Rheological behaviour of AZ91D magnesium alloy in semisolid state

Abstract

The rheological characterisation of semisolid AZ91D magnesium alloy has been successfully studied using a Couette rheometer. Different applied regimes by varying the temperature and the shearing in semisolid state revealed the rheological behaviour of the alloy and were confirmed by microstructural evolution. The apparent viscosity was the critical parameter that was studied during the applied regimes. The change in the morphology of the solid phase during the thermomechanical treatment was the main cause of the evolution of the apparent viscosity. The evolution of the morphology of the solid phase, from dendritic to globular, due to the shearing, had a significant impact on the rheological behaviour of the slurry which was correlated to the operating conditions by an analytical model.

L’étude de la caractérisation rhéologique de l’alliage de magnésium de type AZ91D à l’état semi-solide a révélé une étroite dépendance entre les conditions opératoires et les changements structuraux. Les régimes thermo-mécaniques choisis ont montré que la vitesse de refroidissement ainsi que le cisaillement imposé avaient un effet direct sur la viscosité apparente de l’alliage. Le passage d’une structure conventionnelle dendritique à une structure globulaire a réduit de manière significative la viscosité, tributaire de l’intensité ainsi que du temps de cisaillement selon un modèle analytique.     

Prediction of the structural state of an AD31 alloy extruded section

Computer simulation is used to study the effect of the temperature-rate compression parameters on the structural state in an AD31 alloy radiator section. A method for predicting its structural state is proposed. The developed technique is subjected to experimental-industrial testing. This technique is shown to be promising for predicting the structural state in an extruded section.     

Development of a novel hypereutectic aluminum-siliconmagnesium alloy for die casting

A novel hypereutectic die casting Al-Si-Mg alloy with relatively high Mg content was developed. The optimized microstructure of the alloy is characterized by a fine dispersion of eutectic Si and Mg₂Si particles in an α-Al matrix which is devoid of primary silicon particles. Of the several modifiers used to refine the coarse morphology of the Mg₂Si particles, an optimum combination of strontium and misch metal yielded the best results. The refined microstructure after a standard T6 heat treatment accounts for the enhanced room temperature tensile properties of castings made from this alloy.     

喷射成形含锰高强Al-Zn-Mg-Cu的组织和性能

利用喷射成形和热挤压的方法制备了含锰为2%(质量分数)的高强铝合金Al-8.8Zn-2.9Mg-1.6Cu合金,用X射线衍射(XRD)、光学显微镜(OM)和扫描电镜(SEM)和透射电镜(TEM)等方法研究了铝合金的微观组织,并进行了力学性能测试.研究结果表明:喷射成形制备的含锰铝合金经过热挤压和固溶处理后,基体组织为细小均匀的再结晶晶粒组织,平均尺寸约8μm.MnAl6颗粒在喷射沉积过程中沿晶界析出,经过热挤压后,尺寸大的颗粒破碎,大部分的颗粒则沿挤压方向产生一定的塑性变形.固溶处理对MnAl6颗粒尺寸的影响不明显,但棒状/片状颗粒的边角处发生球化,有利于降低诱发显微裂纹的应力或应变集中.时效后,铝合金达强度为775MPa,延伸率达4.3%,断口以细小的韧窝为主,尺寸小于500nm.     

Texture and anisotropy of mechanical properties of spray-formed alloy tool steels

The initial texture and anisotropy of alloyed tool steel specimens are studied by constructing pole figures and conducting compression tests. Scattered initial axial crystallographic growth texture and weak anisotropy of yield stress and ultimate strength are established. A relationship between deformability and texture is determined. __________ Translated from Poroshkovaya Metallurgiya, Vol. 46, No. 3–4 (454), pp. 26–31, 2007.     

Microstructure and properties of semisolid hypereutectic high chromium cast iron prepared by slope cooling body method

Abstract

To improve the toughness of hypereutectic high chromium cast iron, the morphology of the primary carbides were improved by the slope cooling body method, a semisolid forming process. The semisolid specimens were prepared under different forming techniques, and the microstructure, impact toughness and wear resistance were studied. The results indicate that the morphology of primary carbide is improved, impact toughness initially increases then remains constant as the semisolid forming temperature increases, impact toughness was improved, but wear resistance was reduced.     

