Al 6061-T6 Research Articles

Effect of thermal tempering on microstructure and mechanical properties of Mg-AZ31/Al-6061 diffusion bonding

The objective of this study is to investigate the effect of the types thermal tempering of aluminum alloy on microstructure and mechanical properties of AZ31-O Mg and Al 6061-T6 diffusion bonding. Using Optical Microscope (OM) and Scanning Electron Microscopes (SEM) equipped with EDS analysis and line scan the interfaces of joints were evaluated. The XRD analysis was carried out to characterize phase constitution near the interface zone. The mechanical properties of joints were measured using Vickers micro-hardness and shear strength. According to the results in bonding of AZ31-Mg/Al-6061-O, in less plastic deformation in magnesium alloy, diffusion rate of most magnesium atoms occurred to aluminum alloy and formation of diffusion zone with minimum micro-hardness (140 HV) and maximum shear strength (32MPa) compared to Al 6061-T6/Mg-AZ31 bonding. Evaluation of fracture surfaces indicates an occurrence of failure from the brittle intermetallic phases.

Effect of jet velocity in co-flow water cavitation jet peening

Water cavitation jet peening (WCP) uses cavitation caused by the shear layer created by two concentric co-flowing jets with large velocity difference to introduce compressive residual stresses in the surface layers of metal components subjected to fatigue loading or a corrosive environment. Mass loss and surface alteration in WCP have been shown to be minimal compared to other mechanical surface enhancement techniques, such as shot peening (SP). This paper investigates the effect of concentric jet velocities in cavitation jet peening in a co-flow configuration on cavitation intensity and peening performance, which are characterized by accelerated erosion on Al 1100-O and Al 7075-T6 and a strip curvature test on Al 7075-T6. Accelerated erosion tests reveal that cavitation intensity and associated erosion (measured by mass loss) are greatly affected by the combination of the inner (Vin) and outer (Vout) jet velocities and the normalized standoff distance (sn). Two characteristic erosion patterns are found depending on the relative magnitudes of the jet velocities: one that is focused at the jet center (termed center regime) and another that is concentrated in the surrounding annular region (termed ring regime). Erosion tests on Al 1100-O and Al 7075-T6 give unexpectedly different results in terms of the maximum mass loss as a function of the jet velocities and standoff distance. When compared to strip curvature tests, it is found that the accelerated erosion tests on Al 1100-O do not capture the influence of inner jet velocity Vin and imply misleading trends with regard to outer flow velocity Vout. Erosion and curvature tests on Al 7075-T6 are found to be in good agreement and therefore are believed to be better suited to identify the optimum process conditions in WCP. Notwithstanding the higher mass loss density values observed in the center regime, the resultant strip curvature is found to be higher in the ring regime for a higher inner jet velocity Vin, potentially leading to higher and deeper compressive residual stresses.

Elastic-plastic fracture analysis of notched Al 7075-T6 plates by means of the local energy combined with the equivalent material concept

The main goal of the present research is to analyze tensile fracture in Al 7075-T6 thin plates weakened by blunt V-notches. For this purpose, first, 27 fracture tests are carried out on rectangular plates containing a central rhombic hole with two blunt V-shaped corners horizontally located. The experimental observations indicated that a plastic region initiates from the notch tip and grows as the tensile load monotonically increases, and finally, fracture happens suddenly with a significant opening of the notch tip. By showing significant plastic deformations around the notch tip and also inclined fracture planes, the specimens after fracture confirm well the ductile rupture in V-notched Al 7075-T6 plates. As the main experimental result, the load-carrying capacity of the notched plates corresponding to the onset of crack initiation from the notch tip is recorded. To theoretically predict the experimental results, the equivalent material concept is utilized together with the well-known brittle fracture criterion, namely the averaged strain energy density criterion. Without requiring elastic-plastic finite element analysis, it is shown that the combination of the averaged strain energy density and equivalent material concept is successful in predicting the load-carrying capacity of the V-notched Al 7075-T6 plates that fail by moderate-scale yielding regime.

