6xxx Series Alloys Research Articles

On the Intergranular Corrosion and Hardness Evolution of 6xxx Series Al Alloys as a Function of Si:Mg Ratio, Cu Content, and Aging Condition

The intergranular corrosion (IGC) and hardness associated with 6xxx series aluminum alloy sheets, varying in composition (namely Si:Mg ratio and Cu content), were systematically studied in four different aging conditions. The IGC of each alloy was studied using ASTM G110, scanning electron microscopy, and optical microscopy. Scanning transmission electron microscopy was also used to rationalize the grain boundary chemistry of the relevant alloys. It was revealed that the severity of IGC depends on the continuity and type of grain boundary phases. The property space reported herein reveals prospects for the optimization of 6xxx series alloy compositions.

Microstructure and mechanical properties of near net shaped aluminium/alumina nanocomposites fabricated by powder metallurgy

Near net shaped metal matrix composites were fabricated using high energy cube milling and hot pressing process. Cube milling was up to six times faster than commercially available planetary ball mill in dispersing up to 1 vol.% of Al2O3 nanoparticles in AlMg5 alloy. Combined yield strength of 530 MPa and elongation at rupture of 4% exceed the values of commercially available wrought 5xxx series alloys. Thorough microstructure characterisation of composites revealed bimodal grain structure with an increasing content of finer grains and high angle grain boundaries with higher reinforcement content. The main contributing strengthening mechanisms for achieving high strength were solid solution strengthening, Orowan strengthening and grain boundary strengthening. Coarse grained regions in the bimodal structure, together with low angle grain boundaries were assumed to be the main source of retained ductility.

Influence of copper additions on materials properties and corrosion behaviour of Al–Mg alloy sheet

Abstract Additions of a few tenths of a percent of Cu to Al–Mg alloys give rise to a noticeable age-hardening effect. This can be utilized to overcome the softening during paint curing of conventional Cu-free Al–Mg alloys of the AA 5xxx series for car body applications. In the present study we explore this effect in sheet material of the Al–Mg–Cu alloy AA 5023. Mechanical properties and formability of AA 5023 sheet were determined and compared with results obtained in the Cu-free alloys AA 5019 and AA 5182. The resistance to corrosion, viz. general pitting behaviour, filiform corrosion and, most notably, intergranular corrosion was probed. As to the latter, TEM analysis was performed to assess type, composition and spatial arrangement of grain boundary phases causing intergranular corrosion in AA 5xxx series alloys. The results are discussed as a function of alloy composition, recrystallized grain size as well as final heat treatment, i.e. batch annealing vs. continuous solution heat treatment.

Experimental and Numerical Studies on the Forming Behavior of High Strain Al-Mg-Si(-Cu) Sheet Alloys

Al-Mg-Si(-Cu) heat-treatable alloys are characterized by a significant increase in hardness upon ageing compared to 6xxx series alloys with a copper content of Cu<0.15wt.%. With a higher amount of copper, the formability and ductility of the alloy are typically decreased compared to 6xxx alloys with a low content of Cu.The purpose of this paper is to discuss recent developments of high-strain 6xxx alloys for future automotive applications. The influence of copper content on the forming behavior of Al-Mg-Si(-Cu) sheet alloys was investigated. The examination was confined to four different alloys in total: Three of type AA6111 with varying Cu contents and a reference alloy type AA6016. All alloys were pre-aged at > 80°C after quenching and had a solute content of Si/Mg ratio in balance (wt.%).The forming behavior was characterized by tensile tests, bending tests, cup drawing tests and Nakajima tests. The experimental results are supplemented by numerical simulation. Hocket-Sherby's hardening law and Hill 90 yield criterion were used for the material modeling behavior. The Marciniak-Kuczynski model (M-K-model) approach was chosen to construct forming limit diagrams for all alloys.The study showed that forming behavior is strongly dependent on the type of alloy, the thickness, and natural aging, which is in turn influenced by the storage period and the quenching rate in sheet production. Up to a Cu content of 0.8wt.% the limiting drawing ratio is approximately equal to the reference alloy. With contents greater than 0.8wt%, an exponential drop of the formability is observed. Furthermore, the results of the cup drawing tests are compared to the numerical simulations in order to validate the simulation of parts with higher complexity. The results of this work contribute to the application of high strain 6xxx alloys in automotive applications

