Stretch shaping method and apparatus

A method for stretch shaping an elongated metallic extrusion is disclosed comprising the steps of applying axial tension to an elongated metallic extrusion in an amount sufficient to equal or exceed the yield strength of the extrusion. An external shaping die is applied against a perimetric portion of the outside surface of the extrusion. The die has working faces conforming substantially to a finished cross-sectional shape for the extrusion. While maintaining such tension the die is advanced along a length of the extrusion. An apparatus for stretch shaping an elongated metallic extrusion is also disclosed.

BACKGROUND OF THE INVENTION 
1. Field of Invention 
This invention pertains to a method and an apparatus for shaping an 
elongated hollow or solid article, and more particularly to a method and 
apparatus for performing consistent, accurate spatial dimensional shaping 
of an elongated extruded product. 
2. Description of the Art 
There are a considerable number of operating parameters and conditions 
present in various metal forming methods which cause finished products to 
exhibit dimensional variability. In certain processes the dimensional 
variability is acceptable, while in other processes the dimensional 
variability is unacceptable and requires subsequent metal finishing 
operations. 
In the extrusion process, for example, a heated ingot or billet is forced 
to flow under pressure through a die opening to form an elongated article 
such as a channel, a tube or an angle. In a typical aluminum extrusion 
process the extruded product is forced through the die at forces in the 
500 to 15,000 ton range. The extrusion exits the die of an extrusion press 
at elevated temperatures on the order of 300.degree. to 1200.degree. F. It 
is common to solution heat treat and quench the extruded product in an 
in-line solution heat treating process or by a separate solution heat 
treatment process. Such extruded product may be made to various lengths, 
including lengths in excess of 150 feet, and may be of diverse 
cross-sectional configuration. 
Considering the operating parameters of the extrusion process including 
pressures, temperatures, die condition and product length, and considering 
the effects of subsequent heat treatment and quenching, it is 
understandable that extruded metal products may exhibit considerable 
dimensional variation about the cross-section and over the length of the 
product. It is also understandable that such dimensional variation may be 
present from product cycle to product cycle and from extrusion run to 
extrusion run. It is therefore often necessary to perform subsequent metal 
finishing operations to bring the product within acceptable dimensional 
tolerance. There are some dimensional variations on extruded metal 
products which are not readily correctable by conventional metal finishing 
operations, including bending, roll straightening and hammering. In such 
conventional metal finishing operations, springback is a major concern. 
Such springback may be so extreme, especially in products with substantial 
dimensional variation, that such conventional metal finishing operations 
are inadequate. 
Prior shaping methods and apparatus have provided methods to finish the 
shape of articles, such as extrusions. The tolerances currently 
permissible for such products, as published by the Aluminum Association, 
particularly for thin walled extrusions, are so broad that the products 
may be precluded from certain critical applications. If the dimensional 
deviation could be reduced, the products may be applicable in an increased 
number of applications where dimension is important. Furthermore, the 
dimensional quality of the product in existing applications could be 
dramatically increased. 
Despite prior art attempts to improve the dimensional tolerance and 
minimize dimensional variation in a finishing operation, there is a need 
for further improvement. Accordingly, a stretch shaping method and 
apparatus are desired which results in finish shaping an elongated 
article, such as an extrusion, to minimize cross-sectional and 
longitudinal dimensional deviations from nominal value. 
SUMMARY OF THE INVENTION 
This invention may be summarized as providing an improved method and 
apparatus for finish shaping an elongated metallic extrusion. The method 
comprises the steps of applying axial tension to an elongated metallic 
extrusion in an amount sufficient to exceed the yield strength of the 
extrusion. An external shaping die is applied against a perimetric portion 
of the outside surface of the extrusion. The die has working faces 
conforming approximately to a finished cross-sectional shape for the 
extrusion. While maintaining tension, the die is advanced along a length 
of the extrusion. In a preferred embodiment an internal shaping die may be 
utilized, in conjunction with an external shaping die, against the inside 
surfaces of an extrusion. 
Among the advantages of the present invention is the provision of a method 
for shaping an elongated metallic extrusion with minimal dimensional 
variability. 
