SCR工艺参数对A2017半固态合金组织的影响

采用自行设计的SCR实验机制备A2017半固态合金，研究了SCR工艺参数对A2017半固态合金组织的影响。实验结果表明，合金液浇注温度和轧辊冷却水流量共同影响辊靴间隙中半固态合金的固相率，降低浇注温度或者增大冷却水流量，辊靴间隙高固相率合金区间增长，晶粒尺寸变小；减小辊靴间隙或者提高轧辊转速提高合金层流剪切强度，利于枝晶破碎；增大出口半固态浆料冷却速率能有效抑制晶粒继续生长，保留其完整的非枝晶半固态组织。合金液浇注温度为760～780℃，辊靴间隙为4～6mm，轧辊冷却水流量200～300ml／s条件下，SCR技术可获得浆料状A2017半固态合金，凝固后坯料为均匀、细小的非枝晶等轴晶组织，晶粒大小在40～50μm之间，SCR技术是一种制备半固态加工坯料的理想方法。     

剪切力下半固态Sn-52Bi合金组织演变与塑性的研究

采用机械搅拌的半固态铸造技术制备Sn-52Bi合金,探索了不同固相率下剪切力和作用时间对半固态Sn-Bi合金组织和塑性的影响。结果表明,在近液相线的146℃(固相率为4%),搅拌速度为320 r/min和搅拌6 min的工艺条件下,制备出分布均匀、直径约10μm球状初生相的半固态Sn-52Bi合金,其伸长率为31.3%,与在180℃下的合金金属熔液直接水冷凝固而成的相比提高了74.8%。随着剪切力作用时间的延长和半固态合金浆料中固相率的增加,半固态浆料的初生相易发生团聚,所制备的半固态Sn-Bi合金初生相为分布不均的蔷薇形或椭圆形,合金伸长率下降。     

“半固态+”及其在工业中的应用

随着对半固态金属加工(以下简称半固态)技术发展认识的不断深入,提出了广义半固态以及"半固态+"概念。所谓广义半固态是指从液相线温度上熔体出发,包括固液两相区,通过强剪切熔体处理以及强制均匀凝固的思路,解决控制形核与控制长大的问题,其本质在于组织的细化与成分的均匀化,突破了半固态受合金成分的制约。"半固态+"是指以广义半固态技术为平台,将半固态技术与不同的传统加工技术结合,从而为传统加工技术转型升级提供一种新方法。半固态可以与液态成形相结合,如垂直半连铸、水平连铸、压铸、挤压铸造等;还可以与固态成形相结合,如模锻、挤压、铸轧等。     

半固态加工技术制备金属合金粉末

采用自制半固态搅拌装置研究了Sn．15Pb（质量分数，％下同）合金在搅拌条件下固相分数大于0．6时的连续冷却凝固行为。组织分析结果表明：当搅拌停止温度在合金固相线温度附近时，所获半固态组织出现非常明显的液固两相分离现象，且所获固相组织为分散的类球状颗粒。利用此现象，获得了一种新型金属合金粉末制备方法一半固态金属合金粉末制备法。采用该方法可以实现粉末颗粒内部无夹杂、无孔洞，外部裹有低熔点共晶薄膜。颗粒为以单晶方式生长的金属合金粉末。     

铜钢双金属连铸连轧工艺及设备研究

新型固-液连铸连轧技术的原理是将液态铜合金连续浇注在不锈钢带上,使之在半固态与固态钢带同时进入轧机,在压力的作用下,实现固态钢和半固态铜合金的冶金结合。固-液相复合轧制技术是以液态金属快速非平衡凝固以及半固态合金直接塑性成型为特征的,能有效地控制异种金属复杂接口反应如扩散、溶解、新相的生成和生长等的方向和限度,进而可以保证复合接口的良好结合。     

半固态镁合金微观组织定量分析的应用

为了更好地分析半固态镁合金的金相组织,应用VC 开发出半固态合金金相组织图片的定量分析系统,此系统能够有效分析半固态合金金相组织中的固相率、有效固相率、圆整度、颗粒尺寸及其等级等特征参数,并能对相连的固相颗粒进行有效识别。将此软件用于分析半固态镁合金试样的金相组织,对金相组织中固相颗粒的组分、形貌、大小等进行了描述,较好地评定了半固态合金金相组织的特征信息。经该定量分析系统测定的参数与目测结果基本一致。     

