Age-hardenable Al Alloys Research Articles

時効硬化Al合金のき裂伝ぱ形態に及ぼす大気環境の影響（回転曲げ疲労と超音波疲労）

時効硬化Al合金のき裂伝ぱ形態に及ぼす大気環境の影響（回転曲げ疲労と超音波疲労）

時効硬化Al合金における特異な疲労破壊とその巨視的および微視的様相

時効硬化Al合金における特異な疲労破壊とその巨視的および微視的様相

Enhancing the ductility in the age-hardened aluminum alloy using a gradient nanostructured structure

A high ductility and extra strain hardening are achieved in the age-hardened Al alloy but without strength improvement via the introduction of a gradient nanostructured layer. The strain gradient and different residual stress between gradient layer and matrix are mainly responsible for the enhanced ductility, while the appearance of minority flaws (grain boundary precipitations) accounts for strength change. Compressive residual stress suppresses crack nucleation and propagation and cooperatively activates the partial dislocation mechanism in gradient layer. Combined with tensile residual stress from matrix, strain gradient is further increased during tensile testing. Besides, samples with the gradient layer exhibit significant thickness effect on the tensile properties, due to varying compressive residual stress occurring at flaws. Different contributions from strengthening mechanisms and flaws make the significant difference between pure metal and alloy with a gradient layer. This work provides new understanding of gradient materials, which is imperative to practical application for real complex materials.

Modeling the Microstructural and Yield Strength Evolution of an Age-Hardenable Al Alloy for High Temperature Applications

The mechanical properties of Al alloys are strongly affected by their microstructure: the size and shape of precipitates, their homogeneous distribution and their coherency with the matrix are of primary importance for an effective strengthening of the alloys at room and elevated temperatures. Physically-based models are powerful tools to predict the influence of the mentioned parameters on the mechanical properties of the alloy after age hardening, and also to predict the effect of high temperature service conditions on microstructure evolution. Scope of this work is to model the dimensional kinetic evolution of plate shaped precipitates of an Al-based alloy during aging and after different overaging times at elevated temperature, and use these results to estimate the alloy yield strength. The alloy strengthening response is due to three terms, linearly summed: the intrinsic strength of Aluminum, the contribution from solute in solid solution and the contribution arising from precipitates. The consistency of the model is verified with experimental data obtained from a 2014 Al alloy.

Controlling the high temperature mechanical behavior of Al alloys by precipitation and severe straining

The aim of this work was to investigate the influence of the precipitate distribution on the microstructure and on the room and high temperature mechanical properties of an age-hardenable aluminium AA6082 alloy following severe straining by high-pressure torsion (HPT). With this goal, specimens in the as-cast and T6 peak-aged conditions were processed by HPT using 0.5, 1 and 5 turns at room temperature. At high strain levels (γ>100), similar saturation grain sizes (~250nm), high-angle boundary fractions (~80%) and hardness values (Hv~160) were obtained for both initial conditions. Grain refinement led to significant strengthening and to good ductility values at room temperature. Analysis by TEM and EDS elemental mapping revealed that HPT processing of the as-cast condition led to fracture of the stable β-phase into many small precipitates located preferentially along grain boundaries and triple junctions. By contrast, HPT processing of the T6 peak-aged specimens revealed a partial dissolution of the needle-shaped nanoprecipitates. The different evolutions of the precipitate distributions following straining in the as-cast and peak-aged conditions gave rise to dramatic differences in the mechanical properties and the operative deformation mechanisms at warm temperatures. These results provide evidence that the high temperature mechanical behavior of age-hardenable Al alloys may be conveniently controlled by altering the precipitate distribution followed by severe straining.

Comparison of Age-Hardenable Al Alloy Corrosion Fatigue Crack Growth Susceptibility and the Effect of Testing Environment

A comparison of literature data and laboratory results on transgranular corrosion fatigue (CF) crack growth (also known as environmental fatigue crack propagation) of age-hardenable Al alloys used ...

