WO2001028708A1

WO2001028708A1 - Improved method for hydroforming an aluminum tubular blank

Info

Publication number: WO2001028708A1
Application number: PCT/CA2000/001181
Authority: WO
Inventors: Brian Corcoran; Richard Fleschner; Jeffrey Conklin
Original assignee: Cosma International Inc.
Priority date: 1999-10-15
Filing date: 2000-10-10
Publication date: 2001-04-26
Also published as: EP1220725A1; AR026043A1; CA2388263A1; AU7637400A

Abstract

A method of hydroforming a tubular aluminum alloy blank (T) into a desired configuration, including providing a tubular blank having opposing longitudinal end portions; rapidly heating the blank to an elevated temperature for a brief period of time, the values of the elevated temperature and the brief period of time being selected such that, after the rapid heating, the blank has a reduced hardness below an initial hardness without otherwise significantly changing the microstructure of the blank; quencing the blank while the blank is at or near the elevated temperature; disposing the quenched blank in a die cavity of a hydroforming die (102, 104), the hydroforming die having interior surfaces (106, 108) that provide the die cavity with a shape corresponding to the desired configuration of the tubular blank; sealing the opposing longitudinal end portions of the tubular blank; supplying pressurized fluid to the hollow interior of the tubular blank and relatively forcing the opposing end portions of the tubular blank inwardly toward one another such that the pressurized fluid diametrically expands the temporarily softened portions of the tubular blank such that the temporarily softened aluminum alloy of the blank is caused to flow longitudinally along the blank.

Description

IMPROVED METHOD FOR HYDROFORMING AN ALUMINUM TUBULAR BLANK

Field of the Invention The present invention relates to a method for hydroforming a tubular blank of an aluminum alloy. In particular, the present invention relates to rapidly heating the tubular aluminum alloy blank and then quenching the blank prior to hydroforming the same.

Background of the Invention

In conventional hydroforming of automotive parts and the like, a tubular blank of steel material is placed in a hydroforming die and pressurized with hydroforming fluid in order to expand the diameter of the blank until the shape of the expanded part conforms to the shape of the die cavity defined by the hydroforming die. In more recent years, hydroforming technology have been improved by utilizing higher pressures and by engaging opposite ends of the tubular blank and forcing the ends towards one another during the hydroforming operation in order to replenish/maintain the wall thickness of the blank during tube expansion. This permits fabrication of more complex, more robust, and more cost-effective parts. While hydroforming of steel parts has proven to be very effective and desirable for numerous applications, there has been a recent emphasis in the automotive industry in reducing vehicle weight. To this end, aluminum is becoming a material of choice.

Although hydroforming of aluminum might offer some of the same advantages obtained as with hydroforming of steel, it presents certain difficulties that do not exist with steel. Specifically, when hydroforming where relatively high yield strength is required, it would be desirable to perform the hydroforming process with a high yield strength aluminum alloy that has relatively high ductility to enhance the flow of material within the blank and to prevent rupturing of the blank within the hydroforming die. For automotive applications and other applications requiring high strength and rigidity, 6061 aluminum alloys having a temper of T5 or T6 are ideally suited, but have relatively low elongation properties at these tempers compared to steel. One way of ensuring that the aluminum alloy to be used in hydroforming has a sufficiently high ductility, would be having the blanks delivered fresh from the mill using 'just in time' techniques at or near zero temper just prior to undergoing the hydroforming process. As long as the blanks are delivered just prior to undergoing the hydroforming process, the aluminum alloy of the blanks will remain sufficiently ductile for the hydroforming operation. If there is an extended period of time between the blanks' delivery and the performance of the hydroforming process, the blanks will age-harden to a hardness at which the blanks cannot be hydroformed in a satisfactory manner. One possible alternative to using 'just in time' delivery techniques would be annealing the blanks in a furnace or oven at an elevated temperature. The resulting blanks would remain softened for some time and could then be hydroformed. This approach has two drawbacks. The first drawback is that annealing the blanks in a furnace consumes a significant amount of process equipment, energy, and time. In particular, conventional furnaces take a substantial amount of time to heat up to their operating temperature and must be kept at or near their operating temperature at all times during use to prevent the need for re-heating. The second drawback is that furnace annealed blanks typically need to be heat treated to regain their original hardness and strength after a hydroforming operation. This introduces an extra step into the process, along with its associated costs.