Numerical Simulation of Friction Stir Spot Welding

Friction Stir Spot Welding (FSSW) is a solid state joining process that relies on frictional heating and plastic deformation realized at the interaction between a non-consumable welding tool that rotates on the contact surfaces of the workpieces. Friction Stir Spot Welding (FSSW) is an evolving technique that has received considerable attention from automotive industries to replace electric resistance spot welding, which shows poor weldability for advanced high-strength steels as well as aluminium alloys. Because of the interest shown by the industry towards this process, an attempt to optimize it is imperative. But the experiments are often time consuming and costly. To overcome these problems, numerical analysis has frequently been used in the last years. The purpose of this paper is to develop a three-dimensional fully coupled thermal-stress finite element (FE) model of FSSW process for thin aluminium alloy Al 6061-T6. Numerical simulation being helpful for better understanding and observation of the influence of input parameters on the resulting phenomena. It is described the algorithm and are presented the activities needed to be performed in order to develop a valid numerical model for FSSW. The validation of the numerical model being achieved by comparing the resulted temperatures from the numerical simulation with the experimentally determined temperatures for the same material

Numerical and experimental analysis of particle fracture during solid particle erosion, Part II: Effect of incident angle, velocity and abrasive size

The accompanying paper presented and verified a numerical model to predict the fracture of silicon carbide particles impacting Al 6061-T6 aluminum alloy. In this work, further experiments under conditions that were less likely to cause fragmentation, and for a smaller particle size, confirmed the model׳s predictive capabilities. The model and double pulsed laser shadowgraphy were then used to study the effect of velocity, angle of attack, and particle size on particle fragmentation and rebound kinematics. The predicted percentage of incident particles that fractured was found to correlate with the average particle size after impact. Increases in both the incident particle velocity and the angle of attack were found to increase the propensity for particle fragmentation because they both increased energy transfers perpendicular to the surface. Even at the lowest considered impact angle (30°) and velocity (46m/s), the average particle size after, compared to before impact, still decreased by 11%. A threshold incident kinetic energy perpendicular to the surface was found to exist below which no particles fractured. The average shape (roundness) of the particles changed by no more than 3.3% due to impact under all tested conditions. At a given average incident velocity, larger particles were found more likely to fracture upon impact due to their higher kinetic energy, consistent with observations from the literature. However, at a given incident kinetic energy, the numerical models predicted the smaller particles were more likely to fracture, due to an increased strain localization. Increases in the impact angle or incident velocity generally resulted in a wider range of rebound angles. The ratio of rebound to incident velocity did not depend much on velocity at a given angle, although at a given velocity, decreases in impact angle from 60° to 30° resulted in up to a 1.76 times increase in this ratio. The greater understanding of particle fragmentation provided by this study may have important applications in terms of assessing interference between incoming and rebound particles, the re-use of blasted media in blast cleaning operations, and optimizing removal rates in abrasive jet machining operations.

Investigation into the effects of tangential force on fretting fatigue based on XFEM

Tangential force is one of the most important factors which affect fretting fatigue. In the present paper, the effects of tangential force on fretting fatigue of Al 2024-T351 alloy specimens are investigated by employing extended finite element method combined with cyclic cohesive zone model. It is shown that increasing tangential force can increase the depth of knee point. However, crack initiation life, crack growth life and total fatigue life decrease with the increase of tangential force. In addition, tangential force influences the percentage of crack initiation life and crack growth life in total fatigue life.

