Al 6061-T6 Research Articles

Effect of Laser Shock Peening on surface properties and residual stress of Al6061-T6

The purpose of this study is to investigate the effects of Laser Shock Peening (LSP) on Al 6061-T6. The confined LSP regime using Nd: YAG laser with 1200mJ of energy per pulse and 8ns of pulse width were applied. The treated specimens were evaluated by means of surface integrity with optical microscopy, scanning electron microscope, microhardness, surface roughness and induced residual stress using an X-ray diffraction method. Results showed that by the use of LSP, compressive residual stress could effectively be induced on the surface of treated material. It was also revealed that the hardened depth of the material, up to a maximum depth of 1875μm, could be achieved due to work hardening and grain refinement. In addition, surface roughness measurements showed that the LSP could deteriorate surface quality depending on the LSP parameters. The influences of beam overlap rates, number of laser shots and scanning pattern on microhardness as well as surface roughness are discussed.

Effect fracture resistance on retardation of crack growth

Purpose – The prediction of fatigue crack growth behaviour is an important part of damage tolerance analyses. Recently, the author’s work has focused on evaluating the FASTRAN retardation model. This model is implemented in the AFGROW code, which allows different retardation models to be compared. The primary advantage of the model is that all input parameters, including those for an initial plane-strain state and its transition to a plane-stress-state, are objectively measured using standard middle-crack-tension M(T) specimens. The purpose of this paper is to evaluate the ability of the FASTRAN model to predict correct retardation effects due to high loading peaks that occur during variable amplitude loading in sequences representative of an aircraft service. Design/methodology/approach – This paper addresses pre-setting of the fracture toughness K R (based on J-integral J Q according to ASTM1820) in the FASTRAN retardation model. A set of experiments were performed using specimens made from a 7475-T7351 aluminium alloy plate. Loading sequences with peaks ordered in ascending-descending blocks were used. The effect of truncating and clipping selected load levels on crack propagation behaviour was evaluated using both experimental data and numerical analyses. The findings were supported by the results of a fractographic analysis. Findings – Fatigue crack propagation data defined using M(T) specimens made from Al 7475-T7351 alloy indicate the difficulty of evaluating the following two events simultaneously: fatigue crack increments after application of loads with maximum amplitudes that exceeded J Q and subcritical crack increments caused by loads at high stress intensity factors. An effect of overloading peaks with a maximum that exceeds J Q should be assessed using a special analysis beyond the scope of the FASTRAN retardation model. Originality/value – Measurements of fatigue crack growth on specimens made from 7475 T7351 aluminium alloy were carried out. The results indicated that simultaneously evaluating fatigue crack increments after application of the load amplitude above J Q and subcritical increments caused by the loads at high stress intensity factors is difficult. Experiments demonstrated that if the fatigue crack reaches a specific length, the maximal amplitude load induces considerable crack growth retardation.

Effect of graphite and granite dust particulates as micro-fillers on tribological performance of Al 6061-T6 hybrid composites

Aluminium and its alloys have an ever growing demand in many industries such as aerospace, automotive due to their high strength to weight ratio and corrosion resistance. Our current work focuses on synthesis and tribological studies of precipitation hardened Al 6061–Grp–granite dust hybrid composites. Liquid stir casting technique is used for synthesis, precipitation hardening treatment imparted for maximising the hardness before subjecting to two-body sliding wear tests. The variation of wear for different levels of load, speed and composition along with SEM micrographs of the worn surfaces has been investigated. Hybrid combinations of granite dust (2wt% and 4wt%) with graphite (2wt%) show higher tensile strength, hardness and significantly improved wear resistance as compared to the base alloy.

