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Creep age forming (CAF) process is a novel metal forming method with major benefits including improved mechanical properties and cost reduction for aviation industry applications. CAF happens due to creep phenomenon and stress-release during the artificial aging of heat-treatable aluminum alloys. In this work, the creep age forming of 7075 Aluminum alloy at 120, 150 and 180 °C for 6, 24 and 48 h was done; tensile and hardness tests were used to characterize the samples. Results on spring-back revealed that it was influenced by time and temperature; by increasing the time and temperature, it was reduced from 54.1 to 39.51. Mechanical property evaluation also showed that by increasing the time, the strength and hardness could be enhanced due to microstructural evolution and precipitation during the CAF process. According to the mechanical and CAF results, two samples were selected as the optimum ones and their work hardening behavior and fracture surfaces were investigated

Creep age forming (CAF) process is a novel metal forming method with major benefits including improved mechanical properties and cost reduction for aviation industry applications. CAF happens due to creep phenomenon and stressrelease during the artificial aging of heat-treatable aluminum alloys. In this work, the creep age forming of 7075 Aluminum alloy at 120, 150 and 180 °C for 6, 24 and 48 h was done; tensile and hardness tests were used to characterize the samples. Results on spring-back revealed that it was influenced by time and temperature; by increasing the time and temperature, it was reduced from 54.1 to 39.51. Mechanical property evaluation also showed that by increasing the time, the strength and hardness could be enhanced due to microstructural evolution and precipitation during the CAF process. According to the mechanical and CAF results, two samples were selected as the optimum ones and their work hardening behavior and fracture surfaces were investigated [ABSTRACT FROM AUTHOR]

Creep-age forming (CAF) is a relatively new sheet metal forming method for manufacturing large-scale components with accurate shape and desired performance. This paper fully investigates the strain distribution, springback characteristic, mechanical property, and shape quality of an integrally-stiffened AA2219 alloy plate during CAF process. A new set of unified creep-ageing constitutive equations incorporating the effect of pre-deformation is developed, and its effectiveness in FE solver MSC.Marc is validated by tension creep-ageing simulation. CAF simulation of the stiffened plate is conducted. The results indicate that the final shape of the plate is simultaneously determined by plastic and creep deformations, while the creep deformation plays a major role, with corresponding contribution percentage of 33 and 67%. Comparisons between experiment and modelling show that the deviations are approximately 11% for springback and 3.2% for yield strength, which confirms the validity of CAF modelling. Furthermore, yield strengths on the skin of the plate are more evenly distributed as compared with those on the stiffeners. In addition, the values of generatrix linearity of the formed plate are very small, which demonstrates that the CAF-formed integrally-stiffened plate has high-precision shape quality. [ABSTRACT FROM AUTHOR]

Creep-age forming (CAF) is one of the effective techniques to fabricate extra-large integral airframe panels due to its good forming ability. However, few experimental researches have been conducted on stiffened panel with complex structures under creep-aging condition. In this work, a 1096 × 792-mm stiffened panel with typical structure features, such as stiffeners, the base plate with variable thickness, and the opening, has been formed on the tool of variable curvature at 165 °C for 10 h. Springback of the stiffened panel in static loading process is 91.1%, while it decreases to 82.3% after 10-h creep aging, which shows that plastic and creep deformation play an equivalent role in restraining the springback of the panel. Deformation of the stiffened panel concentrates around the opening with thicker plate and larger curvature, while little deformation occurs in the base plate with varied thickness or stiffeners. It indicates that the size, orientation, and position of the structure features all have significant impact on final shape of the stiffened panel. The accuracy of the corresponding finite element (FE) simulation has been verified by the fact that the proportion of the area, where the gap between the simulated and the experimental part is less than 0.6 mm, accounts for more than 90%. [ABSTRACT FROM AUTHOR]

