Age-hardenable Aluminum Alloys Research Articles

Nondestructive analysis of pitting corrosion characteristics on EN AW-2024-T3 using 3D optical pattern profilometry

ABSTRACTHigh-strength age-hardenable aluminium alloys are susceptible to localised corrosion. It is vital to monitor the evolution of the corrosion depth in order to prevent a critical degradation of the mechanical properties. This study presents the application of 3D optical pattern profilometry for the analysis of the pitting corrosion behaviour of the aluminium alloy EN AW-2024-T3. Therefore, measurements were conducted using a 1 M NaCl solution at different exposure times. The measurements are compared to the results of optical microscopy investigations of metallographic specimens as well as laser scanning microscopy measurements. In immersion bath tests, the 3D pattern profilometry shows a high measurement accuracy of the pitting corrosion. Regarding the pit characteristics, corroded metal sheets exhibit an increase in the pit depth, volume and area with an increasing exposure time of up to 312 h and then decrease. Moreover, a higher increase in the width in rolling direction than perpendicular to it is noticeable.

On the Effect of Natural Aging Prior to Low Temperature ECAP of a High-Strength Aluminum Alloy

Severe plastic deformation (SPD) can be used to generate ultra-fine grained microstructures and thus to increase the strength of many materials. Unfortunately, high strength aluminum alloys are generally hard to deform, which puts severe limits on the feasibility of conventional SPD methods. In this study, we use low temperature equal-channel angular pressing (ECAP) to deform an AA7075 alloy. We perform ECAP in a custom-built, cooled ECAP-tool with an internal angle of 90° at −60 °C and with an applied backpressure. In previous studies, high-strength age hardening aluminum alloys were deformed in a solid solution heat treated condition to improve the mechanical properties in combination with subsequent (post-ECAP) aging. In the present study, we systematically vary the initial microstructure—i.e., the material condition prior to low temperature ECAP—by (pre-ECAP) natural aging. The key result of the present study is that precipitates introduced prior to ECAP speed up grain refinement during ECAP. Longer aging times lead to accelerated microstructural evolution, to increasing strength, and to a transition in fracture behavior after a single pass of low temperature ECAP. These results demonstrate the potential of these thermo-mechanical treatments to produce improved properties of high-strength aluminum alloys.

Influence of Extrusion Temperature on the Aging Behavior and Mechanical Properties of an AA6060 Aluminum Alloy

Processing of AA6060 aluminum alloys for semi-products usually includes hot extrusion with subsequent artificial aging for several hours. Processing below the recrystallization temperature allows for an increased strength at a significantly reduced annealing time by combining strain hardening and precipitation hardening. In this study, we investigate the potential of cold and warm extrusion as alternative processing routes for high strength aluminum semi-products. Cast billets of the age hardening aluminum alloy AA6060 were solution annealed and then extruded at room temperature, 120 or 170 °C, followed by an aging treatment. Electron microscopy and mechanical testing were performed on the as-extruded as well as the annealed materials to characterize the resulting microstructural features and mechanical properties. All of the extruded profiles exhibit similar, strongly graded microstructures. The strain gradients and the varying extrusion temperatures lead to different stages of dynamic precipitation in the as-extruded materials, which significantly alter the subsequent aging behavior and mechanical properties. The experimental results demonstrate that extrusion below recrystallization temperature allows for high strength at a massively reduced aging time due to dynamic precipitation and/or accelerated precipitation kinetics. The highest strength and ductility were achieved by extrusion at 120 °C and subsequent short-time aging.

The influence of heat treatments on electrical conductivity and corrosion performance of AA 7075-T6 aluminium alloy

Heat treatable aluminium alloys such as AA 7075 gains high strength during aging. These alloys have large applications in aircrafts, missiles and automobiles for many components. AA 7075 aluminium alloys is susceptible to exfoliation corrosion due to presence of high content of Zn. Different heat treatment processes are performed on age hardenable aluminium alloys to improve corrosion resistance. The objective of this study is to investigate the effect of different heat treatments such as retrogression and re-aging (RRA) and duplex aging on the corrosion performance and electrical conductivity of heat treatable AA 7075-T6 alloy. On coupons of AA 7075-T6 the retrogression at 200˚C for 2 hr and re-aging at 120 ˚C for 24 hr is performed. Also another treatment known as duplex aging at 163˚C for different time duration such as 4 hr, 8 hr, 16 hr, and 24 hr was performed. This was followed by exfoliation corrosion and the corrosion performance determined. The electrical conductivity is the most important measure associated with age hardenable aluminium alloys, which directly reflect the internal condition (microstructures) of such alloys. The study reveals that duplex aging at 163˚C for 16 hr shows better result for both electrical conductivity and corrosion resistance.

