Papers by Author: Peng Yu

Abstract: Gas-atomized Cu-based metallic glass powder was heat-treated in air from below its glass transition temperature (Tg) to well above its crystallization temperature (Tx). Severe oxidation occurred at temperatures > Tx while at temperatures < Tg oxidation was sluggish. The oxidation products were determined and the mechanism is discussed based on the microstructural findings.

Abstract: Plate samples of Al-2Sc were processed with an electron beam (EB) gun in a vacuum of 10-3 Pa at 50kV and 30 µA. EB processing suppressed the formation of primary Al3Sc, altered the morphology of the eutectic (Al)-Al3Sc, and increased the solubility of Sc in the eutectic (Al). In addition, its cyclic heating effect induced the precipitation of Al3Sc. EB processing resulted in more profound hardening than conventional high temperature solid solution and ageing.

Abstract: An investigation has been made on the infiltration behaviour in rapid prototyping of Al alloy parts under different infiltration atmospheres. An Al 6061 alloy preform was first prepared by selective laser sintering. Then the aluminium precursor powders in the preform were converted into AlN to form a skeletal AlN structure, which was subsequently pressureless infiltrated with a molten Al 6061 alloy under nitrogen, argon or vacuum. The pathway of the infiltrant was determined by density measurements in conjunction with metallographic examination and quantitative image analysis. Detailed comparison of the microstructures at the surface and the centre of the as-infiltrated samples indicates that the pathway into the porous preform is dependent on the infiltration atmosphere. Under vacuum, the infiltrant fills the interior of the preform first and then propagates to the surface. In contrast, under nitrogen or argon, the infiltrant penetrates along the surface prior to filling the interior.

Abstract: Electron beam (EB) direct manufacturing is an additive near-net shape digital fabrication process developed recently. The process offers a promising route for the fabrication of intricate speciality aluminium alloy parts for aircraft and aerospace applications because of the excellent energy coupling between an electron beam and aluminium. As part of a fundamental study on EB manufacturing of Al alloys, this work investigates the effect of the EB processing parameters on the development of the molten pool and the solidification characteristics of Al 2219 and Al 6061 alloys. The samples were processed using a 50kV electron beam gun over a wide range of beam currents (10-40mA) and welding speeds (0.3-0.86m/min) in both the static and oscillation focus modes. In the static focus mode, the molten pool is wedge-shaped; while in the oscillation focus mode, the molten pool is hemispherical, wider and shallower. In both cases, the depth and width of the molten pool increase with increasing beam current but they are less affected by the moving speed of the EB gun in the range 0.3-0.86m/min. Electron beam re-melted and subsequently re-solidified Al 2219 and Al 6061 alloys show microstructural features distinct from those obtained under sand casting and direct chill casting conditions. In particular, fine intermetallic precipitates in the size range 100-200nm are prominent in the equiaxed grains formed in the re-solidified weld beads of Al 6061 compared to the coarse intermetallic particles up to 10m in size prior to EB processing. EB processing offers opportunities for aluminium alloy development.

Abstract: Recent breakthroughs in the sintering of aluminium alloys under nitrogen have opened the way for the in-situ fabrication of Al-AlN composites in a controllable and reproducible fashion over a wide range of volume fractions of AlN. This work reviews the fundamentals for the in-situ fabrication of the Al-AlN composites from metal powders and highlights their technical potential for niche applications because of their excellent resistance to cavitation erosion in water and their unusually low friction coefficient under oil lubrication.