Age-hardenable Aluminum Alloys Research Articles

Enhancing performance through friction welding of dissimilar aluminium 2014 and 7075 hybrid metal-matrix composite: insights into material characterization, crystallographic texture, and mechanical properties

Abstract Welding high-strength, age-hardenable aluminium alloys from the 2XXX and 7XXX series poses challenges when using fusion-based welding processes. Common issues such as hydrogen porosity, hot cracking, stress corrosion cracking, and solidification cracking can lead to a reduction in mechanical properties due to the loss of strengthening precipitates and alloying elements. Consequently, friction stir welding (FSW) has emerged as a promising solid-state welding technique for joining dissimilar aluminium alloys and composites, particularly in aerospace applications where a combination of strength, corrosion resistance, and other desirable properties is required. This study investigates the FSW of dissimilar aluminium alloys, specifically AA2014 and a hybrid metal-matrix composite of AA7075 reinforced with silicon carbide and graphite particles. The microstructural and mechanical properties of the welded joints were characterized, revealing an average grain size of 2.39 μm ± 1.2 in the stir zone (SZ). A high angle grain boundary volume fraction of 47.25% was observed in the SZ, compared to 2.01% in the AA7075 composite and 15.03% in the AA2014 base metal. The presence of shear textures and the distribution of cube, S, and H textures indicate a homogeneous mixing of the base materials and a high shear strain rate during welding. The hardness of the SZ increased by 13%, and the ultimate tensile strength, yield strength, and elongation of the joint were measured at 251 MPa, 183 MPa, and 6.44%, respectively. The flexural strength of the joints improved significantly, with the CW13 sample showing a 67% increase compared to CW1. The dissimilar joint CW13, welded at 12.5 mm min−1, achieved a maximum joint efficiency of 95%.

Influence of multiple friction stir welding passes on the microstructure and mechanical properties of AA2024 age-hardened aluminum alloy

Friction stir welding (FSW) is a promising joining process for age-hardened aluminum alloys. However, when the process is used for welding age-hardened aluminum alloys, the hardness and joint efficiency are poor mainly because the strengthening precipitates become incoherent and/or get dissolved during the welding process. In this novel work, efforts have been made to enable the reprecipitation of the strengthening precipitates using multi-pass FSW. After producing FSW welds on AA2024 alloy, the welds have been processed by the same tool and at the same parameters several times to facilitate the reprecipitation of strengthening precipitates. As the precipitation kinetics of age-hardened aluminum alloys are temperature-dependent, a thermal imaging camera was utilized to record the temperature history during welding and subsequent cooling of the welded plates. The metallurgical examination of welded samples was carried out using optical microscopy, Scanning Electron Microscope (SEM), energy dispersive X-ray spectroscopy (EDS), X-ray diffraction (XRD), and microhardness test. It was observed that grain refinement occurs continuously due to dynamic recrystallization and severe plastic deformation up to the third FSW pass, after which there is no significant change in grain size. Generally, the microhardness value was higher in the stir zone than the heat-affected zone (HAZ), and the hardness in the stir zone increased after 4 passes of FSW. SEM images show clusters of incoherent precipitates, fragmentation, precipitate dispersion, dissolution, and reprecipitation of precipitates during each subsequent pass. The identification of the precipitates was carried out using EDS and XRD analysis.

Nanoscale heterogeneities dictate corrosion pathways in a high-strength aluminum alloy

Micro-segregation and resulting nanoscale microstructural heterogeneities are unavoidable in wrought 7xxx aluminum alloys produced using current casting and thermomechanical processes. This study demonstrates that these nanoscale heterogeneities significantly impact pit growth in an extruded, age-hardened aluminum alloy 7075-T651. The alloy exhibits a complex microstructure with heterogeneously distributed E-Al18Mg3Cr2 dispersoids, which promote the precipitation of coarse ƞ-Mg(ZnAlCu)2 nanoparticles resulting in the formation of solute-depleted regions both within the grain interior and along the grain boundaries in dispersoid-rich areas. These alterations affect resistance to pit growth at the nanoscale, governing the transition of micro-galvanic sites into large pits with potential for crack initiation. This work underscores the necessity for modifying alloy composition and casting processes to develop superior aluminum alloys for critical applications.

