RESEARCHING TECHNOLOGY OF DIE-LESS HYDROFORMING FOR MANUFACTURING METAL SPHERICAL PARTS

RESEARCHING TECHNOLOGY OF DIE-LESS HYDROFORMING FOR MANUFACTURING METAL SPHERICAL PARTS

Duy Dinh Van

Ha Noi University of Science and Technology,

VietNam, Hanoi

Sinh Nguyen Van

Ha Noi University of Science and Technology,

VietNam, Hanoi

Viet Do Duc

Ha Noi University of Science and Technology,

VietNam, Hanoi

Anh Pham Ngoc

Maritime University,

VietNam, Hanoi

ABSTRACT

Spherical hollow metal parts used in architecture, construction, and chemical tanks, can be made by technologies such as metal casting, cutting, machining, stamping, and shaping with a hard-mortar punch. However, these technologies are only suitable for manufacturing small and medium-sized parts, with large-sized parts facing many challenges in terms of time, cost, and manufacturing ability. Research focuses on applicating Die-less hydroforming to make hollow metal sphere parts. The method combines theory and experiment, showing that this technology has many advantages compared to the above technologies, such as high product quality, short production time, and low cost. The research results have illustrated a manufacturing technology process, a method to determine parameters such as forming liquid pressure, size, and geometric shape of the original workpiece – it is necessary to create spherical-shaped products from metal sheets. The obtained results have proven that it is possible to fabricate large-sized spherical parts, which have the potential to apply technology to the fields of chemicals and architecture to meet current reality needs.

Keywords: Hydrostatic forming, spherical products, die-less hydroforming.

Introduction

Hollow spherical components have a strong and optimized construction, which maximizes bearing capacity while reducing weight. This is useful in structural engineering, chemical tanks, weight-bearing capability structures, or iconic buildings in major cities. There are several processing methods for manufacturing spherical parts: metal spinning welding, deep drawing welding [1], and explosive forming welding. However, the disadvantage of the above methods is that only small and medium-sized spherical parts can be manufactured. It is necessary to manufacture other parts with larger sizes by the above methods (diameter over 1 meter); it will require huge machinery and molds, which are costly and not high productivity.

Moreover, the quality of products is not high. With the weld-spinning method, the thickness of the product is uneven due to instability away from the center. With the method of stamping-welding or stamping-welding, the product is easy to wrinkle and tear. The research shows that, with die-less hydroforming technology, it is possible to completely manufacture spherical parts with huge sizes quickly, with high productivity, low cost, high product quality, and precision. When making a spherical part of the desired size, by preliminarily deploying the profile on 2D software, laser cutting, welding the pieces together, and finally pumping the pressure according to the calculation, then, the parts will be made.

Figure 1. Chemical tank with a diameter of 8 meters

1. Materials and Methods

The research method combines theory with experiment. The theory focuses on building the initial billet profile and forming pressure. The investigation is about fabricating 01 hollow sphere model with a diameter of 240mm.

First, The basis for building the initial billet profile and forming pressure

The main content of fabrication: A sealed polyhedral shell is first assembled and sealed with flat sheets or curvature petals. Next, the polyhedron is filled with liquid (usually water) and pressurized through a pressurization system. Under internal pressure, plastic deformation occurs on the shell, gradually expanding to form a sphere.

a)                                                                      b)

Figure 2. Structural polyhedral shell [3]

a) Flat shape b) Simple curved shape

Selecting a polyhedral shell before fabricating [2] is key to making spherical parts. There are two types of structure before hydroforming. One is a planar structure, where flat plates of different shapes, such as the soccer ball create the shell profiles. The second is a single curve structure, such as a basketball (Figure 2.2)

In industry, it is commonly used the form of a planar structure. This structure includes only regular pentagons and ordinary hexagons, which are simple to cut, weld, and assemble. However, compared with the single curve structure with 15 petals, the planar structure has a more significant volume change rate and longer weld length, which is the main shortcoming of the planar structure. Longer welds will easily cause weld cracks during hydroforming.

