J. Cent. South Univ. (2014) 21: 2612−2616 DOI: 10.1007/s11771-014-2220-0

Analysis of isothermal forging process and mechanical properties of complex aluminum forging for aviation

HU Jian-liang(胡建良), YI You-ping(易幼平), HUANG Shi-quan(黄始全)

School of Mechanical and Electrical Engineering, Central South University, Changsha 410083, China

© Central South University Press and Springer-Verlag Berlin Heidelberg 2014

Abstract: Large complex 7A85 aluminum wing-body joint was forged employing isothermal forging process and its mechanical properties were studied. The tensile strength after forging is up to 587.5 MPa in longitudinal direction, 15% higher than that using free forging. Moreover, the tensile strength of the forging is almost the same in three directions. Isothermal forging also performs well on overall fracture toughness, with a maximum value of 39.8 MPa·m1/2, and that of short transverse direction all reaches 36 MPa·m1/2 and above, with a maximum relative error of only 3.6%. The results indicate that the isothermal forging leads to better performance as well as higher uniformity in mechanical properties. Key words: aviation forging; isothermal forging process; uniformity; fracture toughness

1 Introduction

Due to high strength and toughness, excellent thermal processing performance and welding performance as well as good corrosion resistance, 7085 aluminum alloys have been extensively used as aircraft structure materials [1−3]. In the last few years, large 7085 aluminum forgings were applied to important strained components of the Boeing 787 and Airbus A380 aircraft [4-6]. The traditional way of manufacturing complex aviation forgings is machining after free forging, which can reduce the cost of die. However, the free forging process brings about new problems, such as high residual stresses, inhomogeneity of microstructure and mechanical properties, and huge waste of material. As well known, isothermal forging process keeps the die and forging at the same temperature. Thus, the effect of cold die is eliminated and the deformation resistance of materials is greatly reduced. These improve the forming process and minimize the machining allowance of forging, which makes it a near net-shape forming process. Therefore, the isothermal forging process is more adapted to the deformation of aviation forging with complex shape, compared with traditional free forging process [7]. Many researchers have investigated the isothermal forging process of aluminum alloys. In the work by CAI et al [8] and SHAN et al [9], gears and magnesium alloy bracket were isothermally forged,

respectively, and the forgings with complex shapes met the precision requirement well. LIU et al [10] studied the effects of isothermal forging temperature on the microstructure and mechanical properties of 2B70 aluminum alloy, while LI et al [11] analyzed the dynamic recrystallization behavior of 4032 aluminum alloy under different strain rates of isothermal forging process.

In this work, the isothermal forging process was introduced in the forging of 7A85 aluminum wing-body joint to improve the comprehensive mechanical performance and the uniformity of the forgings. The mechanical properties of isothermal forging were measured and analyzed. 2 Isothermal forging experiment

The 7A85 aluminum wing-body joint with high content of reinforcement and thin wall was studied. The overall size is 550 mm×180 mm×174 mm, with a minimum thickness of only 20 mm. If this joint was manufactured through traditional free forging process, the mass of waste material could be 62 kg, which is more than 5 times of its own mass. In addition, due to mechanical processing, the streamline of the forging was cut off, resulting in poor fatigue property of forging and short service life. By contrast, the isothermal forging process forces the streamline of the forging to be along with the geometry of the closed die cavity. During the deformation process, the forging and the die were at the

Foundation item: Project(2010CB731701) supported by the National Basic Research Program of China; Project(2012ZX04010-081) supported by National

Science and Technology Major Program of China Received date: 2013−03−15; Accepted date: 2013−09−01 Corresponding author: YI You-ping, Professor; Tel: +86−731−88830294; E-mail: yyp@csu.edu.cn

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same temperature, which reduced the deformation resistance and thus was conducive to the forming of aviation forging. 2.1 Forging billet design of wing-body joint

