WO2001066991A1

WO2001066991A1 - Locally reinforced hollow structures and method for producing same

Info

Publication number: WO2001066991A1
Application number: PCT/US2000/005802
Authority: WO
Inventors: Stefan Wolfgang Heinemann; Terry Dennis Claar
Original assignee: Fraunhofer USA Inc
Current assignee: Fraunhofer USA Inc
Priority date: 2000-03-06
Filing date: 2000-03-06
Publication date: 2001-09-13
Anticipated expiration: 2002-09-06
Also published as: AU2000238686A1

Abstract

A reinforced tubular structure is manufactured by tacking a metal foam precursor (12) into a tubular member (14) and activating the foaming agent to reinforce a desired segment of the tube. The precursor may also be placed prior to the roll forming of the tube. Alternatively, a molded-to-size pre-foamed metal foam reinforcement may be inserted into place.

Description

LOCALLY REINFORCED HOLLOW STRUCTURES AND METHOD FOR 1'ROϋlJC INC SAME

BACKGROUND OF THE INVENTION

The present invention relates to custom designed structural tubing, also referred to as

tailored tubes. Tubing and thin-walled hollow structures have been recognized as cost-efficient

and weight-efficient structural weight-bearing construction materials. To improve the load

bearing and damping capabilities, or durability of structural tubing, it has been found

advantageous to reinforce or thicken certain sections of structural tubing. This type of modified

tubing is referred to as tailored or custom tubing.

Tailored tubes are made from tubes with different properties, wall thickness and/or

different diameter, in a similar manner as tailored blanks, in which two blanks of different

thickness and properties are welded together. The current state of tailored tube technology is to

use at least two different tubes with the desired properties and sizes which are butt welded

together along the orbital seam as shown in Figure 1. For example, a substantially thicker tube

can be used at the location oi^' a bend or joint and the thinner tubing welded to each end. This

allows the heavier and more expensive tubing to be utilized at the point of greater stress

(particularly non-axial loading), and the less expensive and lighter tubing to be used elsewhere,

reducing both the cost and weight of the structure. Another approach is to put a sleeve around

the tube where reinforcement is needed and which is welded to the main tube as shown in figure

2. This similarly adds material only to the areas where reinforcement is required. These

straightforward approaches are relatively simple methods for tailoring the properties of the finished part, but they also lequπ e processes which tan deliver precise edge qualm and tight

tolerances in creating the l einlorcing pieces to accomplish high-quality welds Foi example, the

butt welding of tubes of \ anous diameters lequnes lixtutmg and space to preciseh position,

clamp and lotate the tubing Also, ty picallv a bulky stiuctuie is l equircd loi supporting a

precision driven laser, as well as sufficient room to load and unload the tubing, and room for any

additional piocessing Plant space has become an increasing portion ol the υ\ eιhead cost

involv ed in manufacturing, as efficiency in manufactuiing processes has improved, thus, it is

preleπed to _leduce or eliminate such space-consuirung operations wherever possible Further,

the equipment to perfoim the necessary orbital welding is expensive and the process is relativel^y

time consuming Further still, subsequent forming typicallv tequires smoothing of the edges

lormed by the reinforcing mateiial. which olten causes problems due to the abrupt 01 step-like

change ol wall thickness at the joint 01 sleeve I his discontinuity acts as a stress user and

impacts the material flow during forming and thus requires special dies and forming

technologies Further still, the material around the seam, in the heat-affected zone, typically

displays a hardness different from the remaining bulk tube material, which also has a negative

impact on the foimabihty of the reinforced area

The reinforcing material used in the present invention is a metal foam which has recently

become commercially available in various shapes, based on a powder metallui gy process

developed and patented by the Assignee of this invention Typical automotiv e uses of these

metal foams are to insulate and stiffen automotive panels, such as the fiont liie all. the rear

luggage compartment panel, and floor panels These foams are also used currently for weight

reduction and increased stiffening in some convertible bodies Metal foams aie typically used to

greatest advantage when used in combination with othei matenals of constiuction Tor example. it has been demonstrated that aluminum foam sandwich panels used as firewalls can provide 8

times the torsional stiffness of a conventional stamped steel part with a 50% reduction in weight.

The unique compression behavior of aluminum metal foams - in which the foams absorbed high

amounts of deformation energy at nearly constant and low mechanical stresses - makes them

very attractive material to absorb crush energy to improve vehicle crashworthiness. Because of

the high amount of controlled porosity, metal foams arc not generally used as stand-alone

materials in components requiring high tensile strengths.

