High Strength Aluminium Alloy Sheets Research Articles

Inter-void shearing effect on damage evolution under plane strain deformation in high-strength aluminum alloy sheet

In this study, the ductile damage responses of high-strength 7000 series aluminum alloy (AA), AA 7075-T6 sheet samples, subjected to the plane strain deformation mode were investigated using finite element (FE) simulations. In the experiments, uniaxial tension (UT) and plane strain tension (PST) tests were conducted to characterize the plasticity and ductile damage behavior of the AA 7075-T6 sheet samples. The limiting dome height (LDH) and V-die air bending tests were conducted to evaluate the ductility of the material subjected to plastic deformation and friction between the tools, and the corresponding fractured samples were qualitatively analyzed in terms of dimples using fractography. FE simulations were performed to predict the ductility of the AA 7075-T6 sheet samples under plane strain deformation using an enhanced Gurson−Tvergaard−Needleman (GTN) model, namely the GTN-shear model. The model was improved by adding the shear dimple effect to the original GTN model. The predicted results in terms of the load–displacement curves and displacements at the onset of failure were in good agreement with experimental data from the aforementioned tests. Furthermore, virtual roll forming simulations were conducted using the GTN-shear model to determine the effect of the prediction on ductile behavior for industrial applications.

Comparative study on the formability and microstructure evolution of different tempered Al–Cu–Li alloy sheets during room and cryogenic temperature forming process

High strength aluminum alloy sheets with excellent formability are necessary for the successful fabrication of complex-shaped components in order to prevent cracking during the forming process. To address this issue, this study investigated the formability of annealed (O) and water-quenched (WQ) Al–Cu–Li alloys at both room and cryogenic temperatures. Our results reveal a significant increase in the forming limit curve, with a 40% enhancement observed as the forming temperature decreased from 298 K to 113 K. Additionally, a 30% increase in formability was observed when transitioning from the O temper to the WQ temper. Thickness measurements reveal that WQ pre-treatment and cryogenic forming promoted continuous work hardening, thus improving deformation homogeneity. The mechanism responsible for the higher work hardening ability was elucidated by examining the microstructure evolution in different regions of the components. The results show that coarse particles in the O tempered alloys accelerated cross slip, resulting in the formation of recovered microstructures (i.e., dislocation cells and Cube texture) while simultaneously hindering dislocation movement and inducing local stress concentration. Conversely, for the WQ tempered alloy deformed at cryogenic temperature, the dissolution of particles and decreased stacking fault energy at low temperature suppressed cross slip and resulted in well-arranged dislocations within the slip system. Consequently, the combination of WQ pre-treatment and cryogenic forming demonstrates enhanced formability in high strength aluminum alloys, offering a novel method for manufacturing complex-shaped components.

An efficient pre-hardened cryogenic forming process for AA7075 aluminum alloy sheets

Currently, due to the requirement of lightweight in the transportation field, the demand for high-strength aluminum alloy is increasing, but it is difficult for the traditional forming processes to balance the three components of formability, strength and efficiency. In order to solve this problem, a novel stamping process for high-strength AA7075 aluminum alloy sheets is proposed, termed Pre-hardened Cryogenic Forming (PCF) process. PCF process is based on the W-temper alloy and to fulfill simultaneously the demands of good cryogenic formability and high post-formed strength through a suitable ratio of microstructure between the solution-treated matrix and the pre-aged second phases, thereby cancelling the additional artificial aging treatment and achieving high-efficiency. The good cryogenic formability is assured by solution-treated matrix, while the initial strength and further artificial age-hardening during paint baking are regulated by means of pre-aged second phases. A critical issue for PCF process is to determine the pre-aging process window, i.e., a suitable combination of pre-aging temperature and time period, because there is a contradiction between the cryogenic formability and post-formed strength under the constraint of high efficiency. The results of tensile tests show that AA7075 aluminum alloy after pre-aging at 120 °C for 1 h (PA-120-1h) can achieve excellent comprehensive mechanical properties of cryogenic plasticity and post-baking strength. Combined with the analysis of micro-experimental results, the optimal pre-aging process window of AA7075 aluminum alloy is to ensure that the volume fraction of fine precipitates is between 2.5 % and 4 %, and the recommended ratio of the fine precipitates (<2 nm/≥2 nm) is about 1.5. The mixed structure of the GP zones and the η′ phases for PA-120-1h alloy is the suitable combination of strengthening precipitates for balancing cryogenic plasticity and post-baking strength. Based on the newly-designed cooling system, stamping tests are conducted on a W-shaped beam component to verify the engineering feasibility. Based on PA-120-1h alloy sheets, the component with improved surface quality can be successfully formed by PCF process. The final post-baking strength exceeds 92 % of the tensile strength of AA7075-T6 alloy without extra heat treatment, and the thickness reduction rate is within 10 %. Regardless of the immersion time of the sheets in liquid nitrogen, the PCF process can be finished within 2 min, greatly improving the forming efficiency.