Numerical simulation of ductile fracture based on mean field homogenization method: Modeling and implementation

A mean filed homogenization method at large deformation is presented to describe the damage softening effects due to nucleation and growth of voids during ductile fracture. The developed homogenization frame is combined with the Nucleation-And-Growth model and the percolation theory based coalescence model for describing ductile damage evolution. A semi-implicit numerical algorithm is developed to solve the ductile damage model. Based on the model, we have carried out numerical simulations of the spall fracture of a Al 2024-T4 plate and a Ti–6Al alloy plate under impact loads. The simulated free surface velocity profiles show good agreement with experimental measures.

Experimental Verification of Effect of Adhesive Layer Thickness Used for Strain Gauge Mounting

Strain measurement from the strain gauge is partially affected by carrier and adhesive materials and their thickness. Effect of adhesive layer thickness has been addressed in this paper. Well characterized tensile experiments have been conducted using Al 6061-T6 specimens attached with strain gauges at mid length of the specimen and strain gauges are attached with different materials namely epoxy and polyurethane to understand the effect of adhesive layer thickness in strain measurement. The strain at a location has been noted for one particular adhesive layer thickness value (0.13 mm) and similar experiments have been carried out with different adhesive thickness values (0.16 mm, 0.18 mm and 0.26 mm). The results obtained from experiments have been compared with analytical results from Basic Strength of Materials approach. Good agreement is seen between the experimental and analytical results. It has been observed that the thickness of the adhesive layer plays significant role for getting accurate strain.

Structure and corrosion behavior of sputter deposited cerium oxide based coatings with various thickness on Al 2024-T3 alloy substrates

Cerium oxide based coatings from ∼100 to ∼1400nm in thickness were deposited onto Al 2024-T3 alloy substrates by magnetron sputtering of a 99.99% pure CeO2 target. The crystallite size of CeO2 coatings increased from 15nm to 46nm as the coating thickness increased from ∼100nm to ∼1400nm. The inhomogeneous lattice strain increased from 0.36% to 0.91% for the ∼100nm to ∼900nm thick coatings and slightly decreased to 0.89% for the ∼1400nm thick coating. The highest adhesion strength to Al alloy substrates was for the ∼210nm thick coating, due to a continuous film coverage and low internal stress. Electrochemical measurements indicated that sputter deposited crystalline CeO2 coatings acted as physical barriers that provide good cathodic inhibition for Al alloys in saline solution. The ∼900nm thick CeO2 coated sample had the best corrosion performance that increased the corrosion resistance by two orders magnitude and lowered the cathodic current density 30 times compared to bare Al 2024-T3 substrates. The reduced defects and exposed surface, along with suppressed charge mobility, likely accounts for the improved corrosion performance as coating thickness increased from ∼100nm to ∼900nm. The corrosion performance decreased for ∼1400nm thick coatings due in part to an increase in coating defects and porosity along with a decrease in adhesion strength.

Microstructural evolution during friction stir welding of dissimilar aluminum alloy to advanced high-strength steel

One type of advanced high-strength steel, transformation-induced plasticity (TRIP) 780 steel, has been successfully welded to aluminum alloy Al 6061-T6 using friction stir welding (FSW) technique. The major Al-Fe interface in the steel side has been analyzed in detail under various welding conditions, where a thin layer of intermetallic compound (IMC) with a thickness of less than 1 μm can be generally observed and the composition was identified to be either FeAl or Fe3Al. This thin IMC layer can be shown to be beneficial for joint strength. Optical microscopy and scanning electron microscopy showed the weld nugget was distributed with sheared-off steel fragments encompassed IMC layers or simply IMC particles. Finally, a stirred-over steel strip embedded in the aluminum matrix was revealed, which would determine the failure mode and is crucial to joint quality based on tensile test results.