Summary of the Invention

Thus, it can be appreciated that there is a need for an improved process for hydroforming aluminum alloy that overcomes the problems associated with the techniques discussed above. To meet this need, the present invention provides a method for hydroforming a tubular aluminum alloy blank into a desired configuration. The method comprises providing a tubular blank made from age-hardenable aluminum alloy and then rapidly heating the blank to an elevated temperature that is high enough to temporarily soften said aluminum alloy below its initial hardness by inducing an electric current within the blank. The heated aluminum alloy blank is then quenched and thereafter disposed in the die cavity of a hydroforming die while its hardness remains lower than the initial hardness. The opposing longitudinal end portions of the tubular blank are then sealed and pressurized fluid is supplied to the hollow interior of the tubular blank. The pressurized fluid diametrically expands portions of the tubular blank against the interior surfaces defining the die cavity to provide the blank with the aforesaid desired configuration. The opposing end portions of the tubular blank arc moved inwardly toward one another such that the aluminum alloy of the blank is caused to flow longitudinally along the blank to thereby replenish the wall thickness of the portions of the blank being diametrically expanded. Then, the diametrically expanded aluminum alloy blank is age hardened, either naturally or artificially, so as to increase the hardness of the aluminum alloy blank to a hardness that is at or near the initial hardness.

Another aspect of the present invention provides a method for hydroforming a tubular aluminum alloy blank into a desired configuration. The method comprises providing a tubular blank made from age-hardenable aluminum alloy that has a plurality of Guinier-Preston (GP) zones distributed throughout its microstructure. The aluminum alloy blank is rapidly heated to an elevated temperature for a brief period of time. The values of the elevated temperature and the brief period of time are selected such that, after the rapid heating, the alloy is in a temporarily softened state wherein the GP zones in the alloy are substantially eliminated and any other precipitates that may be present in the alloy are either reduced or substantially eliminated without otherwise significantly changing the microstructure of the aluminum alloy. The heated aluminum alloy blank is then quenched while the blank is at or near the elevated temperature and the alloy is in its temporarily softened state. The blank is then disposed in the die cavity of a hydroforming die while the alloy is in its temporarily softened state and the opposing longitudinal ends the blank are sealed. Pressurized fluid is thereafter supplied to the hollow interior of the tubular blank and the opposing end portions of the tubular blank are moved inwardly toward one another such that the pressurized fluid diametrically expands portions of the tubular blank against the interior surfaces defining the die cavity to provide the blank with the aforesaid desired configuration and such that the aluminum alloy of the blank is caused to flow longitudinally along the blank to thereby replenish the wall thickness of the portions of the blank being diametrically expanded. The diametrically expanded aluminum alloy blank is then age hardened either naturally or artificially so as to increase the hardness of the aluminum alloy blank to a hardness that is at or near the initial hardness. The methods of the present invention are advantageous over furnace annealing methods for a number of reasons. First, rapidly heating the blank reduces the process time in comparison to furnace heating. Also, softening the alloy of the blank in accordance with the method of the present invention obviates the need for heat treating after the hydroforming piocess Instead, the alloy can simply age- harden back towards its initial hardness After agc-haidcning, a significant position of the alloy's initial hardness and strength will be restored

Brief Description of the Drawings

Figure 1 is a schematic top plan view that illustrates a high frequency induction heating coil being used to rapidly heat a tubular aluminum alloy blank in accordance with the principles of the present invention, Figure 2 is a schematic side elevated view that illustrates a quench πng spraying water onto the heated tubular aluminum alloy blank to cool the blank,

Figure 3 is a schematic top plan view that illustrates the cooled tubular blank disposed in a bending apparatus with a fixed mandrel of the apparatus inserted into one end of the tubular blank, a movable mandrel of the apparatus inserted into the other end of the tubular blank, and an intermediate portion of the tubular blank engaged with a fixed bending post,

Figure 4 is a cross-sectional view taken along line 4-4 of Figure 3 illustrating the concave portion of the fixed bending post that receives the tubular blank,

Figure 5 is a top plan view similar to Figure 3 with the movable mandrel being moved relative to the fixed mandrel along an arcuate path so as to bend the tubular blank around the fixed bending post,

Figure 6 is an elevated sectional view of a hydroforming apparatus with the tubular blank disposed inside the cavity of the hydroforming die,

