Growth restriction effects during solidification of aluminium alloys

The effects of solute elements during solidification on the grain size are very important and can be quantified by the growth-restriction parameter Q, and Q possesses the better correlation with the grain size. Based on the constitutional undercooling generated by the growth of an adjacent grain during the initial solidification, the growth-restriction parameter Q is deduced and a comprehensive physical basis of Q is obtained by using an initial solute distributing equation. For the alloys with more potent nucleants, Q is a suitable predictor of the grain size. For less potent nucleants, the relative grain size(RGS) is a more accurate prediction of the grain size. This prediction coincides with the experimental behaviors for Al-Ti and Al-Cu alloys with lower solute content.     

Mechanisms of enhanced heterogeneous nucleation during solidification in binary Al-Mg alloys

The mechanisms involved in the grain refinement of Al-Mg alloys through varying the Mg content and applying intensive melt shearing were investigated. It was found that the oxide formed in Al-Mg alloys under normal melting conditions is MgAl₂O₄, which displays an equiaxed and faceted morphology with {1 1 1} planes exposed as its natural surfaces. Depending on the Mg content, MgAl₂O₄ particles exist either as oxide films in dilute Al-Mg alloys (Mg < 1 wt.%) or as naturally dispersed discrete particles in more concentrated Al-Mg alloys (Mg > 1 wt.%). Such MgAl₂O₄ particles can act as potent sites for nucleation of α-Al grains, which is evidenced by the well-defined cube-on-cube orientation relationship between MgAl₂O₄ and α-Al. Enhanced heterogeneous nucleation in Al-Mg alloys can be attributed to the high potency of MgAl₂O₄ particles with a lattice misfit of 1.4% and the increased number density of MgAl₂O₄ particles due to either natural dispersion by the increased Mg content or forced dispersion through intensive melt shearing. It was also found that intensive melt shearing leads to significant grain refinement of dilute Al-Mg alloys by effective dispersion of the MgAl₂O₄ particles entrapped in oxide films, but it has marginal effect on the grain refinement of concentrated Al-Mg alloys, where MgAl₂O₄ particles have been naturally dispersed into individual particles by the increased Mg content.     

Effects of low-frequency electromagnetic field on nucleation during the pure aluminium solidification process

The effects of low-frequency electromagnetic field (LFEF) on nucleation during the pure aluminium solidification process are investigated. The experimental results show that the primary structure cannot be refined by applying LFEF on high-temperature liquid phase. The LFEF can dissociate the nuclei from the mould wall and disperse them uniformly when The LFEF is applied during the nucleation stage. However, the temperature at the middle of mould is higher than the liquidus temperature of pure aluminium, and the crystal nuclei cannot be preserved and grow up.     

“节点修正”FVM-CA方法熔积凝固组织模拟

基于Nastac模型建立了一种优化的元胞捕捉机制,即"节点修正"法元胞自动机(CA)模型.较传统的CA模型能够简便有效地模拟任意主轴方向的柱状晶生长;将其与有限体积法(finite volume method,FVM)的宏观温度场模拟相结合,构建了一种改进的FVM-CA宏微观耦合计算方法.用该改进模型对碳钢电弧熔积组织演变过程进行了模拟,并探讨了熔积速度对熔池枝晶间距及生长方向的影响.试验结果证明了计算模型的合理性,该工作将为熔积增材成形金属零件的组织预测与控制提供科学基础.     

Feathery grain growth during solidification under forced flow conditions

The grain morphology developed during solidification of an Al–4.5% Cu alloy is represented generally by columnar or equiaxed dendrites. Twinned feathery grains are found in the structure formed under certain heat and flow conditions during solidification. In this work, these conditions were achieved during solidification in a cavity under forced flow. Feathery grain formation is studied by means of fluid dynamics simulations with solidification included and by experiments. In order to determine the crystallographic orientation of feathery grains, electron backscattered diffraction measurements were performed. The growth features of feathery grains were analyzed by observations made normal and parallel to the growth direction. Some correlations between twinned feathery morphology, flow and solidification parameters were obtained.     

