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Tubular hydroforming of automotive side members with extruded aluminium profiles
Journal of Materials Processing Technology 142 (2003) 93–101
Tubular hydroforming of automotive side members with extruded aluminium profiles Nader Asnafi a,∗ , Tomas Nilsson b , Gunnar Lassl c b
a Volvo Car Corporation, SE-29380 Olofström, Sweden Sapa Profile Bending, Brudabäcksvägen, SE-574 35 Vetlanda, Sweden c Volvo Car Corporation, SE-405 08 Göteborg, Sweden
Received 13 October 2000; received in revised form 13 October 2000; accepted 17 February 2003
Abstract Side member left and side member right, which go from bumper to bumper in a car body, were at the focus in the present study. These side members were produced using straight round (hollow with a circular cross-section) extruded aluminium profiles as tube material. The tubes were bent and hydroformed. Rotary-draw bending yielded the best result. A spread within 8 mm after bending was found to be acceptable provided that the bent tube was hydroformed with a high maximum internal pressure (1300 bar in this study). Pressure-assisted tool closure (hydroforming tool) should be preferred. Such a tool closure prevents formation of buckles, which may be difficult to straighten out completely during hydroforming. Planeness and parallelity of the press tables and adapters play a significant role, as far as the spread and inplaneness of hydroformed components are concerned. The hydroforming tool must be matched in the press that actually will be used. Proper evacuation (of particularly air) is essential, especially in long hydroforming tools. All cross-sections must be deformed at least 2% (average perimeter enlargement) if the hydroformed components are to exhibit a reasonable spread. The critical (fracture) cross-sections predicted by finite-element simulation corresponded to those found in practice. However, the finite-element simulation was not able to predict formation of wrinkles at the tube ends caused by excessively large strokes. Such wrinkles were obtained in practice. © 2003 Elsevier Science B.V. All rights reserved. Keywords: Hydroforming; Bending; Tube; Side member; Automotive; Extrusion; Aluminium; Profile
1. Introduction Lower fuel consumption is of major interest to customers and car producers in the light of legal requirements and brand image. The main parameter that affects fuel consumption is combustion efficiency. Losses in the combustion process accounts for about 83% of all losses. About 50% of the other losses or 9% of all losses are related to the total weight of the vehicle (inertial resistance and rolling resistance), Fig. 1. Weight reduction has been studied in the automotive industry for many years—a work that recently has been accelerated more. In the present study, the body structure is at the focus. In addition to fuel economy and emissions, also other areas or attributes are influenced by a decrease in total weight. However, all options are not available at the same time. At Volvo Car Corporation, fuel efficiency and emis-
∗ Corresponding author. Tel.: +46-454-265334; fax: +46-454-265716. E-mail address: [email protected] (N. Asnafi).
sions/performance are the prime and second driver, respectively, for lower weight. Weight reduction of the body structure and closures facilitates the so-called secondary weight reduction (powertrain, suspension system, steering knuckles, control arms and engine cradles). Available literature indicates a secondary weight saving potential of 50–80% of the primary weight saving on the body structure, Fig. 2. Fig. 3 displays the weight saving potential of different concepts/approaches. In a typical car, the body structure and hang-on parts account for 26% of the total weight or approximately 350 kg (Volvo S70). The largest contribution to this value comes from the load-carrying structure and it is also here that the largest weight saving potential exists, Table 1. By utilising various high strength steel (HSS) grades (yield strength = 210–1200 MPa), Volvo estimates that today’s body weight can be reduced by an additional 10%. Greater weight reduction is not feasible, since HSS is already used to a large extent (45%) in the body structure. However, the weight of the body structure can be reduced by up to 50% using aluminium and new forming and joining techniques, Fig. 3.
0924-0136/$ – see front matter © 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0924-0136(03)00467-9
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N. Asnafi et al. / Journal of Materials Processing Technology 142 (2003) 93–101
Fig. 1. Driving force for weight reduction—complete energy flow in EU combined (5-cyl-gasoline engine).
Fig. 2. The influence of weight reduction on properties and features.
Sapa Profiles and Volvo Car Corporation have, therefore, been studying tube bending and hydroforming with extruded aluminium profiles during the recent years. In a joint project, Sapa and Volvo manufactured a complete underbody. Side member left and side member right (hereafter referred to as
the right and the left side member) were two of the components in this underbody, Fig. 4. These side members, which go from bumper to bumper, were manufactured by: (i) bending a straight (hollow with a circular cross-section) extruded aluminium profile and (ii)
Fig. 3. Weight saving potential—body structure.
N. Asnafi et al. / Journal of Materials Processing Technology 142 (2003) 93–101 Table 1 Weight distribution—complete body Hang-on parts Body structure Other panels
30% 46% 24%
95
Table 2 Mechanical properties of the extruded tubes Doors, trunk lid and hood Load-carrying structure ‘Raincovers’ with limited load-carrying capacity
hydroforming this bent profile. The side members were produced in co-operation with AP&T Lagan AB and SwePart Verktyg AB. The present paper is an account of how these side members were made.
