INTERFACE MORPHOLOGIES AND SOLUTE SEGREGATION OF Al-Li ALLOY 8090 DURING UNIDIRECTIONAL SOLIDIFICATION

The solid-liquid interface morphology and solute segregation behaviour of AI-Li alloy 8090during unidirectional solidification were studied by the liquid metal quenehing method undervaried processing conditions.When solidification rate,RO.75 mm/min (temper-ature gradient,G_L=130℃/cm),the structure revealed of planar or dendritic interfacerespectively.With the increase of R,the interface morphology becomes cellular from planargradually,within a narrow range.And the greater the R,the,finer the dendrite.Segregationof element Cu and impurity elements Fe and Si are quite severe,the interface morphologymarkedly influences on solute segregation.During solidification at coarse dendrite interface,their segregation ratios are rather great and solidified structure is coarse.     

As-cast structure refinement of Ti-46Al alloy by hafnium and boron additions

The influence of Hf and B on the solidification structure of cast Ti-46Al alloys was investigated. The results show that the coupling effect of Hf and B changes the solidification structure morphology and strongly refines the grain size. When the Hf+B contents were increased from 0 + 0.0 to 3 + 0.2, 5 + 0.6 and 7 + 1.0 (in at. %), the solidification structure morphology changed from coarse columnar dendrite to fine columnar dendrite,then to equiaxed dendrite, and further to fine near granular grain whilst the average grain size decreased to 20 μm.It is concluded that the columnar dendrite refinement is due to the effect of Hf and B on the decrease of Al diffusion coefficient in the melt. The fine near granular grain formation is attributed to the combined constitutional supercooling formed by AI and B segregation that is strengthened by Hf and B additions at the solid/liquid interface uring solidification, and the TiB2 precipitates acting as heterogeneous nuclei.     

碳对镍基单晶高温合金AM3凝固组织的影响

研究了碳(C)对第一代镍基单晶高温合金AM3显微组织的影响。结果表明,随着碳含量的增加,枝晶形貌和间距无明显变化,合金中共晶的数量明显减少,一次碳化物逐渐增多。该合金中一次碳化物形貌通常为块状、骨架状和汉字状。当含碳量较高时,碳化物形貌为由骨架状连接形成的网状碳化物(汉字状碳化物)。     

Phase-field-lattice Boltzmann simulation of dendrite growth under natural convection in multicomponent superalloy solidification

The thermosolutal convection can alter segregation pattern,change dendrite morphology and even cause freckles formation in alloy solidification.In this work,the multiphase-field model was coupled with lattice Boltzmann method to simulate the dendrite growth under melt convection in superalloy solidification.In the isothermal solidification simulations,zero and normal gravitational accelerations were applied to investigate the effects of gravity on the dendrite morphology and the magnitude of melt flow.The solute distribution of each alloy component along with the dendrite tip velocity during solidification was obtained,and the natural convection has been confirmed to affect the microsegregation pattern and the dendrite growth velocity.In the directional solidification simulations,two typical temperature gradients were applied,and the dendrite morphology and fluid velocity in the mushy zone during solidification were analyzed.It is found that the freckles will form when the average fluid velocity in the mushy zone exceeds the withdraw velocity.     

Enrichment characteristic of carbon atoms in solid-liquid zone of high carbon steel under different directional solidification rates

To reveal the formation mechanism and main influencing factors of C-segregation in high carbon steel under different solidification rates (40, 80, 160, 200 and 320 µm·s⁻¹), the enrichment characteristics of carbon atoms in the solid-liquid zone of Fe-0.61%C steel were studied using a zone melting liquid metal cooling apparatus and electron probe microanalysis. The relationships among micro-segregation of carbon atoms, solid-liquid interface morphology and solidification rate were fully discussed. The results show that large dendrite spacing and a slow-moving dendritic interface create less resistance and more time for the migration of interdendritic carbon atoms to liquid zone. This results in the continuous enrichment of carbon atoms in liquid zone, further expands the solid-liquid temperature range, prolongs the solidification time of molten steel, and causes the formation of carbon micro-segregation at the solidification end as the solidification rate is 40 µm·s⁻¹. Conversely, abundant and elongated secondary dendrite arms with small spacing seriously impede the diffusion of interdendritic carbon-rich molten steel to liquid zone. As a result, there is only obvious dendrite segregation, but little difference in the carbon content along the solidification direction as solidification rate exceeds 200 µm·s⁻¹.

