# Hydraulic forming study of Hastelloy C22 tube

Aiming at the problems of high forming difficulty, low forming accuracy, and poor product consistency in the production of Hastelloy C22 tubes, the hydraulic forming method is used instead of the collodion welding process to realize the integral forming of shaped tubes. The finite element model of tube hydroforming is constructed with the bent tube as the blank. Through finite element simulation and its experimental verification, the side-pushing replenishment amount, which affects the wall thickness change rate of the tube, is optimized, and the optimal parameters of side-pushing replenishment amount are obtained; comparing the thinning and wrinkling of the tube wall in the forming area, the optimal liquid filling pressure parameters are obtained through simulation and result in comparison experiments. The results show that optimizing the filling pressure and the amount of side-pushing material can significantly improve the forming performance of the tube. The liquid-filling pressure and side-thrust replenishment amount determined by the finite element simulation were applied to the actual production, and the pipe fittings meeting the technical requirements were obtained.

In recent years, in the field of aerospace, the reliability of tubular parts has become more and more demanding to adapt to complex service conditions, especially in engines, and the use of difficult-to-form metallic materials has increased. At the same time, the axes and cross-sectional shapes of tubing are becoming more and more complex. Therefore, plastic forming tubular products’ precision, efficiency, and performance are facing unprecedented challenges. The traditional farming methods (such as push bending, pull bending, bending, etc.) have limitations, with low forming accuracy, and for the large changes in the cross-section of the pipe can not be formed as a whole, the need for peace stamping and then welding processes, resulting in fatigue strength and surface quality of the pipe is difficult to meet the requirements of use. Fig. 1 shows a type of aerospace tube product made of Hastelloy C22 (Ni-Cr-Mo-W alloy) using partial stamping and welding.

Figure.1 Partial piece stamping and then welding forming of Hastelloy pipe fittings

Hydraulic forming technology refers to using water or oil and other fluids as a force transfer medium instead of a rigid concave or convex die so that the billet under the pressure of the liquid medium against the convex or concave die and forming a process. Compared with the traditional process, hydraulic forming technology has the advantages of complex forming parts, high forming accuracy, good surface quality, etc., and can realize the integral forming and manufacturing of complex parts. Therefore, the process is increasingly widely used in advanced manufacturing fields such as aerospace.

In this paper, we simulate the liquid-filled forming process of Hastelloy C22 pipe fittings by finite element and optimize the process parameters, and finally prove through experimental verification that the variable-section pipe fittings can be manufactured integrally by the tube hydroforming process. Compared with traditional piecewise stamping and welding, the forming accuracy of the hydroformed tube has been improved, and the local thinning has been reduced to meet the technical requirements of the product.

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**1. Materials and dimensions of formed fittings**

Table of Contents

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1.1 Hastelloy C22

The metal material of the studied pipe fitting is solid solution C22 (UNS N06022) Hastelloy, whose chemical composition is shown in Table 1. Hastelloy C22 is the most versatile nickel-chromium-molybdenum-tungsten alloy currently available, and it outperforms other nickel alloys, such as alloy C-276, alloy C-4, alloy 20, and alloy 625, in application environments using a variety of mixed industrial chemicals due to improved resistance to uniform and localized corrosion. In addition, Hastelloy C22 offers the following advantages: helps improve resistance to pitting, crevice corrosion, and stress corrosion cracking; and has excellent oxidation resistance to aqueous media (including wet chlorine, mixtures containing nitric acid and chloride ion oxidized acids). Hastelloy C22 is a good substitute when super-austenitic and duplex stainless steels cannot withstand highly aggressive media. However, due to the difficult forming and welding deformation of Hastelloy is difficult to control the characteristics of the size accuracy of the patchwork welded pipe fittings, manufacturing efficiency could be higher, and the processing and inspection process is complicated. Therefore, the traditional first piece stamping and then welding for the whole pipe fitting process is gradually replaced by the overall hydraulic forming process of the pipe.