Figure 7 is a view similar to Figure 6 showing the hydrauhcally dπven tube end engaging structures moved into sealing engagement with the opposing longitudinal ends of the tubular blank,

Figure 8 is a view similar to Figure 7 showing the substantially incompressible hydroforming fluid being supplied to the inteπor of the tubular blank through ports in the tube end engaging structures, Figure 9 is a view similar to Figure 8 showing the upper half of the hydroforming die lowered to its closed, operative position and the fluid mside the tubular blank being pressuπzed so as to diametπcally expand the tubular blank,

Figure 10 is a view similar to Figure 9 showing the tubular blank fully diametπcallv expanded into conformity with the inteπor surfaces of the die cavity and the tube end engaging sti ucturcs being moved inwardly so as to cause aluminum alloy flow and maintain the wall thickness of the portion of the blank being expanded,

Figure 1 1 is a view similar to Tigurc 10 showing the tube end engaging structures disengaged from the tubular blank and the hydroforming fluid drained from the tubular blank

Detailed Description of the Invention

Figure 1 shows an induction coil apparatus, generally indicated at 10 The apparatus 10 compπses a length of tubular copper pipe 12 arranged to form an induction coil 14 Each end of the tubular copper pipe 12 is connected to a control module, schematically indicated at 16, that supplies a flow of water through the pipe 12 and also supplies an a-c electπc current through the pipe 12 The control module 16 is shown schematically and is not intended to be limited to any particular arrangement for supplying fluid and current to the coil 14 An exemplary induction coil apparatus that may be used is disclosed in U S Patent No 4,766,664

A tubular aluminum alloy blank T that is to be hydroformed is disposed within the inteπor of the coil 14 The blank T may be supported in any conventional manner using a non-conductive support When the coil 14 is energized and an a-c electπc current is applied to the copper pipe 12, the coil 14 creates a magnetic field within the coil 14 This magnetic field induces a current within the blank T, which in turn rapidly heats the blank T to an elevated temperature Because the current induced in the blank T has nowhere to conduct due to the use of a non-conductive support, the energy of the induced current transforms into heat and rapidly heats the blank T The use of an induction coil is advantageous because the blank T is heated through its entire thickness rapidly, whereas with a conventional furnace the outer surface of the blank T would heat up first and the heat migrates slowly through thickness of the tube wall

It should be noted that the axial extent of the induction coil 14 is significantly less than the overall axial length of the tubular blank T In this arrangement the blank T is inserted into the coil 14 and advanced axially therethrough along its entire extent This allows a coil 14 of relatively short axial extent in compaπson to the blank's length to be used in the induction heating operation The speed at which the blank T is advanced is determined by the amount of time each portion of the blank T needs to be induction heated to achieve the desired softening As will be discussed later in the application, the time needed to soften each portion of the blank T is dependent on the temperature to which the coil 14 heats the portions of the blank T. Thus, the speed or rate at which the blank is advanced will depend on both time and temperature.

Alternatively, the coil 14 may have the same axial length as the blank T and no advancement of the blank T would be needed. However, this arrangement requires a relatively large coil to ensure that the longest blank T contemplated can be accommodated. Thus, the arrangement wherein the coil 14 has a relatively short axial extent is preferred because it allows for induction heating of blanks of varying lengths. During this rapid heating, the Guinier-Preston (GP) zones within the blank are substantially eliminated. These GP zones are clusters of the alloy's solute that are distributed throughout the blank. GP zones contribute to the hardness of the alloy and substantially eliminating them via the rapid application of heat reduces the hardness of the alloy to a temporarily softened state. In addition, there may be other precipitates present within the alloy, such a needle-shaped or spherical precipitates. These precipitates are also formed from the alloy's solute and will also be either substantially eliminated or reduced in size during the rapid heating, which also contributes to the softening of the alloy. The temperature and time period for the rapid heating are selected based on the specific composition and prior treatment of the alloy. Specifically, the time and temperature are selected so that the GP zones within the alloy are substantially eliminated and any precipitates that may be present in the alloy are either reduced or eliminated without otherwise significantly altering the microstructure of the alloy. In other words, the temperature is high enough to substantially eliminate the GP zones and to either substantially eliminate or reduce any other precipitates, but the period of time is short enough to prevent the remainder of the microstructure from being significantly effected.

