**4. Results and discussion**

*Case study I* 

68 Metal Forming – Process, Tools, Design

2 MPa pre-bulging pressure was applied.

thickness, a mechanical thickness measurement set was used.

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

*FEM simulation* 

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,

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

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

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

certain part with defined shape and thickness, one specific slope was obtained.

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 parts with other hydroforming processes.

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

Developments in Sheet Hydroforming for Complex Industrial Parts 71

**Figure 22.** Desired pressure paths corresponding to different Pmax, to form (a) part A, (b) parts B, C, (c)

The results obtained from experiment and simulation illustrated that for part A with maximum pressures of less than 12.5 MPa bursting occurs in the contact area of the workpiece with punch nose radius. Figure 23 shows a model of part A formed with maximum pressure of 10 MPa along with its simulation results. As it can be seen, bursting has occurred in the workpiece because the low level of forming pressure leads this process

The results indicated that in parts B, C, and D bursting occurs in the same area at maximum pressures less than 7.5, 17.5 and 7.5 MPa, respectively. Figure 24 shows the picture of parts

**Figure 23.** Deformed part A corresponds to pressure path 10MPa, (a) simulation, (b) experiment

to act just like as the conventional deep drawing.

B, C and D in which bursting occurred.

part D

**Figure 20.** Pressure path used to form the lightning industry product

**Figure 21.** Photograph of the hydroformed part in the proposed die-set, (a) external view, (b) internal view

#### *Case study II*

Figure 22 shows the desired paths corresponding to maximum pressures for parts A, B, C and D. For parts B and C the punch velocity, workpiece shape and sheet thickness are the same which leads to the same slope.

Developments in Sheet Hydroforming for Complex Industrial Parts 71

view

*Case study II* 

same which leads to the same slope.

**Figure 20.** Pressure path used to form the lightning industry product

**Figure 21.** Photograph of the hydroformed part in the proposed die-set, (a) external view, (b) internal

(a) (b)

Figure 22 shows the desired paths corresponding to maximum pressures for parts A, B, C and D. For parts B and C the punch velocity, workpiece shape and sheet thickness are the

**Figure 22.** Desired pressure paths corresponding to different Pmax, to form (a) part A, (b) parts B, C, (c) part D

The results obtained from experiment and simulation illustrated that for part A with maximum pressures of less than 12.5 MPa bursting occurs in the contact area of the workpiece with punch nose radius. Figure 23 shows a model of part A formed with maximum pressure of 10 MPa along with its simulation results. As it can be seen, bursting has occurred in the workpiece because the low level of forming pressure leads this process to act just like as the conventional deep drawing.

The results indicated that in parts B, C, and D bursting occurs in the same area at maximum pressures less than 7.5, 17.5 and 7.5 MPa, respectively. Figure 24 shows the picture of parts B, C and D in which bursting occurred.

**Figure 23.** Deformed part A corresponds to pressure path 10MPa, (a) simulation, (b) experiment

Developments in Sheet Hydroforming for Complex Industrial Parts 73

It was observed that increasing the maximum pressure leads to decrease necking. Figure 27 indicates the formed part A corresponding to 25 MPa maximum pressure. Applying this maximum pressure at final stage causes complete forming of the cup without necking defect

**Figure 27.** Deformed part A corresponds to pressure path 25 MPa, (a) simulation, (b) experiment

paths of 17.5, 35 and 20 MPa, respectively, and are shown in Figure 28.

Parts B, C and D were formed without any necking occurrence with the maximum pressure

To have more careful study of thickness distribution, the deformed cups were divided into

**Figure 28.** Deformed workpieces, (a) part B, Pmax = 17.5 MPa, (b) part C, Pmax = 35 MPa, (c) part D, Pmax =

 (a) (b) **Figure 29.** (a) Direction of measuring thickness distribution of conical parts, (b) different regions on the

and creates an accurate geometry.

different regions as it is shown in Figure 29.

20 MPa

formed part

**Figure 24.** Deformed workpieces correspond to different pressure paths, (a) part B, Pmax = 5 MPa, (b) part C, Pmax =15 MPa, (c) part D, Pmax = 5 MPa

In part A with maximum pressure above 12.5 MPa the conical cup was formed. Figure 25 shows the picture of part A formed with maximum pressure of 12.5 MPa. As it is seen, at the end of the process the conical cup was formed, but necking occurred in the workpiece.

