EP1737590A1

EP1737590A1 - Local forming, locally formed work piece and tool for such forming

Info

Publication number: EP1737590A1
Application number: EP05710944A
Authority: EP
Inventors: Børge BJØRNEKLETT; Ole Runar Myhr; Pål VIST
Original assignee: Norsk Hydro ASA
Current assignee: Benteler Automobiltechnik GmbH
Priority date: 2004-02-13
Filing date: 2005-02-11
Publication date: 2007-01-03
Anticipated expiration: 2025-02-11
Also published as: ATE550117T1; EP1737590B1; NO20040660D0; WO2005077560A1

Abstract

A method for local forming of a work piece (19) where a force is used for forming a part of said work piece (19) and where a predefined local region of the work piece (19) is heated and that the heated area (6) is formed while the surrounding cooler structure of the work piece (19) is maintained substantially unaffected of the forming operation. A work piece provided with one or more imprints or protrusions made by such method and a tool for performing such method.

Description

"Local forming, locally formed work piece and tool for such forming"

The invention relates to a method for local forming of a work piece where a stamp, die or pressurised fluid or gas, interacts with a heat source. It also relates to a work piece made by such method and tools used for the forming of a work piece.

It is well known to make local imprints in a work piece with uniform temperature. In conventional forming techniques the final shape of the work piece is defined by the geometry of a stamp and a die or backing tool.

Many conventional forming operations are often performed at ambient temperatures, where the formability is relatively low. Exceeding the critical plastic strain will lead to e.g. cracks, localised necking or formation of Luderbands or even failure of the work piece.

Cold forming will furthermore lead to work hardening in the material being formed. This leads to a lower ductility during the forming operation as well as in the final work piece.

The tool concept, consisting of a stamp and a backing tool, often demands a time consuming and expensive fabrication of necessary tools. The relatively complex tools may also lead to poor reliability in production or high maintenance costs.

Furthermore it is often difficult or even impossible to insert a backing tool inside a hollow work piece due to the limited accessibility. This is particularly the case for long closed profiles such as bumper beams or profiles having a complex geometry. This will add a limitation on the design of such profiles.

Work pieces such as extruded and formed aluminium profiles often exhibit unwanted deviations from the nominal geometry. Such geometric deviations may often disqualify the parts in applications where accurate and consistent geometry is required.

Such deviation may e.g. represent a problem in assembly or joining processes such as welding, brazing, bonding, riveting or other mechanical joining methods, which usually require a good fit up of the parts to be joined.

Such problems may require the use of calibration operations such as reshaping, milling, grinding, cutting or etching. This often represents time-consumiag operations and thereby increased production costs.

Furthermore they also have limited applicability. In the case of milling, grinding, cutting or etching, the calibration operation generates waste material.

Calibration using cold forming is associated with a relatively high degree of elastic spring back of the work piece. Due to the elastic spring back, small geometrical corrections will be difficult to perform. Variations in the spring back e.g. due to inconsistent mechanical properties or geometrical dimensions, will cause deviations in the final geometry of the work piece.

The present invention represents a flexible method for fabrication of e.g. local protrusions or imprints. The principles also enable a higher degree of forming of the work piece and can be performed preferably without any backing tool, thereby enabling processing in regions of the work piece where the access of a backing tool is limited. The method furthermore improves the flexibility of the geometry due to the ability of obtaining small bending radii or sharp edges on the protrusions or imprints.

The relative simple, low cost apparatus also decrease the possibility of failure during production, thereby reducing maintenance costs, increasing the up time of the production and reducing the scrap rate. The method furthermore enables the calibration of end sections without any cutting operations. The present invention utilises the temperature dependency of the mechanical properties of the material. This spatial variation of mechanical properties across the work piece is utilised to allow forming within a locally heated region without distorting the surrounding material, which has a higher resistance to forming. The present invention may for some applications, utilise the thermal field as a virtual die for defining the regions of plastic flow and hence the final shape.

This is done by rapid local heating of selected regions of the profile, which renders the heated material into a soft and ductile state with improved formability. The localised heating can be manipulated to form a sufficiently sharp boundary between the soft and hot material that easily forms plastically and the adjacent material at lower temperature, which has a higher resistance to forming.