The temperature to which the alloys is rapidly heated is preferably any temperature below the solutionizing temperature for that particular alloy. Time and temperature for this process are dependent on one another (specifically, they are inversely related) and thus it can be appreciated that these parameters may be varied to achieve the desired results. For example, as the temperature is increased, the period of time is decreased; and as the temperature is decreased, the period of time is increased. Thus, the present invention is not to be limited to any particular time period or temperature range. Preferably, the entire blank is heated However, tor applications where only a limited longitudinal extent of the blank T is to be diametrically expanded during hydroforming, it may be desirable to heat only those particular portions which arc expanded and undergo longitudinal material flow upon inward forcing of the opposite ends of the tube

The next step in the process of the present invention is to immediately quench the aluminum alloy blank T A water spraying device in the form of an annular quenching ring 20, shown in Fig 2, may be used for such quenching The ring 20 has a series of radially inwardly directed nozzles 22 that spray water (or any other suitable liquid) onto the blank T to cool it down to the ambient temperature As shown in Fig. 2, the quenching ring 20 is disposed adjacent the coil 14 so that the induction heated portions of the blank T enter directly into the quenching ring 20 as they exit the coil 14. As a result, the heating and quenching operations are performed in a single operation without the need for transporting the blank T from the coil 14 to a separate quenching device Alternatively, the blank T may be quenched in any other suitable manner, such as completely immersing the blank T in a vat of water or another suitable liquid to affect the quenching.

This quenching operation should take place as soon as possible after the rapid heating operation while the temperature of the blank T is at or near the elevated temperature to which it was heated duπng the induction heating. As a result of the quenching operation, the microstructure will be temporaπly "frozen" in the state to which it was brought to by the induction heating and the solute particles will be prevented from immediately coming back together to form GP zones and other precipitates, thus remaining in its softened state for a significant period of time before age-hardening. Also, the blank T will be cooled to the ambient temperature so that it can be manually handled. The operation of rapid heating and immediate quenching has been referred to in the art as retrogression heat treatment (RHT)

While the blank T is still in its softened state (i.e., before the GP zones and other solute precipitates have recrystallized), it is then moved to a bending apparatus, shown schematically at 50 in Figs. 3-5, to undergo a bending operation. This bending operation is optional and would not be performed on a blank that is to remain straight. However, for many hydroformed tubular blanks, such as those in the vehicle space frame shown in commonly owned International Patent Application of Jaekel et al., No. WO 99/20516, it is desirable to bend them pπor to the hydroforming operation. Because the blank I stays in its softened, ductile state for an extended period of time after the RI IT operation, the bending operation docs not have to be immediately completed after the softening operation The bending apparatus 50 includes a bending post 52, a fixed mandrel 54, and a movable mandrel 56 The fixed mandrel 54 is fixedly mounted to the floor and the movable mandrel 56 moves relative to the fixed mandrel along a set of tracks or a rail (not shown) under hydraulic power Of course, the two mandrels 54, 56 may both be movable, but it is typical to have one fixed and one movable so that only one set of tracks or rails is needed

Each of the mandrels 54, 56 has a rigid tube end insert 58, 60 that is sized to be received within one of the open ends of the tubular blank T To perform the bending operation, the blank T is mounted as shown in Fig 3 with the tube end inserts 58, 60 received in the respective tube ends and with the intermediate portion to be bent engaged with the bending post 52 As can be best seen in the cross-sectional view of Fig 4, the post 52 has a rigid base 62 with a generally disk-shaped head 64 The head 64 has an annular concave recess 66 extending around the circumference of the head 64 and in which the blank T is nestingly received duπng the bending operation This concave recess 66 prevents movement of the blank T out of its bending plane duπng the bending operation With the blank T mounted as aforesaid, the movable mandrel 56 is moved relative to the fixed mandrel 54 to the position shown in Fig 4 so as to bend the blank T about the post 52 The tube end inserts 58, 60 can then be withdrawn from the tube ends and the bent blank can then be moved onto the hydroforming operation

It should be noted that the relative positions of the bending post 52 and the mandrels 54, 56 is selected based on the length of the blank T and the position of the portion to be bent in relation to the tube ends In the illustrated embodiment, the blank T is being bent halfway between the tube ends and thus the post 52 is positioned halfway between the mandrels For bending the blank T at different points, the position of the post 52 may be changed, and for bending blanks of different lengths, the spacing between the mandrels 54, 56 may be changed Furthermore, for complex configurations, such as U-shaped members with two free legs and a straight beam extending therebetween, two posts 52 and two movable mandrels 56 would be used The positioning of the posts 52 would create the required bend at the two corners where the respective free legs and the straight beam meet One skilled in the art will appreciate the various shapes and geometries that can be created using such various bending techniques.

