Geoff Scamans

Intergranular Corrosion and Stress Corrosion Cracking of Sensitised AA5182

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.

Electrochemical Behavior of the Active Surface Layer on Rolled Aluminum Alloy Sheet

The highly deformed, micrograined layer on the outermost surface of a rolled Al-Fe-Si-Mn model alloy was electrochemically characterized. The thickness of this deformed surface layer in a 1.0 mm thick sheet was approximately 1 μm. Polarization curves in 5% NaCl solution at pH 3.0 and 11.5 were obtained at different depths from the surface using controlled sputtering in a glow discharge optical emission spectrometer for sample preparation. Both the anodic and the cathodic reactivity of the deformed surface layer were significantly higher than that of the bulk. Consistent with this, image analysis of scanning electron microscopy backscattered images revealed an increased number of fine intermetallic particles in the surface layer as compared with the bulk of the material. The corrosion morphology of the outermost surface was characterized by a high density of fine pits, while fewer and larger pits were observed in the bulk. The results highlight the importance of heavily deformed surface layers in controlling corrosion behavior of rolled aluminum products.

Shear enhanced heterogeneous nucleation in some Mg- and Al-alloys

Abstract Intensive shearing was applied to alloy melts at temperatures above their liquidus by using a twin-screw mechanism. The sheared melt was then cast into a TP1 mould for microstructural examination. Alloy melts with or without shearing were also filtered using the Prefil technique developed by N-Tech Ltd in order to analyse oxides and other second phase particles. The experimental results showed a significant grain refinement through enhancement of heterogeneous nucleation. The intensive melt shearing converted oxide films and agglomerates into well dispersed fine particles with a narrow size distribution. It was confirmed that the fine oxide particles can act as potent sites for nucleation during the solidification of the sheared melt. This paper presents the experimental results and theoretical analysis of shear enhanced heterogeneous nucleation during solidification of Mg- and Al-alloys. A multi-step heterogeneous nucleation mechanism has been proposed and discussed.

Significance of Thermomechanical Processing in Determining Corrosion Behavior and Surface Quality of Aluminum Alloys

The paper first gives a brief review of the fundamentals of localized corrosion on multicomponent, multiphase alloys based on a knowledge of the bulk structure and the significance of such knowledge in developing materials with improved corrosion resistance. Recent advances in investigating the near surface microstructure of aluminum alloys are next discussed along with the developing understanding of the relationship between the surface structure, electrochemistry, corrosion behavior, and surface quality in general. In particular, recent electrochemical work and TEM studies of common types of architectural alloys and their model analogues revealed the presence of a surface layer with properties very different from the bulk. The surface layer is a result of the thermomechanical processing of the wrought product, especially exposure to high shear and elevated temperatures, and it is characterized by a refined grain structure with grain sizes one to two orders of magnitude smaller than the bulk grains. These are intermixed with rolled-in oxide particles and a very fine distribution of secondary intermetallics, which precipitate preferentially in the surface layer as a result of heat treatment. The significance of such layers to localized corrosion is discussed along with possible significance also to other type of surface properties, such as in relation to adhesive bonding and optical quality.

Scanning Kelvin Probe Studies of Filiform Corrosion on Automotive Aluminum Alloy AA6016

Scanning Kelvin probe (SKP) potentiometry was used to systematically investigate the effect of surface abrasion and subsequent heat-treatment on the open-circuit potential (OCP) of AA6016 in humid air. SKP is also used to follow the kinetics of filiform corrosion (FFC) 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 paper produces a surface potential ca. 0.2 V lower than the bulk. When the abraded sample was overcoated with a 30 μm layer of PVB (polyvinyl butyral) and exposed to HCl a fast, superficial filiform corrosion was observed in which metal loss was limited to the thickness of the surface layer (i.e., ca. 1 μm). Filiform head OCP values were similar to those of the surface layer, whereas filiform tail OCP values were similar to the bulk. A mechanism is proposed in which the ultrafine grain structure of the surface layer produces an anodic activation, and the potential difference between the surface layer and the bulk provides an increased thermodynamic driving force for corrosion. For postabrasion heat-treatment temperatures between 180 and 350°C the fast filiform process was followed by a slower, deeper form of FFC.

