Patent Application
20150315666
This document relates generally to the metal forming field and, more particularly, to a method for hydroforming a workpiece utilizing induction annealing to provide for enhanced formability.
Hydroforming is a term applied to sheet and tube forming in which the metal is formed against a die by fluid pressure. This may be done with an internal fluid pressure, with an applied axial load to a tube or with a one-sided die in which the sheet metal is formed by a bladder/diaphragm. Hydroforming typically uses conventional, single action hydraulic presses with high ram forces.
When using a multiple stage approach to hydroforming various bending/pre-forming steps may occur prior to the final hydroforming of the part. During these initial deformation stages most of the material formability can be exhausted leaving little plastic strain capability for the hydroforming process itself. One way to mitigate the influence of these localized strains is to apply a recovery heat treatment between stages of the hydroforming process. More specifically, induction annealing allows for local heating of a workpiece to a specified temperature in a specified time using an induction coil.
In one example, hydroformed aluminum tubes are used to form the A-pillar roof rail of a vehicle structure. This type of part may be made with structural or seamless extruded tubes. Structural tubes have better wall and diameter dimensional tolerances and are more efficient to extrude but have lower formability for bending, pre-forming, and hydroforming processes. As a consequence, structural tubes can only be used to form less challenging part shapes in hydroforming. Seamless tubes are less efficient to extrude relative to structural tubes due to scrap losses and seamless press cycle time limitations and have dimensional tolerances on wall and diameter that can be at least two time that of structural round tube. Seamless tubes can also have significantly higher formability which has made them the preferred material for hydroforming parts having complex, variable cross-sections such as A-pillar roof rails. In accordance with our work, it has now been discovered that induction annealing recovers sufficient workpiece formability to allow the less formable structural tubes in the production of A-pillar roof rails. Surprisingly, the formability may be recovered without compromising subsequent heat treatment to strengthen the roof rails.
In accordance with the purposes and benefits described herein, a method of hydroforming a workpiece is provided. That method includes the steps of bending the workpiece into a first preliminary shape, pre-forming the workpiece into a second preliminary shape, induction annealing the workpiece at a temperature between 120-160° C. and hydroforming the workpiece to a desired shape. The method may also include ramping to the induction annealing temperature in 10-30 seconds. Further the method may include completing the induction annealing between the pre-forming and the hydroforming steps. In an alternative embodiment the method includes completing the induction annealing between the pre-bending and the pre-forming. In yet another embodiment, the induction annealing is completed between the bending and preforming steps and between the pre-forming and hydroforming steps.
The method may further include trimming the workpiece to desired length. Further the method may include heat treating the workpiece after hydroforming in order to impart desired T6 strength properties. That heat treating may be completed at 160-200° C. for 4 to 10 hours.
The workpiece may be made from AA6XXX aluminum alloy such as AA6082-T4. Further the workpiece may be an A-pillar roof rail. Accordingly the method may include completing the induction annealing to the A-pillar roof rail at a first bend of an A-pillar portion of the workpiece. That induction annealing may be completed (a) at a temperature of 120-160° C. with a ramp of 20 to 30 seconds, (b) at a temperature of 135-145° C. with a ramp of 20 to 30 seconds or (c) at a temperature of about 140° C. with a ramp of 25 to 30 seconds.
Stated another way a method of hydroforming a workpiece comprises: (a) bending the workpiece into a first preliminary shape; (b) pre-forming the workpiece into a second preliminary shape; (c) induction annealing the workpiece at a temperature between 120-160° C. to improve total elongation of the workpiece while not compromising final strength properties of the workpiece; (d) hydroforming the workpiece to a desired shape; and (e) heat treating the workpiece at about 180° C. for about six hours to improve final strength properties of the workpiece. In one possible embodiment the method may include using an induction annealing temperature of about 140° C. with a ramp time of 20-30 seconds.
These and other embodiments of the present method will be set forth in the description which follows, and in part will become apparent to those of ordinary skill in the art by reference to the following description of the method and referenced drawings.
The accompanying drawings incorporated herein and forming a part of the specification, illustrate several aspects of the present method and together with the description serve to explain certain principles thereof. In the drawings:
a, 3b and 3c illustrate three alternative hydroforming production processes incorporating induction annealing: in the first the induction annealing takes place between the bending and pre-forming steps, in the second the induction annealing takes place between the pre-forming and hydroforming steps, while in the third the induction annealing takes place between the bending and preforming steps and between the pre-forming and hydroforming steps.
Reference will now be made in detail to the present preferred embodiments of the method.
This document relates to a method of hydroforming a workpiece W which incorporates an induction annealing step to enhance formability of the workpiece. In the embodiment illustrated in
In one embodiment, induction annealing is completed at a temperature of between 120-160° C. utilizing a 10-30 second ramp time. In another useful embodiment, induction annealing is completed at a temperature of 130-150° C. utilizing a ramp time of 20-30 seconds. In another useful embodiment, induction annealing is completed at a temperature of 135-145° C. utilizing a ramp time of 20-30 seconds. In yet another useful embodiment, induction annealing is completed at a temperature of about 140° C. utilizing a ramp time of 25-30 seconds.
A first embodiment of a method of hydroforming a workpiece W is illustrated in
This is then followed by the pre-forming of the workpiece W into a second preliminary shape (note step 18). This is then followed by hydroforming the workpiece W to a desired final shape (note step 20). Subsequent to hydroforming, the workpiece W is subjected to trimming to a desired length (note step 22). Following trimming the workpiece W is subjected to a heat treatment in order to impart desired strength properties to the workpiece W (note step 24). In the illustrated embodiment the heat treatment is a T6 treatment at 180° C. for six hours in order to induce or impart an average yield strength of typically 290 MPa to the workpiece W. In alternative embodiments the heat treatment may be completed at temperatures between 160-200° C. for 4 to 10 hours.
In an alternative embodiment of the production method illustrated in
In yet another alternative embodiment illustrated in
Any of the production method embodiments illustrated in
The data presented in
In summary, numerous benefits result from the method 10 of hydroforming of a workpiece W as disclosed herein. Advantageously the method supports high volume automotive manufacturing. Both structural and seamless tubes benefit from the method. In fact, structural tubes may now be readily used in the production of difficult-to-form A-pillar roof rails. Thus, the method allows for the use of a higher tolerance and more manufacturing efficient material for hydroforming roof rails.
As should be appreciated the induction annealing process is restricted to the heated region of interest only: that is, the area of bending where plastic strain capability has been reduced by the bending and/or pre-forming steps or stages 14, 18, 28, 30 of the production process. Heating of the tube is localized to the induction annealed region, therefore, there is no specialized equipment required for material handling of the workpiece W in the unheated regions. Further the induction annealing parameters required to restore formability to the workpiece W do not cause post-hydroformed material heat treatment response damage and the formed workpieces (in the illustrated embodiment, A-pillar roof rails), are still able to demonstrate the desired yield strengths.
The foregoing has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the embodiments to the precise form disclosed. Obvious modifications and variations are possible in light of the above teachings. For example, an incremental induction annealing step may be completed between both (a) the bending and pre-forming step and (b) the pre-forming and hydroforming steps if desired or at other times during production. All such modifications and variations are within the scope of the appended claims when interpreted in accordance with the breadth to which they are fairly, legally and equitably entitled.
