Creep Age Forming Research Articles

A generalised framework for modelling anisotropic creep-ageing deformation and strength evolution of 2xxx aluminium alloys

The 2xxx aluminium alloys are extensively applied in the aerospace industry due to their lightweight and balanced performance characteristics. However, a comprehensive method for modelling both the anisotropic creep deformation and strengthening behaviour in creep age forming (CAF) for 2xxx aluminium alloys remains lacking. This paper presents a generalised framework for establishing constitutive models capable of describing the anisotropic creep deformation coupled with the microstructure and material strength evolutions during creep-ageing of both the original and the pre-deformed 2xxx series Al alloys. This framework extends the rolling direction-based material model to anisotropic scenarios at varying angles between the loading and rolling directions, by employing the non-uniform rational B-splines (NURBS). The details about the anisotropic model calibration and numerical simulation implementation are demonstrated. The feasibility of this method was verified by its application to various 2xxx series aluminium alloys with or without pre-deformation, through constitutive modelling and numerical simulation, with satisfactory agreements between prediction and experimental data. For the first time, the proposed framework provides a generalised routine for establishing anisotropic creep-ageing models for various 2xxx aluminium alloys.

Experimental study and modelling of anisotropic behaviour of aluminium-lithium alloys in creep age forming

The anisotropic deformation behaviour of the 3rd generation aluminium-lithium (Al-Li) alloys can significantly affect the accuracy of prediction and fabrication, especially for fabricating complex shaped components. To address this challenge and shed light on improving the prediction accuracy with the existence of material anisotropy, this study systemically investigated the anisotropic behaviour during deformation and creep-ageing, and proposed a comprehensive anisotropic material model and its implementation method in the numerical simulation, which was verified with four-point bending creep age forming (CAF) tests. The anisotropic behaviours of the material including yielding, hardening, and creep deformation were investigated with uniaxial tensile and creep-ageing tests adopting specimens cutting along 0°, 15°, 30°, 45°, 70° and 90° to the rolling direction. The yield strengths varied in the range of 378 to 456 MPa for different directions, with the highest and lowest values appearing at 0° and 70°. The creep deformation also showed strong anisotropic behaviour and the accumulated creep strain for 90° (0.52%) was four times larger than the value for 0° (0.11%) under 415 MPa at 143 °C for 5 h An anisotropic material model was established based on the modified non-associated quadratic Hill48 yield function and can adequately capture the observed anisotropic behaviour. An implementation strategy using the bisection method with the map returning scheme was proposed for incorporating the proposed model in numerical simulation. A good agreement was achieved between the predicted and experimental results for four-point bending CAF tests, demonstrating the validity of the proposed anisotropic material model and the implementation method.

Effect of natural aging on formability and properties of Al-Cu-Li alloy during creep age forming

As-quenched (AQ) temper due to a good forming potential, is the most common material initial temper used in creep age forming (CAF). The inevitable evolution of microstructure during the natural aging (NA) process may have an effect on the deformation and mechanical properties of alloy during CAF. In this research, a NA regime suitable for CAF of Al-Cu-Li alloy (2195) was obtained by correlation with the creep deformation behavior, microstructures and mechanical properties. The results indicate that the creep deformation of the alloy increases with increasing NA time, after 24 h of NA, the creep deformation reaches a stable value. The creep strain of the NA 24 h temper alloys after creep aging of 21 h is about 1.5 times than that of the AQ temper alloys. The NA 24 h temper alloys present five-stages creep feature, while the creep deformation of the AQ temper presents four-stages creep feature. In addition, this study found that NA before creep aging has a slightly positive effect on age hardening, with increases of 3.9% in yield strength and 3.0% in tensile strength, but with a loss of 5.9% in ductility. The difference in creep deformation and mechanical properties is correlated with dislocation movement and the precipitation and growth of T1 phase. It is recommended to pre-treat Al-Cu-Li alloy (2195) with NA for 24 h to obtain stable creep formability and good mechanical properties.