2. Material The selected material was Sapa 6063-T4, which corresponds to AA6063-T4. Tubes with circular cross-section were used. The outside tube diameter was 110 mm, whilst the tube wall thickness was 2.5 mm. The tubes were extruded in the following manner. The used billets were subject to Sapa quality control before extrusion. The billets were then heated to ≈490 ◦ C, after which they were placed in the container and extruded. The selected extrusion speed was ≥20 m/min. The profile exit temperature was ≈540 ◦ C. The extruded profiles were air-cooled, the cooling rate being ≥1 ◦ C/s. The profiles were stretched after extrusion and air-cooling. The amount of this subsequent stretching was ≤0.5%. The extruded tubes were then allowed to be naturally aged in 3 days. The nominal tube outside diameter was 110 mm (circular cross-section) and the nominal tube wall thickness 2.5 mm. The normal wall thickness tolerance is ±10%. However, the actual wall thickness tolerance was +0.80 to +6.8%. The tolerance on diameter is normally ±1.2 mm. However, the actual tolerance on diameter was −0.10 to + 0.40 mm.
Rp0.2 (MPa) Rm (MPa) Ag (%) A50 (%) r n K (MPa) a
0◦a
45◦a
90◦a
76 159 16.9 21.2 0.517 0.259 295
97 170 18.4 22.1 0.247 0.212 258
88 162 21.1 27.6 2.53 0.215 264
Testing direction with respect to the extrusion direction.
Table 2 displays the mechanical properties of the extruded tubes in different testing directions (with respect to the extrusion direction). In this table, Rp0.2 is the yield strength, Rm the ultimate tensile strength, Ag the uniform elongation, A50 the total elongation, r the plastic strain ratio, n the strain-hardening exponent and K the strength coefficient (Lukwik–Hollomon hardening relationship is assumed to be applicable). The tensile specimen cut 45◦ and 90◦ to the extrusion direction were straightened out before tensile testing. These specimen were therefore pre-deformed approximately 2.3% prior to tensile testing. The grain size varied within 56–77 m in the extrusion direction and within 48–61 m perpendicularly to the extrusion direction.
3. Tube bending Before hydroforming, the tube must first be bent in a tube bender and then placed in the hydroforming tool. The content of this section is valid both for the right and the left side member. The tube bender and the hydroforming equipment were placed in the same workshop. The tube bender is shown in Fig. 5.
Fig. 4. In this study, both the left and the right side members were produced by tube bending and hydroforming. The initial tubes were straight round extruded aluminium profiles (outer diameter = 110 mm). The figure is from [1].
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significant role. Not every bent tube could be hydroformed due to bursting. Fig. 6 displays a cross-section of the hydroforming tool (it is insignificant for this discussion, whether Fig. 6 displays the right or the left hydroforming tool). This tool consisted of three tool parts: upper tool, bottom cavity and cam. Note that Fig. 6 shows the hydroforming tool both as it is open and when it is closed. Fig. 6 depicts how the bent tube should theoretically lie in the hydroforming tool before hydroforming. This was verified by the conducted practical tests. The bent tube must be in contact with the bottom cavity, as shown in Fig. 6, prior to tool closure. This contact should exist not only at one cross-section but also along the whole bent tube. If Fig. 5. The tube bender used in this study.
The length of the straight tube was 6000 mm at delivery. The cut length (initial tube length before bending) was 4870 mm. The straight tube was rotary-draw bent at 10 zones, of which 4 were bends in bend. The same bending tool (the same tool radius) was used at all bends. The bending tool radius and the bending radius (‘centre-line’ radius) lied within 110–160 and 165–215 mm, respectively. The tubes were bent using a mandrel. Soap water was used as lubricant. The tube bender was numerically controlled. During tube bending, it is possible to apply an axial force at the tube end. Rotary-draw bending with and without application of an axial force were simulated and tested. The results of these finite-element simulations and experiments are accounted for in [1]. The best results were obtained, if the tube was rotary-draw bent without application of an axial force. All of the tubes used in this study were, therefore, bent in that fashion [1]. The bent tube shape and how the bent tube was ‘resting’ in the hydroforming tool before hydroforming played an
Fig. 6. The hydroforming tool consisted of three parts: upper tool, bottom cavity and cam. This figure shows how the bent tube should theoretically lie in the tool before hydroforming. The figure is from [1].
Fig. 7. Both the bent tubes and the hydroformed side members were measured in this fixture [1].
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such contact was not established at a certain cross-section (if there was much ‘air’ between the bent tube cross-section and the bottom cavity) prior to tool closure, the risk of fracture was larger at that specific cross-section during hydroforming. In practice, it was not possible (since only one bending tool was used) to carry out the bending so that the bent tube was in contact with the bottom cavity (as shown in Fig. 6) at all cross-sections. The shape of the bent tube must, therefore, be ‘optimised’. The shape of a bent tube was first examined by placing it in the hydroforming tool, as it was open. The tube bending program was then altered so that the next bent tube would ‘rest’ better in the hydroforming tool before tool closure. The bent tube was then hydroformed and examined. The practically ‘optimum’ bending program was found by repeating the procedure above. It should also be mentioned that the left bent tube (the bent tube which was used to hydroform the left side member) was not a mirrored version of the right. A mirrored version did not correspond to the ‘optimum’. After ‘optimisation’ of the bending program, each of the bent tubes were placed in a fixture and measured before hydroforming. Fig. 7 displays this fixture. The dial gauges, which were mounted on this fixture, measured the bent tubes. The maximum spread was approximately 8 mm after bending: • This spread was very significant if the bent tubes were hydroformed with 1100 bar in internal pressure. Seventy percent of the tubes with large deviations after bending also exhibited unacceptably large deviations after hydroforming. • This spread did not exhibit any measurable impact if the bent tubes were hydroformed with 1300 bar in internal pressure.
4. Hydroforming
Fig. 8. Tool set-up: press (nominally 10 000 MT), hydroforming tool and adapters. The figure is partially from [1].
