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Superplastic forming of bellows expansion joints made of titanium alloys
Journal of Materials Processing Technology 178 (2006) 24–28
Superplastic forming of bellows expansion joints made of titanium alloys夽 G. Wang ∗ , K.F. Zhang, D.Z. Wu, J.Z. Wang, Y.D. Yu Department of Materials Engineering, Harbin Institute of Technology, Harbin 150001, PR China Received 16 April 2002; received in revised form 6 September 2005; accepted 19 October 2005
Abstract A new forming technology was developed for bellows expansion joints. This technology uses superplastic forming (SPF) method of applying gas pressure and compressive axial load. It is developed and can be used to manufacture large diameter “U” type bellows expansion joints made of titanium alloys. The forming technology for bellows expansion joints made of titanium alloys is presented to make a two-convolution bellows expansion joint of Ti–6Al–4V alloy as an example. Welded pipe bent by a hot bending method with a set of specific dies and welded by plasma arc welding was used as a tubular blank in the SPF. During the SPF process the tubular blank is restrained in a multi-layer die block assembly which determines the final shape of convolution. The forming load route is divided into three steps in order to obtain optimum thickness distribution. This technology can also be used to fabricate stainless steel bellows expansion joints. © 2006 Published by Elsevier B.V. Keywords: Bellows expansion joint; SPF; Titanium alloy
1. Introduction Bellows expansion joints are used to absorb expansions or contractions in piping systems caused by irregular heat expansion, vibration, non-uniform subsidence of the ground, etc. In general, the usual metal materials for bellows expansion joints include: (1) austenite and dual phase stainless steel; (2) anticorrosion alloys and refractory alloys; (3) colomony; and the manufacturing methods include hydraulic forming, mechanical forming, welding forming and deposit forming. The material performances of titanium alloys namely service temperature, cycle stress, and corrosion resistant are very suitable for making bellows expansion joints. However, because of their large deformation resistance, severe springback, low plasticity and formability at room temperature [1]. Fabrication of bellows expansion joints using mechanical forming methods is complicated and expensive. Up to now there is little literature about this [2]. On the other hand, titanium alloy is a sort of natural superplastic material. The ‘natural’ means that many commercial titanium alloys possess stable equiaxed
fine grain microstructure and superplasticity. The superplasticity of Ti–6Al–4V titanium alloy is the best among of them, for instance the elongation can exceed 1000% [3]. Therefore “U” type bellows expansion joints can be made by using complex SPF technology and applying gas pressure and compressive axial load. A bellows expansion joint of titanium alloys has excellent corrosion-resistant performance and can be used to match a press vessel of titanium alloys and replace that of stainless steel and anticorrosion alloy. The new SPF technology of fabricating bellows expansion joints was first developed by the authors. According to the service requirements of bellows expansion joints, the wall thickness of as-formed parts should have a uniform thickness distribution. The finite element method (FEM) is used to simulate the SPF process. It is in order to predict the thickness distribution, optimum forming pressure and intermediate deformed shapes, and design the dimensions of original material prior to testing. Hence, experimental cost and time can be saved [4–7]. 2. The forming principle
夽
In this paper, the forming technology of titanium alloys bellows, the elastic elements of bellows expansion joints, is presented to make a two-convolution bellows of Ti–6Al–4V alloy as an example. ∗ Corresponding author. Fax: +86 451 86418763. E-mail address: [email protected] (G. Wang). 0924-0136/$ – see front matter © 2006 Published by Elsevier B.V. doi:10.1016/j.jmatprotec.2005.10.005
The bellows profile is shown in Fig. 1. If only gas pressure load is applied during the SPF process, it would produce a large deformation. The result in severe wall-thickness thinning and non-uniform thickness distribution of as-formed bellows. The
G. Wang et al. / Journal of Materials Processing Technology 178 (2006) 24–28
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3. Forming process of the bellows The forming process of the bellows includes a tubular blank fabrication process and SPF process, their flow process diagrams are as follows, respectively: • Tubular blank fabrication process: cutting plate material → bending tubular blank Fig. 1. Bellows profile.
