7xxx Series Aluminum Alloys Research Articles

Grain Boundary Precipitate Modification for Improved Intergranular Corrosion Resistance

Intergranular corrosion is a significant concern for Al-Mg alloys when subjected to a corrosive salt-water environment. To address this issue, the standard composition of a 5XXX series aluminum alloy (AA5083) was modified in an attempt to improve the alloy’s overall corrosion resistance through alloying and thermal processing. The concept being that through alloying and heat treatments, desirable precipitate phases such as τ- and/or τ-copper rich phase(s) that are known to offer corrosion resistance would potentially form that could effectively improve intergranular corrosion behavior. Therefore, the chemical composition of standard AA5083 was modified by adding various amounts of copper and zinc. Sensitization heat treatments were then performed to determine the specific conditions under which these phases would form. LOM, SEM, STEM imaging and conventional TEM were used to analyze microstructural features. Corrosion was attributed to a network of detrimental Mg-rich grain boundary precipitates in the standard alloy. Alloying with Cu and Zn can offer improved intergranular corrosion behavior. The mechanism seems to be either by delaying or eliminating precipitation at the grain boundaries.

Characterisation of early precipitation stages in 6xxx series aluminium alloys

Advanced microscopy techniques such as high angular annular dark field scanning transmission electron microscopy and energy filtered transmission electron microscopy are being explored to study early precipitation stages of 6XXX-series aluminium alloys. Observations are combined with those from 3 dimensional atom probe studies of the same materials. GP-zones are imaged by annular dark field scanning transmission electron microscopy and plasmon mapping. Developed clusters are investigated and quantified by atom probe, and a high number density of clusters with a Mg/Si-ratio close to the alloy composition are confirmed to be present in the alloy.

Microstructural Analysis of 5XXX Series Aluminum Alloys for Optimal Strength and Corrosion Resistance

Extract HTML view is not available for this content. However, as you have access to this content, a full PDF is available via the ‘Save PDF’ action button. Extended abstract of a paper presented at Microscopy and Microanalysis 2005 in Honolulu, Hawaii, USA, July 31--August 4, 2005

Mesoscopic structure control of spray formed high strength Al–Zn–Mg–Cu alloys

Several high solute, high strength 7xxx series aluminum alloys with solute contents close to equilibrium solid solubility limits of the Al–Zn–Mg–Cu system have been produced by rapid solidification using spray deposition (the Osprey process). This process yields massive preforms directly from the liquid state by combining atomization and consolidation into one step. Various elements, including chromium, manganese and silver are incorporated to produce a variety of microstructures and mechanical properties. The zinc to magnesium ratio is also varied to see the effect on the strength. Superior strengths in excess of 849 MPa are achieved and are attributed to two major substructures with different scale; nanometer sized η′ metastable precipitates and slightly larger, but finely distributed dispersoids which provide a fiber-like reinforcement. The remarkable strengthening is predominantly attributed to precipitation hardening and a large coherency strain.

Effect of Cu Content on Corrosion Behavior of 7xxx Series Aluminum Alloys

The corrosion behavior of 7xxx aluminum alloys with various Cu content was investigated using polarization and electrochemical impedance spectroscopy (EIS) techniques. Two breakdown potentials were found in Cu-containing alloys in deaerated chloride solution, and the values increased logarithmically with increasing Cu content. However, in aerated chloride solution, polarization resistance as determined by EIS decreased with increasing Cu content. The first breakdown potential corresponded to transient dissolution associated with attack of the fine hardening particles and the surrounding solid solution in a thin surface layer. The second breakdown potential was associated with combined intergranular and selective grain attack. The correlation between the microstructure, especially Cu content, and corrosion behavior was made by composition analysis of the grain boundary regions including precipitate free zone and grain boundary precipitates as well as the matrix by scanning transmission electron microscopy. The corrosion potential increased with increasing alloy Cu content in aerated chloride solutions because of enhanced rates of oxygen reduction. This degraded the corrosion resistance despite the increase in the breakdown potentials with Cu content. © 2004 The Electrochemical Society. All rights reserved.

