凝固速度对金属基复合材料组织和力学性能的影响

系统介绍了凝固速度对金属基复合材料组织和力学性能的影响，颗粒增强金属基复合材料凝固过程中，固／液界面向前推移时，存在某一临界速度，当实际凝固速度大于临界速度时，颗粒被固－液界面裹入，从而使颗粒在基体中均匀分布，临界速度的大小是地强颗粒的尺寸及含量，凝固界面前沿的温度梯度，凝固速度，界面的表面张力，材料的熔化潜热，导热性和粘滞系数等多种因素的函数。同时，凝固速率强烈地影响金属基复合材料的显微组织，随     

金属铽蒸馏速度的实验测定及理论计算

本文在５×１０－ ２ ～５×１０－ ３ Ｐａ真空条件下和１８２３～２０２３Ｋ 温度范围内对金属铽的蒸馏速度进行了实验测定及理论计算。对比分析了实验测定结果与理论计算结果的差别及其原因，提出了提高金属铽蒸馏速度的措施。     

45号钢二次枝晶臂间距与冷却速度的关系

本文利用可控平均冷却速度的单向凝固实验炉测定了４５号钢在不同平均冷却速度下的冷却曲线，由此得到其在凝固期间的平均冷却速度Ｒ，相应地测量了不同平均冷却速度下试样的二次枝晶臂间距Ｓ２，通过回归，得到了如下关系式：Ｓ２＝７２７Ｒ－０．４１，并将引式成功地应用于实际生产中。     

搅拌剪切作用下凝固组织演变过程的数学描述

分析了凝固组织在搅拌剪切作用下的有效溶质分配系数,将凝固速度和搅拌转速作为凝固组织演变过程的主要因素,建立了凝固组织演变与凝固速度和搅拌转速的数学模型.结果表明:搅拌转速和凝固速度对凝固组织的形貌有较大影响,增大搅拌转速和降低凝固速度有利于提高形状因子.通过与实验结果比较,数学模型与实验结果基本一致.     

新材料与新加工技术(中)

三、铝合金的快速冷却凝固及特性金属铸造时的凝固速度与浇注速度,铸型形状及铸件尺寸有很大关系。最近研制发展的薄壁铸造和压铸中,冷却速度可达到每秒数十度。亨特连续铸造轧制板坯法中,冷却速度达到了数百度/秒。随着凝固冷却速度的逐渐增大,平衡凝固组织破坏,枝晶间距变小,结晶组织细化,合金元素在铝中的固溶极限扩大。另外有时也会出现平衡图上没有的新的非平衡相。     

用差分法求解金属凝固传热问题的一个注记

本文引入固相率指数的概念,并假定该指数是固相率的线性函数,通过适当选取该函数,找到了一种求解金属凝固传热问题时使差分解的凝固速度接近分析解的方法。并发现,对于合金凝固,在固相线温度一定的情况下,液相线温度的高低对凝固速度几乎没有影响。同时,探讨了该法对连铸传热差分计算的影响。     

自耗式振动钢带传热熔化过程的数值模拟

  为了提高铸坯凝固组织的均匀性,减轻宏观偏析,提出了自耗式振动激发金属液形核技术。当振动钢带喂入钢液时,保护渣有可能随之卷入,为此开展了传热数值模拟研究。通过试验可知,对于50mm×5mm的钢带,运动状态下过热度为10℃时,喂入速度小于0.36m/min时下缘中心点的温度能够稳定在钢带的液相线温度以上;当喂入速度小于0.102m/min时,位于钢渣界面2mm以上的钢带窄边表面中心点的温度通过轴向传热能够达到或超过保护渣的熔化温度,此时钢带附近的保护渣有可能呈熔融状态,因此不会被带入钢液内。     

