High-strength Al Alloys Research Articles

Synergizing mechanical properties and damping capacities in a lightweight Al-Zn-Li-Mg-Cu alloy

Alloys with synergized strength and damping capacity are in urgent need to eliminate vibrations and noises. Herein, a lightweight Al-Zn-Li-Mg-Cu alloy which exhibits similar strengths as traditional high-strength Al alloys and considerable damping capacities is developed. The resulting microstructures and precipitation behaviors are systematically characterized in the hot-rolled and cold-rolled alloy. Meanwhile, damping properties at low temperatures and high temperatures are thoroughly studied. Two rolled alloys show the same multi-segment characteristics, in which the effects of the strain amplitude and high temperature on tanφ are attributed to dislocation pinning and grain boundary viscoelastic sliding, respectively. The damping properties and low density, together with superb mechanical properties, demonstrate their practical application where noise and vibrations must be reduced.

High strength Al alloy development for laser powder bed fusion

High strength Al alloy development is the key technique to additive manufacturing (AM) applied on lightweight of aerospace, automotive and military industry. Unlike the conventional wrought Al–Si eutectic alloys available for AM process, the strength of new developed Al alloy can be improved by in situ or additional nano-precipitated phase. This paper presents an overview of high strength Al alloys development including metallic additives, such as Zr, Sc, Mn, Cu, etc., and nanoparticle additives, such as ceramics (TiB2, TiC, LaB6 and TiN) as well as carbon nanotubes (CNTs). The addition of Zr and Sc elements significantly prevents hot tearing and enhances the strength of laser processed Al alloys because the nanoscale Al3Zr, Al3Sc and Al3 (Sc, Zr) precipitated phases generate, facilitate the heterogeneous nucleation of Al matrix and refine the microstructure. Moreover, the addition of Mn and Cu elements provides an increment in the toughness and strength of laser processed Al alloys through the superimposed effect of multi-element solid solution reinforcement and precipitation strengthening role of some Al2CuMg and Al6Mn. The growth process of Al alloy can be interrupted by the addition of nanoceramics particles as additional nucleation site which leads the columnar grain transforms to the equiaxed grain. Furthermore, the mechanism of mutual solubility of LaB6, TiB2, TiC and TiN in Al alloys is systematically studied. Finally, an assessment of the state in laser processed high strength Al alloys and the research demands for the expansion of laser powder bed fusion of Al metallic components are provided.

Microstructure and mechanical properties of two Al alloys welded by linear friction weld

Two high strength Al alloys AA2050 and AA7010 have been joined using linear friction welding (LFW) in this study. Microstructural evolution across the weld line developed during linear friction welding has been examined. Their effects on mechanical properties have also been evaluated. A clean weld region where there is free of microcracking, microporosity, and oxides, was obtained. Dynamic recrystallization (nearly full and partial) took place on the weldment. Dissolution of ‘parent’ precipitates in centre weld zone (CWZ) and thermomechanical affected zone (TMAZ) was observed, thus leading to lower hardness and strength compared with the parent materials.

Formation energies of θ-Al2Cu phase and precursor Al-Cu compounds: Importance of on-site Coulomb repulsion

We investigated the relative stability and structural properties of Al-Cu intermetallic compounds using the density-functional theory (DFT) with different approximations. We reveal the importance of the on-site Coulomb repulsion of Cu 3d electrons on the energetics and structural properties of Al-Cu compounds of free-electron nature. The finite-temperature effect was included by accounting for the vibrational free energy within the Debye model. The present study shows that θ-Al2Cu is the ground state phase, agreeing with the experimental observations in the literature. The DFT + U approach could be useful to predict accurate formation energies of other Cu-containing precipitates in high-strength Al-alloys.