Figs. 6-1 1 illustrate the hydroforming operation that is performed on the tubular blank T and the apparatus 100 therefor. The blank T shown in Figs. 6-1 1 is shown as a straight blank T for clarity of illustration and may be either a straight blank, as shown, or a blank that has undergone the above-described bending operation. Thus, the present invention is not to be limited to the straight blank shown with respect to the hydroforming operation.

The hydroforming apparatus 100 may of any type of construction. The illustrated embodiment is shown for illustrative purposes only and is not intended to be limiting. The apparatus 100 includes upper and lower die halves 102, 104, each having interior surfaces 106, 108 that cooperate to form a die cavity 1 10 when the two die halves 102, 104 are closed. The lower die 104 is preferably fixed on a rigid base and the upper die 102 is attached to a hydraulic press (not shown) for reciprocating movement between its open and closed positions. The lower die half 104 has a drain passage 112 for draining hydroforming fluid after the hydroforming operation has been completed.

The hydroforming apparatus 100 also has a pair of hydraulic ram assemblies 1 14, 1 16 located on opposing sides of the dies 102, 104. Each ram assembly 1 14, 116 includes a sealed housing 118, 120, a hydraulically driven tube end engaging ram 122, 124 mounted for reciprocating movement within its respective housing 114, 116, and a pressure intensifying ram 126, 128 mounted for reciprocating movement within its respective housing 114, 1 16. Each housing 1 18, 120 has an opening formed through the front wall 130, 132 thereof and through which the tube end engaging ram 122, 124 extends. Each housing 118, 120 also includes a first fluid port 134, 136, a second fluid port 138, 140, and a third fluid port 142, 144.

The first port 134, 136 of each housing 118, 120 is fluidly communicated to a first control valve 146, 148 that controls the flow of pressurized hydraulic fluid for extending the tube end engaging ram 122, 124. Specifically, the first control valves 146, 148 control the flow of pressurized fluid to an annular space 150, 152 defined between the housing 118, 120, an annular rear flange 154, 156 of the ram 122, 124, and an annular rear flange 158, 160 of the intensifying ram 126, 128. As fluid is supplied to annular spaces 150, 152, the tube end engaging rams 122, 124 will be moved towards the dies 1 2, 1 4 relative to the housings 1 18, 120 and the pressure intensi fying rams 126, 12S.

The second port 138, 140 of each housing 1 I S, 120 is fluidly communicated to a second control valve 162, 1 4 that controls the flow of pressurized fluid for moving the pressure intensifying ram 126, 128. Specifically, each second control valve 162, 164 controls the flow of pressurized fluid to a rear surface of the pressure intensifying ram 126, 128 so as to force the pressure intensifying ram 126, 128 forwardly with respect to the housing 1 18, 120 and thereby create a space 166, 168 between the rear surface of the pressure intensifying ram 126, 128 and the rear wall of the housing 1 18, 120. Continuing to supply pressurized fluid to the space 166, 168 expands the space 166, 168 and drives the pressurizing ram 126, 128 forwardly relative to the tube end engaging ram 122, 124.