Fe对Al-Ti-B和Al-Ti-C中间合金细化纯铝时晶粒形核与生长行为的影响

分别用Al-Ti-B、Al-Ti-C两种中间合金细化不同Fe含量(w(Fe)=0.2%～1.0%)的铝。通过扫描电镜观察异质形核晶粒核心及围绕核心所形成的晕圈,分析晶粒形核及生长过程。结果表明,两种中间合金加入铝熔体后释放出的TiB2和TiC颗粒都能吸附熔体中的Ti,形成围绕TiB2和TiC颗粒的Ti浓度梯度场。围绕TiB2和TiC颗粒团的晕圈形状存在差异:围绕TiB2颗粒团的晕圈基本为圆形,围绕TiC颗粒团的晕圈基本为花瓣状。在Fe元素存在的条件下,由于晕圈之间的融合以及晶粒核心颗粒不同造成的晕圈形状差异对晶粒的生长方式有重要影响。在晕圈形状相同的条件下,Fe含量对晶粒生长行为影响显著,随着Fe含量增加,晶粒的树枝化程度变大。     

Deflection of columnar grains during solidification of aluminium alloys under forced flow conditions

Abstract

Fluid flow strongly influences the grain morphology promoting growth of the columnar structure deflected towards the incoming flow. The effects of forced flow and initial superheat on columnar grain deflection of aluminium alloys were experimentally investigated in this work. The computer simulations of the flow with solidification included previously validated against experimental temperature measurements were used to interpret experimental results. It is shown that the deflection degree strongly depends on the inlet velocity, melt superheat, and alloy composition. Generally, the deflection angle increases with accelerated flow and at a higher superheat. Some quantitative correlations are obtained. Furthermore, several factors affecting the deflection of the grains are discussed.     

Simulation of nucleation and coalescence of second phase droplets during earth-based solidification process of immiscible alloys

1INTRODUCTION Theproductionofuniform,homogeneous micro compositesfrommaterialshavingdifferentdensities,suchasimmisciblesystemsisdesirable formanyapplications,forexample,asengineering andelectronicmaterialsandself lubricatingbear ings[1].Thesolidifiedmicrostructureofimmiscible alloysstronglydependsonthenucleation,diffu sionalgrowth,collisionandcoalescenceofthesec ondphasedroplets.Therehavebeenseveralat temptstomodelatleastpartofthemicrostructure evolutioninthemiscibilitygap.RogersandDa vis…     

Equations for nucleation of hydrogen gas pores during solidification of aluminium seven weight percent silicon alloy

A statistical prediction model has been applied to the nucleation rate as a function of temperature for an Al7Si alloy based upon observations of pore formation during solidification in an XTGS. A normalised Gaussian function was developed ((1), (2), (3) and (4)) which may be used to describe pore nucleation in mathematical simulations of porosity formation during solidification.     

In-situ observation of porosity formation during directional solidification of Al-Si casting alloys

In-situ observation of porosity formation during directional solidification of two Al-Si alloys (7%Si and 13%Si) was made by using of micro-focus X-ray imaging.In both alloys,small spherical pores initially form in the melt far away from the eutectic solid-liquid (S/L) interface and then grow and coagulate during solidification.Some pores can float and escape from the solidifying melt front at a relatively high velocity.At the end of solidification,the remaining pores maintain spherical morphology in the ne...     

共晶和包晶合金定向凝固过程中共生生长的形态稳定性

近年来对共晶合金共生(耦合)生长形态稳定性的研究取得了一系列重要的成果。这些成果主要包括:片层共晶共生生长在最小过冷度下的超稳定性、片层共晶共生生长的Z字形分叉、熔体流动及非线性动力学对共生生长形态稳定性的影响等。在此基础上人们又开始关注包晶合金定向凝固中的稳态共生生长及其形态稳定性。综述了这些研究成果,并对其发展方向进行了探讨。     

低钛铝合金的电解生产与晶粒细化

在电解低钛铝合金的工业化生产试验中，研究和对比了电解加钛和熔配加钛低钛铝合金的晶粒细化效果及其衰退行为。结果发现：电解质中添加少量TiO2对电解槽铝产量和电流效率影响较小，二者分别维持在1200kg和92％左右，钛的吸收率在95％以上所生产的合金钛含量稳定，晶粒细化效果明显，晶粒尺寸随钛含量的变化趋势与熔配加钛合金相同；钛含量在0．1％～0．2％范围内时，电解加Ti合金晶粒细化效果的抗退化能力比熔配加Ti合金的强。     

