Effects of electromagnetic vibration on the structure and mechanical properties of Al-6%Si alloy during directional solidification

The effects of electromagnetic vibration on the grain refinement in directional solid- ification were investigated. It was found that the electromagnetic vibration applied in the melt not only can refine grains remarkably but also can enhance both tensile strength and ductility values of Al-6%Si alloy. SEM graphs show that coarse dendrite structure was broken up into a somewhat globular structure, and the morphology of eutectic silicon was changed from flaky to fibrous under electromagnetic vibration treatm...     

熔体保温条件下超声波对Al—1％Si合金凝固组织的影响

研究了Al-1%Si合金在保温条件下超声波处理对其凝固组织的影响,考察了不同保温温度、不同处理时间及不同冷却方式条件下Al-1%Si合金凝固组织的变化规律。试验表明:在不同保温温度条件下,当保温温度较高时,超声波处理熔体的晶粒细化效果不明显,晶粒粗大;随着保温温度的下降,铸锭中等轴晶的数量先呈递增趋势;当在两相区内进行超声波处理时,晶粒细化效果降低,等轴晶数量明显减少,可以认为存在一个适当的超声波处理温度。此外,坩埚材质和冷却方式对保温处理后熔体凝固组织有很大的影响。     

Interactions between Nb-silicide based alloy and yttria mould during directional solidification

Pure yttria moulds have been prepared for directional solidifications of Nb-22Ti-16Si-6Hf-2Al-2Cr (at. %) alloy with a withdrawal rate of 1 × 10^-4 m s^-1 at 1850 °C and 1900 °C. This study demonstrates that a mild interface reaction occurred between the yttria mould and hafnium, the most reactive element in the alloy. A sequential reaction layer of HfO₂ and Y₂O₃ was formed at metal-mould interface, and its thickness depended on both heating temperature and holding time. HfO₂-Y₂O₃ inclusions were dispersed in the metal matrix, and the majority of them concentrated around the solid-liquid interface rather than in steady state growth region. Some inclusions were merged with each other. The reaction mechanism between Nb-silicide based alloy and yttria mould also has been described.     

Microstructural control of TiAl-Nb alloys by directional solidification

The lamellar microstructure of TiAl-Nb alloys with and without low boron additions is controlled using double directional solidification (DS). In alloys without the addition of boron, the β phase is seeded during double DS. Complete peritectic transformation occurs in both the dendritic and interdendritic regions, which can lead to the successful alignment of both the high-temperature α phase and the lamellar microstructures. Well-aligned lamellar microstructures can be easily achieved if the alloy composition is close to the peritectic point on the hypo-peritectic side. In alloys with low boron additions, however, the competitive growth of the α phase breaks the continuity of the lamellar microstructure in the region ahead of stable growth, which finally results in columnar grain coarsening and unsuccessful alignment of the lamellar microstructures.     

Microstructure and mechanical properties of Ni3Al intermetallics prepared by directional solidification electromagnetic cold crucible technique

The present work focused on the Ni_3Al-based alloy with a high melting point. The aim of the research is to study the effect of withdrawal rate on the microstructures and mechanical properties of directionally solidified Ni-25 Al alloy. Ni_3 Al intermetallics were prepared at different withdrawal rates by directional solidification(DS) in an electromagnetic cold crucible directional solidification furnace. The DS samples contain Ni_3 Al and Ni Al phases. The primary dendritic spacing(λ) decreases with the increasing of withdrawal rate(V), and the volume fraction of Ni Al phase increases as the withdrawal rate increases. Results of tensile tests show that ductility of DS samples is enhanced with a decrease in the withdrawal rate.     

Rapidly directional solidification of highly undercooled Ni-Fe-Ga shape memory alloy melts

In the present work, high undercooling rapid solidification technique is applied to the directional crystal growth of ternary Ni-Fe-Ga shape memory alloys (SMAs). Experimental results show that the phase selection of undercooled melts strongly depends on the triggering mode and nucleation undercooling. With the planar triggering nucleation at the undercooling of 160 K from the bottom of alloy melt, a directionally solidified sample with a preferred (2 0 0) orientation of γ phase is obtained. When the point triggering mode is employed at 180 K, γ phase is suppressed completely and the directionally solidified sample demonstrates almost identical phase transformation temperatures.     