Design approaches for printability-performance synergy in Al alloys for laser-powder bed additive manufacturing

The unprecedented increase in component design space has led to significant focus on fusion-based additive manufacturing (AM) technologies. The new design possibilities integrate features and functionalities that are not supported by conventional manufacturing, and simultaneously achieve unitization of components. However, fusion-based AM suffers from a lack of diverse alloys, as conventional high-strength Al alloys are prone to hot cracking and other defect formation during printing. New alloy design approaches are being pursued to overcome these limitations. Current strategies for design of printable Al alloys for laser-powder bed fusion (L-PBF) are primarily experimental and revolve around grain refinement and eutectic solidification (ES). Each of these strategies targets hot cracking at only a specific stage of solidification. Consequently, the processing window of the alloy shrinks and fine-tuning of the alloy microstructure becomes difficult, thus prohibiting the activation of multiple deformation mechanisms. On the other hand, strategies that integrate microstructural refinement (MR) and ES attack the problem during multiple stages of solidification. Such MR+ES integrated alloy design strategies allow widening of alloy-processing-window (printability) and activation of multiple deformation mechanisms such as back-stress strengthening and work-hardening, thus producing alloys with excellent synergy of printability and performance.

On the heat treatment and mechanical properties of a high solute Al–Zn–Mg alloy processed through laser powder bed fusion process

Abstract High strength alloys in the Al–Zn–Mg system are primarily strengthened through artificial ageing by the formation of precipitates that are rich in Mg and Zn. In this work, a high solute Al–Zn–Mg alloy with 14 wt% Zn and 3 wt% Mg was prepared using the laser powder bed fusion process (LPBF). Heat treatment of Al–14Zn–3Mg revealed the presence of a high volume fraction of η’ and η-Mg(Zn, Al)2 precipitates in the Al-matrix, accompanied by a compromise of mechanical properties. This was elucidated in the context of microstructure – strength relationship, where the results demonstrate a loss of mechanical properties in aluminium alloys when the area fraction of second phases exceeds ~50%. This study is a discrete effort towards revealing the critical solute limits of Zn and Mg in aluminium that are imperative to achieve high strength through second phase particles – a fundamental concept that maybe exploited in the future design of high strength Al-alloys through additive manufacturing.

Finite element and experimental study of the residual stresses in 2024-T3 Al alloy treated via single toroidal roller burnishing

This article presents the outcomes of finite element (FE) simulations and X-ray stress measurements of residual stresses in high-strength 2024-T3 Al alloy introduced via the single toroidal roller burnishing (STRB) process. In terms of the deforming toroidal roller geometry, STRB is particularly suitable for deep rolling. A 3D FE model was developed using the flow stress concept, and the actual STRB kinematics was simulated to evaluate both hoop and axial residual stresses. The FE model was validated through a comparison of FE and X-ray residual stress distributions. The effects of the burnishing force, feed rate, and number of passes on the residual hoop and axial stresses were studied. It was established that increasing the feed rate leads to a decrease in the residual hoop stresses and an increase in the residual axial stresses. The greater burnishing force increases the compressive zone depth and only slightly increases the surface residual stresses. The FE and X-ray stress analyses confirm the effectiveness of STRB of 2024-T3 Al alloy to introduce significant residual compressive axial and hoop stresses.

Self-Piercing Riveting of Medium- and High-Strength Al and Mg Alloy Sheets Enabled by In-Process Electric Resistance Heating

Abstract Enduring joints of high-strength materials are becoming increasingly important for the manufacturing of highly loaded body-in-white structures, particularly for automobiles, which require structural light-weighting without sacrificing passenger safety. While self-piercing riveting has been proven suitable for various material classes, its application to high-strength, low-ductile materials is hindered by the occurrence of cracks and insufficient penetration. In this work, we demonstrated that these restrictions can be overcome by localized one-sided short-time electric resistance heating. The treatment can be integrated into the riveting process, allowing for short process times. The heating lowers the hardness of the materials to be joined and enables piercing and penetration of the sheets to yield crack-free joints. The local mechanical properties are hardly changed in case of peak-aged sheet material conditions but increase significantly in the close vicinity of the rivet in case of under-aged sheet material conditions. The outstanding static and dynamic mechanical properties of the resulting joints evidence their robustness.

