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Hydraulic forming process of titanium bellows

According to the test data of chemical composition, mechanical properties and bending properties of titanium (TA9/Grade 7), the manufacturing process and process control of титан bellows from blanking, welding, heat treatment to compression molding are designed.

0. Introduction

Pressure element bellows has high requirements for safe use, and its design and manufacture must comply with relevant technical standards. Due to the high chemical activity and special physical and mechanical properties of titanium, the manufacturing of titanium bellows requires special production processes and process control compared with other common metal materials.

1. Technical requirements

(1) The design, manufacture and inspection of TA9 material bellows shall comply with GB16749-1997.
(2) The butt weld of bellows must be subject to 100% radiographic inspection according to Class II in JB/T4730.2-2005.
(3) The formed butt weld must be subject to 100% liquid penetration, and the evaluation shall be carried out according to Grade I in JB/T4730.5-2005.
(4) The corrugated pipe surface shall be free of scratches, obvious unevenness, scratches larger than the negative deviation of the steel plate thickness, welding spatter and other defects, and slight molding parting surface traces are allowed. The section of bellows is shown in Figure 1.

2. Chemical composition and mechanical properties

20230114103537 65231 - Hydraulic forming process of titanium bellows

Fig.1 Section diagram of bellows

  • (1) Material composition. The chemical analysis of the raw material (TA9/Grade 7) sample shows that the main components contain 0.18% Pd, 0.18% Fe, 0.07% C, 0.01% N, 0.005% H and 0.1% O.
  • (2) Mechanical properties. The mechanical property test of raw material samples obtained the transverse room temperature mechanical property of 3mm thick TA9 plate in annealing state: tensile strength σ b=520N/mm2, specified residual elongation stress σR0.2=265N/mm2, elongation σ5=32%。
  • (3) Bending performance. The bending angle of 3mm thick TA9 plate in annealing state was obtained through the bending performance test of raw materials α= 167°。

3. Blanking size

According to the development of the neutral layer of the tube blank, the circumferential dimension of the blank and the ripple development dimension are obtained. According to the characteristics of TA9 material, the stretching of the corrugated part during the hydraulic forming of bellows depends on the surrounding materials, so the expansion size of the corrugated part needs to be lengthened accordingly. According to the empirical formula L (corrugated forming length)=1.04 × [Wave number × (0.5708 × Wave distance+2 × Wave height)], the blank size is 1011 × 400mm (including 350mm of corrugated forming length. Considering the die thickness of 90mm, the blank size is determined as 400mm).

4. Welding and heat treatment

  • (1) Welding. Argon arc welding is adopted. Protective measures shall be taken for welding pool and heat-affected zone>400 ℃ to avoid being polluted by gas in the air. Cylindrical blanks are welded with upper and lower protection boxes after argon protection. Clean the welding groove surface before welding.
  • (2) Heat treatment. The stress relief annealing treatment can eliminate the internal stress generated by TA9 material during welding and avoid cracking during the hydraulic forming process. The annealing can also reduce the material strength and improve the material plasticity. In order to reduce the pollution of various gases on the surface of TA9 material during heat treatment, the lower limit temperature is selected for annealing and anti-oxidation coating is applied on the blank surface. See Fig. 2 for heat treatment process curve.

20230114103850 47981 - Hydraulic forming process of titanium bellows

Fig.2 Heat treatment process curve

5. Hydroforming

Hydroforming is the most commonly used manufacturing method of metal bellows at present. The forming methods include single-wave continuous forming and multi-wave one-time forming. This design adopts multi-wave one-time forming process.

  • (1) Hydroforming process. The hydraulic forming of bellows is divided into two stages. In the first stage, the tubes are constrained to bulge under the action of the liquid pressure in the tube, and the bulge degree is small, so that each forming intermediate template is positioned. In the second stage, when the liquid pressure in the pipe is stable and equal to the bulging pressure, the axial force is applied, and the pipe is compressed and bulged axially under the axial force and internal pressure, and the metal bellows is made. Axial compression bulging can improve the stress and strain state in the bulging zone, which is beneficial to plastic deformation and increase the bulging coefficient.
  • (2) Pressure control. The bulging pressure in the second stage of forming must be stable, and the pressure value is equal to the pressure value when the bulging is constrained. If the bulging pressure value is large, the blank will break. If the bulging pressure is too small, a deflated wave will be formed. The size and shape of the hydroformed bellows can form various waveforms according to the forming die. Compared with other bulging methods, hydroforming has the advantages of easy to obtain higher pressure, uniform pressure action, relatively easy control, smooth surface and good quality; The disadvantage is that the forming device needs to be strictly sealed to prevent the bulging pressure and liquid leakage during bulging.
  • (3) Pressure calculation. The forming force of bellows hydroforming is mainly the liquid unit pressure p and axial compression force P during bulging. When calculating the bulging pressure p, only the circumferential tensile stress during bulging is considered to simplify the calculation σ1. Ignore the smaller tensile stress in the direction of the bus σ2. The simplified bulging pressure calculation formula p=2tσ1/d can be obtained from the force balance condition of the semi-ring shown in Figure 3 by taking a ring in the deformation zone for analysis σ1/d. The condition of plastic deformation of material is σ1≥ σB. Considering the effect of work hardening of materials, it can be used for calculation σd replace σB. The bulging pressure calculation formula p=2tσb/d, where t is the pipe wall thickness (mm), d is the inner diameter of the bellows (mm), σB is the material strength limit (MPa). Axial compression force P=p × F. Where F is the effective area of bellows. According to the thickness of the sheet used (the measured thickness is 3.1mm), the internal pressure required for forming is calculated to be 10.1MPa.
  • (4) Control of pressing process. Compared with carbon steel, stainless steel and other common metals, the pressure processing of titanium and titanium alloy materials has the characteristics of high yield strength, large yield ratio, low elongation and reduction of area, anisotropy, sensitivity to deformation speed, sensitivity to surface defects and tendency to cold work hardening, which makes it very difficult to cold press forming. Especially, the wave height of the bellows is selected according to common materials, and the material stretch is large, which increases the difficulty of press forming. Therefore, TA9 material can only bear cold forming with small deformation. At the same time, due to the small elastic modulus of titanium and the large springback of cold working, a more accurate compensation calculation formula has not yet been obtained. For the bellows with large deformation, in order to restore the plasticity after wave height stretching and prevent cracks (especially the weld and the affected area), it is necessary to adopt sectional pressing and annealing treatment.

6. Pressing results

All inspection results of the TA9 material bellows made by the above process meet the standard requirements. Hydraulic forming, welding and heat treatment of TA9 material bellows are the key to the process. Although titanium material is difficult to be cold-pressed due to its characteristics, as long as the corresponding forming process is formulated according to the material characteristics and the process control of each process is well done, the hydraulic forming of TA9 material bellows can meet the design and use requirements.



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