Oxide bifilms in aluminium alloy castings – a review

Abstract

Aluminium alloy castings are most widely used in automobile industry because of their light weight, better castability and improved properties. The liquid aluminium surface easily oxidises during melting, transferring and pouring operation which may entrain oxide films into the casting. Research work has shown that the entrainment of this surface film and formation of bifilms in castings appear to be the source of most of the casting defects leading to a significant reduction in the mechanical properties of aluminium alloy castings. In this paper, the phenomenon of formation of oxide bifilms in aluminium alloy castings, effect of these bifilms on casting properties and their assessment techniques are discussed. For enhancing the quality of casting, research should focus towards development of process techniques for healing of bifilms in liquid metal during solidification.     

Breakdown of protective scales during the oxidation of thin foils of Fe–20Cr–5Al alloys at high temperatures

Abstract

The oxidation behaviour of alumina-forming Fe–20Cr–5Al and similar alloys containing small concentrations of lanthanum or lanthanum plus molybdenum in air at 1,150°C has been studied, with emphasis on thin (0.05 mm) specimens, where the aluminium reservoir in the substrate is soon depleted to a very low value. Oxidation of these alloys involves establishment and growth of protective alumina scales. However, once the residual aluminium concentration in the alloy drops below a critical level, a layer of chromia is able to develop and grow at the alumina–alloy substrate interface. Eventually, breakaway oxidation occurs and iron-rich oxides form and engulf the specimen.

This paper presents some kinetics of oxidation of these alloys and discusses the growth and breakdown of the protective scales, drawing on the results of detailed examinations of the oxidized specimens using analytical scanning and transmission electron microscopy in cross section. It has been shown that lanthanum increases the time to the onset of breakaway oxidation, probably due to beneficial effects on the mechanical integrity of the scale. Molybdenum additions have been found to decrease significantly the rate at which breakaway oxides are able to penetrate and engulf the alloy substrate. Such additions stabilize the ferrite phase in the substrate at the alloy–scale interface, thereby maintaining a high rate of diffusion of chromium to the interface and facilitating establishment of a healing and partially protective chromium-rich oxide layer at the base of the breakaway oxide scale. In the absence of such additions, depletions of chromium in the substrate adjacent to the alloy/scale interface, arising from oxidation of chromium, enable the austenite phase to be stabilized. The relatively low rate of diffusion of chromium in this phase allows chromium-rich oxide to form as internal precipitates in the alloy rather than as a continuous, healing layer; hence, the breakaway oxide scale is able to penetrate and consume the substrate more rapidly than in the presence of molybdenum additions.     

Investigation on thickness of short time oxide films in Al–1Mg and Al–2Mg alloys

Abstract

The melts of aluminium alloys are very sensitive to oxidation during casting, and the surface oxide film formed during casting can be folded and entrained into the melt due to melt surface turbulence. In this research, sandwiches of oxide–metal–oxide (OMO) formed in a very short time within the cast during solidification were investigated in order to see the effect of magnesium content (i.e. 1 and 2 wt-%) on the oxide film thickness. To form OMO sandwiches within the cast, a certain amount of air was blown into the melt every 0·5 s during casting time by means of a compressor at 0·5 atm pressure. Where bubbles of air collided, they formed a sandwich which later was used for investigating purpose. Both the thickness and the surface of oxide films were studied via SEM. The results showed that the thickness of the short time oxide film varies in the range of 150–250 and 200–300 nm for Al–1Mg and Al–2Mg alloys respectively.     

Selective oxidation of Al rich Fe–Mn–Al–C low density steels

Abstract

The surface quality of steels containing solutes that have a strong affinity for oxygen can be markedly compromised during processing at elevated temperatures. Here, we examine a class of steels that are being developed for low density applications and hence, have relatively large concentrations of aluminium and manganese. These two elements compete for what little oxygen is available in the predominantly hydrogen–nitrogen mixture used to protect the steel during heat treatment. It is found that although the general sequence of oxidation does not seem to vary as the aluminium concentration is increased, the stoichiometry of the oxides changes. Manganese rich oxide is always the first to form for all the dew points examined, but is rapidly overtaken by aluminium oxide when the dew point is kept below ?30°C. In the latter case, the eventual formation of an alumina film also halts the internal oxidation of aluminium. The results obtained using a variety of high resolution microscopy and analytical techniques have been analysed by calculating phase diagrams and by creating a finite difference model to reveal aspects of the internal oxidation problem.     