Direct observation of the dynamic evolution of precipitates in aluminium alloy 7021 at high strain rates via high energy synchrotron X-rays

An improved understanding of the phenomenon of dynamic precipitation is important to accurately model and simulate many industrial manufacturing processes with high strength Al-alloys. Dynamic ageing in 7xxx Al-alloys can occur as a result of both the strain and heat. Small angle X-ray scattering (SAXS) is an advanced technique that allows the precipitation processes to be studied in situ, but to date this has only been possible at lower than industrially relevant strain rates (e.g. < 10−3). In this contribution, we demonstrate the potential of in-situ SAXS studies of metallic alloys at higher strain rates (10−2) than previously, using a high energy synchrotron X-ray. The time resolved SAXS information has been used to evaluate dynamic precipitate evolution models and has demonstrated that at high strain rates a new regime must be considered which includes the more significant effect of vacancy annihilation, leading to a clear strain rate, rather than just strain, kinetic dependence.

Microstructure and Electrochemistry of Al-Rich Primer

Al-rich primer (AlRP) is designed to be anodic relative to high strength Al alloys, so that it drives the mixed potential of Al alloy substrates (e.g. AA2024) into the cathodic region below the breakdown potentials of the Al alloys, preventing localized corrosion. The Al-rich primer (AlRP) under study is composed of spherical Al-Zn-In pigments in an epoxy binder. The 2D and 3D microstructure of the AlRP was studied using a combination of X-ray microscopy, plasma focus ion beam/scanning electron microscopy, and transmission electron microscopy techniques. The primer contained Al-Zn-In particles with various sizes ranging from tens of nanometers to microns, epoxy binder and a significant volume of voids. Potentiodynamic polarization testing was performed to measure the electrochemistry of the Al-Zn-In bulk alloy under neat primer in NaCl solution to simulate the environment which Al-Zn-In pigments are exposed to in the real primer. The bulk alloy under neat primer exhibited higher breakdown potential than the bare bulk alloy exposed to NaCl solution. The polarization curves of the bulk alloy under neat primer provide more accurate boundary conditions for modeling of the galvanic interaction between the AlRP and an alloy substrate. Galvanic corrosion between an AA2024 sample and Al-Zn-In bulk alloy was also studied using a zero resistance ammeter. A trivalent chromium process treatment on galvanic couples can protect AA2024 substrate from cathodic corrosion.Acknowledgements: this project is funded by the Office of Naval Research through the Small Business Innovation Research (SBIR) program. We thank Naval Air Systems Command (NAVAIR) for providing Al-rich primer samples. We acknowledge Dr. Mansoureh Norouzi Rad at Zeiss for 3D microstructure reconstruction on Al-rich primer using X-ray microscope, and Thermo Fisher Scientific for 3D microstructure reconstruction on Al-rich primer using Helios Plasma FIB/SEM. We appreciate Dr. Babu Viswanathan at OSU for STEM characterization, and Swagelok Center for Surface Analysis of Materials (SCSAM) at Case Western Reserve University for Ar ion polishing.

Role of Chromate in the Cathodic Current Availability in an Al Alloy-316 Stainless Steel Galvanic Couple Under Simulated Atmospheric Conditions