双辊铸轧过程轧制力计算的研究

 在对熔池内金属凝固和变形的机理分析的基础上,将铸轧区域分为2个求解区域。采用流体力学中Navier Stokes方程并引入流函数,对液相区、糊状区的流变特性进行分析并建立相应的速度场,同时引入固相百分率计算模型和粘度计算模型,根据速度边界条件推导出该区域单位压力分布解析式。对于凝固区的铸轧力建模仍沿用传统热轧模型。利用上述模型进行铸轧力计算,计算结果与实测结果吻合较好,为建立双辊铸轧过程铸轧力控制策略的提供了有力的支撑。     

圆铸锭连续铸造结晶规律性的探讨

连续铸造铸锭时靠冷却水将热量带走,使熔体金属凝固。金属从熔体转变成固体的结晶过程,直接影响到铸锭的质量。研究结晶过程中铸锭直径、水冷强度、液穴形状、铸造速度、结晶速度以及金属物理性能间的相互关系的问题,称之为结晶规律性。(一)液穴形状从熔体金属转变成固体金属的固相线形状,称之为液穴形状。正常铸造时液穴形状保持一定;当铸造速度、金属物理性能和冷却条件改变时,液穴形状就相应改变。     

基于均匀形核的金属液滴凝固过程

研究了均匀形核的金属液滴凝固过程,应用渐近分析法求得金属液滴内晶核生长数学模型的渐近解,分析了表面张力、界面动力学参数、初始晶核尺寸和过冷度对晶核界面生长速度、晶核半径以及液滴凝固时间的影响.在一定的过冷条件下,表面张力和界面动力学参数显著减缓了晶核界面生长速度.在凝固开始的很短时间内晶核界面生长速度迅速上升,当速度上升到最大值后,随着晶核半径的增大,界面生长速度逐渐减慢,表面张力和界面动力学参数对晶核生长速度的作用也逐渐减小.过冷度越大,液滴凝固时间越短.经过在开始的瞬变凝固阶段之后,温度场从设定的初始分布迅速地调整为由过冷度、表面张力、界面动力学参数等所确定的特定温度分布.      

Improving solidification pattern of ESR ingots combined with energy savings

Most of the metallurgical effects resulting from electroslag remelting of metal may be divided into two groups, namely, the effects due to the slag/metal reactions taking place and the effects due to the special solidification conditions characteristic of this process. Solidification of ESR ingots takes place progressively as heat is removed from the liquid metal pool via the mold walls. By careful matching of the melting rate with the freezing rate, the desired shallow metal pool is attained, leading to the well known directional solidification pattern with consequent improvement in properties of the steel. The choice of power parameters is limited by a compromise between the need for a high melting rate for economic reasons (costs) which may tend to give a rather deep metal pool and the need for a shallow metal pool to obtain optimum metallurgical properties. In this process only a relatively small amount of the total energy input is actually utilized to melt down the metal. The major part of the energy is lost from the slag and metal pool to the water cooled mold. In this paper the results of numerical and experimental investigations are presented, setting out a simple method of saving energy and controlling the solidification pattern of the ingot. This method involves the addition of solid particles to the melt to utilize the surplus energy evolved in the central area of the slag bath.     

纤维/金属复合材料凝固时溶质偏析及数值计算方法

根据合金凝固过程中的守恒定律和传输原理,建立了纤维增强金属基复合材料在凝固过程中溶质传递的数学模型,并建立和推导了复合材料凝固时溶质偏析的数值计算方法。     

Study on Factors Affecting the Structure of High SpeedSteel Ingot Produced by ESR

ＳｔｕｄｙｏｎＦａｃｔｏｒｓＡｆｆｅｃｔｉｎｇｔｈｅＳｔｒｕｃｔｕｒｅｏｆＨｉｇｈＳｐｅｅｄＳｔｅｅｌＩｎｇｏｔＰｒｏｄｕｃｅｄｂｙＥＳＲＬｉＺｈｅｎｇｂａｎｇ；ＣｈｅＸｉａｎｇｑｉａｎＡｂｓｔｒａｃｔ：Ｔｈｅｉｎｆｌｕｅｎｃｅｏｆｔｈｅｍｅｔａｌｐ...     