The rapid local heating is preferably done e.g. by means of an induction coil, which is situated in the neighbourhood or on the surface of the work piece. Material in close proximity of the coil will be heated until a temperature is reached where the yield stress of the material is substantially lower than the surrounding material. Thus, the forming operation takes place in material with an essentially non-uniform, temperature distribution. As the surrounding material have higher strength, the difference in material strength will enable the surrounding material to maintain its original shape.

The local heating is preferably done with a heat source, which is capable to generate sufficiently steep thermal gradients e.g. by applying induction coils fitted with or without intensifiers for optimal distribution of the electro magnetic field. A number of alternative heat source may be applied such as, resistance heating, laser, plasma arc, gas flame, etc.

It is known from the publication "Adapted Mechanical Properties for Improved Formability of Aluminium Blanks by Local Induction Heating" by ?Michael

Kerausch, Marion Merklein and Manfred Geiger (JSAE 20037024), presented at the

International Body Engineering Conference October 2003 in Tokyo, to use induction coils to locally heat an aluminium alloy to modify the material properties of a limited area of the aluminium sheet. The sheet is heated up to a predefined maximum temperature and thereafter cooled down to room temperature. The heating and cooling phase form part of a heat treatment aiming to modify the material properties of the treated sheet. After cooling down the sheet, a cylindrical cup was deep drawn into the material, thereby proving a better formability of the heat treated material.

However, this technique does not take advantage of the possibility of forming parts of the sheet in a heated mode. The heating procedure is used as a material property treatment aiming to change the room temperature properties within a localised area of the profile. Even though this method changes the room temperature formability of the material, the benefit of the process is substantially lower than for forming in a partially hot state.

In contrast to the above described method, the present invention is based on forming of a localised hot area of the work piece, thereby taking advantage of the extended softening of the heated material and the increased difference in material strength between the hot region and the surrounding cooler material.

The present invention gives a number of additional advantages. Applied on a work piece with sufficient geometrical stiffness, the local forming can be performed without the use of a physical die. This principle enables e.g. local forming in regions that are difficult or even impossible to access with a backing tool. The concept increases the forming capability and general freedom of design. This is due to both the increased formability at elevated temperatures as well as the ability to control the thermal field during forming and the corresponding flow stress distribution in the material.

Furthermore the invention requires simpler and cheaper tooling compared to conventional forming. It also requires very low loads for forming, which reduce the investment cost as well as the complexity of the production equipment. In addition, the geometric accuracy of the components and parts fabricated by the method is high due to limited elastic strain as a consequence of the low yield stress at elevated temperature. Normally, the forming is done without use of any lubricants. Another benefit is obtained during the forming since most metallic materials exhibits reduced anisotropy at elevated temperatures, but this depends on the initial texture as obtained from the preceding thermo mechanical process route.

A further benefit of the invention is the possibility to control the mechanical properties in the protrusion or imprint. By the localised heating before and during forming, the work hardening is reduced, thereby ensuring an even better ductility in the formed regions. This is especially advantageous in regions with sharp forming radii which often experience large strains in e.g. impact absorbing members.

The invention will now be further explained by means of figures, where

Fig. 1 shows a first example of a tool set up, for making a local imprint,

Fig. 2 shows the cross section of a profile and forming tool prior to a forming operation, Fig. 3 shows the cross section of a profile and tool during the forming operation, Figs. 4a-b show a first example of a sequence diagram showing the heating power and the tool displacement, and a typical resulting thennal cycle for an arbitrary position in the forming area, Figs. 5a-e Fig a-c show a second example of a tool set up including a supporting die. Fig d and e. show a sequence diagram for the tool displacement, and a typical resulting thermal cycle for an arbitrary position in the forming area, Figs. 6a-b show an example of a local imprint in an extruded profile, Figs. 7a-b show an example of a local imprint in an extruded profile, Figs. 8a-b show an example of a local imprint in a crash absorbing member, Figs. 9a-b show an example of a tool set up,