Anodic oxide growth on aluminium surfaces modified by cathodic deposition of Ni and Co

Surface conditions similar to those found in aluminium alloys of practical use were assessed by cathodic deposition of transitions metals (Ni and Co) from different electrolytes. Fundamental aspects concerning with the growth of anodic oxide films at potentials lower than 10 V in neutral acetate buffer solution on these modified surfaces were analysed by common electrochemical techniques complemented with scanning electron microscopy and transmission electron microscopy. In both potentiodynamic and galvanostatic modes, the growth of aluminium oxide competes with the dissolution of deposited metal particles. The formation of a thin aluminium barrier oxide film beneath them shifts the dissolution potential over to 1.5 V towards more positive values. Some particles get progressively embedded in the matrix of the growing alumina and act as cation sources, increasing the film conductivity and diminishing the established electric field in the oxide. This effect is more pronounced with Co deposits due to its high active dissolution rate, before passivation occurs. Then, the generation of a two-layer film is explained in terms of the precipitation of metal hydroxide at the solution side on the oxide barrier film.     

Improved halide salt process to produce Al–B master alloys

Abstract

Adding KBF₄ salt to molten Al produces a B lean Al–B melt and a B rich dross. A revised practice that relies on the co-addition of Na₃AlF₆ with KBF₄ and mechanical stirring is proposed in the present work to produce Al–B alloys. The highly surface active Na₃AlF₆ helps to break up the boride agglomerates glued together by the spent salt. Mechanical stirring facilitates uniform distribution of the freed borides in molten aluminium. The microstructural features of the Al–3B alloy thus processed and the B recovery were improved in a marked fashion. The boride dispersion in the aluminium matrix was uniform with no evidence of salt residues inside the aluminium matrix. An improved salt addition practice to ensure full B recovery and an Al–B alloy of sufficient quality is thus claimed to comprise the following steps: melting commercial purity aluminium ingot, adding premixed KBF₄ and Na₃AlF₆ salts to molten Al at 800°C gradually to avoid excessive cooling of the melt, holding the melt for 5 min at 800°C to allow the reaction to reach completion, decanting the spent salt and throughly stirring the melt before casting into desired shape.     

The effect of aluminium depletion on the oxidation behaviour of FeCrAl foils

Abstract

In thin FeCrAl foils, the formation of a chromia layer within or underneath the alumina layer has been observed after consumption of the aluminium from the alloy. For Aluchrom I SE, the growth law of the alumina-forming step has been evaluated and an activation energy for oxygen diffusion of 383 ± 36 kJ mol^?1 has been determined. For the growth mechanism of the chromia layer, three models are introduced and discussed. In agreement with a model proposed by H. E. Evans, the measurements of the aluminium content resulted in complete aluminium consumption before the beginning of chromia formation. Because of strong deformation of the thin samples during oxidation, a model is proposed to calculate the alloy thickness based on the amount of aluminium consumption. From a comparison of these calculated values with the measured thicknesses, the elongation of the sample due to creep processes could be determined.     

Various aspects of the air oxidation behaviour of a Ti6Al4V alloy at temperatures in the range 600–700 °C

A study on the air oxidation behaviour of a commercially pure Ti6Al4V alloy between 600 and 700 °C is reported, based on determination of the kinetic curves, microhardness profiles in the metal beneath the scale, and examination of morphology and composition of the scales. The oxidation kinetics shows a gradual transformation dependent on both time and temperature from a diffusive to a nearly linear rate law. It has been observed that such transformation may be associated with the acceleration of oxide scale growth. This phenomenon is accompanied by a parallel change of the oxide morphology, which is essentially manifested with the onset of a duplex-type scale starting from 650 °C. Aluminium is found to pile up near the external surface of the oxide scales, although the presence of a continuous film of alumina may be excluded under any circumstances. Finally, the effect of aluminium and vanadium is beneficial in reducing appreciably the amount of oxygen dissolved in the surface metal layer, compared with unalloyed titanium.     

High spatial resolution imaging of the segregation of reactive elements to oxide grain boundaries in alumina scales

Abstract

Although it is well established that reactive elements such as yttrium and hafnium can segregate to oxide/metal interfaces and oxide grain boundaries in thermally grown oxides, their distribution and role at these sites are less certain. For example, their effect on oxide growth, scale plasticity and spallation is still debated. It has also been reported that hafnium and yttrium rich oxide particles can be present within growing alumina scales and that the growth or shrinkage of these particles can affect the Y and Hf distributions in the aluminium oxide grain boundaries in their vicinity. Hence, we now report the use of very high spatial resolution imaging in the SuperSTEM electron microscope to investigate the distribution of Y and Hf in aluminium oxide grain boundaries at the atomic level.