Table.1 Hastelloy C22 (UNS N06022) chemical composition (%, mass fraction)

C | Mn | P | Co | W | Si | S | Cr | Ni | Mo | V | Fe |

0 | 1 | 0 | 3 | 2.5-3.5 | 0 | 0 | 20.0-22.5 | # | 12.5-14.5 | 0 | 2.0-6.0 |

Young’s modulus of Hastelloy C22 is 205.5 MPa, and Poisson’s ratio is 0.3. The real stress-real strain curve of the material at room temperature is shown in Fig. 2 using the pipe one-way tensile test. The plastic instanton parameters of the material were obtained by fitting as shown in equation (1).

Figure.2 Real stress-real strain curve of Hastelloy C22 in unidirectional tension

σ=Kε

^{n}=1640ε^{0.48}(1)

In the formula: σ is the real stress; ε is the real strain; K is the strength factor; n is the hardening index.

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1.2 Shape and size of the pipe member

The shape and size of the tube are shown in Fig.3; the total length of the axis is 700mm, the shape size is 450mm×230mm, the wall thickness is 1.2mm, the minimum equivalent diameter of the tube section is Φ25mm, the maximum equivalent diameter is Φ41.6mm, the tube billet with diameter Φ25mm is selected as the forming billet.

Figure.3 Shape and size of the tube

To realize the overall forming of the pipe fittings, it is necessary to push the replenishment from both ends of the pipe billet to realize the process replenishment while the pipe is hydroformed. After replenishment of the profile, the change of the tube and section perimeter along the axis direction is shown in Figure.4a. From the distribution curve of the section perimeter increase ratio; it can be seen that the violent deformation of the tube in the process of changing from uniform section to non-uniform section is mainly concentrated at the ends of the tube. Figure.4b gives the shape and dimensions of the tube before and after the deformation of the section at both ends.

Figure.4 Distribution curve of the increased ratio of the circumference of the tube section (a) and the change of the tube section at both ends before and after forming (b)

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**2. Hydraulic forming process**

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2.1 Difficulty analysis of tube forming

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2.1.1 Difficulty analysis of material

Due to previous experience and less research on the hydraulic forming of Hastelloy tubes in the literature, the yield strength of Hastelloy is about 380MPa, the tensile strength is about 760MPa, and the yield-to-tensile ratio is 0.5. The smaller the yield-to-tensile ratio is, the better the rupture resistance, film adhesion, and plasticity of the material, but the hardening index n value of the material is as high as 0.48, and the real stress-real. However, the hardening index n value of the material is as high as 0.48. The real stress-true strain curve shows that the hardening rate is high and fast. Therefore, the pressure control in the expansion process should be accurate, and it is necessary to match the suitable forming parameters to form the part into place quickly. Moreover, the material of this part is thin, the thickness is only 1.2mm, which is sensitive to the filling pressure of tube hydroforming, and the forming process interval is small; combined with the thinning requirements of the part, the filling pressure needs to be analyzed and optimized.

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2.1.2 Difficult analysis of the shape

- (1) At both ends of the tube, the cross-section changes greatly, from the initial cross-sectional perimeter of 78.5mm to the maximum cross-sectional perimeter of 110mm, with a theoretical deformation elongation of 40%. At the same time, the technical requirements for its thinning do not exceed 15%. Hence, the cross-section is prone to rupture and become scrap due to the excess thinning and must be hydraulically formed simultaneously to make up for the timely axial advance.
- (2) As the final part shape for the middle bending, both sides of the alien, and bending parts bending radius is small, need to bend forming and then hydraulic forming. During hydraulic forming, the bending part is no longer deformed. Therefore, it is impossible to realize bilateral replenishment of both sides of the deformed part of the cross-section, but only unilateral replenishment of the deformed part through the tube end to reduce the material thinning of the expanded part. Therefore, the parameters of the closed-loop control curve of the filling pressure and the side-pushing replenishment amount must be continuously optimized and precisely controlled during the hydraulic forming process.
- (3) The cross-section of both ends of the part is composed of asymmetric planes and curved surfaces, and the uniformity of deformation is difficult to control. In addition to hydraulic expansion in the tube, hydraulic flat shaping is also required for filling.

