7xxx Series Aluminum Alloys Research Articles

The Effect of Corrosion Inhibitors on Environmental Fatigue Crack Growth in Al-Zn-Mg-Cu

Corrosion fatigue is an area of concern for the United States Air Force (USAF) and other Department of Defense (DoD) organizations. Often DoD corrosion prevention systems include chromate containing coatings, typically in the form of chromate conversion coatings and polymer primers. Chromate has been used successfully for many years within the DoD to prevent corrosion damage. However the environmental and personnel risks associated with chromate coatings have caused the USAF to pursue non-chromate containing corrosion prevention coatings [1]. To fully quantify chromate replacement coatings, an understanding of the effects that chromate has on corrosion fatigue crack growth rates must be fully characterized. Some researchers have shown that high levels of chromate added to 0.6 M NaCl full immersion corrosion fatigue tests on 7xxx series aluminum alloys slow the fatigue crack growth rate substantially [2,3]. The limitation of that research was that the amount of chromate present in the test solution environment was not connected to expected leach rates of chromate from polymeric coatings and a high solubility salt was used. The majority of DoD assets are protected from corrosion by polymer coatings loaded with corrosion inhibitors. For these coatings to slow fatigue crack propagation the corrosion inhibitors must become mobile as a consequence of hydration of the polymer coating matrix. Based on this mechanism of corrosion inhibitor release, the examination of atmospheric corrosion fatigue becomes important to help understand how inhibitors work in real world situations with hydrated salt layers rather than only fully immersed solutions.

SVET, SKP and EIS study of the corrosion behaviour of high strength Al and Al–Li alloys used in aircraft fabrication

The 2XXX and 7XXX series aluminium alloys are commonly used in aircraft applications where high strength-to-weight ratios are required. The present study aims to compare the corrosion resistance of two novel alloys developed for aircraft industry (AA2198-T851 and 7081-T73511) to that presented by the alternative base alloys (AA2524-T3 and 7050-T7451, respectively). Results indicate that AA2198-T851 may show a superior corrosion performance compared to AA2524-T3, being a potential candidate for its replacement, due to best mechanical properties. In what concerns the 7081-T73511, its resistance to pitting corrosion may be lower than for the base line alloy 7050-T7451.

Mechanism of Crack Initiation and Propagation in Single-side Piercing Process for Hollow Forged Parts

Hollow parts are demanded to reduce the weight of automotive body, and it is required to develop manufacturing technology of hollow parts with high efficiency. Thus the process, which forged part is pierced, has been applied to produce hollow parts since the forging process has advantages compared to conventional processes e.g. casting, machining. However the process, which is combined forging and piercing process, is single-side piercing process since the die is not placed. In this research, the influences of the bottom thickness of pre-form and clearance on the crack propagation are investigated. 6XXX series aluminum alloy is used as a material, and specimen is extruded as a preforming process. Extruded specimens are punched under several clearance conditions. Additionally, finite element analysis is performed to consider the stress and strain state, which affects the initiation and propagation of macroscopic crack. The experimental result shows that sheared surface length increases with increase of clearance between punch and material inner wall. Analytical result indicates that compressive stress state, which occurs at sheared area, interferes the propagation of initiated crack. As a result, secondary sheared surface is generated in case of large bottom thickness of extruded pre-form.

Influence of a Regression Heat Treatment on the Material Properties of a Copper-free 7xxx Series Aluminum Alloy

The alloy used in the investigation is a copper-free Al-Zn-Mg-alloy in the strength optimized T6 temper. The aim of the study is the increase of the typical requirements for formability by a regression heat treatment of the whole sheet. Therefore, the necessary material properties are determined according to temperature and time of the heat treatment to deduce a potential process range. Various test methods of uniaxial and multi-axial stress conditions are used to evaluate a possible influence of the regression heat treatment on the material behavior under different stress states.