Analytical and numerical studies of approximate phase velocity matching based nonlinear S0 mode Lamb waves for the detection of evenly distributed microstructural changes

Most previous studies on nonlinear Lamb waves are conducted using mode pairs that satisfying strict phase velocity matching and non-zero power flux criteria. However, there are some limitations in existence. First, strict phase velocity matching is not existed in the whole frequency bandwidth; Second, excited center frequency is not always exactly equal to the true phase-velocity-matching frequency; Third, mode pairs are isolated and quite limited in number; Fourth, exciting a single desired primary mode is extremely difficult in practice and the received signal is quite difficult to process and interpret. And few attention has been paid to solving these shortcomings. In this paper, nonlinear S0 mode Lamb waves at low-frequency range satisfying approximate phase velocity matching is proposed for the purpose of overcoming these limitations. In analytical studies, the secondary amplitudes with the propagation distance considering the fundamental frequency, the maximum cumulative propagation distance (MCPD) with the fundamental frequency and the maximum linear cumulative propagation distance (MLCPD) using linear regression analysis are investigated. Based on analytical results, approximate phase velocity matching is quantitatively characterized as the relative phase velocity deviation less than a threshold value of 1%. Numerical studies are also conducted using tone burst as the excitation signal. The influences of center frequency and frequency bandwidth on the secondary amplitudes and MCPD are investigated. S1–S2 mode with the fundamental frequency at 1.8 MHz, the primary S0 mode at the center frequencies of 100 and 200 kHz are used respectively to calculate the ratios of nonlinear parameter of Al 6061-T6 to Al 7075-T651. The close agreement of the computed ratios to the actual value verifies the effectiveness of nonlinear S0 mode Lamb waves satisfying approximate phase velocity matching for characterizing the material nonlinearity. Moreover, the ratios derived from the primary and secondary horizontal displacements generated from nonlinear S0 mode Lamb waves are closest to the real value, which indicates that using horizontal displacements is more suitable for detecting evenly distributed microstructural changes in large thin plate-like structure. Successful application to evaluating material at different levels of evenly distributed fatigue damage is also numerically conducted.

Comparative Study of Influence Post weld Aging on the Mechanical Properties of Similar Friction Stir Welded Joints for Al 2024-T3 and Al 7075-T73

The aim of present study is to investigate the effect of precipitation hardening heat treatment at different aging conditions on the mechanical properties and microstructure of similar friction stir welding (FSW) joints ofAA 2024-T3 and AA 7075-T73 aluminum alloys. Friction stir welding was carried out using milling machine with best welding parameters (tool rotation speed 898 rpm, welding speed 45 mm ̸ min and threaded cylindrical pin geometry). Tensile test, microhardness test,microstructure examination,and X-ray radiographic inspection of FSW joints were made.The results indicated that the best aging conditions for similar welded joints of Al2024 and Al 7075 were in sample (natural aging for two week) and sample (artificial ageing at 120°C for 24 h) respectively. ‏Microhardness in the as-welded samples showed fluctuations across the weld zone and minimum hardnessoccurs in the heat affected zone(HAZ) in both alloys while the hardness after heat treatment at best aging conditions was improved across the whole weldment and increased in stir zone,

Surface Characterizations of Fretting Fatigue Damage in Aluminum Alloy 7075-T6 Clamped Joints: The Beneficial Role of Ni-P Coatings.

This paper aims to characterize the surface damage as a consequence of fretting fatigue in aluminum alloy 7075-T6 plates in double-lap bolted joints through XRD, surface profilometry, and SEM analyses. The main focus was on the surface roughness and chemical phase composition of the damaged zone along with the identification of fretting fatigue crack initiations over the surface of the material. The surface roughness of the fretted zone was found to increase when the joint was clamped with a higher tightening torque and tested under the same cyclic loading. Additionally, MgZn2 (η/ή) precipitates and ZnO phase were found to form onto the surface of uncoated aluminum plate in the fretted and worn zones. The formation of the ZnO phase was understood to be a result of frictional heat induced between the surface of contacting uncoated Al 7075-T6 plates during cyclic loading and exposure to the air. The beneficial role of electroless nickel-phosphorous (Ni–P) coatings in minimizing the fretting damage and thus improving the fretting fatigue life of the aluminum plates was also studied. The results showed that the surface roughness decreased by approximately 40% after applying Ni–P coatings to the Al 7075-T6 plates.

