruder together with the wetting and penetration of pure tin grain boundaries submerged and sheared in the semisolid slurry of Sn-15wt%Pb alloy. It is found that the primary dendrites can be fragmented rapidly into small particles and there are penetration grooves existing in the partially fragmented dendrites. Such grooves are similar to that found at the grain boundaries in pre-cast pure tin polycrystals, which have been submerged and sheared in the semisolid slurry of Sn-15wt%Pb alloy.     

Grain refinement of AZ91 alloy by introducing ultrasonic vibration during solidification

Ultrasonic vibration was introduced into the solidification of AZ91 alloy. Various microstructures were produced in this alloy using ultrasonic vibrations at different temperatures of the melt. The coarse dendrite microstructures were obtained with ultrasonic vibrations at temperatures below the liquidus temperature. The fine uniform grains were achieved under ultrasonic vibrations during the nucleation stage, which was mainly attributed to the cavitation and the acoustic flow induced by the ultrasonic vibration.     

低频电磁场对奥氏体不锈钢铸坯组织的影响

在工业实验中研究了低频电磁场对水平连铸奥氏体不锈钢组织的影响,结果表明:在低频电磁场作用下,合适的电磁搅拌参数使奥氏体不锈钢宏观组织在一定程度上得到了明显改善,铸坯的柱状晶和等轴晶得到了显著的细化,消除了穿晶现象,等轴晶区扩大,中心缩孔、中心疏松级别明显降低;多次实验发现:对于奥氏体不锈钢,所需的搅拌强度应高于一般钢种,即使搅拌器的中心磁感应强度达900GS平均值(幅值达到1413GS),采用3～4Hz的频率,搅拌后对钢液的组织影响较小;在同样的磁场强度下,不锈钢液由于粘度大,因而其转速比碳钢液的转速约低20%～30%。     

Modelling mixed columnar-equiaxed solidification with melt convection and grain sedimentation – Part I: Model description

Part I of this two-part investigation presents a volume-averaging multiphase solidification model that accounts for mixed columnar-equiaxed solidification, non-dendritic and dendritic crystal growth, nucleation of equiaxed grains, columnar primary dendrite tip tracking, melt flow, sedimentation of equiaxed crystals, and their influence on macrostructure and macrosegregation. Five distinct thermodynamic phases (phase regions) are defined: solid dendrites in equiaxed grains, the interdendritic melt between equiaxed dendrites, solid dendrites in columnar trunks, the interdendritic melt between trunk dendrites, and the extradendritic melt. These five phase regions are quantified by their volume fractions and characterized by their solute concentrations. The five phase regions are grouped into three hydrodynamic phases: equiaxed grains consisting of solid dendrites and interdendritic melt, columnar trunks consisting of solid dendrites and interdendritic melt, and extradendritic melt. The extradendritic melt is separated from the interdendritic melt with a grain envelope, whose profile connects the primary, secondary or tertiary dendrite tips to form a ‘natural’ enclosure of the equiaxed grains or columnar trunks. The envelope is further simplified as a volume-equivalent sphere for equiaxed grains, or as volume-equivalent cylinder for columnar trunks by use of morphological shape factors. Expansion of the envelopes during solidification is determined by dendrite growth kinetics, using the Kurz–Giovanola–Trivedi model for growth of columnar primary dendrite tips and the Lipton–Glicksman–Kurz model for growth of columnar secondary dendrite tips (radial growth of the columnar trunk) and equiaxed primary dendrite tips. The solidification of the interdendritic melt is driven by the supersaturation of the interdendritic melt and governed by the diffusion in the interdendritic melt region. Illustrative process simulations and model verifications are presented in Part II.     

Sedimentation speed of a free dendrite growing in an undercooled melt

Free equiaxed dendrites in solidifying alloy melts are subjected to hydrodynamic effects as a result of gravity. The sedimentation of dendrites is one such effect and believed to be a cause of macro segregation in partitioning alloys. A novel computational model is proposed to estimate the settling speed of free dendrites at moderate Reynolds numbers. Growth of the dendrite, momentum changes, internal solid fraction evolution within a spherical dendrite envelope of changing diameter, and surface morphology of the dendrite while settling are taken into account in the development of the model. Comparison with results from a series of equiaxed dendrite settling experiments, on solidifying transparent alloy analogues to metals, shows good agreement between predicted and experimental settling speeds. The correlation between surface morphology of the dendrite which affects drag force and the physical parameters of the settling dendrite is studied. The feasibility of applying the proposed model to metallic systems is also explored and the outlook is positive.     

Structure refinement of pure Mg under pulsed magnetic field

Abstract

A pulsed magnetic field (PMF) was introduced into the solidification of pure Mg. Fine uniform equiaxed grains are acquired in the whole ingot from the PMF treatment, in contrast with the coarse columnar grains observed in conventional casting, and the average grain size is refined to 260 μm with a 200 V PMF treatment. Pulsed magnetic field increases melt convection during solidification, and the violent agitation causes warmer liquid to fracture the tip of columnar dendrites or to break off dendrite branches to promote the formation of an equiaxed structure, with the broken pieces transported into the bulk liquid acting as nuclei. In addition, the uniform temperature field resulting from the stirring increases the likelihood of nuclei survival. The Joule heat effect also participates in the structure refinement. The pure Mg produced with a 200 V PMF treatment exhibits improved mechanical properties, such as the ultimate compressive strength (227 MPa) and fracture strain (33·2%).     