4.1. Tool set-up 4.2. Process parameters Fig. 8 displays the tool set-up—a press (with a nominal capacity of 10 000 MT), the hydroforming tool and two adapters. The hydroforming tools were originally planned to be set-up in another press than that shown in Fig. 8. A top cavity was therefore milled on the upper surface of the upper tool. This cavity was filled with the so-called small adapter shown in Fig. 8. The right hydroforming tool (the tool used to hydroform the right side member) is shown in Fig. 9. As shown in Figs. 6 and 9, the hydroforming tool consisted of three parts: bottom cavity, cam and upper tool. The left hydroforming tool was a mirrored version of the right. The hydroforming tools were not hardened and polished.
Tool closure with and without application of a pre-forming (assisting) internal pressure were tested. Large buckles were formed, as the tool was closed without an assisting internal pressure, Fig. 10(a). Signs of these buckles could be observed on the hydroformed side members. Tool closure with application of a pre-forming internal pressure resulted, however, in a shape that did not exhibit any buckles, Fig. 10(b). To find the magnitude of this assisting (pre-forming) internal pressure, the mathematical expressions in [2,3] were used. Three lubricants were tested. The best results were obtained with Iloform BWN 180, which is a mineral oil-containing lubricant.
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Fig. 10. Tool closure without (a) and with (b) application of a pre-forming pressure [1]. Fig. 9. Hydroforming tool: lower tool (bottom cavity and cam) and upper tool. The figure shows the tool for the right side member. This figure is partially from [1].
The hydroforming tools were not hardened and polished. The friction forces are large in such tools. The following tests signify the influence of these large friction forces:
The press force was 9600 MT and the maximum internal pressure 1300 bar. The front-end stroke was 18 mm, whilst the rear-end stroke was 32 mm. The calculated average change in cross-section varied between −0.2 and +6% initially. During the course of the
• A bent tube was lubricated with Iloform BWN 180 before hydroforming. During hydroforming, it burst at 160 bar. • Another bent tube was wrapped up in one layer of plastic film (‘deep-drawing’ film) locally and lubricated with Iloform BWN 180 before hydroforming. This tube burst at the same site as the previous tube. However, bursting occurred at 1100 bar in internal pressure (during hydroforming). As a result of such trials, all of the bent tubes were wrapped up in one layer of plastic film (‘deep-drawing’ film) in some zones and lubricated with Iloform BWN 180 before hydroforming. This is shown in Fig. 11.
Fig. 11. The bent tubes were wrapped up in one layer of plastic film in some zones and lubricated with Iloform BWN 180 before hydroforming [1].
N. Asnafi et al. / Journal of Materials Processing Technology 142 (2003) 93–101
Fig. 12. Rubber pads (thickness = 3 mm) were taped on the bent tube before hydroforming. The figure shows the side member after hydroforming [1].
work, these values were altered to +2 and +5%, respectively, to increase the dimensional stability and to avoid fracture. This issue is discussed in more details in the next section. The left and the right side members were hydroformed in exactly the same fashion (tool closure, lubrication, hydroforming parameters etc.). 4.3. Tool modifications The complete forming operation (bending and hydroforming) was simulated, using the code LS-Dyna. Some of the finite-element results are discussed in [1]. The simulation was not able to predict formation of wrinkles at the tube ends caused by excessively large strokes. However, the predicted critical cross-sections were also critical in practice. At these cross-sections, the magnitude of the plastic deformation was too large and the risk of fracture very high. Rubber pads (thickness = 3 mm) were taped on the tube before hydroforming and putty (filler) was used to find the maximum attainable deformation level at critical sections. At these sections, TIG-welding (and grinding and polishing) then decreased the circumference in the tool, Figs. 12 and 13. There were also cross-sections, at which the magnitude of the plastic deformation was too small. These cross-sections affected the dimensional stability of the side members neg-
Fig. 13. The radii were increased with the aid of putty (filler) [1].
99
Fig. 14. Carving out ‘bananas’ in the tool enlarged the perimeter [1].
atively. At such sections, the perimeter was, therefore, enlarged by carving out ‘bananas’ in the tool, Fig. 14. The left tool was modified in exactly the same fashion as the right. 4.4. Flexing/inplaneness—press, tool and adapters The cam side and bottom side surfaces in the middle straight portion of the side member (both side members), which should be plane, were not completely plane after hydroforming. The buckle depth (inplaneness) was, therefore, measured at the positions shown in Fig. 15 both on the bottom side and the cam side. The maximum buckle depth was 1.05 mm on the cam side and 0.95 mm on the bottom side. The following factors were identified as those responsible for the inplaneness or buckling mentioned above: • There was air left between the tool and the part during hydroforming. • Flexing/inplaneness in the tool/adapters/press table. • Tool matching: the tool was initially matched in another press than that actually used. • Tool separation: Fig. 16 displays schematically how tool separation can cause inplaneness/buckling. • A combination of the factors above. To ease the evacuation of air, small shallow channels were made in the straight middle part of the tool, Fig. 17.
Fig. 15. The positions at which the buckle depth was measured on the cam side and the bottom side [1].
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N. Asnafi et al. / Journal of Materials Processing Technology 142 (2003) 93–101 Table 3 Tool separation measured by sensors 1 and 2 (placed at the tool centre) at different press forces and internal pressures (see also Fig. 18)
Fig. 16. Tool separation: schematic description of changes in the cross-section [1].
Fig. 17. Small shallow channels were made in the tool to ease the evacuation of air [1].
