Zhiliang Ning

The Effect of Initial Microstructure of A356 Alloys on the Mechanical Behavior in the Semisolid State

The quality of semisolid casting largely depends on the formability of the semisolid feedstock. Despite of the semisolid casting process conditions, the initial microstructure of the feedstock plays a significant role in determining the metal formability under the semisolid state. In this study, the effect of initial microstructure of A356 alloy on the mechanical forming response in its semisolid state was investigated. A wide range of the initial microstructures varied from a very coarse dendritic structure to a fine globular structure were produced in A356 alloy using the Controlled Nucleation Method, particularly by controlling pouring temperature during solidification. Cylindrical specimens with 12 mm in diameter and 10 mm in height were compressed to a height reduction of 8 mm at constant strain rates from 1.3910-1 /s to 1.3910-3 /s. Strain rate jump tests were also carried out in order to evaluate the strain rate sensitivity at high fraction solid of 0.9. The materials produced with a low superheat exhibit a fine globular structure. They showed a very low compression stress in the semisolid state compared with the materials poured at high temperatures, which have coarse and dendritic structure.

Hot deformation behavior of spray formed Al-22Si-5Fe-3Cu-1Mg alloy

Abstract The hot deformation behavior of a spray formed Al-22Si-5Fe-3Cu-1Mg alloy was investigated by isothermal compression tests at temperature ranging from 350°C to 500°C and strain rate from 0.001 s−1 to 0.1 s−1. At the beginning of deformation, work hardening is the major factor. The flow stress quickly reaches a peak, followed by a continuous reduction, then a balance is established between the tendency to work harden and the tendency to flow soften. The stress curves appear almost in flat shape. A constitutive equation in the hyperbolic sine function for describing the deformation behavior was established and the hot deformation activation energy was determined to be 312.65 kJ/mol. It can be concluded that the temperature of extrusion should not be more than 500°C in order to avoid primary silicon coarsen.

THE EFFECT OF Y ON THE OXIDATION OF Mg-Zn-Zr ALLOYS

The effects of Y addition up to 5.13% (in weight) to Mg-Zn-Zr alloys on oxidation behaviors under atmospheric ambient conditions were investigated in this study. Samples were heated in the ambient atmosphere at 400°C, with samples cooled to room temperature for mass measurement after some period time of cumulative exposure to obtain kinetic curves. XRD is used to analyze the mixture of oxides. Scanning electron microscopy combined with energy dispersive spectroscopy was used to study the scale microstructure and to obtain composition-depth profiles through the oxide layers. The oxidation results show that the weight gains for the alloys containing low Y concentration are obvious. The oxide films are thick and porous on the alloys containing low Y and thin and protective on alloys containing relatively high Y concentration. The addition of Y into Mg-Zn-Zr alloys results in the formation of protective film which is resistant to oxidation.

Liquid-solid joining of bulk metallic glasses

Here, we successfully welded two bulk metallic glass (BMG) materials, Zr51Ti5Ni10Cu25Al9 and Zr50.7Cu28Ni9Al12.3 (at. %), using a liquid-solid joining process. An atomic-scale metallurgical bonding between two BMGs can be achieved. The interface has a transition layer of ~50 μm thick. The liquid-solid joining of BMGs can shed more insights on overcoming their size limitation resulting from their limited glass-forming ability and then promoting their applications in structural components.

The Formation Mechanism of Porosity for Spray-deposited 7075 Alloy

The distribution model of temperature for the billets was established based on the porosity formation mechanism. By calculating the numerical relationship between the temperature of the deposition layer and the formation ability of the porosity, the distribution of porosity for spray formed 7075 aluminum billets was predicted and the influence of superheat on the formation probability of porosity was analyzed. The results show that a denser billet can be obtained with the average temperature of 813 K of the droplets and a liquid fraction of 50%. The porosity would increase whether the temperature further increased or decreased. Finally, the 7075 aluminum alloy was prepared by spray deposition technique with the optimum process parameters. It can be found that the distribution and types of the porosity were different at different locations. These results are in good agreement with the porosity distribution law.