     

一种镍基单晶高温合金的高温度梯度定向凝固组织及枝晶偏析

采用双区加热和液态金属冷却法 (LMC) 相结合, 对一种含4%Re (质量分数) 的镍基单晶高温合金进行了高温度梯度定向凝固. 结果表明: 与传统的“ 高速凝固法 (HRS) ” (温度梯度G=20-40 K/cm, 抽拉速率V=50-100 μm/s, 一次枝晶间距 λ₁=200-400 μm)相比, 该技术可以显著提高凝固界面前沿的温度梯度 (G=238 K/cm) 和抽拉速率 (V=500 μm/s). 随着抽拉速率的提高, 凝固界面形态呈现出平面、胞状、粗大枝晶和细枝晶形态, 一次枝晶间距不断减小, 通过固态相变析出的γ' 强化相也被显著细化, 当G=238 K/cm, V=500 μm/s时, λ₁和枝晶干γ' 相平均尺寸分别减小到61.3和0.04 μm. 电子探针测定表明, 随着抽拉速率的提高, 枝晶偏析呈现先增大后减小的趋势. 这是高温度梯度条件下, 固相反扩散作用强烈影响元素在枝晶中分布的结果.     

Influence of directional solidification variables on primary dendrite arm spacing of Ni-based superalloy DZ125

The primary dendrite morphology and spacing of DZ125 superalloy have been observed during directional solidification under high thermal gradient about 500 K/cm. The results reveal that the primary dendrite arm spacing decreases from 94 μm to 35.8 μm with the increase of directional solidification cooling rate from 2.525 K/s to 36.4 K/s. The regression equation of the primary dendrite arm spacings λ1 versus cooling rate is λ1=0.013(GV)-0.32. The predictions of Kurz/Fisher model and Hunt/Lu model accord reasonably well with the experimental data. The influence of directional solidification rate under variable thermal gradient on the primary dendrite arm spacing has also been investigated.     

Structure of reciprocal lattice sites of a single-crystal Ni-based superalloy

Volume fractions of the ?? phase are calculated in the form of various geometrical figures depending on the ratio of the volume fraction of the ?? and ?á? phases. The ?? phase in the form of plates prevails at a 70% content of the ?á? phase, which is the most characteristic of the VZhM4 alloy after the complete heat treatment. The qualitative analysis is carried out for the arrangement of satellites in the reciprocal space of the VZhM4 single-crystal Ni-based superalloy. The lattice of the reciprocal space with the satellites caused by the stress state of ?? phase interlayers is constructed in the form of various geometrical figures. The set of experimental sections of the 3D structure of (222), (313), and (202) reciprocal lattice sites is obtained.     

DZ125定向凝固镍基高温合金组织与熔体超温处理时间的相关性

采用熔体超温处理方法,通过改变熔体超温处理时间研究定向凝固镍基高温合金DZ125凝固组织的演化规律。结果表明:熔体超温处理时间由15min延长到30min可以细化枝晶、减小偏析。然而,进一步延长熔体超温处理时间至60min,枝晶却略微粗化。熔体超温处理15min时,碳化物形貌为草书体,而熔体超温处理时间大于30min时碳化物则主要为非连续性棒状和块状。造成这种现象的主要原因是熔体超温处理时间的变化影响到熔体状态,进而影响到凝固组织。     

抽拉速率对DZ951合金组织和性能的影响

系统研究了抽拉速率对DZ951合金组织和性能的影响.试验结果表明:随着抽拉速率的增大,合金的定向凝固组织由粗枝状晶向细枝状晶演变,γ’相体积分数没有明显的变化但尺寸有所减小,而γ’相的形状逐渐趋向于规则立方;共晶含量、疏松水平和偏析程度都随抽拉速率的增加而变大;拉伸强度和伸长率都呈现出先减小而后增大的趋势.     