单相合金定向凝固的特征尺度和过冷度研究

采用透明合金丁二腈（SCN）-1．48％水杨酸苯酯（Salol）（质量分数）进行恒速定向凝固实验,考察了一次间距、尖端半径和尖端温度随凝固生长速度变化的规律．通过考察平界面失稳孕育时间和失稳时界面速度的关系,获得了平界面失稳的临界速度．实验发现,随着生长速度的增大,一次间距先减小到一个极小值然后又增大到一个极大值,随后逐渐下降,呈现S形的变化趋势,并且在从极小值增大到极大值区间发生了胞／枝转变．将实验结果与经典的BHL模型以及自洽数值模型进行了比较,发现BHL模型所预言的尖端温度与实验结果存在较大偏差,而所有实验结果与自洽模型吻合较好．     

Phase-field study of competitive dendritic growth of converging grains during directional solidification

The microstructure evolution of grains with different orientations during directional solidification is investigated by the phase-field method. For converging dendrites, in addition to the usually accepted overgrowth pattern wherein the favorably oriented dendrites block the unfavorably oriented ones, the opposite pattern of overgrowth observed in some recent experiments is also found in our simulations. The factors which may induce this unusual overgrowth are analyzed. It is found that in addition to the difference in tip undercooling, the solute interaction of converging dendrites, which has been ignored in the classical theoretical model, also has a significant effect on the nature of the overgrowth at low pulling velocities. Solute interaction can retard the growth of dendrites at the grain boundary (GB) and induce a lag of these dendrites relative to their immediate neighbors, which gives the unfavorably oriented dendrite the possibility to overgrow the favorably oriented one. However, this unusual overgrowth only occurs when the spacing between the favorably oriented GB dendrite and its immediate favorably oriented neighbor decreases to a certain level through lateral motion. These findings can broaden our understanding of the overgrowth mechanism of converging dendrites.     

Influence of mechanical vibration on the solidification of a lost foam cast 356 alloy

Mechanical vibration was applied to the solidification of a lost foam cast(LFC) 356 aluminum alloy.Effects of mechanical vibration,with different peak acceleration,on the size and morphology of α-Al phase,and also on the mechanical properties of the castings were studied.Results indicated that α-Al dendrites gradually grow into equiaxed grains as the peak acceleration of vibration is increased.When the peak acceleration is between about 1 to 4 g,α-Al phase distribution is uniform and is refined obviously.α-Al dendrites are reduced and the mechanical properties of the castings are improved significantly when compared to those of the castings that are produced without vibration.However,when the peak acceleration is higher than 4 g,strong vibration will lead to defects formation,such as sand adhesion,while the amount and size of pores will be increased.And due to the turbulent flow that caused by strong vibration,the chance of forming large pores in the matrix has been increased significantly.The increase in defects will result in the deterioration of mechanical properties.     

超声振动对Mg-8Li-3Al合金凝固组织及性能的影响

在Mg-8Li-3Al合金的凝固过程中,不同功率的超声波被引入到合金熔体中。在超声作用下研究Mg-8Li-3Al合金的微观结构、耐腐蚀性能和力学性能。基于超声空化作用及声流作用,对超声细化机制进行分析。结果表明：在施加超声波后,合金的α相形貌从粗大的蔷薇状结构变为细小的近球状结构。当超声波功率为170W时,能够获得细小的球状结构。与没有施加超声场的合金相比,施加超声场（功率170W,作用时间90s）后的合金耐腐蚀性明显提高,力学性能也有很大改善,抗拉强度和伸长率分别提高了9.5%和45.7%。     

Effect of electric pulse treatment on solidification structure of an Al-15%Si alloy

1 IntroductionThe study of electric field (EF) modifying themetal microstructure has made some progress sinceAsoka applied electric current when a Pb-Sn alloysolidified [1-8]. Based on the different applying du-rations of EF to melt, there are two kinds o…     

Influence of sub-rapid solidification on microstructure and mechanical properties of AZ61A magnesium alloy

