AA7075-T6 Sheet Research Articles

Numerical simulation analysis of the temperature field in induction heating-assisted flow drilling of the 7075 aluminum alloy

Flow drill riveting (FDR) is a unique method designed for the joining of disparate materials such as aluminum alloys and carbon fiber-reinforced polymers (CFRPs). The success of FDR hinges on the frictional penetration process executed by the blind rivet. When deploying FDR for the joining of high-strength materials, a substantial penetration force often arises, leading to deformations in the workpieces and potentially causing failure in the joining process. To mitigate the excessive penetration force, the use of induction heating to pre-warm the workpiece before the FDR process has been suggested. This led to the development of the induction heating-assisted flow drill riveting (IHFDR) method. In this work, a FEM model was built for the IHFDR penetration process. This model was specifically designed for the AA7075-T6 sheet, utilizing the ABAQUS software. Induction preheating impact on the temperature of the workpiece during the rivet penetration process was scrutinized. The analysis revealed that the workpiece’s peak temperature during the rivet penetration process was elevated due to induction preheating. Furthermore, it was observed that as the preheating elevated the workpiece temperature, the rate of increase for the peak temperature lessened.

High strain rate constitutive and fracture characterization of AA7075-T6 sheet under various stress states

The stress state and strain rate dependent fracture behavior of high strength aluminum sheet, AA7075-T6, is characterized experimentally and numerically. Experiments were performed using shear, hole tension, notch tension, and groove tension specimen geometries at strain rates in the regime 10−2 – 500 s−1. The temperature change on the specimen surface was measured utilizing high speed thermal imaging, while strains were acquired using DIC-based optical measurements. Mild positive rate sensitivity was observed in the flow response for equivalent strains up to approximately 20%, while the flow stress decreased at higher (500 s−1) strain rates as the strain levels increased. An increase in temperature was observed within the gauge region of the specimens during elevated strain rate (500 s−1) tests leading to thermal softening. Positive strain rate sensitivity was observed in the measured axial force response for the hole tension and notch tension specimens. The fracture limits of the AA7075-T6 in the shear, hole tension, and notch specimens decreased between strain rates of 0.01 and 10 s−1, then increased at strain rates between 10 and 500 s−1. A strain rate and temperature dependent Hockett–Sherby (HS-SRT) constitutive model was proposed to capture the dependency of the flow stress on the equivalent plastic strain, strain rate, and temperature. The generalized Drucker-Prager (GDP) fracture model was extended to capture the effect of strain rate on fracture initiation. Numerical simulations of the coupon experiments were then performed to extract the local stress and strain states to calibrate the GDP fracture model across the range of strains considered.

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.

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.

Plasticity and fracture of AA7075 at elevated strain rates and temperatures

The accurate description of the strain rate and temperature dependent response of Aluminium alloys is a perpetual quest in the hot forming industry. In the present study, uniaxial tension, and notched tension experiments are conducted for an aluminium AA7075-T6 sheet metal at various temperatures and strain rates. The experimental campaign covers strain rates ranging from 0.001/s to 100/s, and temperatures ranging from 20°C to 360°C. We observe low strain rate sensitivity at room temperature, with an increase in strain rate sensitivity as temperature is increased up to 360°C. An YLD2000-3D model is employed to describe the anisotropy of the material. A machine learning based hardening model is employed to capture the complex strain rate and temperature effect on the observed hardening response. Counter-example regularization is utilized to guarantee a convergence in the numeric return-mapping algorithm. Comparing the experimental force-displacement curves with the numerical predictions, the neural network model accurately describes the large deformation response of the material in the post-necking range.