→ welding tubular blank → radiographing → sizing tubular blank
complex SPF method of applying gas pressure load and compressive axial load is adapted. In this method the metal can be pushed into a deformation zone during forming. The forming schematic diagram is shown in Fig. 2. In order to avoid the plate’s sticking to dies ahead of time and obtain to uniform thickness distribution. The SPF procedure is designed as follows: (1) The tubular blank is assembled with the die block assembly together. (2) Gas pressure is imposed into the tubular blank to cause suitable plastic deformation. Namely the generating line of the formed tubular blank should be slightly shorter than that of the bellows. (3) The punch is operated downward to close upper, middle and lower dies together. (4) The gas pressure is increased gradually until the whole cylinder wall sticks to the dies. The dimensions of the bellows are controlled by dies. A multi-layer die block assembly is designed to fabricate multiconvolution bellows. So for a two-convolution bellows, forming dies include upper, middle and lower dies. For the convenience of taking out the deformed bellows from the die, all except upper and lower dies have a half structure and the straight segments of both upper and lower dies have a little slope. The whole die block assembly is machined from cast heat-resisting alloy.
→ welding cover with gas entrance connection • SPF process: painting graphite power on dies and high temperature anti-oxidize on tubular blank → assembling → bulging → furnace cooling → demoulding → turning the straight segment → grit blasting The material used was commercial mill-annealed Ti–6Al–4V alloy thin plate of 1.28 mm thickness. 3.1. Tubular blank fabrication According to the results of experiments and simulation, the width W and the length L of the plate blank fabricating tubular blank can be calculated from formulae (1) and (2), respectively, and the diameter D of end cover can be determined from formula (3) W = 2L4 + nC(2πR + 2h + πδ) + 10
Fig. 2. SPF procedure: (a) original state, (b) bulging, (c) dies clamping, (d) shape forming.
(1)
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Fig. 4. Welding line position.
Fig. 3. Hot bending set-up.
Table 1 Hot bending and hot sizing technological parameters Process
Heating temperature (◦ C)
Holding time (min)
Cooling style
Bending tubular blank Sizing tubular blank
650 700
30 45
Furnace cooling Furnace cooling
L = π(D0 − δ − 1) D=
D0
(2)
− 2δ − 1.5
(3)
where L4 is the length of straight segment of bellows, n the convolution number of bellows, h the height of convolution, R the radius of curvature of bellows, D0 the outside diameter of the tubular blank, δ the thickness of original sheet, C is a coefficient whose value ranges from 0.96 to 0.98. In a DN250 two-convolution bellows, the corresponding value of parameters are as follows: L4 = 25 mm, h = 40 mm, R = 9.5 mm, D0 = 273 mm, δ = 1.28 mm and n = 2. So the dimensions of plate blank are as follows: W = 263 mm, L = 850.5 mm and D = 269 mm. The plate was cut by a shearing machine and the covers were by a linear cutting machine. The tubular blank must be bended by stress relaxation forming, because the Ti–6Al–4V cannot be bended at room temperature. The schematic diagram of bending set-up is shown in Fig. 3. The plate blank was assembled into a bending set-up and then was put into a furnace to heat up. The technological parameters are tabulated in Table 1. The cross section of the welded pipe appeared as an elliptical shape generated by the welding deformation. So sizing of the
welded pipe must be performed with a tubal die whose outside diameter is a little smaller than the inner diameter of the welded pipe. The tubal die was inserted into the welded pipe and then was pushed into a furnace to heat up. The processing parameters are also listed in Table 1. The weld metal in the longitudinal direction of the tubular blank is required to possess adequate plasticity to ensure deformation during the SPF process. Among fusion welding methods, namely laser beam welding (LBW), electron beam welding (EBW), manual gas tungsten arc welding (M-GTAW) and plasma arc welding (PAW), PAW was chosen as a better method. This is because both LBW and EBW have the disadvantage of inconvenient operation and high cost although their weld metals possess excellent plasticity. And the weld metal of M-GTAW has low plasticity and needs to be treated with hot-compression processing prior to the SPF process [8–10]. Compared with the former three welding methods, the weld metal of PAW possesses good plasticity that can fulfills the superplastic forming of bellows and also has the advantage of convenience and is economical [11–14]. Automatic plasma arc welding (A-PAW) was used. The tubular blank was welded with square-butt and no gap. All the plate blanks were thoroughly cleaned with a wire brush and emery paper, pickled with H2 O/HNO3 /HF solution (76/20/4 vol.%), and then degreased with acetone prior to welding. Covers were jointed to tubular blanks by M-GTAW and a gas entrance connection was attached to one of covers by M-GTAW also. Strips of 1.2 mm × 1.2 mm × 200 mm were cut from the Ti–6Al–4V plate and used as filler metal. For welding parameters see Table 2 and the welding line position of tubular blank is shown in Fig. 4. All weld metals were visually inspected and also were radiographed for internal soundness prior to the SPF process.