动态凝固Al-Ti-C中间合金组织与细化活性

采用CASTEX连续铸挤技术，对液．固反应法获得的Al-Ti-C中间合金熔体直接进行动态凝固成形，制备细化剂合金线材，计算模拟了动态凝固成形区的温度场和应力场，试验研究了动态凝固组织的形成机制及中间合金细化铝晶粒的特性。结果表明：熔体连续铸挤成形经历动态凝固、半固态挤压和塑性成形3个阶段，为动态凝固与成形过程；该过程对熔体的强烈剪切与热扰动作用，可改善中间合金的组织形态，显著提高其细化活性。     

剪切力对半固态Sn-Bi合金的组织演变与塑性的影响

采用机械搅拌半固态铸造技术制备Sn-62Bi合金,通过XRD、EDS等测试方法探索了不同固相率下剪切力和作用时间对半固态Sn-Bi合金组织和塑性的影响。结果表明:在半固态温度145℃、搅拌速度为500 r/min和搅拌时间为15 min的工艺条件下,制备出分布均匀、尺寸约30μm块状铋初生相的半固态Sn-62Bi合金,拉伸试验下的伸长率为39.9%,与在180℃下的合金金属熔液直接水冷凝固而成的试样相比,其伸长率提高了283.3%。当搅拌温度为150℃时,随着搅拌时间的延长,块状初生相Bi相相互摩擦和碰撞破碎,初生相的尺寸减少,分布较均匀,所制备的半固态Sn-62Bi合金的伸长率得到改善。     

半固态固溶及时效对Mn-Cu合金组织和性能的影响

采用真空感应熔炼法制备Mn65-Cu23.75-Zn3-Al3-Ni3-Fe2-Ce0.05(at%)合金。对该合金进行轧制处理,然后进行均匀化退火。分别在850℃和950~1050℃对合金进行普通固溶及半固态固溶处理,随后在430℃时效0~16 h。研究半固态固溶温度及时效时间对Mn-Cu合金组织、阻尼性能和力学性能的影响。结果表明:普通固溶合金组织由单一γ-Mn Cu相构成,而半固态固溶合金组织则由富Mn和贫Mn的γ-Mn Cu相构成,且随着半固态固溶温度的升高,贫Mn相的含量不断增加。合金阻尼性能随时效时间的延长呈现出先上升后下降趋势。在最优时效条件下,较低的半固态固溶温度可提高合金的阻尼性能,而较高的半固态固溶温度则会降低其阻尼性能。和S850合金相比,S950合金的强塑积提高了约70%。但随着半固态固溶温度的升高,合金强塑积又有所下降。     

半固态成形Al-Fe基合金组织与性能

设计了一种Al-Fe合金的化学成分,采用半固态技术进行了成形,分析了合金铸态、电磁搅拌态、挤压态的组织,检测了合金铸态、半固态成形态的力学性能。试验结果表明:合金元素的加入和凝固过程中外加电磁搅拌以后,合金组织明显细化;经过半固态成形后,合金组织进一步细化,第二相长度和厚度减小,同时尖角发生钝化,且分布较均匀,力学性能比铸态提高了79.7%。     

半固态条件下球晶生长形态的稳定性

采用类金属丁二腈 (SCN) 和H₂O配制的SCN--5%H₂O (摩尔分数) 透明合金研究了不同过冷度和搅拌速率下半固态球晶生长的形态稳定性. 结果表明,随着搅拌速率的提高, 球晶形成的孕育期急剧减小. 在低搅拌速率下, 随着过冷度的提高, 球晶形成的孕育期明 显减小; 而在高搅拌速率下, 过冷度的作用并不明显. 随着搅拌速率的增大, 球晶的固相分数先增加、后减小. 当搅拌速率增加到一定值后, 球晶组织将完全消失. 球--枝转变存在一个临界的过冷度. 采用高搅拌速率得到稳定的孤立球晶后停止搅拌并保温, 可以发现当过冷度为2.9℃时,球晶可以一直生长到100 μm以上时才失去相对稳定性, 并发生球--枝转化; 但过冷度为1.9℃时, 球晶生长到一定尺寸后将不再长大, 而且这个尺寸远远小于100 μm. 根据这些生长行为, 可以通过控制搅拌速率和过冷度从而优化半固态过程的凝固组织形态.     

