Grain structures in aluminum alloy GTA welds

The grain structures in two dimensional GTA welds of a number of commercial aluminum alloys have been studied in order to clarify the mechanisms of grain refinement in welds. At low welding speeds and heat inputs the structures were either axial (continuous grains along the weld centerline) or stray (intermittent new grains). At higher speeds and heat inputs the structures were generally either columnar to the weld centerline, or contained some equiaxed grains at the center. Regression analyses indicated that both stray grains and equiaxed grains were favored by increased titanium content. In several alloys, titanium-rich compounds, and in one alloy, zirconium-rich compounds were found at the centers of dendrites. It is concluded that both stray and equiaxed grains originate by heterogeneous nucleation, with possible secondary effects due to constitutional undercooling. Formerly Graduate Student, University of Waterloo     

Grain structures in gas tungsten-arc welds of austenitic stainless steels with ferrite primary phase

The grain structures were investigated in full penetration gas tungsten-arc (GTA) welds in sheets of 304 and 321 austenitic stainless steels for a range of welding conditions. In type 321 steel welds, fine equiaxed ferrite dendrites were observed in the ferrite phase. The equiaxed structure was ascribed to heterogeneous nucleation of ferrite on Ti-rich cuboidal inclusions present in this steel, since these inclusions were observed at the origin of equiaxed dendrites. In type 304 welds, the ferrite grains were columnar, except in less complete penetration specimens, where a few coarse equiaxed dendrites appeared to originate from the weld surface. The secondary austenitic grain structure was columnar in both steels. In type 304 steel, the columnar austenitic grain structure did not necessarily correspond to the primary ferrite grains. In type 321 steel, the secondary austenite was columnar despite the equiaxed structure of the primary ferrite. Factors which affect the columnar-to-equiaxed transition (CET) are discussed. The failure to form equiaxed austenitic grains in type 321 steel is ascribed to austenite growing across the space between ferrite grains instead of renucleating on the primary equiaxed ferrite.     

双螺旋搅拌法制浆工艺参数对AZ91D合金半固态组织的影响

双螺旋搅拌工艺制备了AZ91D合金半固态浆料,获得金属半固态加工要求的细小、等轴分布均匀的非枝晶组织。研究了熔体温度、搅拌速度和剪切时间对该组织的影响规律。结果表明:采用合适的搅拌工艺,可获得金属半固态加工要求的细小、等轴分布均匀的非枝晶组织。即降低降体温度、搅拌速度及适当剪切时间有利于细小球形晶粒的形成。加大搅拌速度及合理搅拌时间可以细化晶粒,主要是由于是高剪切作用下熔体获得一个非常均匀温度场,从而增加有效形核的缘故。     

Multiphase Solidification Modeling of Solidification Structure Evolution and Macrosegregation of Round Bloom Continuous Casting Process with Mold Electromagnetic Stirring and Final Electromagnetic Stirring

Based on Eulerian–Eulerian method, a 3D/2D multiphase solidification model, which takes into consideration the heat transfer and fluid flow with grains nucleation and crystal growth, is developed to predict the macrostructure evolution and macrosegregation in continuously cast round bloom. The results show that the mold electromagnetic stirring (M-EMS) can accelerate superheat dissipation and promote grain nucleation, but the horizontal swirl induced by M-EMS has a strong washing effect on the solidification front and leads to subsurface negative segregation. When the M-EMS current intensity increases from 200 to 300 A, the subsurface negative segregation ratio decreases from 0.935 to 0.875. The final electromagnetic stirring (F-EMS) not only improves the center segregation, but also leads to the formation of negative segregation zone near the round bloom center due to the enhancement of solute washing induced by F-EMS. As the F-EMS current intensity increases from 150 to 300 A, the center segregation ratio decreases from 1.148 to 1.075, and the negative segregation ratio near the strand center decreases from 0.985 to 0.977. Another phenomenon is found that the nucleation of equiaxed grain ahead of columnar tips restrains the solute diffusion and leads to a local macrosegregation in columnar-to-equiaxed transition zone.     