Effect of Natural Ageing on Strength and Anisotropy in Aluminium Alloy AA 6005C

During processing of age-hardenable AA 6xxx series alloys for automotive applications the sheets may experience significant time spans between solution heat treatment at the aluminium supplier and age hardening upon the final paint bake cycle at the carmaker. Natural ageing during these pause times is known to greatly affect materials properties of autobody sheet. In the present study we explore the impact of natural ageing on the tensile properties and the in-plane anisotropy of alloy AA 6005C. Materials properties at various degrees of natural ageing are modelled with the help of a nanoscale material model NaMo, which consists of a precipitation model simulating the formation of clusters and phases upon natural ageing as input to a mechanical model simulating the evolution of yield strength and work hardening. Plastic anisotropy is modelled from the materials crystallographic texture by a visco-plastic self-consistent polycrystal-plasticity code VPSC.

A Combined TEM and Atom Probe Approach to Analyse the Early Stages of Age Hardening in AA 6016

In this study we aim at combining the results from transmission electron microscopy (TEM) and atom probe tomography (APT) to study the early stages of phase decomposition in the age hardening alloy AA 6016. Samples are subjected to different periods of natural ageing or artificial pre-ageing at elevated temperature in order to produce different types of clusters and early stages of precipitation before age hardening commences. APT is utilized to detect clusters and identify their compositions, whereas TEM is applied to analyse and quantify number density and sizes of the particles during artificial ageing at 185°C. It is shown that the two techniques, TEM and APT, are complementary and a combined approach yields more detailed insight into the early stages of phase decomposition in age hardening 6xxx series alloys than possible by the sole use of either technique individually.

On the Electrochemical and Quasi In Situ Corrosion Response of the Q-Phase (AlxCuyMgzSiw) Intermetallic Particle in 6xxx Series Aluminum Alloys

The electrochemical response and corrosion associated with the Q-phase (AlxCuyMgzSiw) intermetallic compound was studied. Q-phase intermetallics are the principal strengthening phase in a number of Cu-containing 6xxx series (Al-Mg-Si) alloys, and Q-phase has not previously been uniquely studied in regard to its influence on localized corrosion in detail. Herein, quasi in situ scanning transmission electron microscopy was utilized in understanding the localized corrosion response associated with nanoscale Q-phase precipitates in a Cu-containing 6xxx series Al alloy sheet with a composition (in wt%): 97.3Al-0.9Si-0.74Mg-0.84Cu-0.08Fe-0.14Mn. Furthermore, the Q-phase compound was also produced on the microscale within a bulk-synthesized alloy. The electrochemical behavior of the microscale Q-phase was studied using a micro-electrochemical capillary cell and by potentiodynamic polarization. Quasi in situ scanning electron microscopy was also performed on the bulk alloy synthesized to contain microscale Q-phas...

Investigation of Texture Structure and Mechanical Properties Evolution during Hot Deformation of 1565 Aluminum Alloy

The article is devoted to study of mechanical properties and texture evolution during thermal mechanical treatment of new aluminum 5XXX series alloy (1565 ch in Russian naming system). For the investigation of deformation behavior of 1565ch alloy a Gleeble axial compression mode is used. The expression for steady flow stress as the functions of temperature of deformation and strain rate is obtained. For texture evolution investigation industrial mills is used. Texture evaluation is performed using the orientation distribution function (ODF) calculated from the X-ray direct pole figures

Non-destructive ultrasonic examination of root defects in friction stir welded butt-joints

As a solid-state welding process, Friction Stir Welding (FSW) offers a variety of advantages over traditional welding processes. Problems that are typically occurring due to the cooling of the liquid phase, such as solidification cracking and formation of porosity, generally do not occur in FSW. Nevertheless, as a result of suboptimal settings of the welding process parameters and certain uncontrollable conditions, FSWs are still associated with a number of specific flaws, e.g. root flaws and wormholes.Ultrasonic non-destructive testing and evaluation techniques (NDT&E) can be used for quality assessment of friction stir welded joints. In this paper, a novel approach for the detection of root flaws is proposed using an immersion ultrasonic testing method in oblique incidence and backscatter mode. The backscattered energy C-scan images obtained after an empirical positioning and proper time gating can be straightforwardly interpreted by direct comparison with typical ‘flaw’ patterns, allowing for identification and localization of the root flaws in the weld. The method is illustrated for FSW butt joints of the AlZnMgCu (7XXX series) alloy.