Controlled Nucleation Method:A New Process for Semisolid Metal Forming

Semisolid metal forming requires special feedstock material with a fine-grained and globular structure to achieve thixotropic properties. A number of methods have been developed to produce such feedstock materials. Controlled Nucleation Method (CNM) is a new and simple, cost effective method that has been developed by the University of Queensland. The CNM process does not use the conventional stirring process, instead, it uses solidification conditions to control nucleation, nuclei survival and grain growth, thereby produce fine and globular structures suitable for semisolid forming. No specialised equipment is required. The method can produce both semisolid slurry for rheocasting and semisolid billet for thixocasting. It can be applied to a wide range of alloys and can easily be incorporated into existing metal forming installation. Semisolid slurries/billets of hypoeutectic and hypereutectic aluminium-silicon casting alloys, aluminium wrought alloys and a magnesium alloys have been successfully produced.     

AZ31镁合金连续强流变轧制成形过程温度场模拟与优化

采用数值模拟与实验相结合,对AZ31镁合金连续强流变轧制成形过程温度场进行了模拟与优化.结果表明,在倾斜板表面,合金温度从浇注口到出口逐渐趋于线性降低;在横断面上,接触倾斜板一侧合金温度比上侧低,当浇注温度大于690℃时,熔体在倾斜板出口温度高于AZ31合金液相线温度,容易发生制品断裂.在轧制变形区后滑区,主要发生半固态金属变形,合金从孔型入口到出口温度逐渐降低,半固态区间随着浇注温度的升高而增长,温度等值线发生两次弯曲,表层合金温度等值线向孔型出口凸出,而中心合金温度等值线向孔型入口凸出,其弯曲程度从中性面到孔型入口越来越明显;在轧制变形区的前滑区,主要发生固态金属的变形,温度等值线发生一次弯曲,且向孔型出口凸出.在本实验条件下,较合理的浇注温度范围在670—690℃之间.     

A2017合金半固态压缩变形力学特性

采用Gleeble 15 0 0热模拟机 ,对SCR (shearing/coolingroll)技术制备的A2 0 17半固态合金和常规铸造A2 0 17合金在不同变形温度和不同变形速率下进行半固态压缩实验 ,研究了A2 0 17合金半固态压缩变形的力学特性。实验结果表明 ,相同变形温度下 ,SCR技术制备的A2 0 17半固态合金半固态压缩变形抗力低于常规铸造A2 0 17合金半固态压缩变形抗力 ,这主要是SCR技术制备的A2 0 17半固态合金具有的均匀的近球形晶和共晶液相组成的组织优于常规铸造的枝晶组织 ,其压缩变形机制不同于常规铸造枝晶的压缩变形机制。实验结果还表明 ,SCR技术制备的A2 0 17半固态合金半固态压缩变形抗力 ,对压缩变形温度和变形速率具有敏感性 ,随着变形温度降低或者变形速率升高 ,变形抗力增加。     

On the generation of thixotropic structures during melting of Mg-9%Al-1%Zn alloy

The microstructural evolution of Mg-9%Al-1%Zn alloy in the form of mechanically comminuted chips and rapidly solidified granules under the sole influence of external heat was investigated. The mechanisms governing phase and morphological transformations in solid and semisolid states were assessed and verified by a comparison with the behaviour of both conventionally cast and cold-deformed ingots, subjected to similar thermal exposures. It was revealed that the cold deformation introduced during the mechanical fragmentation of chips and chemical microsegregation inherited from a rapid solidification by granules, represent key features which control the generation of equiaxed structures during the very initial stages of melting. Ostwald ripening and coalescence were operative in the semisolid mixture within the entire range of the liquid fraction. As a result, an increase in a volume fraction of the liquid was primarily accompanied by the reduction in the number of solid particles, while the reduction in the particle size was significantly lower than anticipated from a change in the solid/liquid ratio. Some implications for engineering the microstructure during semisolid processing are discussed.     