Yield behavior of commercial Al-Si alloys in the semisolid state

Systematic experimental work and modeling efforts have been conducted to characterize the yield behavior of commercial aluminum alloys in the semisolid state. In this study, extensive compression experiments were performed to measure the yield stress of semisolid aluminum slurries at high solid fractions (0.5 to 1.0), and a cone penetration method was employed to measure yield stress at low solid fractions (<0.5). A functional relationship between yield stress and temperature/solid fraction has been established for these alloys. The effect of the processing route on the resultant yield stress of the material in the semisolid state was studied by evaluating commercial A356 billets manufactured via magnetohydrodynamic stirring, grain refining, and UBE’s new rheocasting (NRC) processes, respectively. Detailed microstructure observations and image analyses reveal that the difference in yield-stress values among the alloys evaluated is intricately related to the semisolid structure. At a given solid fraction, the yield stress of semisolid slurries depends on microstructural indices (i.e., entrapped-liquid content, shape factor of the alpha phase, and the alpha particle size). In addition, numerical simulation results indicate that the finite yield stress of semisolid metals plays a significant role in determining the flow pattern during die filling. Depending on processing conditions, five distinct filling patterns (shell, disk, mound, bubble, and transition) have been identified and confirmed through experimental observations. Recent simulations demonstrate that the finite yield stress is also responsible for flow instabilities encountered in commercial forming operations, such as “toothpaste behavior.” Specifically, most flow instabilities can be avoided by properly controlling processing parameters and the initial semisolid microstructure. A stability map that provides a control guide for semisolid processing has been developed and is presented.     

Inhibited coarsening of solid-liquid microstructures in spray casting at high volume fractions of solid

Experimental studies on coarsening in fine grained solid-liquid microstructures at high volume fractions of solid (f_s) have been carried out to determine if inhibited coarsening under these circumstances could account for the anomalous fine cell sizes observed in spray castings. The materials investigated included a chill-cast dendritic binary alloy of Al-Cu, two spray cast alloys—AA2014 and Cu-Ti, whose grain size was the segregate spacing, and a d.c.-cast alloy Al-4.5 wt% Cu-1.5 wt% Mg in both coarse-grain and grain-refined conditions. The observed segregate spacings after coarsening were smaller than that predicted by empirical correlations of dendrite arm spacing and freezing time. In all cases, the coarsening was found to become slower as the temperature was reduced and f_s increased. Conventional coarsening theories and experiments predict the opposite, i.e. faster coarsening at higher volume fractions of solid. Two additional coarsening models were developed for the grain growth at high volume fractions of solid by processes whose rates are limited by migration of liquid at grain boundaries as liquid films on 2-grain surfaces or liquid rods on 3-grain triple points. In both models, the conventional diffusion-limitedt¹³ coarsening law was reproduced, but the rate constant K contained the term 11−f_s and so also predicted accelerated coarsening asf_s → 1 . Three possible explanations for the observed lower K values at increasing f_s are proposed. The first is the effect of the increasing difference between the solute contents of solid and liquid as the temperature is reduced. This produces a1/X₁ dependence of the coarsening rate constant K. The second inhibiting effect, specific to dendritic structures, is in-grain coalescence of dendrite arms at high f_s which produces isolated liquid particles within the grains. The final possibility is particle-inhibition of grain boundary migration by minority (impurity) particles at the grain boundaries. Such particles were seen, however, for only two of the alloys, viz. the grain defined d.c. cast Al-4.5 wt% Cu-1.5 wt% Mg and the spray case AA2014, but they or gas-filled pores are proposed as strong possibilities to account for the fine grain sizes observed in all spray cast microstructures.     

An investigation of the effects caused by electromagnetic vibrations in a hypereutectic Al-Si alloy melt

In order to investigate the mechanism by which electromagnetic vibrations affect the solidification structure of metallic alloys, an experimental apparatus which enables the simultaneous application of electric and magnetic fields under different cooling conditions (ranging from rapid to furnace cooling) is developed. The objective is followed by inducing vibrations in a hypereutectic Al-Si alloy melt containing suspended silicon particles and interrupting the process at different temperatures before and after the start of solidification by water quenching. Interrupting the process at temperatures higher than the liquidus has revealed the effects of vibrations independent of the influences of the nucleation and growth phenomena. Establishing the conditions for obtaining identical cooling rates in experiments with different experimental conditions has lead to the exclusion of effects resulting from the differences in cooling rates and recognition of the effects caused only by electromagnetic vibrations. It is found that the application of either of the two fields alone has no significant effect on the solidified structure, while profound effects are observed when the two fields are applied simultaneously. An increase in the number of suspended silicon particles and a reduction in their average size are the effects noticed before the start of solidification. In this research, it has for the first time been clearly verified, through microscopic observation of the quenched samples, that these effects are brought about by the cavitation phenomenon. After the start of solidification, particles are locally agglomerated and expelled toward the surrounding walls under a combined influence of electromagnetic vibrations and pinch force squeezing the liquid. The final structure obtained is composed of an almost completely eutectic matrix surrounded by agglomerates of silicon particles along the outer surface.     