Table 2 Welding parameter Welding position
Welding method
Arc current (A)
Arc voltage (V)
Travel speed (m/s)
Argon flow (L/min)
Covers Gas entrance connection Tubular blank
M-GTAW M-GTAW A-PAW
35 30 40
18 20 20
0.1 – 0.06
20 20 25
G. Wang et al. / Journal of Materials Processing Technology 178 (2006) 24–28
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3.2. Superplastic bulging The superplastic bulging experiment was performed on a superplastic forming machine in Harbin Institute of Technology. The procedures of the SPF experiment are as follows. Firstly, both the tubular blank and the die block assembly were heated to 150–200 ◦ C. Which were coated with antioxidize agent on the exterior surface of the tubular blank and painted graphite power on the contact surface of the formed bellows and dies for lubrication [15]. Then the gas entrance connection was welded together with the lower cover by MGTAW. Later the forming apparatus was placed into a furnace and heated to 927 ◦ C for 30 min. Subsequently, 0.2 MPa gas pressure was applied to the inside tubular blank for 30 min to cause suitable plastic deformation in the circumferential direction of the tubular blank. Then the punch was manipulated downward to close the upper, middle and lower dies together tightly. Later gas pressure was pressurized to 2.5 MPa for 10 min, after removing gas pressure, the punch was operated upward to its original position and the die block assembly was moved from furnace to room temperature. Finally, the middle die was taken apart and the bellows formed was taken out from the die block assembly. Then two covers were ground out from formed part. Following that, the straight segments of the bellows were turned to the required length using a turning lathe. And the high temperature anti-oxidize agent and the graphic power from the outside bellows were removed using a grit blast machine. Thus the bellows was formed. A photograph of the formed bellows is shown in Fig. 5. The key points of the forming technology are as follows. During the bulging stage, the inner forming pressure must match with the corresponding forming time. Lower forming pressures and shorter forming times may result in a smaller deformation degree of the tubular blank. The crown of convolutions may produce extra deformation during the shape forming stage to cause larger local thinning rates and non-uniform wall thickness distribution. On the contrary, higher forming pressures and longer forming times may lead to excessive deformation of the tubular blank. The die block assembly cannot be clamped during the dies clamping stage.
Fig. 6. Thickness distribution of as-formed bellows.
During the shape forming stage, the gas pressure applied should be appropriate. On one hand, lower gas pressure may cause the crowns of convolutions not to contact with the die wall, especially at the lower temperature zone or the welding line of high strength. On the other hand, higher gas pressure may force the dies apart which will influence the accuracy of the formed bellows. The thickness measurements are recorded by sectioning the formed bellows and measuring with a digital micrometer, see Fig. 6. The dimensional tolerances are in good agreement with Chinese Standard 16749–1997. All of these performances meet the designed value. 4. Conclusions A new forming technology was developed for bellows expansion joints. Which uses the complex SPF method of applying gas pressure and compressive axial load is developed for the first time. It can be used to manufacture large diameter “U” type bellows expansion joints made of titanium alloys. Welded pipes that were bent by hot bending with a set of specific dies and were welded by PAW are used in tubular blanks of SPF process. A multi-layer die structure is adopted to determine the final shape of convolutions. References
Fig. 5. Photograph of the formed bellows of Ti–6Al–4V alloy.
[1] H. Hucck, Titanium Handbook of International Alloy Compositions and Designations Metal and Ceramics Information Center, vol. I, Columbus, Ohio, 1976. [2] S. Tagami, H. Yamamoto, Expanding of thin wall welded titanium tubes, Furukawa Electric Rev. 69 (1980) 99–106. [3] T. Seshacharyulu, S.C. Medeiros, W.G. Frazier, Hot working of commercial Ti–6Al–4V with an equiaxed ␣/ microstructure: materials modeling considerations, Mater. Sci. Eng. A 284 (2000) 184–194. [4] Akkus, Nihat, Manabe, et al., Finite-element model for the superplastic bulging deformation of Ti-alloy pipe, J. Mater. Process. Technol. 68 (1997) 215–220. [5] Y.X. Li, P. Hu, Y. Cao, Numerical simulation of the superplastic constrained bulging of sheet metals in cylindrical dies, J. Mater. Process. Technol. 59 (1996) 243–249.