Effect of plastic anisotropy on notch deformation behavior of 7075 high-strength aluminum alloy sheet subjected to axial tension

The notch is an inevitable geometric characteristic in thin-walled structural components. However, the anisotropy in the notch deformation behavior of metal sheets has not been thoroughly investigated. To address this gap, we explored the effect of plastic anisotropy on the notch deformation behavior of the AA7075-T6 sheet subjected to axial tension at room temperature and quasi-static conditions. For this purpose, four types of notched tensile specimens are designed and tested along the rolling, diagonal, and transverse directions. The plastic anisotropy is examined by the dog-bone sample every 15° from the rolling to transverse directions. Full-scale numerical simulations for each notch tensile test were performed in ABAQUS/Standard based on the elastic–plastic constitutive framework with yielding depicted by the Yld2004-18p criterion. The results show that the established numerical models high fidelity reproduced the experimental measurements. It is found that the plastic anisotropy strongly affects the notch deformation of the sheet specimen subjected to axial tension, including strain concentration, plastic localization, stress state, and fracture initiation. Besides, the critical location of cracking in the material depends on the notch geometry parameter w/ρ, viz., the ratio of the half distance between roots of two notch (w) to the radius of the depth notch root (ρ). Another crucial contribution is to provide ideas for a researcher to control or design the path of strain localization by producing metals with specific textures to promote or delay fracture initiation. Accordingly, through the investigation of this work, we provide an essential reference for further engineering applications of high-strength aluminum alloy sheet products with notches.

Warm and hot stamping of high-strength aluminum alloy sheets using contact heating

In this paper, the contact heating technology was combined with the warm and hot stamping process of high-strength aluminum alloy, and the corresponding experimental device was developed. For the contact heating-warm stamping process, it’s found that as the heating temperature increases, the mechanical properties of the parts first decrease and then increase, while the springback decreases. Meanwhile, for the contact heating-hot stamping process, it’s found that the aluminum alloy sheets experience large plastic deformation during contact heating, resulting in the refinement of microstructure and the increase in dislocation density, and the performance of the formed parts is improved.

Equivalent characterization of pre-strained material properties and mechanical behavior prediction of steel/aluminum self-piercing riveted joints

Self-piercing riveting formation results in additional straining and property variations of the sheet material and rivet of the joint. As the riveted forming strain was concentrated within a limited area, it was difficult to characterize the properties of the pre-strained materials through conventional tensile tests. The present study dealt with the effects of considering riveted forming strain on the mechanical behavior prediction accuracy of the self-piercing riveted (SPR) joints of DP590 high-strength steel and AC43500-T7 cast aluminum alloy sheets. Main efforts were put on the evaluation of the failure parameters and behaviors of the pre-strained sheet materials. Equivalent specimens of the two sheet materials with different pre-strain levels were prepared via a flat rolling process for the characterization of the sheet deformation during the riveting process. Based on the material parameters obtained from the tensile tests of the rolled sheets and the compression tests of the rivet, a three-dimensional simulation model of the SPR joints was established. The mechanical behaviors of the joints were investigated under three loading conditions of 0° pull-out, 90° lap shear, and 45° tension–shear. The prediction accuracies of pull-off displacement under the three loading conditions reached 94.07%, 91.61% and 94.02%, and were 5.57%, 26.43% and 6.18% higher than those predicted by the model without considering the riveted forming history, respectively. The prediction accuracies of the peak force under the three loading conditions were 98.43%, 89.70% and 95.35%, respectively. The error difference of predicted peak force of the two models was less than 3%.