电磁搅拌作用下CoCrMo合金熔模铸件凝固细晶研究

目的 研究电磁搅拌对CoCrMo合金熔模铸件晶粒尺寸的影响,解决熔模铸造CoCrMo合金铸件晶粒粗大的问题。方法 将CoCrMo合金熔化后,在其凝固过程中分别施加不同工艺参数的电磁搅拌,并对其凝固后的组织进行表征分析。同时,采用有限元法对电磁搅拌在金属熔体中的电磁场和流场进行数值模拟。结果 在不同的电磁搅拌参数下,CoCrMo合金铸件凝固组织出现了不同程度的细晶效果,浇道处的细晶效果优于铸件试棒处的。铸件试棒处的晶粒尺寸最小能控制在1 mm以下,等轴晶率最高能提升至31%。数值模拟结果表明,在电磁搅拌过程中,铸件试棒的磁场、电流和洛伦兹力都呈周期性变化,铸件试棒内部的流速随搅拌时间的延长而增大,最后趋于稳定。结论 电磁搅拌对CoCrMo合金的凝固组织产生了明显的细化效果,促进了柱状晶向等轴晶转变。电磁搅拌的时间越长,铸件凝固组织的细化效果越好,铸件厚大部位的细晶效果越显著。结合实验结果和数值模拟结果发现,在电磁搅拌过程中,熔体流动引发枝晶断裂是晶粒细化的主要原因,而电磁场促进异质形核为次要原因。     

Influence of laser surface remelting on microstructure and degradation mechanism in simulated body fluid of Zn-0.5Zr alloy

In this study, the Zn-0.5 wt%Zr (Zn-Zr) alloy was treated by laser surface remelting (LSR), and then the microstructure and degradation mechanism of the remelting layer were investigated and compared with the original as-cast alloy. The results reveal that after LSR, the bulky Zn₂₂Zr phase in the original Zn-Zr alloy is dissolved and the coarse equiaxed grains transform into fine dendrites with a secondary dendrite arm space of about 100 nm. During the degradation process in simulated body fluid (SBF), the corrosion products usually concentrate at some certain areas in the original alloy, while the corrosion products distribute uniformly and loosely in the LSR-treated surface. After removing the corrosion products, it was found that the former suffers obvious pitting corrosion and then localized corrosion. The proposed mechanism is that corrosion initiates at grain boundaries and develops into the depth at some locations, and then leads to localized corrosion. For the LSR-treated sample, corrosion initiates at some active sites and propagates in all directions, corrosion takes place in the whole surface with distinctly uniform thickness reduction, while the localized corrosion and peeling of bulky Zn₂₂Zr particles were eliminated. The electrochemical results also suggest the uniform corrosion of LSR-treated sample and localized corrosion of original sample. Based on the results, a new approach to regulate the corrosion mode of the biodegradable Zn alloy is proposed.     

Microstructure and interfacial reactions of soldering magnesium alloy AZ31B

In this paper, economic and innoxious solder alloys with low melting temperature were designed for AZ31B. Their chemical composition and relevant parameters were investigated for a high-performance structure of bonding region. Results of microstructure observation showed that Zn-enriched phases disappeared and α-Mg existed in the joints in the form of coarse dendrites by increasing the concentration of Mg in the solder alloys. Water cooling with a high cooling rate was adopted in experiments. Experimental research showed that high cooling rate restricted the grains of α-Mg as the equiaxed dendrites, which was about 1/5 of the coarse dendrite but their number was more than 40–50 times. Both morphology with typical fracture and the analysis on X-ray diffraction fracture indicated that equiaxed dendrites significantly improved the mechanical property of the joints. Necking phenomenon occurred in the bonding region was in favor of the improvement of joint shear strength.     

Grain Size Control of Semisolid A356 Alloy Manufactured by Electromagnetic Stirring

To investigate the possibility of substituting the mechanical stirring system with electromagnetic stirring (EMS) system for aluminum rheo die-casting, the EMS under the different stirring cooling conditions was carried out. It was found that in the early period of solidification, the dendrite breakages led to a fine primary phase. When dendrites grew coarsely, the effect of ripening on grain size overwhelmed that of dendrite breakage. It was also found that the high cooling rate favored large nucleation rate, and led to a fine primary phase. But high cooling rate also made the growth rate of the dendrite arm, which prevented the dendrite arm from being sheared off. Therefore there were a suitable stirring time and suitable cooling rate to obtain the best rheo die-casting structure. Qualified semisolid A356 aluminum alloy was successfully manufactured with short time EMS.     

Microstructure and dendrite morphology of Sip/ZA27 composite

Abstract

The effects of pouring temperature, Si content and P modification on the microstructures and dendrite morphologies of Si_p/ZA27 in situ composites have been investigated. Especially, the dendrite morphologies have been visually shown through observation of shrinkage porosity microstructure. The results indicate that the primary α-Al grains change from equiaxed dendrites to parallel distributed columnar-like dendrites as the pouring temperature rises from 575 to 800°C. The change tendency in dendrite morphology with increasing Si content is opposite to that with rising pouring temperature. However, these parallel distributed dendrites with highly anisotropic morphology are not feathery grains. The P modification promotes the formation of feathery grains, and this kind of grains can form when the P modified 4·76 wt-% Si_p/ZA27 composite is poured at 750°C. There are four lines of secondary dendrite arms around one primary trunk of a feathery grain. Two of them are located in the twin plane (111), and they are easier to grow up than the other two. Therefore, the feathery grains are always in a laminar structure that is overlapped by two-dimensional structured laminar dendrites. The formation process and three-dimensional structure of the feathery grains have also been discussed.