Age-hardenable Aluminum Alloys Research Articles

Laser cladding of aluminum alloys with concurrent cryogenic quenching for improved microstructure and hardness

In this study, a method to deposit quench-sensitive age-hardening aluminum alloy clads is presented, which produces a hardness similar to the T6 temper without the requirement of solution heat treatment. A high-powered diode laser is scanned across the workpiece surface and material feedstock is delivered and melted via off-axis powder injection. The cladding process is immediately followed by quenching with liquid nitrogen, which improves the cooling rate of the quench-sensitive material and increases the hardness response to subsequent precipitation heat treatment. The method was demonstrated on the laser cladding of aluminum alloy 6061 powder on 6061-T6511 extruded bar substrates of 12.7 mm thickness. Single-track single-layer clads were deposited at a laser power of 3746 W, scan speed of 5 mm/s, and powder feed rate of 18 g/min. The in-situ liquid nitrogen quenching improved the clad hardness by 15.7% from 73.1 HV to 84.6 HV and the heat-affected zone hardness by 19.3% from 87.1 HV to 103.9 HV. Extending the process to multi-track multi-layer cladding further increased the clad hardness to 89.3 HV, close to the T6 temper hardness of 90 HV. Transmission electron microscopy revealed the increased precipitate density in the liquid nitrogen quenched clads were responsible for the higher hardness.

Unraveling the precipitate-induced discontinuity of the surface oxide film on Al alloy

Precipitate-induced discontinuity of the surface oxide film is a significant reason for the increasing pitting susceptibility of age-hardening aluminum alloys but lacking of direct characterization. In the present work, the oxide film heterogeneity induced by Mg(Zn,Cu)2 particles with different sizes in an Al-Zn-Mg-Cu alloy and the consequent corrosion behaviors were studied. Quasi in-situ transmission electron microscopy (TEM) observations indicated that the η'-Mg(Zn,Cu)2 precipitates with sizes less than 10 nm were electrochemically inert, while the grown η particles underwent de-alloying and then promoted pitting events of the nearby alloy matrix. By means of cross-sectioning, it was revealed that the surface film on the solid solution and peak-aged alloys were structurally uniform although oxidation of the η' resulted in local Mg enrichment in the surface film. For the grown η particle, the Mg-rich zone was remarkable and covered not only the particle but also the surrounding alloy matrix. The flawed structure in the Mg-rich area should be responsible for the immediately de-alloying of the precipitate and then the corrosion attack of the peripheric alloy.

Studying GPI zones in Al-Zn-Mg alloys by 4D-STEM

A new methodology has been developed to study the fine details of GP zones in age-hardenable aluminium alloys. It is complementary to atomic resolution high-angle annular dark-field scanning transmission electron microscopy imaging, and combines scanning precession electron diffraction with diffraction simulations. To evaluate the method, data was collected from an Al-Zn-Mg alloy in a condition with a dense distribution of GPI zones. Diffraction patterns were recorded in the 〈001〉Al orientation, capturing GPI zones in three projections: along the unique [001]GPI axis, and along the two other mutually orthogonal orientations. The GPI zones viewed along [001]GPI revealed how the truncated octahedron units of the GPI zones were connected in multi-unit GP zones, while the two orientations normal to [001]GPI highlight the internal structure. The stability of the atomic models developed based on the experimental results was verified by density functional theory calculations.

Microstructure and age-hardening effects of aluminium alloys with additions of scandium and zirconium

Abstract The aim of the work presented in this report was to produce age-hardenable aluminium alloys containing scandium and zirconium by a casting process with similar cooling conditions like an industrial casting process. Microstructure, precipitation structure and age-hardening response of different alloys with up to 0.4 wt.% Sc and Zr were investigated. Age-hardening experiments from the as-cast condition without solution annealing showed a significant increase of hardness of about 100% for Sc-rich alloys and of 50% for Zr-rich alloys compared to the as-cast condition. TEM investigations revealed the formation of precipitates of ternary Al3(Sc x Zr1–x) phases with a cubic cP4 crystal structure. In addition to the strengthening effect, a high thermal stability especially of the precipitates in Zr-rich alloys up to 400 °C let these alloys look very promising for high-temperature applications.