Under the action of internal pressure, the stress distribution of the polyhedral shell has a certain correspondence with the radius of curvature (Fig. 2.2). In there, r₁ and r₂ are the first and second principal curvated radius, in turn, t is the thickness of the shell, p is the internal pressure,  is the angle between r2 and the rotated axis (y-axis), a is the equatorial radius (the radius of the non-rotated axis), b is the radius of the rotated axis [3].

Figure 3. Polyhedral shell shape [3]

The ratio of the long axis is defined as . For the ideal elliptical polyhedral shell when subjected to its internal pressure, meridian stress, and latitudinal stress:

Table 1.

Mechanical properties of workpiece

Figure 4. Stress distribution at the polyhedral shell [3]

Stress state at the pole:

Stress state at the equator:

Apply Tressca's law:

is the yield stress of the material (N/mm ² )

The pressures at the poles and equator are calculated:

Second, The experiment of manufacturing model spherical part.

Fabrication of 01 hollow sphere model with a diameter of 240mm using SGC340 material, 0.8mm thick.

The circumference of the sphere to be fabricated:

Figure 5. Ellipse

Ellipse Perimeter:

Due to negligible deformation, perimeter conservation:

AB = 2a, choose a = 120 mm, b = 77.75 mm.

2. Results & Discussion 

First, Geometric size and original workpiece shape.

Divide the number of curve-ture petals: 12 petals.

Using AutoCAD software, drawing the curve-ture blade profile and polar plate:

Figure 6. 2D curve-ture blade and polar

Figure 7. 3D workpiece before hydroforming

When dividing more blades in the equatorial region, the smaller the dihedral angle, the smaller the weld deformation. However, the number of petals in the equatorial region increases the weld length.

Second, Calculation of forming pressure

Pressure at the pole:

Pressure at the equator:

Thus, the required forming pressure is 2.8 MPa.

Figure 8. State of the polyhedral shell at 1.0 Mpa pressure

The first deformation occurs at the polar plate under the action of internal pressure (when the pressure reaches 1.0 Mpa), and the short axis begins to elongate. When the short shaft reaches the designed size, small cracks appear in the equatorial welds due to the continuous increase of the compressive stress value with latitude. With the continuous increase of internal pressure, the elongation of the short axis becomes larger, the polar and temperate regions gradually change to a spherical shell, while the contraction of the equatorial region is relatively small.

The hollow spherical part is made by high-pressure liquid stamping technology with good quality. The thickness is evenly at all points, and the data between calculation and experiment are approximately the same:

Table 2.

Comparison of calculated and experimental data

Third, Manufacturing technology process

Table 3.

Manufacturing technology process

Experimental results show the advantages of the method of manufacturing spherical parts using high-pressure liquid stamping technology:

First, the manufacturing time is short. The time to pump up the forming pressure is in minutes, while other machining methods take a lot of time to prepare the workpiece and set it up.

Second, this method saves production costs. To make any size of the spherical part, it is only to calculate the initial preliminary profile on 2D software, laser cut, and weld the pieces together, so there is no need to use expensive equipment.  

Next, this method gives high accuracy. The error between calculation and experiment is between 3% - 5% due to the properties of each material and the influence of re-elasticity.

3. Conclusion

The research results have given the manufacturing technology process and method to determine parameters such as forming liquid pressure, size, and geometrical shape of the initial workpiece to create spherical products from metal plates. The obtained results have proven that it is possible to fabricate large-sized spherical parts, which has the potential to apply this technology in the fields of chemicals and architecture to meet current real needs.

References:

1. Pham Van Nghe, Hydrostatic Forming Technology, 1st edn. Bach Khoa Publishing House (2007)
2. Harjinder Singh, Fundamentals of Hydroforming, Society of Manufacturing Engineers, 2003.
3. Shijian Yuan, Modern Hydroforming Technology, Springer, 2023.
4. M. Koc, Hydroforming for Advanced Manufacturing, CRC Press, 2008.