Figs. lOa-b show an example of a tool set up with an internal pressurised gas or liquid within a hollow profile, Figs, lla-b show an example of a tool set up with a rolling wheel tool, Figs. 12a-b show an example of a tool set up with a sliding tool, Figs. 13a-c show an example of a local imprint, Fig. 14 shows an example of a tool set up, Fig. 15 shows an example of a local imprint,

Fig. 16 shows the cross section of an end section of a profile and a first embodiment of a calibration tool, Fig. 17 shows the cross section of an end section of a profile and a second embodiment of a calibration tool, Figs. 18a-c show a third embodiment of a calibration tool applied to a bumper system, Fig. 19 shows a side view of a forming tool,

Fig. 20 shows a cross sectional view of the forming tool.

Figure 1 shows a section of a work piece such as a profile 1 prior to forming. An induction coil 2 is in the proximity of or directly at the profile surface 3. The induction coil 2 generates localised heat in the profile side wall 4. The affected region of the profile 1 is heated until a favourable transient temperature distribution is reached. A stamp 5 is thereafter pressed onto the area 6.

Figure 2 shows the cross section of the profile 1 , an induction coil 2 and a stamp 5 prior to a forming operation. The induction coil is situated on the profile surface 3. As the induction coil is turned on, a localised hot area will occur in the profile side wall 4. The stamp 5 will normally be placed above the hot area 6 inside the induction coil 2 prior to forming.

Figure 3 shows the cross section of the profile 1, the induction coil 2 and the stamp 5 during or after the forming operation. The area of the profile side wall 4 surrounding the induction coil 2 will remain mostly unaffected of the heating from the heat source. The profile 1 will be insignificantly heated by the electro magnetic field and as the induction heating process is highly localised, the surrounding structure of the profile 1 remains unaffected during forming. When the stamp 5 is pressed onto the profile 1, the softened hot area 6 will therefore deform while the adjacent unaffected area will resist deformation.

Figure 4a shows an example of the time history of the temperature in the heated area during a local forming operation. Figure 4b shows an example of a time- displacement curve for the stamp related to the temperature cycles in Figure 4a.

Figures 5a-c show a second example of a sequence diagram during forming of an imprint. Fig 5 (d and e) shows the corresponding tool displacement and temperature cycle for the position P. The stamp 5 will start travelling at time A as shown on Figure 5a, where the sheet 7 is at room temperature and the induction coil 2 is just switched on. As the sheet 7 is partially heated at time B as shown on Figure 5b, the part of the sheet 7 in the forming area will become increasingly strained and increasingly heated until it reaches a maximum at time C reaching the position shown on Figure 5c. Figure 5d shows the position of the stamp during the forming operation and 5e shows the temperature cycle during the forming operation.

Figures 6a-b show an example of a section of a profile 12 provided with a local imprint 13. Figure 6a shows a perspective view of the profile and figure 6b shows a cross sectional view of the profile with an imprint. In long closed hollow profiles such as bumpers, there is often a need for local protrusions or imprints 13 where for example lights, sensors or other equipment can be mounted. Such imprints 13 can easily be made by the present method.

There can be as many protrusions or imprints as desired on a work piece, thereby enabling the design of a complicated shape. Even if there is only one imprint 13 in one of the side walls of the profile 12 described on Figure 8, it should be understood that it is possible to make protrusions or imprints on any of the side walls of a profile or other work piece. Figures 7a-b show a section of a profile 12 provided with a local imprint 13. Figure 7a shows a perspective view of the section and Figure 7b shows a cross sectional view of the section with the protrusion. With a heat source enabling the local heating of an edge of a profile, it is possible to make imprints on edges, corners or the like.

Figures 8a-b show a profile provided with local imprints. Figure 8a shows a perspective view of an impact absorbing member 14. In impact absorbing members 14, such as crash boxes, it is often desired to introduce protrusions or imprints 13 which work as triggers to ensure a controlled deformation of the impact absorbing member 14 in an impact situation. The impact absorbing member 14 of the present example is provided with a set of imprints 13 in at least one of the member side walls 15.

Figure 8b shows a cross sectional view of the impact absorbing member 14. On Figure 8 the imprints 13 are made in two opposing member side walls 15. It is also possible to make a set of imprints in two member walls facing directly onto each other. Furthermore, it is possible to make protrusions or imprints in more than two side walls of the member if this is found suitable.