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[6] B.J. Mac Donald, M.S.J. Hashmi, Finite element simulation of bulge forming of a cross-joint from a tubular blank, J. Mater. Process. Technol. 103 (2000) 333–342. [7] M. Ahmed, M.S.J. Hashmi, Finite-element analysis of bulge forming applying pressure and in-plane compressive load, J. Mater. Process. Technol. 77 (1998) 95–102. [8] P.E. Denney, E.A. Metzbower, Laser beam welding of titanium, Weld. J. 68 (1989) 342s–346s. [9] S. Sundaresan, G.D. Janaki Ram, G. Madhusudhan, Microstructural refinement of weld fusion zones in ␣– titanium alloys using pulsed current welding, Mater. Sci. Eng. A 262 (1999) 88–100. [10] K. Yokota, R. Sasano, Fundamental study on electron beam welding of heavy thickness 6Al–4V titanium alloy, Weld. World 26 (1988) 202– 215.
[11] C. Homer, J.P. Lechten, B. Baudelet, Superplastic behavior of welded specimens of the titanium alloy TA6V, Metall. Trans. A 8 (1977) 1191–1192. [12] M. Peters, J.C. Williams, Microstructure and mechanical properties of a welded ␣ +  Ti alloy, Metall. Trans. A 15 (1984) 1589–1595. [13] Ch. Radhakrishna, R.J. Raghurami, R.K. Prasad, Study on microstructures of electron beam and gas tungsten arc Ti–6Al–4V weld metals, Praktische Metallogr. (Pract. Metallgr.) 34 (1997) 194–207. [14] K.F. Zhang, G. Wang, D.Z. Wu, et al., The superplastic capability of butt cover plate of Ti–6Al–4V titanium alloy, Trans. Nonferrous Met. Soc. China 12 (2002) 251–255. [15] L.X. Li, D.S. Peng, J.A. Liu, An experiment study of the lubrication behavior of graphite in hot compression tests of Ti–6Al–4V alloy, J. Mater. Process. Technol. 112 (2001) 1–5.
Superplastic forming of bellows expansion joints made of titanium alloys夽 G. Wang ∗ , K.F. Zhang, D.Z. Wu, J.Z. Wang, Y.D. Yu Department of Materials Engineering, Harbin Institute of Technology, Harbin 150001, PR China Received 16 April 2002; received in revised form 6 September 2005; accepted 19 October 2005
Abstract A new forming technology was developed for bellows expansion joints. This technology uses superplastic forming (SPF) method of applying gas pressure and compressive axial load. It is developed and can be used to manufacture large diameter “U” type bellows expansion joints made of titanium alloys. The forming technology for bellows expansion joints made of titanium alloys is presented to make a two-convolution bellows expansion joint of Ti–6Al–4V alloy as an example. Welded pipe bent by a hot bending method with a set of specific dies and welded by plasma arc welding was used as a tubular blank in the SPF. During the SPF process the tubular blank is restrained in a multi-layer die block assembly which determines the final shape of convolution. The forming load route is divided into three steps in order to obtain optimum thickness distribution. This technology can also be used to fabricate stainless steel bellows expansion joints. © 2006 Published by Elsevier B.V. Keywords: Bellows expansion joint; SPF; Titanium alloy
1. Introduction Bellows expansion joints are used to absorb expansions or contractions in piping systems caused by irregular heat expansion, vibration, non-uniform subsidence of the ground, etc. In general, the usual metal materials for bellows expansion joints include: (1) austenite and dual phase stainless steel; (2) anticorrosion alloys and refractory alloys; (3) colomony; and the manufacturing methods include hydraulic forming, mechanical forming, welding forming and deposit forming. The material performances of titanium alloys namely service temperature, cycle stress, and corrosion resistant are very suitable for making bellows expansion joints. However, because of their large deformation resistance, severe springback, low plasticity and formability at room temperature [1]. Fabrication of bellows expansion joints using mechanical forming methods is complicated and expensive. Up to now there is little literature about this [2]. On the other hand, titanium alloy is a sort of natural superplastic material. The ‘natural’ means that many commercial titanium alloys possess stable equiaxed