The large and small adapters (Fig. 8(b)) and a large ejector plate in the press table were re-milled. After re-milling and tool set-up, it was found that the planeness and parallelity were improved by 55 and 58%, respectively. These values were considered as acceptable. The tool was matched in the actual press both without and with hydroforming. Fig. 9(b) shows that matching colour is painted on the tool to study the contact surfaces. The tool separation was measured using a number of sensors. The maximum separations were obtained at the tool
Internal pressure (bar)
Press force (MT)
Tool separation Sensor 1 (mm)
Sensor 2 (mm)
1300 1200 1100 1100 1100 1100
9600 9600 9600 9800 8300 7000
0.529 0.401 0.274 0.232 0.400 0.607
0.333 0.248 0.064 0.007 0.225 0.452
centre by sensors 1 and 2, Fig. 18. It should also be mentioned that the internal pressure and the press force could not be unloaded simultaneously at the end of hydroforming. A simultaneous unloading was not possible in the used equipment. Table 3 shows how the separation varied with varying press force and internal pressure. As exhibited in Table 3, the separation decreased with increasing press force (the internal pressure kept constant) and decreasing internal pressure (the press force kept constant). Table 3 gives the values obtained with sensors 1 and 2, which were placed at the tool centre (Fig. 18). The separation was also measured at other sites in the tool. However, the separation was equal or very close to zero at these other sites. Table 3 signifies also that 9600 MT are not sufficient to keep the tool closed at 1300 bar in internal pressure. The press capacity was nominally 10 000 MT. However, 9600 MT was the maximum press force that could be obtained repeatedly. For a more detailed discussion on tool separation, see [1]. After the above-mentioned corrective actions, bent tubes were hydroformed with different internal pressures. After hydroforming, the buckle depth (Fig. 15) and the spread were measured. To obtain the spread, the hydroformed tubes were measured in the fixture shown in Fig. 7. Table 4 displays the obtained values. Note in Table 4 that the values in the first row were obtained before the corrective measures described above were taken. As exhibited in Table 4, the above-mentioned corrective measures had a significant positive impact on both the maximum buckle depth and the spread after hydroforming. Table 4 Maximum buckle depth (inplaneness) and maximum spread as function of press force and maximum internal pressure used in hydroforming
Fig. 18. Tool separation measurements at the tool centre: the positions of the sensors 1 and 2.
Internal pressure (bar)
Press force (MT)
1300 1300 1200 1100
9600 9600 9600 9600
Maximum buckle depth Cam side (mm)
Bottom side (mm)
Spread (mm)
1.05a 0.50 0.45 0.40
0.95a 0.55 0.35 0.25
1.75a 0.57 0.70 4.41
a These values were obtained before the corrective actions described above were taken.
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A good repeatability was given the highest priority. The side members (both the right and the left) were therefore hydroformed with 9600 MT in press force and 1300 bar as maximum internal pressure, Table 4.
• A spread within 8 mm after bending is acceptable, provided that the maximum internal pressure in hydroforming is high (1300 bar in this study). • Rapid optimisation of the bent tube geometry requires a tube bender, which admits application of several bending tools with different radii. • Tool closure with a pre-forming pressure (pressure-assisted tool closure) should be preferred. • Planeness and parallelity of press tables and adapters play a significant role, as far as the spread and inplaneness of hydroformed components are concerned. • Proper evacuation is essential, especially in long tools. • All cross-sections must be deformed at least 2% (average perimeter enlargement) during hydroforming, if a reasonable spread is required. • The hydroforming tool must be matched in the press that actually will be used.
5. Post-processing
Acknowledgements
Table 5 The change in maximum spread during post-processing The measurements conducted
Spread (mm)
After hydroforming in city 1a Upon arrival—city 2b After heat treatment—city 2b After end-cutting and hole-making—city 2b After washing—city 2b At Volvo—city 3c
0.57 0.57 0.62 0.72 0.74 0.80
a
The fixture shown in Fig. 7 was used. The fixture was an exact copy of that shown in Fig. 7. c Performed in a coordinate-measuring machine. b
After hydroforming, the side members (both the right and the left) were placed in specially devised transport fixtures and transported to another workshops situated in another city. At these workshops, the side members were moved to heat-treatment fixtures and artificially aged to the temper T6. After artificial ageing, end-cutting and hole-making (two reference holes were made in each side member) were carried out in a CNC milling machine. The side members were then washed, after which they were placed in transport fixtures and transported to Volvo in Göteborg. At Volvo, the side members were subject to a thorough measurement program, these measurements being conducted in a coordinate-measuring machine. Table 5 displays how the spread changed during the post-processing. 6. Conclusions The following conclusions apply: • Rotary-draw bending yields the best results (for hydroforming).
The present work is published with the kind permission of Sapa Profiles and Volvo Car Corporation, which are gratefully acknowledged. The authors would also like to thank the other project partners, AP&T Lagan AB and SwePart Verktyg AB, for a stimulating collaboration.
References [1] N. Asnafi, T. Nilsson, G. Lassl, Automotive tube bending and tubular hydroforming with extruded aluminium profiles, in: Proceedings of the International Body Engineering Conference and Exposition (IBEC 2000), Detroit, MI, USA, October 3–5, 2000, paper number 2000-01-2670. [2] N. Asnafi, Analytical modelling of tube hydroforming, Thin Wall. Struct. 34 (2000) 295–330. [3] N. Asnafi, A. Skogsgårdh, Theoretical and experimental analysis of stroke-controlled tube hydroforming, Mater. Sci. Eng. A 279 (2000) 95–110.