Comparative Study of Creep and Stress Relaxation Behaviour during Ageing of 7050 Aluminum Alloy

This work focuses on quantitatively investigating the relationship between creep and stress relaxation of 7050 aluminum alloy at ageing temperatures. The creep strain rates, creep exponent, n, and activation energy were extracted from both the creep and stress relaxation curves to explore the creep mechanisms. Results showed that higher applied stress promotes the creep and stress relaxation extents. The calculated creep strain rates from creep and stress relaxation curves located in similar ranges and share similar n values (n = 3.7 for creep and n = 3.5 for stress relaxation), indicating that both tests are dominated by the dislocation slip mechanism. Additionally, the n value for creep tests is a constant throughout the tests, indicating that the dominant creep mechanism is unchanged. However, the activation energy changes from 123 kJ/mol to 139 kJ/mol, possibly due to evolved dislocations. Such a new finding could provide a supporting mechanism for building constitutive equations depending on the evolved A value. Furthermore, compared to the single-stage creep ageing, no primary creep was observed in T74 multi-stage creep ageing while similar amount of total creep strains was achieved. This suggests an alternative loading sequence for creep age forming of 7050 aluminum alloy, i.e., loading at the second stage to avoid prolonged exposure to high temperatures, extend the tool life, and achieve similar amounts of inelastic deformations. Based on all the above studies, creep and stress relaxation have a certain relationship. This paper provides a theoretical basis for future research on creep ageing forming of 7050 aluminum alloy.

Non-isothermal creep age forming of Al-Cu-Li alloy: Experiment and modelling

Non-isothermal Creep Age Forming (CAF), including loading, heating, holding, cooling and springback stages, is an advanced forming technique for manufacturing high performance large integral panels at short production period and low cost. However, the creep deformation and aging precipitation during heating stage is often neglected in experiments and modeling, leading to low forming precision. To achieve shape forming and property tailoring simultaneously, a deep understanding of the non-isothermal creep aging behavior and the establishment of predictive models are urgently required. A new five-stage creep feature of Al-Cu-Li alloy during the non-isothermal creep aging is observed. The microstructural interactions between the dislocations, solute atoms, Guinier Preston zones (GP zones) and T1 precipitates are found to dominate the five-stage creep aging behavior. The physical-based model considering temperature evolution history is established to describe the five-stage creep feature. The springback and yield strength of non-isothermal creep age formed plates with different thicknesses are predicted and compared by non-isothermal CAF experiments and corresponding simulations. The CAF experiments show that the springback and yield strength of the non-isothermal creep age formed plate are 62.1% and 506 MPa, respectively. Simulation results are in good agreement with experimental results. The proposed model broadens the application of traditional CAF models that mainly focus on isothermal conditions.

Plastic loading induced high strength-ductility for creep aged Al–Cu–Li alloys

Large-curved or high-ribbed large-size integral panels are often subjected to plastic deformation during the loading stage of creep age forming (CAF). Although this plastic loading helps to improve the creep deformation, whether both good strength and ductility of the creep-aged alloy under plastic loading can be guaranteed remains unclear. Thus, under the applied stress ranging from elastic (230/250/270 MPa) to plastic regions (320/340 MPa) at 180 °C, mechanical properties and microstructural evolution of creep-aged 2195-T4 Al–Cu–Li alloys were systematically investigated. Surprisingly, high yield strength up to 620 MPa and ductility above 9.4% are obtained for peak-aged alloys under plastic loading, increasing by 9.0%∼21.5% and 6.7%∼30.1% over elastic loading and stress-free aging, respectively. High strength is attributed to the combined effect of high density of dislocations, densely distributed fine T1 precipitates in the matrix, and narrowed precipitate free zones (PFZ) at grain boundaries. Meanwhile, it was observed that shearing of T1 plates happens only as single shearing step, and numerous single-shearing events occur at different locations, accompanied by elastic and/or plastic distortion and progressively segmented rotation (accumulated angle up to 20°) of T1 precipitate, which induce high ductility during plastic deformation. The above good mechanical properties were much less sensitive to aging time for plastic-loading creep-aged alloys relative to stress-free or elastic-loading counterparts, which helps to widen the CAF process window. This paper offers a new strategy of tailoring shape and properties during CAF, which would break the limitation of CAF currently only for the small-curved components.

