Papers by Author: Geoff M. Scamans

Abstract: Intensive shearing was applied to AZ91D Mg-alloy melts at temperatures above its liquidus (600oC) by using a twin-screw mechanism. The sheared melt was then cast into a TP1 mould (Aluminium Association’s standard test mould with 3.5K/s cooling rate) for microstructural examination. Alloy melts, both with and without shearing, were also filtered using a pressurized filtration technique to collect oxides and other second phase particles. The experimental results showed a significant grain refinement of AZ91D alloy through enhancement of heterogeneous nucleation. Based on the experimental results, a multi-step heterogeneous nucleation mechanism has been proposed: (1) intensive melt shearing converts oxide films and clusters into fine and well-dispersed particles with a narrow size distribution; (2) the dispersed oxide particles act as potent sites for nucleation of Al8Mn5 intermetallic particles; (3) during the further cooling, Al8Mn5 intermetallic particles nucleate the -Mg phase, resulting in a fine and uniform microstructure. The proposed multi-step nucleation mechanism will be discussed in terms of the effect of forced wetting, particle size and particle size distribution on the enhancement of heterogeneous nucleation.

Abstract: MCAST (melt conditioning by advanced shear technology) is a novel processing technology developed recently for conditioning liquid metal under intensive forced convection prior to solidification processing. The MCAST process uses a twin screw mechanism to impose a high shear rate and a high intensity of turbulence to the liquid metal, so that the conditioned liquid metal has uniform temperature, uniform chemical composition and well-dispersed and completely wetted oxide particles with a fine size and a narrow size distribution. The microstructural refinement is achieved through an enhanced heterogeneous nucleation rate and an increased nuclei survival rate during the subsequent solidification processing. In this paper we present the MCAST process and its applications for microstructural refinement in both shape casting and continuous casting of light alloys. Discussions will be made on the effect of intensive forced convection on the enhanced heterogeneous nucleation. The concept of physical grain refinement will be proposed and discussed in contrast to the conventional chemical grain refinement by addition of grain refiners.

Abstract: High levels of surface shear experienced during rolling, grinding or machining can cause 6000 series aluminium to develop an ultra-fine grained surface layers which dramatically increase susceptibility to filiform corrosion (FFC) under paint films. In-situ Scanning Kelvin Probe (SKP) measurements in humid air are used to compare the kinetics and mechanism of FFC on abraded and lacquer-coated samples of high copper containing AA6111 and low level copper AA6016. FFC is initiated by applying a small volume of aqueous HCl to a penetrative defect on polyvinylbutyral (PVB) coated alloy samples prior to placement in a chamber maintained at constant humidity and temperature. The SKP is then repeatedly scanned over a fixed surface area to produce a time-lapse animation showing the dynamic evolution of localized free corrosion potential patterns. The spatial distribution of potential variation provides insight into the FFC mechanism and the numerical integration of areas of dissimilar potential provides a measure of the time-dependent area of coating delamination. Various possible FFC inhibition strategies are investigated for use under circumstances where removal of the surface layer prior to application of an organic (paint) coating is not feasible. The two strategies shown in this paper are the use of an anti-corrosion pigments based on an intrinsically conducting polymer called polyaniline. An anion-exchange pigment called hydrotalcite is also used.

Abstract: Scanning Kelvin Probe (SKP) potentiometry is used to systematically investigate the effect of surface abrasion and subsequent heat-treatment on the open-circuit potential in humid air of the AA6016 surface. SKP is also used to follow the kinetics of filiform corrosion and to determine characteristic potentials associated with the electrolyte-filled filiform head and dry filiform tail. It is shown that simply abrading with 180 grit SiC produces a surface potential up to 0.5V lower than the bulk. When the abraded sample is overcoated with a 30 micron layer of PVB (polyvinyl butyral) and exposed to HCl a fast, superficial filiform corrosion (FFC) is observed in which metal loss is limited to the thickness of the surface layer. Filiform head OCP values are similar to that of the surface layer, whereas filiform tail OCP values are similar to the bulk. A mechanism is proposed in which the ultra-fine grain structure of the surface layer produces an anodic activation and the potential difference between the surface layer and the bulk provides and increased thermodynamic driving force for corrosion. For post-abrasion heat treatment temperatures up to 350°C the fast filiform process is followed by a slower, deeper form of FFC.

Abstract: Over the past ten years a detailed understanding of the dominant factors controlling the corrosion susceptibility of painted aluminium alloy sheet used for architectural and automotive applications has been developed. Work carried out in this field will be reviewed and the different modes of cosmetic corrosion that can occur-, the mechanisms of surface activation, and the role of different thermo-mechanical processing steps on controlling corrosion susceptibility for different alloy systems, including Al-Mn, Al-Mg and Al-Mg- Si-(Cu) will be discussed. The critical role of chemical or electro-chemical cleaning prior to pre-treatment and coating will be highlighted and prospects for meaningful accelerated testing will be discussed.

Abstract: AA5182 (Al-4.5 wt% Mg) can become susceptible to intergranular corrosion (IGC) with time at moderately elevated service temperatures owing to precipitation of Mg-rich β-phase at grain boundaries, which can lead to stress corrosion cracking (SCC). The IGC and SCC susceptibility of AA5182 was found to depend strongly on sensitisation heat treatments. AFM and TEM studies demonstrated that the degree of precipitation and thus susceptibility to attack for a boundary can be related to its crystallographic misorientation. Low angle boundaries (<20°) are most resistant to attack as they do not show β-phase precipitation. However, higher angle boundaries show highly variable precipitation and corrosion susceptibility: critical factors are the grain boundary plane and precipitate/matrix crystallographic relationship.