Influence of contouring on the microstructure and micromechanical properties of Laser Powder Bed Fusion (LPBF) processed Al6061 enhanced with reactive particles

Age-hardenable aluminum alloys are of interest for powder bed fusion (PBF) additive manufacturing to produce lightweight, high-performance custom components. The successful processing of these alloys depends on specific alloying and print techniques to overcome defects that significantly degrade mechanical properties of PBF aluminum. The Reactive Additive Manufacturing (RAM) process modifieds alloy feedstock with reactive precursors which inoculate equiaxed microstructures during solidification, resulting in a reduction of columnar grain growth and shrinkage cracks. Additionally, surface quality is addressed by contouring, which retraces the perimeters of printed layers to produce smooth, fully fused surfaces. However, the combination of RAM and contouring methods has not been previously described in the literature, raising questions about the extent of refinement which is achieved when RAM particles are remelted by contouring laser sweeps. In this research, X-ray microcomputed tomography (XCT) was used to provide a complete characterization of RAM particles and defects in an AA6061-RAM alloy in 3D. The non-destructive XCT technique was used to quantify pore and particle distributions in the contoured alloy volumes and compared directly to hatched regions. The resulting three-dimensional analysis was correlated to traditional 2D microstructure quantification and nanoindentation experiments, providing a multimodal quantitative description of sub-micron grains, reaction products, and micromechanical properties. The study reveals that RAM particles inhibit cracking while simultaneously reducing the variance in mechanical properties between hatched and contoured regions of the build volume.

Investigation of Two Laser Heat Treatment Strategies for Local Softening of a Sheet in Age-Hardening Aluminum Alloy by Means of Physical Simulation

AbstractCar manufacturers increasingly aid high-strength aluminum alloys for their advantageous weight-to-strength ratio, but their limited formability poses challenges in plastic deformation processes. Tailored heat-treated blanks (THTBs) are a propitious approach to improve formability. Surface laser treatment is the predominant technology for obtaining THTB. To design this process quickly and accurately, without material waste, the use of physical simulation is increasingly promising. It allows replicating the process on a lab-scale and studying posttreatment mechanical and metallurgical properties. By adopting Gleeble® physical simulator, this study investigates the softening effects of local surface laser heat treatment on a EN AW 6082 T6 aluminum alloy blank. Two laser movement strategies—single linear path and multiple rectangular paths—were investigated at two treatment speeds for each. A finite element (FE) model was developed for simulating the process under all explored conditions. FE-derived thermal cycles were reproduced by means of physical simulation. After physical tests, alloy mechanical properties were evaluated. Results show that these properties depend on both the peak temperature thermal cycle and the interaction time between the laser source and the material surface. The comparison between the two strategies revealed that the multiple rectangular paths strategy allows to achieve a wider softened area at comparable interaction times.

Semantic integration of diverse data in materials science: Assessing Orowan strengthening

This study applies Semantic Web technologies to advance Materials Science and Engineering (MSE) through the integration of diverse datasets. Focusing on a 2000 series age-hardenable aluminum alloy, we correlate mechanical and microstructural properties derived from tensile tests and dark-field transmission electron microscopy across varied aging times. An expandable knowledge graph, constructed using the Tensile Test and Precipitate Geometry Ontologies aligned with the PMD Core Ontology, facilitates this integration. This approach adheres to FAIR principles and enables sophisticated analysis via SPARQL queries, revealing correlations consistent with the Orowan mechanism. The study highlights the potential of semantic data integration in MSE, offering a new approach for data-centric research and enhanced analytical capabilities.

