The influences of the microstructure morphology of A356 alloy on its rheological behavior in the semi-solid state

In this paper, the rheological behavior of semi-solid A356 alloy with different solid morphology was studied with an improved static shear test method. The results indicated that the rheological behavior of the alloy was significantly influenced by the structural morphology of the alloy. The alloy had quite different rheological properties even though the same fraction of solid existed in the semi-solid state. The rheological behavior of the alloy fitted a five-element model (H₁–[N₁|H₂]–[N₂|S]) for the as-cast microstructure with developed primary (α-Al dendrites, whereas it fitted a six-element model ([H₁|S₁]–[N₁|H₂]–[N₂|S]) for degenerated dendritic or spheroidal primary α-Al, which had been obtained by electromagnetic stirring and spray deposition, respectively. Computation results showed that the deforming capability and shear rate of the semi-solid alloy increased remarkably with the change of primary α-Al from developed dendrites to degenerated dendrites, and then to spheroidal structures. On the other hand, the temperature dependence of the rheological properties of the semi-solid alloy with spheroidal or degenerated dendritic primary α-Al was much less than that with developed primary α-Al dendrites.     

Study of Microstructure Evolution during Semi-Solid Processing of an In-Situ Al Alloy Composite

In the present work, effect of pouring temperature (650°C, 655°C, and 660°C) on semi-solid microstructure evolution of in-situ magnesium silicide (Mg₂Si) reinforced aluminum (Al) alloy composite has been studied. The shear force exerted by the cooling slope during gravity driven flow of the melt facilitates the formation of near spherical primary Mg₂Si and primary Al grains. Shear driven melt flow along the cooling slope and grain fragmentation have been identified as the responsible mechanisms for refinement of primary Mg₂Si and Al grains with improved sphericity. Results show that, while flowing down the cooling slope, morphology of primary Mg₂Si and primary Al transformed gradually from coarse dendritic to mixture of near spherical particles, rosettes, and degenerated dendrites. In terms of minimum grain size and maximum sphericity, 650°C has been identified as the ideal pouring temperature for the cooling slope semi-solid processing of present Al alloy composite. Formation of spheroidal grains with homogeneous distribution of reinforcing phase (Mg₂Si) improves the isotropic property of the said composite, which is desirable in most of the engineering applications.     

Effects of enhanced solution treatment on microstructure and mechanical properties of Al–Cu–Li–Sc alloy

Two enhanced solution treatments (ESTs) were applied to an Al–Cu–Li–Sc alloy. Results showed that the ESTs reduced the amount and size of the soluble phases, and promoted the recrystallisation of the α–Al matrix and the precipitation of the Al₂CuLi precipitate (T₁), which improved the yield strength, tensile strength and elongation of the alloy. Although the precipitation strengthening of the T₁ phase and the strengthening resulting from grain refinement of the α–Al matrix caused by the recrystallisation contributed equally to the strength increment, the EST process led to a greater proportional increase in the strengthening resulting from grain refinement than it did in the precipitation strengthening of the alloy.     

Hot tensile behavior of an extruded Al–Zn–Mg–Cu alloy in the solid and in the semi-solid state

This is the first reported research into the tensile behavior of as-deformed Al–Zn–Mg–Cu alloy in the semi-solid state. Tensile tests of extruded 7075 aluminium alloy were carried out in the high temperature solid and semi-solid states. Based on the tensile results and microstructural examination, the tensile behavior can be divided into three stages according to the effect of liquid: one behaves in predominantly ductile character between 400 and about 520 °C (f_l ∼ 0.31%), one is governed by both of solid and liquid between 520 and 550 °C (f_l ∼ 2%), and almost completely dominated by liquid above ∼550 °C. A brittle temperature range (519–550 °C) is proposed, in which the as-deformed Al–Zn–Mg–Cu alloy exhibits large crack probability. An equation based on ultimate tensile stress and temperature is proposed.     