Influence of solidification rates and heat treatment on the mechanical performance and joinability of the cast aluminium alloy AlSi10Mg

In modern vehicle chassis, multi-material design is implemented to apply the appropriate material for each functionality. In spaceframe technology, both sheet metal and continuous cast are joined to castings at the nodal points of the chassis. Since resistance spot welding is not an option when different materials are joined, research is focusing on mechanical joining methods for multi-material designs. To reduce weight and achieve the required strength, hardenable cast aluminium alloys of the AlSi-system are widely used. Thus, 85–90% of aluminium castings in the automotive industry are comprised of the AlSi-system. Due to the limited weldability, mechanical joining is a suitable process. For this application, various optimisation strategies are required to produce a crack-free joint, as the brittle character of the AlSi alloy poses a challenge. Thus, adapted castings with appropriate ductility are needed. Hence, in this study, the age-hardenable cast aluminium alloy AlSi10Mg is investigated regarding the correlation of the different thicknesses, the microstructural characteristics as well as the resulting mechanical properties. A variation of the thicknesses leads to different solidification rates, which in turn affect the microstructure formation and are decisive for the mechanical properties of the casting as well as the joinability. For the investigation, plates with thicknesses from 2.0 to 4.0 mm, each differing by 0.5 mm, are produced via sand casting. Hence, the overall aim is to evaluate the joinability of AlSi10Mg and derive conclusions concerning the microstructure and mechanical properties.

Laser beam welding of hot crack sensitive Al-alloys without filler wire by intensity controlled dynamic beam oscillation

Modern fiber-bound or mirror-optical manipulation of the laser beam to adapt intensity distribution in the area of the weld pool dominate recent developments for process stabilization and thus for quality improvement, for example for e-mobility applications. Especially the use of beam manipulation for material-oriented adaptation of the welding process appears very promising. In this paper, the new opportunity of high dynamic beam shaping (DBS) for the prevention of hot cracks in age-hardenable aluminum alloys is described in order to avoid the otherwise necessary addition of filler metal. The results from laboratory experiments show that, depending on the joint geometry and the Al-alloy, typical hot crack formations in the weld metal will be almost completely suppressed. Furthermore, for the first time it is possible to overcome metallurgical limitations in laser beam welding with remote optics by using this intensity-based approach.

Hybrid Heat Treatment for Conventionally Treatable Steel Powder Reinforced Age Hardenable Aluminium Alloy Matrix Composites and Mechanical Property Evaluation

The present work is associated with research concentrating on the innovation and use of hybrid heat treatment for eutectoid steel powder (0.8 wt%) reinforced Al-Zn-Mg (Al 7075) alloy composites. Due to high hardness, wear resistance, tensile strength and flexibility modifying heat treatment, heat treatable aluminium metal matrix composites reinforced with heat treatable hard steel particles may be the choice to explore. In this work, an attempt is made to increase hardness and tensile properties to higher levels through hybrid heat treatment, comprising simultaneous treatment to matrix and reinforcement in 3 different routes (Pearlite, Bainite and Martensite) compared to conventional age hardening treatment. SEM images and microhardness distribution witnessed the phase transformation in both matrix and reinforcement. Aging kinetics in conventional age hardening and hybrid treatments is accelerated by the increase in the quantity of reinforcement and increase in aging temperature. The improvement in hardness and tensile strength obtained by conventional age hardening path is further improved by the hybridisation path. Hybridisation route with martensite reinforcement phase shows excellent result in hardness, strength followed by bainite and pearlite path respectively in the decreasing order. In all heat treatment cycles, lower aging temperature marks greater property enhancement compared to higher temperature. Al 7075 with 6 wt% steel powder reinforced (martensite form) composite showed excellent peak age hardness, tensile strength compared to lesser quantity reinforced composites.