镁合金晶粒异质形核的细化方法

总结了目前镁合金晶粒细化技术中广泛应用的异质形核细化方法,并对其细化机理进行论述。边对边匹配(edge-to-edge matching,简称E2EM)模型与第一性原理计算方法均可以判断镁合金的潜在异质形核核心,是近年来研究异质形核晶粒细化的有效方法。向镁合金中添加溶质元素及中间合金等物质已成为现工业生产中广泛适用的细化技术。为了更好地从微观尺度理解晶粒细化机制,基于密度泛函理论(density functional theory,简称DFT)的第一性原理计算方法不仅能够准确提供界面处原子结合情况,还可以定量预测凝固中异质晶核与初生相之间界面能和黏附功等,为晶粒细化剂的发展提供了理论基础,在镁合金晶粒细化中发挥越来越重要的作用。     

搅拌磁场对Ti1023铸锭宏观组织和成分的影响

研究了真空自耗电弧熔炼Ti1023钛合金过程中,搅拌磁场对铸锭宏观组织和铁元素含量的影响。结果表明,熔融金属在没有搅拌磁场的状态下凝固,铸锭的晶粒粗大且中间等轴晶和柱状晶的界线比较明显。施加搅拌磁场后,晶粒变细。磁场对不同熔化阶段的铸锭组织的影响程度不同,在熔炼阶段对组织影响最强,补缩阶段次之,起弧阶段最弱。未施加磁场时,熔池较窄,施加磁场后,熔池变宽,并且铁元素的偏析有一定程度的减轻。     

The Role of Carbon in Grain Refinement of Cast CrFeCoNi High-Entropy Alloys

As a promising engineering material, high-entropy alloys (HEAs) CrFeCoNi system has attracted extensive attention worldwide. Their cast alloys are of great importance because of their great formability of complex components, which can be further improved through the transition of the columnar to equiaxed grains and grain refinement. In the current work, the influence of C contents on the grain structures and mechanical properties of the as-cast high-entropy alloy CrFeCoNi was chosen as the target and systematically studied via a hybrid approach of the experiments and thermodynamic calculations. The alloys with various C additions were prepared by arc melting and drop cast. The as-cast macrostructure and microstructure were characterized using optical microscopy, scanning electron microscopy, and transmission electron microscopy. The cast HEAs transform from coarse columnar grains into equiaxed grains with the C level increased to ≥ 2 at. pct and the size of equiaxed grains is further decreased with the increasing C addition. It is revealed that the interdendritic segregation of Cr and C results in grain boundary precipitation of M₂₃C₆ carbides. The grain refinement is attributed to the additional constitutional supercoiling from the C addition. The yield stress and tensile strength at room temperature are improved due to the transition of columnar to equiaxed grains and grain refinement.     

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.     

Grain refinement of copper by the addition of iron and by electromagnetic stirring

The effect of iron additions in the range of 0.57 to 7.5 wt pet on the grain size of electromagnetically levitated copper-iron alloys was investigated. The samples were solidified while levitated or quenched in water from the molten state. The addition of iron was found to be effective in reducing the grain size of copper, and the average grain size decreased as the iron content was increased up to the peritectic liquid composition of about 2.8 wt pet Fe. Beyond this composition, the grain size of the samples solidified in the levitated state was insensitive to the iron content, whereas that of the quenched samples continuously decreased with increasing iron content. The results indicate that electromagnetic stirring causes fragmentation of copper dendrites in the hypoperitectic region, and hence enhances grain refinement. In the hyperperitectic region, on the other hand, the stirring has a detrimental effect on the grain refinement by agglomerating the primary iron particles which act as heterogeneous nucleation sites for the copper matrix.     

Grain Refinement of AZ31 Magnesium Alloy Weldments by AC Pulsing Technique

The current study has investigated the influence of alternating current pulsing on the structure and mechanical properties of AZ31 magnesium alloy gas tungsten arc (GTA) weldments. Autogenous full penetration bead-on-plate GTA welds were made under a variety of conditions including variable polarity (VP), variable polarity mixed (VPM), alternating current (AC), and alternating current pulsing (ACPC). AC pulsing resulted in significant refinement of weld metal when compared with the unpulsed conditions. AC pulsing leads to relatively finer and more equiaxed grain structure in GTA welds. In contrast, VP, VPM, and AC welding resulted in predominantly columnar grain structures. The reason for this grain refinement may be attributed to the periodic variations in temperature gradient and solidification rate associated with pulsing as well as weld pool oscillation observed in the ACPC welds. The observed grain refinement was shown to result in an appreciable increase in fusion zone hardness, tensile strength, and ductility.     