Microstructure Control and Performance Evolution of Hypereutectic Al–20Mg2Si Alloy by Novel Al–Ca–Sb Masteralloy

Herein, the microstructure control and performance evolution of hypereutectic Al–20Mg₂Si alloy with the addition of novel Al–3.3Ca–10Sb master alloy are investigated. It is found that AlCa₁₁Sb₉ and CaSb₂ compounds are successfully synthesized through in situ melt reaction of masteralloy. With 0.45 wt% Al–3.3Ca–10Sb master alloy addition, primary Mg₂Si particles in hypereutectic Al–20Mg₂Si alloy are significantly refined from more than 150 μm to 10.7 μm, which are accompanied with the 3D morphologies changing from dendrites to octahedrons. After heat treatment, Brinell hardnesses of Al–20Mg₂Si alloys are remarkably improved to 112 HB. Furthermore, it is also found that the cooling rate of Al–3.3Ca–10Sb master alloys has certain influence on the refinement effect of Al–Mg₂Si alloys. The excellent complex modification of this master alloy on Al–20Mg₂Si alloy can be attributed to the existence of CaSb₂ particles as the heterogeneous nucleation sites of Mg₂Si particles and the inhibiting growth effect of residual Ca atoms adsorbed on the surface of Mg₂Si phase.     

Phase formation sequence of high-temperature Zn–4Al–3Mg solder

Eutectic Zn–4Al–3Mg alloy is one of the potential candidates as high-temperature lead-free solders. The phase formation sequence of eutectic Zn–4Al–3Mg alloy under different solidification conditions were investigated in this work. The results show that the microstructure is strongly affected by the difference of solidification conditions. The microstructure of the furnace-cooled eutectic Zn–4Al–3Mg alloy with a lamellar eutectic structure is composed of (α-Al + η-Zn)_eutectoid, Mg₂Zn₁₁ and η-Zn three phases, while the metastable MgZn₂ phase acts as primary phase during the rapid solidification of the air-cooled and water-cooled alloy specimens, and it evolves into the Mg₂Zn₁₁ phase later through a peritectic reaction ( $ {\text{MgZn}}_{ 2} + {\text{L}} \to {\text{Mg}}_{ 2} {\text{Zn}}_{ 1 1} $ ). Actually, the final solidified microstructure exhibited a feature of the primary MgZn₂ phase surrounded by the Mg₂Zn₁₁ phase due to the incompleteness of the peritectic transformation. Compared with the air-cooled eutectic Zn–4Al–3Mg alloy specimen, the water-cooled eutectic Zn–4Al–3Mg alloy microstructure displayed a dendritic structure resulting from more rapid cooling rate. Furthermore, the difference between the microhardness in the eutectic Zn–4Al–3Mg alloy under various solidification conditions was mainly attributed to the high-hardness phases concluding Mg₂Zn₁₁ and MgZn₂.     

Age hardening and creep resistance of cast Al–Cu alloy modified by praseodymium

The effects of praseodymium on age hardening behavior and creep resistance of cast Al–Cu alloy were investigated. The results indicated that praseodymium facilitated the formation of the θ′ precipitates during the age process and improved the hardness of the Al–Cu alloy. Besides, praseodymium resulted in the formation of the Al₁₁Pr₃ phase in the grain boundaries and among the dendrites of the modified alloy. Because of the good thermal stability of Al₁₁Pr₃ phase, it inhibits grain boundary migration and dislocation movement during the creep process, which contributes to the improvement in the creep resistance of the modified alloy at elevated temperatures.     

Formation and Evolution of Non-dendritic Microstructures of Semi-solid Alloys Prepared by Shearing/Cooling Roll Process

The shearing/cooling roll （SCR） process was adopted to prepare semi-solid A2017 alloy. The formation and evolution of non-dendritic microstructures in semi-solid A2017 alloy were studied. It is shown that the microstructures of semi-solid billets transform from coarse dendrites into fine equiaxed grains as the pouring temperature of molten alloy decreases o.r roll-shoe cavity height is reduced. From the inlet to the exit of roll-shoe cavity, microstructure of semi-solid slurry near the shoe surface is in the order of coarse dendrites, degenerated dendrites or equiaxed grains, but fine equiaxed grains are near the roll surface. Microstructural evolution of semi-solid slurry prepared by SCR process is that the molten alloy nucleates and grows into dendrite firstly on the roll and shoe＇s surface. Under the shearing and stirring given by the rotating roll, the dendrites crush off and disperse into the melt. Under the shearing and stirring on semi-solid slurry with high volume fraction of solid, the dendrite arms fracture and form equiaxed grain microstructures.     