CMSX-2单晶高温合金高梯度定向凝固下过渡区的组织演化特征

用高温度梯度定向凝固装置研究了CMSX－2单晶高温合金初始过渡区的组织演化特征，获得了不同条件下过渡区单晶高温合金的平－胞－枝组织结构，以及相应的一次胞枝晶间距大小，发现初始过渡区的凝固过程影响到最终获得的凝固组织。采用的匀加速抽拉方式有利于提高单晶的引晶率和单晶凝固组织的完整性。     

激光增材制造高Nb含量GH4169合金微观偏析行为研究

目的减轻不同热处理状态下激光增材制造高Nb含量GH4169合金组织中的微观偏析。方法采用激光增材制造方法对球磨Nb合金化后的合金粉末进行快速成形,获得具有较高Nb含量的GH4169合金试样。通过光学显微镜、扫描电子显微镜及能谱分析、维氏硬度测试方法,对沉积态、固溶态和直接时效态试样进行分析,研究因合金中Nb含量变化引起的微观偏析对沉积态和热处理态合金的枝晶组织和显微硬度的影响。结果随着Nb含量的增加,一方面,由于枝晶间的Nb含量增加,枝晶间(γ+Laves)共晶数量增加,且共晶组织形貌更为连续;沉积态试样的显微硬度由228.4HV增大至534.1HV。另一方面,枝晶干Nb元素含量增加,枝晶干与枝晶间Nb元素含量的差异缩小,Nb元素的偏析比由8.59减小至4.13。后续固溶处理后,枝晶结构逐渐消失,枝晶间Laves相的数量随之减少,枝晶干与枝晶间的微观偏析减轻;固溶态试样硬度值随之减小,减小趋势随固溶温度的升高而逐渐平缓。随着Nb含量的增加,直接时效处理后,各试样显微硬度值在微观区域内的均匀性提高,枝晶干与枝晶间强化相的析出差异减小。结论合适的热处理制度既可以实现合金元素的均匀化,还能减小枝晶干与枝晶间强化相的析出差异,减轻激光增材制造高Nb含量GH4169合金组织中的微观偏析。     

冷却水温对GH4169合金电渣重熔凝固过程参数的影响

借助模拟软件MeltFlow,模拟了GH4169合金电渣重熔凝固过程参数的变化,分析了冷却水温变化对凝固过程参数的影响.结果表明,凝固过程参数随冷却水温的升高变化不明显；凝固过程参数沿重熔锭中心至边缘随冷却速度增大逐渐减小；局部凝固时间、二次枝晶间距、Rayleigh数变化趋势一致,在判断凝固组织优劣方面是等同的；计算模拟结果与硫印实验结果吻合良好.     

Effect of chill cooling conditions on cooling rate,microstructure and casting/chill interfacial heat transfer coefficient for sand cast A319 alloy

A combination of experiments and numerical analyses were used to examine the cooling conditions, solidification microstructure and interfacial heat transfer in A319 cast in a chilled wedge format. Both solid copper chills and water cooled chills, with and without a delay in water cooling, were examined in the study. Various chill preheats were also included. The goal of the investigation is to explore methods of limiting heat transfer during solidification directly beside the chill and increasing cooling rates during solidification away from the chill. Within the range of conditions examined in the study, chill preheat was found to have only a small effect on cooling rates between 5 and 50 mm from the chill/casting interface, pour superheat a moderate effect and water cooling a significant effect. In comparison to the results for the solid chill, the solidification time at 5 mm with water cooling applied at the beginning of mould filling is reduced from 56 to 15 s and at 50 mm from 588 to 93·5 s. Furthermore, the average cooling rate during solidification is increased from 1·9 to 7·06°C s^?1 at 5 mm and from 0·18 to 1·13°C s^?1 at 50 mm. At 50 mm, for example, the increased cooling rate achieved with water translates into a reduction in secondary dendrite arm spacing from 40 to 25 μm or ～40%. Delaying the water cooling by 10 s facilitated slow cooling rates at 5 mm (similar to those achieved with a solid chill) and high cooling rates 50 mm from the chill. A temperature based correlation was found to be suitable for characterising the behaviour of the interfacial heat transfer coefficient in the solid shill castings, whereas a time based correlation was needed for the water cooled castings.     

Formation of low angle boundaries in Ni-based superalloys

Abstract

An experimental study has been used to determine the influence of liquid undercooling ahead of dendrite tips on dendrite growth and the accompanying evolution of misorientation resulting in formation of low angle grain boundaries during solidification of a Ni-base superalloy, CMSX-4. The experiment was designed whereby two dendrite envelopes formed by initial branching of the primary front converged along horizontal platforms perpendicular to the withdrawal direction. The magnitude of the undercooling was increased by increasing the length of the platform, ranging from 10 to 60 mm. The position of the dendrite envelope was calculated using a computational thermal model, ProCAST. Misorientation, measured using Electron Back Scattered Diffraction (EBSD), was generally <2·5°, which was consistent with previous studies of dendrite growth in the constrained condition. However, in the longest platform a monotonic increase in misorientation in one dendrite envelope developed leading to formation of a grain boundary of 10°. This monotonic increase was not dependent on the local undercooling ahead of the dendrite envelope and was instead caused by mechanical moments arising from extensive lateral growth of unsupported tertiary dendrite branches growing laterally across the platform.     