Corrosion behavior of age hardening aluminum alloys produced by high-energy ball milling

The influence of high energy-ball milling (HEBM) on corrosion and hardness of age hardening aluminum alloys was investigated. Nanocrystalline age hardening (AA2024, AA6061 and AA7075) alloys were produced by HEBM of pre-alloyed powder and subsequent cold compaction under uniaxial pressure of 3 GPa. Cyclic potentiodynamic polarization and immersion tests were conducted in 0.6 M NaCl solution which revealed significantly increased pitting and protection potentials in the HEBM alloys compared to wrought alloys of same composition. X-ray diffraction analysis and transmission electron microscopy indicated grain refinement below 100 nm in the ball milled alloys which was the major strengthening mechanism in the age hardening HEBM alloys. The superior corrosion resistance and hardness of the age hardening ball milled alloys were attributed to nanocrystalline structure, extended solid solubility and homogenous microstructure- free from coarse intermetallic phases.

Experimental analysis and constitutive modeling of anisotropic creep damage in a wrought age-hardenable Al alloy

Many components of aerospace and automotive engineering manufactured from forged age-hardening aluminum alloys operate at high temperature in the range 0.3Tm–0.5Tm, where Tm is the melting temperature. Experimental data for uni-axial creep on specimens sampled from a forged AA2014 alloy component show that inelastic strain rates depend significantly on the orientation of loading with respect to the material flow direction during the manufacturing stage. Based on the recently developed constitutive model of anisotropic creep, hardening/recovery, softening and overageing processes, this paper addresses experimental analysis and modeling tertiary creep regime up to the final stage of creep rupture. The emphasis is placed on the damage evolution equation in order to account for different damage mechanisms depending on the loading mode and loading direction relative to grain axis: cavitation on elongated grain boundaries as well as decohesions and microcracks around and inside coarse grain particles. Experimental data of creep strain rates for longitudinal and transverse loading directions at 130, 150 and 170 °C have been used to calibrate the model. The validation of the model was carried out by comparing experimental and modeled creep curves in the case of a 30°angle between the loading and the material flow direction during forging. The results show that the assumed damage mechanisms and the developed damage evolution equation are able to reflect both the anisotropic accelerated creep and the time to creep rupture for different stress levels and loading directions.

Effect of precipitation state on plastic anisotropy in sheets of the age-hardenable aluminium alloys AA 6016 and AA 7021

We analyse flow stress, work hardening and plastic anisotropy of two heat-treatable Al alloys of the AA 6xxx and AA 7xxx series in dependence on state and morphology of the age-hardening precipitates. Sheets in the solution heat-treated condition were subjected to various ageing treatments in order to vary the precipitation state while maintaining crystallographic texture and grain shape. Accordingly, differences in the anisotropy obtained for various temper conditions could directly be attributed to the precipitates. Plastic anisotropy was assessed from the in-plane variations in yield strength and plastic strain ratios (r values). The texture-induced anisotropy was further modelled with polycrystal-plasticity simulations; therewith, the anisotropy obtained in the solution heat-treated, i.e. particle-free, sheets could be explained. In alloy AA 6016 with shearable needle-shaped β′′-particles, the effect of precipitates on plastic anisotropy was negligible compared to the pervasive effect of texture. Alloy AA 7021 containing plate-like η′-particles with low aspect ratio displayed a noticeable weakening of the texture-induced anisotropy with progressing heat treatment and the resulting changes in precipitation state. This weakening of anisotropy was attributed to the development of non-directional back-stresses due to the storage of Orowan loops around the non-shearable η′-precipitates during the early stages of straining.