气雾化过程中合金熔滴的凝固行为研究进展

气雾化制粉以其粉末球形度高、氧含量低等优点已经成为现在一种重要的粉末制备方法,制粉过程可粗略分为破碎和凝固两步,但其工艺本身是多相流相互耦合的复杂过程,因此人们对其机理尚在进一步研究中。本文主要描述了凝固阶段熔滴的热交换与凝固行为,并对凝固过程做了详细的介绍。     

冶金凝固过程试验研究方法及应用

 冶金生产条件下的金属凝固问题极为复杂,除了多组元合金凝固过程具有的非平衡自组织现象之外,还受到几何尺寸、异质相、物理场等多种因素影响,研究难度很大。由于凝固对最终制品质量有显著影响,因此研究生产条件下的金属凝固过程,认识其组织、缺陷演变规律,进而优化工艺参数具有重要价值,而试验研究是发现现象、总结规律、验证数值模型的基础和关键,作用无可替代。总结了生产条件下金属凝固过程常用试验研究方法,重点关注了物理模拟、热模拟及工业试验法。物理模拟包括透明物质模拟、水模拟及低熔点金属模拟等,在观察凝固现象、揭示机理方面具有优势,但不能直接反映实际金属在冶金凝固过程中的现象和规律。热模拟方法则直接采用目标金属为试验材料,基于温度场的相似性进行试验模拟研究,能够直接反应冶金凝固过程,是一种成本较低且更为接近实际情况的研究手段。工业试验是验证模型和新技术不可或缺的研究手段,但是也存在成本较高、实施困难、周期长的特点。总体来说,几种试验研究方法各具特点和适用领域,综合运用多种方法开展研究,并进一步融合和发展新的研究手段是未来冶金凝固过程研究方法的发展趋势。     

Effect of welding variables and solidification substructure on weld metal porosity

Porosity is defined as cavity-type discontinuities formed by gas entrapment during solidification. Causes of porosity in fusion welds are the dissolved gases in weld metal and welding process variables that control the solidification rate. To study the mechanisms of porosity formation in weld metal, single-pass gas tungsten-arc weld metal was produced using the bead-on-plate technique on three nickel-copper alloys (80 wt pct Ni-20 wt pct Cu, 65 wt pct Ni-35 wt pct Cu, 35 wt pct Ni-65 wt pct Cu). Four different welding speeds were used under various amounts of nitrogen content in argon-shielding atmosphere. A qualitative model was proposed to characterize the effect of welding variables and solidification substructure on bulk and interdendritic porosity formation. Increasing amounts of nitrogen gas (from 0.2 pct to 6.0 pct in volume) introduced in argon-shielding atmosphere increased the amount of porosity in weld metal. The amount of bulk and total porosity increased as the solubility of nitrogen in the weld metal alloy decreased. The solidification rate of the weld pool is the most important factor controlling the mechanism of porosity formation. The observed amount of bulk pores in this study increased with the increase of welding speed; that is, if the time is insufficient for dissolved and evolved gases to escape during solidification, porosity will result. However, a decrease in the amount of interdendritic pores was observed with increasing welding speed in the 80Ni-20Cu and 35Ni-65Cu alloys. This decrease can be related to the effect of solidification rate on the balance between the disjoining pressure, resistance of the liquid film to be disrupted, repulsion of the bubble from the solidification front, and the hydrodynamic force resisting the movement of the bubble. This balance determines the ability of the cellular solidification front to “equilibrium” capture the pores. Furthermore, the observed decrease of interdendritic porosity with increasing welding speed (80Ni-20Cu and 35Ni-65Cu alloys) can also be related to the time for nucleation and growth of pores in the molten weld metal and their entrapment in the interdendritic channels of a dendritic solidification front. This phenomenon is considered a “nonequilibrium capture” of pores. On the other hand, the 65Ni-35Cu alloy that exhibited a structural transition in solidification substructure with the variation of welding speed showed a slight increase in the amount of interdendritic pores. This increase was correlated to the change of pore-capture mechanism from an equilibrium to a nonequilibrium mode as the solidification substructure changed from cellular to cellular dendritic. To substantiate that the controlling mechanism of interdendritic porosity formation is the nonequilibrium capture, a good correlation between the measured mean pore radius and the interdendritic arm spacing was found.     