弥散相对铝合金冷轧过程中晶粒分裂和织构演变的影响

制备了2种含有相近合金元素含量但不同的弥散第二相密度的铝锰合金冷轧板材。利用背散射电子衍射和取向成像图技术研究了晶粒分裂和微观织构,使用X射线衍射技术测量了宏观织构。研究发现当应变达到3左右时,高密度的小尺寸弥散相会促进铜型和S型织构的形成,同时增加了大角晶界所占比例。在小应变下,弥散相对织构和大角晶界的影响不显著。研究显示弥散相通过钉扎晶界从而促进织构演化,进而影响晶粒分裂和大角晶界的形成。     

Heterogeneous nucleation on potent spherical substrates during solidification

For a spherical-cap nucleus to become a “transformation nucleus”, the linear dimension (d) of the flat substrate must exceed the critical nucleus size (2r¹). This Turnbull criterion (d ⩾ 2r¹) defines a minimum undercooling for grain formation on, and effective inoculation with, flat nucleating substrates. However, for nucleation on potent substrates the spherical-cap model is no longer tenable. The free growth model has in general considered the growth of a two-dimensional nucleus on a potent flat substrate. Inspired by the particle-core structures observed in magnesium alloys after inoculation with nearly spherical zirconium particles, a model has been proposed, on the basis of an adsorption and surface diffusion mechanism, for heterogeneous nucleation and grain formation on potent spherical substrates of d ⩾ 2r¹. The critical undercooling required is found to be approximately the same as that defined by Turnbull’s patch nucleation theory. The model shows excellent agreement with experiments compared from different perspectives.     

Experimental observation of solidification of undercooled single phase alloys

1　ＩＮＴＲＯＤＵＣＴＩＯＮＲｅｌａｔｉｖｅｌｙｌａｒｇｅｕｎｄｅｒｃｏｏｌｉｎｇｉｓｇａｉｎｅｄｉｎｂｕｌｋａｌ ｌｏｙｍｅｌｔｗｉｔｈｍｏｄｅｒａｔｅｃｏｏｌｉｎｇｒａｔｅｂｙｅｌｉｍｉｎａｔｉｎｇｔｈｅｓｏｌｉｄｉｎｃｌｕｓｉｏｎｓｉｎｔｈｅｍｅｌｔａｎｄｂｙｅｌｉｍｉｎａｔｉｎｇｔｈｅｅｆ ｆｅｃｔｏｆｃｏｎｔａｉｎｅｒｗａｌｌｏｎｔｈｅｔｒｉｇｇｅｒｉｎｇｏｆｎｕｃｌｅａｔｉｏｎ .Ｔｈｅａｃｃｕｍｕｌａｔｅｄｄｒｉｖｉｎｇｆｏｒｃｅｆｏｒｃｒｙｓｔａｌｌｉｚａｔｉｏｎｃａｕｓ ｅｓｒａｐｉ…     

Dendrite growth morphologies in aluminium alloys

Different aluminium alloys, in particular Al–Cu, Al–Mg and Al–Si, have been directionally solidified under well-controlled thermal and convection conditions. For relatively high solidification rates, particular growth morphologies were observed. The most common is linked with the formation of feathery grains: these are constituted by series of twinned lamellae, in which the dendrites have 110 trunks cut through by {111} twin planes. These grains undergo a selection mechanism which is similar to that occurring for regular 100 dendritic grains. The dendritic growth along 110 crystallographic directions is supposed to be due to a change in the anisotropy of certain properties of the alloy, such as the solid/liquid interfacial energy and/or the atom attachment kinetics. When solidification conditions become less favourable to 110 growth morphologies, a mixed dendritic form containing 110 trunks and 100 arms may be obtained. In the case of the 5182 Al–Mg type alloy, 110 columnar grains which were not twinned could be observed together with feathery crystals. The possibility of such changes in the growth direction of aluminium alloys was further demonstrated by the observation of dendrites of Al–Cu–Mg solidified in a Bridgman device. In this case, 112 dendrites grow and progressively change their growth direction, thus showing curved shapes.     