Tubular hydroforming of automotive side members with extruded aluminium profiles Nader Asnafi a,∗ , Tomas Nilsson b , Gunnar Lassl c b
a Volvo Car Corporation, SE-29380 Olofström, Sweden Sapa Profile Bending, Brudabäcksvägen, SE-574 35 Vetlanda, Sweden c Volvo Car Corporation, SE-405 08 Göteborg, Sweden
Received 13 October 2000; received in revised form 13 October 2000; accepted 17 February 2003
Abstract Side member left and side member right, which go from bumper to bumper in a car body, were at the focus in the present study. These side members were produced using straight round (hollow with a circular cross-section) extruded aluminium profiles as tube material. The tubes were bent and hydroformed. Rotary-draw bending yielded the best result. A spread within 8 mm after bending was found to be acceptable provided that the bent tube was hydroformed with a high maximum internal pressure (1300 bar in this study). Pressure-assisted tool closure (hydroforming tool) should be preferred. Such a tool closure prevents formation of buckles, which may be difficult to straighten out completely during hydroforming. Planeness and parallelity of the press tables and adapters play a significant role, as far as the spread and inplaneness of hydroformed components are concerned. The hydroforming tool must be matched in the press that actually will be used. Proper evacuation (of particularly air) is essential, especially in long hydroforming tools. All cross-sections must be deformed at least 2% (average perimeter enlargement) if the hydroformed components are to exhibit a reasonable spread. The critical (fracture) cross-sections predicted by finite-element simulation corresponded to those found in practice. However, the finite-element simulation was not able to predict formation of wrinkles at the tube ends caused by excessively large strokes. Such wrinkles were obtained in practice. © 2003 Elsevier Science B.V. All rights reserved. Keywords: Hydroforming; Bending; Tube; Side member; Automotive; Extrusion; Aluminium; Profile
1. Introduction Lower fuel consumption is of major interest to customers and car producers in the light of legal requirements and brand image. The main parameter that affects fuel consumption is combustion efficiency. Losses in the combustion process accounts for about 83% of all losses. About 50% of the other losses or 9% of all losses are related to the total weight of the vehicle (inertial resistance and rolling resistance), Fig. 1. Weight reduction has been studied in the automotive industry for many years—a work that recently has been accelerated more. In the present study, the body structure is at the focus. In addition to fuel economy and emissions, also other areas or attributes are influenced by a decrease in total weight. However, all options are not available at the same time. At Volvo Car Corporation, fuel efficiency and emis-
∗ Corresponding author. Tel.: +46-454-265334; fax: +46-454-265716. E-mail address: [email protected] (N. Asnafi).
sions/performance are the prime and second driver, respectively, for lower weight. Weight reduction of the body structure and closures facilitates the so-called secondary weight reduction (powertrain, suspension system, steering knuckles, control arms and engine cradles). Available literature indicates a secondary weight saving potential of 50–80% of the primary weight saving on the body structure, Fig. 2. Fig. 3 displays the weight saving potential of different concepts/approaches. In a typical car, the body structure and hang-on parts account for 26% of the total weight or approximately 350 kg (Volvo S70). The largest contribution to this value comes from the load-carrying structure and it is also here that the largest weight saving potential exists, Table 1. By utilising various high strength steel (HSS) grades (yield strength = 210–1200 MPa), Volvo estimates that today’s body weight can be reduced by an additional 10%. Greater weight reduction is not feasible, since HSS is already used to a large extent (45%) in the body structure. However, the weight of the body structure can be reduced by up to 50% using aluminium and new forming and joining techniques, Fig. 3.
0924-0136/$ – see front matter © 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0924-0136(03)00467-9
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N. Asnafi et al. / Journal of Materials Processing Technology 142 (2003) 93–101
Fig. 1. Driving force for weight reduction—complete energy flow in EU combined (5-cyl-gasoline engine).
Fig. 2. The influence of weight reduction on properties and features.
Sapa Profiles and Volvo Car Corporation have, therefore, been studying tube bending and hydroforming with extruded aluminium profiles during the recent years. In a joint project, Sapa and Volvo manufactured a complete underbody. Side member left and side member right (hereafter referred to as
the right and the left side member) were two of the components in this underbody, Fig. 4. These side members, which go from bumper to bumper, were manufactured by: (i) bending a straight (hollow with a circular cross-section) extruded aluminium profile and (ii)
Fig. 3. Weight saving potential—body structure.
N. Asnafi et al. / Journal of Materials Processing Technology 142 (2003) 93–101 Table 1 Weight distribution—complete body Hang-on parts Body structure Other panels
30% 46% 24%
95
Table 2 Mechanical properties of the extruded tubes Doors, trunk lid and hood Load-carrying structure ‘Raincovers’ with limited load-carrying capacity
hydroforming this bent profile. The side members were produced in co-operation with AP&T Lagan AB and SwePart Verktyg AB. The present paper is an account of how these side members were made.