An efficient closed-form solution for springback prediction and compensation in elastic–plastic creep age forming

Accurately predicting the amount of springback has always been a prior focus in metal forming industry, particularly for creep age forming (CAF), for its significant effect on tool cost and forming accuracy. In this study, a closed-form solution for CAF springback prediction covering deformation from elastic to plastic loadings was developed by combining the beam theory and Winkler’s theory, based on which an efficient springback compensation method for CAF was proposed. This developed solution extends the application area beyond the traditional beam theory-based springback prediction methods, maintaining its validity with large loading deflection in plastic range. Finite element (FE) simulation and four-point bending CAF tests adopting a 3rd generation Al-Li alloy were conducted in both elastic and plastic forming regions and the results showed close agreement with the closed-form springback predictions. For the proposed compensation method, an adjustment factor was introduced for complex flexible tool CAF to consider its deviation from the uniform stress loading and can be obtained using the closed-form solution. The flexible tool CAF tests using the Al-Li alloy demonstrated the applicability of the proposed compensation method to obtain the target shape within reasonable iterations, which can be further reduced by combining FE simulation.

Semi-analytical study of buckling response for grid-stiffened panels during creep age forming

The elastic–plastic buckling response of panels with grid stiffeners during creep age forming (CAF) has been studied using a semi-analytical method in this research. A simplified model for panels with multiple grid stiffeners under bending has been established and the effective width has been applied to consider the flexible skin effect and multiple stiffener effect. In the semi-analytical method, the equilibrium equation for the simplified model has been solved via differential quadrature method to acquire the buckling stress of grid-stiffened panels, using deformation theory (DT) and incremental theory (IT) of plasticity. The results with IT show good agreements with both the published experimental and non-linear FE results, demonstrating its effectiveness. Based on the method, the effects of geometric dimensions of grid-stiffened panels, multiple stiffeners and temperature on buckling response in the elastic and plastic regions have been studied. It is found that the transverse stiffener provides a 13.9% higher buckling stress of grid-stiffened panels than blade-stiffened panels, and buckling may occur in the heating stage during CAF. The proposed semi-analytical method based on IT provides accurate predictions of buckling stress during either CAF or cold forming, which can guide the parameter optimisation of grid-stiffened panels in design.

Improved creep behaviour for a high strength Al-Li alloy in creep age forming: Experimental studies and constitutive modelling

The creep deformation of 3rd generation 2xxx Al-Li alloys in creep age forming (CAF) is unsatisfactorily low due to its high strength and required low ageing temperature, especially for the commonly used T8 temper. Promoting creep deformation in such process will significantly reduce springback in CAF, making easier the springback compensation in tool design. In this study, two possible process routes of inducing increased creep deformation were explored, one by employing high applied stress to introduce small plastic strain in loading stage; and the other by conducting pre-deformation prior to creep-ageing. The creep behaviour with and without pre-deformation was investigated through creep-ageing tests under stresses ranging from 300 to 430 MPa at the ageing temperature of 143°C. Due to the decrease of threshold stress with introduced plastic strain in the loading stage and/or pre-stretching, a faster increase of creep strain with applied stress was observed for both methods when the applied stress surpassed 400 MPa. The strain promotion of the former was higher than the latter when above 415 MPa. A mechanism-based constitutive model was proposed, with additional work hardening equations to describe the relationship between threshold stress and dislocation density induced by plastic strain in the loading stage and pre-stretching, to model the microstructural evolution and reflect the nonlinear increase of creep strain with stress in both methods. The adequacy of this proposed unified model was demonstrated with good agreement with experimental data utilising both methods. The adaptability of the model in multiaxial case was verified using four-point bending CAF tests with the same Al-Li alloy.

Experimental and numerical investigations on buckling behaviour of stiffened panel during creep age forming

Experimental and numerical studies on the buckling behaviour of stiffened panels under four point bending during creep age forming (CAF) have been carried out in this study for the first time. The experimental programme comprised buckling tests of five different sizes of stiffened panels of aluminium alloy 7050 at room temperature and buckling tests with different loading degrees in CAF including the loading, heating and creep-ageing stages. The buckling mode, the strain distribution and the strain evolution of the stiffened panel were obtained from the experiments, using digital image correlation (DIC) for the room temperature tests and strain gauges for the CAF tests. The effect of stiffener height and stiffener thickness and the effect of the heating and creep-ageing stages in CAF on the buckling behaviour have been investigated and discussed. It was found that buckling mode varied from one half-wave cosine mode in the elastic loading to three half-wave cosine mode with the increase of buckling stress from elastic to plastic region, and during CAF buckling mainly occurred and grew in the heating process. The corresponding non-linear finite element (FE) simulations of stiffened panels at room temperature and ageing temperature (160 °C) have also been carried out, and the FE results of buckling strain and buckling mode shape show a good agreement with experimental results. The non-linear FE method can provide accurate results of buckling strain and formability limits for cold forming and CAF processes, which can be used to guide the structural design of stiffened panels.

