不同条件下脉冲电流对纯铝凝固组织的细化

在金属凝固的过程中施加脉冲电流是一种有效的细化凝固组织的方法。围绕脉冲电流细化晶粒的机制提出了多种理论。利用纯铝作为研究材料,在脉冲电流参数、浇注温度等工艺参数不变的情况下,设计了两组铸型条件不同的试验,发现不同条件下得到的铸锭凝固组织的特征有所不同。结合结晶雨机制讨论了这些特征的形成原因,指出了熔体的流动情况对脉冲电流细化效果有重大的影响。     

超声波处理时间对工业纯铝铸锭结晶组织的影响

研究了超声波处理时间对工业纯铝铸锭结晶组织的影响，即添加除气剂又进行超声波处理的情况。分析了超声波对工业纯铝结晶组织影响的原因。研究结果表明。采用合适的超声波处理时间，可以提高工业纯铝铸锭的细化率。     

AlTiCRE合金细化剂对纯铝的细化作用

介绍了在纯铝中分别加入不同量的AlTiCRE合金细化剂，观察其铸态显微组织和测定其相应铸态试样的力学性能。实验结果表明，在纯铝中加入0．3％的该合金细化剂时，其晶粒最为细小均匀；相应地，铸态拉伸试样的力学性能得以提高。本文还讨论了AlTiCRE合金细化剂的细化机理。     

超声波作用下不同添加剂对高碳钢凝固组织的影响

 主要研究了在超声波作用下不同添加剂对高碳钢凝固组织的影响。结果表明：在超声波作用下,高碳钢液中添加钛铁、铈可以明显细化其凝固组织,但是添加铝对其凝固组织的细化作用不明显。高碳钢凝固组织的细化是由于Ti（C,N）可以作为非均质形核的核心,促进高碳钢液凝固过程的形核,Ti（C,N）有效的钉扎晶界作用,阻止初生奥氏体晶粒的长大;超声空化作用下稀土氧化物、氧硫化物数量的增多,增加了高碳钢液凝固过程中非均质形核核心的数量。     

铝细化剂的研究进展

分析比较了稀土、AlTiC，AlTiB三种铝细化剂各自的制作方法、性能特点，并进一步阐述了它们细化纯铝的机理和存在的问题，由此提出了一条更为有效的、利用复合细化剂进行纯铝细化的新方法。     

冷却速度对稀土锌合金凝固组织和性能的影响

研究了锌铝铜三元共晶合金中冷却速度对稀土锌合金的凝固组织和性能的影响。研究结果表明，稀土能细化锌合金的凝固组织和改善其机械性能，这些效果均受冷却条件的影响；冷却速度大，效果显著。但是，稀土锌合金的塑性比未加稀土的锌合金略低。在不同冷却条件下，锌合金中的稀土最佳加入量可能有所不同。     

Al-Ce中间合金的微观组织和细化机制的研究

利用对掺法制备Al-Ce中间合金,并对工业纯铝进行细化研究。利用X射线衍射(XRD)、能谱仪(EDS)、光学显微镜(OM)等检测手段对Al-Ce中间合金细化剂的微观组织和对工业纯铝细化机制进行研究。结果表明:通过XRD和EDS分析,Al-Ce中间合金的相主要包括α-Al、Al_(11)Ce_3相和Ce相。铝液中加入Al-Ce中间合金,可以细化工业纯铝晶粒。当中间合金添加量低于0.1%时,随着Al-Ce中间合金添加量的增加,对工业纯铝的细化效果越来越好。当加入0.1%Al-Ce中间合金时细化效果最好,晶粒尺寸细化到170.55μm,形状因子为0.68,同时力学性能也最好,抗拉强度和延伸率分别为74.5 MPa,47.64%。添加量超过0.1%时,随着添加量的增多,细化效果会减弱。制备Al-Ce中间合金时共晶反应生成的Al_(11)Ce_3和α-Al具有相似的晶体结构,而且晶格常数也能相对应,所以Al_(11)Ce_3可以作为α-Al凝固时的异质形核点,从而促进细化。     