Modeling anisotropic ductile fracture of AA7075-T6 sheet for sheet metal forming considering anisotropic stress state

Anisotropic ductile fracture is one of the critical issues in the precision plastic forming of lightweight sheet metals. In this work, a new anisotropic ductile fracture model is developed by introducing the anisotropic stress triaxiality into the weight equation of the shear-controlled ductile fracture criterion. In an attempt to consider the effect of material anisotropy on stress state, the proposed fracture model is constructed in the anisotropic stress state form. The proposed anisotropic fracture model is used to characterize the anisotropic ductile fracture behavior of the AA7075-T6 sheet with a thickness of 2.0 mm under various stress states at room temperature. A series of uniaxial tensile tests are conducted using the in-plane shear specimen, the specimen with a central hole, and the plane strain grooved specimen to experimentally investigate the ductile fracture of AA7075-T6 along rolling, diagonal, and transverse directions. The proposed fracture model is calibrated by fracture strains determined from the hybrid experimental–numerical approach. With the calibrated fracture model, the anisotropic ductile fracture behaviors of AA7075-T6 over a wide range of stress states are predicted and compared with experimental measurements. The results show that the proposed fracture model successfully captures the anisotropic ductile fracture behavior of AA7075-T6 under various stress states at room temperature. Furthermore, to further illustrate the fracture predictive capability of the proposed fracture model, the predicted anisotropic fracture forming limit diagram is compared with the experimental forming limit strains obtained from the literature. Reasonable agreement is shown between literature data points and predicted fracture forming limit diagram. Accordingly, the proposed model has a great advantage in quantitative characterizing the anisotropic ductile fracture of high-strength aluminum alloy sheets under various loading conditions.

Strain-Rate Effect on Anisotropic Deformation Characterization and Material Modeling of High-Strength Aluminum Alloy Sheet

Aluminum alloy sheets are widely applied as structure components in automotive, aircraft and other industries to realize lightweight. Nowadays, many high strain rate forming techniques have been developed to improve their formability and widen their application. To ensure the reliability of the aluminum alloy structure components under high strain rate conditions, it is imperative to develop a thorough understanding of the alloy’s mechanical properties. In this paper, taking high-strength 6XXX aluminum alloy sheet as an example, the anisotropic deformation characterization and corresponding material models at various strain-rate conditions are investigated systematically. The material hardening curves and anisotropic plastic yielding stresses were achieved based on the quasi-static uniaxial tensile test and the split Hopkinson tensile bar tests. In this study, the Johnson–Cook hardening model and two anisotropic yield functions are applied to well describe the strain-rate-dependent anisotropic plastic deformation behavior. In addition, the fractographic characterization of the fractured samples at various strain-rate conditions are measured and compared. The study systematically investigates the influence of strain rate on the anisotropic deformation behavior of the high-strength aluminum alloy sheets and gives the basic experimental data for their application in engineering fields in the future.

Bendability assessment of high strength aluminum alloy sheets via V-die air bending method

The bendability of high-strength aluminum alloy (AA) sheets for two different thicknesses was experimentally evaluated by the V-die air bending method and the numerical simulation procedure was attempted to comprehend the characteristics of the V-die air bending method. Despite similar bendability along the material direction for a given punch radius in the experiment, the distinguished fracture strains were predicted for each direction based on the punch force and displacement information. For the numerical simulation, V-die air bending was modeled with the full 3D non-quadratic anisotropic yield criterion and compared with the results of the isotropic yield function estimating that bendability along the rolling direction is higher than those along the transverse direction. The thinner material showed higher anisotropy in bendability and the characteristics of V-die air bending were also discussed suggesting a maximum value as an upper bound of the bendability. As a comparison, an anisotropic analytical formulation was also newly developed combined with the kinematic assumption and the limitation was also covered in the study. The analytical fracture strain was discussed as the equivalence to the maximum value suggested in the research.