SUMMARY OF THE INVENTION

The present invention relates to a new structurally reinforced tubing and a method of

making same. The invention is cost effective and structurally superior as it allows tor tubing to

be easily reinforced, thus reducing or eliminating the need for multiple sizes of tubing^,

expensive manufacturing floor space, and time consuming processing.

The invention involves incorporating a metal foam insert within the tubing at critical

loading areas where structural reinforcement is desired. In one embodiment, the metal foam is

activated inside the hollow tube, causing the metal foam to expand in volume and thus fill the

interior of the tube. This provides substantial stiffening, which may be particularly necessary in

areas such as bends in the tubing or at joints which result in non-axial, torsional. or bending

loading of the tubing. In another embodiment to produce ready-to-use custom tubing, the metal

foam can be activated inside a shaped tool cavity to form a molded foam insert^, which is

subsequently inserted into the tubing.

This new method of reinforcing structural tubing takes advantage of the newly emerging

lightwciuht metal foam materials technology. All sections requiring higher sti ffness, increased damping, increased torsional strength, increased bending strength or increased crashworthiness

can be internally reinforced using metal foam. Further, the resultant reinforced tubing is

lightweight and has an aesthetically pleasing uniform outer diameter.

This innovative technology does not have the aforementioned drawbacks of the state-of-

the art and combines tailoring of the mechanical properties of the finished part with easy

formability. uniform exterior profile and low production costs.

BRIEF DESCRIPTION OF THE DRAWINGS

Figures 1 and 1 Λ illustrate tailored tubes made according to the known art . showing

tubes with different wall thickness butt welded together.

Figure 2 shows another tailored tube according to the known art. having a reinforcing

sleeve around a base tube.

Figure 3 shows a tube according to the claimed invention with metal foam precursor

inserts.

Figure 4 shows the tube of Figure 3 inserted after tacking and forming of the tube.

Figure 5 shows the tube of Figure 4 after the metal foam is activated.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The present invention allows the use of a single tube for shaping into a finished part that

is locally reinforced to meet the varying structural requirements of the finished part. A typical

tube reinforced with metal foam in specific sections according to the present invention is shown

in Figures 3-5. illustrating the placement of the foam preform, the forming of the tube and the

activation of the foam to fill a section of the tube. The two essential steps of the present invention are to position a foam '"preform" 12 at a

desired position within the tube 14. and to activate the preform to fill the tube and thus reinforce

the desired area. To allow for reinforcement of tubes or hollow shapes of varying cross-sectional

geometry, a metal foam precursor is inserted in the tubing before activation and before the

tubing is formed into its desired structural shape (see Fig. 4). This allows the tubing with

inserted metal preform to be formed with conventional methods, such as bending or

hydroforming. After the tubing has been formed into the desired geometry, the foam can be

activated, typically by temperature. Upon thermal activation, the metal foam preform typically

expands by a factor of about 400% in volume to fill the tube interior.

The foam preforms can be cut to size to allow proper filling of the tubular section for

achieving the designed stiffness. The stiffness to be achieved will be designed by tailoring the

material, the mechanical properties and the amount of metal foam put into the specific section.

Metal foam is commercially available in a wide variety of different materials and characteristics.

Aluminum foams can be produced in essentially any alloy composition or commercial grade for

which metal powders are available. For example, aluminum foams have been produced from Al-

Si. Al-Mg. Al-Cu, AA6061. ΛA506. and AA7093 alloys, among other. Foams can be fabricated

to have porosity levels over the broad range 40 - 90 vol% porosity. Cell sizes typically vary

over the range of 1 to 10 mm. A widely used metal foam alloy is Al-Si which metallurgically

bonds to the inner surface of AA6061 tubes during the in-situ foaming activation step. Other

foams in aluminum, steel, zinc, and other alloys are also available. It is contemplated that other

foam compositions, such as aluminum alloys, steels, ceramic particle-reinforced aluminum metal

matrix composites, copper alloys, zinc alloys, titanium, intermetallics. could also be utilized as

the foamiim reinforcement. The foamable piecursor is produced in extruded lengths containing a foaming agent that

is activ ated bv heating to elev ated temperature The precuisoi pietoims can be extruded in

v arious cioss-sectional shapes and sizes foi the specific component design T 01 example, the

preform can be tapered to increase the amount ot material toward the centei of the tube or could