Effect of isothermal ageing on the semi-solid microstructure of rheoprocessed and partially remelted of A390 alloy with 10% Mg addition

The semi-solid microstructure of commercial A390 (Al—17%Si—4.5%Cu—0.5%Mg) hypereutectic Al–Si alloy with an addition of 10% Mg was investigated for two different processing routes: 1) rheocasting after stirring with rotation speed of 260 rpm and 2) partial remelting after fast cooling in a steel mould. The results show that the morphology of α-Al grains becomes globular during isothermal holding time for both cases. However, at the same isothermal condition, the size of the α-Al phase particles for rheocast samples are larger and their morphology are more globular than for the samples examined after the partial remelting process. The microstuctural evolution, size and shape of the primary Mg₂Si as well as the silicon particles during isothermal ageing in the semi-solid region was also investigated for the two processing conditions.     

Effects of Zr and B on the structure and tensile properties of Al–20%Mg alloy

In current research, the effects of different Zr and B contents on the structure and tensile properties of Al–20%Mg alloy have been investigated by using Al–15Zr and Al–8B master alloys. Optical and scanning electron microscopy (SEM) were utilized to study the microstructures and fracture surfaces. Microstructural analysis of the cast alloy showed dendrites of primary α-phase within the eutectic matrix which consists of β-Al₃Mg₂ intermetallic and α-solid solution. After tensile testing, the optimum amounts for both Zr and B were found to be 0.5 wt.%. Ultimate tensile strength (UTS) value of the unrefined alloy increased from 168 MPa to 243 MPa and 236 MPa by adding 0.5% Zr and 0.5%B, respectively. The main mechanism for UTS enhancement was found to be due to the refinement of grains and also altering large dendrites of Al(α)-phase to finer structure. The study of fracture faces revealed that B/Zr addition changes the mode of fracture from brittle to rather ductile.     

Effect of boron on the microstructure of near-eutectic Al–Si alloys

The effect of boron on the microstructure of a near-eutectic Al–Si alloy (ZL109) was investigated by scanning electron microscopy (SEM) and electron beam microprobe analysis (EPMA). It was found that α-Al dendrites and eutectic clusters were significantly refined by the addition of boron. Another interesting discovery is that the near-eutectic alloy exhibited hypereutectic structure characteristics when the level of boron added exceeds 0.3%, i.e., primary Si is precipitated in the eutectic microstructure. A new type of nucleation substrate for the primary Si is found, Al_xCa_mB_nSi. This appears to be the main reason for the precipitation of primary Si.     

Conversion of uranium dioxide and real spent nuclear fuel into nitrates

Conversion of UO₂ into uranyl nitrate in liquid N₂O₄ in the presence of water was studied. The completeness of conversion of oxides in spent nuclear fuel (SNF) into nitrates is determined by the temperature and reagent ratio. The conversion of uranium oxides is complete in 2–6 h at 70–120°C and molar ratio [UO₂]: [N₂O₄]: [H₂O] = 1: (2–4): (6–8). The conversion of real oxide SNF (RBMK, 14–20 GW day/t U) in a mixture of liquid CO₂ and NO₂ was carried out.     

Influence of rare earth additions on rheological behaviour of Al–5Cu alloy in solid-liquid coexistence zone

Abstract

A rheological model was constructed and the influence of rare earth (RE) additions on the rheological behaviour of an Al–5 wt-%Cu alloy in the solid-liquid coexistence zone (SLCZ) was investigated, using an apparatus specially designed by the authors for studying the rheological properties of aluminium alloys. It was found that the rheological behaviour of an Al–5Cu alloy can be described by a combination model (the series arrangement of the Hooke model with the Kelvin and Bingham models). It was shown that RE additions can increase the values of the rheological parameters (shear modulus G, viscosity η, and yield strength τ_s) of Al–5Cu in the SLCZ to some extent, and can decrease the temperature at which the retardation time η₂/G₂ of the Kelvin model is a minimum and the Kelvin model temperature T_K. Also, RE additions can reduce the effective temperature zone ?T_K of the Kelvin model.