Survey of Detection, Mitigation, and Repair Technologies to Address Problems Caused by Sensitization of Al-Mg Alloys on Navy Ships

Structural aluminum alloys are used in marine applications where light weight and increased speed are paramount. The 5xxx series alloys are the ideal choice, providing an optimum combination of as-welded strength and general corrosion resistance. A particular concern is sensitization in service. Aluminum is considered sensitized when a nearly continuous network of β phase forms along the grain boundaries. The β phase is anodic to the Al matrix, and, when exposed to sea water and sufficient loading, provides a clear pathway to stress corrosion cracking (SCC). The degree of β-phase precipitation is driven by a combination of time and elevated temperature. For over a decade, SCC of Al superstructures has plagued Navy ships. To address this issue, relevant technologies have been adopted, adapted, and developed to combat this issue. More specifically, the technology areas involve the detection of sensitization, the mitigation of SCC, and the replacement of corrupt materials. The technologies discussed herein a...

A Survey of Sensitization in 5xxx Series Aluminum Alloys

The 5xxx series (Al-Mg-based) aluminum alloys suffer from intergranular corrosion and intergranular stress corrosion cracking when the alloy has become “sensitized.” Sensitization refers to insidious precipitation of β phase (Mg2Al3), which is problematic when present at grain boundaries. The β phase is electrochemically active and may preferentially dissolve. This paper reviews the relevant works that have documented the degree of sensitization for various 5xxx series alloys, providing a holistic overview of the issue, along with attention to the bulk composition, heat treatment, and microstructure.

Exploring the electrochemistry of 6xxx series aluminium alloys as a function of Si to Mg ratio, Cu content, ageing conditions and microstructure

The electrochemical response of 6xxx series Al-alloys, which are based on the Al-Mg-Si-(Cu) system, was systematically studied. Nine custom Al-Mg-Si-(Cu) alloys were produced on the basis of varying Si to Mg ratio and Cu content. The alloys were further aged at four different ageing conditions, and analysed by potentiodynamic polarisation and careful quantification of metastable pitting events from potentiostatic transient testing. The relevant alloy microstructures were characterised using transmission electron microscopy, revealing differences in nanoscale precipitates that evolve as a function of alloy composition and ageing conditions. It was revealed that the pitting events depend primarily on the thickness of the precipitates, rather than their length or number density. In the case of Cu-containing 6xxx alloys, precipitates were found to be surrounded by an apparently single atomic layer of segregated Cu atoms, which appeared to inhibit the precipitate growth, thus inhibiting the number of pits occurring. This study provides significant insights into both the understanding of, and the control of, the genesis of pitting corrosion in 6xxx series Al alloys.

Warm HydroForming of the heat treatable aluminium alloy AC170PX

In this work the Warm HydroForming process of a 6xxx series Al alloy has been investigated in order to find suitable process parameters for successfully forming a benchmark component displaying varying deformation ratios due to its geometry. Since the attention was focused on the 6xxx series alloy AC170PX, belonging to the group of the age-hardenable Al alloys, the process conditions affecting the ageing, such as temperature and duration of heating exposure, the rate to increase the oil pressure, had to be determined. A preliminary mechanical characterization on artificially aged specimens was thus necessary both at room temperature and in warm conditions to investigate the influence of the ageing phenomenon on the mechanical and deformation behaviour at temperatures ranging from 150°C to 350°C. Based on the results of the tensile tests, preliminary Warm HydroForming experiments were conducted by changing the most influencing process parameters: working temperature, heating time and the time needed to increase the oil pressure up to the maximum value. It was found that the optimum working temperature was 200°C and that the exposure of the material to the employed warm-forming temperature had to be minimized. Final Warm HydroForming tests were thus carried out using different rates to increase the oil pressure for comparison purposes, revealing that employing higher rates, i.e. increasing the strain rate, was beneficial, in terms of die cavity filling and sheet thinning, to obtain sound products.