be cuived to bettei fit within the tube

The pielorms are sold in sheets of desned thickness which can be cut to size depending on the

expansion characteristics ot the foam used and the size tube filled These foam pretoim pieces

are inserted into the tube and tacked into position Trom the outside of the tube using high power

laser beams (as shown in Figure 4) Another preferred approach for positioning the foam

preform is to set the ptelorm in place upon the fiat tube blank prior to lorming the blank into a

tube In this case the inserts would be placed and tacked into position belore the tube is closed

by lollers and sealed by longitudinal welding, typically by means of a lasei beam This allows

veiy cost-effective production due to minimized pait handling effoit and very high production

lates of the tube manufacturing process The preform can also be positioned with a spot weld or

adhesiv e or any other known positioning means It should be appreciated that the tacking

operation is not a precision process, as the tacking need only be sufficient to hold the foam

preform pieces in place duiing the tube forming, shipment and assemblv The tacking is not a

stiuctuial bond and need not be precisely located, and could also be perfoimed by brazing,

soldering or adhesive bonding

After the preform or preforms have been fixed in position, the tube can be formed or bent

into the desired structural shape through anv ot the various know industiial methods, such as a

conventional hvdrofoiming piocess Plastic or composite tubmg can be foimed utilizing known

heating and bending processes l o assuie that the material How ol the lube dui ing loiming is not at all impacted by the inserted preform, the volume of individual foam preforms is preferably

kept relatively small and the tacking is located toward the center of the preform to minimize

stress on or exerted by the preform. Thus, it is preferred to use multiple small preforms that each

fill only a fraction of the cross-sectional area or a fraction of the intended length of reinforcement

at any point, rather than one large preform. Once activated, the metal foam generated from these

multiple preforms will bond into an homogenous foam reinforcement. Further, tacking

performed toward the center of the preform will allow the ends of the preform to separate from

the tube as the tube is bent. Further still, it should be appreciated that the preforms are preferably

affixed to the portion of the tubing that will form the inner radius of a bend or curve as shown in

Figure 4 to enable this separation as opposed to the deformation that would occur to the preform.

Another approach is to have extruded preforms of a tapered or keystone shape which would

allow for tube bending without effecting the preform. Since the outer surface of the tube is

smooth, no special dies or technologies need to be developed for the hydroforming process.

In order to position the preform of foamable precursor material in existing tubing, it is

anticipated that any type of pusher rod or sufficiently stiffened wire would suffice, with the

distance the preform is to be inserted to be measured on the pusher rod or wire. A disk of

circumference slightly less than the inner diameter of the tube can be included at the end of the

pusher rod or wire to facilitate insertion. In this type of application, the preform can be attached

to one or more soft foam disks of circumference slightly greater than the inner diameter of the

tubing, which creates an interference fit within the tubing sufficient to hold the preform in place

until the preform is activated, but which allows easy insertion and movement of the preform

within the tube. These soft foam disks would merely effect the necessary temporary positioning

of the preform and would have no structural effect. Another method of positioning the foam precursor is to tack the foam inserts into a

position prior to roll forming of the tube. In this case the inserts would be placed and tacked

into position before the tube is closed by the rollers and sealed by longitudinal welding, typically

by means of a laser beam. This allows very cost-effective production due to minimized part

handling effort and very high production rates of the tube manufacturing process.

After the tube with the foam precursor inserts has undergone forming (such as

hydroforming) the tube will be heated at an elevated temperature (of about 650-700 degrees

Celsius for aluminum foam) with a laser or other means to cause the foam inserts to expand and

to fill the tube in a section where it is needed. The foam can also be activated with microwaves,

chemically or through other activation means known in the art. The composition, density, cell

size, and resulting mechanical properties of foam material utilized in the tailored tubes

determine the final mechanical properties of the finished component in the sections of the

shape. It is anticipated that this process can be used for tubing of various materials, such as

steels, aluminum alloys, aluminum metal matrix composites, high temperature superalloys.

titanium, and copper alloys. Of course, there is an advantage to choose a foam that will bond to

the tubing interior, which will significantly improve the strength and stiffness of the tubing at the

reinforcement, and thus the preferred choice of metal foam will vary with the type of tubing

materials. ^'To increase this bonding, it is preferred with certain tubing to pre-tieat the tube

interior as is known in the art.