Microstructural evaluation of melt conditioned twin roll cast Al–Mg alloy

Abstract Aluminium alloy strip has been successfully cast using the melt conditioned twin roll casting (MC-TRC) process. The liquid metal to be strip cast was fed into a melt conditioner, where it was intensively sheared under conditions of high shear rate and high turbulence induced by corotating twin screws. Melt conditioning provides a melt which has a uniform temperature and chemical composition and well dispersed nuclei. The conditioned melt was then fed into a twin roll caster. The MC-TRC process produced high quality Al–Mg alloy strip with fine and uniformly distributed intermetallics throughout the cross-section with minimal centreline segregation compared to conventional TRC strip. The Erichsen height value of MC-TRC sheet was superior to that of TRC sheet. The lower formability of the TRC sheet is attributed to the severe centreline segregation.

Grain refinement of DC cast AZ91D Mg alloy by intensive melt shearing

Abstract Melt conditioning by advanced shear technology (MCAST) is a new process for microstructural refinement of both cast and wrought magnesium alloys. Melt conditioned direct chill (MCDC) casting combines the MCAST process with conventional direct chill (DC) casting. In the present work, melt conditioning has been combined with permanent mould casting to simulate the production of DC cast AZ91D billets and slabs. The results show that the MCDC process can achieve significantly finer grain size and more uniform microstructure than conventional DC process for both billets and slabs. Grain refinement in the MCDC process is due to the fine and well dispersed oxide particles produced after processing in the MCAST unit.

Extremely fine and uniform microstructure of magnesium AZ91D alloy sheets produced by melt conditioned twin roll casting

Abstract Magnesium AZ91D alloy strips consisting of equiaxed grains with a mean size less than 100 μm were fabricated successfully by using the melt conditioned twin roll casting (MC-TRC) process. A melt conditioning by advanced shear technology (MCAST) process for conditioning liquid metals at temperatures either above or below the alloy liquidus using a high shear twin screw mechanism was combined with the twin roll casting (TRC) process to form an innovative technology, namely, the MC-TRC process for casting Al alloy and Mg alloy strips. During the MC-TRC process, liquid melt with a specified temperature is continuously fed into the MCAST machine. By intensive shearing under the high shear rate and high intensity of turbulence, the liquid is transformed into conditioned melt with uniform temperature and composition throughout the whole volume. The conditioned melt is then fed continuously into the twin roll caster for strip production. The experimental results show that the AZ91D MC-TRC strips with different thicknesses have a fine and uniform microstructure. It consists of equiaxed grains with a mean size of 60–70 μm, and also displays extremely uniform grain size and composition throughout the entire cross-section. Investigation also shows that the MC-TRC processes are extremely effective to reduce the formation of defects, particularly the formation of the central line segregations.

Solidification behaviour of an AA5754 Al alloy ingot cast with high impurity content

Abstract In view of the recycling of aluminium scrap for automotive sheet application, we have investigated the solidification behaviour of AA5754 alloy containing additional amounts of impurity elements such as Si, Fe, and Cu. Ingot casts with a high impurity content resulted in coarse α-Al dendrites and complex-shaped secondary phases. A large volume of coarse Chinese script and needle-type Fe-bearing intermetallic phases were observed to form at the centre of an ingot. In addition to the grain-boundary eutectic, spherically shaped rosette-type eutectic phases were observed within the Al grain in the high-impurity alloy. The more uniform size distribution of the Fe-bearing intermetallics observed in the Al–Ti–B grain refined alloy is attributed to the presence of a large fraction of α-Al grain boundaries which distributes the eutectic liquid where the Fe-bearing intermetallic forced to nucleate and grow.

Inhibition of Filiform Corrosion on AA6111-T4 Using In-Coating Phenylphosphonic Acid

Phenylphosphonic acid (H 2 PP) dissolved in a polyvinylbutyral coating is shown to profoundly inhibit filiform corrosion (FFC) on coated AA6111-T4. Following FFC initiation by application of aqueous HCl to a penetrative coating defect, the time-dependent extent of coating delamination was determined both optically and by repeated in situ measurements using a scanning Kelvin probe. FFC inhibition is shown to become almost complete when H 2 PP is ≥0.5% w/w. In-coating H 2 PP is also shown to depress Volta potential values associated with the intact (uncorroded) coated surface by up to 0.35 V.