Papers by Author: Ma Qian

Abstract: The development of grain refinement technologies began in the 1930s in response to the need to improve the mechanical properties of as-cast components. Commercial grain refining technologies were developed by industrial and experimental trials often with good success including the production of effective master alloys. In parallel, researchers developed theories to explain the mechanisms of refinement in order to improve the efficiency of refiners and develop new better performing grain refining master alloys. This research continues today. Here we briefly present the history of these developments. It is shown that many developments in our understanding were based on assumptions arising from experimental and industrial observations and the prevailing solidification theories of the time.

Abstract: TiAl alloy powder is difficult to sinter unless assisted with pressure and/or pulsed current. This paper investigates the effect of a small addition of iron on the sintering behaviour of γ-TiAl alloy powder at 1350 °C in vacuum. Thermodynamic calculations using Thermo-Calc and the Ti-alloy database TTTI3 predict that iron is a potential sintering aid for TiAl powder. The relative sintered density (RSD) increased with increasing Fe content and peaked at an addition of 2at.%Fe, at which the RSD increased from ~ 60% theoretical density (TD) without iron to ~ 97%TD. The enhanced densification is attributed to liquid formation induced by iron based on both thermodynamic predictions and differential scanning calorimetry (DSC) analysis. The as-sintered microstructures and phase constituents were analysed by scanning electron microscopy (SEM) equipped with an energy dispersive spectroscopy (EDS) microanalysis system and X-ray diffraction (XRD) analysis.

Abstract: The inexpensive hydrogenated–dehydrogenated (HDH) titanium powder made from the Kroll sponge titanium provides a cost-affordable basis for powder metallurgy (PM) Ti alloy development. The design targets we hope to achieve are low feedstock cost (< $25/kg) including all alloying elements; low fabrication cost based on cold compaction and pressureless sintering, and wrought grades of properties of Ti-6Al-4V in the as-sintered state. Relevant issues are considered. These include alloying with inexpensive elements such as Fe and Si, grain size control during heating, isothermal sintering and cooling, chemical homogeneity of as-sintered microstructure, and simultaneous scavenging of oxygen and chlorine. In addition, it is proposed that achieving 6% of tensile elongation will be adequate for most PM Ti applications, compared to PM steels (normally < 2%), PM aluminium alloys (mostly < 4%) and the requirements for wrought Ti-6Al-4V armour plates (≥ 6%). This will allow the use of HDH Ti powder that contains relatively high oxygen (~0.35wt.%) and direct more efforts towards improving other properties.

Abstract: The Interdependence Theory is a theoretical description of grain formation that links heterogeneous nucleation to grain growth early in the initial transient of a previously nucleated grain. Thus nucleation is the result of a repeating cycle of growth and nucleation events moving towards the thermal centre of a casting. The principles of this theory are introduced and then the Interdependence equation that embodies the Interdependence Theory, is applied to the prediction of experimental grain size data for aluminium, magnesium and titanium-based alloy systems.

Abstract: Turnbull’s transformation nucleus model initiated the concept of a growth barrier for a spherical-cap crystal nucleus growing on a small flat substrate. The recently developed free growth model provided a clear physical picture of the growth barrier concept with experimental and modelling support. Fletcher’s spherical substrate model enhanced the understanding of the geometrical effect of a substrate on nucleation. A recent novel analysis of Fletcher’s model furnished new insights into the similarities and differences between nucleation on spherical and flat substrates. It is necessary to distinguish between the undercooling required for nucleation and that for overcoming the growth barrier; the greater one determines the early stages of grain formation.

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: Effective sintering of titanium requires the use of a high sintering temperature (≥1200 °C), preferably in high vacuum (< 10-2 Pa). This confines the heating and cooling rates to ~ 4 °C/min because of the limited thermal shock resistance of ceramic tube furnaces. Consequently, it leads to lengthy sintering cycles (10-12 hr). This work presents an assessment of microwave (MW) sintering of titanium. Titanium powders in the size ranges of <20 µm, 45-63 µm, and 100-150 µm were used to make green samples with compaction pressures ranging from 200-800 MPa. Sintering was carried out at 1200 °C for 2 hr in a 3 kW MW furnace with a 2.45 GHz multimode cavity under a vacuum of 2-6×10-3 Pa. The characteristics of MW heating of green titanium samples in vacuum are described in terms of the heating rate, vacuum fluctuations, and sparking discharge. The actual MW heating rate achieved from 350 °C to 1200 °C was 34 °C/min. The attendant densities are comparable to those attained by conventional vacuum sintering. Cross-sectional examinations revealed a fairly uniform pore distribution in MW-sintered samples made from either the coarse or fine titanium powder.

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: A TEM study of aluminium nitride formed during sintering of powder injection moulded aluminium under nitrogen is presented. A polycrystalline layer consisting of fine, rod-shaped crystallites of hexagonal AlN formed on the Al powder surfaces. The grain boundaries exhibit a double layer of AlN separated by a thin layer of Al. The structure of the AlN is characterised and its influence upon sintering discussed.