chapter 4 hydroforming experiments -...

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CHAPTER 4

HYDROFORMING EXPERIMENTS

In this chapter, the hydroforming of tubular specimens is discussed.

A 100-ton hydraulic press was used in hydroforming experiments.

A description of the principle of working of the hydraulic press follows.

Press tonnage is the force that the ram is able to exert safely. It is

the piston area multiplied by the oil pressure in the cylinder. Tonnage can be

varied by changing the oil pressure. Stroke is the reciprocating motion of a

press slide, usually specified as the number of inches between terminal points

of the motion. Stroke is constant on a mechanical press but adjustable on a

hydraulic press. Shut height is the distance from the top of the bed to the

bottom of the slide with the stroke down and the adjustment up. The thickness

of the bolster plate must be taken into consideration while determining the

maximum die height. Shut height of the die must be equal to or less than the

shut height of the press. Shut height of the press is always given with the

adjustment up. Figure 4.1 shows a hydraulic press at work. The specification

of the hydraulic press used in this work is tabulated below (Table 4.1).

Figure 4.1 Experimental setup in hydraulic press

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Table 4.1 Specification of the hydraulic press

1 Capacity 100 ton

2 Ram stroke 500 mm

3 Ram speed

(a) Approach 52 mm/sec

(b) Pressing 10 mm/sec

(C) Return 135 mm/sec

4 Table 800 800 mm

5 Motor 20 HP

6 Overall size 3400 3200 1300 mm

7 Weight 7000 kg

8 Power supply 440 V

9 Maximum working pressure 163 kg/cm2

4.1 DESIGN AND FABRICATION OF DIE

The die was made of low carbon steel (EN19 grade) and of split

type with a top and a bottom portion. Length of top portion was 54 mm;

bottom portion, 142 mm; outer diameter, 80 mm; inner diameter, 38 mm;

flange diameter, 128 mm.

By varying the two machining parameters – maximum diameter of

die cavity (D) and semi-cone angle ( ), an extensive experimental plan could

be obtained. The required bulging diameter is 54 mm for semi-cone angle of

20° and 48 mm for semi-cone angle of 12°. The die and punch layout is

shown in Figure 4.2. The top and bottom dies and its assembled view are

shown in Figure 4.3. A groove was made on the top portion of the die cavity

for proper seating of ‘O’ ring to avoid leakage while hydroforming. At the

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bottom surface of the die cavity, a hole was machined to permit the

measurement and control of the fluid pressure inside the tube.

Ø38

Ø54

Ø80

Ø128

Ø104

1024

30

24

28

10

92

117

25

Ø50

M66 Holes

Die Set Assembly

Sealing Material

20

25

20

Ø150

Ø38

Punch

All Dimensions in mm

Figure 4.2 Die and punch layout

(a) (b) (c)

Figure 4.3 (a) Top die (b) Bottom die (c) Assembled setup

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4.1.1 Punch Attachment

The punch was made using low carbon steel (EN19 grade). At the

front of punch, a provision was made to fix punch attachments that help

increase the fluid pressure inside the tube. Punch attachments of different

heights (33, 32, 30 mm) were used (Figure 4.4) in order to achieve a wide

pressure range in hydroforming. Punch attachments were provided with an

‘O’ ring to avoid leakage.

Figure 4.4 Punch attachments of different heights

Tubular specimens of aluminum, copper and brass in different

thicknesses and lengths were prepared so as to achieve an extensive

experimental plan. Specimens were prepared from seamless tubular materials

in order to ensure that a crack formed during experiments is only due to fluid

pressure inside the tube, and not due to defects in the tubes. The outer

diameter of all tubes was constant at 38 mm.

4.2 CHEMICAL TESTS

The results of chemical analyses for aluminum, brass and copper

tubular materials in terms of % of weight are shown in Tables 4.2–4.3.

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Table 4.2 Composition of aluminum

Si Fe Cu Mn Mg Zn Ni Cr Pb Sn Ti Al

0.42811 0.35 0.0184 0.0249 0.4506 <0.0020 <0.0010 <0.0010 <0.0010 0.0051 0.0056 98.77

From chemical analysis, the aluminum material confirms the grade

of AA6063.

Table 4.3 Composition of brass

Cu Pb Sn Zn

58.54 0.19 0.07 41.2

Table 4.4 Composition of copper

Cu Pb Sn Zn

98.91 0.07 0.36 0.98

4.3 EXPERIMENTAL WORK

The experimental work was conducted to evaluate the deformation

characteristics of tubular materials and the influence of annealing on

hydroforming outcomes. The initial size of tubular specimens was diameter

38 mm, thickness 1.5 mm and length 106 mm.

