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  Tube bending and hydroforming
Posted by: God on Friday, November 09, 2001 - 10:06 PM
News Published in Svetsaren nr. 2, 2000, pp. 34-35 by Nader Asnafi, Sapa Technology, Sweden

For production of low-weight, high-energy absorbent, and cost-effective structural automotive components, the hydroforming of aluminium extrusions is now regarded as the only method in many cases. The hydroforming of aluminium extrusions has also demonstrated significant potential in other applications.

The principle of tube hydroforming is shown in Fig. 1. The hydroforming operation is either force-controlled (the axial forces are varied with the internal pressure) or stroke-controlled (the strokes are varied with the internal pressure). See Fig. 1. See also refs. [1] & [2].

Fig. 1 Click here to enlarge the image.
Figure 1

Tube hydroforming offers both technical and economical benefits compared to conventional fabrication. The Ford Mondeo engine cradle can be mentioned as a good example. Studies conducted at Ford (concerning the above-mentioned engine cradle) have shown that (ref. [3])

the number of pieces was reduced from 6 to 1,
the number of process stages was decreased from 32 to 3,
the component weight was lowered from 12 kg to 8 kg, and
the cost per component was reduced from £20 to £10,
as tube hydroforming was selected instead of conventional fabrication.
Sapa has been working with tube bending and hydroforming in many years. In one of the ongoing projects, Sapa and its partners - AP&T and Swepart Verktyg AB are prototyping a number of bent and hydroformed components for the automotive industry, Fig. 2.

Fig. 2 Click here to enlarge the image.
Figure 2

The starting tube was a straight aluminium extrusion with a circular cross-section, as the component shown in Fig. 2 was produced. This straight tube was first bent and then placed in the hydroforming tool. Fig. 3 shows a section of the bent tube and the hydroforming tool (both when the tool is open and as it is closed). After placing the bent tube in the tool (Fig. 3), the tube was hydroformed. The hydroforming was conducted in the following fashion:

a preforming internal pressure of 30-35 bar was built up
the tool was closed and the press force increased to 1,800 metric tonnes and
the internal pressure was increased to 950 bar.

Fig. 3 Click here to enlarge the image.
Figure 3

The whole process was modelled by finite-element simulation. The simulation results were used to identify the critical zones (the zones at which the risk of fracture was high) where tool adjustments could be regarded as necessary. Fig. 4 shows the effective plastic strains after tube bending and hydroforming predicted by finite-element simulation.

Fig. 4 Click here to enlarge the image.
Figure 4

Sapa regards the bending and hydroforming of aluminium extrusions as significant forming methods which offer great potential and intends to continue using these methods also in future commercial projects.

N. Asnafi: Analytical Modelling of Tube Hydroforming, Thin-Walled Structures 34 (1999) 295-330.
N. Asnafi & A. Skogsgårdh: Theoretical and experimental analysis of stroke-controlled tube hydroforming, Materials Science and Engineering A279 (2000) 95-110.
D. Eldred, R. F. Malkin & T. Barringer: Vari-Form a hydroforming technique for manufacturing complex tubular components, Technische Mitteilungen Krupp, 1/1994 (English edition April 1994), pp. 45-50.

About the author:
Dr. Nader Asnafi was graduated from Luleå University of Technology (LUTH) in 1984. He has been working at Esselte Dymo, LUTH, Swedish Institute for Metals Research and Industrial Development Centre in Olofström. Since April 1999, Dr. Asnafi is Technology Area Manager at Sapa Technology. He is currently involved in hydroforming projects at Sapa.

Published with permission of Dr. Nader Asnafi and the Svetsaren (Welder) journal.


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