**3. Case study: Introducing two complex industrial workpieces**

#### *Case study I [9]*

Figure 7 shows the photograph of the product that is used in lightning industry. The part is made of Al 1100 sheet with a thickness of 1mm and with strain hardening equation of σ=610 ε0.24 [MPa]. Currently, it is produced in the collaborating industry by the conventional deep drawing process in several stages.

Three main features of the workpiece are:

1. It has a very complex shape with special internal profile that should be produced with high precision and surface finish.


**Figure 7.** Case study 1; The part of lightning industry (after trimming and coating): (a) internal view, (b) external view

### *Case study II [17]*

60 Metal Forming – Process, Tools, Design

operations.

technology:

1. Reduction in weight

associated fixturing)

ease of applying internal pressure.

drawing process in several stages.

Three main features of the workpiece are:

high precision and surface finish.

8. Good surface finish

**workpiece** 

*Case study I [9]* 

2. Increase in stiffness and rigidity 3. Economic material utilization

4. Complex shaped and various part types

6. Reduction in overall cost per part or cost of assembly

improving structural integrity, strength and rigidity. In addition, this process satisfies these

Saving in tooling, material, design, production and assembly will altogether contribute reducing the overall cost of a sheet hydroforming part. Elimination or decrease of welds and

A reduction in number of production steps and components in an assembly will be obtained with this process. This would reduce dimensional variations, and facilitate assembly

Following is a list of potential advantages gained with the use of sheet hydroforming

5. Reduction in number of steps during manufacture and assembly (reduced welding and

The range of the applications of any sheet hydroforming process is limited. Not all the processes can be used for complex industrial parts. In this section, the criteria in selecting a specific process will be explained, such as the high drawing ratio, control of wrinkling, and

Figure 7 shows the photograph of the product that is used in lightning industry. The part is made of Al 1100 sheet with a thickness of 1mm and with strain hardening equation of σ=610 ε0.24 [MPa]. Currently, it is produced in the collaborating industry by the conventional deep

1. It has a very complex shape with special internal profile that should be produced with

7. Tight tolerances with good dimensional characteristics and less variation

In contrast, longer process cycle and higher tool cost are limitations of this process.

**2.1. Selection of a sheet hydroforming process for a complex industrial** 

**3. Case study: Introducing two complex industrial workpieces** 

requirements with utilizing the common and available material efficiency.

welding operations is an additional of the overall cost.

Drawing of conical parts is considerably more difficult than the deep drawing of cylindrical cups. Conical shape forming through conventional deep drawing is shown in Figure 8. As it is shown in the figure, due to small contact area of the punch tip with the blank at initial stages of deformation, high stresses are applied to this area of the sheet. This may cause bursting in the sheet. In addition, when forming conical cups through conventional deep drawing, wrinkling occurs on the sheet wall because the blank is free between the punch and the die [18, 19]. Thus, conical parts are normally formed by multi-stage deep drawing [18], spinning [20] or explosive forming [21].

Because of the large radial tensile stresses, small drawing ratios must be used in each stage when making conical components in multi-stage deep drawing. In addition, the ratio of the sheet thickness to the initial blank diameter influences the limiting drawing ratio to a greater extent than when drawing cylindrical parts. The limiting drawing ratio also depends on the cone angle and the ratio of the largest to the smallest cone diameter [18].

### *Experimental set-up*

In all the hydroforming processes a universal testing machine (Figure 9(a)) and a hydraulic unit (Figure 9(b)) are used. In addition, the hydraulic unit was used as the pressure medium to form the workpiece. A control valve with a maximum pressure of 50MPa regulated the liquid flow to maintain the required pressure.