Mechanisms of the Origin of Fine and Non-Dendritic Grains at the Sonotrode–Liquid Metal Interface During Ultrasonic Solidification of Metals

Proposed grain refinement mechanisms during ultrasonic solidification have been explained in terms of refinement between cavitation enhanced nucleation and fragmentation of dendrites according to the casting conditions. Solidification studies also describe the activation of nucleation under pressure pulses after bubble implosion as an additional supporting mechanism for grain refinement. This study clarifies some overlooked concepts and proposes a plausible grain refinement mechanism explaining the role of cavitation in pure Zn and a Mg–6 wt pct Zn alloy. Equivalent grain size and grain density have been obtained in pure Zn and the Mg–6 wt pct Zn alloy (grain size distribution ranging from 40 to 200 µm) when UST was applied after the onset of solidification. These fine, non-dendritic grains originate from the cavitation zone beneath the sonotrode. Significant thermal undercooling surrounding the low superheat sonotrode in contact with the melt is responsible for the formation of a solidified layer (typically the thickness is equivalent to the average grain diameter) at the sonotrode–melt interface. High-frequency vibrations with or without cavitation at this interface assist the separation of these fine grains, which are then carried into the melt by acoustic streaming. A possible mechanism for the separation of fine grains produced from the cavitation zone is explained with the help of established concepts reported for the ultrasonic atomization process.

     

Modeling of the Coupling of Microstructure and Macrosegregation in a Direct Chill Cast Al-Cu Billet

The macroscopic multiphase flow and the growth of the solidification microstructures in the mushy zone of a direct chill (DC) casting are closely coupled. These couplings are the key to the understanding of the formation of the macrosegregation and of the non-uniform microstructure of the casting. In the present paper we use a multiphase and multiscale model to provide a fully coupled picture of the links between macrosegregation and microstructure in a DC cast billet. The model describes nucleation from inoculant particles and growth of dendritic and globular equiaxed crystal grains, fully coupled with macroscopic transport phenomena: fluid flow induced by natural convection and solidification shrinkage, heat, mass, and solute mass transport, motion of free-floating equiaxed grains, and of grain refiner particles. We compare our simulations to experiments on grain-refined and non-grain-refined industrial size billets from literature. We show that a transition between dendritic and globular grain morphology triggered by the grain refinement is the key to the explanation of the differences between the macrosegregation patterns in the two billets. We further show that the grain size and morphology are strongly affected by the macroscopic transport of free-floating equiaxed grains and of grain refiner particles.

     

Formation of Fine Clusters in High-Temperature Oxidation of Molten Aluminum

High-temperature oxidation of molten aluminum was investigated by high-resolution electron microscopes in order to determine the possibility of heterogeneous nucleation of aluminum grains on oxide for the grain refinement and structural uniformity of intensively melt-sheared aluminum alloys. High-resolution observations detect initial amorphous phase and gamma-alumina phase and show fine clusters with size of about 150 to 200 nm composed of extremely fine aluminum grains and gamma-alumina or amorphous aluminum oxide. Furthermore, high-resolution lattice images and diffraction patterns show no orientation relationship, although there is a specific orientation between gamma-alumina and aluminum along (111)[110] with high potency of heterogeneous nucleation. The volumetric shrinkage by the transformation of gamma- into alpha-alumina causes the surface oxide films to repeatedly rupture and leads to the creation of channels to the base melt surface for further oxidation of fresh metal. Based on the observations, the mechanism of high-temperature oxidation of molten aluminum and formation of the fine clusters as well as the possibility of the heterogeneous nucleation of aluminum grains are discussed.     