Experimental calibration of ductile fracture parameters and forming limit of AA7075-T6 sheet

A new method with digital image correlation (DIC-method) is proposed for the calibration of ductile fracture parameters and a partition optimizing method based on multi-group fracture parameters is utilized for the establishment of forming limit diagram (FLD). Both of the two methods are verified with the application of Lou-Huh ductile fracture criterion on AA7075-T6 sheet in this paper. A comparison study with conventional method combining simulation with experiment (SE-method) shows that DIC-method can provide objective experimental data for fracture parameter calibration without virtual simulation. It is also indicated that the plastic flow of AA7075-T6 sheet has stress state dependence and it is necessary to build related material subroutine into FEA software. A research on the establishment of FLD is conducted with the calibrated fracture parameters. It is demonstrated that the FLD of AA7075-T6 sheet could not be accurately built by a single group of fracture parameters. Fracture morphology shows the reason is that the fracture of AA7075-T6 is not pure ductile fracture. The partition optimizing method based on multi-group fracture parameters could better improve the FLD of AA7075-T6. The optimized FLD is well verified by the result of punch-stretch experiment.

Plasticity and fracture of aluminium 7075 at high strain rates and elevated temperatures

Slow, intermediate and high strain rate experiments with UT geometries are performed on aluminum AA7075-T6 sheet metal at various temperatures. The comprehensive experimental program characterizes the plasticity response at temperatures ranging from 20°C to 360°C and at strain rates ranging from 0.001/s to 150/s. The elevated temperature - elevated strain rate experiments are performed on a hydraulic tensile testing machine and a Split Hopkinson Pressure Bar system with a Load Inversion Device along with a custom-made induction heating system. A machine learning based modified Johnson-Cook plasticity model is calibrated to capture the complex strain rate and temperature effect of the observed hardening response.

Comparative study on forming limit prediction of AA7075-T6 sheet with M−K model and Lou−Huh criterion

Abstract In order to effectively predict the fracture of AA7075-T6 sheet, the forming limit curves of AA7075-T6 high-strength sheet were drawn according to Morciniak−Kuczyski (M−K) model and Lou−Huh criterion, respectively. The errors between the predicted values of the two theoretical prediction models and experimental values were calculated by error analysis. The forming limit curves were verified by the punch−stretch test to evaluate the prediction accuracy of M−K model and Lou−Huh criterion. The error analysis results show that the mean error of Lou−Huh criterion with the optimal parameters for all tensile specimens is 25.04%, while the mean error of M−K model for all tensile specimens is 74.24%. The prediction accuracy of Lou−Huh criterion in predicting the fracture of AA7075-T6 sheet is higher. The punch−stretch test results show that the forming limit curve drawn by Lou−Huh criterion can effectively predict the fracture of AA7075-T6 sheet, but the prediction accuracy of M−K model is relatively poor.

Forming limit prediction of AA7075-T6 sheet based on ductile fracture criterion and the error analysis of parameters calibration

It is worthy to investigate how it will affect the parameters calibration using an average stress state variable before the practical application of a ductile fracture criterion. In order to study this problem, ten notch specimens of AA7075-T6 sheet were designed to implement the tension tests and a parallel simulation of each test was run to obtain the variation of related variables such as stress triaxiality, Lode parameter and fracture strain. The Lou-Huh criterion was selected to research the prediction error through the difference between the damage value calculated by integral expression and that calculated by analytical expression. Based on the error analysis method, a clear answer was given on how to choose the tension tests of notch specimens in the calibrating process of fracture parameters. The studying results show that the stress state variation of notch specimens has a significant influence on the calibration result. It turns out that how to choose specimens from the ten notch specimens to calibrate the fracture parameters also has big influence on the result. Therefore, it is necessary to conduct an error analysis after the calibration of fracture parameters. Based on the error analysis results, the fracture parameters of AA7075-T6 sheet were optimized and its forming limit diagram (FLD) was deduced based on the optimized parameters. The predictive result of FLD is safe compared with the experimental forming limit results.

Role of tool design on thermal cycling and mechanical properties of a high-speed micro friction stir welded 7075-T6 aluminum alloy

AA7075-T6 sheet welds that were 0.5 mm thick were fabricated by micro friction stir welding (μFSW) using a pinless tool as well as a pin tool. The experiments were conducted at a higher rotational speed of 6000 rpm and a higher welding speed of 1200 mm/min than those that are used in conventional FSW. The heat generation, peak temperature and elevated-temperature exposure time (t150) of the joints fabricated by the pin tool were higher than those fabricated by the pinless tool at the same shoulder penetration depth. The minimum ratio of the thickness reduction at the center of the friction stir weld was 0% for the pinless tool at a penetration depth of 0.14 mm. In comparison to those for the joints fabricated by the pin tool, the joint fabricated by the pinless tool showed less softening and better tensile properties. For the welds fabricated by the pin tool, the tensile properties increased with increasing penetration depth. In the case of welds made by the pinless tool, the tensile specimens failed at the transition region between the stir zone and the thermomechanically affected zone on the retreating side due to the local thickness reduction. However, the tensile specimens failed at the center of the stir zone due to kissing bond defects in the joints produced by the pin tool.