Microstructure and rheological properties of a semisolid A356 alloy with erbium addition

The effects of erbium addition on the rheological properties and microstructure of a semisolid A356 alloy were studied. The semisolid slurries were prepared through the serpentine channel technique before they were thixoformed using parallel-plate compression with cylindrical discs. The grain and globule size decreases as the Er content increases, resulting in an improved and uniform distribution of spherical primary α-Al phase within the semisolid slurry. The addition of the Er modifies the grain morphology and size of the α-Al grains, resulting in a better and more uniform distribution of spherical primary α-Al phase within the semisolid slurry. As a result, rheocast quality index increases with the addition of Er, which is suitable for the thixoforming process. The A356 alloy without Er has the highest viscosity herein. The viscosity decreases, and the flow characteristics of the semisolid feedstock are expected to improve when Er is added as a result of the refinement of primary α-Al and modification of eutectic silicon. Furthermore, the refined semisolid A356 alloys with Er show a slightly larger fraction of high-angle grain boundaries compared to that for the unre?ned alloy.     

Microstructural features and heat flow analysis of atomized and spray-formed Al-Fe-V-Si alloy

Microstructural features of rapidly solidified powders and preforms of Al₈₀Fe₁₀V₄Si₆ alloy produced by spray forming process have been studied. The atomization and spray deposition were carried out using a confined gas atomization process and the microstructural features were characterized using scanning electron microscopy and transmission electron microscopy (TEM) and X-ray diffraction (XRD) techniques. The microstructure of a wide size range of atomized powders invariably revealed cellular and dendritic morphology. The extent of dendritic region and the dendritic arm spacing were observed to increase with powder particle size. The TEM investigations indicated the presence of ultrafine second-phase particles in the intercellular or interdendritic regions. In contrast, the spray deposits of the alloy showed considerable variation in microstructure and size and dispersion of the second-phase particles at specific distances from the deposit-substrate interface and the exterior regions of the deposit. Nevertheless, considerable homogeneity was observed in the microstructure toward the center of the spray deposit. The formation and distribution of a cubic phase α-Al(Fe,V)Si has been characterized in both atomized powders and spray deposits. A one-dimensional heat flow model has been used to analyze the evolution of microstructure during atomization and also during spray deposition processing of this alloy. The results indicate that thermal history of droplets in the spray on deposition surface and their solidification behavior considerably influence the micro-structural features of the spray deposits.     

Dispersion strengthening in a hypereutectic Al-Si alloy prepared by extrusion of rapidly solidified powder

When a hypereutectic aluminum-silicon alloy containing 16 wt pct silicon was rapidly solidified into powder using the spinning water atomization process, the individual powder grains were predominantly aluminum that was supersaturated with silicon and also contained well-dispersed 0.02-μm silicon particles. Although the silicon particles grew when the powder was extruded into a bar at temperatures from 673 to 803 K at an extrusion ratio of 4.3 and an extrusion speed of 0.9 mm/s, the average diameter was maintained on a submicron level. When the extrusion temperature was decreased from 803 to 673 K, the average diameter of the silicon particles in the extruded bar decreased from 0.8 to 0.5 μm, while the Vickers hardness (HV) and the ultimate tensile strength of the extruded bar increased from 120 to 160 (HV) and from 330 to 500 MPa, respectively. Both the hardness and the tensile strength of the extruded bars were several times higher than those of conventionally cast bars of the same alloy with cooling rates from 10⁻¹ to 10² K/s. On the other hand, the elongation decreased from 5.5 to 3.1 pct when the extrusion temperature was decreased from 803 to 673 K.     

Oxide coarsening and its influence on recrystallization in a mechanically alloyed Fe-base oxide-dispersion-strengthened alloy

Analytical transmission electron microscopy has been used to determine whether yttria particles readily coarsen in oxide-dispersion-strengthened ferritic stainless alloy (PM2000), in an attempt to contribute to a model that explains the directionality of the microstructure observed when this material undergoes recrystallization under isothermal conditions. Likewise, the role of Al₂O₃ particles in the recrystallization processes of alumina-enriched PM2000 alloy has been studied via the addition of 1 wt pct Al₂O₃ particles as dispersoids. A carbon extraction replica technique was used to show that no coarsening of the yttria dispersion occurred even under exaggerated heat treatment at 1380 °C for 1 hour. By contrast, a significant ripening of alumina particles occurred apparently without influence on the recrystallization processes.     