On the deformation and failure of Al 6061-T6 at low triaxiality evaluated through in situ microscopy

We investigate deformation and failure of Al 6061-T6 at low triaxiality conditions through in situ scanning electron microscopy. The global behavior of the specimen, as well as the local deformations of the matrix material, second phase particles, and preexisting voids, are observed with a combination of high temporal/low spatial resolution images and low temporal/high spatial resolution images. It is found that the matrix dominates the deformation response, with the second phase particles and voids imparting little influence on the deformation. Tracking of second phase particles is used to determine the local strain fields averaged over a suitable gage length and to correlate to the macroscopic behavior.

Structure and Corrosion Behavior of Sputter Deposited Ce-Al-O Coating on Al 2024-T3 Alloy Substrates

Deposition of Ce-Al-O coatings on Al 2024-T3 alloy substrates was conducted by magnetron co-sputtering of CeO2 and Al targets. The incorporation of Al into the CeO2 film resulted in an amorphous or nanocrystalline structure with binding energy states different from Al, Al2O3, and CeO2. Electrochemical measurements indicated that the amorphous or nanocrystalline Ce-Al-O coating increased the corrosion resistance of bare Al 2024-T3 substrates by more than three orders of magnitude, and exhibited a very different cathodic and anodic protective mechanism compared to Al and CeO2 coated and as-polished Al 2024-T3 samples. The oxygen reduction cathodic reactions on the Al alloy surface have been significantly suppressed, and the tendency for pitting corrosion of Al alloy substrate has been significantly reduced by the Ce-Al-O coating. The improved corrosion performance is attributed to the unique microstructure and the resulting interaction and redox reactions of the Ce, Al, and O atoms within the Ce-Al-O coatings.

Fabrication of Al7075 / B4C Surface Composite by Novel Friction Stir Processing (FSP) and Investigation on Wear Properties

Friction stir processing (FSP), a process, derived from the friction stir welding (FSW) process, is an emerging novel, green and energy efficient processing technique to fabricate surface composite. In the present investigation, FSP technique has been used for fabrication of surface composites, using aluminium 7075 as parent metal and Boron Carbide (B4C) powder particles as reinforcement., Aluminium 7075 has been selected as matrix phase, as being widely used by automotive and aerospace application and having the highest strength among all commercial Al alloys. In present paper, details about the fabrication of Al 7075-T651- B4C surface composite for various combination of tool rotation, tool travel speed and number of passes have been discussed. The same being intended to improve hardness and thereby wear resistance. The fabricated surface composites are examined for microstructure using image analyser, and found friction stir processed zone with a few defects. It is also observed that the average hardness of friction stir processed surface composite was 40 – 70% higher than that of parent metal (75 – 80 HV). Wear Resistance is found to be improved by 100% compared to parent metal. The increase in same is attributed to B4C particles dispersed in aluminium matrix and grain strengthening mechanism.

Fine tuning of dwelling time in friction stir welding for preventing material overheating, weld tensile strength increase and weld nugget size decrease

After successful welding, destructive testing into test samples from Al 2024-T351 friction stir butt welds showed that tensile strength of the weld improve along the joint line, while dimensions of the weld nugget decrease. For those welds, both the base material and the welding tool constantly cool down during the welding phase. Obviously, the base material became overheated during the long dwelling phase what made conditions for creation of joints with the reduced mechanical properties. Preserving all process parameters but varying the dwelling time from 5-27 seconds a new set of welding is done to reach maximal achievable tensile strength. An analytical-numerical-experimental model is used for optimising the duration of the dwelling time while searching for the maximal tensile strength of the welds

Investigation of mixed mode-I/II fracture problems - Part 2: evaluation and development of mixed mode-I/II fracture criteria