2. Material The selected material was Sapa 6063-T4, which corresponds to AA6063-T4. Tubes with circular cross-section were used. The outside tube diameter was 110 mm, whilst the tube wall thickness was 2.5 mm. The tubes were extruded in the following manner. The used billets were subject to Sapa quality control before extrusion. The billets were then heated to ≈490 ◦ C, after which they were placed in the container and extruded. The selected extrusion speed was ≥20 m/min. The profile exit temperature was ≈540 ◦ C. The extruded profiles were air-cooled, the cooling rate being ≥1 ◦ C/s. The profiles were stretched after extrusion and air-cooling. The amount of this subsequent stretching was ≤0.5%. The extruded tubes were then allowed to be naturally aged in 3 days. The nominal tube outside diameter was 110 mm (circular cross-section) and the nominal tube wall thickness 2.5 mm. The normal wall thickness tolerance is ±10%. However, the actual wall thickness tolerance was +0.80 to +6.8%. The tolerance on diameter is normally ±1.2 mm. However, the actual tolerance on diameter was −0.10 to + 0.40 mm.
Rp0.2 (MPa) Rm (MPa) Ag (%) A50 (%) r n K (MPa) a
0◦a
45◦a
90◦a
76 159 16.9 21.2 0.517 0.259 295
97 170 18.4 22.1 0.247 0.212 258
88 162 21.1 27.6 2.53 0.215 264
Testing direction with respect to the extrusion direction.
Table 2 displays the mechanical properties of the extruded tubes in different testing directions (with respect to the extrusion direction). In this table, Rp0.2 is the yield strength, Rm the ultimate tensile strength, Ag the uniform elongation, A50 the total elongation, r the plastic strain ratio, n the strain-hardening exponent and K the strength coefficient (Lukwik–Hollomon hardening relationship is assumed to be applicable). The tensile specimen cut 45◦ and 90◦ to the extrusion direction were straightened out before tensile testing. These specimen were therefore pre-deformed approximately 2.3% prior to tensile testing. The grain size varied within 56–77 m in the extrusion direction and within 48–61 m perpendicularly to the extrusion direction.
3. Tube bending Before hydroforming, the tube must first be bent in a tube bender and then placed in the hydroforming tool. The content of this section is valid both for the right and the left side member. The tube bender and the hydroforming equipment were placed in the same workshop. The tube bender is shown in Fig. 5.
Fig. 4. In this study, both the left and the right side members were produced by tube bending and hydroforming. The initial tubes were straight round extruded aluminium profiles (outer diameter = 110 mm). The figure is from [1].
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significant role. Not every bent tube could be hydroformed due to bursting. Fig. 6 displays a cross-section of the hydroforming tool (it is insignificant for this discussion, whether Fig. 6 displays the right or the left hydroforming tool). This tool consisted of three tool parts: upper tool, bottom cavity and cam. Note that Fig. 6 shows the hydroforming tool both as it is open and when it is closed. Fig. 6 depicts how the bent tube should theoretically lie in the hydroforming tool before hydroforming. This was verified by the conducted practical tests. The bent tube must be in contact with the bottom cavity, as shown in Fig. 6, prior to tool closure. This contact should exist not only at one cross-section but also along the whole bent tube. If Fig. 5. The tube bender used in this study.
The length of the straight tube was 6000 mm at delivery. The cut length (initial tube length before bending) was 4870 mm. The straight tube was rotary-draw bent at 10 zones, of which 4 were bends in bend. The same bending tool (the same tool radius) was used at all bends. The bending tool radius and the bending radius (‘centre-line’ radius) lied within 110–160 and 165–215 mm, respectively. The tubes were bent using a mandrel. Soap water was used as lubricant. The tube bender was numerically controlled. During tube bending, it is possible to apply an axial force at the tube end. Rotary-draw bending with and without application of an axial force were simulated and tested. The results of these finite-element simulations and experiments are accounted for in [1]. The best results were obtained, if the tube was rotary-draw bent without application of an axial force. All of the tubes used in this study were, therefore, bent in that fashion [1]. The bent tube shape and how the bent tube was ‘resting’ in the hydroforming tool before hydroforming played an
Fig. 6. The hydroforming tool consisted of three parts: upper tool, bottom cavity and cam. This figure shows how the bent tube should theoretically lie in the tool before hydroforming. The figure is from [1].
Fig. 7. Both the bent tubes and the hydroformed side members were measured in this fixture [1].
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such contact was not established at a certain cross-section (if there was much ‘air’ between the bent tube cross-section and the bottom cavity) prior to tool closure, the risk of fracture was larger at that specific cross-section during hydroforming. In practice, it was not possible (since only one bending tool was used) to carry out the bending so that the bent tube was in contact with the bottom cavity (as shown in Fig. 6) at all cross-sections. The shape of the bent tube must, therefore, be ‘optimised’. The shape of a bent tube was first examined by placing it in the hydroforming tool, as it was open. The tube bending program was then altered so that the next bent tube would ‘rest’ better in the hydroforming tool before tool closure. The bent tube was then hydroformed and examined. The practically ‘optimum’ bending program was found by repeating the procedure above. It should also be mentioned that the left bent tube (the bent tube which was used to hydroform the left side member) was not a mirrored version of the right. A mirrored version did not correspond to the ‘optimum’. After ‘optimisation’ of the bending program, each of the bent tubes were placed in a fixture and measured before hydroforming. Fig. 7 displays this fixture. The dial gauges, which were mounted on this fixture, measured the bent tubes. The maximum spread was approximately 8 mm after bending: • This spread was very significant if the bent tubes were hydroformed with 1100 bar in internal pressure. Seventy percent of the tubes with large deviations after bending also exhibited unacceptably large deviations after hydroforming. • This spread did not exhibit any measurable impact if the bent tubes were hydroformed with 1300 bar in internal pressure.
4. Hydroforming
Fig. 8. Tool set-up: press (nominally 10 000 MT), hydroforming tool and adapters. The figure is partially from [1].