The oxide scales studied were detached from Fe – 20Cr – 5Al alloy substrates doped with Y and Hf, which had been oxidised for up to 100 h at 1250°C in laboratory air. The scales were ion beam thinned prior to examination in the STEM, and a series of tilting experiments and through focal series were used to map out the distributions of the reactive elements. The influence of electron beam/sample interactions was also studied and some evidence for the movement of Hf and Y atoms along the grain boundaries to the surfaces of the thin oxide foils is also reported.     

Parameters affecting transient oxide formation on FeCrAl based foil and fibre materials

Abstract

The effects of oxidation temperature and atmosphere on the formation of alumina scales on two commercial FeCrAl foil materials have been investigated. The oxidized specimens were characterized using a range of surface analysis techniques including SEM, XRD, laser induced optical spectrometry (LIOS), AES and XPS. During oxidation at temperatures exceeding 1000°C, the protective oxide scales formed on FeCrAl-alloys consist mainly of alpha alumina. At lower temperatures, however, formation of transient alumina modifications, has been observed. Although after longer oxidation times transformation into the stable alpha alumina occurs, the high initial growth rate of the metastable oxide phases could lead to a critical depletion of the Al-reservoir in thin walled (e.g. 20 (m) components, resulting in early breakaway failure. The occurrence of metastable oxides cannot simply be correlated with alloy composition.     

TEM analysis of the growth of oxide scales at different temperatures in FeAl grade 3 intermetallic alloy

Abstract

Upon the isothermal oxidation of an ODS FeAl Grade 3 intermetallic alloy, a structured oxide scale is developed between 800 and 950°C. TEM studies have revealed a three-layered structure with a top nanoequiaxed alumina, a central alumina and a bottom Fe-Al spinel. At these temperatures, aluminium outward diffusion does not seem to be suppressed neither by the spinel sub-layer nor by the yttria present in the superalloy. However, the formation of metastable θ-Al₂O₃ seems to be significantly hindered. On the contrary, at 1000°C, the spinel phase no longer forms but a columnar alumina layer topped up with a nanoequiaxed structure. Yttria seems then to segregate at the scale/alloy interface and to randomly coarsen to produce an Y-Al oxide phase.     

The halogen effect for improving the oxidation resistance of TiAl-alloys

Abstract

Alloys based on TiAl intermetallics are potential candidates for high temperature applications in e.g. aero engines or automotive engines because of their low specific weight and good high temperature strength. To improve their oxidation resistance at temperatures up to 1000°C the halogen effect offers an innovative and cost-effective way. The addition of small amounts of halogens into the surface leads to the preferential formation of gaseous aluminium halides which are oxidised to aluminium oxide during their outward migration forming a dense, protective and slowly growing alumina scale on the surface. In this paper two methods were used to apply halogens to the surface, ion implantation (F and Cl) and a liquid phase process (F). Ion beam analysis with detection limits in the ppm-range was applied to quantify the needed amount of halogens to achieve the halogen effect. Thermocyclic oxidation experiments at 900°C were performed in laboratory air and wet air. Depth concentration profiles of fluorine were measured by PIGE within the first 1.4 μm without destruction of the sample before and after oxidation. Furthermore, the loss of fluorine during heating up and oxidation was measured characterising the stability of the effect. Simultaneous RBS-measurements of the O-, Al- and Ti-depth profiles prove the formation and growth of an almost pure alumina scale. Correlation with the fluorine profiles validates the proposed model for the halogen effect. Furthermore, metallographic methods, REM, EPMA, AES and the proton micro beam (PIXE) were applied to study cross-sections. A virtually pure alumina scale was found after F-treatment and oxidation up to 1500 hours at 900–1000°C in air. The fluorine depth profiles after ion implantation and liquid phase treatment, respectively, show similar levels for both methods before and after oxidation. The development of the fluorine interfacial concentration underneath the oxide scale as a function of oxidation time and temperature was recorded. The results are discussed in the light of the existing model considerations on the halogen effect and with regards to differences in the behaviour between F- and Cl-doping.     