fine grain microstructure and superplasticity. The superplasticity of Ti–6Al–4V titanium alloy is the best among of them, for instance the elongation can exceed 1000% [3]. Therefore “U” type bellows expansion joints can be made by using complex SPF technology and applying gas pressure and compressive axial load. A bellows expansion joint of titanium alloys has excellent corrosion-resistant performance and can be used to match a press vessel of titanium alloys and replace that of stainless steel and anticorrosion alloy. The new SPF technology of fabricating bellows expansion joints was first developed by the authors. According to the service requirements of bellows expansion joints, the wall thickness of as-formed parts should have a uniform thickness distribution. The finite element method (FEM) is used to simulate the SPF process. It is in order to predict the thickness distribution, optimum forming pressure and intermediate deformed shapes, and design the dimensions of original material prior to testing. Hence, experimental cost and time can be saved [4–7]. 2. The forming principle
夽
In this paper, the forming technology of titanium alloys bellows, the elastic elements of bellows expansion joints, is presented to make a two-convolution bellows of Ti–6Al–4V alloy as an example. ∗ Corresponding author. Fax: +86 451 86418763. E-mail address: [email protected] (G. Wang). 0924-0136/$ – see front matter © 2006 Published by Elsevier B.V. doi:10.1016/j.jmatprotec.2005.10.005
The bellows profile is shown in Fig. 1. If only gas pressure load is applied during the SPF process, it would produce a large deformation. The result in severe wall-thickness thinning and non-uniform thickness distribution of as-formed bellows. The
G. Wang et al. / Journal of Materials Processing Technology 178 (2006) 24–28
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3. Forming process of the bellows The forming process of the bellows includes a tubular blank fabrication process and SPF process, their flow process diagrams are as follows, respectively: • Tubular blank fabrication process: cutting plate material → bending tubular blank Fig. 1. Bellows profile.
→ welding tubular blank → radiographing → sizing tubular blank
complex SPF method of applying gas pressure load and compressive axial load is adapted. In this method the metal can be pushed into a deformation zone during forming. The forming schematic diagram is shown in Fig. 2. In order to avoid the plate’s sticking to dies ahead of time and obtain to uniform thickness distribution. The SPF procedure is designed as follows: (1) The tubular blank is assembled with the die block assembly together. (2) Gas pressure is imposed into the tubular blank to cause suitable plastic deformation. Namely the generating line of the formed tubular blank should be slightly shorter than that of the bellows. (3) The punch is operated downward to close upper, middle and lower dies together. (4) The gas pressure is increased gradually until the whole cylinder wall sticks to the dies. The dimensions of the bellows are controlled by dies. A multi-layer die block assembly is designed to fabricate multiconvolution bellows. So for a two-convolution bellows, forming dies include upper, middle and lower dies. For the convenience of taking out the deformed bellows from the die, all except upper and lower dies have a half structure and the straight segments of both upper and lower dies have a little slope. The whole die block assembly is machined from cast heat-resisting alloy.
→ welding cover with gas entrance connection • SPF process: painting graphite power on dies and high temperature anti-oxidize on tubular blank → assembling → bulging → furnace cooling → demoulding → turning the straight segment → grit blasting The material used was commercial mill-annealed Ti–6Al–4V alloy thin plate of 1.28 mm thickness. 3.1. Tubular blank fabrication According to the results of experiments and simulation, the width W and the length L of the plate blank fabricating tubular blank can be calculated from formulae (1) and (2), respectively, and the diameter D of end cover can be determined from formula (3) W = 2L4 + nC(2πR + 2h + πδ) + 10
Fig. 2. SPF procedure: (a) original state, (b) bulging, (c) dies clamping, (d) shape forming.
(1)
26
G. Wang et al. / Journal of Materials Processing Technology 178 (2006) 24–28
Fig. 4. Welding line position.
Fig. 3. Hot bending set-up.