Differential scanning calorimetry of age-hardenable aluminium alloys: effects of sample preparation, experimental conditions, and baseline correction

Precipitation processes in age hardenable aluminium alloys are often investigated by differential scanning calorimetry (DSC). The endothermic and exothermic peaks of the DSC signal correspond to the dissolution and formation of phases, respectively. However, parasitic effects can lead to an unintended curvature of the DSC signal. Although a baseline correction can be used, some imperfections typically remain. Additionally, sample preparation and experimental conditions can influence the precipitation sequence itself and, therefore, the DSC curve. In this study, we investigate the influence of sample preparation by milling and punching on DSC curves for three different aluminium alloys: EN AW-2024, EN AW-6082, and EN AW-7075. Additionally, the influence of quenching and natural ageing was investigated for EN AW-6082. We found that deformation introduced by punching the DSC samples with a piercing die after heat treatment leads to a change in the precipitation kinetics in 2xxx, 6xxx, and 7xxx series alloys. The influence was strongest for punching the samples after solution heat treatment and less significant for punching after artificial ageing. The influence of sample preparation can be avoided by punching the samples before solution heat treatment. If this is not practicable, milling of the samples is a good alternative. The choice of quenching medium and short storage at room temperature before measurement (5 min) had only small effects on the DSC curves. Moreover, the start temperature of the measurement was found to be crucial. For observing phases forming below 100 °C and for low-bias baseline correction, the measurement should start at low-temperatures (i.e. ∼\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\sim$$\\end{document} − 50 °C).

The dynamic recrystallization microstructure characteristics and the effects on static recrystallization and mechanical properties of Al–Mg–Si alloy

It is essential to clarify the dynamic recrystallization microstructure characteristics of age-hardened aluminum alloys and the effects on the static recrystallization microstructure and aging mechanical properties, with the aim of selecting the deformation conditions reasonably. This work investigated the evolution of the hot deformation and solid solution microstructures and aging mechanical properties of Al–Mg–Si alloy with deformation temperatures of 400–550 °C, strain rates of 0.01–10 s−1, and true strains of 0.2–0.8. Research has shown that it is advantageous for the DDRX and CDRX mechanisms to annihilate and rearrange dislocations simultaneously for forming deformation microstructure with uniform grain size at higher deformation temperatures and lower strain rates (such as 500 °C, 0.01 s−1). Additionally, the lower dislocation density in deformation microstructure facilitated the inhibition of static recrystallization during subsequent solid solution treatment, whereas lower deformation temperature and higher strain rate conditions should result in a greater degree of static recrystallization and reduce the uniformity of the solid solution microstructure. Hardness test results demonstrated that the deformation temperature of 480–520 °C and strain rate of 0.03–0.2 s−1 were the appropriate conditions for Al–Mg–Si alloy to attain finer and uniform grains and superior properties after solid solution treatment. The extruded samples within the above conditions (500 °C, 0.032 s−1) exhibited higher UTS and YS after solid solution and aging treatment, reaching 444 ± 2 MPa and 426 ± 4 MPa, respectively.

Decoding stress-strain parameters in FCC metals using digital constitutive analyses to devolve dynamic obstacle-strength factor and diffuse necking

Simulation codes use constitutive relations of work hardening to virtually predict shape change during metal forming. Recent analysis has shown that the modified Hollomon-type relation correlates to stress-aided thermal activation at obstacles for dislocation movement. The sweeping action of dislocation gives rise to strain and the dislocation intersections to strain rate sensitivity during work-hardening. The constitutive relation analyses (CRA) encompass fitting parameters which remain constant with strain and its validation is the precision to replicate the measured stress-strain diagram. In this study, digital constitutive analyses (DCA) are examined whereby the modelled-fit parameters are simultaneously numerically adjusted as strain proceeds. For bulk properties such as expended work, volume fraction of vacancy creation and mean slip velocity, CRA predictions have been validated. However, DCA enables the identification of defects being created with strain using derived obstacle-strength factor (αγ). The changes in mechanisms during work-hardening can be decoded using the αγ – γ plot whereby constant αγ indicate steady-state deformation and its rapid decrease, the start of diffuse necking. Thus, αγ is a composite factor of defects being continuously created whereas conventional α is a measure of the stored work up to that strain. The tensile data from polycrystalline super-pure aluminum tested at 78 K were used to validate the derived relations which were applied to the DCA of age-hardenable aluminum alloys tested at 298 K. The present work shows that integral replication of stress-strain diagram is essential, but a differential analysis is required to devolve the creation of crystal defects with strain.