Numerous studies have demonstrated that the Cu-rich intermetallic particles (IMPs) are the major facilitators of localized corrosion of high strength Al alloys in chloride-containing environments.[1-4] In the context of cathodic activity, these Cu-rich IMPs catalyze the fast oxygen reduction reaction (ORR) rates required to sustain anodic dissolution of the peripheral Al matrix and/or preferential dealloying of active elemental constituent(s) of the Cu-rich IMPs. The latter process results in Cu replating on the surface which increases the cathodic surface area. The situation is exacerbated when these Al alloys are coupled with more noble fasteners such as stainless steel (SS) as found in aircraft structures; macro-galvanic interactions between the Al plates and SS fasteners lead to substantial damage, including within the fastener holes.[5-8] The SS fastener is speculated to increase the driving force for Cu replating, and the replated Cu in turn sustains fast cathodic reaction kinetics.[4] Traditionally, soluble chromates have been used in conversion coatings and as pigments in protective coatings to mitigate localized corrosion of Al alloys.[9-10] Chromate cathodically reduces to form an insoluble Cr3+-rich film on the alloy surface that blocks cathodic sites and hinders the ORR as well as the dealloying of the S-phase (Al2CuMg).[10-13] While chromate is well known to be carcinogenic, and efforts are underway to replace it with an environmentally-friendly inhibitor, efforts to date on potential alternatives have proven generally unsuccessful.[14] Consequently, chromate remains the choice inhibitor for Al alloy applications particularly in the aerospace industry. As such, it remains of utmost academic and industrial interest to continue to gain more in-depth understanding of chromate inhibition mechanism(s), especially as it pertains to the corrosion behavior of complex structures with micro- and macro-galvanic couples with the intent to enhance the knowledge required to facilitate the establishment of non-toxic chromate replacement systems.In the present work, the rotating disk electrode (RDE) technique is used for a comparative study of the inhibitive effect of sodium chromate on the ORR on polished and pre-corroded AA7050 electrodes. The results will serve as boundary conditions for the computational modeling of galvanic corrosion inhibition of AA7050 coupled to 316SS in a plate-fastener arrangement. The RDE is used to simulate thin electrolyte films with varying diffusion boundary layer thicknesses as rotation rate is increased. In the inhibitor-free chloride solution, a Pt working electrode is used to determine boundary layer thickness as a function of rotation rate, eliminating the effects of a surface oxide film. Initial results on polished electrodes show that at optimal chromate concentrations where the cathodic current density is significantly reduced on AA7050 and 316SS, the ORR kinetics on pure Cu is still substantial. This implies that higher inhibitor concentrations may be required to suppress the ORR on corroded AA7050 replated with Cu. AA7050 pre-corrosion is simulated by employing two approaches: 1) free corrosion at the open circuit potential in inhibitor-free solution and 2) electroplating Cu onto AA7050. The results are compared to the ORR kinetics on pure Cu. Furthermore, the scanning vibrating electrode technique (SVET) will be carried out on a typical AA7050/316SS galvanic couple to map local current density distributions as a function of chromate concentration. Surface characterization will be performed using optical microscopy and scanning electron microscopy (SEM) and surface film analyses will be conducted utilizing x-ray photoelectron spectroscopy (XPS).

Friction stir dovetailing of AA7099 to steel with AA6061 interlayer for reduced Zn embrittlement at dissimilar interface

Abstract Joining of high strength Al alloy (AA7099) to steel (rolled homogeneous armor) with and without AA6061 interlayer was achieved using a friction stir dovetailing (FSD) method. Detailed investigations were conducted to establish correlations between mechanical performance and microstructural evolution of FSD joints. Lap shear tensile testing revealed that incorporation of the AA6061 interlayer combined with tailored processing methodology significantly improved both mechanical interlocking and load transmission across the dovetail interface. Following incorporation of the interlayer, the maximum load carrying capacity and peak extension (of the AA7099/AA6061/steel joint) are 60% and 210% higher, respectively, than results obtained from an AA7099/steel joint without an interlayer. Without an AA6061 interlayer, formation of zinc rich intermetallic compounds led to brittle interfacial failure of the AA7099-steel FSD joint. Incorporation of novel tool and joint configuration eliminated asymmetric joint performance as a function of loading direction, which is generally observed in friction stir lap welding. The efficacy of the use of an AA6061 interlayer in joining AA7099 to steel was demonstrated through mechanical testing as well as high resolution analytical electron microscopy characterization.