The results indicated that in parts B, C, and D necking occurred in the same area at maximum pressures of 7.5, 17.5 and 7.5 MPa, respectively. Figure 26 indicates the picture of the parts in which necking occurred.

**Figure 25.** Deformed part A correspond to maximum pressure 12.5 MPa, (a) simulation, (b) experiment

**Figure 26.** Deformed workpieces corresponds to pressure paths, (a) part B, Pmax = 7.5 MPa, (b) part C, Pmax = 17.5 MPa, (c) part D, Pmax = 7.5 MPa

It was observed that increasing the maximum pressure leads to decrease necking. Figure 27 indicates the formed part A corresponding to 25 MPa maximum pressure. Applying this maximum pressure at final stage causes complete forming of the cup without necking defect and creates an accurate geometry.

72 Metal Forming – Process, Tools, Design

part C, Pmax =15 MPa, (c) part D, Pmax = 5 MPa

the parts in which necking occurred.

Pmax = 17.5 MPa, (c) part D, Pmax = 7.5 MPa

**Figure 24.** Deformed workpieces correspond to different pressure paths, (a) part B, Pmax = 5 MPa, (b)

In part A with maximum pressure above 12.5 MPa the conical cup was formed. Figure 25 shows the picture of part A formed with maximum pressure of 12.5 MPa. As it is seen, at the end of the process the conical cup was formed, but necking occurred in the workpiece.

The results indicated that in parts B, C, and D necking occurred in the same area at maximum pressures of 7.5, 17.5 and 7.5 MPa, respectively. Figure 26 indicates the picture of

**Figure 25.** Deformed part A correspond to maximum pressure 12.5 MPa, (a) simulation, (b) experiment

**Figure 26.** Deformed workpieces corresponds to pressure paths, (a) part B, Pmax = 7.5 MPa, (b) part C,

**Figure 27.** Deformed part A corresponds to pressure path 25 MPa, (a) simulation, (b) experiment

Parts B, C and D were formed without any necking occurrence with the maximum pressure paths of 17.5, 35 and 20 MPa, respectively, and are shown in Figure 28.

To have more careful study of thickness distribution, the deformed cups were divided into different regions as it is shown in Figure 29.

**Figure 28.** Deformed workpieces, (a) part B, Pmax = 17.5 MPa, (b) part C, Pmax = 35 MPa, (c) part D, Pmax = 20 MPa

**Figure 29.** (a) Direction of measuring thickness distribution of conical parts, (b) different regions on the formed part

Figure 30 shows the thickness distribution curve for part A with maximum pressure of 25 MPa. As it is seen from the figure, there is a good correlation between the results of simulation and experiment.

Developments in Sheet Hydroforming for Complex Industrial Parts 75

**Figure 31.** Thickness distribution curves for formed parts in different pressure paths, obtained from

**Figure 32.** Thickness reduction curve versus maximum pressure for region B of conical parts, (a) part

experiments, (a) part A, (b) part B, (c) part C, (d) part D

A, (b) part B, (c) part C, (d) part D

**Figure 30.** Thickness distribution curves for part A in maximum pressure of 25 MPa

In Figure 31 the thickness distribution curves obtained from experiment for different maximum pressures for four parts are displayed. As it can be found from this figure in the top of the conical cup, region A, the thickness reduction is very small. The most thickness reduction occurred in B and D regions. This thickness reduction is because of the bending occurrence in these regions. Region B is the critical zone as it was indicated in the previous results. In C and E regions the thickness increases and this thickness increase becomes greater toward the edge. It is obvious that the pressure increasing has a great effect on thickness reduction at different points of cup, especially in the critical region B.

For obtaining the best forming pressure path to produce a cup with better thickness distribution and quality, the maximum thickness reduction curve in B region was compared for different pressures. As stated previously, this is the most critical region of conical formed parts. Figure 32 shows the thickness distribution curves in B region corresponding to pressure paths with different maximum fluid pressures. As it can be found in Figure 32 (a), in part A the greatest thickness reduction is related to maximum pressure path 12.5 MPa. At this pressure, necking defect occurred in region B. It can be seen in the figure that by increasing the maximum pressure to 25 MPa, thickness reduction decreases with sharp slope. From the maximum pressure of 25 MPa and greater, the slope will not change considerably. Thus, maximum pressure more than 25MPa does not have any positive effect on the cup thickness in region B. Also, in Figure 32 (b), (c), (d) it can be found that the similar behavior happened for other parts but the greatest thickness reduction and minimum thickness reduction are different.

simulation and experiment.