Flexible rolling of aluminium alloy sheet—Process optimization and control of materials properties

Continuously increasing demands for consumption-optimized automotive concepts require steady development of weight-saving potentials. In the area of steel lightweight design, flexible rolling of tailor-rolled blanks, TRB, is a well-established technology applied worldwide in all automotive categories. In the future, with the current tendency towards an increased application of aluminium sheets in the car body and chassis, flexible rolling of aluminium will become more and more attractive.In the present paper, the status of the process and materials development of Al-TRB is presented and the prospective of the future development is outlined. First, the possible development directions are addressed, followed by an outline of the process chain for the Al-TRB fabrication together with several application examples. Considering work-hardening AA 5xxx series and age-hardening AA 6xxx series alloys for automotive applications, lab-scale experiments have been conducted in order to elucidate the range of properties which can be realized through flexible rolling while maintaining reproducible constant materials properties according to specification. Large-scale trials were performed to demonstrate that the manufacturing of Al-TRBs is feasible, in that these alloys can be processed in industrial scale to provide homogeneous materials properties over the relevant thickness range for automotive applications.

Critical Factors Affecting Intergranular Corrosion of AA5083 Under Atmospheric Exposures

Al-Mg 5XXX series alloys are widely used for marine applications due to their low cost, high strength-to-weight ratio and good weldability. These alloys can become susceptible to intergranular corrosion (IGC) due to precipitation of a more anodic β-phase (Al3Mg2) along grain boundaries after exposure to hot environments during service. A number of studies [1-4]have been conducted to study the role of metallurgical and electrochemical factors on intergranular corrosion penetration. Orientation propagation, the degree of sensitization (DoS) and exposure time have been shown to affect the rate of IGC propagation. Unfortunately, majority of the aforementioned IGC studies have been conducted in full immersion, but much of the marine infrastructure is exposed to atmospheric conditions. There is a need to extend the full immersion corrosion framework and conduct IGC studies under atmospheric and/or thin film conditions. The present work is aimed at identifying critical factors affecting IGC of AA5083 under atmospheric conditions, quantifying their effects and understanding the corrosion mechanisms to better predict IGC of 5XXX alloys in the field. Preliminary results indicate that oxidizing strength of the environment and material system govern IGC behavior in the thin film conditions. Both persulphate and ozone have been used in laboratory studies to control the oxidizing behavior of the environment. Figure 1 shows the optical microscopic images depicting the IGC of AA5083 exposed to 95% RH and 30⁰C with different persulphate thickness for 100hours. It can be clearly seen that mass of persulphate affects IGC of AA5083 under thin film as the concentration of persulphate is same in both cases. Another notable result is that the cathode area has a strong influence on IGC of AA5083 under thin film. Figure 2 shows the maximum IGC length for AA5083-H131 exposed to 6ppm ozone at 98% RH and room temperature. The samples were loaded with 1000 µg/cm2NaCl. The graph shows that the maximum length of IGC has a strong correlation with the sample size. Further work will be presented from electrochemical measurements under thin film conditions and a study of the cathodic kinetics for atmospheric exposures. The effects of critical factors like ozone and persulphate concentration, salt loading density, solution film thickness and aerosol size will be presented to develop a better understanding of IGC corrosion under atmospheric conditions. Intergranular corrosion depth and surface morphology will be used as corrosion indicators. Acknowledgements The authors would like to thank Daniel J. Dunmire, Director, DoD Corrosion Policy and Oversight, Office of the Secretary of Defense, for financial support, via the US Air Force Academy Contract#FA7000-12-2-0017.

Exploitation Load Impact on Electrochemical Properties of Shipbuilding 7xxx Series Aluminum Alloy Joints Prepared with TIG and FSW Methods

The paper presents the results of investigations on mechanical properties and electrochemical potential distribution within arc welded (TIG) and friction stir welded (FSW) joints subjected to slow strain rate tests. The materials investigated were high-strength 7xxx series (7020 and its modification 7020M) aluminum alloys intended for shipbuilding. The objectives were joint quality assessment and comparison of the advantages of new FSW method with the traditional TIG methods commonly utilized in shipbuilding industry. Joint quality was evaluated based on mechanical investigations, hardness distribution tests and simultaneous electrochemical corrosion potential measurements at various locations within the welded joints.Initiation of corrosion processes on TIG and FSW joints was identified as a radical decrease in corrosion potential related to load followed by oxide layer cracking. Arc welded (TIG) joints of 7xxx series alloys undergo corrosion at lower values of tensile load applied as compared to the FSW joints. Superior mechanical properties and higher corrosion resistance of the FSW joints make this technology well-suited for joining high-strength 7xxx series alloys.