The microstructure of sub-rapid solidification processed AZ61A magnesium alloy was presented and discussed. The results show that the grain size of the foil is significantly refined, and the grain morphology is cellular or globular. The eutectic transformation L→α-Mg+β-Mg17Al12 and microsegregation in conventionally solidified AZ61A alloy are suppressed to a great extent.The β-Mg17Al12 phases located in the α-Mg grain boundaries are largely decreased due to high solidification cooling rate. As a consequence, the alloying elements Al, Zn, Mn show much higher solid solubility and the sub-rapid solidification microstructure dominantly consists of supersaturated α-Mg solid solution. The mechanical properties and fractographic analysis reveal that the fracture mechanism and corresponding morphology of the rapture surface of tensile bars are linked to the microstructure obtained and depend on the sub-solidification processes.     

电磁搅拌法连铸半固态铝合主其凝固组织分析

采用电磁搅拌连续铸造装置,制备具有触变性非合晶组织的兰固态铝合金圆棒。棒料内部组织形貌为具有类圆形轮廓的初生相晶粒浸润在细共晶体中,与传统的凝固枝晶组织有显著差异。对兰固态非枝晶组织的演化机理进行了探讨,结果表明处于交变电磁场中的金属液体,可视为导电介质流,它与磁场交互作用,在导电介质充中产生彻体力。半固态非枝晶组织是在晶体正常的形核和长大过程中,受彻体力强烈的混合抑制作用而形成的。     

超声振动对5356铝合金组织及力学性能的影响

为探究超声振动对5356铝合金组织与性能的影响,改变变幅杆插入深度、超声处理时间等工艺制备出5356铝合金,通过对显微组织与断口形貌的观察、密度和室温力学性能的测试研究了5356铝合金超声处理对组织性能的影响,试验结果表明:未加超声处理的5356铝合金组织中存在大量的树枝晶,还有许多杂质包括氧化物夹杂。熔体超声处理后的5356铝合金晶粒组织均匀,大部分杂质被除去。当熔体温度为720℃,变幅杆插入深度30mm,超声处理时间2～5min时,5356铝合金力学性能最好,铸锭抗拉强度为225MPa,比未加超声处理试样提高7.1%,伸长率为11.7%,提高了52%,密度达到2.598g·cm-3。     

Solidification structure of eutectic Al-Si alloy under a high magnetic field-aid-electromagnetic vibration

An electromagnetic vibration was generated by simultaneously imposing a strong static magnetic field (up to 10 T) and an alternative electricity current to the metal. Its effects on the solidification structure of eutectic Al-Si alloy have been investigated experimentally. It is found that the eutectic structure has been refined by solely imposing high magnetic field while it is coarsened under the electromagnetic vibration. Furthermore, polyhedral Si grains and non-dendritic α-Al appeared when the electrom...     

Effect of electric pulse treatment on solidification structure of an A1-15%Si alloy

The effect of electric pulse modifying on the solidification structure of an Al-15%Si alloy was investigated. The result shows that the primary silicon disappears sometimes and the eutectic phase is refined after the treatrnent of EP （electric pulse） though there are different modalities in different treating durations. DSC （differential scanning calorimetry） analysis indicates that the super-cooling texture decreases and the freezing range narrows evidently after the electric pulse treatment.     

机械振动对消失模铸造镁合金组织及力学性能的影响

研究机械振动对消失模铸造AZ91合金微观组织和力学性能的影响,并分析凝固过程中机械振动细化晶粒的机制。结果表明:机械振动能显著地细化消失模铸造AZ91合金的组织,合金组织由α-Mg固溶体、沉淀Mg17Al12和新相Al13Mn12组成,其铝锰相有别于不施加振动时的Al32Mn25相;机械振动较大幅度改善了合金的力学性能,当激振力为1.5kN时,AZ91合金抗拉强度相对于不振动时提高27%,其合金力学性能最优;而当激振力进一步增加时,由于组织中存在着微观缩松,强度有所降低。机械振动法通过增大过冷度、剪切力碎化枝晶和使枝晶臂熔断来细化晶粒。