The solidification characteristics of 6061 and A356 aluminum alloys and their ceramic particle-reinforced composites

The solidification curve and viscosity are the two most important solidification characteristics. The solidification curves of the 6061 and A356 alloys and their ceramic particle-reinforced composites are determined by using differential thermal analysis (DTA) coupled with mathematical modeling. In applying this method, the cooling curves of the alloys and composites are first determined by using DTA, then mathematical modeling is used to simulate the cooling curves, and their solidification curves can thus be calculated. It has been found that the principal characteristics of the solidification curves of the composites and their matrix alloy are similar. The viscosities of these alloys and composites are determined by using a Searle-type viscometer. The viscosity of the semisolid slurry increases with increasing solid content. A sharp increase in the viscosity is observed during solidification, and the value of the critical solidified fraction is a function of the shear rate. Both the aluminum alloys and their respective composites displayed similar critical solidified fractions, and these values are hardly affected by the addition of the ceramic particles.     

Solidification studies on sand cast Al 6061–SiCp composites

Most of the automotive components are cast and their performance depends very much on the solidification phenomenon. Solidification is primarily a process of achieving solid crystals from the liquid melt by promoting zones possessing very high cooling rates to ensure super cooling of the melt. Till date enormous data is available as regards the solidification behaviour of popular light alloys such as Al 6061 and A 356 with regard to the casting process, mould materials used and other important processing parameters. Effect of chills on the solidification behaviour of the above materials has also been reported suggesting chills to be an important promoter of directional solidification. Directional solidification results in minimized solidification defects. However, there is a lack of information regarding the effect of chills on solidification behaviour of aluminium based metal matrix composites which are currently the most potential candidate materials in automotive industries as a replacement for conventional light alloys. In the light of the above, this work is aimed at experimentally studying the solidification behaviour of Al 6061–SiC_p castings in sand mould using copper and mild steel chills. Further, commercially available finite element analysis (FEA) software has been used to predict the cooling curves with and without the use of chills for the developed composite. The experimental and predicted cooling rates of the developed composites are not in good agreement. Use of copper chills resulted in promoting higher cooling rates during the solidification of developed composites.     

金属半固态加工

半固态加工是生产形状复杂的近终形产品的一种新方法。从半固态金属的基本物理性能,阐述了半固态金属浆液的制备、半固态金属的成形、半固态加工产品的机械性能、应用及未来发展趋势。     

Melt conditioning by advanced shear technology (MCAST) for refining solidification microstructures

Abstract

MCAST (melt conditioning by advanced shear technology) is a novel processing technology developed recently for conditioning liquid metal prior to solidification processing. The MCAST process uses a twin screw mechanism to impose a high shear rate and a high intensity of turbulence on the liquid metal, so that the conditioned liquid metal has uniform temperature, uniform chemical composition and well-dispersed and completely wetted oxide particles with a fine size and a narrow size distribution. The microstructural refinement is achieved through an enhanced heterogeneous nucleation rate and an increased nuclei survival rate during the subsequent solidification processing. In this paper we present the MCAST process and its applications for microstructural refinement in both shape casting and continuous casting of light alloys.     

Mushy/Semi-Solid Metal Forming Technology - Present and Future

Mushy, semi-solid and/or thixo processing of metals (alloys) is becoming popular as a new potential manufacturing technology for parts and components in automobile, electronic and machine industries. Internal structures and mechanical properties of those metals that include solid and liquid fractions are quite different from those of hot or molten metals. Diversified possibilities are known today to process those metals based on die casting, hot metal forming or polymer injection technologies, each of which has its own specific advantages and disadvantages. Up to now thixocasting and thixomolding have been used in industrial applications for light metal alloys. The potentials of those processes are wider by far however. They include the processing of specially designed alloys and composites, the combination of forming and joining processes as well as reduction of production costs and energy consumption.