大直径铝锭热顶铸造中超声施振深度的细晶机制

在直径为650 mm的铝合金热顶半连续铸造过程中施加双源超声振动系统, 研究3种超声辐射杆浸入深度对铸锭宏观凝固组织的影响.基于铝合金铸锭凝固组织形貌的检测结果以及ANSYS等有限元软件对铸造过程中声场的仿真结果, 深入探讨了超声辐射杆在不同的施振深度下对铝合金铸锭凝固组织细化机制的影响.结果表明: 随着超声辐射杆施振深度的增加, 铸锭截面组织整体进一步细化, 晶粒形状由发达的枝晶变为等轴枝晶; 由于超声辐射杆端面以及柱面存在几个固定位置处振动波峰, 在铝熔体中不同的超声施振深度下存在不同的超声空化范围, 进而导致凝固组织的细化机制也不同.      

稀土导电铝合金的铸态组织和相组成

本文利用光学显微镜、扫描电镜、Ｘ射线衍射仪等，研究了Ｌａ、Ｃｅ对工业纯铝铸态组织和相组成的影响。结果表明，Ｌａ、Ｃｅ能细化工业纯铝的铸态组织，并和Ｆｅ、Ｓｉ等元素结合生成了二元或三元稀土化合物相。     

TiO_2含量对Al-TiO_2-C细化剂组织及细化效果的影响

以TiO_2粉、C粉、Al粉为原料,采用放热弥散法制备Al-TiO_2-C晶粒细化剂,并进行晶粒细化试验。采用X射线衍射仪(XRD)、扫描电子显微镜(SEM)及能谱分析(EDS)等方法研究了不同TiO_2含量细化剂的显微组织。结果表明,Al-TiO_2-C晶粒细化剂由α-Al,Al_3Ti,Al_2O_3和Ti C相组成。TiO_2含量不同,Al-TiO_2-C晶粒细化剂内第二相形貌及分布也不同。当TiO_2含量为12%(质量分数)时,细化剂组织中Al_3Ti内包裹有大量的Al2O3和Ti C颗粒,这些颗粒将Al_3Ti分割开来,Al_3Ti相呈现圆块状,此时组织相对分布均匀。TiO_2含量不同,AlTiO_2-C晶粒细化剂对工业纯铝的细化效果也不同。随着TiO_2含量的增加,Al-TiO_2-C细化剂对工业纯铝的细化效果呈现先变好后变差的变化趋势,其中当TiO_2含量为12%时,其细化效果达到相对最好。本文细化剂的最佳添加量为0.3%。当细化剂中TiO_2含量为12%时,0.3%的细化剂添加量可使工业纯铝细化到约154μm。     

Study on Improving Solidification Structure of Rare Earth Al-Si Alloys with Electropulse Acting on Liquid Alloy

Electropulse modification (EPM) process, a new physical field method for improving the solidification structure of metals was introduced.Different from other research, EPM is only acting pulse current on melt under liquid state.The solidification structure of Al-Si alloys, A1-Cu alloys,cast iron and steel can be modified obviously with this method: the solidification structure of ZL101 alloy presented the Na and Sr modification and the mechanical properties were enhanced; a large number of primary silicon appeared in the microstructure of ZL109 alloy; the equiaxed grain zone was expanded and the grains were fined in Al-5.0wt% Cu alloy; the graphitization took place in solidification process of molten cast iron; the grain sizes of solidification structure of T8 steel were reduced significantly and the shape of steel pearlites also changed; the equiaxed grain zone increased to 88% from original untreated 19%, the equiaxed grains were fined and the intercrystalline crack was avoided in concasting billet by continuously treating liquid electrical sheet steel in tundish.Effects of rare earths on casting Al-Si alloys were also summarized.The method of modifying the solidification structure of rare earth Al-Si alloys with EPM in producing the alloys was proposed.     