Mechanical properties of β″ precipitates containing Al and/or Cu in age hardening Al alloys

Abstract

Casting and Homogenization of AlCu3.3Mg1.5Mn Alloy for Aircraft Industry

Heat treatable aluminum alloys and other alloys with similar properties are the most widely used light alloys in aeronautical construction [1-3]. These alloys have (i) high resistance to crack initiation and propagation, (ii) resistance to developing a strong lift force of the wings, (iii) high temperature, (iv)creep and fatigue vibration resistance. These age hardening Al alloys, are designed to take the mechanical stresses of the aircraft during flight. To achieve and continually improve these characteristics are necessary studies and researches on optimizing the manufacturing technological process of these types of Al alloys. This paper presents theoretical and experimental development of aluminum prealloys and high purity 2xxx Al alloys series elaboration for the aviation industry. We aimed to obtain higher energy efficiency, a better protection against gases, development of melt refining decontamination during casting. We are taking into account that the AlCu3.3Mg1.5Mn alloy has in chemical composition besides the alloying elements also a limited percentage of impurities (eg. 0.15% Si and 0.20% Fe), impurities difficult to remove from molten aluminum. In these conditions, the manufacturing processing requires some precautions such as: cleaning melting furnace and the correct choice of materials composing the charge in order to prevent contamination of the bath. The casted and homogenized Al alloys were analyzed in terms of composition and microstructure.

Warm HydroForming of the heat treatable aluminium alloy AC170PX

In this work the Warm HydroForming process of a 6xxx series Al alloy has been investigated in order to find suitable process parameters for successfully forming a benchmark component displaying varying deformation ratios due to its geometry. Since the attention was focused on the 6xxx series alloy AC170PX, belonging to the group of the age-hardenable Al alloys, the process conditions affecting the ageing, such as temperature and duration of heating exposure, the rate to increase the oil pressure, had to be determined. A preliminary mechanical characterization on artificially aged specimens was thus necessary both at room temperature and in warm conditions to investigate the influence of the ageing phenomenon on the mechanical and deformation behaviour at temperatures ranging from 150°C to 350°C. Based on the results of the tensile tests, preliminary Warm HydroForming experiments were conducted by changing the most influencing process parameters: working temperature, heating time and the time needed to increase the oil pressure up to the maximum value. It was found that the optimum working temperature was 200°C and that the exposure of the material to the employed warm-forming temperature had to be minimized. Final Warm HydroForming tests were thus carried out using different rates to increase the oil pressure for comparison purposes, revealing that employing higher rates, i.e. increasing the strain rate, was beneficial, in terms of die cavity filling and sheet thinning, to obtain sound products.

Early stage ageing effects and shallow positron traps in Al–Mg–Si alloys

Room temperature ageing, so-called natural ageing, of Al–Mg–Si alloys has a subtle but striking influence on the mechanical properties achievable by subsequent ageing at more elevated temperatures. Though strongly debated, different clustering processes are generally accepted to give rise to this effect. Using temperature-dependent positron lifetime measurements of naturally aged Al–Mg–Si alloys, it is shown that in the early stages of ageing, small clusters of alloying atoms without embedded vacancies take part in the decomposition process. These clusters serve as shallow positron traps with a binding energy of about 55(10) meV, grow in the course of natural ageing and transform to deep positron traps with binding energies well above thermal energies. Thus, results of positron annihilation spectroscopy techniques need to be interpreted carefully with respect to the microstructure of age-hardenable Al alloys. Moreover, it is shown that a simple approach to bind positron states using a three-dimensional potential well and (bulk) positron affinities cannot explain the present findings.

Influence of ECAP on the fatigue behavior of age-hardenable 2xxx aluminum alloy

The fatigue behavior under load control and the mechanical properties of commercial 2011 aluminum as an age-hardenable Al alloy was studied. To estimate the effects of the equal channel angular pressing (ECAP) process, solution heat treatments, and aging on the fatigue life, tests were conducted at four different stages: furnace cooling; furnace cooling plus one ECAP pass; solid solution heat treatment, quenching, one ECAP pass plus aging at peak age level; and the T6 condition. Only one pass was possible at room temperature because of the high strength of the material. The fracture surface morphology and microstructure after fatigue were evaluated by scanning electron microscopy (SEM). The experimental results revealed that the optimum fatigue life under load control, the tensile strength, and the Vickers hardness of the material were interdependent. The optimum fatigue life under load control was achieved by increasing the tensile strength and hardness of the material.