Development of the cryogenic processing cycle for age hardenable AA7075 aluminium alloy and optimization of the process for surface quality using gray relational analysis

The research on cryogenic processing of cutting tools has led to the implementation of the treatment in industrial applications.The application of cryogenic to age hardenablealuminium alloy in a proper sequence is a complex issue as they are sensitive to temperature and time due to ‘aging’, this work comprises cryogenic treatment of 7075 aluminium alloy, with effect of soaking period on this alloy was assessed in term of surface quality for hardness and surface roughness using design of experiments by Taguchi and optimization by gray relational analysis. The work concludes by identifying the effect of cryogenic processing and optimizing the treatment parameters for getting best combination of hardness and surface roughness after the processing.

Strength differential effect in age hardened aluminum alloys

The strength differential (SD) effect, as reported in literature over the past decades, is a discrepancy between the axial stresses in compression and tension. This study investigates the SD effect in aluminum alloys using both experiments and numerical simulations. We present compressive and tensile tests of four aluminum alloys in several tempers with yield strengths varying from 27 MPa to 373 MPa: a total of thirteen different material configurations. The axial stresses measured in compression tests are significantly higher than corresponding tensile stresses for nearly all material configurations. In our tests, the SD effect generally increases with material strength, indicating that aluminum alloys are pressure sensitive. The physical mechanism responsible for the SD effect was not investigated in this paper, but a plasticity model based on the hypothesis that dislocation motion is affected by hydrostatic pressure, as put forth by several authors, gives an accurate description of the material behavior in compression and tension.

Microstructure and mechanical properties of an AA6060 aluminum alloy after cold and warm extrusion

Severe plastic deformation techniques can produce ultrafine-grained aluminum alloys with high strength and ductility, but further processing often requires costly and/or time-consuming machining. In this study, we investigate the potential of an alternative processing route that operates below recrystallization temperature and that can produce fine-grained materials. Cast billets of the age hardening aluminum alloy AA6060 were solution annealed and then extruded at room temperature or at 170°C (which corresponds to the aging temperature of the alloy, and allows for simultaneous forming and aging). The materials were then subjected to an aging treatment. Electron microscopy and mechanical testing were performed to characterize the resulting microstructural features and mechanical properties. Both extruded profiles exhibit similar, strongly graded microstructures with submicron-sized grains in the highly deformed surface layers, and a mixture of coarse and fine grains in the center regions. Despite different processing routes (i.e., cold vs. warm extrusion), both materials are characterized by similar mechanical properties in terms of maximum hardness, strength and ductility; these results are discussed in the light of the relevant microstructural deformation and precipitation mechanisms.

Effect of concurrent precipitation on the texture evolution during continuous heating of multi directionally forged solution treated Al-Cu-Mg alloy

Concurrent precipitation in an age hardenable aluminum alloy determines recrystallization behavior especially grains structure and recrystallization texture during continuous heating. In this study, to investigate the effect of concurrent precipitation on the recrystallization texture and grains structure, a solution treated Al-Cu-Mg alloy has been multi-directionally forged at room temperature and continuously heated with different heating rates (800, 1600 and 2500K·min−1) up to 450°C using Gleeble 3800 thermo-mechanical simulator. At lower heating rates (800K·min−1), Goss texture component {110} 〈001〉 (Φ1=90°/0°, Φ=90°/45°, Φ2=45°/90°) is developed during continuous heating and increasing the heating rate leads to gradual diminishing of Goss texture component. The results demonstrate that at lower heating rate, concurrent fine S′/S precipitation takes place during heating and increasing the heating rate hinders concurrent precipitation. The intensity of Goss texture depends on the extent of concurrent precipitation. The strong concurrent precipitation leads to strong Goss texture component, while without any precipitation there is no Goss texture and randomly orientated microstructure has been developed. The presence of low and high angle grain boundaries after heating at lower heating rate and concurrent precipitation of fine S′/S particles suggest that extended recovery occurs. The association of constituent large particles like Al7Cu2Fe and randomly orientated microstructure after heating at higher heating rate implies the occurrence of particle stimulated nucleation (PSN).