Effects of Pulse Current on Solidification Structure of Austenitic Stainless Steel

The 1Cr18Ni9Ti specimens were treated respectively with pulse current under 520V and 2600V during solidification and the solidification structure was observed. The results showed that pulse current can refine solidification grains, cut primary dentrities remarkably and reduce second dentritic arm spacing. The mechanism and effect are changed with operation parameters.     

The effect of lead and thallium on eutectic grain structures in zinc-aluminum alloys

Abstract

The influence of small additions of lead or thallium on the eutectic grain structure in zinc-aluminum alloys has been studied. With slow solidification, thallium or lead at low concentrations cause large increases in the grain size. With more rapid solidification they promote columnar grains. Autoradiography has shown that at slow growth rates, when thallium is present in concentrations so low that there is no increase in grain size, the eutectic grains grow with planar interfaces. At higher thallium contents, where grain size enhancement becomes noticeable, the grains grow with non-planar interfaces. It is argued that the non-planar growth form leads to the observed increased grain size.

Résumé

Nous avons ;amp;#x00E9;tudié l'effet de petites additions de plomb et de thallium sur la structure des grains d'eutectique d'alliages de zincet d'aluminium. Quand la vitesse de solidification est faiblede telles additions ont pour effet une augmentation considérable de la taille des grains. Quand les additions de thallium sont suffisamment faibles pour ne pas modifier la taille des grains, nous voyons, par autoradiographie que, à faible vitesse de solidification, les grains d'eutectique croissent selon une interface plane.A plus forte concentration de thallium, là où on note une augmentation de la taille des grains, la croissance n'est plus plane. Cette croissance selon une interface non plane serait la cause de l'augmentation de la taille des grains.     

高锰高铝钢凝固组织的实验研究

为研究高锰高铝钢的凝固组织,在实验室条件下采用真空感应炉冶炼了三种不同成分的高锰高铝钢。研究结果表明,高锰高铝钢的柱状晶组织比较发达,其晶体尺寸比较粗大;增加钢中的Al含量可以降低柱状晶区比例和晶体尺寸,随着钢中Al含量的增加,钢的热塑性也明显增强;在奥氏体晶界析出的铁素体会降低钢的热塑性,是诱发连铸坯角部裂纹的主要原因。     

Mechanisms of eutectic solidification in Al–Si alloys modified with Ba,Ca, Y and Yb

The eutectic solidification mechanisms in an A356.0 (Al–7%Si–Mg) alloy modified by barium, calcium, yttrium and ytterbium have been determined. The crystallographic orientations of aluminium in the eutectic and the surrounding aluminium dendrites were measured by electron backscattering diffraction mapping, and samples were also quenched at different stages during the eutectic arrest and examined by optical microscopy. The combination of these two techniques shows that each of the elements added promote heterogeneous nucleation of eutectic grains in the interdendritic liquid, while the aluminium in the unmodified alloy grows epitaxially from the dendrites. Furthermore, calcium and yttrium result in a strong dependency of eutectic solidification on the thermal gradient, i.e. the eutectic evolves from the walls towards the centre of the sample on a macro-scale. These differences in eutectic solidification mode show a correlation with some thermal characteristics of the eutectic arrest.     

节铬型铁素体不锈钢连铸板坯凝固过程热模拟

连铸板坯凝固传热主要在厚度方向进行,这造成了连铸坯大部分区域由侧面向中心凝固,因此可以近似地用非稳态定向凝固进行热模拟。利用自主研发的水平式连铸坯枝晶生长热模拟装置研究了新型节铬铁素体不锈钢连铸坯凝固组织,以期在工业生产前预测连铸工艺对其凝固组织的影响。热模拟试样热端温度采用连铸坯心部冷却曲线进行控制,并通过调节冷却水流量控制热模拟试样冷端的冷却强度,从而实现由冷端向热端的非稳态定向凝固。实验发现过热度和冷却强度对热模拟试样的等轴晶率及其平均晶粒尺寸影响不显著,但大的冷却强度会导致柱状晶长度增加。     