Atomic-scale pathway of early-stage precipitation in Al–Mg–Si alloys

Strengthening in age-hardenable alloys is mainly achieved through nano-scale precipitates whose formation paths from the atomic-scale, solute-enriched entities are rarely analyzed and understood in a directly verifiable way. Here, we discover a pathway for the earliest-stage precipitation in Al–Mg–Si alloys: solute clustering leading to three successive variants of face-centered cubic (fcc) clusters, followed by the formation of non-fcc GP-zones. The clusters, which originally assume a spherical morphology (C1), evolve into elongated clusters and orient themselves on {111}Al (C2) and subsequently on {100}Al planes and 〈100〉Al directions (C3). We also analyze the association of quenched-in dislocations with clustering phenomena. The results of this work can open a new frontier in advancing alloy-process-property design for commercially important age-hardenable Al alloys.

Fine-grained AA 7075 processed by different thermo-mechanical processings

Grain refinement by two classical thermo-mechanical processings (RI-ITMT and J-ITMT) and an alternative short-cycled thermo-mechanical processing (NTMT) of high strength AA 7075 alloy based on particle-stimulated nucleation of recrystallization (PSN) was investigated. The main difference between those TMPs was investigated in detail, especially the specific grain refining procedure of NTMT was carefully characterized. Their mechanical properties after TMPs were studied as well. The results indicated that the finest grain structure (~8.2 μm) can be achieved by RI-ITMT, and a similar structure (~9 μm) can be obtained by the present NTMT while a much coarser structure (~20 μm) was obtained by J-ITMT. As a result, the strength anisotropy can be eliminated by NTMT and RI-ITMT, and a significant increase in elongation can be achieved by all the processings while maintaining high strength. Warm deformation can accelerate MgZn 2 precipitation, coarsening and spheroidization, and more uniform distribution of MgZn 2 throughout the Al matrix can be accomplished in reference to the one with only overaging. Deformation-enhanced precipitation and PSN played a key role in grain refinement during NTMT. NTMT combines two ordinary procedures – warm rolling and continuous rolling – in present industry together, which makes grain refinement of age-hardenable Al alloys more time-saving.

The Heat Treatment of Rheo-High Pressure Die Cast 6xxx Series Al-Mg-Si-(Cu) Alloys

Limited information is available regarding the heat treatment response of age-hardenable wrought Al-alloys produced by semi-solid metal forming. In this study, the effects of chemical composition, natural pre-ageing and artificial ageing on the T6 properties of rheo-high pressure die cast 6xxx series Al-Mg-Si-(Cu) alloys are investigated. A Cu-free alloy 6082 is compared with Cu-containing alloys 6013 and 6111. The effects of Mg:Si ratio of the alloys are also quantified by means of a comparison between alloys 6111 and 6013. Artificial ageing curves following either no or 5 days of natural pre-ageing are also presented.

Correlation between Supersaturation of Solid Solution and Mechanical Behaviour of Two Binary Al-Si-Alloys

Cooling of age-hardening Al-alloys after solution annealing is a critical step with respect to distortion and residual stresses. In order to predict their extent by simulation models, the mechanical behaviour must be known in a wide range of conditions and compositions. Therefore, experimental data is needed both for calibration and validation of the mechanical model. It is known for Al-Mg-Si alloys that supersaturation of the solid solution leads to a significant increase of strength during cooling. In order to understand the influence of single alloying elements on the strengthening effect, the mechanical properties of different binary alloys are investigated experimentally. The precipitation behaviour during cooling was investigated by Differential Scanning Calorimetry in a wide cooling rate range. A methodology to determine the degree of supersaturation of the solid solution based on the calorimetric results is presented. This approach is compared to atom probe tomography data. The mechanical behaviour of the alloys after various heat treatments was analysed by mechanical tests performed in a quenching and deformation dilatometer. Flow curves with high resolution at small strains (< 3 %) were measured at different temperatures. The results of the different experimental techniques are discussed in comparison and with respect to their testing limitations.