It may be preferable in certain applications to provide metal foam reinforcing inserts

activated prior to insertion. Due to the stiffness of the activated foam, this approach is generally

limited to linear tubing segments. However, if it is desired to reinforce joints such as ^' "

sections, or to reinforce a tube before forming a bend, and the activation step is preferably eliminated, an activated "slug^" of metal foam can be inserted to the structural tube. This slug can

be manufactured by placing the desired amount of foamable preform material into a mold having

an inner diameter the same as the specified tubing and activating the foam. Although the foam

reinforcement will not bond with the tubing when inserted, due to the tight tolerances possible

with the foam, any deformation of the tube under loading (or forming) will clamp the slug in

place. It is also contemplated that a second activation could be achieved once the slug is in

place, whether by completing an incomplete reaction from the molding phase or by performing a

second independent reaction utilizing a secondary foaming agent.

The resulting reinforced tubing can be used in a variety of structural applications. For

custom structures, the tubing can be delivered in straight pieces of stock lengths with one or

more preforms unactivated. so that the customer can form the desired shape and activate the

necessary preforms to reinforce joints or bends and cut the tubes to length.

As can be appreciated from the disclosure, there is a wide variety of anticipated

applications and embodiments of the present invention.

While the best modes for carrying out the invention have been described in detail, those

familiar with the art to which the invention relates will recognize various alternative designs and

embodiments for practicing the invention as defined by the following claims.

Claims

WHAT IS CLAIMED IS:

1. A method to reinforce sections of a hollow structure comprising;

inserting into the hollow structure at least one preform of foamable precursor material

containing a foaming agent, said preform expanding in volume when said foaming agent is

activated;

positioning and holding said preform in place; and

activating the foaming agent to cause said foam to expand and substantially fill said

section of the hollow structure.

2. The method according to claim 1 where the hollow structure is a tube having a constant

wall thickness and outer diameter.

3. The method according to claim 1 wherein said holding is performed by using a laser

beam to tack said foam preform in position.

4. The method according to claim 1 wherein said hollow structure is a metal tube.

5. The method according to claim 1 further comprising the step of forming said hollow

structure containing said foam preform.

6. The method according to claim 1 further comprising the step of forming said hollow

structure after activating said foaming agent.

7. The method according to claim 1 wherein a plurality of preforms are positioned and held

into place in spaced apart relation to each other.

8. The method of claim 7 wherein said plurality of preforms are circumferentially spaced.

9. The method of claim 7 wherein said plurality of preforms are linearly.

10. The method according to claim 1 wherein said foaming agent is chemically activated.

1 1. The method according to claim 1 wherein the step of activating the foaming agent is

performed by applying heat to said hollow structure.

12. The method according to claim 1 wherein said foam preform is held into position on a

flat strip and said strip is formed into said hollow structure.

13. The method according to claim 12 wherein said hollow structure is further formed into a

non-linear structural component having at least one bend having an inner radius and an

outer radius.

14. The method according to claim 13 wherein said preform is held on a portion of said strip

which forms a segment of said inner radius of said bend.

I I

15. A hollow, elongated structural support comprising a shell of uniform construction and a

metal foam reinforcement at at least one section of said shell, said metal foam

substantially filling said hollow support at said section.

16. The structural support of claim 15 wherein said shell is of relatively uniform cross- section.

17. The structural support of claim 16 wherein said shell comprises tubing.

18. The structural support of claim 15 wherein said section is non-linear.

19. A hollow, non-linear, elongated structural support comprising a uniform shell material

and a metal foam precursor material located at at least one section.said metal foam

precursor material being foamable to expand to substantially fill said hollow support at

said section.

20. The structural support of claim 17 wherein said shell is of relatively uniform cross-

section.

21. The structural support of claim 20 wherein said shell comprises tubing.

22. The structural support of claim 19 wherein said section is non-linear.

23. The structural support of claim 22 wherein said metal cursor foam precursor material is

located on the radially inward portion of said non-linear section.

24. The structural support of claim 19 wherein said precursor material has a tapered cross-

section.

25. A generally cylindrical activated metal foam reinforcing insert for structural tubing.

26. The insert of claim 25 having an outer diameter substantially the same as the inner

diameter of said structural tubing.

27. A reinforcement for a tubular structure comprising:

at least one foam backer forming a compression fit within said tubular structure, and

a foamable precursor material containing a foaming agent, said foamable precursor

material substantially filling a section of said tubular structure when said foaming agent is

activated.

28. The reinforcement of claim 27 wherein said foam backer is disk shaped.

29. The reinforcement of claim 28 wherein said foam precursor material is affixed between a

pair of said foam backers.