Cryorolling and warm forming of AA6061 aluminum alloy sheets

Cryorolling is a severe plastic deformation (SPD) process used to obtain ultrafine-grained aluminum alloy sheets along with higher strength and hardness than in conventional cold rolling, but it results in poor formability. An alternative method to improve both strength and formability of cryorolled sheets by warm forming after cryorolling without any post-heat treatment is proposed in this work. The formability of cryorolled AA6061 Al alloy sheets in the warm working temperature range is characterized in terms of forming limit diagrams (FLDs) and limiting dome height (LDH). Strain distributions and thinning in biaxially stretched samples are studied. Hardness of the formed samples is correlated with ultimate tensile strength to estimate post-forming mechanical properties. The limit strains and LDH have been found to be higher than in the case of the conventional processing route (cold rolled, annealed and formed at room temperature), making this hybrid route capable of producing sheet metal parts of aluminum alloys with high strength and formability. In order to combine the advantages of enhanced formability and better post-forming strength than the conventional cold rolled and annealed sheets, warm forming at 250°C has been found to be suitable for this alloy in the temperature range that has been studied.     

Effect of laser-arc hybrid welding on fracture and corrosion behaviour of AA6061-T6 alloy

The welding condition of the hybrid laser-gas metal arc (GMA) welding for AA6061-T6 alloy was optimized by tensile test. Formability performance was checked by the bend test. Fractographic analysis indicates a large number of fine ductile type voids in the fracture surface. The microstructure measurements exhibit a dendritic cellular structure in the weld fusion zone (WFZ) and a partially melted zone adjacent to the fusion boundaries. The corrosion behaviour of the weldment and the base alloy were investigated by weight-loss test in nitric acid solution. The WFZ suffers more severe pitting than the rest regions in the weldment. It shows that corrosion cracking is owing to the precipitation of intermetallic phases and the formation of galvanic corrosion couplings in the weldment of AA6061-T6 alloy.     

Tensile behavior of friction stir welded AA 6061-T4 aluminum alloy joints

In this investigation response surface methodology based on a central composite rotatable design with three parameters, five levels and 20 runs, was used to develop a mathematical model predicting the tensile properties of friction stir welded AA 6061-T4 aluminum alloy joints at 95% confidence level. The three welding parameters considered were tool rotational speed, welding speed and axial force. Analysis of variance was applied to validate the predicted model. Microstructural characterization and fractography of joints were examined using optical and scanning electron microscopes. Also, the effects of the welding parameters on tensile properties of friction stir welded joints were analyzed in detail. The results showed that the optimum parameters to get a maximum of tensile strength were 920 rev/min, 78 mm/min and 7.2 kN, where the maximum of tensile elongation was obtained at 1300 rev/min, 60 mm/min and 8 kN.     

Experimental and numerical evaluation of forming limit diagram for Ti6Al4V titanium and Al6061-T6 aluminum alloys sheets

The forming limit diagram (FLD) is a useful concept for characterizing the formability of sheet metal. In this work, the formability, fracture mode and strain distribution during forming of Ti6Al4V titanium alloy and Al6061-T6 aluminum alloy sheets has been investigated experimentally using a special process of hydroforming deep drawing assisted by floating disc. The selected sheet material has been photo-girded for strain measurements. The effects of process parameters on FLD have been evaluated and simulated using ABAQUS/Standard. Hill-swift and NADDRG theoretical forming limit diagram models are used to specify fracture initiation in the finite element model (FEM) and it is shown that the Hill-swift model gives a better prediction. The simulated results are in good agreement with the experiment.     

Ductile failure behavior of polycrystalline Al 6061-T6

Ductile failure in polycrystalline aluminum alloys is explored through uniaxial tension and notched tension experiments. Specimens obtained through tests interrupted at various stages of deformation and failure evolution are examined through microscopy to discern the mechanisms of failure and to evaluate the local strain evolution quantitatively. Fractographic observations are used to identify the onset and evolution of damage processes during deformation and failure of these aluminum alloys. Local strain levels are estimated from measurements of the change in grain size with deformation and used to indicate that the local values of failure strains are likely to be much larger than that estimated from strains averaged over characteristic specimen dimensions such as the gage length or the specimen diameter. Lower bound estimates of the failure strain at moderate triaxiliaties are obtained from the experiments.     

Nanomechanical properties of friction stir welded AA6082-T6 aluminum alloy

Lightweight alloys are of major concern, due to their functionality and applications in transport and industry applications. Friction stir welding (FSW) is a solid-state welding process for joining aluminum and other metallic alloys and has been employed in aerospace, rail, automotive and marine industries. Compared to the conventional welding techniques, FSW produces joints which do not exhibit defects caused by melting. The objective of the present study is to investigate the surface hardness (H) and elastic modulus (E) in friction stir welded aluminum alloy AA6082-T6. The findings of the present study reveal that the welding process softens the material, since the weld nugget is the region where the most deformations are recorded (dynamic recrystallization, production of an extremely fine, equiaxial structure), confirmed by optical microscopy and reduced nanomechanical properties in the welding zone. A yield-type pop-in occurs upon low loading and represents the start of phase transformation, which is monitored through a gradual slope change of the load-displacement curve. Significant pile-up is recorded during nanoindentation of the alloy through SPM imaging.     