The Properties of 7xxx Series Alloys Formed by Alloying Additions

Abstract Currently there is a constant development in the field of aluminium alloys engineering. This results from, i.a., better understanding of the mechanisms that direct strengthening of these alloys and the role of microalloying. Now it is microalloying in aluminum alloys that is receiving a lot of attention. It affects substantially the macro- and microstructure and kinetics of phase transformation influencing the properties during production and its exploitation. 7xxx series aluminum alloys, based on the Al-Zn-Mg-Cu system, are high-strength alloys, moreover, the presence of Zr and Sr further increases their strength and improves resistance to cracking. This study aims to present the changes of the properties, depending on the alloy chemical composition and the macro- and microstructure. Therefore, the characteristics in the field of hardness, tensile strength, yield strength and elongation are shown on selected examples. Observations were made on ingot samples obtained by semi-continuous casting, in the homogenized state. Samples were prepared from aluminum alloys in accordance with PN-EN 573-3: 2013. The advantage of Al-Zn-Mg-Cu alloys are undoubtedly good strength, Light-weight and resistance to corrosion. As widening of the already published studies it is sought to demonstrate the repeatability of the physical parameters in the whole volume of the sample.

Effect of natural ageing and pre-straining on strength and anisotropy in aluminium alloy AA 6016

During processing of age-hardenable AA 6xxx series alloys for automotive applications the sheets may experience significant time spans between solution heat treatment at the aluminium supplier and age hardening upon the final paint bake cycle at the carmaker. Natural ageing during these pause times and intermediate deformations by sheet levelling or forming operations both affect materials properties of autobody sheet. In the present study we explore the impact of a combination of natural ageing and pre-straining on the tensile properties and the in-plane anisotropy of alloy AA 6016. Materials properties are modelled with the help of a nano-scale material model called NaMo, which consists of a precipitation model simulating the formation of clusters and phases upon natural ageing as input to a mechanical model simulating the evolution of yield strength and work hardening as a function of natural ageing and pre-straining. Plastic anisotropy is modelled by polycrystal-plasticity simulations under consideration of latent hardening to account for strain path change effects introduced by the pre-straining.

Correlation of texture and intergranular corrosion in Al-Mg 5xxx series alloys

Aluminium-alloys of the AA 5xxx series with Mg contents in excess of 3% may suffer from intergranular corrosion (IGC) when exposed to temperatures in the range 60 to 200°C. At these temperatures Al-Mg alloys are rendered susceptible to IGC by precipitation of β-Al8Mg5 phases along the grain boundaries. Accordingly, susceptibility to IGC will depend on grain size as well as type and orientation of the grain boundaries present in the material, that is, on the crystallographic texture of the material at final gauge.Therefore, it is of great interest to study the correlation of texture and precipitation of β-AlMg phases and, therewith, susceptibility to IGC. For this purpose, different AA 5182 samples were processed so as to produce different crystallographic textures and characterized with respect to microstructure and resistance against IGC. EBSD local texture analysis was applied to provide information about the grain boundary character distribution. Eventually, this may enable Al industry to reduce the susceptibility of Al-Mg alloys to IGC by proper control of the final gauge texture, such that higher Mg-contents may be used in IGC-critical applications.

Fatigue Behavior of Aerospace Al-Cu, Al-Li and Al-Mg-Si Sheet Alloys

In the present work, an experimental study was performed to characterize and analyze the tensile and constant amplitude fatigue mechanical behavior of several aluminum alloys, namely 2024 (Al-Cu), 2198 (Al-Li) and 6156 (Al-Mg-Si). Al-Li alloy was found to be superior of 2024 in the high cycle fatigue and fatigue endurance limit regimes, especially when considering specific mechanical properties. Alloy 6156 was found to have superior constant amplitude fatigue performance that the respective 6xxx series alloys; more than 15% higher endurance limit was noticed against 6061 and almost 30% higher than 6082. Alloy 6156 presented only a marginal increase in fatigue life for the HCF regime.