According to the characteristics of the part and the difficulty of forming, it is necessary to bend the tube by CNC and then carry out two sub-forming. Therefore, set the part forming process: CNC bending partial hydraulic expansion filling hydraulic flat shaping, as shown in Figure.5. The focus of attention in this paper is on the last two steps, namely, hydraulic expansion and filling hydraulic flat shaping, and considering the middle part of the part without cross-sectional changes, therefore, mainly intercepted the two ends of the variable cross-sectional part to study.

Figure.5 Schematic diagram of the pipe fitting forming process.

(a) CNC pipe bending; (b) local hydraulic expansion; (c) hydraulic flattening and shaping

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2.2 Hydraulic forming process steps

Step 1: hydraulic expansion, as shown in Figure.6. First of all, the tube billet formed by CNC bending is placed in the expansion die, the upper and lower die after the mold, both sides of the push head in the side of the push cylinder to move forward until contact with the billet to achieve extrusion seal; then, through the booster to apply certain water pressure to the tube p; finally, in the push head fill and water pressure under the joint action of the billet local diameter expansion. The main purpose of this step is to increase the cross-sectional diameter from Φ25mm to Φ35mm at the maximum cross-section of the pipe billet; secondly, it also has the role of shaping because there is a rebound in the pipe billet after bending, and it cannot be put directly into the die of the hydraulic filling flat shaping process, this process can play a role in eliminating the rebound of the bent pipe and part of the shaping. In hydraulic tube fitting forming, the possible defects are wrinkling and rupture, which requires precise control of the forming process of the amount of lateral pushing and filling pressure matching relationship.

Figure.6 Schematic diagram of the hydraulic swelling process and tooling.

(a) hydraulic expansion mold; (b) initial stage; (c) expansion stage; (d) shaping stage

Step 2: Hydraulic flattening and shaping. Based on the first step of hydraulic expansion, the expanded part of the pipe billet is flattened by the support of internal pressure and finally shaped to meet the shape and size of the part.

This paper focuses on optimizing the process parameters in the hydraulic forming process of the tube end part of the pipe fitting on the impact of the part forming.

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2.3 Key process parameters

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2.3.1 Fluid filling pressure

In tube hydroforming, the internal liquid pressure mainly radially expands the tube. According to the size of the tube billet and the yield stress of the material, the formula for the filling pressure can be established as follows

p=<2t(1-ξ)/d>R

_{eL}(2)

In the formula: p is the internal liquid filling pressure of the billet; t and d are the thickness and internal diameter of the billet, respectively; ξ is the thickness thinning rate of the billet forming part; R_{eL} is the initial yield strength of the material.

The thickness of the billet t is 1.2mm, the diameter of the billet d is Φ25mm, and the initial expansion is considered as no wall thickness thinning, so ξ is taken as 0. The yield strength of the material R_{eL} is 380MPa, measured by the single pull test, and the initial filling pressure is about 36.5MPa obtained by calculation.