On reliable measurement of micro drilling forces and identification of different phases

Mechanical micro-drilling process is often used in the manufacture of miniature components and molds. Frequent breakage of micro-drill is a major problem necessitating a thorough investigation into the process. One of the important responses measured during micro-drilling process is variation of thrust force and torque. Since the values of thrust force and torque are very low in micro-drilling, reliable measurements have to be carried out using high-resolution dynamometer. In the present work, micro-drilling is carried out using 0.5mm diameter drill on Al 6061-T6 sheet of 3mm thickness. The measurement results obtained from multi-component dynamometer are discussed first. Subsequently, a torque sensor mounted on the multi-component dynamometer is used to measure the torque, while the thrust force is acquired from latter. Since it is practically impossible to start acquiring the signals at a given time instant, the signals have to be synchronized in time domain using a novel signal processing technique. Different phases in micro-drilling, namely entry, full penetration and dwell are identified uniquely.

Failure Mode Analysis of Aluminium Alloy 2024-T3 in Double-Lap Bolted Joints with Single and Double Fasteners; A Numerical and Experimental Study

This paper aims to study the mechanical behaviour and failure mode of Al 2024-T3 in double-lap bolted joints. To accomplish this, the effect of geometric parameters was investigated in both configurations of single and double fasteners. Using ABAQUS/Standard, a three-dimensional finite element model was developed and verified against the experimental results of the joints loaded in tension. In general, double bolt joints were found to have greater load carrying capacities than single bolt joints (by 40%–49%). In single bolt joints, the plate width had insignificant effect on the behaviour of the joint under tensile loading; whereas, increasing the distance of the hole from the edge, considerably enhanced the strength of the joint. In double bolt joints, changing the edge distance had almost no effect on the behaviour of the joint. However, increasing the plate width from 25.4 to 30 mm increased the load carrying capacity by 28%. This study showed that in single bolt connections, with increasing the edge distance, the failure mode can favourably shift from shear-out to bearing. Also, double bolt joints with wider plates (increased width) can beneficially shift the failure mode from net-tension to bearing. The geometric parameters were found to play an important role in controlling the failure mode so that catastrophic failure modes of net-tension and shear-out can be prevented in bolted joint.

Continuous dynamic recrystallization during severe plastic deformation

Severe plastic deformation (strains>100%) has been shown to create significant grain refinement in polycrystalline materials, leading to a nanometric equiaxed crystalline structure for such metals as aluminum, copper and nickel alloys. This process, termed continuous dynamic recrystallization, is governed by evolution of the dislocation structure, which creates new grain boundaries from dislocation walls. In the proposed model, plasticity occurs which firstly involves dislocation multiplication, leading to strain hardening limited by dynamic recovery. After a critical dislocation density is reached new grain boundaries are formed by condensation of walls of dislocations, creating a new stable configuration that is favored due to a reduction of the system free energy. This evolution of the microstructure continues to develop, with a consequent progressive decrease in the average grain diameter. The proposed model provides a quantitative prediction of the evolution of the average grain size, as well as the dislocation density, during continued plastic strain. The model can be calibrated by use of results from any experiment that involves large plastic deformation of metals, subject to negligible annealing effects. In this paper, the model has been calibrated, and consequently validated, through experiments on machining of Al 6061-T6.

An engineering model for the penetration of a rigid-rod into a Cowper–Symonds low-strength material

The impact of a rigid-rod into a low-strength target is an ubiquitous research problem in many scientific and engineering fields. Typically, this impact has been modeled by the expansion of a pressurized spherical cavity in an unbounded elasto-plastic medium. In this paper, an engineering penetration model was developed using the dynamic spherical cavity expansion theory for an elasto-plastic, compressible, strain-dependent and strain-rate-sensitive material. The material plastic flow behavior was modeled using the Cowper–Symonds strength model. The engineering model was numerically solved, and its predictions were compared with results of computational finite-elements simulations performed in ANSYS/AUTODYN. Additionally, engineering and computational models were validated with experimental data using spherical-nosed, M300 steel projectiles impacting a semi-infinite Al 6061-T651 plate. Comparisons showed good agreement among engineering model results, computational model results and experimental data.