Effect of Cu and Sn additions on the thermal stability of Al–Mg–Si alloy

Age-hardening aluminum alloys generally exhibit poor thermal stability, making them unsuitable for prolonged use at temperatures around the aging range. This study investigated the effect of 0.08 wt% Cu and 0.05 wt% Sn on the thermal stability of peak-aging Al–0.9Mg–1.1Si (wt.%) alloy. After an exposure of 150 °C for 1000 h, the combined effects of Cu and Sn result in a higher yield strength (∼322 MPa) than that of the Cu/Sn-free alloy (∼311 MPa). This improvement is mainly attributed to the Sn-induced fine precipitate morphology. Specifically, precipitates with higher aspect ratios exhibit rapid coarsening rates, significantly promoting the response of experimental alloys to thermal exposure. In contrast, precipitates with lower aspect ratios coarsen at a slower rate and maintain a higher level of strengthening during thermal exposure. Furthermore, Sn atoms may also contribute to the formation of the thermostable Cu-containing phases (L phase). On the one hand, the Sn-induced higher precipitate density will increase the amount of L phase. On the other hand, the special Sn-containing sub-units may serve as heterogeneous nucleation sites to promote the formation of L phase. In summary, the combined effects of Cu and Sn enhance the thermal stability of the experimental alloys near the aging temperature range. The present study provides a straightforward and feasible approach to enhancing the thermal stability of Al–Mg–Si alloys.

Assessment on structural integrity of aluminium alloy AA6061 water tank used in satellite launch vehicles

AA 6061 is an age-hardenable aluminium alloy with 1wt% magnesium and 0.6wt% silicon as major alloying constituents and is used for manufacture of toroidal water tanks of earth storable liquid rocket engines of ISRO’s launch vehicles. These water tanks are used for storing water to generate steam in gas generator as well as for cooling various components of liquid rocket engine. As a part of the developmental efforts of a new source for fabrication of water tanks, a qualification test procedure was laid down, wherein a quarter portion of the water tank, namely the test article, is designed, fabricated and subjected to series of acceptance and qualification tests, including proof pressure test, cyclic pressure tests with intermediate non-destructive tests. These were finally followed by burst testing of the test article for conforming structural design margins and quality standards adopted for realizing the hardware. A burst test is a destructive test, performed to determine the overload capacity of the pressure vessels depicting margin of safety with respect to its mechanical strength limit. This work highlights the detailed manufacturing processes viz. forming, machining and welding including heat treatment of shells, metallurgical analysis carried out on the parent and weld specimens as well as results from proof pressure and pressure cyclic tests. Further, fractographic studies were carried out on the burst tested hardware, wherein specimens collected from different locations of the burst tested hardware were analyzed using optical scanning electron microscopy, to locate the origin of crack initiation and nature of the failure.

Controllable precipitation behavior near grain boundaries enabled by artificial strain concentration in age-hardening aluminum alloys

Regulating the precipitation behavior in the vicinity of grain boundary (GB) is crucial for improving the mechanical property and corrosion resistance of age-hardening aluminum alloy. In this work, we innovatively propose a strain-concentration controlled strategy to realize the controllable fabrication of the thinner precipitate-free zone (PFZ), associated with the finer grain boundary precipitates in Al–Zn–Mg–Cu alloy. Specifically, the primary PFZ with certain solute atoms is introduced by short pre-aging treatment, followed by the subsequent cyclic deformation at room temperature to induce the localized plastic deformation in the primary PFZ. Compared with the traditional aging, the Al–Zn–Mg–Cu alloy treated by the current process manifests superior combination of impact toughness and corrosion properties, as well as the slightly improved unidirectional tensile properties. Furthermore, the atomic-scale mechanism of the unique precipitation behavior near GB is revealed using the molecular dynamics simulation. A large number of dislocations are generated in PFZ, which acts as the favorable precipitation sites and contribute to the secondary precipitates in the primary PFZ. The present work provides a novel design route and an atomic-scale understanding for controlling the precipitation behavior in the vicinity of GB, and it has the potential to direct the optimization of mechanical performance and corrosion resistance in a board spectrum of aging-hardening alloys.