Figure 1 shows the designed wing-body joint for forging. Because of the complex shape, it is difficult to come into shape under one forging process, and some defects such as disordered streamline and eddy of metal may appear. Therefore, a two-step forming process was used in this work. The forging billet design of the joint follows the principle of equal-area, which means forging and forging billet should have the same area on each cross section along the length direction. A series of cross sections along the length direction were chosen. The deformation on each section during the forging process was seen as a plane deformation to calculate the forging billet shape of each section. The shapes of forging billet on each section were then connected to form a three-dimensional forging billet. Finally, the isothermal forging process was simulated by Deform-3D software to optimize the shape of the forging billet (Fig. 2) as well as to predict the load curve (Fig. 3). 2.2 Isothermal forging experiment

Isothermal forging experiment of the wing-body joint was carried out on the 40 MN isothermal forging hydraulic machine. The die and the forging billet were heated to 450 °C, and held for 4 h. Water-based lubricant

Fig. 1 Designed 7A85 aluminum joint

Fig. 2 Designed forging billet of 7A85 aluminum joint

Fig. 3 Simulation load curve of 7A85 aluminum joint under

isothermal forging process

(tungsten disulfide + water + additives) was taken as the lubricant between forging billet and die. Figures 4 and 5 show the forging billet and final forging through isothermal forging process, respectively. It can be inferred that metal flows smoothly and thus there are no folding, delamination, crack or other defects of isothermal forging.

Figure 6 shows the plot of velocity and load versus stroke during isothermal forging experiment. Accordingly, the process can be divided into four stages. In the first stage, the contact area of die and forging billet

Fig. 4 Forging billet of 7A85 aluminum joint

Fig. 5 7A85 aluminum joint forged by isothermal forging

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Fig. 6 Plot of velocity and load versus stroke during isothermal

forging

is small, and the forging billet is in the free upsetting process, so the load and the growth of it are very small. In the second stage, when the stroke reaches 65 mm, the forging load begins to increase sharply, since the die contacts completely with forging billet. From then on, the forging steps into a stress state of backward extrusion. In the third stage, when the stroke arrives at 72 mm, the velocity of upper die decreases to 0.002 mm/s according to the speed setting program. There is a drop in load at this point in Fig. 6. This is because the flowing speed of metal in forging billet could not decrease instantaneously due to the relaxation effect of elastic-plastic body, causing a difference in velocity between the forging billet and the die. After that, the load increases sharply; in the last stage, the forging load rises to a maximum load of around 3000 t at the end of the isothermal forging process.

3 Mechanical properties of forgings 3.1 Experimental method

To compare the mechanical properties of the forgings by isothermal forging process with those by free forging process, both forgings were heat treated with the same heat treatment, involving solution treating at 470 °C for 4 h, water quenching and double-ageing treatment at 120 °C for 4 h and 157 °C for 8 h, respectively. Specimens for tensile tests were cut from four locations (Fig. 5) of the isothermal forging. For each region, there were three groups of specimens coming from three directions, which are longitudinal direction (L direction), long transverse direction (T direction), short transverse direction (S direction). Tensile tests were carried out according to the standard test method (GB 228—2002), at temperature of 30 °C and tensile speed of 2 mm/min. The fracture toughness at S direction was investigated for the forging has the weakest fracture toughness in S direction. The fracture toughness experiments were based on standard test method for plane-strain fracture toughness of metallic materials (GB 4161—1984). 3.2 Experimental results

The measured mechanical properties of the forgings by isothermal forging and free forging are summarized in Table 1. The tensile strengths at three directions of all locations by isothermal forging are significantly higher than those by free forging, even that in S direction of location 2 in Fig. 5 with a minimum tensile strength of 517.5 MPa. The maximum tensile strength of isothermal

Table 1 Mechanical properties of forgings by different forging processes

Forging

method

Location

in Fig. 5 Direction Tensile strength/MPa Yield strength/MPa Fracture elongation/%

Fracture toughness,

KIC/(MPa·m1/2)