Wettability of preoxidised TiAl substrates by liquid aluminium,copper, and silver

Abstract

The process of wetting of preoxidised TiAl alloy substrates by aluminium, copper, and silver filler metals has been examined. Two types of wetting behaviour were observed in these systems, one in which the filler metal undermined the oxide layer on the substrate and the other in which the filler metal spread on the outer surface of the substrate oxide layer. The mechanisms leading to these two different types of wetting behaviour are discussed. The effects of filler metal reactivity and of thickness of substrate oxide layer on the wetting process are discussed     

Aluminium based composites reinforced with alumina coated carbon fibres

Abstract

A fine Al₂0₃ coating could be obtained from alumina sols modified by chelator acetylacetone, with exact control of parameters. Coating with alumina by the sol–gel method on a carbon fibre surface was investigated in detail to improve the oxidation resistance of carbon fibre. Further study focused on making the alumina coated fibre reinforced aluminium composite prefabrication. X-ray diffraction and SEM methods were used to analyse the alumina gels and the carbon fibre/aluminium (CF/Al) preformed wire. After the coating treatment, oxidation resistance of carbon fibres is enhanced, the wettability between the fibres and melting aluminium is greatly improved, and the tensile strength of CF/Al preformed wire is increased.     

Fabrication of Al–TiC master composites and their dispersion in Al,Cu and Mg melts

Abstract

TiCpowders have been spontaneously infiltrated by molten Al with the aid of a K-Al-Fflux. The fluxdissolves the oxide film on the surface of molten Al, facilitating wetting between 'clean' Al and TiC particle surfaces, enabling liquid to be rapidly drawn into the network of TiC particles by capillary forces. The resulting master alloy was readily incorporated and dispersedinto molten Al, Cu, and Mg, indicative that the additive was readily wetted by all three molten metals and that the particles in the additive were not joined together by strong bonds. The use of a flux to facilitate cleaning of TiC, dissolution of Al in the meltand the avoidance of direct contactbetween TiC and melt surface oxides, all contribute to improved wetting. The slightly poorer quality of the particle distribution and the lowest yield in the Cu based alloy, suggest that wetting is worst in this system.     

Microstructural evaluation of dross formation on Mg- and non-Mg-containing Al alloys from industrial furnaces

Abstract

The microstructural aspects of dross formation in industrial Al re-melt furnaces for several different alloy compositions have been evaluated. The Al alloys used in this study included 1350 (no Mg), 3004 (low Mg), and 5182 (high Mg). Dross specimens were collected directly from the different Al alloy melts in industrial furnaces using a consistent sampling protocol in order to compare the microstructure and phase development of the dross as a function of melt temperature and composition. The results showed that the sequence of phases formed during the re-melt process was the same for all the alloys examined; amorphous-Al₂O₃ forms first followed by either α-Al₂O₃+AlN (for non-Mg-containing alloys) or cubic MgO, then MgAl₂O₄, and lastly α-Al₂O₃ (for low- or high-Mg content). The formation of MgAl₂O₄ is associated with accelerated oxidation rates (known as breakaway oxidation) and this reaction proceeds until the Mg is depleted at the molten surface. At this point, aluminum oxidation is predominant and occurs at a significantly lower oxidation rate. The results obtained in this study are consistent with models developed for dross grown on similar Al alloys in laboratory environments and show that Mg oxidation (and the accelerated formation of MgAl₂O₄) dominates the oxidation process during Al melting, whether the Al contains low or high Mg contents. The oxide morphology within the dross layer differed according to the particular alloy being melted and thus the amount of Al recovery from dross can vary with composition.     

The influence of the moisture content of the atmosphere on alumina scale formation and growth during high temperature oxidation of PM2000

Abstract

Preliminary studies have been undertaken on cyclic and isothermal oxidation at 1,300°C of thin (125 μm) samples of commercial ODS alloy PM2000 for up to 350h in two different oxidising environments; dry and moist air. Scanning electron microscopy (SEM) and electron microprobe analysis (EPMA) have been used to study the influence of such environments on alumina scale formation and growth. Initial mass gain observations showed that the alumina scale, which formed on the samples oxidised in air+2.5vol% H₂O grew faster in the early stages of oxidation than in the case of dry air. However the SEM analysis revealed that the scale morphologies in both dry air and air+2.5vol% H₂O were similar. In both cases the scales consisted of equiaxed grains at the scale–gas interface with Ti-rich particles in the outermost part of the scale. The major factor for the total scale failure, the formation of non-protective iron oxide, is the depletion of Al levels to a critical value, below which no protective alumina scale can form; and this occurred slightly faster in moist air than in dry air     

A life prediction model for the chemical failure of FeCrAlRE alloys: preliminary assessment of model extension to lower temperatures

Abstract

FeCrAl alloys are being deployed increasingly for industrial applications at elevated temperatures, primarily because their environment protection derives from the formation of an alumina scale. However, such protection is limited ultimately by chemical failure of the scale resulting in catastrophic, non-protective corrosion rates. The accurate prediction of component failure, therefore, is essential for economic and safety reasons.