The process parameters considered were punch displacement, fluid

medium, and semi-cone angle of 20 and 12 . The required bulging diameter

of tubes is 54 mm for 20 semi-cone angle and 48 mm for 12 semi-cone

angle. Below, a brief of experimental work is provided.

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A 100-ton hydraulic press was used to conduct experimental trials.

The assembled die setup was mounted on the table of the hydraulic press and

the punch was fitted with the ram of the press. The tubular specimen was

inserted into the die and the tube was completely filled with the fluid

(forming) medium (water or oil). A Teflon sealing was provided on the front

face of the punch and tube to avoid leakage of fluid through side wall and

joints. A pressure gauge attached to the bottom portion of the die was used to

record pressure forming as the punch travels through the tube. The movement

of the punch causes the fluid pressure to increase. As the punch forces the

fluid downwards, thereby compressing the fluid, the development of high

pressure causes the tube to bulge.

A detailed description of the experimental work follows.

Figure 4.5 shows the schematic diagram of tube hydroforming. The

assembled view of die with specimen is shown in Figure 4.6. The top and

bottom dies were attached together and screwed on the base plate, and the

whole assembly was mounted on table of the press. The punch was mounted

on the ram plate of the press. Pressure gauge and relief valve were attached to

the bottom portion of the die through the ‘T’ joints. This arrangement was

inspected to ensure no possible leakage.

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Figure 4.5 Schematic diagram of hydroforming

Figure 4.6 Assembled view of die with specimen

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Subsequently, the tubular specimen was inserted into the die. The

fluid medium (or working fluid) was filled into the tube up to die level. After

filling with the working fluid, the ram was pressed down to the die, and as it

comes down, punch attachments were advanced into the inner wall of the tube

to come in contact with the fluid. Sealing in the threaded areas was provided

by Teflon tape. Then, the punch came into contact with the top inner surface

of the tube to provide axial feeding (punch displacement).

Axial feeding is important because it pushes the tubular material

into the die cavity during forming; without proper axial feeding, material

thinning can occur. The deformed tubular specimen was taken out from the

split die by separating the top and bottom portions.

A number of experimental trials were conducted by varying the

process parameters, such as tube thickness, tube length, working fluid, punch

attachments, and punch displacement.

At the end of each experiment, each deformed tube was put to

closer observation and necessary modification was made in further trials to

improve the outcomes. After each forming experiment, the fluid inside the

tube was drained out.

4.3.1 Formability at Die Semi-cone Angle of 20 Without Annealing of

Specimens

Formability experiments were performed without annealing of the

specimens. The original diameter of the tubular material at different locations

was measured using a coordinate measuring machine and found to be uniform

for the entire length. The fluid medium used water or SAE 90 oil, and punch

displacements of 5, 10, 13, 15 and 25 mm were applied. Three trials were

performed, and deformation zone behaviour of bulge region was studied.

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Figure 4.7 Punch and die in semi-cone angle of 20

Trial 1

Experimental work was performed on aluminum, brass and copper

tubular materials of thickness 1.5 mm, length 106 mm and diameter 38 mm.

The required bulging diameter of tube was 54 mm. The punch displacements

for aluminum, brass and copper were 10 mm, 10 mm and 5 mm, respectively.

The change in pressure during the forming process was recorded (Table 4.5).

Failures were due to leakage of fluid through pipe joints. The deformed

specimens thus obtained are shown in Figure 4.8.

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Table 4.5 Effect of punch displacement on pressure development

without annealing of tubular materials

S.No. Material Working

medium

Punchdisplacement

(mm)

Pressure(MPa) Observation

1 Brass Water 10 22.06 Wrinkle failure

2 Aluminum SAE 90 oil 10 15.85 Wrinkle failure

3 Copper SAE 90 oil 5 15.85 Wrinkle failure

Figure 4.8 Deformation with defect (wrinkling) in trial 1

Trial 2

Different values of punch displacement (10, 13 and 15 mm) were

applied to aluminum tubular specimens. The influence of material length and

thickness and punch displacements on pressure development was studied

(Table 4.6). The modes of failure observed were wrinkling and bursting,

which may be due to leakage of oil through pipe joints and side wall of the

tube. The deformed specimens are shown in Figure 4.9.

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Table 4.6 Effect of thickness, length of material and punch

displacement without annealing of tubular materials

MaterialThickness

(mm)Length(mm)

Workingmedium

Punchdisplacement

(mm)


Aluminum 1 101 SAE 90 Oil 10 20.68 Wrinklingfailure

Aluminum 1 96 SAE 90 Oil 13 19.30 Initiation offorming

Aluminum 1.5 94 SAE 90 Oil 15 20.30 Initiation offorming

Aluminum 0.7 96 SAE 90 Oil 13 18.2 Tear andbuckling

Figure 4.9 Defects in trial 2

Trial 3

After proper sealing by means of a Teflon tape, experiments were

conducted. A punch displacement of 25 mm was considered for this trial. Due

to increase in the axial feed, the pressure inside the tube increased (Table 4.7),

which initiated partial deformation. The partially deformed specimen is

shown in Figure 4.10.