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.     

高周疲劳载荷下6061-T6铝合金的温度演化

为了研究金属材料在疲劳载荷下的温度变化,采用红外热像系统对高周疲劳载荷下6061-T6铝合金的温度演化进行分析,用热像图对疲劳裂纹尖端的塑性区进行测量.结果显示,疲劳加载作用下,循环次数达到107次时6061-T6铝合金试样表面温度的变化分为四个阶段:初始温升阶段、温度缓降阶段、温度二次缓慢上升阶段和温度快速上升阶段.结合热弹性理论、铝合金塑性变形的微观机制分析并预测疲劳载荷下温度的演化和宏观裂纹扩展时裂纹尖端塑性区域大小.宏观裂纹开始扩展时,裂纹尖端的塑性区域可达3.6 mm2,红外热像仪测得结果为3.46 mm2,测试结果与理论结果吻合.     

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.     

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.     

Numerical simulation and experimental investigation on friction stir welding of 6061-T6 aluminum alloy

A modified three-dimensional model was established to simulate the friction stir welding of the 6061-T6 aluminum alloy. A detailed calculating method of the heat generation was proposed by taking account of the contact conditions between the tool and the work-piece. The results show that the heat mainly generated within the region close to the shoulder, the high temperature exists within the upper portion of the weld and decreases along the thickness direction. The strong material flow mainly occurs within the region around the tool and the material ahead of the tool sweeps toward the RS and finally deposits behind the tool. During this procedure the material is extruded to experience different shear orientations, and a defect-prone region exists in the region where material flow is weak. The temperature field and material flow behaviors predicted by the simulation method are in good agreement with the results obtained by the experiments.     

Time-dependent fracture criteria for 6061-T6 aluminum under stress-wave loading in uniaxial strain

New spallation threshold data for 6061-T6 aluminum were obtained under stress-wave loading conditions in uniaxial strain, covering the range of tensile pulse durations of 60 to 200 nsec. This range of pulse duration was achieved by using exploding-foil techniques to accelerate thin Mylar plates against thin aluninum specimens. A comparison was made between exploding-foil spallation tests on 6061-T6 aluminum in air and vacuum. The data indicate that the spallation threshold of 6061-T6 aluminum is sensitive to the tensile pulse duration, amplitude, and impulse at the spall location. The exploding-foil impact conditions were reduced to stress-pulse loading parameters by using a one-dimensional elastic-plastic hydrodynamic computer code. The time-dependent aspects of the spallation threshold of 6061-T6 aluminum were found to obey failure theories which were rate process oriented, and which combine the effects of tensile-pulse duration, peak tensile stress, tensile impulse, and tensile-pulse shape. The present data have been used to quantitatively establish failure relationships for 6061:T6 aluminum. Where applicable, supplemental information in the literature concerning dynamic fracture of 6061-T6 aluminum was utilized.     

Growing and critical ovalization for sharp-notched 6061-T6 aluminum alloy tubes under cyclic bending

In this paper, we present results from experiments dealing ovalization in 6061-T6 aluminum alloy tubes which have been notched and subjected to cyclic bending are presented. It was discovered that the tubes continuously ovalized to a critical quantity when they buckled. Tubes with a smooth surface and five different notch depths were considered. The ovalization–curvature curve exhibited a symmetrical and ratcheting increase with the number of bending cycles. Deeper notch depths caused greater ovalizations. In addition, the trend of the ovalization at negative extreme curvature and number of bending cycles relationship was distinguished into three stages. Finally, the empirical form proposed by Lee, Hung, and Pan in 2010 was employed for describing the above-mentioned correlation in the first two stages. It was found that the experimental and simulated data agreed quite well.     

Formation of zinc phosphate coatings on AA6061 aluminum alloy

Conditions for forming zinc phosphate conversion coatings on AA6061 aluminum alloy have been investigated by characterizing coatings formed for different parameters of the coating bath. Morphological and compositional information on the coatings was assessed by SEM, EDX and XPS, and simple adhesion tests were undertaken to indicate the strengths of coating attachment. The emphasis was to identify conditions that give high coverage, uniform coatings of small, strongly adhered, zinc phosphate crystals. The use of low-zinc solutions (e.g. an atomic Zn/P ratio of 0.07) and normal-zinc solutions (Zn/P ratio 0.25) were compared; coatings formed by the two solution types appear comparable at pH 2, although at pH 4 the low-zinc solution is more effective. Fluoride in the concentration range 200–400 ppm is indicated to be a useful additive for the normal-zinc coating bath and in the 600–1000 ppm range for the low-zinc process. The use of acid etching in the pre-treatment appears to yield better coatings than when mechanical polishing alone is used.     

