Formation of Nondendritic Primary Aluminum Phase in Hypoeutectic Alloys in Controlled Diffusion Solidification (CDS): A Hypothesis

Controlled diffusion solidification (CDS) is a novel process wherein specific Al alloys can be cast by mixing two precursor alloys of specific compositions and temperature and subsequently casting the resultant mixture. This process enables a nondendritic morphology of the primary Al phase in the cast samples, which is beneficial in mitigating hot tearing tendencies and enabling castability of dilute Al (wrought) alloys to obtain castings with superior mechanical and performance properties. In this study, a hypothesis is proposed to describe the mechanism of the CDS process, specifically the processes of mixing two precursor alloys and a subsequent solidification process. Al – 4.5 wt pct Cu was used as an example alloy system to propose a hypothesis and to verify the various features in the mechanism of mixing two alloys. Experimental results show that the mixing process naturally causes copious nucleation of one of the alloys mixed and that the turbulence energy during mixing distributes these nuclei uniformly to enable a favorable solidification condition for a nondendritic cast microstructure. It is critical that the alloy with the higher thermal mass (mass and temperature) is mixed into the alloy with lower thermal mass to obtain a valid CDS process and that the reverse will not yield a favorable homogeneous cast sample. Certain critical parameters during the CDS process have also been identified and quantified for a favorable microstructure of the casting.     

高纯铝定向凝固提纯技术应用研究

高纯铝是目前最主要的电子新材料,通过定向凝固工艺获得的高纯铝具有纯度高、成本低的优势,该法目前被产业界广泛采用。简单介绍了定向凝固技术的原理,重点分析了国内外高纯铝定向凝固技术的新工艺、新方法,最后对高纯铝定向凝固技术存在的问题及发展趋势进行了分析总结。     

铸铁中各形态石墨间的相互转变

采用定向凝固方法,并借助金相显微镜和扫描电镜等手段,研究了铈含量的变化对铸铁中各形态石墨间相互转变的影响。发现:在控制界面推进速度不变的条件下,随铈含量的增加,可从片状石墨依次向过冷石墨、蠕墨、枝晶间石墨、球缺状石墨连续转变,这种转变是逐渐进行的。与文献﹝1﹞的结论相结合,可以得出:球墨、蠕墨、片墨之间的转变是可逆的。此外,还发现在铈含量高的情况下,石墨与奥氏体仍可进行密合共晶生长。     

Microstructure,Intermetallic Phases,and Fractography of the Cast Al-5.8Zn-2.2Mg-2.5Cu Alloy by Controlled Diffusion Solidification

Metallurgical and Materials Transactions A - The cast specimens for this study were prepared with a final composition of Al-5.8Zn-2.2Mg-2.5Cu alloy by using a relatively new casting process that is...     

Prediction of Cooling Curves for Controlled Unidirectional Solidification Under the Influence of Shrinkage: A Semi-analytical Approach

Metallurgical and Materials Transactions B - Quality of casting products can be significantly improved by controlling the crystal growth rate. Both dendritic and equiaxed grain structures depend...     

Morphology Transition from Dendrites to Equiaxed Grains for AlCoCrFeNi High-Entropy Alloys by Copper Mold Casting and Bridgman Solidification

The AlCoCrFeNi high-entropy alloys (HEAs) were prepared by the copper mold casting and Bridgman solidification. X-ray diffraction (XRD) results verify that the main phase was body-centered-cubic (bcc) solid solution by these two solidification processes, indicating its good phase stability. Interestingly, the metallographic photos show a morphology transition from dendrites to equiaxed grains after Bridgman solidification, which was considered to have a strong dependence on the parameter of the G/V (the temperature gradient to the growth rate ratio). Compared to the as-cast sample, the plasticity of alloys synthesized by Bridgman solidification was improved by a maximum of 35?pct.     

Solidification of a binary eutectic in a three-component system

The change in mass and composition of the phases during the equilibrium solidification of eutectic alloys in three-component systems is considered. For the model system A-B-C, we show that equilibrium solidification of binary eutectics in three-component systems is determined by processes of decomposition and interaction, as in the crystallization of a solid solution. Eutectic transformation is generally associated with equilibrium solidification, since decomposition dominates over interaction. These findings are confirmed by studying solidification in the Al-Si-Cu system.     

Observation of Misoriented Tertiary Dendrite Arms During Controlled Directional Solidification in Aluminum-7 Wt?pct Silicon Alloys

Electron backscattered diffraction (EBSD) examination of transverse cross sections from a directionally solidified aluminum-7 wt?pct silicon alloy revealed tertiary dendrite arms that appeared in place but in reality had orientations that significantly differed from their parent arm. The maximum extent of spuriously orientated arms occurred at an intermediate growth velocity and was more pronounced at subgrain boundaries that separated uniquely oriented dendritic arrays. Mechanisms for tertiary arm misorientations are discussed, and attention is called to the practical consequences of these in-situ defects.     