Figure 30 shows the thickness distribution curve for part A with maximum pressure of 25 MPa. As it is seen from the figure, there is a good correlation between the results of

**Figure 30.** Thickness distribution curves for part A in maximum pressure of 25 MPa

thickness reduction at different points of cup, especially in the critical region B.

minimum thickness reduction are different.

In Figure 31 the thickness distribution curves obtained from experiment for different maximum pressures for four parts are displayed. As it can be found from this figure in the top of the conical cup, region A, the thickness reduction is very small. The most thickness reduction occurred in B and D regions. This thickness reduction is because of the bending occurrence in these regions. Region B is the critical zone as it was indicated in the previous results. In C and E regions the thickness increases and this thickness increase becomes greater toward the edge. It is obvious that the pressure increasing has a great effect on

For obtaining the best forming pressure path to produce a cup with better thickness distribution and quality, the maximum thickness reduction curve in B region was compared for different pressures. As stated previously, this is the most critical region of conical formed parts. Figure 32 shows the thickness distribution curves in B region corresponding to pressure paths with different maximum fluid pressures. As it can be found in Figure 32 (a), in part A the greatest thickness reduction is related to maximum pressure path 12.5 MPa. At this pressure, necking defect occurred in region B. It can be seen in the figure that by increasing the maximum pressure to 25 MPa, thickness reduction decreases with sharp slope. From the maximum pressure of 25 MPa and greater, the slope will not change considerably. Thus, maximum pressure more than 25MPa does not have any positive effect on the cup thickness in region B. Also, in Figure 32 (b), (c), (d) it can be found that the similar behavior happened for other parts but the greatest thickness reduction and

**Figure 31.** Thickness distribution curves for formed parts in different pressure paths, obtained from experiments, (a) part A, (b) part B, (c) part C, (d) part D

**Figure 32.** Thickness reduction curve versus maximum pressure for region B of conical parts, (a) part A, (b) part B, (c) part C, (d) part D

The drawing ratio of conical part is the ratio of initial blank diameter to the minimum diameter of the conical potion. The drawing ratio for parts A, B and C is 4.875 and for part D is 8. The relation between the drawing ratio and the lowest maximum forming pressure has been studied through simulation which is shown in Figure 33. In this figure it can be seen that with the sheet diameter decreasing, the conical part forming will be possible at lower pressures. When the sheet diameter or drawing ratio is increased, the forming pressure increases too, but with increasing the pressure, the sheet diameter increases to some extent. In the workpiece A, the sheet with the maximum of 87 mm in diameter can be formed through increasing the pressure but forming a blank with a diameter greater than 87 mm is not possible.

Developments in Sheet Hydroforming for Complex Industrial Parts 77

**Figure 34.** Force–punch stroke curve

Part C, (d) Part D

forming a workpiece with high quality.

**Figure 35.** Maximum punch force versus maximum pressure corresponding to (a) Part A, (b) Part B, (c)

As it was stated, a pressure increase leads to a thickness reduction and on the other hand pressure increase makes the punch force more and there is a need to have bigger tonnage of press. So, obtaining the optimum pressure and punch force has a great importance in

To examine the effect of cone angle, different punches with 450, 600 and 750 angles were manufactured for part D geometry. Figure 36 shows %thinning in B region corresponding to

In Figure 33 the maximum drawing ratio for parts A, B, C and D are 5.4, 6.06, 5, and 9.67, respectively. In conical part forming through HDDRP, the sheet thickness, the conical angle, punch tip radius (B region) and sheet properties have great and considerable influence on the drawing ratio.

Punch force is related to the forming force and internal pressure in vertical direction. As it can be seen in Figure 34, the punch force increases when it moves down to reach a maximum force, and as the punch continues to move downward the punch force decreases.

By increasing the maximum pressure, the more punch force is needed. This is because the vertical direction force increases. Figure 35 illustrates the effect of maximum fluid pressure on punch force for parts A, B, C and D.

**Figure 33.** The drawing ratio corresponding to maximum pressure, Pmax, (a) part A, (b) part B, (c) Part C, (d) part D

the drawing ratio.

C, (d) part D

on punch force for parts A, B, C and D.