The mechanical properties of high strength Al alloy sheet under quasi-static-dynamic loading

To investigate the mechanical properties of high strength Al alloy, the AA2024 was selected and conducted the uniaxial tensile tests at room temperature under different loading modes, namely pure quasi-static (QSD) loading mode, pure dynamic loading mode and quasi-static-dynamic loading mode. The mechanical properties such as yield stress (YS), ultimate tensile stress (UTS), total elongation and strain rate sensitivity were compared. The results shown that the UTS under QSD loading mode is larger than that of quasi-static and pure dynamic loading mode, and dependent on prestrain levels. However, the YS at pure dynamic loading mode is less than that of QSD loading mode at prestrain of 10% but larger than that of prestrain of 6%. In addition, the QSD loading mode would further improve the total elongation of AA2024 compared to the elongation enhancement under pure dynamic loading mode. Finally, the variation of strain rate sensitivity under different loading modes was analyzed to explain the evolution of tensile properties of AA2024.

Development of an Al 7050-10 vol.% alumina nanocomposite through cold consolidation of particles by high-pressure torsion

An Al 7050-10 vol.% Al2O3 nanocomposite was produced by cold consolidation of Al 7050 gas-atomized powder with alumina particles by means of severe plastic deformation using high-pressure torsion (HPT). The processing was conducted at room temperature under a pressure of 6.0 GPa, for 50 revolutions with a rotation speed of 2 rpm. The purpose of this work was to consolidate at room temperature one high strength Al alloy with Al2O3 particles without any prior or post sintering. In order to characterize the product, the disk microstructure was examined after consolidation using scanning and transmission electron microscopy and X-ray diffraction was used to determine the presence of different phases. Energy-dispersive X-ray spectroscopy (EDS) was undertaken to provide chemical analysis of the phases and hardness tests were performed to check the strength. The results show that the alumina particles in the periphery of the disk are more finely dispersed than in the central regions. The average grain size of the Al 7050 matrix ranged from ∼50 to ∼200 nm and the hardness measurements varied from ∼237 Hv to ∼307 Hv with disk edges having higher hardness values than in the central area.

Architecturally Arranged Precipitates Yield a New High Strength Al-Cu-Sc-Zr-Mn Alloy with Exceptional Thermal Stability

The refinement and thermal stability of intermediate theta-prime ( θ' ) precipitates are critical in the development of new high strength 2xxx series aluminium-copper (Al-Cu) alloys for high temperature applications. In this work, we use trace additions of Sc, Zr and Mn in an Al-6.5wt.%Cu alloy to refine and stabilise the θ' precipitates. The Al 3 (Sc, Zr) core/shell dispersoids significantly refine the θ' precipitates by acting as preferential nucleation sites during artificial ageing. Adding Mn results in a significant increase of hardness during ageing at 190 °C. Furthermore, hardness is maintained for up to 24 h of thermal exposure at 280 °C. Transmission electron microscopy (TEM) reveals that the addition of Mn leads to a finer and denser distribution of θ' precipitates, and greatly slows the growth and coarsening of the θ' precipitates at elevated temperatures. Differential scanning calorimetry (DSC) shows that this can be attributed to an enhanced nucleation and improved coarsening resistance of the θ' precipitates in the presence of Mn. A combination of energy dispersive spectroscopy (EDS) and atom probe tomography (APT) reveals that Mn, Zr and Sc segregate at the Al/ θ' semi-coherent interfaces. This segregation lowers the interfacial energy of the θ' precipitates and provides a barrier for solute drag, thus lowering the nucleation energy and enhancing their thermal stability. A final benchmark reveals that the Al-Cu-Sc-Zr-Mn alloy surpasses other common elevated temperature high strength Al alloys.

Friction stir butt welding of a high strength Al-7050 alloy with a metastable transformative high entropy alloy

A metastable transformative Fe39Mn20Co20Cr15Si5Al1 high entropy alloy (Al-HEA) was friction stir butt welded with Al-7050 alloy. Extensive mechanical mixing was evident in the weld nugget wherein sheared HEA particles were dispersed in Al-alloy matrix. Detail investigation of the weld interface showed suppression of intermetallic formation thereby approaching a clean interface. Tensile testing of the weld nugget showed a strength of 400 MPa with ~ 10% ductility whereas corrosion testing showed no sign of extensive pitting at the Al-HEA/Al-7050 weld interface. Therefore, use of high entropy alloy is an effective pathway for obtaining strong dissimilar joints with high strength Al alloys for lightweight applications.