4.1. Tool set-up 4.2. Process parameters Fig. 8 displays the tool set-up—a press (with a nominal capacity of 10 000 MT), the hydroforming tool and two adapters. The hydroforming tools were originally planned to be set-up in another press than that shown in Fig. 8. A top cavity was therefore milled on the upper surface of the upper tool. This cavity was filled with the so-called small adapter shown in Fig. 8. The right hydroforming tool (the tool used to hydroform the right side member) is shown in Fig. 9. As shown in Figs. 6 and 9, the hydroforming tool consisted of three parts: bottom cavity, cam and upper tool. The left hydroforming tool was a mirrored version of the right. The hydroforming tools were not hardened and polished.
Tool closure with and without application of a pre-forming (assisting) internal pressure were tested. Large buckles were formed, as the tool was closed without an assisting internal pressure, Fig. 10(a). Signs of these buckles could be observed on the hydroformed side members. Tool closure with application of a pre-forming internal pressure resulted, however, in a shape that did not exhibit any buckles, Fig. 10(b). To find the magnitude of this assisting (pre-forming) internal pressure, the mathematical expressions in [2,3] were used. Three lubricants were tested. The best results were obtained with Iloform BWN 180, which is a mineral oil-containing lubricant.
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Fig. 10. Tool closure without (a) and with (b) application of a pre-forming pressure [1]. Fig. 9. Hydroforming tool: lower tool (bottom cavity and cam) and upper tool. The figure shows the tool for the right side member. This figure is partially from [1].
The hydroforming tools were not hardened and polished. The friction forces are large in such tools. The following tests signify the influence of these large friction forces:
The press force was 9600 MT and the maximum internal pressure 1300 bar. The front-end stroke was 18 mm, whilst the rear-end stroke was 32 mm. The calculated average change in cross-section varied between −0.2 and +6% initially. During the course of the
• A bent tube was lubricated with Iloform BWN 180 before hydroforming. During hydroforming, it burst at 160 bar. • Another bent tube was wrapped up in one layer of plastic film (‘deep-drawing’ film) locally and lubricated with Iloform BWN 180 before hydroforming. This tube burst at the same site as the previous tube. However, bursting occurred at 1100 bar in internal pressure (during hydroforming). As a result of such trials, all of the bent tubes were wrapped up in one layer of plastic film (‘deep-drawing’ film) in some zones and lubricated with Iloform BWN 180 before hydroforming. This is shown in Fig. 11.
Fig. 11. The bent tubes were wrapped up in one layer of plastic film in some zones and lubricated with Iloform BWN 180 before hydroforming [1].
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Fig. 12. Rubber pads (thickness = 3 mm) were taped on the bent tube before hydroforming. The figure shows the side member after hydroforming [1].
work, these values were altered to +2 and +5%, respectively, to increase the dimensional stability and to avoid fracture. This issue is discussed in more details in the next section. The left and the right side members were hydroformed in exactly the same fashion (tool closure, lubrication, hydroforming parameters etc.). 4.3. Tool modifications The complete forming operation (bending and hydroforming) was simulated, using the code LS-Dyna. Some of the finite-element results are discussed in [1]. The simulation was not able to predict formation of wrinkles at the tube ends caused by excessively large strokes. However, the predicted critical cross-sections were also critical in practice. At these cross-sections, the magnitude of the plastic deformation was too large and the risk of fracture very high. Rubber pads (thickness = 3 mm) were taped on the tube before hydroforming and putty (filler) was used to find the maximum attainable deformation level at critical sections. At these sections, TIG-welding (and grinding and polishing) then decreased the circumference in the tool, Figs. 12 and 13. There were also cross-sections, at which the magnitude of the plastic deformation was too small. These cross-sections affected the dimensional stability of the side members neg-
Fig. 13. The radii were increased with the aid of putty (filler) [1].
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Fig. 14. Carving out ‘bananas’ in the tool enlarged the perimeter [1].
atively. At such sections, the perimeter was, therefore, enlarged by carving out ‘bananas’ in the tool, Fig. 14. The left tool was modified in exactly the same fashion as the right. 4.4. Flexing/inplaneness—press, tool and adapters The cam side and bottom side surfaces in the middle straight portion of the side member (both side members), which should be plane, were not completely plane after hydroforming. The buckle depth (inplaneness) was, therefore, measured at the positions shown in Fig. 15 both on the bottom side and the cam side. The maximum buckle depth was 1.05 mm on the cam side and 0.95 mm on the bottom side. The following factors were identified as those responsible for the inplaneness or buckling mentioned above: • There was air left between the tool and the part during hydroforming. • Flexing/inplaneness in the tool/adapters/press table. • Tool matching: the tool was initially matched in another press than that actually used. • Tool separation: Fig. 16 displays schematically how tool separation can cause inplaneness/buckling. • A combination of the factors above. To ease the evacuation of air, small shallow channels were made in the straight middle part of the tool, Fig. 17.
Fig. 15. The positions at which the buckle depth was measured on the cam side and the bottom side [1].
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N. Asnafi et al. / Journal of Materials Processing Technology 142 (2003) 93–101 Table 3 Tool separation measured by sensors 1 and 2 (placed at the tool centre) at different press forces and internal pressures (see also Fig. 18)
Fig. 16. Tool separation: schematic description of changes in the cross-section [1].
Fig. 17. Small shallow channels were made in the tool to ease the evacuation of air [1].