Effect of Chemical Composition Variation on Microstructure and Mechanical Properties of a 6060 Aluminum Alloy

The 6XXX series aluminum alloys (Al-Mg-Si) are widely used in many different engineering and architectural applications. These alloys usually undergo a thermal treatment, which consists of a heat treatment solution and artificial aging, since the desirable mechanical properties depend on the microstructural state of the material. The recycling of materials has been increasing recently for economic and ecologic reasons. By using scrap was raw material, important reductions in energy and total costs can be achieved, and, at the same time, negative environmental impacts can be greatly reduced. In the present work, the possibility of using a larger amount of scrap as raw material in the production of an AA 6060 alloy is evaluated by analyzing the difference in microstructure and mechanical properties between a commercial 6060 alloy and a variation with higher Fe and lower Si contents that was specially produced for this study. Both materials were placed into a heat treatment solution at 560 °C for 1 h, and then underwent water quenching followed by artificial aging at 180 °C for different periods of time. Hardness and tension tests were used to evaluate the mechanical properties. Light and transmission electron microscopy have been used to determine important features such as grain size before and after being placed into the heat treatment solution, and the characteristics of the second-phase particles in the two materials. This study leads to the conclusion that a higher amount of scrap material can be used in the production of 6060 Al alloy without significant changes in mechanical properties compared with the more usual compositions.

Quantitative analysis of texture evolution in cold-rolled, continuous-cast AA 5xxx-series aluminum alloys

The texture evolution of continuous-cast AA 5xxx-series aluminum alloys during cold rolling was investigated by X-ray diffraction. Texture volume fractions were calculated by an improved integration method. The results show that the relationship between the texture volume fractions and true strain can be quantified by mathematical formulae. The effect of alloy composition as well as initial microstructure and texture on rolling-texture evolution can be evaluated by the ki and ni values in the Johnson-Mehl-Avrami-Kolmogorov (JMAK) equation. The ki value reflects the rate of formation or disappearance of the texture components. The high-Fe AA 5754 aluminum alloy exhibits the lowest formation rate of the β fiber among the AA 5005, 5052, 5754, 5182, and high-Fe 5754 aluminum alloys.

Fatigue analysis of thin AlMgSi welded joints under constant and variable amplitude block loadings

This paper reports the fatigue behaviour of thin AlMgSi1 aluminium alloy weldments and the improvement in fatigue strength due to post-weld treatments. Several fatigue tests were performed using two distinct types of thin welded joints, T and single lap, manufactured using a 6xxx series aluminium alloy, and the efficiency of fatigue life improvement techniques, such as post-weld heat treatment and weld toe’s burr dressing, was studied. Both the post-weld heat treatment and the burr dressing produced a fatigue strength improvement. This fact is discussed, for both geometries, in the context of the initiation and propagation phases contribution to total fatigue life. Finally, several fatigue tests were performed under variable amplitude block loading using T6 post-weld heat-treated single lap joints, and the correspondent fatigue lives were compared with the predictions of Miner’s rule.

The effect of retrogression and reaging on the properties of the 7249 aluminum alloy

The retrogression and reaging (RRA) heat-treatment process and recent developments in high-strength 7xxx series aluminum alloys are summarized in this article. The results of experimental work indicate that RRA 7249 aluminum has the strength equivalent to or greater than 7249-T6 and superior corrosion resistance. This work is the result of collaborative efforts between the U.S. Navy and Loyola Marymount University.

Kinetics of the formation of the β fiber rolling texture in continuous cast AA 5xxx series aluminum alloys

The texture evolution in cold rolled continuous cast AA 5xxx series aluminum alloys was investigated by X-ray diffraction. The volume fraction of the β fiber component was calculated by integration of the ODF intensities within 15.5° of the center position of the β fiber. The relationship between the volume fraction of the β fiber component and true strain follows the Avrami equation.

