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.     

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.     

Microstructure and tensile properties of friction welded aluminum alloy AA7075-T6

Solid-state welding processes like friction welding and friction stir welding are now being actively considered for welding aluminum alloy AA7075. In this work, friction welding of AA7075-T6 rods of 13 mm diameter was investigated with an aim to understand the effects of process parameters on weld microstructure and tensile properties. Welds made with various process parameter combinations (incorporating Taguchi methods) were subjected to tensile tests. Microstructural studies and hardness tests were also conducted. The results show that sound joints in AA7075-T6 can be achieved using friction welding, with a joint efficiency of 89% in as-welded condition with careful selection of process parameters. The effects of process parameters are discussed in detail based on microstructural observations.     

Corrosion-fatigue behaviour of 7075-T651 aluminum alloy subjected to periodic overloads

The corrosion-fatigue behaviour of 7075-T651 aluminum alloy subjected to periodic overloads was examined. This aluminum alloy is typically used in aerospace structural components such as the wing spars of aircraft. Axial fatigue specimens were subjected to a loading spectrum that consisted of a fully reversed periodic overload of near-yield magnitude followed by 200 smaller cycles at high R-ratio. The specimens were fatigue tested while they were fully immersed in an aerated and recirculated 3.5 wt% NaCl simulated seawater solution.The results for the corrosion-fatigue testing were compared to data obtained for the same overload spectrum applied in laboratory air. A damage analysis showed that the presence of the corrosive environment accelerated the damage accumulation rate to a greater extent than that observed in air, particularly at low stress ranges. This resulted in a reduction in the fatigue strength of the material when it was simultaneously subjected to overloads and a corrosive environment. It is believed that the reduced fatigue life was due primarily to corrosion pit formation and a combination of anodic dissolution at the crack tip and hydrogen embrittlement. For practical purposes, the endurance-limit of the material disappears under these conditions.     

Environmental effects on low cycle fatigue of 2024-T351 and 7075-T651 aluminum alloys

The environmental effects on the low cycle fatigue (LCF) behavior of 2024-T351 and 7075-T651 aluminum alloys were studied at room temperature. The specimens were subjected to identical LCF tests at strain ratio R of −1 and frequency of 5 Hz in three environments: vacuum, air and 1% NaCl solution of pH 2. A separate group of specimens was pre-corroded in 1% NaCl solution and then LCF-tested in air. Their strain–life relations and cyclic stress–strain responses were investigated and compared. Furthermore, the fracture surface morphology was evaluated to find the association of LCF behavior and fractographic features under different environmental conditions.     

Fatigue of 7075-T651 aluminum alloy under constant and variable amplitude loadings

The fatigue crack growth (FCG) behavior of 7075-T651 aluminum alloy was studied under constant and variable amplitude loadings in vacuum, air and 1% NaCl solution. In the study of constant amplitude loading fatigue, the stress ratios were 0.1 and 0.85 and the loading frequency was 10 Hz. In the study of variable amplitude loading fatigue, the load spectrums were tension type and tension–compression type, and the average loading frequency was about 5 Hz. The results of FCG tests, under constant and variable amplitude loadings, validated the unified two parameter driving force model, accounting for the residual stress and stress ratio effects on fatigue crack growth.     

Effect of tool pin eccentricity on microstructure and mechanical properties in friction stir welded 7075 aluminum alloy thick plate

Four different tools with the pin eccentricity of 0.1 mm, 0.2 mm, 0.3 mm and 0.4 mm were designed to friction stir weld 10 mm thick AA7075-O plate. The effect of pin eccentricity on microstructure, secondary phase particles transformation and mechanical properties of the joints was investigated. The results show that the nugget area (A_NZ) increases firstly and then decreases with increasing the pin eccentricity. When the pin with 0.2 mm eccentricity is applied, the A_NZ is the largest; meanwhile the grains size is the smallest which is about 3 μm and secondary phase particles are the most dispersive in nugget zone compared with other tools. While the grains are coarsened to 7–11 μm as the eccentricity is more than 0.4 mm, some coarse hardening particles get to cluster in the thermo-mechanically affected zone. The joints produced by the pin with 0.2 mm eccentricity perform the highest tensile strength and elongation, which is attributed to better interfaces, finer grains and more dispersive secondary phase particles.     

