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  Performance of lubricants in internal high pressure forming of tubes
Posted by: Hydro_Publishing on Wednesday, January 29, 2003 - 10:32 PM
Information from the PtU Darmstadt In an Internal High Pressure Forming (IHPF) process several forming zones are distinguished based on contact normal stress, sliding velocity, and degree of deformation. The precise design of an IHPF process requires an extensive investigation of the tribological conditions in these different zones. So far laboratory investigation of tribology in IHPF processes has involved pushing a pressurized tube through a tool in order to measure the friction force or determine the friction coefficient. However, such experimental set-ups only represent the guiding zone of IHPF. The prevailing loads require a careful consideration of the individual forming zones. This led to the establishment of further experimental setups for the investigation of transition and expanding zones. These experimental setups, in particular those for the expanding zone, permit a more accurate examination of the friction properties of lubricants. Thus, performance differences that would be indiscernible using the traditional guiding zone test are revealed. Oil and slide lacquer were compared to one another. Differences in friction coefficients in the guiding zone area were not too large and the potential of the slide lacquer was first revealed in experiments in the expanding zone.

1 Introduction

1.1 IHPF process

Internal High Pressure Forming (IHPF) serves as a valuable forming used in large series production since the early 1990s. It belongs to the group of forming technologies involving liquid media and can be subdivided into the following: internal high-pressure forming, hydropiercing and joining, and formation of hollow sections and sheet metals. IHPF processes are primarily performed on straight or preformed tubes. In the steel sector, tubes are mainly seam-welded. Extruded aluminum profiles are also used.
In IHPF, the tubular blank is placed into one half of a die in a split construction. The die cavity corresponds to the final shape of the component. After closing the die halves, two sealing rams are moved into the ends of the tube. The internal pressure is increased, and combined with the action of the sealing rams, any additional tool elements, and auxiliary punches, this generates the desired component geometry.

1.2 IHPF tribology

In order to achieve a high level of accuracy in the design of an IHPF process, interactions between the system parameters, the process parameters and the friction coefficient µ need to be known. Quantitative insights into the behaviour of the friction coefficient µ are particularly interesting, since the FE-simulation used for process design is only as accurate as its inputs. In addition to affecting deformation, variations of the friction coefficient also has an effect on the wall thickness distribution of the part. For these reasons, the modelling of tribological boundary conditions plays an important role in reducing time and cost factors associated with production and development of a part. A promising method for determining accurate friction coefficients and evaluating their dependence on process and system parameters is a reproducible and homogeneous modelling of typical stress conditions during the IHPF process. Hereunto a distinction is made between the following forming zones as shown in Figure 1: guiding zone, transition zone and expanding zone. Results from a FEA of the IHPF process justify the aforementioned classification. Figure 2 shows the result of a T-shape FE-simulation. The upper part of Figure 2 displays the max. plastic strain, the normal stress, and the velocity distribution along the tube contour. The guiding zone, starting from the tube end to point 1, is characterized by a constant progress of stress. The sliding velocity at the tube-workpiece interface is maximal in this zone and a surface expansion of the tube, characterized by the “max. plastic strain”, is negligible. The transition zone, located between point 1 and point 2, is characterized by a very inhomogeneous distribution of the contact normal stress. The bending of the tube, due to expansion, causes a decrease in contact normal stress between tube and workpiece at the beginning of the radius. A stress maximum occurs close to the end of the radius when the tube comes into contact with the die. Here, the sliding velocity is less than in the guiding zone and surface is compressed. In the expanding zone of the current model, a free expansion is present, after point 2. However, looking at other part geometries, in which contact between die and workpiece prevails in this zone, the contact normal stress is determined by internal pressure and the properties of the tube during contact with the die contour. In this example the free expansion sliding velocity is very high. If, however, there is die-workpiece contact in this zone, the sliding velocity attains a minimum value. Workpiece surface expansion reaches its maximum in this zone.

Figure 1: Model of a IHPF process

Figure 2: Results of a T-shape FE-Simulation

Lubricants that are applied in IHPF processes can be categorized according to their performance as follows: oils and emulsions, polymer dispersions, and slide lacquer. Oils are either mineral or synthetic-based. Pressure additives are used in order to adjust the properties and attain the required performance. Polymer dispersions consist of a fluid substrate medium (e.g. water) and solid elements which disperse in the substrate. In contrast to oils, polymer dispersions are more difficult to apply and to remove. Slide lacquer belongs to the group of dry lubricants, such as MoS2 or graphite in an organic or inorganic medium. Although slide lacquers are more effective, their application and removal is expensive compared with that of oils and polymer dispersions. To hydroform big features or to maintain a uniform wall thickness distribution, a high system performance is targeted. A high system performance can be quantified by low coulomb coefficients of friction µ. This enables one to push more material into the expansion zone. The objective of several investigations is to develop oils which enable low friction coefficients in IHPF applications while at the same time having the benefit of easy application and removal.

2 Friction test setups for IHPF processes

Chapter 2 describes test setups to simulate the tribological conditions prevailing in an IHPF process. These setups permit the detection of characteristic process parameters for different tribological systems.