#### *Tool set-up: Case study I*

Figure 10 illustrates the schematic illustration of the new tool set-up proposed for this part. Figure 11 shows the photograph of the punch used for the part. The manufactured die-set is

Developments in Sheet Hydroforming for Complex Industrial Parts 63

**Figure 10.** Schematic illustration of the tool set-up in the proposed method

**Figure 11.** Photograph of the manufactured punch for case study I

The operational sequence for the new hydroforming process is given below:

1. The die was clamped on the machine anvil. The rubber diaphragm was located on the die, the steel sheet was put on the rubber, and the O-ring was located in the groove

**Figure 8.** Schematic illustration of forming a conical part in conventional deep drawing

**Figure 9.** (a) Testing machine, (b) hydraulic unit

shown in Figure 12. It consists of a punch, a blank holder, a steel sheet ring, a die (oil container), a rubber diaphragm and an O-ring, which seals the liquid in the pressure chamber (die). As it is seen from the figure, the rubber diaphragm is only used in the region between the blank holder and die. Therefore, no diaphragm is used in the deformation region of the blank. In this region, the liquid is in direct contact with the blank. Thus, while the diaphragm is prevented from any deformation and tearing, lower pressure is required to form the workpiece, compared with the case in the standard hydroforming.

In contrast to the conventional deep drawing, the sheet metal is not in direct contact with the die in the proposed method. Also, the blank holding system in this method is a soft-tool one. Therefore, wrinkles can be controlled to high extent, compared with the case in the hydromechanical and hydro-rim processes.

**Figure 10.** Schematic illustration of the tool set-up in the proposed method

62 Metal Forming – Process, Tools, Design

**Figure 8.** Schematic illustration of forming a conical part in conventional deep drawing

shown in Figure 12. It consists of a punch, a blank holder, a steel sheet ring, a die (oil container), a rubber diaphragm and an O-ring, which seals the liquid in the pressure chamber (die). As it is seen from the figure, the rubber diaphragm is only used in the region between the blank holder and die. Therefore, no diaphragm is used in the deformation region of the blank. In this region, the liquid is in direct contact with the blank. Thus, while the diaphragm is prevented from any deformation and tearing, lower pressure is required to

(a) (b)

In contrast to the conventional deep drawing, the sheet metal is not in direct contact with the die in the proposed method. Also, the blank holding system in this method is a soft-tool one. Therefore, wrinkles can be controlled to high extent, compared with the case in the

form the workpiece, compared with the case in the standard hydroforming.

**Figure 9.** (a) Testing machine, (b) hydraulic unit

hydromechanical and hydro-rim processes.

**Figure 11.** Photograph of the manufactured punch for case study I

The operational sequence for the new hydroforming process is given below:

1. The die was clamped on the machine anvil. The rubber diaphragm was located on the die, the steel sheet was put on the rubber, and the O-ring was located in the groove machined in the die. Then, the die cavity was filled with the pressure medium. The circular blank was then located on the steel sheet ring.

Developments in Sheet Hydroforming for Complex Industrial Parts 65

Density, *ρ* (kg/m3) [16]

Poisson's ratio [16]

Yield stress, *σ* (MPa) [16]

In this study, pure copper and St14 sheets with different thicknesses and different initial diameters were used to form conical-cylindrical cups in one stage by HDDRP process. Mechanical and physical properties for the sheets are shown in Table 1. Based on the experimental observations, the copper sheet did no behave any anisotropy. To characterize the material properties and anisotropy for steel sheets, according to ASTM-A370 standard different specimens were cut at different orientations to the rolling directions (0o, 45o, and 90o). Tensile specimens were used to determine the stress–strain curves and the sheet anisotropy parameters, r-values. Plastic strain ratio (r-value) and yield stress ratio (R-value)

The schematic of the hydroforming die set used in this research is shown in Figure 14. Figure 15 shows the parametric geometry of the workpieces examined. Initially, three types of blanks with similar geometries but with variation in material or sheet thickness were selected. The specifications of these blanks and the formed parts are shown in Table 3 as parts A, B and C and with strain hardening equation of σ=k εn, where k and n are specified