Effect of cooling rates on dendrite spacings of directionally solidified DZ125 alloy under high thermal gradient

The dendrite morphologies and spacings of directionally solidified DZ125 superalloy were investigated under high thermal gradient about 500 K/cm. The results reveal that, with increasing cooling rate, both the spacings of primary and secondary dendrite arms decrease, and the dendrite morphologies transit from coarse to superfine dendrite. The secondary dendrite arms trend to be refined and be well developed, and the tertiary dendrite will occur. The predictions of the Kurz/Fisher model and the Hunt/Lu model accord basically with the experimental data for primary dendrite arm spacing. The regression equation of the primary dendrite arm spacings λ₁ and the cooling rate V _c is λ₁ = 0.013V _c^−0.32. The regression equation of the secondary dendrite arm spacing λ₂ and the cooling rate V _c is λ₂ = 0.00258V _c^−0.31, which gives good agreement with the Feurer/Wunderlin model.     

Effect of melting rate on microsegregation and primary MC carbides in M2 high-speed steel during electroslag remelting

Large-size primary MC carbides can significantly reduce the performance of M2 high-speed steel. To better control the morphology and size of primary MC carbides, the effect of melting rate on microsegregation and primary MC carbides of M2 steel during electroslag remelting was investigated. When the melting rate is decreased from 2 kg·min⁻¹ to 0.8 kg·min⁻¹, the columnar dendrites are gradually coarsened, and the extent of segregation of Mo and V is alleviated, while the segregation of Cr becomes severe. At 2 kg·min⁻¹, the number of primary MC carbides per unit area with the sizes in the range of 2 µm to 6 µm accounts for about 75% of all MC carbides, while the carbides are mainly concentrated on the size larger than 8 µm at 0.8 kg·min⁻¹. Thermodynamic calculations based on the Clyne-Kurz (simplified to C-K) model shows that MC carbide can be precipitated in the final solidification stage and a smaller secondary dendrite arm spacing caused by higher melting rate (2 kg·min⁻¹ in this experiment) facilitates the refinement of primary MC carbides.

     

Current efficiency of Ga electrodeposition under different anions concentrations

The gallium electrodeposition from alkaline solution has a very low current efficiency,the reason for which is still not quite understood.The effects of electrode materials used for gallium electrodeposition,as well as the effects of Na OH concentration and the anions concentrations in the solution,including SO_4~(2-),SiO_3~(2-),CO_3~(2-),AlO_2~-,F~-,and Cl~-,on the deposition were analyzed in this study.The suitable materials of SUS316–SUS316 were suggested for the gallium electrodeposition.Based on the electrode couples,the Na OH concentration of 4 mol L~(-1)for gallium electrodeposition exhibits the greatest current efficiency.Moreover,the current efficiency would decrease in the electrolyte along with the increasing concentration of the anions,except that,0.2 mol L~(-1)Cl-in the solution slightly improves the current efficiency of gallium electrodeposition.Moreover,the gallium deposited on the cathode from the solution with 0.6 mol L~(-1)SiO_3~(2-)appears tiny black in color and coarse.Meanwhile,SUS304 is shown to be not suitable to be used as cathode for the gallium electrodeposition from the alkaline solution.     

Simulation of Grain Growth during Solidification of Twin-Roll Continuously Cast Pure Aluminum with a Modified CA Model

基于枝晶生长的基本传输过程和元胞自动机(Cellular Automaton,简称CA)-有限元(Finite Element,简称FE)模型基本原理,建立了适应双辊连续铸轧纯铝薄带工艺特点的凝固过程形核和晶体生长的数学模型。模型耦合了宏观温度场和微观组织模拟计算,考虑了溶质扩散、曲率过冷和潜热释放等重要因素的影响,能够模拟凝固过程中枝晶生长的形态。应用本模型对双辊连续铸轧纯铝薄带凝固过程中等轴晶生长、等轴晶多晶粒生长及柱状晶生长、柱状晶向等轴晶演化进行模拟并与实验结果进行对比,模拟结果与实验结果吻合较好