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2.3.2 Axial advance filling volume (side push filling volume)

During hydraulic expansion, the billet expands rapidly in the radial direction under internal liquid pressure. In contrast, the natural contraction rate in the axial direction is often smaller than that in the radial direction. Therefore, a corresponding axial thrust is required to contract the material to compensate for the rapid material contraction due to radial expansion. When the filling pressure or speed of filling material applied by the pushing head is low, it is impossible to form a uniform and effective filling material for the smooth deformation of the billet in the radial direction. The billet is locally equivalent to pure expansion, the thickness will be rapidly thinned, and necking or even fracture will occur. When the filling pressure or speed is high, the billet deformation in the axial direction is too large and too fast, and it is too late to expand the shape in the radial direction, leading to local instability or wrinkle failure. Therefore, the pushing head motion at both ends of the billet axially is critical in tube hydroforming and is the key to process design and the process parameter to focus on in subsequent finite element simulations. In this paper, the expansion area of the tubing is at both ends, so the filler can only be pushed axially close to the tube end. The calculation of the side push filler volume is shown schematically. As shown in Figure.7, the theoretical side-push replenishment amount is calculated according to the principle of equal surface area before and after forming.

πdL

_{0}=πd(L_{1}-L_{2})+πDL_{3}+π<(D^{2}-d^{2})^{/}2sinα>(3)

In the formula: L0 is the initial length of the pipe billet; L1 is the length of the tube after forming; L2 is the length of the expansion area; L3 is the length of the maximum diameter area; α is the half cone angle of the transition area; D is the maximum outside diameter after forming.

Figure.7 Schematic diagram for calculating the amount of side push replenishment.

Both sides of formula (3) are divided by πd to obtain:

L

_{0}=L_{1}-L_{2}+DL^{3}/d+<(D^{2}-d^{2})/2dsinα>(4)

The side-thrust replenishment Δl is

Δl=L0-L1=L1-L2+DL

^{3}/d+<(D^{2}-d^{2})/2dsinα>-L_{1}(5)

Bringing the tubing size into equation (5) yields a theoretical lateral pushing replenishment of 24.54 mm.

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**3. Finite element modeling and verification experiments**

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3.1 Finite element modeling

The finite element simulation of the tube hydroforming was performed on the software DynaForm. First, a geometric model of the mold and the bent pipe billet is created in the CAD software. Since the pipe is symmetric from left to right, a symmetric model is created for the simulation to save analysis time. Then, the geometry model was imported into DynaForm pre-processing. Finally, the finite element model of the tube hydroforming is established. The hydraulic expansion model consists of the upper die, lower die, push head, and the tube billet, and the filled hydraulic flat shaping model consists of the upper die, lower die, push head, floating block, and the expanded tube billet, as shown in Figure.8. Compared with the plastic deformation of the billet, the elastic deformation of both the die and the pusher head is small enough to be negligible and are regarded as rigid materials. The billet material is modeled using the plastic ontology of Hastelloy C22 of equation (1). The billet is discretized using a four-node shell unit. The friction between the billet and the die is modeled by the Coulomb friction model with a constant friction factor of f=0.15.

Figure.8 Finite element model of hydroforming

(a) Hydraulic expansion; (b) hydraulic flattening and shaping

The finite element simulation analysis was used to verify the effect of different side push filler amounts on the part forming and to determine the optimal side push filler amount. Single-factor analysis was used to analyze the beneficial wrinkles caused by the axial shrinkage deformation of the pipe fitting after the end of replenishment (which can be straightened in the subsequent expansion) and the amount of wall thinning after the final lamination under the same filling pressure. The initial filling pressure is 36.5 MPa, calculated by equation (2), so the simulated forming process parameters are shown in Fig. 9 with five different side-push filler amounts (15, 20, 25, 30, and 35 mm). In comparison, the filling pressure is kept constant at 36.5 MPa.

Figure.9 Simulated forming process parameter settings with different side-thrust charge amounts.

In addition, five sets of different filling pressures were set for simulation and comparison with the same amount of side-push charge to determine the optimal filling pressure. The simulation scheme is shown in Figure.10. The forming process is divided into 3 stages: the first stage is the filling stage (0-0.05s), which is mainly to fill the tube with liquid after the side push sealing so that the tube pressure reaches the set filling pressure; the second stage is the replenishment forming stage (0.05-0.10s), the side push replenishment amount in this stage is 25mm constant, to realize the filling pressure and side push replenishment amount To realize the formation of beneficial wrinkles under the reasonable cooperation of liquid filling pressure and side pushing charge, five groups of liquid filling pressure (36.5, 40.0, 50.0, 60.0 and 70.0 MPa) are set; the third stage is the shaping stage (0.10-0.15s), which keeps the side pushing charge constant and increases the liquid filling pressure inside the tube until the tube billet is completely fitted to the mold.