Defect Formation in Twin Roll-Cast AA 3xxx and 5xxx Series Aluminium Alloys

Defect Formation in Twin Roll-Cast AA 3xxx and 5xxx Series Aluminium Alloys

Microstructural Modelling of a Commercially Processed 6xxx Series Aluminium Alloy

Microstructural Modelling of a Commercially Processed 6xxx Series Aluminium Alloy

Electron backscatter diffraction analysis of microstructural evolution in hot-deformed 6xxx series aluminum alloys

The electron backscatter diffraction (EBSD) technique is used to analyze the crystallographic grain orientation of deformed microstructures for 6xxx series aluminum alloy extrusions. In a partially extruded billet of aluminum, deformation zones with different crystallographic characteristics can be seen. Using compression testing performed on a Gleeble thermal-mechanical simulator under various conditions, the formation of different deformation zones is simulated. The EBSD technique is used to characterize samples deformed in compression testing to analyze the different deformation zones present in the extrusion process. Metal-flow analysis using EBSD is also applied to study the influence of different die designs on the surface texture of an extrudate. Microstructures from a profile that was formed with different die designs are examined. Analysis of a longitudinal weld present in typical hollow profiles is performed to determine the effect of metal-flow conditions on the resultant texture in the extrudate. The texture (or microtexture) measured from individual grains can be correlated to the orientation changes that occur during deformation.

524 Experimental evaluation of the 6xxx series aluminium alloys extrudability

6xxx series aluminium alloys are mainly used in door- and window- frame manufacturing. Commonly the raw material is recycled aluminium with the correct Mg and Si additive content. The aluminium profile beams extrusion process presents several failure defects, which are obviously associated with various forming parameters such as die geometry and material mechanical properties affected by its chemical constitution. In the present paper, several aluminium alloys were subjected to an experimental procedure in order to estimate the way the above-mentioned parameters affect the extrusion load. Various aluminium alloys were used with Si rating between 0.2% and 0.6% p. w. and Mg rating between 0.5% and 1.3% p. w. This procedure also focused on semi-angle die cone effect, as well as temperature conditions. The most significant result concerns the decrease in the extrusion load observed in aluminium alloys with Mg rate over 0.9% p. w. and Si rate between 0.2% and 0.3% p. w. Extruded through small semi-angle cone die, maintaining improved mechanical characteristics.

Strain-rate sensitivity limit diagrams and plastic instabilities in a 6xxx series aluminum alloy Part I: Analysis of temporal stress–strain serrations

A method for the construction of “processing windows” to avoid negative strain-rate sensitivity and associated serrated flow in some aluminum alloys is described. The method is based on modern approaches and techniques of non-linear dynamics – linear stability analysis, amplitude Ginzburg–Landau equations, and bifurcation diagrams. The general instability area is identified as possessing a negative strain-rate sensitivity coefficient (SRS). The mathematical technique developed in the present paper was applied to a specific aluminum alloy, Al–0.4%Mg–0.2%Si, and yielded good results in terms of predicting the negative strain-rate sensitivity regions in the “strain rate–temperature” parameter space. It was clearly demonstrated that even though the instability area is located in the region of intermediate strain rates, a qualitative difference exists between the areas of (relatively) fast and (relatively) slow strain rates: while in the first case the dynamic behavior of the system is supercritical, in the second case it is subcritical. The second case is highly undesirable because it causes a sudden onset of stable stress serrations that are difficult to suppress (at a given temperature), while in the first case the development of instability is gradual and, consequently, more easily controllable.

Dispersoid precipitation and process modelling in zirconium containing commercial aluminium alloys

Small dispersoid particles inhibit recrystallisation and are thus critical in controlling the grain structure of many high strength commercial aluminium alloys. A general, physical model has been developed for the precipitation of Al 3Zr dispersoids in aluminium alloys. The predictions of the model have been compared with results of an experimental investigation of Al 3Zr precipitation in 7050. The model has been shown to faithfully reproduce the distribution of dispersoids observed in this alloy, correctly predicting dispersoid free zones observed in interdendritic regions and at grain boundaries. Furthermore, the predicted precipitation kinetics agree well with experimental observation. The model has been used to study the effects of homogenisation conditions and alloy composition on dispersoid formation and has been shown to be a powerful tool for optimising the dispersoid distribution in 7xxx series aluminium alloys.