It should also be noted that imprints or protrusions can be made in the member end plate 10 of the impact absorbing member 14. It is also possible to make one or more protrusions or imprints in the member flanges 17 of the impact absorbing member 14.

Figures 9a-b show a cross sectional view of a second embodiment of a tool set up. A ferritic lens 18 can be used to redirect and amplify the electro magnetic field. The iron lens 18 can at the same time work as a forming tool, as the induction coil 2 with the ferritic lens 18 is pressed in to the work piece 19. Figure 9a shows the iron lens 18, induction coil 2 and stamp 5 prior to forming. Figure 9b shows the iron lens 18, induction coil 2 and stamp 5 during the forming operation. The iron lens with ferromagnetic properties may also consist of other appropriate dielectric material capable of redirecting the electro magnetic field. Figures lOa-b show a cross sectional view of a tool set up. Figure 10a shows a work piece and tool set up prior to a forming operation. The pressure Pi inside the work piece 19 is equal to the pressure Pi outside the work piece. Figure 10b shows the work piece 19 during or after a forming operation with an internal working pressure P₂ above the external pressure Pi. After the forming operation, a protrusion 20 is made in the surface. It is possible to make as many protrusions 20 as desired in the surface of a work piece. It is also possible to make two or more protrusions simultaneously provided that more than one tool set up is available.

By applying an induction coil 2 in combination with a die 21, hydro forming can be done with less tooling and internal pressure than conventional hydro forming. A work piece 19 is filled with an adequate liquid or gas which can be given a higher inside pressure P₂ than the external pressure Pi. The work piece 19 can be hydro formed if the inside pressure P₂ results in a stress in the material of the work piece 19 which exceed the yield stress of the material.

Hydro forming is a complex and expensive operation, where a high pressure has to be applied. Due to the reduced yield stress of the hot area 6 of the work piece 19, the pressure P of the liquid or gas inside the work piece can be considerably reduced relative to the conventional methods. The inside pressure P₂ of the work piece 19 will move the side wall to fit with the stamping die 21. The cold material outside the hot area 6 will withstand the pressure of the liquid or gas, and will not be deformed.

Figures lla-b show a tool set up. A rotating tool 22 in combination with an induction coil 2 which moves relative to a work piece, can be used to make an imprint. The direction of the imprint on the work piece can be arbitrary and also curved. The shape of the imprint can change as a function of the shape of the rotating tool 22.

Figure 11a shows the rotating tool 22 in a perspective view. A forming wheel is situated in the area affected by an induction coil 2. As the induction coil 2 is moved along the work piece 19 (see Fig. lib), at the same time as a force is acting on the axle an imprint will be made. Figure 1 lb shows the rotating tool in a cross sectional view. The axis 25 situated in the center 23 of the forming wheel 24 (see Fig. 11a) can be mounted separately from or directly onto the induction coil 2.

Figures 12a-b show a tool set up. A sliding tool 26 in combination with an induction coil 2 which moves relative to a work piece can be used to make an imprint. The direction of the imprint in the work piece can be freely chosen. Elevating or lowering the sliding tool 26 relative to the surface of the work piece 19 can continuously change the depth of the imprint (see Fig. 12b).

Figure 12a shows a perspective view of such sliding tool 26 and Figure 12b shows a cross sectional view of the sliding tool. The sliding tool 26 is situated in an induction coil 2 and slides over the surface of a work piece 19 while it is pressed down onto the work piece 19. The sliding tool 26 will function as described above for the rotating tool.

Figures 13a-c show a fourth example of a local imprint 13. By using a rotating tool 21 or a sliding tool 25 as described above, any design of the imprint 13 is possible. Figure 13a shows a perspective view of an oval imprint 13 made in the surface of a section of a work piece 19. The cross sectional shape B-B as shown on Figure 13b is formed by the cross section of the sliding or rotating tool. Figure 13c shows the cross sectional shape C-C of an imprint in a sheet. The shape C-C is formed by the movement of the sliding or rotating tool.