The drawing ratio of conical part is the ratio of initial blank diameter to the minimum diameter of the conical potion. The drawing ratio for parts A, B and C is 4.875 and for part D is 8. The relation between the drawing ratio and the lowest maximum forming pressure has been studied through simulation which is shown in Figure 33. In this figure it can be seen that with the sheet diameter decreasing, the conical part forming will be possible at lower pressures. When the sheet diameter or drawing ratio is increased, the forming pressure increases too, but with increasing the pressure, the sheet diameter increases to some extent. In the workpiece A, the sheet with the maximum of 87 mm in diameter can be formed through increasing the

In Figure 33 the maximum drawing ratio for parts A, B, C and D are 5.4, 6.06, 5, and 9.67, respectively. In conical part forming through HDDRP, the sheet thickness, the conical angle, punch tip radius (B region) and sheet properties have great and considerable influence on

Punch force is related to the forming force and internal pressure in vertical direction. As it can be seen in Figure 34, the punch force increases when it moves down to reach a maximum force, and as the punch continues to move downward the punch force decreases. By increasing the maximum pressure, the more punch force is needed. This is because the vertical direction force increases. Figure 35 illustrates the effect of maximum fluid pressure

**Figure 33.** The drawing ratio corresponding to maximum pressure, Pmax, (a) part A, (b) part B, (c) Part

pressure but forming a blank with a diameter greater than 87 mm is not possible.

**Figure 35.** Maximum punch force versus maximum pressure corresponding to (a) Part A, (b) Part B, (c) Part C, (d) Part D

As it was stated, a pressure increase leads to a thickness reduction and on the other hand pressure increase makes the punch force more and there is a need to have bigger tonnage of press. So, obtaining the optimum pressure and punch force has a great importance in forming a workpiece with high quality.

To examine the effect of cone angle, different punches with 450, 600 and 750 angles were manufactured for part D geometry. Figure 36 shows %thinning in B region corresponding to different pressure paths for conical part with different cone angles. As it can be found, in conical part with 450 angle, the greatest thickness reduction is related to maximum pressure 18MPa. At maximum pressure less than 18MPa bursting occurs in B region. It can be seen in the figure that by increasing the maximum pressure to 25MPa, thickness reduction decreases with sharp slope. From the maximum pressure of 25MPa and greater, the slope will not change considerably. In conical workpiece with 750 angle, without applying any pressure, no bursting was observed. With increasing the pressure to almost 20MPa the thickness reduction decreases sharply. At maximum pressure of approximately 20MPa and beyond this, the slope will be horizontal. Figure 36 shows %thinning for the three different angles. It can be observed that with increasing the conical angle the thickness reduction will be decreased in B area. Moreover, as the conical angle increases, bursting occurs at lower pressure in such a way that beyond one specific angle, say 750, the conical workpiece can be formed in the die chamber without applying any pressure.

Developments in Sheet Hydroforming for Complex Industrial Parts 79

**Figure 37.** % thinning versus cone angle

considerably specially in B region.

**Figure 38.** Thickness distribution curves versus punch friction coefficient, punch angle 600

blank holder friction coefficient of more than 0.3 results in bursting in region B.

Figure 39 shows %thinning curves in A and B regions corresponding to different punch friction coefficients. From the figure it is observed that as the punch friction coefficient increases the thickness reduction decreases in the two regions. It can be seen in the figure that by increasing the punch friction coefficient to 0.3, thickness reduction decreases with sharp slope. From the punch friction coefficient greater than 0.3 the slope will not change

Figure 40 illustrates the effect of the blank holder friction coefficient. As it shows, changes in the blank holder friction coefficient affect all regions. For more accurate study, the thickness reduction in B area was investigated and the results are shown in Figure 41. It is observed that by increasing the blank holder friction coefficient, the thickness reduction increases. At higher blank holder friction coefficients, necking occurs in B region. In this research, the

**Figure 36.** % thinning for different cone angles

Figure 37 shows the failure, thinning and safe forming regions for different conical angles. As shown in the figure, by increasing the punch conical angle, the bursting region becomes smaller. At angle of 750 and higher the part did not fail even without applying the fluid pressure. In other words, by decreasing the conical angle, the pressure level should increase to prevent the failure. This analysis is valid for safe forming limits too. It means that by increasing the angle, the maximum pressure level decreases for forming an accurate part without any defect. At the angle of less than 350, bursting occurs in the part at any fluid pressure.