行波磁场铸流搅拌提升不锈钢板坯等轴晶率

为揭示各种行波磁场铸流搅拌的电磁冶金效果,基于计算域分段法建立了断面1280 mm×200 mm板坯连铸电磁、流动、传热和凝固的耦合模型,利用电气参数和磁感应强度的实测值和预测值的对比验证了模型的可靠性。研究表明:行波磁场搅拌器因电磁推力的方向性特点在板坯二冷区搅拌过程中均表现有不同程度与特征的端部效应,辊后箱式搅拌器(Box-typed electromagnetic stirrer, B-EMS)的单侧安装形式导致板坯内弧侧磁感应强度远大于外弧侧,辊式搅拌器(Roller-typed electromagnetic stirrer, R-EMS)的对辊安装形式则使磁感应强度呈现对称分布。在400 kW和7 Hz的相同电气参数下,R-EMS的电流强度比B-EMS高75 A;尽管箱式电磁搅拌的有效作用区域较辊式电磁搅拌大,铸坯中心钢液过热耗散区域大,但辊式搅拌推动钢液冲刷凝固前沿形核作用则明显大于箱式搅拌。两者均具有较好的抑制柱状晶生长、促进凝固前沿等轴晶形核与发展的能力,将不锈钢板坯等轴晶率提高至45%的门槛值以上,其中间隔型反向辊式搅拌器下的等轴晶率比箱式搅拌高约17%。综合表明,基于行波磁场铸流搅拌的间隔型反向辊式搅拌器有望更好地消除铁素体不锈钢板材表面皱折缺陷。      

Study of Soft Impingement of Diffusion Fields and Its Effect on Cast Microstructure

Rigorous Mullins and Sekerka perturbation theory has been applied to two solidification interfaces. The results indicate that as the distance between the two interfaces decreases, the rate of amplitude growth of perturbations on the planar solid–liquid interface becomes negative, which is a further verification of the stability of a planar interface due to soft impingement of diffusion fields during solidification. This work conclusively shows that spheroidal grains form due to grain refinement of Mg alloys by Zr addition because of the high density of nucleation.     

Resistance-Spot-Welded AZ31 Magnesium Alloys: Part I. Dependence of Fusion Zone Microstructures on Second-Phase Particles

A comparison of microstructural features in resistance spot welds of two AZ31 magnesium (Mg) alloys, AZ31-SA (from supplier A) and AZ31-SB (from supplier B), with the same sheet thickness and welding conditions, was performed via optical microscopy, scanning electron microscopy (SEM), X-ray diffraction (XRD), and transmission electron microscopy (TEM). These alloys have similar chemical composition but different sizes of second-phase particles due to manufacturing process differences. Both columnar and equiaxed dendritic structures were observed in the weld fusion zones of these AZ31 SA and SB alloys. However, columnar dendritic grains were well developed and the width of the columnar dendritic zone (CDZ) was much larger in the SB alloy. In contrast, columnar grains were restricted within narrow strip regions, and equiaxed grains were promoted in the SA alloy. Microstructural examination showed that the as-received Mg alloys contained two sizes of Al₈Mn₅ second-phase particles. Submicron Al₈Mn₅ particles of 0.09 to 0.4 μm in length occured in both SA and SB alloys; however, larger Al₈Mn₅ particles of 4 to 10 μm in length were observed only in the SA alloy. The welding process did not have a great effect on the populations of Al₈Mn₅ particles in these AZ31 welds. The earlier columnar-equiaxed transition (CET) is believed to be related to the pre-existence of the coarse Al₈Mn₅ intermetallic phases in the SA alloy as an inoculant of α-Mg heterogeneous nucleation. This was revealed by the presence of Al₈Mn₅ particles at the origin of some equiaxed dendrites. Finally, the columnar grains of the SB alloy, which did not contain coarse second-phase particles, were efficiently restrained and equiaxed grains were found to be promoted by adding 10 μm-long Mn particles into the fusion zone during resistance spot welding (RSW).     