Microstructure and Texture Evolution during Single- and Multiple-Pass Friction Stir Processing of Heat-Treatable Aluminum Alloy 2024

Microstructure and crystallographic texture evolution during single- and multiple-pass friction stir processing (FSP) of an age-hardenable aluminum alloy 2024 (Al-Cu-Mg) was investigated. Multiple-pass experiments were carried out using two different processing strategies, multi-pass FSP, and multi-track FSP. Effect of a post-FSP heat treatment above and below the solutionizing temperature of the alloy was also studied. FSP experiments were carried out using an optimal set of parameters. Characterization tools used in the study include scanning electron microscopy (SEM), electron back-scattered diffraction (EBSD), electron probe micro-analyser (EPMA), and X-ray diffraction (XRD). Microstructural features indicate the occurrence of particle stimulated nucleation (PSN) assisted dynamic recrystallization (DRX) as the dominant microstructural evolution mechanism in the nugget zone. Geometrical coalescence occurred, leading to the formation of some larger grains in the nugget zone. Heterogenous micro-texture distribution was observed in the nugget zone with the bulk textures consisting of FCC shear texture components dominated by A (1)*/A (2)* and C. Microstructure and texture in the nugget zone remained stable after both routes of multiple-pass processing, demonstrating the possibility of FSP to produce bulk volume of fine-grained materials. Post-FSP heat treatment indicated the stability of microstructure and texture up to 723 K (450 A degrees C) owing to relatively lower strain energies retained after FSP.

A modified Taylor model for predicting yield strength anisotropy in age hardenable aluminium alloys

Coherent precipitates have long been known to alter the anisotropy in mechanical properties of rolled products with respect to rolling direction, which is manifested in terms of yield strength anisotropy (YSA) of age hardenable wrought aluminium alloys. The effect of anisotropy of matrix due to texture and of precipitate due to its morphology has been captured previously using elastic and plastic inclusion approaches with limited success. In the present investigation, yield strength of polycrystalline sheet specimen of Al-Mg-Si alloy in different ageing conditions was calculated by considering solid solution, grain boundary and shearable precipitate strengthening. Effect of coherent directional precipitates on yield strength anisotropy was analyzed using both elastic as well as plastic inclusion models. Additional elastic or plastic work required to deform the precipitates is taken into account along with work done in deforming the matrix to predict YSA. The concept of effective Taylor factor that accounts for anisotropic contribution from precipitates and crystallographic texture of matrix is proposed and incorporated in the existing strengthening models to predict YSA in aged samples. It is observed that modified plastic inclusion model that incorporates anisotropy in flow stress of precipitates is able to capture the experimental YSA in differently aged samples. Universal nature of modified plastic inclusion model is demonstrated by extending the concept to Al-Zn-Mg-Cu alloy.

Rough-Fuzzy-GA-based design of Al alloys having superior cryogenic performance

ABSTRACTMulti-objective genetic algorithm (GA) is employed for the optimal design of novel heat-treatable aluminum alloys with superior performance at cryogenic temperatures. Existing database on age-hardenable aluminum alloys is utilized to create a learning model. Composition and processing parameters of the alloys are considered as the inputs, whereas mechanical properties, viz. YS (Yield Strength), UTS (Ultimate Tensile Strength) and %Elongation tested at subzero temperatures, are used as the outputs, which characterize the performance of the alloy. Data-driven models are developed using the hybrid rough-fuzzy approach. While rough set brings out the most significant variables and formulates if-then rules to explain the relationships between inputs and outputs, fuzzy inference system (FIS) serves as the predictive model. Strength and ductility of the Al alloys at low temperature being conflicting in nature are simultaneously optimized using multi-objective GA to design alloys having an optimal blend of the two properties.

Influence of the heat treatment condition of alloy AlCu4Mg1 on the microstructure and properties of anodic oxide layers

Due to their outstanding specific mechanical properties, high-strength, age-hardenable aluminum alloys offer a high potential for lightweight security-related applications. However, the use of copper-alloyed aluminum is limited because of their susceptibility to selective corrosion and their low wear resistance. These restrictions can be overcome and new applications can be opened up by the generation of protective anodic aluminum oxide layers. In contrast to the anodic oxidation of unalloyed aluminum, oxide layers produced on copper-rich alloys exhibit a significantly more complex pore structure. It is the aim of the investigation to identify the influence of microstructural parameters such as size and distribution of the strengthening precipitations on the coating microstructure. The aluminum alloy EN AW-2024 (AlCu4Mg1) in different heat treatment conditions serves as substrate material. The influence of the strengthening precipitations’ size and distribution on the development of the pore structure is investigated by the use of high-resolution scanning electron microscopy. Integral coating properties are characterized by non-destructive and light-microscopic thickness measurements and instrumented indentation tests.