In this study, experimental and numerical results of compact tension shear (CTS) specimen and a new specimen type under in-plane mixed mode (Mode-I/II) loading conditions are compared with existing inplane mixed mode fracture criteria to investigate and understand the nature of fracture behavior properly. The material used in numerical and experimental analyses is Al 7075-T651 aluminum machined from rolled plates in the L-T rolling direction (crack plane is perpendicular to the rolling direction). In Part 1 of the study, results from numerical and experimental analyses are given. Having computed the mixed mode stress intensity factors from the numerical analyses, fracture loads are predicted and compared with different mixed mode-I/II fracture criteria. The experimental and numerical results show that many criteria are in good agreement with each other for predominately mode I to moderate mixed mode conditions. However, existing criteria increasingly differ from the experimental measurements for highly mode-II conditions. Using the computational and experimental results obtained, improved empirical mixed mode I/II fracture criteria for fracture condition and angle are also proposed.

Investigation of mixed mode - I/II fracture problems - Part 1: computational and experimental analyses

In this study, to investigate and understand the nature of fracture behavior properly under in-plane mixed mode (Mode-I/II) loading, three-dimensional fracture analyses and experiments of compact tension shear (CTS) specimen are performed under different mixed mode loading conditions. Al 7075-T651 aluminum machined from rolled plates in the L-T rolling direction (crack plane is perpendicular to the rolling direction) is used in this study. Results from finite element analyses and fracture loads, crack deflection angles obtained from the experiments are presented. To simulate the real conditions in the experiments, contacts are defined between the contact surfaces of the loading devices, specimen and loading pins. Modeling, meshing and the solution of the problem involving the whole assembly, i.e., loading devices, pins and the specimen, with contact mechanics are performed using ANSYSTM. Then, CTS specimen is analyzed separately using a submodeling approach, in which three-dimensional enriched finite elements are used in FRAC3D solver to calculate the resulting stress intensity factors along the crack front. Having performed the detailed computational and experimental studies on the CTS specimen, a new specimen type together with its loading device is also proposed that has smaller dimensions compared to the regular CTS specimen. Experimental results for the new specimen are also presented.

Failure Analysis of Fasteners in a Remotely Operated Vehicle (ROV) System

A failure analysis study was carried out on AISI 204 Cu stainless steel full-threaded fasteners and end-threaded SS 316L fasteners used in a deep water work class ROV system. The AISI 204 Cu stainless steel full-threaded fasteners were used in a Ti–6Al–4V alloy pressure case. The end-threaded SS 316L fasteners were used for attaching the bottom fender in the tether management system (TMS) made of Al 6061-T6 alloy. The mechanical property of both the fasteners was found to be within standard specification. The SEM study of the full-threaded fasteners revealed that the corrosion pit formation in the unfractured interface area is the cause of crack initiation. Subsequently, the crack propagated by stress corrosion cracking and finally, failed by fast overload cleavage fracture. It can be reasoned that a lower wt.% of molybdenum in AISI 204 Cu stainless steel reduced the pitting corrosion resistance of the full-threaded fasteners. The end-threaded fastener failed by stress corrosion cracking, but corrosion pit was not found in the fracture surface, due to the lack of its interface area with the unfractured region. It can be inferred that the chlorine ions of seawater break the oxide film of stainless steel bolts to form corrosion pits. These corrosion pits act as a stress concentration point to initiate a crack, and subsequently the bolts fail by stress corrosion cracking, showing a brittle appearance.

Numerical Study of the Effects of Residual Stress on Fretting Fatigue Using XFEM.

Residual compressive stress can improve fretting fatigue strength. In this paper, the effects of residual stress on fretting fatigue of Al 2024-T351 alloy specimens are studied using a numerical approach. The extended finite element method combined with the cyclic cohesive zone model is adopted to model fretting fatigue crack growth behavior. It is shown that residual stress changes the fretting fatigue crack growth path and enhances fretting fatigue life. Crack initiation angle, depth of knee point, crack initiation life, crack propagation life and total life are greater for specimens with residual stress compared to specimens without residual stress. The effects of residual stress are more remarkable for specimens with a high intensity of residual stress. However, the effects of residual stress reduce at a high bulk load level.