Precipitates evolution of the AA7075-H18 alloy sheet during the application of the solution heat treatment process

The hot stamping technology of the high-strength aluminum alloy sheet has attracted increased attention from both the scientific community and industry due to the rapidly growing demand for lightweight automobiles. However, the development of a proper solution process suitable for the hot stamping technique remains a great challenge. Along these lines, in this work, the precipitates evolution of the AA7075-H18 alloy sheet with a thickness of 2 mm during the implementation of the solution process was systematically studied. The insoluble particles (i.e., Al23CuFe4 and Mg2Si) and the soluble particles (η-MgZn2) were clearly identified from the aluminum matrix. Moreover, the transformation sequence during the solution heat treatment was determined, and a reasonable solution temperature window was also established.

Optimization of fiber laser welding parameters for high strength aluminium alloy AA7075-T6

High strength aluminium alloy sheets are widely used in automobile industry for manufacturing super lightweight vehicles. The high strength to weight ratio possessed by these Al alloys helps in reducing the total weight of the vehicle without affecting the overall performance, thereby increasing the fuel economy and reducing the carbon emission levels. However, joining these high strength aluminium alloy sheets for mass production in automobile industry is a challenging task and is not feasible by using conventional welding processes. In the present study fibre laser welding of high strength aluminium Alloy AA 7075-T6 sheets of 1.5 mm thickness has been carried out with a capacity of 2 kW. The welding parameters namely welding power, transverse speed & frequency have been optimized with output parameters of tensile strength, micro-hardness at fusion zone and micro-hardness at heat affected zone. VIKORs optimization technique has also been applied to the experimental results to obtain the optimal solution. Taguchi method in MINITAB Software has been used for validating the results obtained.

Springback and deformation uniformity of high-strength aluminum alloy sheet using electromagnetic forming

High-strength aluminum alloy has been widely used in aerospace and other industries. In the traditional bending process, a large springback occurs for high-strength aluminum alloy sheet due to its higher yield strength and lower elastic modulus. In the past, many scholars found that electromagnetic forming (EMF) can significantly reduce the springback effect. However, the electromagnetic-assisted bending and low-yield-strength aluminum alloy were used in the past researches. If the electromagnetic force is used to bend the high-strength aluminum alloy, what is the different of plastic deformation laws and springback effect both in electromagnetic forming and traditional bending? Those are the research interest of this paper. Thus, we have studied the influence of electromagnetic direct forming, electromagnetic drive forming, and traditional stamping on deformation uniformity and springback of high-strength aluminum alloy sheets. The deformation and stress distribution were analyzed by numerical simulations. The accuracy of the simulation was verified by experimental results. The results reveal that both electromagnetic direct forming and traditional stamping endow a large springback effect. It must also be mentioned that the high-strength aluminum alloy sheet dented on the center after electromagnetic direct forming, which reduces the sheet deformation uniformity. However, a small springback occurs on sheets due to small residual stress and the final deformed sheet has a smooth profile by electromagnetic drive forming. Thus, the electromagnetic drive forming is an effective mean to reduce springback and enhance deformation uniformity for high-strength aluminum alloy sheets.

Mechanical clinching and self-pierce riveting for sheet combination of 780-MPa high-strength steel and aluminium alloy A5052 sheets and durability on salt spray test of joints