Table 1 Hot bending and hot sizing technological parameters Process
Heating temperature (◦ C)
Holding time (min)
Cooling style
Bending tubular blank Sizing tubular blank
650 700
30 45
Furnace cooling Furnace cooling
L = π(D0 − δ − 1) D=
D0
(2)
− 2δ − 1.5
(3)
where L4 is the length of straight segment of bellows, n the convolution number of bellows, h the height of convolution, R the radius of curvature of bellows, D0 the outside diameter of the tubular blank, δ the thickness of original sheet, C is a coefficient whose value ranges from 0.96 to 0.98. In a DN250 two-convolution bellows, the corresponding value of parameters are as follows: L4 = 25 mm, h = 40 mm, R = 9.5 mm, D0 = 273 mm, δ = 1.28 mm and n = 2. So the dimensions of plate blank are as follows: W = 263 mm, L = 850.5 mm and D = 269 mm. The plate was cut by a shearing machine and the covers were by a linear cutting machine. The tubular blank must be bended by stress relaxation forming, because the Ti–6Al–4V cannot be bended at room temperature. The schematic diagram of bending set-up is shown in Fig. 3. The plate blank was assembled into a bending set-up and then was put into a furnace to heat up. The technological parameters are tabulated in Table 1. The cross section of the welded pipe appeared as an elliptical shape generated by the welding deformation. So sizing of the
welded pipe must be performed with a tubal die whose outside diameter is a little smaller than the inner diameter of the welded pipe. The tubal die was inserted into the welded pipe and then was pushed into a furnace to heat up. The processing parameters are also listed in Table 1. The weld metal in the longitudinal direction of the tubular blank is required to possess adequate plasticity to ensure deformation during the SPF process. Among fusion welding methods, namely laser beam welding (LBW), electron beam welding (EBW), manual gas tungsten arc welding (M-GTAW) and plasma arc welding (PAW), PAW was chosen as a better method. This is because both LBW and EBW have the disadvantage of inconvenient operation and high cost although their weld metals possess excellent plasticity. And the weld metal of M-GTAW has low plasticity and needs to be treated with hot-compression processing prior to the SPF process [8–10]. Compared with the former three welding methods, the weld metal of PAW possesses good plasticity that can fulfills the superplastic forming of bellows and also has the advantage of convenience and is economical [11–14]. Automatic plasma arc welding (A-PAW) was used. The tubular blank was welded with square-butt and no gap. All the plate blanks were thoroughly cleaned with a wire brush and emery paper, pickled with H2 O/HNO3 /HF solution (76/20/4 vol.%), and then degreased with acetone prior to welding. Covers were jointed to tubular blanks by M-GTAW and a gas entrance connection was attached to one of covers by M-GTAW also. Strips of 1.2 mm × 1.2 mm × 200 mm were cut from the Ti–6Al–4V plate and used as filler metal. For welding parameters see Table 2 and the welding line position of tubular blank is shown in Fig. 4. All weld metals were visually inspected and also were radiographed for internal soundness prior to the SPF process.
Table 2 Welding parameter Welding position
Welding method
Arc current (A)
Arc voltage (V)
Travel speed (m/s)
Argon flow (L/min)
Covers Gas entrance connection Tubular blank
M-GTAW M-GTAW A-PAW
35 30 40
18 20 20
0.1 – 0.06
20 20 25
G. Wang et al. / Journal of Materials Processing Technology 178 (2006) 24–28
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3.2. Superplastic bulging The superplastic bulging experiment was performed on a superplastic forming machine in Harbin Institute of Technology. The procedures of the SPF experiment are as follows. Firstly, both the tubular blank and the die block assembly were heated to 150–200 ◦ C. Which were coated with antioxidize agent on the exterior surface of the tubular blank and painted graphite power on the contact surface of the formed bellows and dies for lubrication [15]. Then the gas entrance connection was welded together with the lower cover by MGTAW. Later the forming apparatus was placed into a furnace and heated to 927 ◦ C for 30 min. Subsequently, 0.2 MPa gas pressure was applied to the inside tubular blank for 30 min to cause suitable plastic deformation in the circumferential direction of the tubular blank. Then the punch was manipulated downward to close the upper, middle and lower dies together tightly. Later gas pressure was pressurized to 2.5 MPa for 10 min, after removing gas pressure, the punch was operated upward to its original position and the die block assembly was moved from furnace to room temperature. Finally, the middle die was taken apart and the bellows formed was taken out from the die block assembly. Then two covers were ground out from formed part. Following that, the straight segments of the bellows were turned to the required length using a turning lathe. And the high temperature anti-oxidize agent and the graphic power from the outside bellows were removed using a grit blast machine. Thus the bellows was formed. A photograph of the formed bellows is shown in Fig. 5. The key points of the forming technology are as follows. During the bulging stage, the inner forming pressure must match with the corresponding forming time. Lower forming pressures and shorter forming times may result in a smaller deformation degree of the tubular blank. The crown of convolutions may produce extra deformation during the shape forming stage to cause larger local thinning rates and non-uniform wall thickness distribution. On the contrary, higher forming pressures and longer forming times may lead to excessive deformation of the tubular blank. The die block assembly cannot be clamped during the dies clamping stage.