Improved surface integrity from cryogenic machining of Al 7050-T7451 alloy with ultrafine-grained structure

Materials with ultrafine-grained (UFG) structure become the prioritised selections for specific applications where high strength, hardness, wear/corrosion resistance, etc. are sought. Machining materials with UFG structure requires further research as their mechanical and material properties are different from the ones with regular coarse grains. In this study, Al 7050-T7451 aerospace alloy was first processed by friction stir processing (FSP) with UFG structure for use in subsequent machining experiments. Cryogenic machining using liquid nitrogen as the coolant was applied in the experimental work of machining Al 7050-T7451 alloy with UFG structure. Surface integrity characteristics including microstructure, grain size and hardness were investigated in order to understand the differences of these properties induced by dry and cryogenic machining. Smaller grains with the grain size of around 1.6 μm were observed in the machined surface generated by cryogenic machining compared to the grains of bulk materials and the ones acquired by dry machining. On the other hand, higher hardness measured from the surface and subsurface regions of the machined samples show a harder layer formation by cryogenic machining. Forces and temperatures were measured during dry and cryogenic machining. Cutting and thrust force components for both dry and cryogenic condition were found to be almost identical due to the homogeneous UFG structure achieved by the previous FSP while cryogenic machining generated lower cutting temperature. Also, microstructure and hardness of the chips formed by machining were investigated to understand the effects of cryogenic machining on materials with UFG structure from the dynamic recrystallisation and grain growth points of view. Grains with approximately 1 μm grain size existed in the machined chips, and the hardness of the chips was about 42% higher than the bulk material. Overall, the experimental findings demonstrate that the proposed approaches were beneficial to enhance surface integrity characteristics of this material.

On the Ability of the Equivalent Material Concept in Predicting Ductile Failure of U-Notches under Moderate- and Large-Scale Yielding Conditions

In the present research, first, the equivalent material concept (EMC), proposed originally by the first author to equate a ductile material with a virtual brittle material, was utilized in conjunction with two stressbased brittle fracture criteria, namely the point-stress (PS) and the mean-stress (MS) criteria, to develop two combined failure criteria capable of predicting tensile crack initiation from U-notches in ductile materials. Then, to verify the two failure criteria, several rectangular thin plates weakened by a central bean-shaped slit with two U-shaped ends and made of aluminum alloys Al 7075-T6 and Al 6061-T6 were tested under tension. Experimental observations indicated that Al 7075-T6 plates failed by moderate-scale yielding while Al 6061-T6 by largescale yielding. It was found that the EMC-MS criterion could predict the experimental results of both materials successfully. Meanwhile, the EMC-PS criterion was found to be accurate for moderate and large notch radii, particularly for Al 6061-T6 material.

Numerical simulation of fatigue crack propagation in friction stir welded joint made of Al 2024-T351 alloy

In this work, fatigue crack propagation in thin-walled aluminium alloy structure with two friction stir welded T joints has been simulated numerically. Crack propagation in stiffened part of the structure between two friction stir welded T joints is analysed by using the eXtended Finite Element Method (XFEM), including software ABAQUS, as well as MORFEO, for modelling and results display. Tensile fatigue loading is applied, with stress ratio R=0, and maximum stress σmax=10MPa. Material properties (Al 2024-T351, as used in aeronautical industry) in different welded joints zones are adopted from available literature data. Following results are obtained by numerical analysis: stress–strain and displacement state in the structure, position of the crack tip and value of stress intensity factor for every crack propagation step, as well as the structural life estimation, i.e. number of load cycles, N, also for each crack propagation step. Using these results the number of cycles at which the crack starts to propagate in an unstable manner is predicted.