Investigation of stress effect on creep, precipitation and dislocation evolution of Al–Li alloy during creep age forming

The 3rd generation Al–Li alloy is a good candidate for large panel fabrication adopting creep age forming (CAF) because of its balanced synergy of high strength, good toughness, corrosion resistance and light weight. In this study, the creep-ageing behaviour of a 2xxx series 3rd generation Al–Li alloy in T8 state has been experimentally investigated under different stresses ranging from 300 to 430 MPa at 143 °C for up to 20 h. The corresponding evolution of precipitate size, its distribution and dislocation density during CAF has been analysed using transmission electron microscopy (TEM) and X-ray diffraction (XRD) tests. With the increase of the applied stress, the dominant deformation mechanism transformed from diffusion creep to dislocation creep at around 400 MPa, with corresponding stress exponent n = 2.9 in the former and n = 8.4 in the latter. A strong stress dependent characteristic has been observed in creep deformation and evolutions of dislocation density and precipitate size during creep-ageing. The creep strain increased dramatically above 400 MPa and the accumulated creep strain at 430 MPa was 7 times higher than that at 400 MPa after 20 h creep-ageing. Much higher dislocation density was observed at the applied stress above 400 MPa according to XRD measurements, which accelerates the precipitation kinetics. It was also found that the dominance of T1 over θ′ precipitates increased with applied stress and ageing time during creep-ageing at 143 °C.

Thermomechanical pretreatment of Al-Zr-Mg-Cr alloy to improve the performance through creep-age forming

High-strength alloys are commonly treated with thermomechanical treatment to concurrently improve the alloy's forming and performance. Improved CAF processing method is presented in this research, which blends thermomechanical pre-treatment and CAF into a single process. CAF-treated Al–Zr–Mg–Cr alloys are projected to benefit from the suggested processing approach, which is expected to improve the alloy's forming efficiency and overall characteristics. This experiment looked at the creep-aging behaviour and characteristics of AA7475. Temperature mechanical pre-treatment causes more creep deformation than regression and re-aging or 3 percent pre-strain alone, according to the study's findings. Creep-aged materials with thermomechanical pre-treatment had mechanical, electrical, and erosion properties that were virtually equivalent to regression and re-aging-treated samples. In the creep-aged regression and re-aging pre-treatment sample, precipitates were observed at the coarse and discrete grain boundaries, as well as in the usual GP/phases and a 2-phase. Because of the reduced solute concentration in the matrix of the regression and re-aging pre-treatment sample, dislocations were able to move more easily, resulting in increased creep strain. This was more than made up for by the toughening effects of next stage and the hardening of disruptions. The thermomechanical pre-treatment procedure improves the creep forming and all the characteristics alloys comprising Al—Zr—Mg—Cr by providing an acceptable initial temper. The study's findings on Al–Zr–Mg–Cr alloys could boost CAF technology.

Creep Age Forming of Fiber Metal Laminates: Effects of Process Time and Temperature and Stacking Sequence of Core Material.

Fiber metal laminates (FMLs) are a type of hybrid materials interlacing composites and metals. In the present work, FMLs with aluminum alloy 6061 as the skin and E-glass fiber-reinforced polypropylene (PP) as the core material are fabricated and formed by the creep age forming (CAF) process. The effects of time and temperature as the process parameters and thickness and stacking sequences of composites layers as the FML parameters are evaluated on the springback of glass-reinforced aluminum laminates (GLARE) FMLs. After the CAF process, the springback of creep age-formed FMLs is calculated. The results show that the FMLs can be successfully formed with the CAF process by considering appropriate time and temperature. In addition, the stacking sequence of composite layers can affect the springback behavior of FMLs significantly.