Figure.10 Simulated forming process parameters setting under different filling pressure

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3.2 Verification experiments

The equipment used in the test mold is a 1000t hydraulic forming press (Fig.11a), and the expansion mold is shown in Fig.11b. The main parameters of the equipment are shown in Table 2.

Figure.11 Hydraulic forming equipment (a) and partial hydraulic expansion mold (b)

Table.2 Parameters of 1000t hydroforming equipment

Parameter | Numerical value |

Equipment nominal pressure/kN | 10000 |

Booster system pressure/MPa | 125 |

High voltage control accuracy/MPa | ≤0.5 |

Lateral pushing system pressure/MPa | 25 |

Lateral push position control accuracy/mm | ≤0.3 |

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**4. Results and analysis**

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4.1 The shape and wall thickness change of the pipe fittings

By comparing the simulation results with the corresponding experimental test results, the causes and laws of wall thickness variation are analyzed. The formed part is cut along the axis direction. The wall thickness distribution of the tube along the axis direction is measured as shown in Fig.12. The formed part can be divided into 3 areas: the patching area, the expanding area, and the unchanged area. The center of the expanding area is taken as the origin, and the measurement is made and recorded every 10mm. The initial wall thickness of the part is 1.2mm, and the maximum wall thickness of the patch zone is 1.44mm, which is 20% thicker; the minimum wall thickness of the expanded zone is 1.03mm, which is 14.2% thinner; the wall thickness of the unchanged zone is 1.2mm, which is the same as the initial wall thickness. In the patching area, the actual material wall thickness is larger than the simulation result, which shows that the actual side-pushing patching effect is worse than the simulation, resulting in serious thickening of the material in the patching area. In the swelling area, the actual wall thickness is smaller than the simulation result because the material does not enter the swelling area exactly like the simulation result, resulting in serious thinning in the actual swelling area. Still, there is some undulation in the wall thickness at the position of -10mm in the swelling area due to the beneficial wrinkles generated during the patching process. The position is exactly at the crest of the wrinkles, so the thinning at this point is smaller than the thinning around it. The comparative analysis with the results of numerical simulation shows that the trend of actual wall thickness distribution is consistent with the trend of simulation results, and the simulation results have guiding significance for the actual process design.

Figure.12 Simulated and experimentally measured wall thickness thinning results of the expanded part.

(a) the section surface and wall thickness measurement points of the expanded part; (b) the measured wall thickness data of the expanded part

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4.2 Influence of side-thrust replenishment on tube forming

A comparison of the finite element simulation and experimental results of the tube end is shown in Figure.13. In the case of constant filling pressure of 36.5MPa, when the side-push charge amount is 15mm (Fig.13a), the wrinkles on the pipe billet only appear in the transition part of the section, and the wrinkle diameter is small; with the increase of the side-push charge amount, the number of wrinkles on the part increases (Fig.13b), and an obvious “beneficial wrinkle” can be formed. When the amount of side-pushing material reaches 25mm (Fig.13c), the wrinkle shape is more uniform; when the amount of side-pushing material exceeds 25mm (Fig.13d and Fig.13e), the wrinkling tendency of the part increases and “dead wrinkles” appear.

