Ji Yong Yao

Effect of melt cleanliness on the formation of porosity defects in automotive aluminium high pressure die castings

The effect of melt cleanliness on the formation of porosity defects in automotive aluminium high pressure die castings (TA Transmission Case) was investigated experimentally. The experiments were conducted under actual industrial production conditions at Nissan Casting Australia Pty Ltd. (NCAP). It was found that the probability of rejection due to excessive porosity present at critical locations in the castings (determined using a real-time X-ray radiographic method) increased as the number of inclusions in the melt (measured with LiMCA II) increased. The types of inclusion in the melt were identified as mainly amorphous oxides, oxide films and sludge particles

Swelling during liquid phase sintering of Al–Mg–Si–Cu alloys

Abstract Swelling during liquid phase sintering under nitrogen of Al–xMg–ySi–0·25Cu alloys from elemental powder blends (where x and y vary from 0 to 2 wt-%) has been characterised using dilatometry and differential scanning calorimetry. It is shown that swelling in these alloys occurs upon the formation of liquid phases. It is concluded that swelling is neither a consequence of unbalanced diffusion rates (the Kirkendall effect) nor the result of the specific volume expansion of the elemental powders during melting. It is suggested that swelling occurs as a result of insufficient wetting of the aluminium particles due to the presence of an oxide layer on their surface, and the subsequent separation of the particles under the influence of the surface tension.

Characterisation of AlFeSi intermetallics in 6000 series aluminium alloy extrusions

Alloys designed to optimise strength and extrudability have a lower alloy Mg to Si ratio than has commonly been used in AA6060 and AA6063 alloys. Intermetallic phases have an impact on alloy design since they tie up some of the Mg and Si alloy content. The effect of Mg and Si alloy content on the type of intermetallic phases present has been investigated using TEM, SEM and Thermocalc analysis. Results for Al-Mg-Si-Fe alloys with 0.46 - 0.70 wt%Mg, 0.27 - 1.24 wt%Si and 0.09 - 0.22 wt%Fe are presented. The occurrence of a-AlFeSi (various stoichiometries), b- Al5FeSi, p-Al8FeSi6Mg3, Mg2Si and Si has been found to depend on alloy composition within the ranges examined.

Interfacial Microstructures in InxGa1-xAs/GaAs Strained Layer Structures

The interfacial microstructures of lattice strained In x Ga l-x As/GaAs multiple layer structures, that were grown by molecular beam epitaxy (MBE) on GaAs (100) substrates, have been investigated and characterised by transmission electron microscopy (TEM). A g 3ii weak beam imaging technique has been used to study structural imperfections at the heterointerfaces. The morphology of rough heterointerfaces, which resulted from the growth of the In x Ga l-x As layers (strained layer) either in a two dimensional (2D) or in a three dimensional (3D) growth mode via island formation, was imaged using this technique. A transition from 2D to 3D growth was found to occur at a certain critical layer thickness which decreased with increasing indium fraction. In thicker layers, dislocation complexes, which may have been caused bythe formation of islands, were also observed. These complexes were primarily composed ofstacking faults bounded by partial dislocations.

Interfacial Microstructure of InxGa1-xAs/GaAs Strained Layers

The topography and defect structure of interfaces in InGaAs/GaAs strained-layer structures grown by molecular beam epitaxy were investigated by transmission electron microscopy. A large range of indium fractions, x, was investigated: 0.16 ≥ x ≥ 1.00, and three different critical layer thicknesses, corresponding to the formation of three different types of defects, were observed with increasing thicknesses of the strained layers. These defects were: rough interfacial topographies resulting from an onset of 3-dimensional growth for the InGaAs layers; misfit dislocations of the 60° mixed type; and dislocation complexes consisting of planar defects on {111} planes. Compared with results obtained from photoluminescence measurements it was found that both types of defects involving dislocations resulted in severe degradation of the optical properties of the strained-layer structures.

Pressure Assisted Sintering of an Aluminium Alloy

An aluminium alloy was sintered using a conventional press and sinter process, at various gas pressures, to observe the effect of sintering gas pressure on the densification rate. Compacts of aluminium alloy 2712 (Al-3.8Cu-1Mg-0.7Si-0.1Sn) were prepared from elemental powders and sintered in a horizontal tube furnace under nitrogen or argon at 590°C for up to 60 minutes, and air cooled. The gas flow was adjusted to achieve specific gas pressures in the furnace. It has been found that increasing the nitrogen pressure at the start of the isothermal holding stage to 160kPa increased the densification rate compared to standard atmospheric pressure sintering. Increasing the nitrogen pressure further, up to 600kPa, had no additional benefit. The densification rate was increased significantly by increasing the gas pressure to 600kPa during both heating and isothermal holding. Under argon the elevated pressure did not increase the densification rate. Results seem to suggest that the beneficial effect of the elevated pressure on the rate of densification is related to nitride formation.