Corrosion-Fatigue Crack Growth in Age-Hardened Al Alloys: Examples of Failures and Explanations for Fractographic Observations

The characteristics of corrosion-fatigue in age-hardened Al alloys, e.g. brittle striations on cleavage-like facets, are described, with reference to two examples of component failure. Mechanisms of corrosion fatigue (and explanations for fracture-surface features) are then reviewed. New observations of corrosion-fatigue crack growth for 7050-T7451 alloy compact-tension specimens tested in aqueous environments using a constant (intermediate) ΔK value but different cycle frequencies are then described and discussed. These observations provide additional support for a hydrogen-embrittlement process involving adsorption-induced dislocation-emission from crack tips.

Phase-field method and Materials Genome Initiative (MGI)

Predicting and controlling the microstructure evolution within a material are considered as the “holy grail” of materials science and engineering. Many important engineering materials are designed by controlling their phase transformations and microstructure evolution. Examples include the improvement of mechanical properties through solid-state precipitation reactions in Ni-based superalloys and age-hardened Al alloys, the useful dielectric properties and electro-mechanical coupling effects by manipulating the phase transitions in ferroelectric crystals, and the memory effect of shape-memory alloys by utilizing martensitic transformations. Phase-field method has become the method of choice for modeling three-dimensional microstructure evolution for a wide variety of materials processes. This short article briefly discusses the potential roles that phase-field method can play in the Materials Genome Initiative.

PROPAGATION BEHAVIOR OF A FATIGUE CRACK OF HIGH STRENGTH Al ALLOY

In order to investigate the effect of humidity change on growth behavior of a fatigue crack of an extruded and age-hardened Al alloy 7075-T6, rotating bending fatigue tests were carried out using plain specimens in relative humidity of 25% and 85%. In constant humidity, a crack propagated in a tensile mode macroscopically in low humidity and in a shear mode in high humidity. The crack growth rate was accelerated by high humidity. By changing humidity, the growth rate and the growth mode of a crack were changed to those corresponding to the changed humidity. That is, cumulative fatigue life in humidity change may be estimated by the fatigue life in constant humidity.

EFFECT OF MICROSTRUCTURE ON FATIGUE PROPERTIES OF Al ALLOYS

Effects of a texture and atmospheric moisture on fatigue properties of an extruded bar and a drawn one of age-hardened Al alloy were investigated under rotating bending in relative humidity of 25% and 85%. The extruded alloy had a marked texture of (111) plane, but the drawn one did not have a specified orientation. Fatigue strength was largely decreased by high humidity in both alloys. The decrease in fatigue strength was larger in the extruded alloy than in the drawn one. Macroscopic growth mode of a crack of the extruded alloy was a shear mode inclined 35° to the specimen axis in high humidity, though those were a tensile mode in low humidity and in both humidity in the drawn alloy. The fracture surface yielded by the shear mode crack was occupied by many slip planes and voids and was (100) plane, though the fracture surface formed by the growth of a tensile mode crack was mainly covered with striations. The shear mode crack was related to the marked texture of the extruded Al alloy and promoted by high humidity.

Fatigue properties of age-hardened Al alloy 2017-T4 under ultrasonic loading

Fatigue properties of age-hardened Al alloy 2017-T4 under ultrasonic loading frequency (20 kHz) were investigated and compared with the results under conventional loading of rotating bending (50 Hz). The growth of a crack retarded at about 500µm in surface length under ultrasonic loading, while at about 20µm under rotating bending. Although striations being a typical fracture mechanism were observed under conventional loading, most of fracture surface was covered with many facets under ultrasonic loading. These facets were also observed under rotating bending in nitrogen gas. The difference in growth mechanism depending on the loading frequency and the retardation of a crack growth under ultrasonic loading may be caused by the environment at the crack tip due to high crack growth rate under ultrasonic loading.