If the work piece 19 is an open profile, it is possible to use the moving tools on all sides of the work piece, thereby enabling the forming of both imprints or protrusions in the surface of a work piece. It is also possible to make one or more longer grooves in a work piece.

Figure 14 shows a sixth example of a tool set up. Depending on the shape of the rotating tool 22, it is possible to vary the geometry of the imprint during forming. Figure 15 shows an imprint 13 in a section of a work piece 19 made by such rotating tool having a certain designed circumference. Figure 16 shows the cross section of an end section 28 of a work piece 19 and a calibration die 29. The calibration die 29 is formed as a compact die with an opening for each end section being calibrated. As an end section 28 is entered into an opening, the end section is clamped inside the calibration die 29. The end section 28 of the work piece 19 can be heated by an induction coil 2 after or prior to being entered into the calibration die 29. In the latter case, the heated end section 28 is thereafter introduced into a calibration die 29 where the desired geometry is obtained by pressing or drawing.

Figure 17 shows the cross section of an end section 28 of a work piece 19 and a second embodiment of a calibration die 29. The calibration die 29 is formed as an outer shelf 30 with integrated induction coils 2 surrounding an inner opening in the shelf 30. An internal expanding tool 31 is situated inside the shelf 30 forming the outer wall of the calibration die 29. An end section 28 is introduced to the shelf of the calibration die 29 in which the induction coil 2 is moulded. The end section 28 is situated between the shelf 30 and the internal expanding tool 31. As the internal expanding tool 31 expands, the end sections 28 will be clamped between the shelf 30 and the expanding tool 31. The end section 28 is heated by the induction coil 2 and calibrated by the expanding internal tool 30. The heated end section 28 can be prolonged, shortened or deformed in any desired manner in the calibration die 29.

Figures 18a-c show a forming tool. A section of a sheet 7 is situated between a profile 1 and a stamp 5. The induction coil 2 can be integrated in a tool 32. In the present embodiment, the induction coil 2 is integrated in a tool 32 which is placed on the sheet 7 on the opposite side of the stamp 5. As the induction coil 2 is turned on, the sheet 7 is heated and can be formed. The sheet in the present embodiment is formed by pressing the stamp 5 situated on one side of the sheet 7, towards the profile 1 and the tool 32 situated on the opposite side of the sheet 7, away from the profile 1.

Figure 18a shows the forming tool prior to the forming operation. A backing device 33 can be placed on one side of the sheet. A surface of the backing device 33 can then be used as a datum 34 to ensure a precise result after the forming operation. Figure 18b shows the forming tool during and after the forming operation. The tool 32 is pressed against an outer section 35 of the sheet 7 while the stamp 5 is pressed onto an inner section 36 of the sheet 7. In the present embodiment, the outer section 35 is pressed in one direction and the inner section 36 in the opposite direction, thereby forming a protrusion in the sheet 7. After the forming operation, it is possible to use the backing device 33 in other operations such as punching or spot welding, as shown on Figure 18c.

The process described on Figures 18a-c can be used for the assembly of a crash system. The method enables an individual fabrication of bumper beam and crash- box, including welding of back-plate to body of box without any consideration of weld distortions.

Welding of bumper beams to crash boxes is a common process and is often performed without any sort of calibration. The resulting bumper system can often suffer from relatively large deviations in geometry due to the whole stack-up of geometric deviations resulting from the different operations like extrusion, sawing, stretch-bending and welding before the crash system is assembled. Hence, a final calibration of the assembly would have been desirable.

The bumper beam can then be fixated at point positions. Figure 18a illustrates the initial positioning of the crash system with a profile 1 or bumper beam, a crash box 37 and a sheet 7 working as a back plate. There will be small deviations in all the parts of the crash system. The deviations between the actual and desired configuration of the system can be added-up as a resulting deviation in plainness of the back plate. By applying moving tools such as a ceramic tool 32 with integrated induction coils 2, the sheet 7 or back plate can be forced to fit against a backing device 33 by allowing small amounts of plastic deformations. The next step will be to punch holes in the back plate at fixed positions relative to the datum point.