Negative to positive strain rate sensitivity in 5xxx series aluminum alloys: Experiment and constitutive modeling

Uniaxial tension and compression tests, at both quasi-static and dynamic ranges of loadings, are utilized to study the thermo-mechanical response of an AA5182-O sheet. The strain rate at quasi-static regime ranges from 10−4 to 100 (s−1) and at dynamic loading is chosen to be around 3500 (s−1). Measurements are reported at temperatures of 296K, 338K, 373K, 423K, and 473K. While at temperatures lower than the critical temperature of Tc, the flow stress decreases with increase of strain rate during quasi-static loading; above this temperature, the measured response shows positive strain rate sensitivity at the same strain rate range. In contrast, significant positive strain rate sensitivity prevails at dynamic loading over the whole range of temperature. A constitutive model is proposed to predict the stress–strain response of the alloy over the studied strain rate and temperature ranges. The developed model is also calibrated for another AA5xxx alloy series, AA5754-O, to evaluate the capability of the developed model in capturing transition from negative to positive strain rate sensitivity.

Development of continuous cooling precipitation diagrams for aluminium alloys AA7150 and AA7020

Two commercial 7xxx series aluminium alloys with different solute contents and different quench-induced precipitation behaviour have been investigated by using a specialised differential scanning calorimetry (DSC) technique to record exothermal heat outputs during continuous cooling. Together with hardness testing and microstructural analysis, this DSC method was used to develop continuous cooling precipitation (CCP) diagrams for alloys AA7150 and AA7020. The results show that the total precipitation heat for each alloy decreases with increasing cooling rate. However, the excess specific heat at a given cooling rate in alloy AA7150 is much higher than that in alloy AA7020. It is evident that there are atleast three different quench-induced reactions in different temperature regimes for alloy AA7150 cooled at various linear cooling rates, but only equilibrium MgZn2 (η-phase) and Al2CuMg (S-phase) particles were observed by scanning electron microscopy (SEM). There are at least two main precipitation peaks that can be found for alloy AA7020, which correspond to Mg2Si and MgZn2 (η-phase). Furthermore, a method is developed to evaluate the quench sensitivity of an alloy based on a determination of the critical cooling rate. The maximum hardness values are reached at cooling rates that are faster than or similar to the critical cooling rate.

General aspects related to the corrosion of 6xxx series aluminium alloys: Exploring the influence of Mg/Si ratio and Cu

Abstract Specially prepared 6xxx series aluminium alloys (Al–Mg–Si) with different Mg/Si ratios and Cu contents were investigated in the naturally aged, under-aged, peak-aged and over-aged conditions. Potentiodynamic polarisation and immersion testing in 0.1 M NaCl were used to investigate the corrosion behaviour of such alloys in a general sense. Results reveal a significant dependence of the corrosion rate and morphology on the ageing conditions and solute content. Whilst contributing to broader understanding of 6xxx alloy corrosion, we note that corrosion rate is lowest in the naturally aged condition and is principally influenced by Cu and subsequently by the Mg/Si ratio.

Integrated Modeling of Strength Evolution in Al-Mg-Si Alloys during Hot Deformation

In the present work we develop a physically based model of strength evolution during hot deformation of Al-Mg-Si alloys. The goal is to predict a change of material strength taking into account the impact of microchemistry, i.e. the influence of solutes and precipitates on strengthening and softening mechanisms. The material strengthening is considered in the present work in terms of solid solution strengthening (the Labusch-Naborro model), work hardening (the advanced one-parameter Kocks model), as well as precipitation strengthening due to the stress contribution of non-deformable particles, i.e. dispersoids (the Orowan by-pass). The material softening is described by dynamic recovery through thermal activation of dislocation climb. For the precipitation kinetics the computational thermodynamics code MatCalc (Materials Calculator) was used. The model was validated by comparison with experimental data of compression tests of the 6xxx series aluminium alloys and a reasonable agreement of the simulated and measured flow stress curves was found.

Formation of Fine Spheroidal Microstructure of Semi-Solid Al–Zn–Mg–Cu Alloy by Hyperthermally and Subsequent Isothermally Reheating

The high strength 7xxx series aluminium alloys are usually difficult to be prepared as semi-solid feedstock, because some dispersoid particles (soluble only in liquid) could pin at grain and subgrain boundary to make the alloy difficultly recrystallise. In this work, a novel multistep reheating regime is developed for recrystallisation and partial melting (RAP) route to prepare fine spheroidal microstructure of semi-solid Al–Zn–Mg–Cu alloy. After reheating to 665 °C, holding for 4 min and subsequent isothermally holding at 620 °C with total heating time of 15 min, fine spheroidal microstructures with grain size of ∼40 μm were prepared without remained unrecrystallised grains and directionality, which are much finer than the conventionally obtained microstructures (∼100 μm).