Figure 38 shows the effect of punch friction coefficient on thickness distribution of the conical part. As it is shown in the figure, changes in the punch friction coefficient only affect region A and B, and it has no significant effect on other areas. It was observed that with increasing the punch tip radius the thickness reduction decreases in this region.

For more detailed review, the effect of punch friction coefficient in region A and B were studied for different punch friction coefficients.

**Figure 37.** % thinning versus cone angle

formed in the die chamber without applying any pressure.

**Figure 36.** % thinning for different cone angles

studied for different punch friction coefficients.

pressure.

different pressure paths for conical part with different cone angles. As it can be found, in conical part with 450 angle, the greatest thickness reduction is related to maximum pressure 18MPa. At maximum pressure less than 18MPa bursting occurs in B region. It can be seen in the figure that by increasing the maximum pressure to 25MPa, thickness reduction decreases with sharp slope. From the maximum pressure of 25MPa and greater, the slope will not change considerably. In conical workpiece with 750 angle, without applying any pressure, no bursting was observed. With increasing the pressure to almost 20MPa the thickness reduction decreases sharply. At maximum pressure of approximately 20MPa and beyond this, the slope will be horizontal. Figure 36 shows %thinning for the three different angles. It can be observed that with increasing the conical angle the thickness reduction will be decreased in B area. Moreover, as the conical angle increases, bursting occurs at lower pressure in such a way that beyond one specific angle, say 750, the conical workpiece can be

Figure 37 shows the failure, thinning and safe forming regions for different conical angles. As shown in the figure, by increasing the punch conical angle, the bursting region becomes smaller. At angle of 750 and higher the part did not fail even without applying the fluid pressure. In other words, by decreasing the conical angle, the pressure level should increase to prevent the failure. This analysis is valid for safe forming limits too. It means that by increasing the angle, the maximum pressure level decreases for forming an accurate part without any defect. At the angle of less than 350, bursting occurs in the part at any fluid

Figure 38 shows the effect of punch friction coefficient on thickness distribution of the conical part. As it is shown in the figure, changes in the punch friction coefficient only affect region A and B, and it has no significant effect on other areas. It was observed that with

For more detailed review, the effect of punch friction coefficient in region A and B were

increasing the punch tip radius the thickness reduction decreases in this region.

**Figure 38.** Thickness distribution curves versus punch friction coefficient, punch angle 600

Figure 39 shows %thinning curves in A and B regions corresponding to different punch friction coefficients. From the figure it is observed that as the punch friction coefficient increases the thickness reduction decreases in the two regions. It can be seen in the figure that by increasing the punch friction coefficient to 0.3, thickness reduction decreases with sharp slope. From the punch friction coefficient greater than 0.3 the slope will not change considerably specially in B region.

Figure 40 illustrates the effect of the blank holder friction coefficient. As it shows, changes in the blank holder friction coefficient affect all regions. For more accurate study, the thickness reduction in B area was investigated and the results are shown in Figure 41. It is observed that by increasing the blank holder friction coefficient, the thickness reduction increases. At higher blank holder friction coefficients, necking occurs in B region. In this research, the blank holder friction coefficient of more than 0.3 results in bursting in region B.

Figure 42 shows the effect of sheet thickness on the thickness reduction in region B. As it is obvious, up to the maximum pressure of 20MPa the changes in the sheet thickness affect the thickness reduction. From the maximum pressure of 20MPa this effect is not considerable. Also, it can be observed that by decreasing the sheet thickness the possibility of thickness reduction reduces.

Developments in Sheet Hydroforming for Complex Industrial Parts 81

**Figure 42.** % thinning versus sheet thickness

chamber without applying any punch radius.

**Figure 43.** Thickness distribution curves versus punch tip radius, punch angle 600

Bending radius is a very effective factor on thickness distribution. Figure 43 shows the effect of punch tip radius on thickness distribution of the conical part. As it is shown in the Figure, changing the punch tip radius only affects region B and it has no significant effect on other areas. Figure 44 shows %Thinning at different punch radiuses. As it can be observed, with increasing the punch tip radius the thickness reduction decreases in B region. With increasing the conical angle the thickness reduction will be decreased in region B. Moreover, as the conical angle increases, bursting occurs at lower punch radius in such a way that beyond one specific degree angle, say 750, the conical workpiece can be formed in the die

**Figure 39.** % thinning of the conical part for different punch friction coefficient, (a) region A, (b) region B.

**Figure 40.** Thickness distribution curves for different blank holder friction coefficient, punch angle 600

**Figure 41.** % thinning versus blank holder friction coefficient in region B

**Figure 42.** % thinning versus sheet thickness

reduction reduces.

