In last few years the use of hydrostatic pressure in forming has got greater interest of researchers mainly because of the requirement to manufacture light weight structures for automobiles. Reduction of body weight of an automobile can be achieved by using lighter gage sheets of steel or by using sheets of lighter material like aluminum alloys. In both the cases forming by hydrostatic pressure is of immense help.
The processes using hydrostatic pressure may be grouped into following two categories:
(i) Hydro-mechanical forming
These are the processes which are carried out with help of tools similar to the conventional tools but modified to get additional help which the hydrostatic pressure can provide. Use of hydrostatic counter pressure in deep drawing is one such example which is illustrated in Fig. 11.34. In this case the cup drawing is carried out in a chamber full of oil or water. As the drawing proceeds the pressure in the chamber increases.
The pressurized fluid presses the sheet on to punch thus enhancing the friction between punch and sheet, which restrains the movement of sheet and hence inhibits neck formation. Besides, the oil under pressure, does better lubrication of die surfaces and hence reduces friction between die and the sheet.
Even dies filled with water are quite effective, in which case, the process is called Aqua-draw. In this process there is no expense of oil because water is used. The problem of rusting may be reduced by using rust inhibitors.
A scheme of apparatus in which the hydrostatic pressure is used to simply press the sheet on to the punch is illustrated in Fig. 11.34. In this scheme the punch draws into a pressurized fluid chamber fitted with a relief valve to keep the maximum pressure in the chamber under control. The blank edge is free and open to atmosphere.
The oil leak provides lubrication to sheet underside. The sheet at the punch profile radius is pressed against the punch by fluid pressure thus inhibiting the slip and necking. As a result the LDR increases.
The useful effect of hydrostatic pressure on LDR has also been observed by other researchers. Some results are given in the Table 11.4. From the table it is clear that the useful effect reaches a saturation value at a pressure of 20.7 MPa (3000 psi) and further increase in pressure does not result in further increase in LDR.
In the scheme shown in Fig. 11.34, it is believed that the useful effect is only because of better lubrication. This view is supported by the experimental fact that by coating the same sheet by Teflon a similar increase in drawability could be obtained.
In the design shown in Fig. 11.35 the edges of blank are also subjected to high pressure. This provides additional push to the flange and thus acts against the frictional force at the die surface. It greatly enhances the drawability of sheet metal. The edge pushing acts against the friction forces during flange drawing,
Along with better lubrication of die by hydrostatic pressure there is additional benefit of reducing the cup wall stress as given by Eqn. (11.29). Naturally it would increase drawability.
For instance, we take the Example 11.3 and introduce a pressure of 30 N/mm2, with µ reducing from 0.15 to 0.075 on die surface and with bend angle π/2 it would reduce the cup wall stress by as much as = (-5.561/2 + 30) e0.075 × π/2 = 30.62 N/mm2. If the blank diameter is increased to bring the stress to initial level, the increase in diameter would be 7.2 mm which will increase the drawability from 1.818 to 1.983 i.e. increase of 0.165. A component is made out of stainless steel in single stroke of press by hydro-mechanical forming.
Figure 11.37 shows the hydro-mechanical reverse redrawing operation. In this scheme the chamber pressure is connected to provide the edge pressure. There is significant advantage in terms of increase drawability. Authors claim a better performance in case of deep drawing of aluminum in which case they obtained a LDR of 1.94 in the first draw and 4.46 in the second draw.
In hydro-forming the hydrostatic pressure is directly used to deformed the work piece. The process has been used for bulging of tubes as shown in Fig. 11.39 (a, b). In Fig. 11.39(a) only hydro-pressure is used.
The maximum height of bulge obtained before bursting is small. The axial compression in Fig. 11.39(b) helps in bulge formation and the resultant bulge height is much bigger. The range of products that can be produced by bulging tubes is rather limited.
The need to manufacture light weight structures with aim to reduce the body weight of automobile has given a fresh impetus to hydro-forming. Newer methods for manufacturing a greater range of flat and irregularly shaped structural components are being researched. In this regard the work of Geiger et al. deserves a serious consideration. The authors have explored a technique to manufacture component in pairs or a complete product consisting of double sheet layer.
In this technique two pieces of sheet cut to the required size, are pressed and arrangement is made to connect the space between them to pump delivering high pressure fluid. The process is carried out in two stages. In the first stage the component are pre-formed. In this the sheets may also get drawn in because the holding pressure at the edge is kept low for draw-in of sheet.
After this stage the edges of the two sheets are welded and high pressure is applied in the space between them to size the component and to form the fine details. The formed component is removed from dies. If it consists of two components, the edges may be trimmed to separate them.
With this technique the authors have successfully manufactured engine cradle and suspension triangle for an automobile. The authors hope that other box like structures such as fuel tank, longitudinal beam, cross beam, seat frame etc. may also be similarly developed.
Another variant of hydro-forming involves use of polymer membrane pressure chambers called bran. It could be one chamber or several chambers. The membranes are made of casting polyurethane. This material was chosen because the material properties particularly hardness may be varied and it is easier to cast it in different shapes.
The multiple chambers have the advantage that if a membrane is damaged due wear or by sharp corners of work piece only that particular chamber membrane has to be replaced or repaired instead of the large single chamber.
The other advantages of using membrane are as follows:
(i) There is no problem of sealing because fluid is contained in membrane chambers. So the set up is quicker than conventional hydro-mechanical process.
(ii) One surface of sheet is in contact with soft polymer. So pre-painted sheets may be used.
(iii) With proper design of punch the under cuts on component may also be realized.
(iv) In case of deep drawing, high normal pressure which forces the sheet on to the punch enhances the drawability.
(v) The process has additional advantages over hydro-mechanical process as given below-
(a) There is no direct contact of component with hydraulic fluid
(b) The membrane may be installed on any side of component. So component need not be turned upside down or side ways.
(c) If the process includes cutting operation also, there is no loss of pressure due to leakage.