High cycle fatigue characteristics of 2124-T851 aluminum alloy

The fatigue crack growth rate, fracture toughness and fatigue S-N curve of 2124-T851 aluminum alloy at high cycle fatigue condition were measured and fatigue fracture process and fractography were studied using optical microscopy (OM), X-ray diffraction (XRD) technique, transmission electron microscopy (TEM) and scanning electron microscopy (SEM). The results show that at room temperature and R = 0.1 conditions, the characteristics of fatigue fracture could be observed. Under those conditions, the fatigue strength and the fracture toughness of a 2124-T851 thick plate is 243 MPa and 29.64 MPa · m^1/2, respectively. At high cycle fatigue condition, the higher the stress amplitude, the wider the space between fatigue striations, the faster the rate of fatigue crack developing and going into the intermittent fracture area, and the greater the ratio between the intermittent fracture area and the whole fracture area.     

The investigation of abnormal particle-coarsening phenomena in friction stir repair weld of 2219-T6 aluminum alloy

The single-pass friction stir weld of aluminum 2219-T6 with weld-defects was repaired by overlapping friction stir welding technique. However, without any post weld heat treatment process, it was found that the phenomena of abnormal particle-coarsening of Al₂Cu had occurred in the overlapping friction stir repair welds. The detecting results of non-destructive X-ray inspection proved that not only one group of repair FSW process parameters could lead to occurrence of the abnormal phenomena. And the abnormally coarsened particles always appeared on the advancing side of repair welds rather than the retreating side where the fracture behaviors occurred after mechanical tensile testing. The size of the biggest particle lying in the dark bands of ‘Onion-rings’ was more than 150 μm. After the related investigation by scanning electron microscope and X-ray energy spectrometer, three types of formation mechanisms were proposed for reasonably explaining the abnormal phenomenon: Aggregation Mechanism, Diffusion Mechanisms I and II. Aggregation Mechanism was according to the motion-laws of stir-pin. Diffusion Mechanisms were based on the classical theories of precipitate growth in metallic systems. The combined action of the three detailed mechanisms contributed to the abnormal coarsening behavior of Al₂Cu particles in the friction stir repair weld.     

Precipitate evolution in friction stir welding of 2219-T6 aluminum alloys

Precipitate evolution in friction stir welding of 2219-T6 aluminum alloys was characterized by transmission electron microscopy. In the weld nugget zone and the thermo-mechanically affected zone some metastable precipitates overaged to equilibrium phase while others solutionized into the aluminum solid solution. In the heat-affected zone the precipitates coarsened.     

Constitutive descriptions for hot compressed 2124-T851 aluminum alloy over a wide range of temperature and strain rate

The compressive deformation behaviors of 2124-T851 aluminum alloy were investigated over a wide range of temperature and strain rate on Gleeble-1500 thermo-simulation machine. The results show that the true stress–true strain curves exhibit a peak stress at a very small strain, after which the flow stresses decrease until high strains, showing a dynamic flow softening. The measured flow stress was modified by friction correction, and the corrected flow stresses are lower than the measured ones, which nicely reflect negative effects of the interfacial friction on the flow stress. A revised model is proposed to describe the relationships of the flow stress, strain rate and temperature of 2124-T851 aluminum alloy at elevated temperatures. The stress–strain values of 2124-T851 aluminum alloy predicted by the proposed model well agree with experimental results, which confirmed that the revised deformation constitutive equation gives an accurate and precise estimate for the flow stress of 2124-T851 aluminum alloy.     

Mean stress effects on crack propagation rate and crack closure in 7050-T76 aluminum alloy

High strength aluminum alloys that are cyclically loaded in tension are known to show increased fatigue crack propagation rates when the mean stress is increased. It has been suggested that this increase in growth rate may be due to a lower crack closure stress. A smaller closure stress results in a higher effective fatigue crack propagation stress. Measurements of the closure stress were made using an extensometer placed across the crack in order to study the effects of crack closure on the fatigue crack growth rate. The closure stress was determined from the change in slope of load-displacement curves. The growth rate data at various mean stresses were plotted as a function of effective stress intensity factor. In all tests the cyclic stress amplitude was 80 MPa. For specimens run at mean stress levels of 44 MPa and 70 MPa the crack faces were found to close, but on increasing the mean stress to above 120 MPa no closure was observed. The growth rate data from specimens run at mean stresses from 44 MPa to 120 MPa coincided when plotted vs effective stress intensity factor but that crack growth rates for a mean stress of 226 MPa were higher than those obtained at lower mean stresses. We concluded that effects other than crack closure also influence the crack growth rate. Much greater scatter was found in the fatigue crack growth rates of our thicker specimens (5.6mm thick) than in our thinner specimens (2.5 mm thick). Indirect measurements of the length over which the crack faces were closed showed that the length closed decreased sharply with increasing mean stress. At a mean stress of 44 MPa the length closed at minimum stress was several millimeters, while at 70 MPa it had been reduced to less than 1mm.     