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Table 4.7 Change in punch displacement and its outcome

Material Thickness(mm)

Length(mm)

Workingmedium

Punchdisplacement

(mm)


Aluminum 1 106 SAE 90oil 25 14.47

Partialdeformation

without defects

Figure 4.10 Partially deformed specimen

4.3.2 Formability in Die Semi-cone Angle of 20 after Annealing of

Specimens

Annealing is a softening process that involves heating a material to

annealing temperature in a furnace for 2–3 hours and allowing it to cool

slowly in the furnace. The tubular specimens of aluminum, brass and copper

were annealed in a heavy-duty electric furnace, as shown in Figure 4.11. The

annealing temperatures considered were 413 C for aluminum, 525 C for

copper and 550 C for brass. The holding time in the furnace was 2 hours.

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The type of temper used for this annealing is T4. It means that heat

treated and naturally aged. The mechanical properties of T4 obtained from

tensile test results and type of alloy obtained from chemical composition

results. The process parameters considered for the trials were punch

displacement, fluid medium (SAE 90 oil) and die semi-cone angle of 20 .

Figure 4.11 Annealing of tubular materials in furnace

Trial 1

With no leakage detected in the setup, experiments were carried out

with the tubular specimen of diameter 38 mm, length 106 mm, thickness 1.5

mm and oil as fluid medium. A punch displacement of 30 mm was applied.

On completion of formability test, the specimen was removed from the die.

Using a coordinate measuring machine, the bulge diameter was measured as

53.45 mm. The non-uniformly deformed specimen without defect is shown in

Figure 4.12, and the pressure data are shown in Table 4.8.

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Table 4.8 Effect of punch displacement on pressure development in

annealed aluminum specimen

S. NoPunch

displacement(mm)


1 30 10.29 Flaw-free deformation2 30 9.80 Flaw-free deformation

Figure 4.12 Non-uniform bulging

Trial 2

In a repeated trial with a punch displacement of 30 mm, however,

the pressure formed was measured to be 10.78 MPa, and deformation

was better. However, at a punch displacement of 32 mm, a high pressure

of 12.25 MPa was generated, causing the component to wrinkle. The

deformed specimen is shown in Figure 4.13, and the pressure data are

shown in Table 4.9.

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S. NoPunch

displacement(mm)


1 30 10.78Better deformation in the expansion

zone but non-uniform thickness

2 32 12.25 Wrinkling

Figure 4.13 (a) Uneven bulging (b)uneven bulging with wrinkle

Trial 3

When a punch displacement of 33 mm was applied, pressure

developed was 14.84 MPa and caused wrinkle in the tubular specimen. On the

other hand, when a punch displacement of 30 mm was applied, pressure

developed was 9.6 MPa and the expected expansion zone geometry was

achieved. The deformed specimens are shown in Figure 4.14. The pressure

data are shown in Table 4.10. The bulge diameter of 53.74 mm without defect

was obtained from this trial.

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S. NoPunch

displacement(mm)


1 33 14.84 Wrinkle

2 30 9.6 Required deformation achieved

Figure 4.14 (a) Wrinkle defect (b) deformed without defect

Trial 4

When a punch displacement of 30 mm was applied on brass

specimen, a high pressure of 22.06 MPa was developed and bursting of

specimen occurred. Whereas, on applying the same punch displacement in

copper specimen, pressure developed was 16.54 MPa and better formability

was obtained. The deformed specimens are shown in Figure 4.15, and the

pressure data are shown in Table 4.11.

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annealed brass and copper specimens

Sl.No Material

Punchdisplacement

(mm)

Pressure(MPa)

Bulgediameter

(mm)

1 Brass 30 22.06 44.45

2 Copper 30 16.54 48.66

(a) (b)

Figure 4.15 (a) Bursting in brass (b) safe deformation in copper

4.3.3 Formability in Die Semi-Cone Angle of 12 after Annealing

The formability characteristics of tubular materials in die semi-cone

angle of 12 was studied. The punch and die arrangement is shown in Figure

4.16. The process parameters considered for these experiments were punch

displacement, SAE oil as fluid medium and die semi-cone angle of 12 .

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Figure 4.16 Punch and die in semi-cone angle of 12

For experiments with die semi-cone angle of 12°, separate die

attachments (top and bottom) made of nylon were used. The attachments were

suitably machined (Figure 4.17) to get the required taper.

Figure 4.17 Die attachment

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The attachmentnts were then kept inside the top and bottom die

portions, and tubular specimen was inserted into the die before running

the trial. The specimen fitted into the inside of die attachments as shown in

Figure 4.18.