Variant selection during nucleation and growth of γ-massive phase in TiAl-based intermetallic alloys

This work deals with the mechanisms of nucleation and growth of γ-massive phase in TiAl-based intermetallic alloys. In particular, it focuses on the process of variant selection operating at both stages of the transformation. Small γ-massive domains produced by rapid cooling are extensively characterized by high-resolution electron backscatter diffraction. This large data set allows a statistical analysis of nucleation sites, according to different crystallographic configurations. It is established that, whatever the nucleation sites, i.e., grain boundaries or triple junctions, a coherent facet is always found presenting a Blackburn orientation relationship (BOR) between the γ nucleus and the α parent grain. Moreover, some γ nuclei can additionally present another semicoherent facet with an approximate BOR with the α host grain. A new nucleation mechanism, called “co-nucleation”, is highlighted for this type of double-faceted nucleus. Variant selection during nucleation is discussed for both types of nuclei, in terms of minimization of interface energy. In addition, it is shown that growth of γ-massive domains from their initial nucleus always involves successive {1 1 1} twinning. Variant selection also occurs either at the nucleus growth stage or during the development of successive twin generations, and is discussed in terms of interphase boundary mobility.     

Real-time corrosion monitoring of aluminium alloys under chloride-contaminated atmospheric conditions

In this study, the use of electrical resistance (ER) sensors to monitor the corrosion of Al₉₄Cu₆ alloy is assessed and compared with 2024-T3 coupons. Under uniform corrosion, a good correlation was found between the ER sensors and mass loss on coupons. Three different chloride depositions are studied: (i) pre-contamination with dry/wet cycles, (ii) Volvo standard accelerated corrosion test and (iii) neutral salt spray test. The obtained results show good reproducibility of the ER sensors under all tested conditions. This suggests that ER sensors more levelled the effect of localised corrosion through a large surface evaluation compared with cross-sections. The corrosion thickness obtained with the ER sensors does not correspond to the mean depth obtained by cross-sections. This can be explained by the distribution and size of the localised corrosion events according to a finite element model proposed. The ER method allows obtaining useful real-time corrosion data for the understanding of the corrosion mechanisms and the development of accelerated tests. The chloride concentration, the frequency of salt application and wet/dry cycles have a strong influence on the corrosion rate of aluminium alloys.     

包晶合金定向凝固中的共生生长

对包晶合金定向生长特性进行分析，并与共晶系经典共生生长模型（Jackson-Hunt模型）所描述的共生生长特性进行对比，提出包晶稳态过热共生生长模型，同时以Fe-Ni合金系为对象，对其亚包晶和过包晶成分在不同温度梯度G和生长速度1，下进行定向凝固实验及组织分析。研究结果表明：在定向凝固条件下，包晶系两相严格的共生生长只有在进入稳态且温度高于包晶相变温度时才可能进行；包晶合金两相的共生生长尤其在G／v值高的平／胞界面形态，确实发生在Tp温度以上，与提出的模型所预测的一致。     

Direct observation of the effect of strontium on porosity formation during the solidification of aluminium-silicon alloys

Strontium is added to many aluminium-silicon casting alloys at the level of a few hundred parts per million in order to modify the structure of the eutectic. However, the addition of strontium also alters the amount and morphology of the porosity formed during solidification. The effect of strontium on porosity formation is not well understood and there is no consensus upon the cause of the changes observed. This paper investigates the effect of strontium, and its interaction with titanium-boride grain refiner, upon pore nucleation and growth using an experimental technique that allows the in situ observation of pore formation. The technique is a combination of a traditional temperature gradient stage¹ and real time micro-focus radiography, termed an x-ray temperature gradient stage or XTGS.

The addition of strontium was found to depress the eutectic temperature causing an increased total time for pore growth; however, the initial growth rate of the pores was also reduced, producing little net change in final size. Strontium additions also altered the nucleation and stability of pores within the semi-solid region. Strontium in combination with titanium-boride grain refiner produced an even stronger change in the nucleation and stability of the pores, allowing pores that floated out of the semi-solid region to remain stable, rather than dissolving back into the liquid aluminium.