模具钢电渣重熔铸坯凝固组织形貌的分形特征

为了更为精细化地表达及控制铸坯质量，以H13模具钢电渣重熔铸坯凝固组织为研究对象，引入分形维数对其主体形貌特征进行定量描述。结果表明，基于数盒子法计算得到的分形维数可定量表征凝固组织形貌的自相似复杂程度，其值从柱状晶向中心等轴晶先减小后增大；凝固组织分形维数可作为衡量铸坯偏析程度（偏析率大小）的指标，且分形维数越大，对应区域偏析率越小，偏析越轻；通过凝固组织分形维数、偏析率和偏析点平均面积与二次枝晶间距关系的研究发现，使用二次枝晶间距表征铸坯凝固组织形貌差异并由此反映偏析程度的方法存在局限性。     

Microsegregation and Secondary Phase Formation During Directional Solidification of the Single-Crystal Ni-Based Superalloy LEK94

A multicomponent phase-field method coupled to thermodynamic calculations according to the CALPHAD method was used to simulate microstructural evolution during directional solidification of the LEK94 commercial single-crystal Ni-based superalloy using a two-dimensional unit cell approximation. We demonstrate quantitative agreement of calculated microsegregation profiles and profiles determined from casting experiments as well as calculated fraction solid curves with those determined in differential thermal analysis (DTA) measurements. Finally, the role of solidification rate on dendrite morphology and precipitation of the secondary phases is investigated and a new measure of the dendrite morphology is presented to quantify the effect of back diffusion on the amount of secondary phases.     

Evaluating the Properties of Dissimilar Metal Welding Between Inconel 625 and 316L Stainless Steel by Applying Different Welding Methods and Consumables

The current work was carried out to characterize welding of Inconel 625 superalloy and 316L stainless steel. In the present study, shielded metal arc welding (SMAW) and gas tungsten arc welding (GTAW) with two types of filler metals (ERNiCrMo-3 and ERSS316L) and an electrode (ENiCrMo-3) were utilized. This paper describes the selection of the proper welding method and welding consumables in dissimilar metal joining. During solidification of ERNiCrMo-3 filler metal, Nb and Mo leave dendritic cores and are rejected to inter-dendritic regions. However, ERSS316L filler metal has small amounts of elements with a high tendency for segregation. So, occurrence of constitutional super-cooling for changing the solidification mode from cellular to dendritic or equiaxed is less probable. Using GTAW with lower heat input results in higher cooling rate and finer microstructure and less Nb segregation. The interface between weld metal and base metal and also unmixed zones was evaluated by scanning electron microscopy and energy dispersive X-ray (EDX) analysis. Microhardness measurements, tensile test, and Charpy impact test were performed to see the effect of these parameters on mechanical properties of the joints.     

The solidification of pure metals (and eutectics) under uni-directional heat flow conditions: II. Solidification in the presence of superheat

The generalized integral-profile method previously developed² to predict the solidification rates of pure metals and eutectic alloys under a range of cooling conditions has been extended to treat solidification rates in the presence of super heat. Heat transfer within the liquid metal has been characterized in terms of a single parameter for which a heat balance on the liquid yields an ordinary nonlinear equation. This equation is additional to the two equations derived previously for the growth of the solid metal, and the three equations have been solved simultaneously using a Runge-Kutta technique. Solutions have been obtained for heat transfer conditions that can be reproduced accurately in laboratory experiments. The results of a series of such experiments are also presented and shown to agree very well with the theoretical predictions. In carrying out the analysis, it has been shown that a number of different cooling and solidification modes can occur during the solidification of superheated liquid metal, the solidification process following one of two possible routes through these modes. This presents a useful approach to the analysis of this solidification problem and, indeed, of more complicated problems since the logic of the computer program used in the analysis is closely related to the logic of the solidification process. The work described in this paper was carried out while the authors were with the John Percy Research Group in Process Metallurgy at Imperial College, London.