MST/1357     

Microstructure and mechanical properties of Al–Si cast alloy with additions of Zr–V–Ti

The microstructure and tensile properties at temperatures up to 300 °C of an experimental Al–7Si–1Cu–0.5Mg (wt.%) cast alloy with additions of Ti, V and Zr were assessed and compared with those of the commercial A380 grade. The microstructure of both alloys consisted of Al dendrites surrounded by Al–Si eutectic containing, within its structure, the ternary Al–Al₂Cu–Si phase. Whereas the Al₁₅(FeCrMn)₃Si₂ phases were present in the A380 alloy, Ti/Zr/V together with Al and Si phases, Al(ZrTiV)Si, were identified in the experimental alloy. As a result of chemistry modification the experimental alloy achieved from 20% to 40% higher strength and from 1.5 to 5 times higher ductility than the A380 reference grade. The role of chemistry in improving the alloy thermal stability is discussed.     

Hydrotalcite-like compounds obtained by anion exchange reactions

The synthesis of nickel aluminium hydroxypicrate, [Ni₃Al(OH)₈] (C₆O₇N₃H₂)·nH ₂O, and lithium aluminium hydroxypicrate, [Al₂Li(OH)₆] (C₆O₇N₃H₂)·nH ₂O by anion exchange is described. Picric acid and the corresponding hydroxycarbonates were used as starting materials. The new compounds were characterized by chemical analyses, electron microscopy, infrared spectroscopy and X-ray diffraction. The results obtained indicate that both are hydrotalcite-like compounds where the picrate anion lies between the basic layers. The thermal decomposition of the compounds was studied by differential thermal and thermogravimetric analysis.     

Effects of Sc(Zr) on the microstructure and mechanical properties of as-cast Al–Mg alloys

Both the addition of 0.6% Sc and simultaneous addition of 0.2% Sc and 0.1% Zr exerted a remarkable effect on grain refinement of as-cast Al–Mg alloys, changing typical dendritic microstructure into fine equiaxed grains. Such effect was found to be related to the formation of primary particles, which acted as heterogeneous nucleation sites for α-Al matrix during solidification. Primary particles formed in Al–Mg–Sc–Zr alloy could be identified as the eutectic structure consisting of multilayer of ‘Al₃(Sc,Zr)?+?α-Al?+?Al₃(Sc,Zr)’, with a ‘cellular-dendritic’ mode of growth. In addition, an attractive comprehensive property of as-cast Al–5Mg alloy due to the addition of 0.2% Sc and 0.1% Zr was obtained.     

Thermomechanical treatment of A380 aluminum alloy for globular thixo feedstock

The main aim of this paper is to present and evaluate thermomechanical treatment for semi-solid feedstock production of A380 (AlSi₉Cu₃(Fe)) aluminum alloy. Equal channel angular pressing (ECAP) is used to heavily deform and refine as cast dendritic microstructure with needle like eutectic appearance. The deformed microstructure is easily changed to globular microstructure by recovery and recrystallization mechanisms during heating at the semi-solid temperature range which is a crucial factor for quality semi-solid feedstock preparation. The microstructural evolutions investigations during semi-solid heating at different holding times is carried out utilizing optical microscope and scanning electron microscope accompanied by energy dispersive x-ray spectroscopy. Homogeneous distribution of the silicon particles and intermetallic compounds around globular primary aluminum phase is observed. The used approach is appropriate and seems highly promising for preparation of the quality semi-solid feedstock.     