Experimental evaluation and prediction of forming limit of FSW blanks made of AA 6061 T6 sheets at different weld orientations and weld locations

The main objective of the present work is to predict the forming limit of friction stir welded (FSW) sheets made of AA 6061T6, having different weld orientations, weld locations, and made at two different welding speeds. The predicted forming limit curves (FLCs) are validated with experimental FLCs. The thickness gradient based necking criterion (TGNC) and major strain‐rate ratio based necking criterion (MSRC) are used to predict the forming limit. The significance of single zone model and double zone model in FLC prediction is discussed. A decrease in hardness is witnessed in the weld zone as compared to base material. With increase in shoulder diameter and decrease in rotational speed, hardness has improved in the weld zone. The forming limit predictions of un‐welded sheets and FSW sheets coincide well with experimental results. The predicted FLCs of FSW sheets from TGNC and MSRC are equally accurate as compared to experimental FLCs in all the weld locations. Both TGNC and MSRC predict almost the same forming limit in 90° weld orientation, while TGNC showed better prediction in 45° weld orientation. FSW sheets with double zone models show better prediction accuracy than single zone models in most of the cases, except in the case of weld at centre location and at longitudinal orientation. There is only slight deviation between single zone and double zone model predictions. The failure location and failure pattern predictions are also agreeing well with the experimental FLCs.     

Fatigue behavior of AA7075-T6 aluminum alloy coated with ZrN by PVD

The present investigation has been conducted in order to study the effect of the deposition of a ZrN coating, of 3 μm in thickness, on the static mechanical properties and fatigue behavior of a 7075-T6 aluminum alloy substrate. It has been determined that the coating deposition process gives rise to a significant decrease in such properties, which is not fully compensated for the presence of the film. When fatigue tests were carried out in a 3 wt.% NaCl solution at low alternating stresses, the ZrN film partially compensated for the decrease in fatigue properties of the coated substrate. Extensive delamination of the coating from the substrate was observed under the action of cyclic stresses greater than approximately 220 MPa. Below this stress and in the presence of NaCl, the behavior of the coated material approached that of the uncoated alloy, which highlighted the good corrosion resistance of the ZrN coating and its ability to protect the substrate when it remained adhered to the latter.     

Flow stress and ductility of AA7075-T6 aluminum alloy at low deformation temperatures

The present investigation has been conducted in order to develop a rational approach able to describe the changes in flow stress of AA7075-T6 aluminum alloy with deformation temperature and strain rate, when this material is deformed at temperatures in the range of 123-298 K at strain rates in the range of 4 × 10⁻⁴ to 5 × 10⁻² s⁻¹. The constitutive formulation that has been advanced to accomplish these objectives represents a simplified form of the mechanical threshold stress (flow stress at 0 K) model developed at Los Alamos National Laboratory (Los Alamos, New Mexico, USA). Thus, it is assumed that the current flow stress of the material arises from both athermal and thermal barriers to dislocation motion. In the present case, the effect of three thermal barriers has been considered: solid solution, precipitation hardening and work-hardening. The first two effects do not evolve during plastic deformation, whereas the last one is considered as an evolutionary component of the flow stress. Such an evolution is described by means of the hardening law earlier advanced by Estrin and Mecking (1984) [20]. The law is implemented in differential form and is integrated numerically in order to update the changes in strain rate that occur during tensile tests carried out both at constant and variable crosshead speed. The extrapolation of the hardening components from 0 K to finite temperatures is accomplished by means of the model earlier advanced by Kocks (1976) [19]. The results illustrate that the constitutive formulation developed in this way is able to describe quite accurately both the flow stress and work-hardening rate of the material, as well as temperature and strain rate history effects that are present when deformation conditions change in the course of plastic deformation. The evaluation of the ductility of the alloy indicates that the changes in this property are mainly determined by deformation temperature rather by strain rate. When deformation temperature decreases from 298 to 123 K, ductility also decreases from ∼35 to 24%. However, despite these relatively small variations, significant changes in the fracture morphology could be observed on the fracture surfaces of the examined specimens, with the predominance of a mixed ductile-brittle mechanism at lower temperatures.     

Friction Stir Weldabilities of AA1050-H24 and AA6061-T6 Aluminum Alloys

The friction stir weldabilities of the strain-hardened AA1050-H24 and precipitate-hardened AA6061-T6 aluminum alloys were examined to reveal the effects of material properties on the friction stir welding behavior. The experimental results are obtlained. (1) For AA1050-H24, the weld can possess smoother surface ripples; there is no elliptical weld nugget in the weld; there is no discernible interface between the stir zone and the thermomechanically affected zone; and the internal defect of the weld looks like a long crack and is located in the lower part of the weld. (2) For AA6061-T6, the weld usually possesses slightly rougher surface ripples; an elliptical weld nugget clearly exists in the weld; there are discernible interfaces among the weld nugget, thermomechanically affected zone and heat affected zone; and the internal defect of the weld is similar to that of the AA1050-H24 weld. (3) The effective range of welding parameters for AA1050-H24 is narrow, while the one for AA6061-T6 is very wide. (4) T