Microstructural characterization of Al-7075-T651 chips and work pieces produced by high-speed machining

High-speed orthogonal machining produced a series of Al-7075-T651 chips and work pieces for microstructural charcterization. The orthogonal machining conditions included two rake angles, three cuttings speeds, and a constant feed rate. The chips and work pieces were characterized using optical metallography, transmission electron microscopy (TEM), and microhardness measurements. TEM observations indicated growth of the η′ precipitates and showed recrystallized equiaxed grains within the shear bands of the chips. Microhardness profiles of the work pieces showed a decrease in hardness at the work surface. The thickness of this deformation layer increased with cutting speed. Microstructural characterizations were correlated with calculated temperature–time profiles to investigate possible mechanisms for the observed microstructure changes.     

Damage modelling for a laminated ceramic composite

A general theory of damage mechanics is applied to a fibre-reinforced ceramic matrix composite, SiC/MAS-L, in order to describe its non-linear mechanical behaviour up to failure. This study is limited to the case of quasi-monotonic loading at room temperature. The model, which uses an anisotropic damage theory previously applied to other composites, is a mesoscopic-scale model which has been developed using tension–compression tests on different stacking sequences and applies in the case of multiaxial loading. It includes the marked differences observed between mechanical behaviour in tension and in compression, and is also able to predict the failure values.     

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.     

Thickness direction inhomogeneity of mechanical properties and fracture toughness as observed in aluminium 7075-T651 plate material

The variation of the mechanical properties, hardness and fracture toughness in the thickness direction of an aluminium 7075-T651 plate with a thickness of 25.4 mm is investigated. The results indicate that these properties show a distinct inhomogeneity in this direction. The influence of this inhomogeneity on fatigue crack growth rate and crack closure is investigated also. Explanations for this inhomogeneous behaviour are discussed, including observations on inhomogeneity of grain size and chemical composition.     

Comparative study on local and global mechanical properties of bobbin tool and conventional friction stir welded 7085-T7452 aluminum thick plate

7085-T7452 plates with a thickness of 12 mm were welded by conventional single side and bobbin tool friction stir welding (SS-FSW and BB-FSW, respectively) at different welding parameters. The temperature distribution, microstructure evolution and mechanical properties of joints along the thickness direction were investigated, and digital image correlation (DIC) was utilized to evaluate quantitatively the deformation of different zones during tensile tests. The results indicated that heat-affected zone (HAZ), the local softening region, was responsible for the early plastic deformation and also the fracture location for SS-FSW samples, while a rapid fracture was observed in weld nugget zone (WNZ) before yield behavior for all BB-FSW specimens. The ultimate tensile strength (UTS) of SS-FSW joints presented the highest value of 410 MPa, 82% of the base material, at a rotational speed of 300 rpm and welding speed of 60 mm/min, much higher than that of BB-FSW joints, with a joint efficiency of only 47%. This should be attributed to the Lazy S defect produced by a larger extent of heat input during the BB-FSW process. The whole joint exhibited a much higher elongation than the slices. Scanning electron microscopic (SEM) analysis of the fracture morphologies showed that joints failed through ductile fracture for SS-FSW and brittle fracture for BB-FSW.     

Fatigue life variability due to variations in interference fit of steel bushings in 7075-T651 aluminum lugs

Fatigue of metallic components is a concern in rotary wing aircraft. There exists a potential for loss of an aircraft if a component fatigue life does not take into account all factors than can cause life variation. Much literature was located dealing with the impact of large interference fits and cold working. Literature covering small changes was lacking. This study focused on small changes in interference such as those that could come from a tolerance on a drawing. Testing was completed on 7075-T651 Aluminum alloy lugs with steel bushings of varying interference fit. Testing three different levels of interference fit revealed three different S–N curves even though the variation from fit to fit was small. Significant improvements can be achieved in the S–N curve simply by small changes in interference. From a safety point of view these changes should be looked at as potential reductions.     