Effect of Dy addition on microstructure and mechanical properties of Mg-4Y-3Nd-0.4Zr alloy

Abstract Minor Dy element was added into a Mg-4Y-3Nd-0.4Zr alloy, and its effects on the microstructure and the mechanical properties at elevated temperatures were investigated. Scanning electron microscope (SEM) and transmission electron microscope (TEM) were used to observe the microstructures. The results indicated that the as-cast eutectic and isolated cuboid-shaped Mg-RE phases were Mg5RE and Mg3RE17, respectively, and distributed mainly along grain boundaries. After a solution treatment, the eutectic Mg5RE phases were dissolved into the matrix, whereas the Mg3RE17 compound still remained. After peak aging, fine Mg-RE phases were precipitated homogeneously within the matrix of the alloys containing Dy. Dy addition can result in a significant improvement in the tensile strength at both room and elevated temperatures, and a slight decrease in the elongation.

Plasticity improvement of a Zr-based bulk metallic glass by micro-arc oxidation

Mciro-arc oxidation ( MAO) was used to coat porous films on the surface of a Zr-based bulk metallic glass sample. The compressive test results indicated that, compared with the as-cast sample, the MAO treated one exhibited higher deformation capacity, associated with multiple shear bands with higher density on the side surface and well-developed vein patterns with smaller size on the fractured surface. The pore in the MAOed film and the matrix/coating interface initiated the shear bands and impeded the rapid propagation of shear bands, thus favoring the enhanced plasticity of the MAO treated sample. The obtained results demonstrated that MAO can be considered as an effective method to finely tune the mechanical performance of monolithic bulk metallic glasses.

Effect of Double Oxide Film Defects on Mechanical Properties of As-Cast C95800 Alloy

The morphology of double oxide film defects and their influence on the tensile mechanical properties of a commercial Cu–Al (C95800) alloy were investigated in this study. Plane castings were produced with two types of pouring systems, and their tensile properties were measured and then analyzed by means of Weibull statistics method. The fracture surfaces of the tensile specimens were examined using scanning electron microscopy equipped with energy-dispersive spectroscopy. A large amount of double oxide film defects were observed on the tensile fractured specimens of the top-filled plane castings, and their chemical composition is identified to be Al2O3. Weibull statistics analyses showed that the double oxide film defects significantly reduce mechanical properties of the castings investigated. Furthermore, the ultimate tensile strength is more obviously deteriorated by double oxide film defects than elongation.

Effect of depressurizing speed on mold filling behavior and entrainment of oxide film in vacuum suction casting of A356 alloy

Abstract The effect of depressurizing speed on mold filling behavior and entrainment of oxide film of A356 alloy was studied. The mold filling behavior and velocity fields were recorded by water simulation with particle image velocimetry. The results show that the gate velocity first increased dramatically, then changed with the depressurizing speed: the gate velocity increased slowly at relatively high depressurizing speed; at reasonable depressurizing speed, the gate velocity kept unchanged; while at lower depressurizing speed, the gate velocity decreased firstly and then kept unchanged. High gate velocity results in melt falling back under gravity at higher speed. The falling velocity is the main factor of oxide film entrainment in vacuum suction casting. The design criterion of depressurizing rate was deduced, and the A356 alloy castings were poured to test the formula. The four-point bend test and Weibull probability plots were applied to assessing the fracture mechanisms of the as-cast A356 alloy. The results illuminate a method on designing suitable depressurizing speed for mold filling in vacuum suction casting.

Graphite Morphology Evolution during Melt Holding of Ductile Iron

Evolution processes of graphite morphology in ductile iron were investigated by quenching specimens during a long time holding of iron melt in a Ar atmosphere. Results show that spheroidal graphite is only observed at the early stage of melt holding. There are no evident changes in morphology of spheroidal graphite with increasing holding time up to 180 min. Subsequently chunky graphite precipitates directly after holding for 240 min as spheroidizing ability (Mg residual and RE residual) is insufficient. The number and size of eutectic chunky graphite cells increase with prolonged holding time. It should be noted that vermicular graphite forms around eutectic chunky graphite cells after holding for 360 min. When holding time reaches 420 min, graphite morphology is flake-like together with some chunky graphite. The graphite morphology in ductile iron changes from spherical to chunky, then chunky to vermicular, finally to flake with an increase in melt holding time. Both spheroidizing ability and numbers of effective nucleus decrease with prolonged holding time of melt, which affect graphite morphology.