Da Shu

First-principles calculations on the stability of Al∕TiB2 interface

The atomic structure, adhesion, and interfacial energy of Al∕TiB2 interfaces were studied using first-principles calculations. The preferable interfacial atomic structure is that wherein the Al atoms continue the natural stacking sequence of the bulk TiB2. The interface adhesion for Ti-terminated interface is stronger than that for B-terminated interface. The structural stabilities of two interface terminations depend greatly on the Ti interfacial chemical potential. Calculation of the interfacial energies provides theoretical evidence for the hypernucleation and duplex nucleation grain refinement mechanisms.

First-principles study of TiB2(0001) surfaces

The TiB(2)(0001) surfaces are calculated using the first-principles total-energy plane-wave pseudopotential method based on density functional theory. It is found that there are large relaxations within the top three layers for both termination surfaces, and the outermost and second interlayer relaxations for B-terminated surfaces are much larger than those for Ti-terminated surfaces. The charge depletion in the vacuum and the charge accumulations in the interlayer region between the first and second layers reinforces the interlayer Ti-B chemical bonds and reduces the outermost interlayer distance. Simultaneously, the charge accumulation for B-terminated surface is more than that for Ti-terminated surface, and the interlayer Ti-B bonds between the second and third layers are weakened more for the B-terminated surface. The Ti-terminated surface is thermodynamically more favourable in most of the range of μ(B)(slab).

Effects of melt thermal treatment on hypoeutectic Al-Si alloys

Abstract A new refining method—melt thermal treatment without additive to melt is studied in this paper. Microstructure analysis and property evaluation of hypoeutectic Al–Si alloys treated with this method show that the solidification microstructure can be refined significantly with a considerable increase in elongation ratio and strength. Effects such as cooling rate, holding time and alloy composition on the solidification microstructure and mechanical properties have been evaluated. It is shown that the strengthening and toughening effects on the treated samples vary with alloy composition. The property increment of the alloy rich of iron is relatively more remarkable than those rich of Cu or Mg element. Specifically, the structure of the low temperature melt is identified as a primary factor, on which the solidification structure of the treated melt is dependent.

Continuous separation of non-metallic inclusions from aluminum melt using alternating magnetic field

Experiments to continuously separate inclusions from aluminum melt were conducted on a laboratory-scale apparatus by using high-frequency alternating magnetic field. It was found that the separating efficiency of 6-μm alumina inclusions can exceed 95%. The separating efficiency for the square separator channels is significantly higher than that for the circular separator channels under the close processing conditions, which supports that the separating efficiency can be improved by disturbance in the melt.

Synchrotron radiation X-ray imaging of cavitation bubbles in Al-Cu alloy melt

Cavitation bubbles in Al-10 wt.%Cu melt has been investigated by adopting synchrotron radiation X-ray imaging technology. In-situ observation reveals that most of bubbles concentrate within an intense cavitation zone nearby the radiation face. The measured near-maximum bubble radii obey a similar truncated Gaussian distribution as in water but increase by nearly the magnitude of one order due to higher ultrasonic intensity applied in aluminum melt.

Numerical calculation of the electromagnetic expulsive force upon nonmetallic inclusions in an aluminum melt : Part II. Cylindrical particles

The finite-element method was employed to explore the electromagnetically engendered fluid streaming around a spherical inclusion particle suspended in aluminum melt, and numerical integration of the pressure over the particle surface was made to calculate the electromagnetic expulsive force exerted on the particle. It was found that convection flows with four cells appear around the particle along the direction of the electromagnetic force. The change of pressure gradient is confined within the thin-skin layer around the particle, while the perturbance of the velocity field widely spreads out. Compared with the theoretical results derived from the analytical method, the numerical results agree well in the range of small particle sizes or low-intensity force densities. For large particle sizes or high-intensity force densities, the calculated expulsive force is only a little bit larger than the theoretical result. The influence of a boundary effect and proximity effect on the electromagnetic expulsive force is insignificant, except for a slight increase in the expulsive force when the particle approaches a boundary or other particles.

Performance improvement of industrial pure aluminum treated by stirring molten fluxes

Removal of inclusions from molten aluminum (A001) and aluminum grain refinement were studied by thermodynamic theory and a surface renewal kinetic model. After molten aluminum poured into the rotated molten flux in batches, the inclusions were transferred into the stirred molten flux, at the same time the trace impurities in molten flux diffused into the molten aluminum droplets. Thermodynamic theory shows that wettability of the inclusions in molten aluminum is worse than that in the molten active flux. Migration of the inclusions and diffusion of the trace impurities were described with the surface renewal kinetic model. Tensile tests show that the tensile strength of treated tensile sample (as-cast) was increased by 8.59%. The fracture cracks and the dimples were homogeneous and small as shown in SEM (scanning electron microscopy) photographs. Moreover, the quantities and sizes of the inclusions in the treated tensile samples were obviously fewer and smaller than that of in the untreated tensile samples as shown in metallographs and statistical results of an image analysis software. Meanwhile, the grain sizes of treated tensile samples were smaller than that of untreated tensile samples.

Corrosion resistance properties of enamels with high B2O3–P2O5 content to molten aluminum

Abstract Anticorrosive properties of borophosphate and boron-free enamels to molten aluminum were investigated using SEM and electron probe. Carbonates of alkali metal and alkaline earth metal were added in an appropriate weight ratio to achieve desired melting temperature of the enamels. SEM examination on the solidified interface between the enamels and aluminum alloy show that the enamels can spread slightly on aluminum alloy. For anticorrosive sample of borophosphate enamel, phosphorus was not detected by electron probe at the side of aluminum alloy near the interface, but silicon was detected in the silica-free enamels side. For the sample of boron-free enamels, however, phosphorus was found at the side of aluminum alloy near the interface. It was revealed that the enamels with high B 2 O 3 –P 2 O 5 content have high corrosion resistance to molten aluminum.

Study on iron purification from aluminium melt by Na2B4O7 flux

Abstract A study was carried out to investigate the effect of Na2B4O7 on the reduction of Fe concentration from commercial purity aluminium. Experiments were conducted and the double barrier layers theory was used to study the kinetics of the purification process. Results showed that the iron reduction ratio increased with the addition of Na2B4O7 and the holding time, which follows the first order exponential decay law. The addition of Na2B4O7 containing flux could reduce iron content from 0·14 wt-% to less than 0·1 wt-% and achieve an optimal ratio of 44%. Thermodynamic calculations and the XRD, SEM and EDX analyses of the molten sludge indicated that formation of Fe2B could occur spontaneously in molten aluminium with the addition of Na2B4O7 containing flux.

Effects of silicon content on microstructure and stress corrosion cracking resistance of 7050 aluminum alloy

Evolution of microstructure and stress corrosion cracking (SCC) susceptibility of 7050 aluminum alloy with 0.094%, 0.134% and 0.261% Si (mass fraction) in T7651 condition have been investigated. The results show that the area fraction of Mg2Si increases from 0.16% to 1.48% and the size becomes coarser, while the area fraction of the other coarse phases including Al2CuMg, Mg(Al,Cu,Zn)2 and Al7Cu2Fe decreases from 2.42% to 0.78% with Si content increasing from 0.094% to 0.261%. The tensile strength and elongation of 7050-T7651 alloys is decreased with the increase of Si content by slow strain rate test (SSRT) in ambient air. However, electrical conductivity is improved and SCC susceptibility is reduced with the increase of Si content by SSRT in corrosion environment with 3.5% NaCl solution.