Shear-Based Deformation Processing of Age-Hardened Aluminum Alloy for Single-Step Sheet Production

Abstract Shear-based deformation processing by hybrid cutting-extrusion and free machining are used to make continuous strip, of thickness up to 1 mm, from low-workability AA6013-T6 in a single deformation step. The intense shear can impose effective strains as large as 2 in the strip without pre-heating of the workpiece. The creation of strip in a single step is facilitated by three factors inherent to the cutting deformation zone: highly confined shear deformation, in situ plastic deformation-induced heating, and high hydrostatic pressure. The hybrid cutting-extrusion, which employs a second die located across from the primary cutting tool to constrain the chip geometry, is found to produce strip with smooth surfaces (Sa < 0.4 μm) that is similar to cold-rolled strip. The strips show an elongated grain microstructure that is inclined to the strip surfaces—a shear texture—that is quite different from rolled sheet. This shear texture (inclination) angle is determined by the deformation path. Through control of the deformation parameters such as strain and temperature, a range of microstructures and strengths could be achieved in the strip. When the cutting-based deformation was done at room temperature, without workpiece preheating, the starting T6 material was further strengthened by as much as 30% in a single step. In elevated-temperature cutting-extrusion, dynamic recrystallization was observed, resulting in a refined grain size in the strip. Implications for deformation processing of age-hardenable Al alloys into sheet form, and microstructure control therein, are discussed.

Internal Stress and Dislocation Interaction of Plate-Shaped Misfitting Precipitates in Aluminum Alloys.

The fine misfit precipitates in age-hardenable aluminum alloys have important roles due to their excellent age-hardening ability, by their interaction with dislocations. The present study focused on the internal stress field of plate-shaped misfitting precipitates to evaluate their roles in dislocation overcoming the precipitates by means of micromechanics based on Green’s function method. The stress field of misfit precipitates on {001} and {111} habit planes were reproduced by homogeneous misfit strain (eigenstrain) of the precipitate (Eshelby inclusion method), and the dislocation motion vector on the primary slip plane was predicted by the force acted on the dislocation by the Peach–Koehler formula. According to simulation results, the dislocation interaction strongly depends on the stress field and geometry of misfit precipitates; repulsive and attractive forces are operated on the dislocations lying on the primary slip plane when the dislocation approaches the misfit precipitates. The hardening ability of different orientations of precipitation variants was discussed in terms of interaction force acted on the dislocation.

Review of Techniques for Improvement of Softening Behavior of Age-Hardening Aluminum Alloy Welded Joints.

The softening phenomenon of age-hardening aluminum alloy-welded joints is severe during conventional fusion welding, which increases the likelihood of stress and strain concentration in the joint during the period of service, significantly reduces the mechanical properties compared to the base metal, and represents an obstacle to the exploration of the potential structural performance. This review paper focuses on an overview of the softening phenomenon. Firstly, the welding softening mechanism and the characteristics of age-hardening aluminum alloys are clarified. Secondly, the current main research methods that can effectively improve joint softening are summarized into three categories: low-heat-input welding, externally assisted cooling during welding, and post-weld treatment. The strengthening mechanism and performance change rule of age-hardening aluminum alloy joints are systematically analyzed. Finally, this paper considers the future development trends of further research on joint softening, and it is expected that interest in this topic will increase.

κ-Carbide assisted nucleation of B2: A novel pathway to develop high specific strength steels

Controlling morphology of B2 phase as nano-sized particles that are homogenously distributed within austenite matrix has been remained as a challenging task particularly for hot rolled lightweight steels, where the nucleation sites for B2 precipitation are less abundant than in cold rolled ones. Utilizing the phenomena occurring in age-hardenable aluminum alloys, i.e., heterogeneous nucleation of precipitates from pre-existing GP zones, here we solved this challenge by using κ-carbide nanoparticles to promote the formation and to engineer the morphology of B2. This strategy has been applied to a hot rolled Fe-21Mn-10Al-1C-5Ni model steel resulting in the formation of a large fraction of B2 nanoparticles (sizes of 20-500 nm) uniformly distributed within austenite grains. Besides their contribution to precipitation strengthening, these B2 nanoparticles lead to a dramatic improvement of work hardening capability of the steel. As a result, novel precipitation hardened hot rolled lightweight steel with excellent combination of ultrahigh specific strength and ductility has been fabricated. Such properties of the present steel are not only better than those of other hot rolled lightweight steels strengthened by B2 and/or κ-carbide but also comparable to those of the steels subjected to additional cold rolling, and it is expected to expand the applications of B2-strengthened steels in response to an increasing demand for lightweight constructions.

