Method of hydroforming a structural member

Description

[0001] The invention relates to a method of manufacturing a shaped and hollow structural member, in particular of aluminium, via a hydroforming process. Further the invention relates to a product obtained via such a method.

[0002] As is conventional, the term "aluminium" refers also to aluminium alloys. In the industry, and in particular in the automotive sector, forming is an important tool in manufacturing complex shapes with as main materials steel and aluminium. Suppliers are urged to improve the formability of their materials and to integrate their processes because of, amongst others, costs and environmental aspects. A part of this objective can be achieved using a relative novel forming technology such as hydroforming. In a typical hydroforming operation, for example a cylindrical metal tube is bent into the overall shape desired for the final part, then placed between a pair of dies. The dies provide a cavity around the tube which has an interior shape matching the exterior shape desired for the part. Then, the ends of the tube are sealed and it is internally pressurised to expand it into the shape of the dies' cavity. A similar approach is used when hydroforming one or more planar metal sheets. This technique is becoming an important tool in the forming technology for automotive and other applications. By means of hydroforming different forming steps can be integrated and much more complex shapes can be formed which in turn is saving costs and efforts compared with conventional made products. There is a need in the industry, in particular in the automotive industry, for methods of hydroforming which allow more complex hydroformed shapes of the final product, such as structural members.

[0003] An object of the invention is to provide an improved method of manufacturing a shaped and hollow structural member via a hydroforming process using an extruded or welded tubular or hollow metal product as starting material.

[0004] Another object of the invention is to provide an improved method of manufacturing a shaped and hollow structural member via a hydroforming process using an extruded or welded tubular or hollow aluminium product as starting material.

[0005] To achieve this object there is provided in accordance with the invention a method comprising the steps of subsequently:-

• providing an extruded or welded tubular or hollow intermediate metal product;
• placing the intermediate metal product between a pair of hydroforming dies forming an interior space that matches the desired exterior cross sectional shape of said structural member;
• pressurising the interior of intermediate metal product so as to force it out into the interior space of said dies by the introduction of a heated pressure medium having a temperature in the range of 50 to 300°C, preferably in the range of 140 to 300°C, and more preferably in the range of 160 to 270°C.

[0006] With this method there is achieved that more complex hydroformed members can be manufactured as compared to hydroforming at ambient temperatures. In particular the combination of the initial shape of the intermediate product and the hydroforming at elevated temperatures results allows to the manufacture of complex hydroformed members.

[0007] In an embodiment of the method according to the invention the pressure medium used during hydroforming is a hydraulic oil which can withstand the relatively high temperatures.

[0008] In a further embodiment of the method according to the invention the intermediate metal product and/or the hydroforming dies are pre-warmed, when possibly they may are be pre-warmed locally. This further improves the method and optimises the hydroforming behaviour of the metal used. The metal product can be pre-heated, locally, to a temperature in a range of 200 to 400°C in for example an air circulating furnace prior to placing the metal product in the hydroforming dies.

[0009] It has been found that the method in accordance with the invention is particularly suitable for aluminium alloys, and more in particular for aluminium alloys from the Aluminium Association (AA)5xxx-series having Mg as major alloying element in a typical range of 2 to 6 weight percent, and preferably produced on an industrial annealing furnace with a controlled heating-rate between 2 to 200 °C/sec, and a soaking time in the range up to 40 sec. at a soaking temperature in the range of 400 to 550°C, followed by a controlled cooling rate in the range of 10 to 500 °C/sec. A typical property of a good warm hydroforming behaviour aluminium alloy are those which show a good necking resistance (high m at elevated temperature) with relative moderate strain rates (ε'), such as AA5182 material in an O-temper condition providing good dimensional rigidity. In this embodiment the improved forming characteristics of aluminium at elevated temperature are used to manufacture complex structural members via hydroforming. In particular in the given temperature range the ductility is superior to standards steel grades under ambient conditions. Another suitable aluminium alloy is the aluminium alloy obtained from the method as described in the European patent no. EP-A-0818553, which is incorporated here by reference.