Prediction of the As-Cast Structure of Al-4.0 Wt Pct Cu Ingots

A two-stage simulation strategy is proposed to predict the as-cast structure. During the first stage, a 3-phase model is used to simulate the mold-filling process by considering the nucleation, the initial growth of globular equiaxed crystals and the transport of the crystals. The three considered phases are the melt, air and globular equiaxed crystals. In the second stage, a 5-phase mixed columnar-equiaxed solidification model is used to simulate the formation of the as-cast structure including the distinct columnar and equiaxed zones, columnar-to-equiaxed transition, grain size distribution, macrosegregation, etc. The five considered phases are the extradendritic melt, the solid dendrite, the interdendritic melt inside the equiaxed grains, the solid dendrite, and the interdendritic melt inside the columnar grains. The extra- and interdendritic melts are treated as separate phases. In order to validate the above strategy, laboratory ingots (Al-4.0 wt pct Cu) are poured and analyzed, and a good agreement with the numerical predictions is achieved. The origin of the equiaxed crystals by the “big-bang” theory is verified to play a key role in the formation of the as-cast structure, especially for the castings poured at a low pouring temperature. A single-stage approach that only uses the 5-phase mixed columnar-equiaxed solidification model and ignores the mold filling can predict satisfactory results for a casting poured at high temperature, but it delivers false results for the casting poured at low temperature.     

Effect of electric current pulse on solidification of silicon steel pertinent to twin roll casting

Abstract

An experimental study with respect to the influence of electric current pulse (ECP) on the solidification structure of molten silicon steel was investigated with a copper mould designed to simulate the twin roll casting process. The experimental results showed that the application of ECP can increase the proportion of equiaxed grains up to 67·5%. The mechanism of exerting ECP is also discussed. It can be confirmed that the application of ECP on twin roll casting could effectively improve the solidification structure of silicon steel strip.     

合金元素对20CrMnTi钢凝固组织影响的模拟

 为了改善20CrMnTi小方坯的凝固组织,采用移动边界法先对20CrMnTi钢的温度场进行模拟,在此基础上,采用元胞自动机有限元法（CAFE法）对其凝固组织进行了数值模拟。当模拟结果与实际结果基本吻合时,进一步探讨了硅、铬、锰、钛等合金元素对20CrMnTi钢凝固组织的影响规律。模拟结果表明,在该钢号的合金元素规定范围内,适当减少硅元素的质量分数,能够提高铸坯中心等轴晶比例,使晶粒数目增加,晶粒平均半径减小。适当添加铬元素能够扩大铸坯中心等轴晶区,从而减小晶粒平均半径,增加晶粒数量。增加锰元素质量分数可以提高铸坯的中心等轴晶率。最后根据所得模拟结果对该钢种的合金元素进行调整,达到了扩大铸坯中心等轴晶区和细化晶粒的目的。     

Effects of alloy composition and casting speed on structure formation and hot tearing during direct-chill casting of Al-Cu alloys

Effects of casting speed and alloy composition on structure formation and hot tearing during direct-chill (DC) casting of 200-mm round billets from binary Al-Cu alloys are studied. It is experimentally shown that the grain structure, including the occurrence of coarse grains in the central part of the billet, is strongly affected by the casting speed and alloy composition, while the dendritic arm spacing is mostly dependent on the casting speed. The hot cracking pattern reveals the maximum hot-tearing susceptibility in the range of low-copper alloys (1 to 1.5 pct) and at high casting speeds (180 to 200 mm/min). The clear correlation between the amount of nonequilibrium eutectics (representing the reserve of liquid phase in the last stage of solidification) and hot tearing is demonstrated. A casting speed-copper concentration-hot-tearing susceptibility chart is constructed experimentally for real-scale DC casting. Computed dimensions of the solidification region in the billet are used to explain the experimentally observed structure patterns and hot cracking. Thermomechanical finite-element simulation of the solidifying billet was used as a tool for testing the applicability to DC casting of several hot-tearing criteria based on different principles. The results are compared to the experimentally observed hot tearing. It is noted that hot-tearing criteria that account for the dynamics of the process, e.g., strain rate, actual stress-strain situation, feeding rate, and melt flow, can be successfully used for the qualitative prediction of hot tearing.