Kinetics of discontinuous coarsening of cellular precipitate in a Cu-15 wt% in alloy

The morphology and growth kinetics of the cellular precipitate and discontinuously coarsened cellular precipitate have been studied in the temperature range 573–731 K by utilizing optical and scanning electron microscopy and X-ray diffraction. In order to avoid precipitation of the Widmanstatten precipitate phase, which has a retarding effect on the rate of growth of primary cells, isothermal aging of the alloy was preferred. The Cu-In alloy was observed to decompose completely by cellular precipitation reaction into a lamellar structure consisting of alternate lamellae of the α and δ phases at all aging temperatures. The fine lamellar structure of the primary cells decomposed into a coarse lamellar structure consisting of the same two phases by a discontinuous coarsening or secondary reaction. Lattice parameter measurements indicated that whereas the depleted matrix was richer in solute than the equilibrium solvus during the primary reaction, it was very close to the equilibrium solvus during the secondary reaction. Analysis of the growth kinetics both of the primary and secondary cellular reaction indicated that the transformations are controlled by diffusion through the cell boundaries.     

热挤压对喷射沉积7055铝合金显微组织和力学性能的影响

采用全自动控制往复喷射成形工艺制备工业规格7055铝合金锭坯,研究热挤压工艺对喷射成形7055铝合金的显微组织和力学性能的影响。采用电子背散射衍射技术对经不同热挤压后7055铝合金的织构进行研究。结果表明,喷射沉积锭坯组织为等轴状晶粒,均匀细小(30~50μm),基体中不存在枝晶型偏析。由于喷射沉积工艺本身的特点,在合金中存在大量的显微疏松缺陷。沉积锭坯经过热挤压致密化后,合金力学性能显著提高,抗拉强度σb为390 MPa,伸长率δ为13.3%,表明热挤压工艺可有效消除疏松缺陷,从而充分发挥出喷射沉积工艺的优越性。EBSD分析表明,挤压后沿着挤压轴方向形成丝织构,主要为?001?与?111?两种织构。     

Mechanical behavior of aluminum matrix composite during extrusion in the semisolid state

Semisolid extrusion experiments have been carried out on SiC particle reinforced aluminum composite. Different die diameters and ram velocities have been used in order to obtain a range of extrusion conditions at a constant temperature of 580 °C. The experimental results were fitted to a multiple linear regression to obtain a constitutive equation describing the behavior of the material in these conditions. From the regression equation, it was possible to model the material by finite element analysis. The predicted values fo the extrusion force resulting from the numerical analysis are consistent with the experimental values for the different conditions. The flow of primary phase particles through the die observed on micrographs has been compared with the predicted pattern and shows good agreement. These results also justify the use of a frictional stress factor corresponding to a sticking condition at the interface between the billet and the die components.     

Solidification of hypereutectic Al-38 Wt Pct Cu alloy in microgravity and in unit gravity

Solidification in microgravity aboard the space shuttle Endeavour resulted in a dramatic change in the morphology of the primary Al₂Cu phase compared to ground-based solidification in unit gravity. An Al-38 wt pct Cu ingot directionally solidified at a rate of 0.015 mm/s with a temperature gradient of 1.69 K/mm exhibited large, well-formed dendrites of primary Al₂Cu phase. Ingots solidified under similar conditions in unit gravity contained primary Al₂Cu phase with smooth, faceted surfaces. The primary Al₂Cu phase spacing in the microgravity ingot was much greater than that in the unit gravity ingot, 670 μm compared to 171 μm. It is suggested that thermosolutal mixing in the unit gravity ingot reduces the buildup of an Al-rich layer at the solid/liquid interface, which increases the stability of the interface resulting in smooth, faceted particles of Al₂Cu phase. It is also suggested that the large difference in primary phase spacings is due mostly to the difference in morphology rather than changes in parameters that might influence dendrite ripening mechanisms. The presence or absence of gravity had no effect on the interlamellar spacing of the inter-Al₂Cu phase eutectic. The ingot solidified in microgravity exhibited almost no longitudinal macrosegregation, in agreement with the theory of inverse segregation in the absence of thermosolutal convection. The ingot solidified in unit gravity exhibited considerable longitudinal macrosegregation, with the chilled end having about 6 wt pct more Cu than the average composition. It is not clear whether the segregation results from thermosolutal convection during solidification or from sedimentation during melting.