[0010] When using an extruded or welded tubular or hollow aluminium alloy intermediate metal product a suitable process comprises more preferably the following subsequent processing steps:-

• providing an extruded or welded tubular or hollow intermediate metal product;
• bending the intermediate aluminium product, preferably by radial draw bending;
• preforming of the bend intermediate aluminium product, this is an intermediate forming step between the bending and hydroforming operation where the step from bending to directly hydroforming is to big;
• in the case of an AA6xxx-series aluminium alloy metal product an annealing or solution heat treatment of the preformed intermediate aluminium product followed by rapid cooling, preferably by means of water quenching, to below 50°C, and in the case of an AA5xxx-series aluminium alloy metal product it is preferred to preheat the product to a temperature in a range of 200 to 400°C.
• placing the intermediate metal product between a pair of hydroforming dies forming an interior space that matches the desired exterior cross sectional shape of said structural member;
• pressurising the interior of intermediate metal product so as to force it out into the interior space of said dies by the introduction of a heated pressure medium having a temperature in the range of 50 to 300°C, preferably in the range of 140 to 300°C, and more preferably in the range of 160 to 270°C.

In particular by annealing or solution heat treating and water quench the aluminium alloy prior to hydroforming improved forming characteristics are obtained allowing the manufacture of more complex hydroformed members. For an AA6xxx-series aluminium alloys such heat treatments are typically carried out by holding the intermediate product at a temperature in the range of 460 to 540°C for a period of up to 8 hours, preferably up to 3 hours, and more preferably in the range of 0.15 to 2 hours. The heating-up rate is typically in a range of 10 to 60°C/hour and a fast water quench cooling rate is applied. For an AA5xxx-series aluminium alloys such heat treatments are typically carried out by holding the intermediate product at a temperature in the range of 350 to 550°C for a period of up to 8 hours, preferably up to 3 hours, and more preferably in the range of 0.15 to 2 hours. The heating-up rate is typically in a range of 10 to 100°C/hour and a cooling rate in the range of 20 to 100°C/hour.

[0011] In a further aspect of the invention there is provided in a structural component or members for a vehicle obtained via the method of the invention as set forth above. A particular embodiment of such a structural component or member is the A-, B-, or C-pillar of a vehicle.

[0012] While the invention has been described in conjunction with the exemplary embodiments described above, many equivalent modifications and variations will be apparent to those skilled in the art when given this disclosure. Accordingly, the exemplary embodiments of the invention set forth above are considered to be illustrative and not limiting. Various changes to the described embodiments may be made without departing from the spirit and scope of the invention.

Claims

1. Method of manufacturing a shaped and hollow structural member via a hydroforming process, comprising the steps of subsequently:-

- providing an extruded or welded tubular or hollow intermediate metal product;

- placing the intermediate metal product in a die for hydroforming;

- hydroforming the intermediate metal product to a hydroformed structural member by the introduction of a heated pressure medium having a temperature in the range of 50 to 300°C.

2. Method according to claim 1, wherein the pressure medium is an oil.

3. Method according to any one of claims 1 to 2, wherein the heated pressure medium has a temperature in the range of 160 to 270°C.

4. Method according to any one of claims 1 to 3, wherein the intermediate metal product is of an aluminium alloy.

5. Method according to claim 4, where the aluminium alloy is an AA5xxx-series aluminium alloy.

6. Method according to claim 5, wherein the aluminium alloy in an AA5182 alloy or a modification thereof.

7. Method according to any one of claims 4 to 6, wherein the intermediate metal product is annealed prior to hydroforming.

8. Method according to any one of claims 1 to 7, wherein the intermediate metal product is pre-formed prior to placing it in a die for hydroforming.

9. Structural component for a vehicle obtained via the method in accordance with any one of claims 1 to 8.

10. Structural component according to claim 9, wherein the structural component is an A-, B-, or C-pillar of a vehicle.