Quasi-brittle fracture during structural impact of AA7075-T651 aluminium plates

The stress–strain behaviour of the aluminium alloy 7075 in T651 temper is characterized by tension and compression tests. The material was delivered as rolled plates of thickness 20 mm. Quasi-static tension tests are carried out in three in-plane directions to characterize the plastic anisotropy of the material, while the quasi-static compression tests are done in the through-thickness direction. Dynamic tensile tests are performed in a split Hopkinson tension bar to evaluate the strain-rate sensitivity of the material. Notched tensile tests are conducted to study the influence of stress triaxiality on the ductility of the material. Based on the material tests, a thermoelastic–thermoviscoplastic constitutive model and a ductile fracture criterion are determined for AA7075-T651. Plate impact tests using 20 mm diameter, 197 g mass hardened steel projectiles with blunt and ogival nose shapes are carried out in a compressed gas-gun to reveal the alloy's resistance to ballistic impact, and both the ballistic limit velocities and the initial versus residual velocity curves are obtained. It is found that the alloy is rather brittle during impact, and severe fragmentation and delamination of the target in the impact zone are detected. All impact tests are analysed using the explicit solver of the non-linear finite element code LS-DYNA. Simulations are run with both axisymmetric and solid elements. The failure modes are seen to be reasonably well captured in the simulations, while some deviations occur between the numerical and experimental ballistic limit velocities. The latter is ascribed to the observed fragmentation and delamination of the target which are difficult to model accurately in the finite element simulations.     

Microstructural aspects of crack nucleation during cyclic loading of AA7075-T651

A series of fatigue test samples made of 7075-T651 aluminum were interrupted at various life fractions and the number of debonded, cracked particles and cracks in the metal matrix was determined quantitatively as a function of load cycles. It was found that only cracked constituent particles nucleate a matrix crack. The crystallography of one individual crack and its three-dimensional shape was determined by serial sectioning in a scanning electron microscope by applying focused ion beam (FIB) milling in combination with orientation imaging microscopy (OIM). The limited data suggest that the initial growth direction of a crack is influenced by the crystallographic orientation of the matrix into which the crack is growing.     

7075-T651铝合金喷丸工艺的仿真与试验研究

为探究喷丸工艺参数对材料残余应力分布的影响,以7075-T651铝合金材料为研究对象,基于铝合金材料特性和试验条件,利用ANSYS/LS-DYNA建立喷丸有限元模型,获得了不同喷射距离、喷射压力和弹丸直径下残余应力的分布情况。结果表明：增大喷射压力和弹丸直径可以显著提高最大残余压应力和残余压应力影响深度,且喷射距离应保持在适当的范围内,最后将应力计算值和实测值进行对比,最大偏差为(10.0±12.5) MPa,偏差率不超过6.9%。分析认为通过有限元模型能够准确地获得残余应力的分布情况,有助于开展喷丸工艺参数对残余应力分布规律的影响研究。     

Fracture mechanisms of aluminium alloy AA7075-T651 under various loading conditions

The fracture behaviour of the aluminium alloy AA7075-T651 is investigated for quasi-static and dynamic loading conditions and different stress states. The fracture surfaces obtained in tensile tests on smooth and notched axisymmetric specimens and compression tests on cylindrical specimens are compared to the fracture surfaces that occur when a projectile, having either a blunt or an ogival nose shape, strikes a 20 mm thick plate of the aluminium alloy. The stress state in the impact tests is much more complex and the strain rate significantly higher than in the tensile and compression tests. Optical and scanning electron microscopes are used in the investigation. The fracture surface obtained in tests with smooth axisymmetric specimens indicates that the crack growth is partly intergranular along the grain boundaries or precipitation free zones and partly transgranular by void formation around fine and coarse intermetallic particles. When the stress triaxiality is increased through the introduction of a notch in the tensile specimen, delamination along the grain boundaries in the rolling plane is observed perpendicular to the primary crack. In through-thickness compression tests, the crack propagates within an intense shear band that has orientation about 45° with respect to the load axis. The primary failure modes of the target plate during impact were adiabatic shear banding when struck by a blunt projectile and ductile hole-enlargement when struck by an ogival projectile. Delamination and fragmentation of the plates occurred for both loading cases, but was stronger for the ogival projectile. The delamination in the rolling plane was attributed to intergranular fracture caused by tensile stresses occurring during the penetration event.     

Fracture of laminates combining 2024-T3 and 7075-T6 aluminum alloys

Crack growth resistance curves have been determined for crack-divider laminates in which layers of 2024-T3 aluminum alloy are adhesively bonded to layers of 7075-T6 alloy. Results are compared with the fracture resistance of laminates consisting wholly of each material, the layer thickness being the same (1.54 mm) in all cases. The initial portions of the resistance curves are similar for both alloys; however those for 2024-T3 have steeper slopes at longer effective crack lengths. As a result, laminates consisting entirely of 2024-T3 alloy exhibit greater amounts of stable crack extension and higher toughnesses at instability. This is attributed in part to the greater strain hardening rate in 2024-T3 material. Laminates combining 2024-T3 and 7075-T6 layers are intermediate between those consisting entirely of one or the other alloy.