Creep aging properties variation and microstructure evolution for 2195 Al–Li alloys with various loading rates

Creep age forming (CAF) technology has been successfully applied to the manufacture of large aluminum alloy components. As the component size is large, the loading rate of different parts of the component has a large deviation. In order to fully understand the CAF process of large components, the effects of loading rate on creep aging (CA) treatment of Al–Li alloy were investigated at 180 °C for 18 h, including the deformation, mechanical properties, and microstructure evolution. Compared with different loading rates (2 N/s, 1 N/s and 0.5 N/s) in the CA experiment, the loading rate can be found to effectively affect the CA behavior of the materials. On the one hand, higher loading rate (2 N/s) is beneficial to the accumulation of creep strain in the process of CA, the increase of loading rate will cause the creep strain to increase suddenly. On the other hand, the increase of loading rate will promote the nucleation of T1 phase, refine the precipitate, limit the precipitation of θ′ phase, shorten the peak-aged time, reduce the peak aging strength and increase the elongation. In addition, the increase of loading rate also promotes the transition from brittle fracture to mixed fracture. The evolution of microstructure during aging was observed by transmission electron microscope (TEM). It is confirmed that the effect of loading rate on creep behavior is mainly caused by the change of dislocation due to the difference of loading rate, which leads to the change of evolution law of precipitated phase. Moreover, the mechanism of material properties and creep strain change is explained.

HR-STEM investigation of atomic lattice defects in different types of η precipitates in creep-age forming Al–Zn–Mg–Cu aluminium alloy

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. • 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.

Thermomechanical pretreatment of Al-Zn-Mg-Cu alloy to improve formability and performance during creep-age forming

Abstract 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 η2 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.

Development and trend of unified mechanism-based materials modelling for creep age forming of aluminium alloys

Materials modelling plays an important role for sprinback compensation and tool surface design in creep age forming (CAF). This review aims to provide perspective on the development and trend of materials modelling for CAF of aluminium alloys. Recently proposed unified constitutive equations for creep-ageing behaviour of aluminium alloys during CAF by integrating microstructural evolution and hardening effects are reviewed. The modelling methods and developing trend to quantitively reflect the relationship between the microstructural evolutions and macro creep-ageing behaviour of materials during CAF are discussed.

Review on Fatigue Performance of Aluminum Alloys after Creep Age Forming

The process and characteristics of creep age forming technology was introduced. The influence of the creep age forming process on the fatigue performance on aviation aluminum alloy was discussed. Researches from three aspects were reviewed respectively–the fatigue damage mechanism of the creep age forming material, the influence of creep age forming process parameters on the performance of material fatigue, and creep age formed material life prediction mode. The trend of the creep age forming technology and the problems that may exist in its application in the field of aerospace were analysed. Finally, the major research direction was pointed out that fatigue properties of aluminum alloy materials and components based on the creep age forming technology should be studied deeply.

Investigation of anisotropic creep-ageing behaviour of Al-Cu-Li alloy AA2050

Creep age forming (CAF) of an Al-Cu-Li alloy (AA2050-T34) considering its anisotropic creep-ageing behaviour has been experimentally and numerically investigated in this study. A series of uniaxial creep-ageing tests of AA2050-T34 has been carried out in both tension and compression conditions. The creep-ageing results in longitudinal and transverse samples show some anisotropic behaviour in both the room temperature yield strength and the creep strain, in which more strengthening occurs at the intermediate stage and less creep strain is produced in the transverse samples than the longitudinal samples under the same loading condition. It is also found that compressive loading results in much higher anisotropic behaviour than tensile loading. A material model has been adopted for the creep-ageing of AA2050 and shows a good fit to experimental data. Based on the model, a four-point bending virtual test has been developed in PAM-STAMP for simulating the CAF process of AA2050. The results indicate that the anisotropic behaviour contributes an apparent effect on both creep-aged properties and springback of the CAF process and need be considered for CAF manufacture of AA2050 components.

Non-linear finite element investigation of formability limit by buckling in creep age forming of stiffened panels

This paper focuses on the prediction of the formability limit of the stiffened panels during the loading stage and heating stage of the creep age forming (CAF) process. A set of non-linear finite element (FE) simulations has been carried out to investigate the elastic-plastic buckling behaviour of stiffened panels subjected to pure bending at the room temperature and the ageing temperature (155 ℃) for AA2050. From the non-linear FE simulations, the effects of main geometric parameters (stiffener thickness and stiffener height) on the critical buckling stress and the critical radius of the stiffened panel have been investigated at both temperatures. Increasing the stiffener thickness or decreasing the stiffener height leads to an increase of the critical buckling stress and an increase of formability limit. The critical buckling stress at the ageing temperature is smaller than that at the room temperature when the stiffened panel enters the plastic region, however, the formability increases slightly at the ageing temperature.