Design of novel age-hardenable aluminium alloy using evolutionary computation

Abstract This work considers the experimental data of tensile properties as a function of composition and processing of the three series of age-hardenable aluminium alloys, i.e. 2XXX, 6XXX and 7XXX, for designing new age hardenable alloy. Computational approach of designing better alloys with desired properties is employed with a target of breaking the barrier of class or series of age hardenable Al alloys. The alloy designed with the help of two evolutionary computation tools, viz. genetic programming and multi-objective genetic algorithm, having better combination of properties, i.e. strength and ductility, is experimentally developed. The designed Al-Zn-Cu-Mg alloy with complex aging characteristics and encouraging tensile properties seem to have the potential for further study.

Effect of the initial state on nanostructuring and strengthening of middle- and high-strength age-hardenable aluminum alloys under severe plastic deformation (Review)

General requirements to the initial structure/phase state of the commercial middle- and high-strength age-hardenable aluminum alloys, especially to precipitates of aluminides of transition metals and main strengthening phases, under thermomechanical processing, involving severe plastic deformation (SPD), preliminary aging and re-aging, and aiming at nanostructuring the alloy matrix, are reviewed. The data on the alloy structure, hardness and tensile strength, realized in disc-shape samples severely deformed via room temperature high pressure torsion (HPT), have been taken into consideration. Influence of the nature, morphology and densities of secondary phases on efficiency of the alloys nanostructuring and strengthening are analyzed. The role of main strengthening phases in the alloys (sub)grain refinement to nano sizes, as well as in complete suppression of the matrix nanostructuring, are discussed. Nature of the alloys structure transformations and structure-strength relations owing to evolution of the grain and second phase structures due to SPD, preliminary and post-SPD heat treatment are considered. It was concluded that the «direct» impact of the formation of nanosized net of grain boundaries on the alloys strength is not exceeding the impact of its dispersion hardening under conventional aging. The main alloy hardening effect under HPT is due to an increasing the crystal defects densities, predominantly of dislocations.

Tensile Properties Improvement on Friction Stir Welded Age-Hardenable Aluminum Alloys: An Evolutionary Approach using RSM based GA and SA

Tensile Properties Improvement on Friction Stir Welded Age-Hardenable Aluminum Alloys: An Evolutionary Approach using RSM based GA and SA

Parametric optimization of friction stir welding process of age hardenable aluminum alloys−ANFIS modeling

A comparative approach was performed between the response surface method (RSM) and the adaptive neuro-fuzzy inference system (ANFIS) to enhance the tensile properties, including the ultimate tensile strength and the tensile elongation, of friction stir welded age hardenable AA6061 and AA2024 aluminum alloys. The effects of the welding parameters, namely the tool rotational speed, welding speed, axial load and pin profile, on the ultimate tensile strength and the tensile elongation were analyzed using a three-level, four-factor Box-Behnken experimental design. The developed design was utilized to train the ANFIS models. The predictive capabilities of RSM and ANFIS were compared based on the root mean square error, the mean absolute error, and the correlation coefficient based on the obtained data set. The results demonstrate that the developed ANFIS models are more effective than the RSM model.

Temporal evolution of coherent precipitates in an aluminum alloy W319: A correlative anisotropic small angle X-ray scattering, transmission electron microscopy and atom-probe tomography study