Natural and artificial ageing in aluminium alloys – the role of excess vacancies

Non-equilibrium excess vacancies quenched-in after solutionising age-hardenable aluminium alloys at high temperature are known to play an important role in precipitation at lower temperatures. However, knowledge is still lacking on the extent of the contribution to precipitation at various temperatures. In this work, we revisit this classical problem and study the role of excess vacancies in an Al-Mg-Si alloy in natural and artificial ageing experimentally by hardness measurements, differential scanning calorimetry, and positron annihilation lifetime spectroscopy. We then apply a precipitation model involving a simulation of vacancy loss and solute diffusion based on parameters calculated by first principles. The experiments show that excess vacancies are largely removed in the initial seconds of artificial ageing with hardly any corresponding hardening, i.e., subsequent hardening is triggered primarily by equilibrium vacancies unlike natural ageing, where hardening is mainly driven by excess vacancies. We reproduce the anomaly that hardening for a given time can be faster at lower temperatures and explain this by the different activation energies of vacancy annihilation and solute diffusion. The role of excess vacancies demonstrated in Al-Mg-Si alloys could be similar in other age-hardenable alloys and hence be a universal phenomenon.

Artificial aging time influencing the crack propagation behavior of the aluminum alloy 6060 processed by equal channel angular pressing

The combination of a severe plastic deformation (SPD) by equal-channel angular pressing (ECAP) with a subsequent heat treatment significantly influences the precipitation characteristics of age-hardenable aluminum alloys. The aging kinetics is accelerated and the morphology of the precipitates is affected by the induced strain. In the present study, the influence of the artificial aging time and the resulting precipitation morphology on the fatigue threshold was investigated for the aluminum alloy 6060 with conventional grain size (CG) and after ECAP processing. Four artificial aging conditions are compared for the as-extruded and ECAP processed material, respectively. The resulting microstructure is examined by transmission (TEM) and scanning transmission electron microscopy (STEM) and related to the processing condition and the aging time. The mechanical properties are characterized by tensile and hardness testing and crack propagation tests are conducted in order to determine the threshold ΔKth. The ECAP processed conditions exhibit a lower resistance against crack propagation regardless of the aging time, when compared to the as-extruded conditions. Therefore, the grain size is determined as the major influencing factor on the fatigue threshold. In contrast, the effect of the aging time and the precipitation morphology is only minor. When compared to incoherent precipitates, coherent ones promote planar slip resulting in an increased threshold against crack propagation for both CG and finer grained, bimodal microstructures. However, the size of the coherent precipitates and the resulting lattice distortion is decisive to achieve higher fatigue thresholds.

Decipher Ultrasonic Signals from Defects in Thick Aluminum Extrusions to Identify Type and Origin of Defects

Manufacturing of thick seamless pipes of age-hardenable aluminum alloys are a specialized technology since their application is limited to specific hi-tech areas. Quality criteria for their inspection are propriety items of the very few production facilities that develop these criteria in-house. Present study relates ultrasonic signals reflecting back from non-continuities in the thickness of seamless pipes with their microstructural features. Detailed study of defects leads to the source of their formation and will ultimately help to systematically control them. Signals from ultrasonic testing trace defects as UT waves reflect back from discontinuities in the material. Defective sections of seamless pipes were cut with precision to reveal the defects. The sectioned surfaces were subjected to metallographic preparation and revealed defects were studied using Optical and Field Emission Scanning Electron Microscopes (FESEM). Defects are grouped based on the shape of UT signals as well as the defect morphology as revealed by microscopic studies. Most of the observed cracks are found to grow in the direction of extrusion. Energy Dispersive Spectroscopy (EDS) analysis was conducted to determine the composition of inclusions in the vicinity of the defects. Data from elemental analysis is used to identify the potential sources. The study recommends measures to control the defects and improve the yield.