A component with different geometry from those of the above mentioned parts was also

Strength coefficient, K

Cu 99.9 % 117 0.44 530.98 123 0.32 8940 St 14 210 0.35 638.96 190 0.3 7850

Thickness (mm) R0 R45 R90 R22 R33 R12

A DMG (Denison Mayes Group) universal testing machine with 600kN capacity was used in

Figure 16 shows the components of the die and the assembled die-set. Figure 17 shows the parametric dimensions of the die set used for forming the parts. The dimensions of the die

1 1.79 2.27 1.01 1.0402 1.24897 1.0789

selected to have a wider study. This is specified in Table 3 as part D.

Strain hardening exponent, n

**Table 1.** Mechanical and physical properties for pure copper and St14 sheets

**Table 2.** Plastic strain ratio (r-value) and yield stress ratio (R-value) for St14 sheet

*Tool set-up: Case study II* 

in the table.

Blank material

the experiments.

set are given in Table 4.

for St14 sheet are illustrated in Table 2.

Young's modulus, E (GPa) [16]


**Figure 12.** Photograph of the manufactured die set

Figure 13(a) illustrates the variations of internal pressure with punch stroke for the hydroforming of part in case study 1. As it is seen from the figure, the maximum forming pressure is about 5.5 MPa that is very low, compared with the results of reference [16], which formed simple parts with other hydroforming processes. As it can be seen from Figure 13(a), the internal pressure in the final stage of hydroforming oscillates. This is due to forming the special internal profile of the workpiece. Figure 13(b) illustrates the load-punch stroke curve of the workpiece. As it is seen from the figure, the maximum load is about 60kN which is not so high.

**Figure 13.** (a) Internal pressure-punch stroke curve, (b) load –displacement curve, correspond to case study I

#### *Tool set-up: Case study II*

64 Metal Forming – Process, Tools, Design

preloading on the blank.

machined in the die. Then, the die cavity was filled with the pressure medium. The

2. The blank was clamped between the die and blank holder by four screws. At this stage, an initial blank holder force was applied to the blank and a pre-hydroforming pressure was exerted on the lower surface of the rubber diaphragm and blank, lead to a small

3. The punch that was attached to the machine ram, moved down, pressurized the

Figure 13(a) illustrates the variations of internal pressure with punch stroke for the hydroforming of part in case study 1. As it is seen from the figure, the maximum forming pressure is about 5.5 MPa that is very low, compared with the results of reference [16], which formed simple parts with other hydroforming processes. As it can be seen from Figure 13(a), the internal pressure in the final stage of hydroforming oscillates. This is due to forming the special internal profile of the workpiece. Figure 13(b) illustrates the load-punch stroke curve of the workpiece. As it is seen from the figure, the maximum load is about

**Figure 13.** (a) Internal pressure-punch stroke curve, (b) load –displacement curve, correspond to case

(a) (b)

circular blank was then located on the steel sheet ring.

medium, and the deformation was started.

**Figure 12.** Photograph of the manufactured die set

60kN which is not so high.

study I

In this study, pure copper and St14 sheets with different thicknesses and different initial diameters were used to form conical-cylindrical cups in one stage by HDDRP process. Mechanical and physical properties for the sheets are shown in Table 1. Based on the experimental observations, the copper sheet did no behave any anisotropy. To characterize the material properties and anisotropy for steel sheets, according to ASTM-A370 standard different specimens were cut at different orientations to the rolling directions (0o, 45o, and 90o). Tensile specimens were used to determine the stress–strain curves and the sheet anisotropy parameters, r-values. Plastic strain ratio (r-value) and yield stress ratio (R-value) for St14 sheet are illustrated in Table 2.

The schematic of the hydroforming die set used in this research is shown in Figure 14. Figure 15 shows the parametric geometry of the workpieces examined. Initially, three types of blanks with similar geometries but with variation in material or sheet thickness were selected. The specifications of these blanks and the formed parts are shown in Table 3 as parts A, B and C and with strain hardening equation of σ=k εn, where k and n are specified in the table.