Multi-stage heat treatment of aluminum alloy AA7049

High strength 7xxx series aluminum alloys in the T6 temper are known to be highly susceptible to stress corrosion cracking (SCC). Retrogression and re-aging (RRA) is a heat treatment by which the resistance to SCC is enhanced without losing yield or tensile strength. The influence of multi-stage heat treatment on the properties and microstructure of AA7049 aluminum alloy was investigated. The microstructure of the alloy in various temper conditions was correlated to the electrical conductivity measurements, DSC analysis and TEM observation. The DSC analysis shows that RRA treatments lead to significant change in the microstructure and the resultant RRA precipitate microstructure is significantly different from both the T6 and T73 microstructures and is able to retain the strength of T6 temper while attaining the thermodynamic stability of the T73 temper.

Direct visualization of β phase causing intergranular forms of corrosion in Al–Mg alloys

Abstract For a more effective examination of microstructure in Al–Mg alloys, a new etching solution has been developed; dissolved ammonium persulfate in water. It is demonstrated how β phase (Al3Mg2) in Al–Mg alloys respond to this solution using samples of a binary Al–Mg alloy and a commercial 5083 aluminum alloy. Nanometer sized β phase is clearly visualized for the first time using scanning electron microscopy (SEM) instead of transmission electron microscopy (TEM). It is anticipated that direct and unambiguous visualization of β phase will greatly augment intergranular corrosion research in 5xxx series aluminum alloys.

Quench Sensitivity and Quench-Induced Precipitation in High Strength 7xxx Series Aluminum Alloys

This paper investigates the quench sensitivity of some selected 7xxx series Al alloys based on a Jominy End Quench method. The precipitate microstructures as a function of cooling rate during quenching are also characterized by using transmission electron microscopy (TEM). The results indicated that quench sensitivity and therefore the mechanical properties inhomogeneity in large plates or forgings can be predicted more accurately by the simultaneous combination of hardness and electrical conductivity measurements based on Jominy end quench. The hardness drop and conductivity increase in the novel alloy following a low cooling rate are much reduced compared to AA7050 and 7B04 because of a lower sensitivity to quench-induced precipitation on dispersoids. The novel alloy exhibited the least quench sensitivity, and the 7B04 Al alloy was the most quench sensitive. If the 90% of the maximum hardness is defined as the depth of quenching, the depth of 7B04 Al alloy, AA7050 through Jominy end quenching is about 20 and 55 mm respectively. Meanwhile, the depth of greater than 150 mm is achievable in the novel alloy, and hence it can be recommended to fabricate large section plates or forgings without compromising properties in the center of the part after a slow cool.

Robotic welding of thin wall components made of 6xxx and 2xxx series aluminium alloys using low-energy methods

The application of low-energy methods of metal inert gas welding (ColdArc and CMT) for the manufacture of joints in hard-weldable aluminium alloys is presented. The purpose of the technological research was to show the usefulness of the CMT and ColdArc methods for production of butt and T-joints in 2.0 mm thick aluminium alloy plates. Basic difficulties related to welding of 6xxx and 2xxx series aluminium alloys and the specificity of welding by low-energy methods are discussed. Selected results of metallographic macro- and microscopic examination as well as those of tensile and bend tests are given; detailed analysis of the results is also carried out. The article shows also that ColdArc and CMT methods provide high quality and aesthetic welded joints in aluminium alloys that are considered to be non-weldable or of limited weldability.

CMT and MIG-Pulse Robotized Welding of Thin-Walled Elements Made of 6xxx and 2xxx Series Aluminium Alloys

The article presents the course and the results of research on material and technological welding conditions of different aluminium alloys using standard (MIG-Pulse) and low energy welding method (CMT) as well as discusses the properties of welded joints and the application fields of modern low energy welding devices for joining thin aluminium sheets.