Isothermal

forging

1

L 587.5 517.5 7.5 —

T 542.5 460 6.4 —

S 522.5 472.5 6.4 36.3

2

L 560 545 9.17 —

T 570 507.5 8.33 —

S 517.5 445 5.28 38.8

3

L 565 500 9.44 —

T 571.67 461.67 10.92 —

S 555 510 5.28 39.8

4

L 561.67 451.67 12.41 —

T 537.5 500 10.28 —

S 560 551.6 9.07 37.0

Free forging

L 510 474 11.00 —

T 511 483 10.54 —

S 490 441 4.37 28.1

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forging occurs at L direction of location 1 in Fig. 5, up to 587.5 MPa. It is increased by 15% compared with that by free forging. Figure 7 shows the bar chart of the experimental data in Table 1. It can be seen that the tensile strengths at three directions of four locations by isothermal forging are basically the same, showing a high homogeneity. The fracture elongation by free forging at three directions varies widely, with a maximum value of 11% and a minimum value of only 4.37%, indicating serious anisotropy. Compared with free forging, the isothermal forgings develop more uniform fracture elongation. In a word, the results of tensile test show that isothermal forging process can significantly improve the tensile strength as well as the homogeneity.

Fig. 7 Tensile strength in three directions of four locations

It can be seen from Table 1 that the isothermal forgings have better overall fracture toughness, all reaching above 36 MPa·m1/2. The maximum value is 39.8 MPa·m1/2, which is 41.5% higher than that of free forgings. As shown in Fig. 8, the fracture toughness along S direction of different locations shows a high homogeneity, with a max relative error of only 3.6%.

Fig. 8 Fracture toughness in S direction by isothermal forging

4 Discussion

Due to the effect of cold die [12−13], the forgings through free forging process have lower deformation temperature than isothermal forgings. Dynamic recovery takes place slightly during the deformation. Meanwhile, a lot of strain energy is preserved, which lowers the recrystallization temperature. Therefore, during the solid solution and aging treatment, recrystallization occurs to a large extent, leading to coarser grains and lower tensile strength [14]. Instead, during the isothermal forging process, the forging temperature is maintained constantly, so that the strain energy preserved in the process of plastic deformation is relatively small. The recrystallization during heat treatment is greatly inhibited and the tensile strength is thus improved.

Eliminating the effect of cold die during the isothermal forging process, the temperature field and the stress field of forgings are uniform, so that the metal flows more smoothly and deforms more evenly. Figure 9

Fig. 9 Streamline (a) and grain structures of location 1 (b) and

location 2 (c) on cross section of isothermal forging

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shows that the isothermal forging has fine streamline on the cross section in L direction as well as uniform grain structure at different locations. The uniformity of the temperature field and the stress field is far superior to that of free forging process, so dislocations and vacancies generated during the deformation are evenly distributed. According to LI et al [15], the GP area and η′ phase are more prone to form around dislocation and vacancy of low strain energy and interface energy. Therefore, it can be concluded that they are uniformly precipitated in the matrix by isothermal forging, which improves the uniformity of mechanical properties. The above analysis shows that the isothermal forging process can not only improve the mechanical properties of forging, but also significantly improve the degree of uniformity. 5 Conclusions

1) Isothermal forging process is employed to form complex wing-body joint precisely. It reduces the deformation resistance of materials, and greatly improves the utilization of aeronautical materials.

2) The tensile strengths of isothermal forgings in different directions are significantly higher than that of free forgings. The maximum tensile strength of isothermal forgings is up to 587.5 MPa in L direction, which is 15% higher than that of free forgings. The tensile strength of isothermal forgings in three directions is basically the same, showing high homogeneity. The isothermal forging process can significantly improve the tensile strength as well as the homogeneity of the forgings.

3) The isothermal forgings have better overall fracture toughness, all reaching above 36 MPa·m1/2, with a maximum value of 39.8 MPa·m1/2, which is 41.5% higher than that of free forgings. The fracture toughness in S direction of different locations by isothermal forging shows a high homogeneity with a max relative error of only 3.6%. References [1] POOLE W J, WELLS M A, LLOYD D J. Advanced aluminum and

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(Edited by FANG Jing-hua)