A component with different geometry from those of the above mentioned parts was also selected to have a wider study. This is specified in Table 3 as part D.


**Table 1.** Mechanical and physical properties for pure copper and St14 sheets


**Table 2.** Plastic strain ratio (r-value) and yield stress ratio (R-value) for St14 sheet

A DMG (Denison Mayes Group) universal testing machine with 600kN capacity was used in the experiments.

Figure 16 shows the components of the die and the assembled die-set. Figure 17 shows the parametric dimensions of the die set used for forming the parts. The dimensions of the die set are given in Table 4.

Developments in Sheet Hydroforming for Complex Industrial Parts 67

Values related to parts Parameter Parts A, B, C Part D

Die inside diameter, Dd A=46, B=C=44 75 diameter, Dh Blank holder inside 42 71 Blank diameter, Db 78 120 Blank holder entrance radius, rh 3 3 Die entrance radius, rd 4 5 , G Gap between die and blank-holder A= 2.2, B=C=1.2 2.2

In the HDDRP, before the punch goes down, a small pre-bulging can be created on the sheet to improve the drawing process [16]. A hydraulic unit was used to create the pre-bulging pressure. When the punch moves down, the blank is forced into the die cavity filled with oil or other liquids. A control valve was used for controlling the maximum pressure. The liquid in the die cavity was pressurized so it pushes the blank tightly onto the punch surface. After

**Figure 16.** (a) components of the die, (b) assembled die set mounted on the test machine

**Figure 17.** Parametric dimensions of the hydroforming die set

**Table 4.** Parameters of the die corresponds to Figure 17 (Dimensions in mm)

**Figure 14.** Schematic of the hydroforming die set used

**Figure 15.** Parametric dimensions of formed parts


**Table 3.** Parameters of the part corresponds to Figure 15 (Dimensions in mm)

Developments in Sheet Hydroforming for Complex Industrial Parts 67

**Figure 16.** (a) components of the die, (b) assembled die set mounted on the test machine

**Figure 17.** Parametric dimensions of the hydroforming die set

66 Metal Forming – Process, Tools, Design

**Figure 14.** Schematic of the hydroforming die set used

**Figure 15.** Parametric dimensions of formed parts

Values related to parts Type and dimensions of the part Parts (A, B, C) Part (D)

Type of material A, B=Pure Copper, C= St14 Pure copper 40 Height of conical portion, h1 <sup>20</sup>

Height of cylindrical portion, h2 18 16 Flat head radius, R1 8 7.5 Cylindrical radius section, R2 20.75 35.35 Nose radius, r1 3.5 10.5 Conical-cylindrical radius, r2 5.5 6.5 Initial blank thickness t0 A=2, B=1, C=1 2

**Table 3.** Parameters of the part corresponds to Figure 15 (Dimensions in mm)


**Table 4.** Parameters of the die corresponds to Figure 17 (Dimensions in mm)

In the HDDRP, before the punch goes down, a small pre-bulging can be created on the sheet to improve the drawing process [16]. A hydraulic unit was used to create the pre-bulging pressure. When the punch moves down, the blank is forced into the die cavity filled with oil or other liquids. A control valve was used for controlling the maximum pressure. The liquid in the die cavity was pressurized so it pushes the blank tightly onto the punch surface. After

reaching a maximum pressure, the control valve was opened and the pressure remained constant during the forming process. The liquid in the die cavity leaks out dynamically from the interface between the blank-holder and the die. The interface between die and blankholder was grinded metal contact and no o-ring was used in the die. At the same time, the liquid leaking out from this interface creates a pressure around the outside rim of the blank. Therefore, it is impossible to create high pre-bulging pressure in this die set. In this research, 2 MPa pre-bulging pressure was applied.