Buckling and longitudinal cracks in electromagnetically accelerated hollow cylinders

Hollow cylindrical Al-6061 T6 projectiles, driven in a coilgun system, suffer radial compression and buckle into quadrilateral or pentagonal cross sections. In some cases, the projectiles fail by developing longitudinal cracks in the compressed region. Simulations of the coilgun-projectile system, using Johnson–Cook and Bao–Wierzbicki failure models, reproduce buckling and formation of longitudinal cracks via localization of plastic strain and high temperatures around the bends of the buckled geometry. Failed specimens were micro-graphically investigated and the cause of failure attributed to synergistic effect of buckled geometry and localized high temperature zones.

A stress-based approach to analyse fretting fatigue life behaviour of electroless Ni–P coated Al 7075-T6

The purpose of this study is to examine the fretting fatigue performance of Ni–P coated (EN coated) Al 7075-T6 plates. In addition, the effect of two stress-based fretting mechanical parameters (clamping pressure and friction force) on the fretting fatigue performance is evaluated using a finite element stress analysis of the contact region. It was found that the fretting fatigue life of the EN coated specimens tested at high cycle fatigue improved significantly by up to 54±5% and 69±22% when clamped at 2kN and 5.6kN, respectively. The harder EN coated surface was found to reduce the contact stress due to the clamping pressure and thus assisted in improving the fretting fatigue life. The EN coated surface was also found to decrease the friction coefficient which assisted in mitigating surface wear damage, thus reducing the probability for fretting cracks to initiate. Furthermore, fractographic analyses revealed the presence of coating defects which were found to be the cause of fatigue failure at low cycle fatigue. These defects could possibly initiate fatigue cracks earlier, thus overruling the fretting phenomenon all together.

AN ANALYSIS OF MECHANICAL PROPERTIES OF ANODIZED ALUMINUM FILM AT HIGH STRESS

In this paper, a new environmental-friendly electrolyte containing sulfuric acid and tartaric acid has been used as the substitute of chromic acid for anodization. The work discussed the influence of anodizing voltages on the fatigue life of anodized Al 2024-T3 by performing fatigue tests with 0.1 stress ratio (R) at 320 MPa. Meanwhile the fatigue cycles to failure, yield strength, tensile strength and fracture surface of anodic films at different conditions were investigated. The results showed that the fatigue life of anodized and sealed specimens reduced a lot compared to aluminum alloy, which can be attributed to the crack sites initiated at the oxide layer. The fracture surface analyses also revealed that the number of crack initiation sites enlarged with the increase of anodizing voltage.

EFFECT OF SPECIMEN DIAMETER AND NOTCH DIAMETER ON THE FRACTURE TOUGHNESS OF AL7075 T6 ALLOY- AN EXPERIMENTAL APPROACH

In this paper fracture toughness (KIC) of Al7075 T6 alloy has been determined by using circumferentially cracked round bar (CCRB) specimen having three specimen diameters (D) 12mm, 10mm,8mm and three notch diameters(d) 10mm,8.3mm,6.7mm respectively with constant D/d ratio as 1.2. The purpose of the study was to determine the effect specimen diameter and notch diameter on the fracture toughness of the Al7075 T6 alloy. The obtained fracture toughness results for different notch diameter are compared with ASTM standards. The result shows that the notch diameter has significant effect on the value of fracture toughness (KIC).

A Unified Model of Crack Propagation in Scratched and Unscratched Samples of Al 2024 - T3 Clad

An experimental study of the effect of scratches on the fatigue life and fatigue crack growth of Al 2024-T3 clad material has been conducted. The result showed that Al 2024-T3 clad material is a scratch sensitive material. It presented a significant drop in fatigue life, 95% with a scratch depth of 100 µm in comparison with the unscratched material. Also, it has been found that cracks propagate faster in scratched samples than unscratched and at a lower stress intensity factor. A model that predicts the fatigue crack propagation of scratched samples is proposed. Furthermore, scanning electron micrographs showed microcracks, generated by the presence of scratches, responsible in decreasing the fatigue life of the material.

