Chapter 1

INTRODUCTION

Hydroforming is a cost-effective way of shaping ductile metals such as aluminum, brass,

low alloy steels, stainless steel into lightweight, structurally stiff and strong pieces. One of

the largest applications of hydroforming is the automotive industry, which makes use of the

complex shapes possible by hydroforming to produce stronger, lighter, and more

rigid unibody structures for vehicles. This technique is particularly popular with the high-

end sports car industry and is also frequently employed in the shaping of aluminum tubes

for bicycle frames. Hydroforming is a specialized type of die forming that uses a high

pressure hydraulic fluid to press room temperature working material into a die.

Hydroforming allows complex shapes with concavities to be formed, which would be

difficult or impossible with standard solid die stamping. Hydroformed parts can often be

made with a higher stiffness-to-weight ratio and at a lower per unit cost than traditional

stamped or stamped and welded parts. Virtually all metals capable of cold forming can be

hydroformed, including aluminum, brass, carbon and stainless steel, copper and high

strength alloys.

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Chapter 2

2.1 HYDRO-FORMING PROCESS

A hydroforming press operates like the upper or female die element. This consists of a

pressurized forming chamber of oil, a rubber diaphragm and a wear pad. The lower or male

die element is replaced by a punch and ring. The punch is attached to a hydraulic piston, and

the blank holder, or ring, which surrounds the punch.

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The hydroforming process begins by placing a metal blank on the ring. The press is closed

bringing the chamber of oil down on top of the blank. The forming chamber is pressurized

with oil while the punch is raised through the ring and into the forming chamber. Since the

female portion of this forming method is rubber, the blank is formed without the scratches

associated with stamping.

The diaphragm supports the entire surface of the blank. It forms the blank around the rising

punch, and the blank takes on the shape of the punch. When the hydroforming cycle is

complete, the pressure in the forming chamber is released and the punch is retracted from

the finished part.

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TYPES OF HYDROFORMING

There are four main types of hydroforming:

Hydroforming of tubes, usually at low pressure, is the most widely used technology

at present, with Hydroformed tubular parts offering improved integrity and structural

performance.

Low pressure hydroforming simply re-shapes tubes, producing a very good shape,

but is not as useful if better cross-section definition is required.

High-pressure hydroforming totally changes the tube shape and alters the length to

circumference ratio by up to 50%. It gives very good tolerance control, being a

highly robust process.

Panel hydroforming at high pressures is used in the aerospace industry, and is

expected to be used for applications in the automotive industry in which

hydroforming is needed to get the right material flow.

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Manufacturing Costs: Hydroforming Versus Deep Draw Stamping

Tooling - With low volume runs, tooling is often the most important cost consideration.

With hydroforming, a male die, or punch, and a blank holding ring are the only tools

required as the rubber diaphragm and pressurized forming chamber act as the female die. As

a result, hydroforming tooling is typically 50% less expensive than matched die tooling.

With hydroforming, most punches are made from cast iron as opposed to the hardened tool

steels used for match die drawing punches. Finally, hydroforming tools are easily mounted

and aligned, making set-ups fast and efficient.

Development Costs - Proto-typing is often a necessary step in the manufacturing process.

Changes in material type or wall thickness specifications can typically be accommodated

with hydroforming without creating a need for new tooling.

Reduced Press Time - Complex parts requiring multiple press cycles in matched die

operations can be drawn in a single hydroforming cycle. Hydroforming presses frequently

achieve reductions of 60-70% compared to 35-45% for conventional matched die presses.

Finishing Costs - Aerospace, medical and commercial cookware applications often demand

parts with outstanding surface finishes. Unlike matched die metal forming, which can leave

scratches and stretch lines, the flexible diaphragm utilized in hydroforming eliminates

surface blemishes, reducing the need for costly finishing processes like buffing.

2.2 Metal Flow

Metal flow is controlled largely by several basic factors: draw ratio, blank holder pressure,

the die addendum, and the part geometry. One of the most influential factors, especially

when using high-strength material, is the draw ratio. The draw ratio is defined as the direct

relationship between the forming punch and the blank. If the blank is too far from the edge

of the forming punch, very little or no metal flow occurs. This will most likely result in

stretching and fracturing of the blank. If the blank is close enough to the punch contact area,

the metal flows inward toward the punch, resulting in much less material stretching

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2.3 LIMITING DRAWING RATIO (LDR)

The limiting drawing ratio (LDR) is defined as the maximum value of the ratio

dodp

for which a complete cup without cracks or wrinkles can be drawn. A specific LDR has to

be determined for each material. The following parameters influence the value of the LDR;

- Material properties,

- Sheet thickness,

- Punch and die geometry,

- Lubrication.

Calculate the required force to deep draw a piece of sheet metal based on the drawing ratio,

sheet thickness, and the ultimate tensile strength of the material. The drawing ratio is a

measure of the severity of the drawing operation and is the ratio of the initial blank diameter

to the punch diameter. For a given material, the limiting drawing ratio (LDR) is a measure

of that material’s deep draw-ability and is calculated from the largest blank that can be

completely deep drawn for a given punch diameter. The amount of draw can also be

represented as the percent reduction of the blank diameter.