Figure 19 shows a side view of a second embodiment of a stamp 5. Figure 20 shows a cross sectional view of the same stamp 5. An induction coil 2 is integrated in the stamp 5. The stamp 5 is placed directly on or in the immediate neighbourhood of the surface of a work piece 19. As current is turned on in the induction coil 2, the region of the profile located below the stamp 5 is heated. The heating can continue as the stamp 5 is pressed onto the work piece 19, thereby forming an imprint or protrusion in the surface. Some regions of the stamp 5 may preferably consist of electrically low-conductive material.

The method can be used to any local reshaping of a work piece and is especially suitable for local forming in thin walled open or closed profiles such as extruded or rolled profiles. Typical applications include automotive structures such as bumpers, crash boxes, engine cradles and other frame structures.

Even if the preferred embodiment of the method is to use induction heating of the profile, it should be noted that any other sources for local heating which is capable to generate sufficiently steep thermal gradients can be used e.g. by applying resistance heating, laser, plasma arc, gas flame, etc.

It should also be noted that the method can be used on any material being affected by a heat treatment, such as aluminium alloys, other metals such as steel, magnesium and alloys of these, polymers and the like.

The method can also be used to make imprints or protrusions on already formed imprints or protrusions.

Claims

1. A method for local forming of a work piece (19) where a force is used for forming a part of said work piece (19), characterised in that a predefined local region or area of the work piece (19) is heated and that this heated area (6) is formed while the surrounding cooler structure of the work piece (19) is maintained substantially unaffected of the forming operation.

2. A method according to claim 1, characterised in that an induction coil (2) is placed in the immediate neighbourhood of or on the surface of the work piece (19) for heating of the work piece (19).

3. A method according to claim 1 or 2, characterised in that a forming operation is performed by pressing a stamp (5) or the like onto the heated area (6) of the work piece (19).

4. A method according to any of the preceding claims 1-3, characterised in that at least one protrusion (20) is made forcing the heated area (6) of the work piece (19) to expand due to an internal pressure P inside the work piece (19) which is above the external pressure Pi outside the work piece (19).

A method according to any of the preceding claims 1-4, characterised in that the forming operation is performed by pressing a rotating tool (21) on to the surface of the work piece (19).

6. A method according to any of the preceding claims 1-5, characterised in that the forming operation is performed by pressing a sliding tool (25) onto the surface of a work piece (19).

7. A method according to any of the preceding claims 1-6, characterised in that at least one imprint (13) is made in the surface of the work piece (19) during the forming operation.

8. A method according to any of the preceding claims 1-7, characterised in that at least one protrusion (20) is made in the surface of the work piece (19) during the forming operation.

9. A method according to any of the preceding claims 1-8, characterised in that the at least one imprint (13) and/or the at least one protrusion is made in the surface of a bumper beam.

10. A method according to any of the preceding claims 1-9, characterised in that the at least one imprint and/or the at least one protrusion is made in the surface of an impact absorbing member (14).

11. A method according to claim 1 or 2, characterised in that at least one section (27) of a work piece (19) is calibrated by local forming after local heating of the at least one section.

12. A method according to claim 11, characterised in that the calibration is performed by a calibration die (28).

13. A work piece (19) having at least one formable surface, characterised in that at least one imprint (13) and/or protrusion (20) is formed in the surface of the work piece (19) according to any of the preceding claims 1-12.

14. A work piece (19) according to claim 13, characterised in that it is an aluminium profile.

15. A work piece (19) according to claim 13 or 14, characterised in that it is an extruded aluminium profile.

16. A work piece (19) according to any of the preceding claims 13-15, characterised in that it is a bumper provided with at least one imprint and/or protrusion according to any of the preceding claims 1-11.

17. A work piece (19) according to any of the preceding claims 13-16, characterised in that it is an impact absorbing member provided with at least one imprint or protrusion according to any of the preceding claims 1-5.

18. Tool for local forming of a work piece (19) where a force is used for forming a part of said work piece (19), characterised in that an induction coil (2) is integrated in a stamp (5).

9. Tool for local forming of a work piece (19) where a force is used for forming a part of said work piece (19), characterised in that an induction coil (2) is integrated in a calibration die (28).