To increase the usage of high-strength steel and aluminium alloy sheets for lightweight automobile body panels, the joinability of sheet combinations including a 780-MPa high-strength steel and an aluminium alloy A5052 sheets by mechanical clinching and self-pierce riveting was investigated for different tool shapes in an experiment. All the sheet combinations except for the two steel sheets by self-pierce riveting, i.e., the two steel sheets, the two aluminium alloy sheets, and the steel-aluminium alloy sheets, were successfully joined by both the joining methods without the gaps among the rivet and the sheets. Then, to show the durability of the joined sheets, the corrosion behaviour and the joint strength of the aged sheets by a salt spray test were measured. The corrosion and the load reduction of the clinched and the riveted two aluminium alloy sheets were little. The corrosion of the clinched two steel sheets without the galvanized layer progressed, and then the load after 1176 h decreased by 85%. In the clinched two galvanized steel sheets, the corrosion progress slowed down by 24%. In the clinched steel and aluminium alloy sheets, the thickness reduction occurred near the minimum thickness of the upper sheet and in the upper surface on the edge of the lower aluminium alloy sheet, whereas the top surface of the upper sheet and the upper surface of the lower sheet were mainly corroded in the riveted joint. The load reduction was caused by the two thickness reductions, i.e., the reduction in the minimum thickness of the upper sheet and the reduction in the flange of the aluminium alloy sheet. Although the load of the clinched steel without the galvanized coating layer and aluminium alloy sheets decreased by about 20%, the use of the galvanized steel sheet brought the decrease by about 11%. It was found that the use of the galvanized steel sheets is effective for the decrease of strength reduction due to corrosion.

Selection of intense energy welding process for high strength aluminum alloy using AHP

In the automobile industry, the reduction of the overall weight of vehicles faced a global challenge in terms of achieving better fuel economy and reducing the co2 emission levels. Aluminum alloys are widely used in the automotive industry due to their high strength to weight ratio than steel. In recent years, high strength aluminum alloy sheets are used in the aeronautical and automotive industries for constructing body structures. However, joining this high-strength aluminum alloy during mass production is difficult by using conventional welding processes. The family of intense energy welding like plasma arc welding, electron beam welding, and laser beam welding is generally employed to weld the high-strength aluminium alloys. This study aims in evaluating and selecting the best intense energy welding process to produce high strength aluminum alloy joints for automotive application by using the Analytical Hierarchical Process (AHP). Thus, the proposed AHP model was also verified with SuperDecision v3.2 software.

Development of Pulsed Cold Metal Transfer and Gas Metal Arc Welding Techniques on High-Strength Aerospace-Grade AA7475-T761

High-strength aluminum alloy (AA7475) sheets are widely used in aerospace sectors due to its better performance, global suitability, and multifunctional features. The welding of AA7475 in the fusion welding process like gas metal arc welding (GMAW) is difficult due to cracks generation. However, these problems could be overcome by different variants of GMAW process, like pulsed and normal cold metal transfer (PCMT and NCMT) techniques. The main objective of this research work is to deliver a better insight into mechanical and metallurgical properties of butt joint welded AA7475 through PCMT, NCMT, and GMAW techniques with ER5356 and ER4043. The defects-free butt joints were observed for PCMT, NCMT, and GMAW processes with ER5356, whereas the presence of porosities is found for ER4043. The microsegregation of Fe, Si, Mg, Zn, and Cu leads to the emergence of Mg2Cu, Mg2Si, Al2Cu, Al2CuMg, MgZn2, Mg2Zn11, and Fe2Al6 phases in the AA7475 weldments. The development of binary brittle intermetallic Mg2Si phase affects the tensile strength of all weldments made with ER4043. Better joint efficiency (96.11%), impact toughness (12.1 ± 0.8 J), and microhardness (119 ± 7.4 HV) are found in PCMT with ER5356 compared to other processes. Because of the low heat input utilised, PCMT provides better joint strength and microstructural integrity than NCMT and GMAW.

Microstructure and Mechanical Properties of TIG and Friction Stir Welded Joints of Sc-contained High Strength Aluminum Alloy Sheet

Microstructure and Mechanical Properties of TIG and Friction Stir Welded Joints of Sc-contained High Strength Aluminum Alloy Sheet

Fast solution heat treatment of high strength aluminum alloy sheets in radiant heating furnace during hot stamping

Lightweight of automobile has drawn increasing attention due to the demand for energy saving and emission reduction. High strength aluminum alloy sheet shows great potential in the automobile industry for its high strength and low density. Hot stamping has been developed for high strength aluminum alloy to improve the formability and reduce the springback. However, the solution time of this technology is too long. In order to reduce the solution time, several experiments have been carried out. The heating curves of bare sheets and BN or graphite coated sheets in radiant heating furnace were measured and the theoretical heating efficiency was calculated. The results showed that the heating efficiency was improved about 7.4 times with BN coating and 9.2 times with graphite coating depending on the heating temperature, so the heating rates of the coated sheets were faster than that of bare sheets. Further, hot stamping of bare and coated sheets was carried out and the mechanical properties of hot stamped sheets were tested. Hot stamping results showed that the solution time of 6061 and 7075 decreased from 10 min to 5 min by spraying BN or graphite on the sheets, reducing the solution time by 50%.