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Chapter 3

3.1 EXPERIMENTAL PROCEDURE

The geometry of the aluminum alloy complex shaped component is shown in Fig. 1. The

surface of the component consists of a cylindrical surface at the top and a doubly curved

surface at the left end, and a flange exists. The depth of the component is 118 mm, and the

opening distance from the left to the right is 194 mm. The fillet radius is 32.5 mm in the left

of opening, and the radius is 131 mm at the transition from the curved arc to the straight

line. The radius is 105 mm at the transition from the flange to the top, and the radius of

cylindrical surface at the top is 78 mm. The radius of transition corner between the curved

surface and the flange is 5 mm. The size is 156 mm from the front to the back of opening. A

rectangular blank with cut corners was used in the paper. The dimension is 440 mm×360

mm×160 mm.

Fig. 1 Geometry of component (unit: mm)

The material used in the experiment is 2A12 aluminum alloy with 1.5 mm thickness and the

mechanical properties of the material are shown in Table 1.

Conventional deep-drawing Hydroformed

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strong local thinning of the material

inhomogeneous distribution of material thicknesses

less internal stress of the formed part

less internal stress and less tendency to return to its original shape

homogeneous strength and less amount of waste

high dimensional accuracy

The advantages of sheet hydroforming are as follows:1. Requires fewer operations to make certain part geometries2. Does not require lower or upper draw punch or cavity3. Uses water, a widely available resource4. Forces material to distribute stretch or strain more evenly5. Reduces springback6. Reduces material consumption7. Forms higher strength materials Inexpensive tooling costs and reduced set-up time.8. Reduced development costs.9. Shock lines, draw marks, wrinkling, and tearing associated with matched die

forming are eliminated.10. Material thinout is minimized.11. Low Work-Hardening12. Multiple conventional draw operations can be replaced by one cycle in a

hydroforming press.13. Ideal for complex shapes and irregular contours.14. Materials and blank thickness specifications can be optimized to achieve cost

savings.

The disadvantages of sheet hydroforming are as follows:1. Requires expensive equipment2. Cycle times are generally poor3. Operators often get wet

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Chapter 4

4.1 RESULTS AND DISCUSSION

To investigate the effect of air pressing on the deep drawability, the LDR is obtained at each

process condition. For the calculation of LDR, the maximum blank diameter, this diameter

being that below which the blanks will be drawn successfully and above which tearing will

occur in the cup wall is determined. Figure 4.1 shows the variation of LDR with increasing

air pressure for AI-1050.

Fig 4.1 Obtained LDR with increasing air pressure (a) (b)

Fig. 4.2 Comparison of deep drawn depth

(a) LDR=1.862(No air pressing)

(b) LDR=1.915(Air pressure=70kg/cm2 )

Fig. 4.2 shows photograph of deep drawn cups at given process conditions.

Above figures show that higher LDR is obtained at higher internal air-pressure. Overall

thickness of deep drawn cup and the degree of thickness variation at rounding part are

decreased at the air pressure of 70 kg/mm2. The relatively steep decrease in thickness at

rounding part that had touched with punch nose radius reflects the local strain has been

concentrated on this part. Therefore, the decrease in the degree of thickness variation at the

rounding part confirms that the local strain concentration has been relaxed by air pressing.

In general, an increase in the drawing force is observed for larger blank diameters due to the

enlargement of fictional interfaces such as the die-blank and blank holder- blank interfaces.

While the figure indicates that the maximum drawing loads are not so significantly

increased even with increasing maximum blank diameters at higher air pressure. It means

that the internal air-pressing contribute to the reduction of drawing load possibly by

reducing friction between punch and blank. In other words, the internal air-pressing itself

does not alter the deformation resistance of deforming sheet blanks, but has an effect on the

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manner of the transmission of the load at punch nose radius part and increases LDR of

blank.

Although the experimental conditions used in this study may not be the optimum for the

highest LDR, the trends obviously show that the internal air pressing is advantageous for

higher LDR. The effectiveness of the air pressing process depends on how well the metal

can be pressed. Therefore, the effect of air-pressing process will be more prominent for

aluminum alloy sheets than mild steel sheets.

Chapter 5

CONCLUSION

The air-pressing method is very effective in increasing the deep drawability of Al-1050. The

increased LDR is mainly caused by the relaxation effect of local strain concentration at

punch nose radius area. The results that have been described above shown that air-pressing

method also have the potential to increase the LDR of other metal alloy sheets.

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REFERENCES

H.Mohammadi Majd, M.Jalali Azizpour, M. Goodarzi “Prediction the Limiting Drawing

Ratio in Deep Drawing Process by Back Propagation Artificial Neural Network” World

Academy of Science, Engineering and Technology -2011

“Examination of the Strength and Ductility of Aa-1050 Material Shaped with the Multi-

Stage Deep Drawing Method” archives of metallurgy and materials -2011

Young Hoon Moon*t, Yong Kee Kang, Jin Wook Park, Sung Rak Gong “Deep Drawing

With Internal Air-Pressing to Increase The Limit Drawing Ratio of Aluminum Sheet”

KSME International Journal; VoL 15 No.4, pp. 459- 464, 2001

M. Jain a,*, J. Allin a, M.J. Bull b “Deep drawing characteristics of automotive aluminum

alloys” Materials Science and Engineering A256 (1998) 69–82

Die design handbook - David Alkire Smith

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