High-Temperature Equal-Channel Angular Pressing of a T6-Al-Cu-Li-Mg-Ag-Zr-Sc Alloy

Equal-channel angular pressing (ECAP) is known to induce significant grain refinement and formation of tangled dislocations within the grains. These are induced to evolve to form low-angle boundaries (i.e., cell boundaries) and eventually high-angle boundaries (i.e., grain boundaries). On the other hand, the precipitation sequence of age hardening aluminum alloys can be significantly affected by pre-straining and severe plastic deformation. Thus, ECAP is expected to influence the T6 response of aluminum alloys. In this study, a complex Al-Cu-Mg-Li-Ag-Zr-Sc alloy was subjected to ECAP following different straining paths. The alloy was ECAP at 460 K via route A, C, and by forward-backward route A (FB-route A) up to four passes. That is, ECAP was carried out imposing billet rotation between passes (route A), billet rotation by +90° between passes (route C), and billet rotation by +90° and inversion upside down between passes (FB-route A). The alloy was also aged at 460 K for different durations after ECAP. TEM microstructure inspections showed a marked influence of the different shearing deformations induced by ECAP on the alloy aging response. The precipitation kinetics of the different hardening secondary phases were affected by shearing deformation and tangled dislocations. In particular, the T1-Al2CuLi phase was the one that mostly showed a precipitation sequence speed up induced by the tangled dislocations formed during ECAP. The T1 phase was found to grow with aging time according to the Lifshitz-Slyozov-Wagner low-power regime.

Cracks detection in age hardenable aluminum alloys

Abstract Friction Stir Welding is one of the most important welding processes which can be able to yield better quality joints at the time of welding. Friction Stir Welding is the most favorable procedure because it need not require any usable filler materials and also cost effective in nature. Though it has more advantages, the welded materials are supposed to do get fractured. In order to investigate the presence of fracture in the welded metals, the defected area must be located in a proper manner. The main objective of this present investigation is to identify the defected area with low cost. A web camera is placed which is revolved at 360 degree over the welded metal surface. This captured image of the welded zone is used to analyze the defected location by using Matlab. The existing methods for identifying the defected area such as ultrasonic testing, infrared inspection, XRD and Nondestructive are expensive. In this present exploration, a method has been proposed and it helps the researchers to identify the cracks in an economical manner with minimum time consumption.

Processing of an AA7050/Al Hybrid Composites Reinforced with Multiwalled Carbon Nanotubes

Abstract The development of carbon nanotubes reinforced metal matrix composites merits relevant attention due to its capacity of improve the hardness and strength of the metal. However, the production of this kind of composite presents many challenges, such as the dispersion and adhesion of carbon nanotubes in the metallic matrix, wich may affect the composite consolidation. The present study aimed the development of composites using an age hardening aluminum alloy reinforced with 0.5 wt.% of multiwalled carbon nanotubes (MWCNT) as well as a hybrid composite based on the mixture of pure aluminum powder and the former composite. AA7050 alloy and pure aluminum matrix composites were processed by ultrasonic mixing, hot pressing and hot extrusion. Subsequently, the materials were submitted to aging heat treatments. The samples were analysed by Scanning Electron Microscopy, X-Ray Diffraction, Atomic Force Spectroscopy, Raman spectroscopy, and Vickers microhardness. It was verified the feasibility in manufacturing the composites by the powder metallurgy route proposed in this work. The AA7050 composite reinforced with 0.5 wt.% MWCNT presented higher hardness than other similar composites in the literature with higher carbon nanotubes content. The aged composite hardness increased 72% in comparison with the present one in the as extruded condition.