2.1 Friction test setup for the guiding zone

A test setup was constructed to simulate IHPF conditions in the guiding zone. A schematic represen-tation of this test setup is illustrated in Figure 3. A pressurized tube is pushed through a closed die. The die halves are located on bearings in the axial direction. Shear stresses that occur between the die and workpiece push the die against the load cells. Press closing force and the force acting between both die halves are recorded. The difference between press closing force and the force acting between the upper and lower die makes it possible to measure the contact normal force FN. Knowing the friction force FF and normal force FN, the Coulomb friction coefficient µ can be calculated according to equation (1).

FF = µ * FN (1)

Figure 3: Friction test stand representing the guiding zone of a IHPF process [1]

2.2 Friction test setup for the transition zone

In order to determine interface friction in the transition zone, a new test setup was developed that enables the forming of T-fittings. As shown in Figure 4, a sensor is placed close to the radius of the T-fitting so that the forces in this zone can be measured. The friction coefficient can then be determined by using the measured forces and a suitable mathematical model. The tool consists of four vertically stacked parts: a top and a base plate, on which three dummy load cells and one load cell are mounted, and two die halves, where the workpiece is formed, located between the two plates. A carved recess resembling two tubes meeting in a T-shape is located in the die halves. The axes of the T-branches lie in the separation plane. The tubes have a diameter of 48 mm. The radii are accessible in the separation plane in four places, two in each die half, where receptacles for probe heads are positioned. Each probe head can be connected with a load cell. During the hydroforming of the T-fitting, the tube material flows along the tip of the probe head, thereby transmitting the forces to the 4-component load cell. The three orthogonal forces as well as the torque along the vertical axis are recorded. Using Equation (2), derived from the Figure 5, it is possible to determine the friction coefficient.


Figure 4: Friction test stand representing the transition zone of a IHPF process

Figure 5: schematic of the prevailing forces on the test plate

2.3 Friction test setup for the expanding zone

To simulate the conditions in the expanding zone, the test setup for the guiding zone was modified. A tube with an outer diameter of 48 mm is partially expanded of 60 mm. The test procedure is similar to the guiding zone test described in Chapter 2.1. In this process, an O-ring which is integrated in the sealing stamp seals the workpiece. Consequently, additional sealing forces that might lead to spurious friction force measurements are avoided. To accomplish partial expanding, the tube is placed between the sealing stamp and the expansion ring. The friction coefficient is then determined in using the method described in Section 2.1.

Figure 6: Friction test setup representing the expanding zone of a IHPF process

3 Results of experiments

3.1 Results guiding and expanding zone

The following examinations were conducted using the test setups described in Chapter 2.1 and 2.3:

constant: workpiece guiding zone:
tube, 200 x 60 x 2 (lt x dta x tt) 1.4541 cold rolled
workpiece expanding zone:
tube, 200 x 48 x 2 (lt x dta x tt) 1.4541 cold rolled
tool: 1.2343 hardened and nitrided, Diameter 60 mm
sliding velocity: 3 mm/s
lubricant quantity: oil – 5 g/m²; slide lacquer – 15 g/m²

varied: lubricant: oil, slide lacquer
normal stress: 50 N/mm² (» 750 bar) , 75 N/mm² (» 1000 bar)

The specified friction coefficients were recorded after a sliding distance of 25 mm. They represent averaged values from 5 separate measurements. The notation used in each test consist of the initial tube diameter ( guiding: 60 mm, expanding: 48 mm), the contact normal stress ([N/mm²]), and the used lubricant (oil, slide lacquer). As indicated in Figure 7 , the friction coefficients for oil and slide lacquer differ only slightly in the guiding zone, whereas the difference in the expanding zone is considerable. This difference can be explained by the roughening up of the tube“s surface which results from the expansion process. Oil flows into the grooves of the roughened areas and is thus no longer available to separate the contact interface between the tube and die. In contrast to this phenomenon, the lubrication efficiency of slide lacquer, a dry lubricant, was not decreased by roughening of the surface. This is due to the fact that slide lacquer cannot flow into the emerging grooves of the tube“s surface.

Figure 7: Results comparing guiding and expanding zone

3.2 Results transition zone

The following examinations were conducted using the test stand described in Chapter 2.2:

constant: workpiece: tube, 200 x 48 x 2 (lt x dta x tt) 1.4541 cold rolled
tool: T-shape, 1.2343 hardened, Diameter 48 mm
process parameters
lubricant quantity: oil – 5 g/m²; slide lacquer – 15 g/m²

varied: lubricant: oil, slide lacquer

The friction coefficients were determined after the T-shape was in complete contact with the probe head. The friction coefficients averages µ » 0,08 for oil and µ » 0,07 for slide lacquer. Even slight deviations in the positioning of the probe head strongly influence on the results. Thus, the determined friction coefficients show some variance.

4 References

[1] Prier, M., Die Reibung als Einflussgröße im Innenhochdruck-Umformprozess,
Dissertation, Technische Universität Darmstadt, Berichte aus Produktion und
Umformtechnik, Band 46, Shaker Verlag 2000

Performance of lubricants in internal high pressure forming of tubes, Proceedings of the 7th ICTP, Oct. 27 - Nov. 1, 2002, Yokohama, Japan

Contact at the PtU University

Dipl.-Ing. Armin Peter
Phone: 0049 (0) 61 51 / 16 - 35 57
Fax: 0049 (0) 61 51 / 16 - 30 21


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