Fig. 6. Thickness distribution of as-formed bellows.
During the shape forming stage, the gas pressure applied should be appropriate. On one hand, lower gas pressure may cause the crowns of convolutions not to contact with the die wall, especially at the lower temperature zone or the welding line of high strength. On the other hand, higher gas pressure may force the dies apart which will influence the accuracy of the formed bellows. The thickness measurements are recorded by sectioning the formed bellows and measuring with a digital micrometer, see Fig. 6. The dimensional tolerances are in good agreement with Chinese Standard 16749–1997. All of these performances meet the designed value. 4. Conclusions A new forming technology was developed for bellows expansion joints. Which uses the complex SPF method of applying gas pressure and compressive axial load is developed for the first time. It can be used to manufacture large diameter “U” type bellows expansion joints made of titanium alloys. Welded pipes that were bent by hot bending with a set of specific dies and were welded by PAW are used in tubular blanks of SPF process. A multi-layer die structure is adopted to determine the final shape of convolutions. References
Fig. 5. Photograph of the formed bellows of Ti–6Al–4V alloy.
[1] H. Hucck, Titanium Handbook of International Alloy Compositions and Designations Metal and Ceramics Information Center, vol. I, Columbus, Ohio, 1976. [2] S. Tagami, H. Yamamoto, Expanding of thin wall welded titanium tubes, Furukawa Electric Rev. 69 (1980) 99–106. [3] T. Seshacharyulu, S.C. Medeiros, W.G. Frazier, Hot working of commercial Ti–6Al–4V with an equiaxed ␣/ microstructure: materials modeling considerations, Mater. Sci. Eng. A 284 (2000) 184–194. [4] Akkus, Nihat, Manabe, et al., Finite-element model for the superplastic bulging deformation of Ti-alloy pipe, J. Mater. Process. Technol. 68 (1997) 215–220. [5] Y.X. Li, P. Hu, Y. Cao, Numerical simulation of the superplastic constrained bulging of sheet metals in cylindrical dies, J. Mater. Process. Technol. 59 (1996) 243–249.
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G. Wang et al. / Journal of Materials Processing Technology 178 (2006) 24–28
[6] B.J. Mac Donald, M.S.J. Hashmi, Finite element simulation of bulge forming of a cross-joint from a tubular blank, J. Mater. Process. Technol. 103 (2000) 333–342. [7] M. Ahmed, M.S.J. Hashmi, Finite-element analysis of bulge forming applying pressure and in-plane compressive load, J. Mater. Process. Technol. 77 (1998) 95–102. [8] P.E. Denney, E.A. Metzbower, Laser beam welding of titanium, Weld. J. 68 (1989) 342s–346s. [9] S. Sundaresan, G.D. Janaki Ram, G. Madhusudhan, Microstructural refinement of weld fusion zones in ␣– titanium alloys using pulsed current welding, Mater. Sci. Eng. A 262 (1999) 88–100. [10] K. Yokota, R. Sasano, Fundamental study on electron beam welding of heavy thickness 6Al–4V titanium alloy, Weld. World 26 (1988) 202– 215.
[11] C. Homer, J.P. Lechten, B. Baudelet, Superplastic behavior of welded specimens of the titanium alloy TA6V, Metall. Trans. A 8 (1977) 1191–1192. [12] M. Peters, J.C. Williams, Microstructure and mechanical properties of a welded ␣ +  Ti alloy, Metall. Trans. A 15 (1984) 1589–1595. [13] Ch. Radhakrishna, R.J. Raghurami, R.K. Prasad, Study on microstructures of electron beam and gas tungsten arc Ti–6Al–4V weld metals, Praktische Metallogr. (Pract. Metallgr.) 34 (1997) 194–207. [14] K.F. Zhang, G. Wang, D.Z. Wu, et al., The superplastic capability of butt cover plate of Ti–6Al–4V titanium alloy, Trans. Nonferrous Met. Soc. China 12 (2002) 251–255. [15] L.X. Li, D.S. Peng, J.A. Liu, An experiment study of the lubrication behavior of graphite in hot compression tests of Ti–6Al–4V alloy, J. Mater. Process. Technol. 112 (2001) 1–5.