以党的十九大精神为引领,奋力开创钨协工作新局面——中国钨业协会六届六次理事会工作报告

回顾了中国钨业协会2017年所做的工作,分析了协会改革发展中存在的问题和面临的形势,提出了2018年工作指导思想和重点工作。2017年,中国钨业协会始终履行服务宗旨,紧盯行业发展态势,不断加强行业自律、行业协调和国际交流,积极反映行业企业诉求,为行业发展和会员企业生产经营架桥、搭台、干事,各项工作取得了新的进展和成效。2018年,中国钨业协会要认真学习贯彻落实党的十九大精神,以习近平新时代中国特色社会主义思想为行动指南,牢固树立和贯彻落实新发展理念,围绕建设钨工业强国目标,不断创新和优化服务内容、服务手段和服务方式,加强服务品牌建设,提升服务能力、影响力、凝聚力和公信力,为加快推进产品结构调整和产业转型升级,提高钨业发展质量和效益做出新的贡献。     

Study of Solidification Cracking in a Transformation-Induced Plasticity-Aided Steel

In situ high-temperature laser scanning confocal microscopy is applied to study solidification cracking in a TRIP steel. Solidification cracking was observed in the interdendritic region during the last stage of solidification. Atom probe tomography revealed notable enrichment of phosphorus in the last remaining liquid. Phase field simulations also confirm phosphorus enrichment leading to severe undercooling of more than 160 K in the interdendritic region. In the presence of tensile stress, an opening at the interdendritic region is difficult to fill with the remaining liquid due to low permeability and high viscosity, resulting in solidification cracking.     

Solidification of a thick slab in a mold with supplementary cooling

A model is proposed for the solidification of thick ingots with supplementary cooling by means of mandrels. On that basis, the overheating of the liquid steel in the classical system may be eliminated. The effectiveness of supplementary cooling by means of mandrels is shown in a computational experiment. The solidification rate may be significantly modified by that means.     

电解金属锰渣中重金属的固化新技术

以湘西某电解金属锰厂的电解金属锰渣为原料,在分析其浸出特性的基础上,研究硫化钙焙砂固化工艺对电解金属锰渣中重金属离子浸出率的影响。结果表明,利用硫化钙焙砂能有效地固定锰渣中的重金属,固化体浸出液中的重金属浓度明显降低。当硫化钙焙砂用量为电解金属锰渣质量的15％,固化反应时间为3．0h时,固化后电解金属锰渣的浸出毒性符合相关国家标准。     

Solidification behaviour of Pb droplets embedded in a Cu matrix

A hypomonotectic alloy of Cu-5wt%Pb has been manufactured by melt spinning and the resulting microstructure examined by transmission electron microscopy. As-melt spun hypomonotectic Cu-5wt%Pb consists of a homogeneous distribution of faceted 20–100 nm diameter Pb particles embedded in a matrix of Cu, formed during the monotectic solidification reaction. The Pb particles show a cube-cube orientation relationship with the surrounding Cu matrix and a truncated octahedral shape bounded by {111} and {100} facets. The kinetics of Pb particle solidification have been examined by heating and cooling experiments in a differential scanning calorimeter over a range of heating and cooling rates. Pb particle solidification is nucleated catalytically by the surrounding Cu matrix, with an undercooling of 0.5 K and a contact angle of 4°. Analysis of the nucleation kinetics of Pb particle solidification seems to indicate a breakdown of the classical spherical cap model of heterogeneous nucleation.     

Progress in Research on Solidification in a Strong Static Magnetic Field

Strong magnetic fields available from superconducting magnets are opening a way to new phenomena that could lead to new methods in materials processing including solidification. The principal research involving solidification in strong static magnetic fields is emphasizing four aspects: control of crystal orientation, convection damping, thermoelectric magnetohydrodynamics (TEMHD) and change in thermodynamics. Under high magnetic intensity, aligned structural textures are induced in both magnetic and non‐magnetic materials. Since in strong magnetic field the melt flow is suppressed by convection damping, the microstructure being formed during solidification is affected heavily; this phenomenon applies to eutectic, monotectic and peritectic alloys as well as to dendritic morphologies typical of directional solidification. If strength and orientation of a magnetic field are controlled appropriately, this strong damping effect will generate more homogeneous crystals as a result of achieving diffusion‐controlled solute transport conditions. TEMHD more easily occurs in strong magnetic fields, resulting in equiaxed crystals even under directional solidification. It is evidenced experimentally and theoretically that the thermodynamics of phase transformation and nucleation are changed by strong magnetic fields.