Investigations on the microstructure and mechanical properties of laser welded dissimilar galvanized steel–aluminum joints

In this study, galvanized high-strength steel and aluminum alloy sheets were laser-welded in zero-gap lap joint configuration. In order to determine the influences of the heat input levels, microstructural and mechanical properties of the joints were investigated. The weld bead geometry, microstructure, and intermetallic phases at the interface of welded joints were investigated using an optical microscope and scanning electron microscopy (SEM) with energy-dispersive spectroscopy (EDS) at different heat input levels. Mechanical properties and microhardness distribution of the welded joints were examined according to the weld bead dimension. The results revealed that there is a correlation between the weld seam geometry and intermetallic phase formation. At relatively high heat input level, the penetration depth increased, and thick Al-rich intermetallic layer was observed at the interface of the weld seam, which deteriorated the tensile strength of the joint. It has been found that without the need for any additional precaution, the thickness of the IMC layer can be limited to 5–15 μm when the optimum welding parameters were conducted. The experimental results showed that with limited heat input and penetration depths, up to 108.7-N/mm tensile strength could be achieved and fracture initiated at the weld seam–steel interface.

Mechanical Properties and Formability of Heat-Treated 7000-Series High-Strength Aluminum Alloy: Experiments and Finite Element Modeling

The feasibility of room-temperature (cold) forming of peak-aged 7075 high-strength aluminum alloy (7075-T6) sheets after solution heat treatment followed by water quenching was investigated. The anisotropic plastic properties of the heat-treated aluminum sheet (7075-WT) were experimentally characterized by performing uniaxial tensile tests (along different loading directions) and a balanced biaxial tension test. Limit dome height (LDH) and cylindrical cup drawing experiments were conducted to evaluate the formability at room temperature. With the obtained anisotropic mechanical properties, different yield functions, including Yld2000-2d, Hill 1948, and von-Mises, were determined and implemented in the finite element simulations to numerically predict the formability at room temperature. The formability of the 7075-WT sheet at room temperature could be significantly enhanced owing to the prior heat treatment, and the finite element simulations performed using the anisotropic yield functions and Voce hardening law were in good agreement with the measured LDH. The LDH of the 7075-WT sheet in the plane strain and biaxial stress modes and the earing in the circular cup drawing tests could be better predicted non-quadratic Yld2000-2d function than those by isotropic and Hill 1948 quadratic yield functions.

Seam-Joining between High-Strength Steel Sheet and Aluminum Alloy Sheet by Magnetic Pulse Welding

In this paper, we describe the process parameters and the joining property of seam-welded sheets obtained by magnetic pulse welding. In this method, it is possible to weld an aluminum alloy sheet to a high-strength steel sheet (SPFH), for example, A5052-H34/SPFH or A6061-T6/SPFH. The aluminum sheet is deformed to a convex, because the electromagnetic force generated in the sheet is symmetrical with respect to the central axis. Therefore, the seam-welded sheet is joined by two lines, not a plane. Collision times of the three kinds of the aluminum sheets are examined for various gap lengths at 2.0 kJ discharge energy in experiments, and collision velocities are calculated using the collision times. The collision times of the three sheets are approximately the same at gap lengths less than or equal to 2.48 mm, because magnetic pulse welding is a phenomenon with a high strain rate. It is difficult to weld an aluminum sheet with a large flow stress to a high-strength steel sheet. Therefore the choice of process parameters is important, the influential parameters are discharge energy and gap length. We should choose the values of these two parameters such that the first collision occurs before the maximum discharge current.