Warm Forming Characteristics of AA7050-T6

The formability of AA7050 aluminum alloy sheet is characterized under isothermal forming conditions at room temperature and 150 °C. The material was tested in the T6 condition. Nakazima limiting dome height (LDH) tests in conjunction with stereoscopic digital image correlation (DIC) strain measurements are used for the formability assessment. Four-point forming limit curves (FLCs) were developed at the two test temperatures. AA7050-T6 is shown to respond positively to elevated temperature forming with a 0.19 major true limit strain for a 76.2 mm Nakazima specimen, for example, compared to 0.13 at room temperature. The current results for AA7050-T6 are compared to the formability of AA7075-T6 sheet tested under similar conditions; a slightly higher formability was observed for the AA7075-T6 alloy at the elevated temperature.

Formability and forming force in incremental sheet forming of AA7075-T6 at different temperatures

The AA7075-T6 sheet recently received attention, owing to its low weight and strength for use fabricating automotive parts (e.g., body, motor case). Owing to its low ductility and high strength at room temperature, it is difficult to form; a heating process is required. In this paper we investigate the mechanical properties of a 1.0-mm-thick AA7075-T6 sheet at different temperatures via an uniaxial tensile test, examining the forming characteristics of incremental sheet forming (ISF) at different temperatures. In the ISF experiment, a sheet clamped on a fixed die was heated and maintained at the designed temperature. The forming force was measured by an in situ force monitoring system. The forming parameters included tool diameter, vertical step, temperature, and feed speed. Vertical step and temperature were selected at three levels for experimental design. The formability was evaluated by varying the shapes of wall-angle conical frustums. When the temperature increased to 200 °C, the formability increased dramatically. The fracture-forming limit curve in ISF was defined for this material at different temperatures. This fracture criterion was loaded into ABAQUS software to predict fracture. The comparison between experimental and finite-element method simulation shows good agreement.

Retrogression and Reaging Applied to Warm Forming of High-Strength Aluminum Alloy AA7075-T6 Sheet

This study introduces retrogression forming, a new warm-forming methodology for high-strength aluminum alloys such as AA7075-T6. Retrogression forming combines a retrogression heat treatment with simultaneous warm forming at approximately 200 °C to improve upon the room-temperature ductility of AA7075-T6, approximately 10 pct in tension, and can then use the automotive paint-bake cycle to restore nearly peak-aged strength. Experimental data indicate that retrogression forming at 200 °C provides a tensile ductility of 20 pct, approximately double that of room temperature, and that subsequent reaging by a simulated paint bake restores hardness to within 10 pct of the original peak-aged (T6) hardness. To explore the retrogression-forming concept, the retrogression behaviors of two AA7075-T6 sheet materials from different suppliers are characterized between 200 °C and 350 °C. The times and temperatures of retrogression treatments suitable for simultaneous warm forming are determined, and tensile ductilities under these conditions are measured. Tensile ductilities during high-temperature deformation (up to 520 °C) are measured and compared to those possible under retrogression-forming conditions. Plastic flow at temperatures of 460 °C to 520 °C is governed by dislocation-climb-controlled creep, a mechanism different from the behaviors observed under retrogression-forming conditions.