The third port 142, 144 of each housing 1 18, 120 is fluidly communicated to a third control valve 170, 172 that controls the flow of pressurized fluid for moving the tube end engaging rams 122, 124 and the pressure intensifying rams 126, 128 back to their original, retracted positions away from the dies 102, 104 and the tubular blank T. Specifically, each of the third control valves 170, 172 controls the flow of pressurized fluid to an annular space 174, 176 that is defined between the forward wall 130, 132 of the housing 1 18, 120 and the annular flange 154, 156 of the tube end engaging ram 122, 124. When it is desired to disengage the tube end engaging rams 122, 124 from the ends of the tubular blank T, the control valves 170, 172 are operated so as to supply pressurized fluid to the annular spaces 174, 176. This pressurized fluid forces the tube end engaging rams 122, 124 to move away from the tubular blank T and rearwardly relative to the housings 1 18, 120 and the pressure intensifying rams 126, 128. As the tube end engaging rams 122, 124 travel rearwardly, the annular flanges 154, 156 thereof engage forwardly facing annular shoulder surfaces 178, 180 on the pressure intensifying rams 126, 128 so as to move the pressure intensifying rams rearwardly along with the tube end engaging rams 122, 124 until the rear surfaces of the pressure intensifying rams engage the rear walls of the housings 1 18, 120. Each tube end engaging ram 122, 124 includes a tubular body 182, 184 with a nosepiece 186, 188 secured to the end of the body 182, 184 by a set of fasteners 190. Each tubular body 182, 184 has a fluid port 192, 194 that fluidly communicates to a fourth control valve 196, 198. The nosepieces 186, 188 each have a disk-shaped base plate 200, 202 with a cylindrical protrusion 204, 206 extending from the base plate 200, 202 A fluid passageway 208, 210 extends through the length of each nosepiece 186, 188 At the free end of the protrusion 204, 206 on each nosepiece 186, 1 88 is a cylindrical nipple mcmbci 212, 214 that is si/cd to be received with the end of the tubular blank T The nipple member 212, 214 of each nosepiece 186, 1 88 is of smaller diameter than its associated cylindrical protrusion 204, 206 so as to define a shoulder surface 216, 21 8 that engages the end of the tubular blank T during the hydroforming operation

As can be seen in the Figures, the tube end engaging rams 122, 124 are assembled within the respective housings 1 18, 120 with a cylindrical protrusion 220, 222 of the pressure intensifying ram 126, 128 shdably, seahngly received within the hollow interiors of the tube end engaging rams 122, 124 During operation, the pressure intensifying rams 126, 128 are moved relative to the tube end engaging rams 122, 124 to pressurize the fluid that is disposed inside the pressurizing cavities 228, 230 defined between the forwardly facing pressuπzing surfaces 224, 226 of protrusions 220, 222 and inteπor surfaces of rams 122, 124

To perform the hydroforming operation, the upper and lower dies 102, 104 are opened and the quenched, softened blank T is disposed in the die cavity, as shown in Fig 6 Then, as shown in Fig 7, the first control valve 146, 148 is opened to supply pressurized fluid to the annular spaces 150, 152 so as to move the tube end engaging rams 122, 124 into sealed engagement with the opposing longitudinal ends of the blank T It can be seen from Fig 7 that the shoulder surfaces 216, 218 provided on the nosepieces 186, 188 engage the ends of the blank's wall and the nipple members 212, 214 are disposed inside the blank T It is to be understood that a swaging or sizing operation may need to be performed on the ends of the blank T before loading the blank T into the die cavity to ensure a tight fit between the nosepieces 186, 188 and the ends of the blank T

Next, the first control valves 146, 148 are closed to prevent the pressuπzed fluid from flowing out from the annular spaces 150, 152 so as to maintain the tube end engaging rams 122, 124 m engagement with the ends of the blank T The fourth control valves 196, 198 are then opened to supply fluid to the pressuπzing cavities 228, 230 of the tube end engaging rams 122, 124 This fluid flows from the pressuπzing cavities 228, 230 to fill the inteπor of the blank T via passageways 208, 210, as shown in Fig 8 The tight, sealed engagement between the nosepieces 186, 188 and the ends of the blank T prevents fluid leakage The fourth control valves 196, 198 arc then closed to prevent fluid from flowing back out of the prcssuri/ing cavities 228, 230 during the following steps in the process. As shown in Figs. 9 and 1 , the second control valves 162, 164 arc then opened and pressurized fluid is communicated to the rear surfaces of the pressurizing rams 126, 128 via ports 138, 140 to move the intensifying rams 126, 128 relative to the tube end engaging rams 122, 124. As the intensifying rams 126, 128 are moved away from the rear wall of the housing 1 18, 120, spaces 166 and 168 arc fonncd and the second control valves 162, 164 continue supplying pressurized fluid to these spaces 166, 168. As a result of the intensifying rams 126, 128 being moved, two effects are realized. The first effect is that the pressurizing surfaces 226, 228 of the intensifying rams 126, 128 pressurizes the fluid in the pressuring cavities 228, 230 and the blank interior to a level sufficient to diametrically expand the blank T, which can be best seen by viewing Figs. 9 and 10 in sequence. This pressurizing is a result of the intensifying rams 126, 128 reducing the combined volume of the pressurizing cavities 228, 230 and the blank interior. As can be seen from Fig. 10, the fully hydroformed blank T is expanded into conformance with the interior surfaces of the dies 102, 104.