Identification and Validation of a Low Cycle Fatigue Damage Model for Al 7075-T6 Alloy

In this paper, the behavior of 7075-T6 aluminum alloy under low cycle fatigue (LCF) loading is experimentally and numerically investigated using continuum damage mechanics (CDM). An experimental procedure is established to identify the damage parameters for Al 7075-T6. A damage-coupled explicit finite element code is developed using the experimentally extracted damage parameters to study the material behavior under LCF loading. Moreover, fractographic examinations are conducted to identify the fatigue crack initiation locations and propagation mechanisms. The model is employed for life-time assessment of stringer-skin connection of a fuselage and the results are compared with the data available in the literature.

Surface Layer Modification by Cryogenic Burnishing of Al 7050-T7451 Alloy and Validation with FEM-based Burnishing Model

As one of the chipless machining processes, burnishing has been performed on manufactured components as the final operation to improve the surface integrity, including reduced surface roughness and increased surface and subsurface hardness. Refined surface layers with ultra-fine grains or nano-grains could be generated during the burnishing process due to imposed severe plastic deformation and the associated dynamic recrystallization (DRX). These harder layers with compressive residual stresses induced by the burnishing process also provide added benefits by enhancing wear/corrosion resistance and increasing the fatigue life of the components. The research findings presented in this paper show the effects of cryogenic burnishing with roller burnishing tool on Al 7050-T7451 alloy, using liquid nitrogen as the coolant. Burnishing forces and temperatures are measured to compare the differences between dry and cryogenic burnishing. Higher tangential forces and lower temperatures are observed from cryogenic burnishing due to the work-hardening and the rapid cooling effects introduced by cryogenic burnishing. Also, refined layers with nano-grains (grain size of approximately 40nm) are formed in the cryogenicallyburnished surface, in which an average hardness increase of 9.5%, 17.5% and 24.8% within the 200μm depth are achieved in comparison with the hardness values obtained from dry burnishing, at the corresponding burnishing speeds of 25, 50 and 100 m/min. A finite element model (FEM) is developed to simulate the burnishing forces and temperatures for validation of the experimental results, based on the modified Johnson-Cook flow stress model combined with the constitutive equations concerning DRX. Good agreement is obtained between the predicted and experimental results.

Fatigue Behaviour of Al 6082-T6 Friction Stir Welded Tubular Joints under Torsional Loading

The aim of the present research is to investigate the fatigue behaviour of friction stir (FS) welded tubular joints in aluminium alloy subjected to torsional fatigue loading. To manufacture the samples which were tested, an MTS I-STIR process development system was equipped with a retracting tool specifically designed for these tubular welds. Al 6082-T6 FS welded samples were tested under cyclic torsion with a nominal shear stress ratio equal to-1 and 0. The test data show that, strictly speaking, the presence of non-zero mean shear stresses has a detrimental effect on the overall torsional fatigue strength of the FS welded joints. However, the reanalysis discussed in the present paper suggests that, from a statistical point of view, any detrimental effect is small. This means that fatigue assessment under torsional fatigue loading of these FS welded tubular connections can be performed with little loss of accuracy by neglecting the presence of non-zero mean shear stresses.

Short fatigue crack initiation and growth modeling in aluminum 7075-T6

A series of fatigue tests is carried out to investigate the behavior of microcrack initiation and propagation in Al 7075-T6. Plate specimens with semi-circular single edge notch are tested under uniaxial loading at different load amplitudes. Optical microscopy is used to detect the initiation and growth of short cracks. Continuum damage mechanics is used to model the crack initiation. After the initiation, crack growth is studied using fracture mechanics models. For both regions of crack initiation and growth, Bayesian estimation method is used to account for uncertainties in the parameters of the model. Results of the entire fatigue life, including initiation and growth, are compared with experiments. Good agreement is observed.