Strain rate and thermal softening effects in shear testing of AA7075-T6 sheet

Shear tests were performed at strain rates ranging from quasi-static (0.01 s-1) to 500 s-1 for AA7075-T6 sheet metal alloy at room temperature. A miniature sized shear specimen was used in this work to perform high strain rate shear testing. Digital image correlation (DIC) techniques were employed to measure the strains in the experiments. At maximum in-plane shear strains greater than 20%, the AA7075-T6 alloy demonstrated a reduced work hardening rate at elevated strain rates. At lower strains, the AA7075-T6 alloy showed mild positive rate sensitivity. The strain to localization (using the Zener-Holloman criterion), measured using the DIC technique, decreased with strain rate in shear loading. The strain at complete failure, however, exhibited an increase at the highest strain rate (500 s-1). The current work also focused on characterization of the thermal conditions occurring during high rate loading in shear with in situ high speed thermal imaging. Experimental results from the highest strain rate (500 s-1) tests showed a notable increase in temperature within the specimen gauge region as a result of the conversion of plastic deformation energy into heat.

High-Speed Friction Stir Welding of AA7075-T6 Sheet: Microstructure, Mechanical Properties, Micro-texture, and Thermal History

Friction stir welding (FSW) is a cost-effective and high-quality joining process for aluminum alloys (especially heat-treatable alloys) that is historically operated at lower joining speeds (up to hundreds of millimeters per minute). In this study, we present a microstructural analysis of friction stir welded AA7075-T6 blanks with high welding speeds up to 3 M/min. Textures, microstructures, mechanical properties, and weld quality are analyzed using TEM, EBSD, metallographic imaging, and Vickers hardness. The higher welding speed results in narrower, stronger heat-affected zones (HAZs) and also higher hardness in the nugget zones. The material flow direction in the nugget zone is found to be leaning towards the welding direction as the welding speed increases. Results are coupled with welding parameters and thermal history to aid in the understanding of the complex material flow and texture gradients within the welds in an effort to optimize welding parameters for high-speed processing.

Stress-strain analysis on AA7075 cylindrical parts during hot granule medium pressure forming

Hot granule medium pressure forming (HGMF) is a technology in which heat-resistant granules are used to replace liquids or gases in existing flexible-die forming technology as pressure-transfer medium. Considering the characteristic of granule medium that seals and loads easily, the technology provides a new method to realize the hot deep-drawing forming on high strength aluminum alloy sheet. Based on the pressure-transfer performance test of granule medium and the material performance test of AA7075-T6 sheet, plastic mechanics analysis is conducted for the areas, such as the flange area, force-transfer area and free deforming area, of cylindrical parts deep-drawn by HGMF technology, and the function relation of forming pressure is obtained under the condition of nonuniform distribution of internal pressure. The comparison between theoretical result and experimental data shows that larger deviation occurs in the middle and later period of forming process, and the maximum theoretical forming force is less than the experimental value by 24.6%. The variation tendency of the theoretical thickness curve is close to the practical situation, and the theoretical value basically agrees well with experimental value in the flange area and the top area of spherical cap which is in the free deforming area.

Warm formability and post-forming microstructure/property of high-strength AA 7075-T6 Al alloy

Formability of peak-aged AA 7075-T6 sheet across a temperature range from room temperature to 250°C as well as post-forming microstructure/property were investigated in this paper. It showed that the optimized formability was obtained at 200°C, due to the higher ductility and work hardening capacity at this temperature. The dominant phases in 7075-T6, 200°C warm formed, and 250°C warm formed samples were fine η′ and GP zones, well-developed η′, and coarser η phases, respectively. Post-forming and post-paint-baking mechanical property of 7075 samples was closely associated with those corresponding microstructures. Specifically, 200°C warm forming caused limited coarsening of matrix precipitates (MPts) and generated a certain number of dislocations, due to the combined effect of the decreased precipitation strengthening and increased dislocation strengthening, the inherent high strength of 7075-T6 was perfectly preserved after 200°C forming. However, 250°C forming led to severely decreased hardness due to the sharply coarsened MPts. Furthermore, the paint-baking treatment exerted little influence to MPts, grain boundary precipitates (GBPs) and precipitate free zone (PFZ), thus the hardness loss caused by paint-baking was unobvious. Overall, 200°C was the appropriate temperature under which the peak-aged 7000 alloys exhibited enhanced formability and maintained high post-forming strength, after paint baking, the drawn component possessed microstructure/property similar to those of retrogression and re-aged (RRA) samples.