The second effect is that the rear flanges 158, 160 of the intensifying rams 126, 128 pressurize the annular spaces 150, 152 so as to cause the tube end engaging rams 122, 124 to be moved inwardly with respect to the blank T, as best seen in Fig. 10. Such inward movement of the tube end engaging rams 122, 124 moves the ends of the blank T inwardly toward one another such that the aluminum alloy of the blank T flows longitudinally along the blank T to replenish the thickness of the portions being diametrically expanded. The temporary softening of the alloy is especially advantageous during this portion of the process because it enhances the flow of the alloy that replenishes the wall thickness. It can be appreciated that without such replenishment, the thickness of the wall would decrease as its diameter expands, which results either in a weak finished product or a blank that ruptures within the hydroforming cavity. It is preferred that the blank ends be moved inwardly toward one another so as to maintain the wall thickness of the diametrically expanded portions within +/- 10% of its original wall thickness.

After the blank T has been diametrically expanded, the third control valves 170, 172 are opened and pressurized fluid is supplied to annular spaces 174, 176, as shown in Fig, 1 1. Supplying pressurized fluid to these spaces 174, 176 causes the tube end engaging rams 122, 124 to move towards their initial positions and out of engagement with the ends of the blank T. With the tube end engaging rams 122, 124 disengaged from the ends of the blank T, the fluid inside the blank T will drain therefrom and flow through drainage path 1 12. As the tube end engaging rams 122, 124 continue to retract, the rear flanges 154, 156 thereof engage the shoulder surfaces 178, 180 on the intensifying rams 126, 128 so that the intensifying rams 126, 128 continue to move back to their original positions along with the tube end engaging rams 122, 124. The dies 102, 104 are then moved apart and the hydroformed blank T is unloaded from the die cavity. The next step in the process is to age harden the hydroformed blank T. Such age hardening may be allowed to occur naturally by allowing the blank T to age harden at ambient temperature. Usually, natural age hardening takes four to five days to fully complete, although approximately 90% of the alloy's final hardness will normally be achieved within twenty four hours. Alternatively, the blank T may be artificially age hardened by heating it in an oven to an elevated temperature, usually within the range of 200 to 500 degrees Fahrenheit, for a period of time that is dependent on the particular alloy being used. Such artificial age hardening is well known in the art and need not be detailed herein. The advantages of artificial age hardening is that it is quicker than natural age hardening, but natural age hardening is more cost effective because it occurs naturally without the expense of the energy and equipment required to perform artificial age hardening. Either method of age hardening may be used depending on the particular needs and desires of the individual manufacturer.

As a result of the process performed in accordance with the principles of the present invention, the age hardenable blank T should fully recover the initial hardness and strength it had immediately prior to the heating operation. This hardness recovery takes place as a result of the solutes recrystalling to form GP zones and other precipitates. However, there may be only partial recovery of hardness, but such partial recovery should preferably be more than approximately 85% of the alloy's initial hardness. Thus, it can be appreciated that the method of the present invention allows the aluminum alloy blank to be easily hydroformed without rupturing and with little or no loss in hardness. The cost savings, hardness, and strength recovery associated with rapid heating and quenching followed by age hardening make the method of the present invention superior lo conventional furnace annealing operations

It will be realized that the foregoing preferred embodiment of the present invention has been shown and described in detail for the purposes of illustrating the functional and structural principles of the present invention is subject to change without departure from such principles Therefore, the present invention includes all modifications encompassed within the scope of the appended claims.

Claims

WHAT IS CLAIMED:

1 . A method for hydroforming a tubular aluminum alloy blank into a desired configuration, comprising the steps of: providing a tubular blank made from aluminum alloy having an initial hardness; applying a retrogression heat treatment to said tubular blank to temporarily soften the tubular blank to a reduced hardness below said initial hardness; hydroforming said tubular blank to said desired configuration while said alloy blank is in said temporarily softened state; and age-hardening said hydroformed tubular blank.

2. The method according to claim 1 wherein said step of applying a retrogression heat treatment comprises steps of: rapidly heating said tubular blank to an elevated temperature for a brief period of time, the values of said elevated temperature and said brief period of time being selected such that, after said rapid heating, said aluminum alloy is in said temporarily softened state wherein GP zones in said alloy are substantially eliminated and any other precipitates that may be present in said alloy are either reduced or substantially eliminated to provide said alloy with said reduced hardness without otherwise significantly changing a microstructure of said aluminum alloy; and quenching said heated tubular blank.