Effect of friction stir welding parameters on microstructural characteristics and mechanical properties of 2219-T6 aluminum alloy joints

A 2219-T6 aluminum alloy was friction stir welded in the present study. The results indicate that the recrystallized grains in the weld nugget zone (WNZ) of the joints exhibit the largest size in the middle part and the smallest size in the lower part. Furthermore, the void defect is formed in the joint when the rotation speed or welding speed is quite high. As the rotation speed or welding speed increases, the tensile strength of the joint firstly increases to a maximum value and then sharply decreases due to the occurrence of void defect. During tensile test, the defect-free joints welded at lower rotation speed are fractured in the WNZ, while those welded at relatively high rotation speed tend to be fractured in the heat affected zone (HAZ) adjacent to the thermal mechanically affected zone (TMAZ) on the retreating side.     

A correlation between acoustic emission and the fracture toughness of 2124-T851 aluminum

The paper reports on an experimental study of the possible application of acoustic emission as an inspection technique, to determine whether 2124-T851 aluminum plate meets certain, minimum, fracture toughness specifications. Unflawed specimens, taken in the three principal directions, from three separate plates are tested in compression and tension. The-true-mean-square or the root-mean-square voltage of the continuous acoustic emission generated during the tests is recorded as a function of specimen strain. The acoustic emission, chemical analysis, fracture surfaces, and large (1–20 μm) second-phase particles are studied in relation to fracture toughness. Results indicate that the generated acoustic emission from a longitudinal compression test, long-transverse tension test and short-transverse tension test all correlate with changes in plate fracture toughness. Based on the experimental results, the monitoring of acoustic emission during relatively simple tension or compression tests of aluminum may be useful in checking whether the material meets a fracture toughness specification. Also, the results indicate that acoustic emission is useful in studies to develop fructure toughness models.     

Hot deformation of AA6082-T4 aluminum alloy

High-temperature tensile deformation of 6082-T4 Al alloy was conducted in the range of 623–773 K at various strain rates in the range of 5 × 10⁻⁵ to 2 × 10⁻² s⁻¹. Stress dependence of the strain rate revealed a stress exponent, n of 7 throughout the ranges of temperatures and strain rates tested. This stress exponent is higher than what is usually observed in Al–Mg alloys under similar experimental conditions, which implies the presence of threshold stress. This behavior results from dislocation interaction with second phase particles (Mg₂Si). The experimental threshold stress values were calculated, based on the finding that creep rate is viscous glide controlled, based on creep tests conducted on binary Al–1Mg at 673 K, that gave n a value of 3. The threshold stress (σ _o) values were seen to decrease exponentially with temperature. The apparent activation energy for 6082-T4 was calculated to be about 245 kJ mol⁻¹, which is higher than the activation energy for self-diffusion in Al (Q _d = 143 kJ mol⁻¹) and for the diffusion of Mg in Al (115–130 kJ mol⁻¹). By incorporating the threshold stress in the analysis, the true activation energy was calculated to be about 107 kJ mol⁻¹. Analysis of strain rate dependence in terms of the effective stress (σ − σ _o) using normalized parameters, revealed a single type of deformation behavior. A plot of normalized strain rate () versus normalized effective stress (σ − σ _o)/G, on a double logarithmic scale, gave an n value of 3. Ehab A. El-Danaf—on leave from the Department of Mechanical Design and Production, College of Engineering, Cairo University, Egypt.     