Small-angle X-ray scattering (SAXS), transmission electron-microscopy (TEM), and atom-probe tomography (APT) are utilized correlatively to study the evolution of nanometer length-scale coherent precipitates in a commercial age-hardenable aluminum alloy, W319. SAXS, with its high-field-of-view and resultant large statistics, perfectly complements TEM and APT at the nanometer length-scale. The combined results provide a comprehensive description of the evolution of the fine precipitates. W319 exhibits two types of fine coherent precipitates on aging namely, θ′ and Q, whose temporal evolution has been studied using time-resolved in situ SAXS analyses at three different aging temperatures: 438, 463, and 533 K. Complementary information about the morphology, dimensions and number density of the precipitates were determined from ex situ APT and TEM analyses. Coherent precipitates from a single grain of the α-Al-matrix of W319 give rise to an anisotropic SAXS signal, which is subsequently simulated, taking into account the relative orientation of the sample and the crystallographic orientation relationships of the different precipitates. The simulated anisotropic SAXS intensity, based on the results from APT and TEM, is validated utilizing the experimentally obtained 2D SAXS patterns. This permits us to determine the dimensions and number densities of the precipitates from anisotropic SAXS signals, and a complete quantitative analysis of the in situ temporal evolution of the coherent precipitates is performed. Additionally, the present study demonstrates an alternative technique for extracting structural parameters from anisotropic SAXS analyses. The methodology presented herein will be useful for age-hardening analyses of commercial age-hardenable Al-based alloys and Ni-based super alloys.

Modeling of the mutual effect of dynamic precipitation and dislocation density in age hardenable aluminum alloys

A model has been proposed to capture the complex strain rate effect on dynamic precipitation of GP zones in an age-hardenable aluminum alloy. The contributions of vacancies and dislocations to dynamically formed GP zones have been specified in the model. It has been demonstrated that the proposed model is capable for predicting the contribution of each dynamic precipitation mechanisms, accurately, which are acting during deformation. Furthermore, the vacancy and dislocation evolutions during deformation have been considered in this modeling. The effect of strain rate by considering different mechanisms of dynamic precipitation of GP zones has been studied and confirmed by experimental data reported in the work of other researchers. Both experimental results and model outputs demonstrate that at high strain rate regime, strain rate dependency of dynamic precipitation is increased and in comparison with vacancy-assisted nucleation, dislocation-assisted nucleation of GP zones is encouraged. In this study, the mutual effects of dislocation density and dynamic precipitation have been considered. Outputs of model demonstrate that there is a weak negative strain rate sensitivity in the age-hardenable supersaturated aluminum alloy.

The effect of pin profiles on the microstructure and mechanical properties of underwater friction stir welded AA2519-T87 aluminium alloy

AA2519-T87 is a new armour grade aluminium alloy employed in the fabrication of light combat military vehicles. Joining of this material using fusion welding processes, results in the formation of solidification defects like porosity, alloy segregation and hot cracking. In order to overcome the solidification related problems, solid state welding processes such as friction stir welding (FSW) can be used. Though the joining takes place below the melting temperature of the material, the thermal cycle experienced by the thermo-mechanical-affected zone (TMAZ) and heat-affected zone (HAZ) is causing grain coarsening and precipitates dissolution in the age-hardenable aluminium alloys, which deteriorate the joint properties. To get rid of this problem, underwater friction stir welding (UWFSW) process can be employed. The water cooling reduces the heat, and thus, the thermal softening required taking place in TMAZ and HAZ. Therefore, the material flow is entirely different in FSW and UWFSW. In this investigation, a comparative study is made to understand the influence of tool pin profiles, namely straight cylindrical (STC), taper cylindrical (TAC), straight threaded cylindrical (THC) and taper threaded cylindrical (TTC) on stir zone characteristics and the resultant tensile properties of both FSW and UWFSW joints. From this investigation, it is found that the joint made by taper threaded pin profiled tool underwater cooling medium exhibited higher tensile properties of 345 MPa and joint efficiency of 76 %. The enhancement in the strength is attributed to the precipitation hardening, grain boundary strengthening and narrowing of lower hardness distribution region (LHDR).

Experimental and numerical investigation on under-water friction stir welding of armour grade AA2519-T87 aluminium alloy

Abstract Friction stir welding (FSW) is a promising welding process that can join age hardenable aluminium alloys with high joint efficiency. However, the thermal cycles experienced by the material to be joined during FSW resulted in the deterioration of mechanical properties due to the coarsening and dissolution of strengthening precipitates in the thermo-mechanical affected zone (TMAZ) and heat affected zone (HAZ). Under water friction stir welding (UWFSW) is a variant of FSW process which can maintain low heat input as well as constant heat input along the weld line. The heat conduction and dissipation during UWFSW controls the width of TMAZ and HAZ and also improves the joint properties. In this investigation, an attempt has been made to evaluate the mechanical properties and microstructural characteristics of AA2519-T87 aluminium alloy joints made by FSW and UWFSW processes. Finite element analysis has been used to estimate the temperature distribution and width of TMAZ region in both the joints and the results have been compared with experimental results and subsequently correlated with mechanical properties.