3. A method according to claim 2, wherein said tubular blank is rapidly heated by inducing an electric current in said tubular blank.

4. A method according to claim 3, wherein the electric current is caused to flow in said tubular blank by disposing said tubular blank within an induction coil and energizing said coil so as to induce the aforesaid electric current in said tubular blank.

5. A method according to claim 4 wherein said step of hydroforming comprises: disposing said quenched blank in a die cavity of a hydroforming die while said tubular blank is in said temporarily softened state, said hydroforming die having interior surfaces that provide said die cavity with a shape corresponding to the desired configuration of said tubular blank; sealing said tubular blanks al opposing longitudinal end portions thereof; supplying pressurized fluid to a hollow interior of said tubular blank and relatively forcing the opposing end portions of said tubular blank inwardly toward one another while said opposing end portions of said tubular blank arc scaled and said alloy is in said temporarily softened state such that said pressurized fluid diametrically expands said tubular blank against the interior surfaces defining said die cavity to provide said blank with the aforesaid desired configuration and such that the tubular blank is caused to flow longitudinally to thereby replenish the wall thickness of the portions of said tubular blank being diametrically expanded.

6. A method according to claim 5, further comprising: after said quenching step and before disposing said tubular blank in said die cavity, bending said tubular blank.

7. A method according to claim 6 wherein said step of age hardening comprises a step of allowing the tubular blank to remain at ambient temperatures until the tubular blank has a hardness of about 85% of the initial hardness.

8. A method according to claim 6 wherein said step of age hardening comprises a step of heating the tubular blank to a temperature in a range of 200 to 500° F until the tubular blank has a hardness of at least 85% of the initial hardness.

9. A method for hydroforming a hollow aluminum alloy blank into a desired configuration, comprising: providing a hollow blank made from age-hardenable aluminum alloy and having opposing longitudinal end portions, said aluminum alloy blank having an initial hardness; rapidly heating said aluminum alloy blank to an elevated temperature that is high enough to temporarily soften said aluminum alloy below said initial hardness by inducing an electric current in said aluminum alloy blank; quenching said heated aluminum alloy blank while said aluminum alloy blank is at or near said elevated temperature; disposing said aluminum alloy blank in a die cavity of a hydroforming die while the hardness of said aluminum alloy blank remains lower than said initial hardness, said hydroforming die having interior surfaces that provide said die cavity with a shape corresponding to the desired configuration of said tubular blank; sealing the opposing longitudinal end portions of said aluminum alloy blank while the hardness of said aluminum alloy blank remains lower than said initial hardness; supplying pressurized fluid to the hollow interior of said aluminum alloy blank and relatively forcing the opposing end portions of said aluminum alloy blank inwardly toward one another while said opposing end portions of said aluminum alloy blank are sealed and the hardness of said aluminum alloy blank remains lower than said initial hardness such that said pressurized fluid diametrically expands temporarily softened portions of said aluminum alloy blank against the interior surfaces defining said die cavity to provide said aluminum alloy blank with the aforesaid desired configuration and such that the temporarily softened aluminum alloy of said aluminum alloy blank is caused to flow longitudinally along said blank as a result of said end portions being relatively moved inwardly toward one another to thereby replenish the wall thickness of the portions of said aluminum alloy blank being diametrically expanded; and age-hardening said diametrically expanded aluminum alloy blank so as to increase the hardness of said aluminum alloy blank to a level approximating said initial hardness.

10. A method according to claim 9, wherein the electric current is caused to flow in said aluminum alloy blank by disposing said aluminum alloy blank within an induction coil and energizing said coil so as to induce the aforesaid electric current in said aluminum alloy blank.

1 1. A method according to claim 9, further comprising: after said quenching step and before disposing said tubular blank in said die cavity, bending said tubular blank. 12 A method according to claim 9 wherein said step of age hardening comprises a step of allowing the tubular blank to remain at ambient tempci aturcs until the tubular blank has a hardness of about 85% of the initial hardness

13 A method according to claim 9 wherein said step of age hardening comprises a step of heating the tubular blank to a temperature in a range of 200 to 500° F until the tubular blank has a hardness of at least 85% of the initial hardness