Experimental Investigation on Creep Forming of A New Al-Li-Cu-Mg Alloy Plate

Aluminium-lithium(AL-Li) alloys has comprehensive advantages including relatively low density, high elastic modulus, high specific strength and stiffness, excellent fatigue strength, superior corrosion resistance and good weld ability, and lower cost than composite materials. After three generations of development, the new Al-Li alloys are applied in modern aerial and space vehicles to meet their requirement of weight reduction [1-4]. For example, Aluminium alloy AA2124 is replaced by Al-Li alloy 2197 in the rear fuselage and other parts of F-16 Fighting Falcon to reduce weight for 5%, improve fracture toughness for 7% [5-6]. Almost all parts for fuselage of Bombardier’s C Series airplanes are manufactured by Al-Li alloys, and its Al-Li alloy usage rate accounts for 23% of aircraft materials. Al-Li alloy 2060 is applied to fuselage of C919 airplane of China for reducing weight. Creep forming is a kind of new manufacturing technology, which restrain a component to a specific shape during heat treatment allowing the component to relieve stresses and creep to contour. There are many advantages for creep forming, including lower residual stress, good shape stability, high stress corrosion resistance, good surface quality and so on. Thereby, it can be applied to manufacture skin components and integral stiffened panels of airplane [7-10]. In this study, fundamental creep forming experiments are carried out on a new Al-Li-Cu-Mg alloy. After just one heat treatment process, the material is age-hardened, and the part is also shaped. The effect of creep aging parameters on springback rate is studied, and a function relationship between springback rate and the process parameters is obtained. Experiment on typical part of welding fuselage panel is used to validate the model, which establish the technical basis for the manufacturing of large Al-Li alloy structures especially of welded fuselage panels.


Introduction
Aluminium-lithium(AL-Li) alloys has comprehensive advantages including relatively low density, high elastic modulus, high specific strength and stiffness, excellent fatigue strength, superior corrosion resistance and good weld ability, and lower cost than composite materials.After three generations of development, the new Al-Li alloys are applied in modern aerial and space vehicles to meet their requirement of weight reduction [1][2][3][4].For example, Aluminium alloy AA2124 is replaced by Al-Li alloy 2197 in the rear fuselage and other parts of F-16 Fighting Falcon to reduce weight for 5%, improve fracture toughness for 7% [5][6].Almost all parts for fuselage of Bombardier's C Series airplanes are manufactured by Al-Li alloys, and its Al-Li alloy usage rate accounts for 23% of aircraft materials.Al-Li alloy 2060 is applied to fuselage of C919 airplane of China for reducing weight.
Creep forming is a kind of new manufacturing technology, which restrain a component to a specific shape during heat treatment allowing the component to relieve stresses and creep to contour.There are many advantages for creep forming, including lower residual stress, good shape stability, high stress corrosion resistance, good surface quality and so on.
In this study, fundamental creep forming experiments are carried out on a new Al-Li-Cu-Mg alloy.After just one heat treatment process, the material is age-hardened, and the part is also shaped.The effect of creep aging parameters on springback rate is studied, and a function relationship between springback rate and the process parameters is obtained.
Experiment on typical part of welding fuselage panel is used to validate the model, which establish the technical basis for the manufacturing of large Al-Li alloy structures especially of welded fuselage panels.The experiment material is a new kind of Al-Li-Cu-Mg alloy which is aged to T8 status, with t=1.8 mm in thickness.
Its chemical components are listed in table 1, and its mechanical properties in longitudinal direction (LD) and transversal direction (TD) of rolling fibre at room temperature are listed in table 2 which shows a little anisotropy properties.

Springback ratio
The contour precision of part component is one of the most important objective for sheet metal forming and creep forming is no exception.The general contour error is 0.5 mm or less for sheet panel component.Experiments are carried out to obtain the relationship between the process parameters and springback ratio (SR), which can established the fundamental technical base for precise creep forming.The effects of key parameters on creep forming in the dominant deforming direction are investigated using range method.The springback is defined as shown in figure 2.

Fig. 2 Definition of springback
So the calculation of springback rate is shown as follows, The experimental range analysis results are shown in table 4. In this table, K ij under column A is the sum of SR with A i (i=1,2,3,4), K ij under column B is the sum of SR with B i (i=1,2,3,4), K ij under column C is the sum of SR with C i (i=1,2,3,4).K ij /4 is the average of K ij .T means the sum of SR with the whole column.R j under column A is the difference between the maximum and the minimum of K ij /4 under column A. Comparing the effect of each parameter on forming result, the most influential factor is the temperature of creep forming, the next is time period of creep forming, and the last is radius of pre-bent.The influential ability of each parameter on creep forming can be obtained by the experimental data analysis.As shown is figure 3, the horizontal component is parameter level of each factor and the vertical component is the springback ratio which shows as K ij /4 in figure 3. The springback ratio decreases with the time period of creep forming and the temperature, increased with the radius of pre-bend, which shows the viscoplastic deformation characteristic from T8 status.The regressive analysis method is used to analyse the exact relationship between each parameters and springback ratio for optimization.The experiments are designed with 4 variables and 4 response levels, and each variable has an effect where and p is times of repetition, n is number of level.
Thorough the formula deduction, the equation with values for the relationship between springback ratio and process parameters is, .3275 0.54625 18 -0.50985170 0.0023 3 1020 85200 0.06455 200 where A is time period of creep forming (hour), B is temperature of creep forming (ºC), C is radius of pre-bend (mm).
The equation model shows that the springback ratio decreases with the time period of creep forming and the temperature, increases with the radius of pre-bend, which are in agreement with previous analysis.Substituting the process values of experiments into the equation model, the regressive springback ratio are calculated and compared with the experimental data.As shown in Fig. 4, the regressive springback ratio from the equation model and the measured experimental data are very close.So this equation model derived from experimental data is accurate and can be referenced in engineering manufacturing.

Conclusion
For creep forming on the new Al-Li-Cu-Mg alloy, the most influential factor is the temperature, the next is time period, and the last is radius of pre-bend.The springback ratio decreases with the temperature and time period, increases with radius of pre-bend.
The equation model derived from experimental data by regressive analysis method is accurate and can describe the relationship between the springback ratio and creep parameters, which is validated by creep forming experiment of a typical welded reinforced panel component.

Fig. 3
Fig. 3 Relationships between parameters and springback ratio

Fig. 7 Fig. 8
Fig. 7 Microstructure of the material before and after creep forming

Fig. 9 Fig. 10 6 .
Fig. 9 Pre-bend part with tool Fig. 10 and 11 show the part component before and after experiment.There are gaps of 15 mm and 18.5 mm between the bottom surface of part and the plane before creep forming.After forming process, the deformation caused by welding is reduced obviously due to the stress relief during creep aging.The gap between gauge and part surface is less than 0.5mm in the acceptable error range, which confirmed the effective of equation model derived from fundamental experimental data.The main creep deformation occurs perpendicular to the friction stir welding direction, so the effect of welded region on creep forming can be ignored.The uniaxial tension samples were cut from the creep formed structural panel and table6shows the mechanical properties.The tension strength and yield strength of both directions are increased, which demonstrate the effect of the process.

Table 1 .
Chemical components of the new Al-Li alloy

Table 2 .
Mechanical Properties of the new Al-

Table 4 .
Experimental data and range analysis results of creep forming