Effect of nickel on the microstructure and mechanical property of die-cast Al–Mg–Si–Mn alloy

The effect of nickel on the microstructure and mechanical properties of a die-cast Al–Mg–Si–Mn alloy has been investigated. The results show that the presence of Ni in the alloy promotes the formation of Ni-rich intermetallics. These occur consistently during solidification in the die-cast Al–Mg–Si–Mn alloy across different levels of Ni content. The Ni-rich intermetallics exhibit dendritic morphology during the primary solidification and lamellar morphology during the eutectic solidification stage. Ni was found to be always associated with iron forming AlFeMnSiNi intermetallics, and no Al₃Ni intermetallic was observed when Ni concentrations were up to 2.06 wt% in the alloy. Although with different morphologies, the Ni-rich intermetallics were identified as the same AlFeMnSiNi phase bearing a typical composition of Al_[100–140](Fe,Mn)_[2–7]SiNi_[4–9]. With increasing Ni content, the spacing of the α-Al–Mg₂Si eutectic phase was enlarged in the Al–Mg–Si–Mn alloy. The addition of Ni to the alloy resulted in a slight increase in the yield strength, but a significant decrease in the elongation. The ultimate tensile strength (UTS) increased slightly from 300 to 320 MPa when a small amount (e.g. 0.16 wt%) of Ni was added to the alloy, but further increase of the Ni content resulted in a decrease of the UTS.     

Effect of carbon content on solidification behaviors and morphological characteristics of the constituent phases in Cr-Fe-C alloys

A combination of transmission electron microscopy, electron backscatter diffraction and wavelength dispersive spectrum has been used to identify crystal structure, grain boundary characteristic and chemical composition of the constituent phases in Cr-Fe-C alloys with three different carbon concentrations. Depending on the three different carbon concentrations, the solidification structures are found to consist of primary α-phase and [α + (Cr,Fe)₂₃ C₆] eutectic in Cr-18.4Fe-2.3 C alloy; primary (Cr,Fe)₂₃ C₆ and [α + (Cr,Fe)₂₃ C₆] eutectic in Cr-24.5Fe-3.8 C alloy and primary (Cr,Fe)₇ C₃ and [α + (Cr,Fe)₇ C₃] eutectic in Cr-21.1Fe-5.9 C alloy, respectively. The grain boundary analysis is useful to understand growth mechanism of the primary phase. The morphologies of primary (Cr,Fe)₂₃ C₆ and (Cr,Fe)₇ C₃ carbides are faceted structures with polygonal shapes, different from primary α-phase with dendritic shape. The primary (Cr,Fe)₂₃ C₆ and (Cr,Fe)₇ C₃ carbides with strong texture exist a single crystal structure and contain a slight low angle boundary, resulting in the polygonal growth mechanism. Nevertheless, the primary α-phase with relative random orientation exhibits a polycrystalline structure and comprises a massive high-angle boundary, caused by the dendritic growth mechanism.     

Segregation behaviors of Sc and unique primary Al3Sc in Al-Sc alloys prepared by molten salt electrolysis

This work mainly deals with the segregating behaviors of Sc and the growth of unique primary Al₃Sc in Al-Sc alloys prepared by molten salt electrolysis. The alloys contain 0.23–1.38 wt%Sc where Sc segregation is observed. It is found that a high current density and long electrolysis time are in favor of high Sc content, and so do the high temperature and the addition level of Sc₂O₃. Sc content at the edge of Al based alloy (average Sc content: 0.75 wt%) can be as high as 1.09 wt%, while it is merely 0.24 wt% at the central area. The cooling rates have a strong impact on the morphology and particle size of primary Al₃Sc, but a weak influence on Sc segregation. The cusped cubic and dendritic primary Al₃Sc can precipitate in the prepared Al-Sc alloys. In a slightly hypereutectic Al-0.67 wt%Sc alloy, a large and cusped dendrite grows from the edge into the center. The primary and secondary dendritic arms can be as long as 600 and 250 μm, respectively. The Sc segregating behaviors in Al-Sc alloys is due to the mechanism controlled by the limited diffusion rate of Sc in liquid Al. This can involve the establishment of a near spherical discharge interface between liquid Al and the electrolyte. The Sc rich layer near Al-molten salt interface may provide the potential primary nuclei and sufficient Sc atoms for the growth of large dendritic primary Al₃Sc.