Figure 4.18 Die attachments with tube inserted

Trial 1

Aluminum and copper tubes of thickness 1.5 mm and length

106 mm were used for formability evaluation under punch displacement of

30 mm. Although the pressure developed during forming was high, defect-

free components were produced. The deformed components are shown in

Figure 4.19, and the pressure data are shown in Table 4.12.

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die semi-cone angle of 12 with annealed aluminum and

copper specimens

S.No.

Material Workingmedium

Punchdisplacement

(mm)

MaximumPressure

(MPa)

Bulge diameter(mm)

1 Aluminum Oil 30 22.4 46.685

2 Aluminum Water 30 19.99 47.690

3 Copper Oil 30 28.95 46.355

Figure 4.19 Bulging in aluminum and copper specimens

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Trial 2

When punch displacements of 15 mm and 20 mm were applied, the

pressure developed during forming was closer to theoretical pressure. The

defect-free bulged specimens are shown in Figure 4.20, and the pressure data

are shown in Table 4.13.


die semi-cone angle of 12 with annealed aluminum and

brass specimens

S. No MaterialWorking

medium

Punch

displacement

(mm)

Maximum

Pressure

(MPa)

Bulge diameter

(mm)

1 Aluminum Oil 15 7.1 44.064

2 Aluminum Oil 20 11.03 46.905

3 Brass Oil 20 23.43 45.487

Figure 4.20 Bulging in aluminum and brass specimens

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4.4 FORMING LIMIT DIAGRAM

Forming limit diagram is a graphical representation of the limits of

principal strain a material may suffer without failure due to localized necking,

fracture or wrinkling. To minimize experimental trials, Keeler and Goodwin

proposed a grid strain analysis. The method involves etching a pattern of fine

circles on the sheet metal before pressing. After pressing, the circles will

deform into ellipses (Figure 4.21), which can be measured to find the major

and minor strains of the material. Failure can be predicted using a plot of the

major and minor strains over a wide range of conditions. A forming limit

diagram indicates the limiting strains that a sheet metal can sustain over a

wide range of major-to-minor strain ratios.

Figure 4.21 Circles deforming into ellipses

A grid pattern of circles of diameter of 2.5 mm was prepared on

stencil paper (Figure 4.22b) and pasted over the annealed aluminum tubular

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material. The tube was then soaked in an etching solution (NaCl) for

electrochemical etching.

On hydroforming, the pattern of circles got deformed into ellipses.

The major axis and minor axis lengths were measured using a flexible steel

rule, and the major and minor strains were determined by using the formula in

Figure 4.21. The strain values were then plotted in the forming limit diagram.

The necking strain values were connected to determine the forming limit

curve. Since the strain values fell below the forming limit curve, the material

was considered to be in safe zone.

(a) (b)

Figure 4.22 (a) A grid circle patterns on stencil sheet (b) grid circle

patterns (diameter 2.5 mm) pasted over the tubular specimen

4.4.1 Electrochemical Etching

The principle of electrochemical etching is shown in

Figure 4.23. The anode of battery (12V) was connected to the tube and the

cathode was connected to a nail, and current was passed between the

electrodes (Figure 4.24). The tube was stirred, for about less than 2 minutes,

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until an etched grid pattern of circles was produced over the outer surface of

the tube. Then the formability analysis was done with the tube.

Figure 4.23 Block diagram of electrochemical etching

Figure 4.24 Electrochemical etching in process

4.4.2 Major and Minor Strains

Upon deformation of the tube, the grid pattern of circles turned

elliptical. The major and minor axes were measured, strain values were

determined (Table 4.13) and a forming limit drawing was produced

(Figure 4.25).

ELECTRODE

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Grid circle measurement was done carefully using transparent

scale which was to match the curvature of the profile. More than 20 strain

values were considered and plotted, out of which as many as 15 values were

overlapping with the already plotted points and hence did not show up in the

FLD.

Table 4.14 Major and minor strain values

Major axislength (mm)

Minor axislength(mm)

Majorstrain (%)

Minorstrain (%)

3 2 20 -20

3.5 2.5 40 0

3.5 3 40 20

2.5 2 0 -20

3 2.5 20 0

2 2 -20 -20

3 1.5 20 -40

2.5 3.5 0 40

Figure 4.25 Forming limit diagram

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4.5 SUMMARY

Experimental trials were performed with aluminum, copper and

brass tubular materials before and after annealing. The formability of tubular

materials was determined with reference to process parameters such as die

semi-cone angle, fluid medium, and punch displacement. The results suggest

that the required bulge diameter was achievable in annealed specimens.

Moreover, a closer look at processed tubular specimens revealed thinning in

the bulge region.

chapter 4 hydroforming experiments -...

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