Creep-age forming (CAF) is an interesting process for the airframe industry, as it is able to form or shape panels into smooth, but complex, curvatures. In the CAF process, the ageing cycle of the alloy is used to relax external loads imposed to the part, through creep mechanisms. Those relaxed stresses impose a new curvature to the part. At the end of the process, significant spring back (sometimes about 70%) is observed and the success in achieving the desired form depends on how the spring back can be predicted in order to compensate it by tooling changes. Most of the applications relate to simple (non stiffened) panels. The present work deals with the CAF of aluminum panels for aircraft wing skin application. CAF was performed using vacuum-bagging autoclave technique in small scale complex shape stiffened panels, machined from an AA7475 alloy plate. An analytical reference model from the literature was employed estimate the spring back effect in such panel geometry. This model that deals with simple plates was adapted to stiffened panels using a geometric simplification, resulting in a semi-empirical model. The results demonstrate that CAF is a promising process to form stiffened panels, and the spring back can be roughly estimated through a simple model and few experiments.

Springback behaviour of surface-machined 7B04-T651 aluminium plates of 3–8 mm thicknesses, creep-age formed under a single curvature bending radius of 1200 mm, has been experimentally investigated. The surface residual stress levels of the plates have been measured and the typical residual stress distribution and values in 7000-series aluminium alloys have been reviewed and analysed. Finite element models have been developed to simulate creep-age forming (CAF) processes and predict the amount of springback in the physical CAF test. Machining-induced residual stresses and distortion have been considered in the finite element models. This paper presents a first attempt to study the effect of machining-induced residual stresses on the errors of springback prediction in CAF of 7000-series aluminium alloy panel components. It has been found that the effect of machining-induced residual stresses on the accuracy of springback prediction is related to the initial loading stress levels in CAF. In addition, when the initial distortion conforms to the loading curvature, an opposing effect on the material’s deformation is observed. [ABSTRACT FROM AUTHOR]

Creep-age forming (CAF) is one of the relatively new forming techniques that has been proven viable for the production of extra-large integral airframe structures. However, experimental studies on forming stiffened structures under creep-ageing conditions remain scarce. In this work, 200 mm × 48 mm integrally stiffened plates of aluminium alloy 2219 have been formed on an end clamp device that has a bending radius of 156 mm and creep-aged at 175 °C for 18 h. Three different stiffener designs, namely the beam stiffened, waffle and isogrid plates, are tested alongside the flat plates. Utilisation of PTFE pocket fillers and intermediate sheets to reinforce and protect stiffeners during forming operation is found effective as demonstrated by the defect-free formed parts with smooth curvature. Springback of the plates ranged from 12.2 to 15.7% in the experimental studies for different stiffener designs. Using the CAF material constants determined for this alloy, corresponding finite element models have been developed and experimentally validated using the measured profiles of the creep-age formed plates. Up to 6.3% difference in springback is observed when forming workpieces with different pre-form age condition alone. [ABSTRACT FROM AUTHOR]

Creep-ageing behaviour of aluminium alloy 7B04-T651 at 115 °C under a range of tensile stress levels has been experimentally investigated and numerically modelled for creep-age forming (CAF) applications. Creep strain, yield strength evolution and precipitate growth of creep-aged specimens were investigated. The alloy was modelled using a set of unified constitutive equations, which captures its creep deformation and takes into account yield strength contributions from three creep-age hardening mechanisms. Applications of the present work are demonstrated by implementing the determined material model into a commercial finite element analysis solver to analyse CAF operations carried out in a novel flexible CAF tool. Stress relaxation, yield strength, precipitate size and springback were predicted for the creep-age formed plates. The predicted springback were further quantified and compared with experimental measurements and a good agreement of 2.5% deviation was achieved. This material model now enables further investigations of 7B04 under various CAF scenarios to be conducted inexpensively via computational modelling. [ABSTRACT FROM AUTHOR]