2219铝合金薄壁曲面件拉形过程变形均匀性EI北大核心CSCD

为改善2219铝合金薄壁拉形曲面件的变形均匀性,建立基于Hill 1990各向异性屈服准则的有限元模型,利用ABAQUS软件对曲面件的应变分布规律进行数值模拟,分析加载路径和板坯形状对拉形变形均匀性的影响规律。结果表明:加载路径和板坯形状对曲面件的变形均匀性有较大影响。采用折线路径,开始加载时使板材发生压缩失稳从而形成一定拱形,不仅可缓解左侧钳口附近的破裂倾向,还可增加曲面件右侧变形量,从而提高其变形均匀性。此外,减小变形量不足位置对应的板坯宽度,如采用中间窄板坯或左侧宽板坯,使其在拉形时所受应力增加,从而提高其变形量,也可实现变形均匀性的改善。最终,利用矩形板坯,经两次转折的实验路径进行拉形,获得了表面质量良好的高性能2219铝合金薄壁曲面件。     

Microstructure-based multistage fatigue modeling of aluminum alloy 7075-T651

The multistage fatigue model for high cycle fatigue of a cast aluminum alloy developed by McDowell et al. is modified to consider the structure-property relations for cyclic damage and fatigue life of a high strength aluminum alloy 7075-T651 for aircraft structural applications. The multistage model was developed as a physically-based framework to evaluate sensitivity of fatigue response to various microstructural features to support materials process design and component-specific tailoring of fatigue resistant materials. In this work, the model is first generalized to evaluate both the high cycle fatigue (HCF) and low cycle fatigue (LCF) regimes for multiaxial loading conditions, with appropriate modifications introduced for wrought materials. The particular microstructural features of relevance to fatigue in aluminum alloy 7075-T651 include micron-scale Fe-rich intermetallic particles and rolling textures. The model specifically addresses the role of local constrained cyclic microplasticity at fractured inclusions in fatigue crack incubation and microstructurally small crack growth, including the effect of crystallographic orientation on crack tip displacement as the driving force. The model is able to predict lower and upper bounds of the fatigue life based on measured inclusion sizes.     

Characterization of 2024-T3: An aerospace aluminum alloy

The 2024-T3 aerospace aluminum alloy, reported in this investigation, was acquired from a local aerospace industry: Royal Malaysian Air Force (RMAF). The heat treatable 2024-T3 aluminum alloy has been characterized by use of modern metallographic and material characterization techniques (e.g. EPMA, SEM). The microstructural characterization of the metallographic specimen involved use of an optical microscope linked with a computerized imaging system using MSQ software. The use of EPMA and electron microprobe elemental maps enabled us to detect three types of inclusions: Al–Cu, Al–Cu–Fe–Mn, and Al–Cu–Fe–Si–Mn enriched regions. In particular, the presence of Al₂CuMg (S-phase) and the CuAl₂ (θ′) phases indicated precipitation strengthening in the aluminum alloy.     

Effect of deformation temperature on microstructure evolution in aluminum alloy 2219 during hot ECAP

The effect of deformation temperature on microstructure evolution during equal channel angular pressing (ECAP) was studied in a coarse-grained aluminum alloy 2219 in a wide temperature interval from 250 to 475 °C. The structural changes taking place during ECAP up to strains of 12 are classified into the following three stages irrespective of deformation temperatures: i.e. (1) an incubation period for formation of the embryos of deformation bands (DBs) at low strains; (2) development of large-scale DBs followed by grain fragmentation at moderate strains; (3) rapid development of new grain at high strains. Microstructure development in stages 1 and 2 is hardly influenced by temperature, while that in stage 3 is most significantly affected at higher temperature. An increase in the pressing temperature leads to decreasing the volume fraction of new grains and increasing the average grain size in stage 3. This can be attributed to relaxation of strain compatibility between grains due to frequent operation of dynamic recovery and grain boundary sliding at higher temperature. The mechanism of grain refinement is discussed in detail.     

Surface effects in chromate conversion coatings on 2024-T3 aluminum alloy

Chromate conversion coatings formed on samples of 2024-T3 aluminum alloy, which had been given different pre-treatments, were examined by transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM) and corrosion tests. Two pre-treatments were considered, namely a simple mechanical polish, and polishing followed by an etch in a HF-H₂SO₄ solution. The latter treatment leads to significant Cu enrichment at the oxide-alloy interface, and this in turn can lead to a deleterious effect on the corrosion protection afforded by a subsequently applied chromate coating. Discussions are given of the behaviour of Cu in the coating formed on the sample that received an acid etch in the pre-treatment. This involves both migration through the coating and a non-uniform redeposition of Cu on to the coating surface. By contrast, the sample that initially was given just the mechanical polish in the pre-treatment does not show a Cu enrichment in the surface region, and the subsequently applied coating appeared stable after a 24 h immersion in a NaCl test solution.     