Use of Electron Backscatter Diffraction Technique in Characterization of 6XXX Aluminum Alloy Extrusions

Abstract Electron Backscattering Diffraction (EBSD) is an important tool for analyzing the crystal grain orientation of a microstructure and can be used to formulate conclusions about microtexture, texture determined from individual grains. This technique has been used to study a 6xxx series aluminum alloy's response to the deformation of the extrusion process. Extrusion is the process by which a billet of material is forced, under high pressure, through a die. The material undergoes a significant decrease in cross sectional area, and is formed into a shape equivalent to the geometry of the die orifice. Different bearing lands are shown in shown in Figure 1. These surfaces form the part, and are designed to control the metal flow making it uniform through the die, thus yielding good mechanical properties. This research was focused on the resultant microstructure. The shaded regions of Figure 2 show the two surface regions where EBSD measurements were taken.

Residual Stress Field in a Friction Stir Welded Aluminium Extrusion

Friction stir welding (FSW) is a relatively new welding technique, which makes it possible to weld low fusion weldability materials with very little or no post weld distortion, high mechanical strength and good fatigue performance. This is particularly important for welding high strength precipitation strengthened aluminium alloys of the 7xxx series previously not recommended for welding. Residual stress measurements have been made, using the synchrotron X-ray diffraction transmission technique, on a 7xxx series aluminium alloy plate made by friction stir butt welding two extruded T-profiles along their middle section. Measurements were also carried out on a non-welded extruded section to determine the pre-welded strain state. Comparisons have been made with holographic interferometry method. Hot extrusion of thin walled sections of 7xxx series alloys results in substantial preferred orientation. The grains appear mainly equiaxed in the transverse and highly elongated in the longitudinal directions, which is described as fibre texture. During friction stir welding, the original extrusion texture is destroyed. In the stirred region, fine grained recovered microstructure is observed, whereas in the thermo-mechanically affected zones, it is intermediate. This strongly textured microstructure makes is difficult to accurately determine the residual stress fields using the traditional measurement techniques but non-destructive synchrotron strain scanning produced excellent results over a full cross-section of the weld.

The Influence of Microstructure on the Formation of Stress Corrosion Cracks in 7XXX Series Aluminium Alloys

The stress corrosion cracks (SCC) formed in a 7010 Al alloy aged to different tempers have been examined using transmission electron microscopy. The data described centres on an investigation of those features of the microstructure of the 7010 that determine how the material degrades during SCC. Comparisons are made between the roles played by intermetallic particles, the microchemistry of the alloy grain boundaries and different ageing treatments and the results are discussed in relation to the way in which these parameters can lead to homogenisation either of the deformation or of the electrochemical potential differences which thereby reduce the susceptibility of the alloy to SCC.

Processing-Property Correlation in Friction Stir Welds

Friction Stir Welding (FSW) is a relatively new process for joining of metals. The process was invented in 1992 at The Welding Institute, Cambridge, UK. FSW is particularly well suited to use in highly alloyed aluminum because it is a solid state process; therefore, no undesirable, brittle, low-melting eutectic phases are formed during the welding process. The microstructure of a typical friction stir weld is that of a wrought product. The details of the weld formation mechanism have been the subject of some debate. Previous work on elucidation of flow patterns in friction stir welds indicated that the process may be described as an in-situ extrusion. Material flow in two friction stir welds produced using different welding parameters was visualized using embedded marker materials and the fidelity of the visualization technique was demonstrated. Flow of material in the friction stir welds was observed to depend strongly on the temperature of the weld. In this paper, welding temperature measurements are combined with tensile test data to further promote understanding of the process in 2XXX, 5XXX, 6XXX, and 7XXX series aluminum alloys. Weld parameters (spindle rotation rate and welding speed) and time-temperature histories are correlated with the global and local tensile properties, and microstructure of the resulting welds.