Figure.13 Finite element simulation and experimental testing of the wrinkling situation of the billet with different side thrusts

(a) Δl=15mm; (b) Δl=20mm; (c) Δl=25mm; (d) Δl=30mm; (e) Δl=35mm

When hydroforming a pipe billet, the radial expansion under the high pressure in the pipe cavity will thin the pipe wall, while the axial advance feeding will contribute to its thickening, the maximum thickening rate and the maximum thinning rate of the wall thickness of the hydroformed tubing under different side pushing replenishment amounts simulated by finite elements are shown in Figure.14. The maximum wall thinning rate is 23.22% when the side-thrust charge is 15mm which exceeds the manufacturing requirement of thinning and has the risk of cracking. With the increase of side-pushing material, the wall thinning rate is gradually reduced. For example, when the amount of side-push material exceeds 25mm, the change of wall thinning rate is very small; when the amount of side-push material is 30 and 35mm, the thinning tends to be the same, and the maximum thinning rate is 12.2%. When the amount of side pushes replenishment on the maximum wall thickening rate of the law’s impact is the opposite. When the amount of side push replenishment is small, the thickening rate of slow changes, but with the increase in the amount of side push replenishment, the wall thickening becomes more serious. When the amount of side push replenishment is more than 25mm, the tube forming area forms a “dead wrinkle,” leading to its scrap.

Figure.14 Influence of side-thrusting replenishment on the wall thickness of the formed pipe

Figure.15 Distribution of wall thickness thinning rate of the tube by finite element simulation

(a) After filling and expanding (b) After high pressure shaping

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4.3 Influence of filling pressure on tube forming

According to the design of different filling pressures, the hydraulic forming of the part is simulated, and one group of results with the smallest thinning is selected for analysis. The simulation results are shown in Figure.15. The maximum wall thinning rate is 9.47%, which is located at the nearest wrinkle crest from the end of the tube (Figure.15a); the maximum thickening rate is 18.26%, which is located at the end of the tube filling area (Note: Figure.15 shows the thickening rate, the parameter at the thinning is negative). The maximum wall thinning rate of 12.86% after high-pressure shaping, located in the location of the wrinkle valley after the filler expansion, the location of the plastic pressure under the role of expansion against the mold, so the largest thinning (Figure.15b); shaping stage of the tube no thickening, the maximum thickening or in the location of the tube end after the filler expansion, the maximum thickening rate of 18.16%.

The maximum rate of thinning and maximum rate of thickening of the part after shaping under different filling pressure is shown in Figure.16. The simulation results show that: with the increase of filling pressure, the maximum thinning rate of the tube becomes smaller; and when the filling pressure inside the tube increases, the wrinkles produced by the filling material become more uniform, and the best effect of beneficial wrinkles is achieved when the filling pressure is 60MPa, and the thinning after the expansion is also minimal, and the maximum thinning rate of the tube is 12.86%. When the filling pressure is too large or too small, it will increase the local thinning of the final expanded part.

Figure.16 Effect of different filling pressures on the wall thickness of the formed tube

The optimal process parameters were obtained by optimizing the side-push charge amount and filling pressure: 25mm side-push charge amount and 60.0MPa filling pressure, which were experimentally verified on 1000t hydraulic forming equipment. Through finite element simulation, the process parameters were optimized, the test production of the part was completed, and after inspection, its size and shape met the process requirements.

Figure.17: Simulated and experimental parts of each process

(a) Simulation result; (b) Experimental result

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**5. Conclusion**

Finite element simulation was used to analyze the forming process of the Hastelloy C22 tube, and reasonable key process parameters were obtained. The optimal amount of side-push replenishment was determined to be 25 mm by comparing the effects of different side-push replenishment amounts on the forming effect and the thinning rate of the part; the optimal filling pressure was determined to be 60.0 MPa by comparing the effects of different filling pressures on the forming of the part with the same side-push replenishment amount and obtaining the effect of the change in filling pressure on the replenishment of the part. The process parameters determined based on the finite element simulation were verified experimentally, fittings meeting the manufacturing requirements were formed, and the production commissioning cycle was shortened, providing solutions and reference experience for studying the tube hydroforming process.

Authors: Shi Lijun, Fang Gang