The large and small adapters (Fig. 8(b)) and a large ejector plate in the press table were re-milled. After re-milling and tool set-up, it was found that the planeness and parallelity were improved by 55 and 58%, respectively. These values were considered as acceptable. The tool was matched in the actual press both without and with hydroforming. Fig. 9(b) shows that matching colour is painted on the tool to study the contact surfaces. The tool separation was measured using a number of sensors. The maximum separations were obtained at the tool
Internal pressure (bar)
Press force (MT)
Tool separation Sensor 1 (mm)
Sensor 2 (mm)
1300 1200 1100 1100 1100 1100
9600 9600 9600 9800 8300 7000
0.529 0.401 0.274 0.232 0.400 0.607
0.333 0.248 0.064 0.007 0.225 0.452
centre by sensors 1 and 2, Fig. 18. It should also be mentioned that the internal pressure and the press force could not be unloaded simultaneously at the end of hydroforming. A simultaneous unloading was not possible in the used equipment. Table 3 shows how the separation varied with varying press force and internal pressure. As exhibited in Table 3, the separation decreased with increasing press force (the internal pressure kept constant) and decreasing internal pressure (the press force kept constant). Table 3 gives the values obtained with sensors 1 and 2, which were placed at the tool centre (Fig. 18). The separation was also measured at other sites in the tool. However, the separation was equal or very close to zero at these other sites. Table 3 signifies also that 9600 MT are not sufficient to keep the tool closed at 1300 bar in internal pressure. The press capacity was nominally 10 000 MT. However, 9600 MT was the maximum press force that could be obtained repeatedly. For a more detailed discussion on tool separation, see [1]. After the above-mentioned corrective actions, bent tubes were hydroformed with different internal pressures. After hydroforming, the buckle depth (Fig. 15) and the spread were measured. To obtain the spread, the hydroformed tubes were measured in the fixture shown in Fig. 7. Table 4 displays the obtained values. Note in Table 4 that the values in the first row were obtained before the corrective measures described above were taken. As exhibited in Table 4, the above-mentioned corrective measures had a significant positive impact on both the maximum buckle depth and the spread after hydroforming. Table 4 Maximum buckle depth (inplaneness) and maximum spread as function of press force and maximum internal pressure used in hydroforming
Fig. 18. Tool separation measurements at the tool centre: the positions of the sensors 1 and 2.
Internal pressure (bar)
Press force (MT)
1300 1300 1200 1100
9600 9600 9600 9600
Maximum buckle depth Cam side (mm)
Bottom side (mm)
Spread (mm)
1.05a 0.50 0.45 0.40
0.95a 0.55 0.35 0.25
1.75a 0.57 0.70 4.41
a These values were obtained before the corrective actions described above were taken.
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A good repeatability was given the highest priority. The side members (both the right and the left) were therefore hydroformed with 9600 MT in press force and 1300 bar as maximum internal pressure, Table 4.
• A spread within 8 mm after bending is acceptable, provided that the maximum internal pressure in hydroforming is high (1300 bar in this study). • Rapid optimisation of the bent tube geometry requires a tube bender, which admits application of several bending tools with different radii. • Tool closure with a pre-forming pressure (pressure-assisted tool closure) should be preferred. • Planeness and parallelity of press tables and adapters play a significant role, as far as the spread and inplaneness of hydroformed components are concerned. • Proper evacuation is essential, especially in long tools. • All cross-sections must be deformed at least 2% (average perimeter enlargement) during hydroforming, if a reasonable spread is required. • The hydroforming tool must be matched in the press that actually will be used.
5. Post-processing
Acknowledgements
Table 5 The change in maximum spread during post-processing The measurements conducted
Spread (mm)
After hydroforming in city 1a Upon arrival—city 2b After heat treatment—city 2b After end-cutting and hole-making—city 2b After washing—city 2b At Volvo—city 3c
0.57 0.57 0.62 0.72 0.74 0.80
a
The fixture shown in Fig. 7 was used. The fixture was an exact copy of that shown in Fig. 7. c Performed in a coordinate-measuring machine. b
After hydroforming, the side members (both the right and the left) were placed in specially devised transport fixtures and transported to another workshops situated in another city. At these workshops, the side members were moved to heat-treatment fixtures and artificially aged to the temper T6. After artificial ageing, end-cutting and hole-making (two reference holes were made in each side member) were carried out in a CNC milling machine. The side members were then washed, after which they were placed in transport fixtures and transported to Volvo in Göteborg. At Volvo, the side members were subject to a thorough measurement program, these measurements being conducted in a coordinate-measuring machine. Table 5 displays how the spread changed during the post-processing. 6. Conclusions The following conclusions apply: • Rotary-draw bending yields the best results (for hydroforming).
The present work is published with the kind permission of Sapa Profiles and Volvo Car Corporation, which are gratefully acknowledged. The authors would also like to thank the other project partners, AP&T Lagan AB and SwePart Verktyg AB, for a stimulating collaboration.
References [1] N. Asnafi, T. Nilsson, G. Lassl, Automotive tube bending and tubular hydroforming with extruded aluminium profiles, in: Proceedings of the International Body Engineering Conference and Exposition (IBEC 2000), Detroit, MI, USA, October 3–5, 2000, paper number 2000-01-2670. [2] N. Asnafi, Analytical modelling of tube hydroforming, Thin Wall. Struct. 34 (2000) 295–330. [3] N. Asnafi, A. Skogsgårdh, Theoretical and experimental analysis of stroke-controlled tube hydroforming, Mater. Sci. Eng. A 279 (2000) 95–110.