Reasons for superior mechanical and corrosion properties of 2219 aluminum alloy electron beam welds

Electron beam welds of aluminum alloy 2219 offer much higher strength compared to gas tungsten arc welds of the same alloy and the reasons for this have not been fully explored. In this study both types of welds were made and mechanical properties were evaluated by tensile testing and pitting corrosion resistance by potentio dynamic polarization tests. It is shown that electron beam welds exhibit superior mechanical and corrosion properties. The weld metals have been characterized by scanning electron microscopy; transmission electron microscopy and electron probe micro analysis. Presence of partially disintegrated precipitates in the weld metal, finer micro porosity and uniform distribution of copper in the matrix were found to be the reasons for superior properties of electron beam welds apart from the fine equiaxed grain structure. Transmission electron micrographs of the heat affected zones revealed the precipitate disintegration and over aging in gas tungsten arc welds.     

Microstructural evolution in friction self-piercing riveted aluminum alloy AA7075-T6 joints

Friction self-piercing riveting(F-SPR)is an emerging technique for low ductility materials joining,which creates a mechanical and solid-state hybrid joint with a semi-hollow rivet.The severe plastic deforma-tion of work materials and localized elevated temperatures during the F-SPR process yield complex and heterogeneous microstructures.The cut-off action of the work materials by the rivet further compli-cates the material flow during joint formation.This study employed the F-SPR process to join AA7075-T6 aluminum alloy sheets and systematically investigated the microstructural evolutions using electron backscatter diffraction(EBSD)techniques.The results suggested that as the base material approached the rivet,grains were deformed and recrystallized,forming two distinct fine grain zones(FGZs)surround-ing the rivet and in the rivet cavity,respectively.Solid-state bonding of aluminum sheets occurred in the FGZs.The formation of FGZ outside the rivet is due to dynamic recrystallization(DRX)triggered by the sliding-to-sticking transition at the rivet/sheet interface.The FGZ in the rivet cavity was caused by the rotation of the trapped aluminum,which created a sticking affected zone at the trapped aluminum/lower sheet interface and led to DRX.Strain rate gradient in the trapped aluminum drove the further expansion of the sticking affected zone and resulted in grain refinement in a larger span.     

Load interaction effects on fatigue crack propagation in 2024-T3 aluminum alloy

Variable amplitude fatigue studies have been conducted within a linear-elastic fracture mechanics framework in order to systematically examine the effect of complex loading on fatigue crack retardation in 2024-T3 aluminum alloy. Complex loading conditions were simulated by introducing a second tensile or compressive peak load after the crack had extended various distances, a', into the region affected by a previously applied high load excursion.

Maximum interaction between single peak overloads resulted when the two peak load cycles were separated by a small distance, a' _min, where the fatigue crack propagation rate resulting from a single overload reached a minimum. This behavior was attributed in part to interference of tensile displacements produced during the first peak load cycle which was verified from fractographic observations. Crack growth rate retardation was related also to the development of a favorable compressive residual stress at the crack tip. Peak loads were found to act as completely isolated events only when they were separated by a distance approximately three times the plastic zone size resulting from a single overload. Comparable findings resulted when 10 cycle block overloads were employed in place of single peak excursions.

When a single peak overload was followed by a compressive cycle, retardation was found to decrease to a minimum; however, when the loading sequence was reversed, the effect was less damaging. In addition, as the distance between positive and negative peak loads was increased, the number of delay cycles quickly approached that associated with a single high load excursion.     

A study of fatigue crack growth of 7075-T651 aluminum alloy

Both standard and non-standard compact specimens were employed to experimentally study the crack growth behavior of 7075-T651 aluminum alloy in ambient air. The effects of the stress ratio (R), overloading, underloading, and high–low sequence loading on fatigue crack growth rate were investigated. Significant R-ratio effect was identified. At the same R-ratio, the influence of specimen geometry on the relationship between crack growth rate and stress intensity factor range was insignificant. A single overload retarded the crack growth rate significantly. A slight acceleration of crack growth rate was identified after a single underload. The crack growth rate resumed after the crack propagated out of the influencing plastic zone created by the overload or underload. A parameter combining the stress intensity factor range and the maximum stress intensity factor can correlate the crack growth at different stress ratios well when the R-ratio ranged from −2 to 0.5. The parameter multiplied by a correction factor can be used to predict the crack growth with the influence of the R-ratio, overloading, underloading, and high–low sequence loading. Wheeler’s model cannot describe the variation of fatigue crack growth with the crack length being in the overload influencing zone. A modified Wheeler’s model based on the evolution of the remaining affected plastic zone was found to predict well the influence of the overload and sequence loading on the crack growth.