Developments in Sheet Hydroforming for Complex Industrial Parts 69

elements along the thickness was 4. The die set was modeled using a rigid four- node shell

The die and the blank holder were constrained fully and the punch could move only along the vertical direction, corresponding to the central axis of the punch. Pressure constrains were applied on the whole bottom surface and also on the rim of the blank. The gap between the die and the blank holder was fixed. The punch motion was prescribed with a constant velocity. Because of the consideration of pre-bulging, the loading of liquid pressure in the die cavity was used as a two-step linear profile. The friction coefficient on the blank and the punch interface was considered to be 0.14 in the simulations. The coefficient on the other surfaces was considered to be 0.04. Penalty contact interfaces were used between the sheet metal and the tooling elements.

The schematic of the modified die-set for case study I is shown in Figure 19. The photograph of the used punch is shown in tool set-up section. To form this part, several pressure paths have been examined by FE simulation and the appropriate pressure path is shown in Figure 20. As it can be seen in the figure, the maximum forming pressure is about 5.5MPa which is very low, in comparison to the results of the other relevant references, which formed simple

Figure 21 shows the photograph of the workpiece formed in the new die-set. The initial blank is a round one with a diameter of 140 mm. As it can be seen in this figure, the workpiece is formed quite well to the final required height, only in one step. The sharp region of the workpiece is formed successfully. The internal surface of the product is formed with high precision and good surface finish and there is no, even one small, wrinkle on the flange area.

**Figure 19.** Schematic illustration of the proposed set-up for a complex part

Table 1 shows the properties of pure copper and St14 sheets which have been used.

element (R3D4).

*Case study I* 

**4. Results and discussion** 

parts with other hydroforming processes.

Figure 18 shows the typical pressure path used in this study. In this path, OA is the initial pre-bulging pressure (2 MPa) applied before the punch moves down. BC is the constant maximum pressure. The liquid outflows from control valve by applying this pressure. SAE10 hydraulic oil with a viscosity of 5.6 cSt was used as the pressure medium. Due to the strain-rate sensitive behavior of the viscous medium, the punch velocity has significant effect on the internal pressure generation. Thus, in the pressure path of Figure 18, AB is the linear pressure path and its slope depends on punch velocity and workpiece shape and thickness. In this research, a punch velocity of 200mm/min was applied. To measure the cup thickness, a mechanical thickness measurement set was used.

The typical pressure path in this paper was shown in Figure 18. According to the figure, for each certain maximum pressure, a pre-bulging pressure, OA, and a pressure path AB with different slopes are definable. The slope of AB changes with punch velocity, workpiece shape and sheet thickness. The punch velocity was fixed at 200mm/min. Thus, for each certain part with defined shape and thickness, one specific slope was obtained.

**Figure 18.** The typical pressure path applied in the investigation

#### *FEM simulation*

The commercial software, ABAQUS 6.7/Explicit, was used for the simulation. For pure copper sheet, the material behavior was assumed to be isotropic as the experimental results have verified this assumption. For St14 sheet, the anisotropy factors mentioned in the previous section were used in the simulation. 3D models were used for the simulation. The blank was modeled deformable with eight-node solid element (C3D8R). The number of elements along the thickness was 4. The die set was modeled using a rigid four- node shell element (R3D4).

The die and the blank holder were constrained fully and the punch could move only along the vertical direction, corresponding to the central axis of the punch. Pressure constrains were applied on the whole bottom surface and also on the rim of the blank. The gap between the die and the blank holder was fixed. The punch motion was prescribed with a constant velocity. Because of the consideration of pre-bulging, the loading of liquid pressure in the die cavity was used as a two-step linear profile. The friction coefficient on the blank and the punch interface was considered to be 0.14 in the simulations. The coefficient on the other surfaces was considered to be 0.04. Penalty contact interfaces were used between the sheet metal and the tooling elements. Table 1 shows the properties of pure copper and St14 sheets which have been used.