Developing a high-strength Al-Mg-Si-Sc-Zr alloy for selective laser melting: Crack-inhibiting and multiple strengthening mechanisms

To develop high-strength Al alloys for selective laser melting (SLM) additive manufacturing, we designed a series of Al-Mg(-Si)-Sc-Zr alloys and additively manufactured them using atomized alloy powders. In the absence of Si, the developed Al-xMg-0.2Sc-0.1Zr (x = 1.5, 3.0 and 6.0 wt%) alloys are all susceptible to hot cracking and the average crack density increases with increasing Mg content. The addition of 1.3 wt% Si into the Al-6Mg-0.2Sc-0.1Zr alloys effectively inhibits hot cracking during SLM and simultaneously refines the microstructure, and thus leading to enhanced mechanical properties in the as-printed samples. By further fine-tuning the alloy compositions, we designed a new alloy Al-8.0Mg-1.3Si-0.5Mn-0.5Sc-0.3Zr. This new alloy demonstrates significantly refined microstructure consisting of submicron cells with coherent Al3(Sc,Zr) nano-particle (2–15 nm) residing in the cell and intergranular Al-Mg2Si eutectic (Mg2Si diameter 10–100 nm). High-density stacking faults and a unique 9R phase are formed in the as-printed sample. The tensile strength and elongation of the as-printed sample are up to 497 MPa and 11%, respectively. After the aging treatment, the tensile strength reaches 550 MPa, while the ductility ranges from 8% to 17%, depending on the aging conditions. In addition to solid solution strengthening, grain boundary strengthening and nanoparticle strengthening, the high-density stacking faults also contributes to strengthening.

Grain boundary segregation and precipitation in an Al-Zn-Mg-Cu alloy

High strength Al-alloys are highly susceptible to intergranular embrittlement, which severely limits their lifetime. This article summarizes our recent work on the effect of solute segregation in the precipitation behavior at grain boundaries (GBs) compared to the grain interiors. Solute segregation could accelerate the precipitation behavior at GBs, which causes the formation of coarse precipitates and precipitate free zones along GBs. Furthermore, the interplay of solute segregation and the local structure at GBs has been considered. We show that the distinct segregation and precipitation behavior occurs within the same GB, which makes the GB excess of solutes at one facet significantly higher than the other facet. This paper enriches the current understanding on the role of chemistry and structure at GBs related to intergranular fracture and corrosion resistance in high strength Al-alloys.

Defect Tolerance Assessment Method of Fusion Welded Medium and High Strength Al Alloy Joints

Defect Tolerance Assessment Method of Fusion Welded Medium and High Strength Al Alloy Joints

Microstructure and corrosion evolution of additively manufactured aluminium alloy AA7075 as a function of ageing

Additively manufactured high strength aluminium (Al) alloy AA7075 was prepared using selective laser melting (SLM). High strength Al-alloys prepared by SLM have not been widely studied to date. The evolution of microstructure and hardness, with the attendant corrosion, were investigated. Additively manufactured AA7075 was investigated both in the “as-produced” condition and as a function of artificial ageing. The microstructure of specimens prepared was studied using electron microscopy. Production of AA7075 by SLM generated a unique microstructure, which was altered by solutionising and further altered by artificial ageing—resulting in microstructures distinctive to that of wrought AA7075-T6. The electrochemical response of additively manufactured AA7075 was dependent on processing history, and unique to wrought AA7075-T6, whereby dissolution rates were generally lower for additively manufactured AA7075. Furthermore, immersion exposure testing followed by microscopy, indicated different corrosion morphology for additively manufactured AA7075, whereby resultant pit size was notably smaller, in contrast to wrought AA7075-T6.