B.

Figure 42 shows the effect of sheet thickness on the thickness reduction in region B. As it is obvious, up to the maximum pressure of 20MPa the changes in the sheet thickness affect the thickness reduction. From the maximum pressure of 20MPa this effect is not considerable. Also, it can be observed that by decreasing the sheet thickness the possibility of thickness

**Figure 39.** % thinning of the conical part for different punch friction coefficient, (a) region A, (b) region

**Figure 40.** Thickness distribution curves for different blank holder friction coefficient, punch angle 600

**Figure 41.** % thinning versus blank holder friction coefficient in region B

Bending radius is a very effective factor on thickness distribution. Figure 43 shows the effect of punch tip radius on thickness distribution of the conical part. As it is shown in the Figure, changing the punch tip radius only affects region B and it has no significant effect on other areas.

Figure 44 shows %Thinning at different punch radiuses. As it can be observed, with increasing the punch tip radius the thickness reduction decreases in B region. With increasing the conical angle the thickness reduction will be decreased in region B. Moreover, as the conical angle increases, bursting occurs at lower punch radius in such a way that beyond one specific degree angle, say 750, the conical workpiece can be formed in the die chamber without applying any punch radius.

**Figure 43.** Thickness distribution curves versus punch tip radius, punch angle 600

Developments in Sheet Hydroforming for Complex Industrial Parts 83

In this chapter, a new sheet hydroforming is proposed and applied for forming of two industrial parts that currently are produced in industry by conventional deep drawing and stamping in several stages. With the new method, these two parts were produced in one stage and without any defects. In addition, it is shown that the forming pressure and load

In addition, for case study II, the effects of tool parameters such as the radius of the punch tip, punch-cylindrical radius, friction between punch and sheet, friction coefficient between blank holder and sheet, sheet thickness and punch angle, on formability and thickness distribution of the conical parts were studied through using hydrodynamic deep drawing assisted by radial pressure. It was observed that with increasing the conical workpiece angle the thickness reduction will decrease in B area. Moreover, as the conical angle increases, bursting occurs at lower pressure in such a way that beyond one specific angle, the conical workpiece can be formed in the die chamber without applying any pressure. Also, with the cone angle increasing, the thickness distribution will be improved and the likelihood of

*Faculty of Mechanical Engineering, Babol University of Technology, Babol, Mazandaran, Iran* 

The authors would like to thank Dr. M. Hosseinzade for the provision of valuable

[2] Koc M (2008) Hydroforming for advanced manufacturing, Woodhead publishing

[3] Elyasi M, Bakhshi-Jooybari M, Gorji A (2009) Mechanism of improvement of die corner filling in a new hydroforming die for stepped tubes. Materials and Design. 30: 3824-

[4] Elyasi M, Bakhshi-Jooybari M, Gorji A, Hossinipour SJ, Norouzi S (2009) New die design for improvement of die corner filling in hydroforming of cylindrical stepped tubes.

[5] Bakhshi-Jooybari M, Elyasi M, Gorji A (2009) Numerical and experimental investigation of the effect of the pressure path on forming metallic bellows. Proc. IMechE, Part B: J.

are very low compared with those of other hydroforming methods.

**5. Conclusions** 

bursting decreases.

**Author details** 

**Acknowledgement** 

**6. References** 

3830.

limited, 396 p.

information and product data.

M. Bakhshi-Jooybari, A. Gorji and M. Elyasi

[1] Singh H (2003) Fundamental of hydroforming, SME, 219 p.

Proc. IMechE, Part B: J. Engineering Manufacture. 223: 821-827.

Engineering Manufacture. 224: 95-101.

**Figure 44.** % thinning versus punch tip radius for region B

Figure 45 illustrates the effect of the radius of conical-cylindrical region on the thickness distribution. As it shows, changes in the radius only affects region D and it has no effect on other regions of the part. For more accurate study, the thickness reduction in region D was investigated and the results are shown in Figure 46. As it can be seen, by increasing the radius of region D and the conical angle the thickness reduction will be decreased.

**Figure 45.** Thickness distribution curve versus radius of conical–cylindrical region

**Figure 46.** % thinning versus radius of conical-cylindrical region