Recrystallization in Severely Deformed Aluminum

Qualitative and quantitative texture changes are investigated in a severely deformed 5xxx series aluminum alloy. Unusual recrystallization textures are observed after a rolling strain of more than 97% in thickness reduction. The influence of both strain amplitude and strain mode on the development of the deformation and recrystallization textures is discussed based on experimental data and results of crystal plasticity calculations.

Effect of Cerium-Rich Mischmetal Content on the Mechanical Properties and Fracture Morphology of New 5XXX Series Aluminum Alloys

Preparation process of new 5XXX series aluminum alloys containing cerium-rich mischmetal wasintroduced .The effects of cerium-rich mischmetal on fracture morphology and mechanical properties of aluminium alloy were investigated in detail by scanning electronic microscope (SEM), and tensile test.The results show that alloys tensile strength and elongation with the increase of the content of mischmetal first increased, then down. When the mischmetal content is increased up to 0.30%, the tensile strength and elongation are 115 MPa and 25.9% respectively, meanwhile, the fractograph exhibited typical ductile dimple fracture pattern. Then the alloy performance is best. Mischmetal added into the alloy can improve the mechanical properties of materials, but too much mischmetal will induce the decrease in the material performance.Becase it may generate more the coarse Al11(Ce ,La)3 particle.

Effect of Process Variables on the Formation of Streak Defects on Anodized Aluminum Extrusions: An Overview

AbstractStreak defects are often present on anodized extrusions of 6xxx series aluminum alloys, increasing the fabrication cost of these products. Moreover, streaking often only becomes visible after etching and anodizing treatments, rather than in the as-extruded condition, making it difficult to identify the original causes and influencing factors of these defects. In this paper, various process variables that influence the formation of streak defects on anodized aluminium extrusions are reviewed on the basis of a literature review, industrial practice and experimental results. The influencing factors involved in various processing steps such as billet quality, extrusion process, die design and etching process are considered. Effective measures for preventing the formation of streak defects in industrial extrusion products are discussed.

Effect of process variables on Mg-Si particles and extrudability of 6xxx series aluminum extrusions

The Mg-Si particles are one of the major constituents in the microstructure of 6xxx series aluminum alloys. The size, distribution, and morphology of the Mg-Si particle have significant impact on extrudability in terms of mechanical properties and surface quality of aluminum extrusions. The characteristics of the Mg-Si particles are influenced by various process parameters in the production of extrusions. This paper reviews the effects of the major process variables on the Mg-Si particle characteristics and extrudability of aluminum extrusions.

Thermal expansion/contraction behavior of AA7050 alloy in the as-cast condition relevant to thermomechanical simulation of residual thermal stresses

Abstract As-cast 7xxx series aluminum alloys experience a severe loss in ductility upon cooling and become brittle below 300 °C. In order to simulate the thermal stresses arising during casting, a thorough understanding of the thermal contraction behavior during solidification and cooling is required. In this study, some thermal properties of the AA7050 alloy are determined experimentally in the “as-cast” condition. Solidification contraction tests were performed to determine the onset of thermal contraction in the grain refined alloy and dilatometry tests were conducted to estimate the coefficient of thermal expansion/contraction of the alloy. Our simulation results showed that although the temperature dependence of the coefficients of thermal expansion/contraction obtained from the tests differ from the predictions of thermodynamic data-base, these differences are not crucial to the results of the thermomechanical simulations.

Study of Effect of Ce-Rich Mixed Rare Earth on Corrosivity of New 5XXX Series Aluminium Alloys

In this paper, AL-Mg-Mn-RE series alloys are becamed object of study.They are prouced with pure alumimum, pure Mg,electrolytic Mn and mix rich Ce rare earth. The effect of rare earth content on the resistance of the Al-Mg-Mn-RE series alloys are investigated by using Anodic polarization curves, electrochemical impendance spectroscopy. The results showed that their corrosivity is influenced by RE content in aluminum alloys.NO.3.3 impedance of Al-Mg-Mn-RE alloys is larger in the Nyquist diagram and its corrosion current of alloy decreases obviously. Then corrosivity of alloy is the best and the RE content is 0.3%,When the RE content is higher or lower than it that corrosivity isn’t improved but descended.