Modelling the spatial evolution of excess vacancies and its influence on age hardening behaviors in multicomponent aluminium alloys

Excess vacancies play crucial roles in the precipitation of age-hardening precipitates in aluminium alloys, but their spatial evolution across grains during heat treatments is less known. In this work, a numerical model is developed to predict the spatial evolution of non-equilibrium excess vacancies during cooling from solution treatment and during ageing heat treatments of multicomponent aluminium alloys. A finite volume scheme is applied to derive the spatial distribution of vacancy site fraction across grains by solving the diffusion equations of vacancies under the influence of solute elements. Binding energies between solute atoms and vacancies predicted by first-principles calculations has been used to handle the trapping of excess vacancies by solute atoms and atom clusters. The annihilation rates of excess vacancies at grain boundaries and at dislocation jogs have been derived based on a rigorous description of the annihilation mechanisms of vacancies. The evolution of the density of dislocation jogs due to vacancy annihilation has been taken into account. The model is successfully applied to interpret the age hardening behaviors of experimental alloys subjected to different thermomechanical processing conditions. This model will help to reach a deeper understanding of the roles of excess vacancies in precipitation kinetics and therefore is important to further optimize thermomechanical processing parameters and alloy composition to improve the macroscopic mechanical properties of age hardening aluminium alloys.

Mechanical behaviors of metallic alloys dominated by thermo-kinetic synergistic effects upon materials processing

For most metallic alloys, improving strength is usually accompanied by losing ductility (and via versa) even if the work hardening rate keeps unchanged, however, the behind reasons still remain elusive to date. In the present work, we explore the underlying physics for such mechanical performance paradox from the perspective of synergistic effects between thermodynamics and kinetics of phase transition and deformation involved in materials processing. With age-hardenable aluminum alloys as typical examples, the hardening and softening effects are analyzed in terms of system energy accumulation and dissipation during precipitation nucleation and growth along with dislocation multiplication and annihilation. The key thermo-kinetic parameters of driving force and energy barrier are determined by performing relevant atomistic calculations. Analytical models for bridging mechanical performances and thermo-kinetics in terms of microstructural characteristics, which derive from classical phase transition and deformation theories, endow connotation for directing precipitation design, as confirmed by a 6xxx series alloy at the pre-determined aging temperature with ideal strength and ductility. Our work offers an insightful perspective toward understanding the strength and ductility paradox, which can benefit to further establish quantitative connections among processing routes, microstructures and mechanical performances of metallic alloys.

Mechanical and Microstructure Characterisation of the Hypoeutectic Cast Aluminium Alloy AlSi10Mg Manufactured by the Twin-Roll Casting Process

Multi-material designs (MMD) are more frequently used in the automotive industry. Hereby, the combination of different materials, metal sheets, or cast components, is mechanically joined, often by forming joining processes. The cast components mostly used are high-strength, age-hardenable aluminium alloys of the Al–Si system. Here, the low ductility of the AlSi alloys constitutes a challenge because their brittle nature causes cracks during the joining process. However, by using suitable solidification conditions, it is possible to achieve a microstructure with improved mechanical and joining properties. For this study, we used the twin-roll casting process (TRC) with water-cooled rollers to manufacture the hypoeutectic AlSi10Mg for the first time. Hereby, high solidification rates are realisable, which introduces a microstructure that is about four times finer than in the sand casting process. In particular, it is shown that a fine microstructure close to the modification with Na or Sr is achieved by the high solidification rate in the TRC process without using these elements. Based on this, the mechanical properties increase, and especially the ductility is enhanced. Subsequent joining investigations validate the positive influence of a high solidification rate since cracks in joints can be avoided. Finally, a microstructure-property-joint suitability correlation is presented.

Nucleation and Phase Development of Precipitates in Age-hardenable Aluminium Alloys Studied by 4D-STEM.

Nucleation and Phase Development of Precipitates in Age-hardenable Aluminium Alloys Studied by 4D-STEM.