A model for the chemical failure of alumina scales has been developed, which predicts the lifetimes of commercial FeCrAlRE alloys in oxidising environments. This model is based on the consumption of aluminium in the alloy through scale growth and also takes into account the effect of mechanical scale failure/spallation accelerating the rate of aluminium depletion – over the temperature range (1100–1400°C) this model has been shown to be applicable, when the scale formed was α-Al₂O₃. Its growth and failure processes are essentially similar over the range, although, of course, rates are temperature dependent.

In other potential technological applications, for example automotive catalytic converters, service temperatures can be much lower, in some instances as low as 750°C, while component sections are considerably thinner than for the high temperature (≥1100°C) structural applications.

At these lower temperatures scale growth mechanisms are more complex, involving the formation initially of transitional aluminas, which in some instances transform with time into the more stable α alumina. Additionally, the role of the reactive element (RE) and also of the mechanical interaction between substrate and scale can vary over the lower temperature range.

Component lifetime prediction at these temperatures is vital also. So, to initiate this action, this paper will present a preliminary assessment of the ability of the existing high temperature model to predict the lifetime of FeCrAlRE alloys in oxidising environments at temperatures in the range 750–1050°C.     

Continuous separation of Fe–Al–Zn dross phase from hot dip galvanised melt using alternating magnetic field

Abstract

Experiments to continuously separate Fe–Al–Zn dross phase from hot dip galvanising zinc melt were conducted on a laboratory scale apparatus by using high frequency alternating magnetic field. Effects of processing time (t) on separation efficiency were investigated. The experimental results show that using the electromagnetic repulsive force resulting from the electrical conductivity difference between zinc melt and Fe–Al–Zn dross phase, the deleterious zinc dross particles causing surface defects of galvanising steel sheets can be continuously separated from the zinc bath under alternating magnetic field, and the separation efficiency increases with the increase in processing time. When the magnetic frequency is 17·5 kHz, the effective magnetic flux intensity is 0·1 T, the cross-section of the ceramic square pipe is 10 × 10 mm, and the processing time is 0·6–2·5 s, the separation efficiency of zinc dross varies from 43·76 to 85·71%, and the experimental results are in reasonable agreement with the theoretical results.     

The influence of reactive element additions to β-NiAlCr alloys on the morphology of thermally grown oxides

Abstract

The effect of the reactive elements (REs), Y and Zr, on oxidation of β-NiCrAl alloy at 1373 K in a gas mixture of argon with 20 vol.% oxygen at atmospheric pressure was evaluated using X-ray diffractometry, scanning electron microscopy, electron probe X-ray microanalysis and secondary ion mass spectroscopy. The oxide surfaces and interface morphologies, compositions and growth kinetics were studied for alloys with 0.32 at.% Zr and 0.24 at.% Y additions and for an undoped alloy. The oxide layer produced on the three different alloys contains mainly α-alumina and some intermediate alumina modification, Cr₂O₃ and RE-oxides. A needle-like morphology was seen on top of the oxide layer for the undoped and Zr alloy. Needle formation on the Y alloy was suppressed by the formation of a thin Y₂O₃ layer during the initial stage of oxidation. Needles were maintained to long oxidation times for the undoped alloy, but disappeared on the doped alloys indicating that some cation diffusion is possible when REs are present. Fewer intermediate alumina modifications are seen for the oxide layers on the RE alloys showing that the REs promote the formation of the α-alumina phase. Oxide layer growth occurs in two stages for all alloys. Initially, oxide growth is rapid with outward diffusion of aluminium. The second stage of oxidation is slow and is initiated by the formation of a closed α-alumina layer limiting further oxidation to inward oxygen diffusion. This stage is characterised by parabolic growth kinetics associated with a constant aluminium interface concentration. The oxide layer is thinnest for the Y alloy due the fine Y₂O₃ layer acting as a diffusion barrier. The oxide/alloy interface for the undoped alloy is flat and shows many voids, whereas voids are not seen for the RE alloys. This is due to the promotion of a closed α-alumina layer giving predominantly inward growth early in the oxidation process. Oxide pegs of the RE are also seen growing into the alloys. The lack of voids and the oxide pegs are advantageous for oxide layer adhesion to the doped alloys.