Grain refinement of aluminum alloys: Part II. Confirmation of, and a mechanism for, the solute paradigm

In Part I of this article, the literature underpinning both the nucleant and solute paradigms was explained, and the validity of the paradigm shift toward the solute paradigm, as a more complete understanding of grain refinement, was presented. In this Part II, experimental work is presented which confirms the validity of the solute paradigm. TiB₂ particle additions were found to refine the columnar zone of pure aluminum; however, an equiaxed structure was only observed when a small amount of titanium was added as solute. The potency of nucleant particles was confirmed by thermal analysis, which showed that additions of TiB₂ to pure aluminium removed the nucleation undercooling. Upon the addition of more TiB₂ particles and titanium as solute, the grain size continued to decrease until an apparent minimum grain size was achieved, past which little further refinement occurs. That the segregating ability of solute elements in general is essential for grain refinement, and not only that of titanium in particular, was confirmed by comparison of the Al-2Si and Al-0.05Ti systems. Finally, a mechanism of grain refinement is presented that incorporates both nucleant particles and solute segregation as essential for effective grain refinement. The solute is required to form a constitutionally undercooled zone in front of the growing solid/liquid interface to facilitate further nucleation on the substrates present. The potency of the nucleants dictates the probability of nucleation occurring for a given degree of constitutional undercooling.     

The effect of alloy composition and welding conditions on columnar-equiaxed transitions in ferritic stainless steel gas-tungsten arc welds

The columnar-equiaxed transition (CET) was investigated in full penetration gas-tungsten arc (GTA) welds on ferritic stainless steel plates containing different amounts of minor elements, such as titanium and aluminum, for a range of welding conditions. In general, the fraction of equiaxed grains increased, and the size of the equiaxed grains decreased with increasing titanium contents above 0.18 wt pct. At a given level of titanium, the equiaxed fraction increased, and the size of the equiaxed grains decreased with increased aluminum content. The CET was ascribed to heterogeneous nucleation of ferrite on Ti-rich cuboidal inclusions, since these inclusions were observed at the origin of equiaxed dendrites in the grain refined welds. Titanium-rich cuboidal inclusions, in turn, were found to contain Al-Ca-Mg-rich inclusions at their centers, consistent with observations by previous investigators for other processes. The welding conditions, in particular, the welding speed, were observed to affect the occurrence of the CET. Increasing the welding speed from 3 to 8 mm/s increased the equiaxed fraction noticeably, but a further increase in speed to 14 mm/s had a smaller additional effect. A finite element model (FEM) of heat transfer was used to examine the role of the welding conditions on the local solidification conditions along the weld pool edge. The results are compared with existing models for the CET. Formerly Postdoctoral Fellow, Department of Mechanical Engineering, University of Waterloo     

Microstructure Formation in AlSi7Mg Alloys Directionally Solidified in a Rotating Magnetic Field

To produce high stressed automotive components like engine frames and cylinder heads in foundry industry often AlSi7Mg alloys are used. During mould filling and casting melt flow affects the development of the microstructure, which defines the mechanical properties. In this paper the microstructure formation in AlSi7Mg0.3 and AlSi7Mg0.6 alloys during directional solidification is investigated. To induce a forced melt flow a rotating magnetic field is applied. For that purpose a Bridgman‐type gradient furnace is equipped with a rotary ring magnet. For detailed investigation of the shape of the solid‐liquid interface and the primary dendrite spacing a decanting device is used. As a result, the forced melt flow substantially changes the dendritic solidification microstructure. The rotating magnetic field generates a radial secondary flow in and ahead of the mushy zone, which causes an enrichment of eutectics in the centre of the samples. At lower solidification velocities this locally leads to the transition to mixed columnar‐equiaxed or even to equiaxed growth. In that case the solid‐liquid interfaces of the decanted samples show a significant depression in the centre part. In the out‐of‐centre region columnar growth still exists and the primary dendrite spacing decreases with increasing melt flow.     

Effects of forced electromagnetic vibrations during the solidification of aluminum alloys: Part II. solidification in the presence of colinear variable and stationary magnetic fields

The influence on grain refinement of electromagnetic vibrations imposed during solidification of various aluminum alloys has been examined. The vibrations were produced, without any material contact with the solidifying alloys, by the simultaneous application of a stationary magnetic fieldB ₀ and a periodic magnetic fieldb(t) of 50 Hz frequency. Extensive grain refinement has been observed in both continuous casting and batch-type mold casting. This investigation shows that the mean grain size obtained by this electromagnetic vibrational method is smaller than that produced by the variable magnetic field acting alone (electromagnetic stirring), particularly when the alloys are characterized by a narrow freezing range.