Analysis of a phase‐field finite element implementation for precipitation

AbstractPrecipitation hardening is an essential mechanism in materials design of age‐hardenable aluminium alloys. The occurrence and distribution of nano‐sized particles in such alloys can lead to superior material properties. During thermo‐mechanical processing, these particles evolve dynamically as function of temperature and applied load. Therefore, sophisticated modelling frameworks are required to study the underlying phenomena of this microstructural evolution in depth. Phase‐field method based on the diffuse interface approach has been successfully employed in literature to study particle nucleation and growth, as well as equilibrium particle shapes. Although phase‐field models provide reliable results due to the flexible adaption of the free energy, the method is computationally expensive, requiring efficient solution schemes. The finite‐element discretization in deal.II can overcome scalability disadvantages and can outperform standard finite‐difference codes. In this work, we used adaptive mesh refinement and adaptive time‐stepping and investigate how AMR and the use of the same stiffness matrix for a certain amount of time steps affect the performance of the phase‐field model. Particle growth simulations are performed to outline the major benefits of the finite element phase‐field model. The numerical strategy is shown to be effective regardless of the initial particle shape by considering different particle morphologies. The results illustrate a significant increase in simulation performance with the applied numerical techniques.

Influence of Local Microstructural Variations on the Bendability of Aluminum Extrusions: Experiments and Crystal Plasticity Analyses

Abstract The bendability of extruded profiles of an age hardenable aluminum alloy is investigated using mechanical tests on flat tensile specimens and bending specimens. Two profile geometries are considered, where the profiles exhibit different grain structures. The microstructure of the profiles in terms of the crystallographic texture and constituent particles is otherwise comparable. While the tensile properties are not that different for the two profiles, their bendability is strongly dependent on the grain structure and is about twice as high for one profile than for the other. A newly proposed coupled damage and single crystal plasticity model is used in finite element analyses of the mechanical tests to investigate the influence of the grain structure on the bending behavior, and the numerical results are compared to the experimental tests. The crystallographic texture and the grain morphology of the profiles, found by the electron back-scatter diffraction technique, are explicitly represented in the finite element models. The crystal plasticity simulations capture the difference in the bendability of the two profiles, and in agreement with the experiments predict a considerably higher bendability for one of the profiles. It is found that the grain structure affects the shear band formation in these profiles, but also the local texture where the shear bands are located is important for crack initiation and propagation as grains with certain crystallographic orientations may have a higher fracture resistance.

Superfunctional Materials by Ultra-Severe Plastic Deformation.

Superfunctional materials are defined as materials with specific properties being superior to the functions of engineering materials. Numerous studies introduced severe plastic deformation (SPD) as an effective process to improve the functional and mechanical properties of various metallic and non-metallic materials. Moreover, the concept of ultra-SPD-introducing shear strains over 1000 to reduce the thickness of sheared phases to levels comparable to atomic distances-was recently utilized to synthesize novel superfunctional materials. In this article, the application of ultra-SPD for controlling atomic diffusion and phase transformation and synthesizing new materials with superfunctional properties is discussed. The main properties achieved by ultra-SPD include: (i) high-temperature thermal stability in new immiscible age-hardenable aluminum alloys; (ii) room-temperature superplasticity for the first time in magnesium and aluminum alloys; (iii) high strength and high plasticity in nanograined intermetallics; (iv) low elastic modulus and high hardness in biocompatible binary and high-entropy alloys; (v) superconductivity and high strength in the Nb-Ti alloys; (vi) room-temperature hydrogen storage for the first time in magnesium alloys; and (vii) superior photocatalytic hydrogen production, oxygen production, and carbon dioxide conversion on high-entropy oxides and oxynitrides as a new family of photocatalysts.

In situ laser-ultrasonic monitoring of elastic parameters during natural aging in an Al-Zn-Mg-Cu alloy (AA7075) sheet

Solution heat treatment increases the formability of 7xxx series aluminum alloys by dissolution of nanoscale hardening phases. However, subsequent natural aging at room temperature partly reverses this process within a few hours, posing a logistical challenge to industrial operations as formability decreases and springback increases. Hence, non-destructive monitoring of mechanical properties may help facilitate forming of parts from 7xxx series alloys, but no suitable non-destructive direct measurement technique has been established so far. Here we demonstrate an in situ laser ultrasonic method based on the generation and detection of plate resonances to continuously monitor small changes (<1%) of Young’s modulus, shear modulus, and Poisson’s ratio during natural aging of an age-hardenable aluminum alloy (AA7075) sheet. Additionally, we followed plastic mechanical properties by tensile testing and we intermittently measured electrical conductivity and performed differential scanning calorimetry to assess precipitation during natural aging. We find qualitative correlation between Guinier-Preston zone formation and an increase in elastic moduli and material strength.