Thermomechanical treatment is often conducted to improve the formability and performance of high-strength aluminum alloys simultaneously. This paper presents an improved creep-age forming (CAF) processing technology that combines thermomechanical pretreatment and CAF. The proposed processing technique is expected to enhance the forming efficiency and comprehensive properties of Al–Zn–Mg–Cu alloys subjected to CAF. Creep-aging behavior and properties of the 7150 aluminum alloy undergoing retrogression and re-aging (RRA) with 3 % pre-strain treatment was experimentally investigated in this study. Results obtained reveal greater creep deformation in thermomechanically pretreated samples than in those pretreated using RRA or 3 % pre-strain exclusively. Mechanical properties, electrical conductivity, and exfoliation corrosion of creep-aged samples with thermomechanical pretreatment were closely match those of the traditional RRA-treated sample. Microstructure observation discovered η 2 phase in the creep-aged RRA pretreated sample in addition to conventional GP / η' phase, η phase and coarse and discrete grain-boundary precipitates. Low concentration of solutes in matrix in RRA pretreated sample impeded dislocation motion less and pre-strain-induced dislocations increased the creep strain significantly. Moreover, hardening effect of η2 phase and dislocation hardening compensated the strength loss due to precipitation coarsening. The proposed thermomechanical pretreatment process provides an appropriate initial temper for CAF of Al–Zn–Mg–Cu alloys, thereby improving the creep formability and comprehensive properties remarkably. The findings of this study could aid in the development of CAF technology for Al–Zn–Mg–Cu alloys, especially in terms of manufacturing aviation components. [ABSTRACT FROM AUTHOR]

High-resolution (HR) high-angle annular dark-field (HAADF) scanning transmission electron microscopy (STEM) has revealed the atomic lattice defects in different types of η precipitates in the Al–Zn–Mg–Cu aluminium alloy subjected to creep-age forming treatment (with a constant stress lower than its room-temperature yield strength during ageing). Along the zone axes of [110] Al //[2 1 ‾ 1 ‾ 0]η of η 1 and η 12 , [112] Al //[2 1 ‾ 1 ‾ 0]η of η 2 and [100] Al //[2 1 ‾ 1 ‾ 0]η of η 13 , atomic projections of (2 1 ‾ 1 ‾ 0)η have been investigated. In those types of η, elongated hexagonal lattice defects (labelled as Type I defects) can be found; they are apparently related to local disorder in atomic stackings. Furthermore, in η 12 , elongated hexagonal lattice defects with a much higher aspect ratio (labelled as Type II defects) are uniquely observed. These atomic lattice defects are presumably pertinent to the lattice accommodation in the course of creep-age forming. Additionally, in η 1 and η 12 , the features of a Penrose tiling defect connecting with Type I defects are observed, and these complex defects obviously affect the growth direction of the precipitate, resulting in a nearly spherical morphology. Alternatively, several entirely-passed faulted layers in a new type of precipitate, η 14 , consequently bring about a new orientation relationship: (51 3 ‾) Al //(0001)η 14 and [112] Al //[2 1 ‾ 1 ‾ 0]η 14. Moreover, in an atomic STEM image of η 14 , the significant Z-contrast gradient adjacent to the transformation front of η 14 elucidates the Zn/Cu diffusion from the matrix to the precipitate along {1 1 ‾ 1 ‾ } Al planes at the interface. [Display omitted] • Along the [2110]η zone axis, the edge-on atomic configurations of η 1 , η 2 , η 12 , η 13 and η 14 precipitates have been investigated to explore their atomic lattice defects. • Within the spherical-like morphologies of η 1 and η 12 , Penrose tiling features jointed with elongated hexagonal lattice defects were identified. • In η 12 precipitates, two types of elongated hexagonal lattice defects of different sizes were found. • Differently-oriented elongated hexagonal lattice defects presumably affect the local stackings, bringing about differently-oriented types of η precipitates. • In the newly-typed η 14 precipitate, some entirely-passed stacking fault layers presumably cause a new specific orientation relationship with respect to the Al matrix. [ABSTRACT FROM AUTHOR]