Another advantage of the present invention is the provision of a method and 
an apparatus for performing consistent spatial dimensional corrections to 
an extruded product in a process which involves minimal, if any, 
springback. 
An objective of this invention is to provide a final shaping method which 
can be readily employed in-line with an extrusion process. 
A feature of the method of this invention is that an extruded product is 
shaped to finished dimension without inducing significant residual 
stresses in the product. 
Another feature of this invention is that extruded aluminum product can be 
shaped to a dimensional tolerance better than the dimensional tolerance 
currently accepted by the Aluminum Association and within the tighter 
dimensional tolerance currently accepted European aluminum standards. 
Another advantage of this invention is the production of extrusions within 
previously unattainable tolerance, which permits the use of extrusions in 
new, dimensionally critical applications. 
These and other objectives, features and advantages of the invention will 
be more thoroughly understood and appreciated with reference to the 
following description and the accompanying drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The present invention is directed to a method and apparatus for shaping 
elongated products, such as extrusions, into final dimension with close 
dimensional tolerance. Because of the number and the complexity of the 
various operating parameters for extruded product, including quenching, 
extrusions are typically characterized by wide dimensional variability. 
Such dimensional variability is due to a lack of consistent control of the 
extrusion and quenching process, tooling design and maintenance, and 
thermal distortion. Prior reworking processes to correct the dimensional 
variation were costly and inefficient. The present invention overcomes 
those deficiencies by providing a method for consistently correcting the 
axial and cross-sectional dimensional variation of straight length, 
elongated extrusions. 
Straight length extrusions include both complicated and simple shapes, and 
include complex hollow to simple solid structures. FIG. 1 illustrates a 
relatively simple four walled hollow extrusion 10. FIG. 3 illustrates a 
relatively simple solid (open) angle extrusion 30. Preferred extrusions of 
the present invention include, but are not limited to, thin walled 
extrusions, i.e., those having a wall thickness of less than about 4 mm, 
which typically exhibit more distortion during extrusion and quenching 
than thick wall extrusions. Such preferred extrusions include highly 
ductile extrusions, such as 6XXX series aluminum alloys, and harder 
aluminum alloys in the 2XXX and 7XXX series, as well as aluminum-lithium 
alloys. Extrusions which are preferred for applications in the automobile 
and aircraft industries and may be stretch shaped by the process of the 
present invention include, but are not limited to, 2024, 6061, 6063, 6009 
and 7075 aluminum alloys. 
In the process of the present invention an elongated extrusion is stretch 
shaped to final dimension. The starting workpiece is the extrusion 
typically after the product has been solution heat treated. Such 
extrusions are elongated, and may extend to lengths which exceed as much 
as 150 feet in length. Longer extrusions minimize the end scrap losses as 
a percentage of total finished product and are therefore desirable. An 
extrusion in the extruded and quenched condition may exhibit a warp, bow, 
wave, bulge or, as shown in FIG. 1, an out-of-dimension distorted 
cross-sectional condition as a result of the variables in the extrusion 
and quenching process. It is understandable that in addition to the 
out-of-dimension cross-sectional configuration, as illustrated in FIG. 1, 
the extrusion may exhibit bow, waves or twist along the length thereof. 
Such extrusions must first be put into axial, or longitudinal, tension in 
the process of the present invention. In the present invention an 
elongated extrusion is transferred to a stretch shaping apparatus. The 
extrusion is gripped, typically at longitudinal end portions of the 
extrusion by an appropriate gripper mechanism, such as stationary gripper 
70 and adjustable gripper 72 shown in FIG. 6. It should be understood that 
both grippers may be adjustable in the present invention. In a preferred 
embodiment the engaging faces of the gripper mechanism match the contour 
and shape of the extrusion to enhance the grip. Such gripping devices are 
called custom grippers and tend to minimize or eliminate adverse end 
effects by insuring that substantially uniform tension is applied over the 
entire cross-section, along the entire length of the extrusion, including 
the end portions which are inbound of the gripping device. To further 
enhance the grip, the engaging faces of the gripper mechanism may be 
provided with a treated finish such as a knurled or saw tooth finish, or 
with a rubber, rubberized or elastomeric or polymeric surface treatment. 
Certain saw tooth structures act to enhance the holding effects as tension 
is applied to the gripped product. 
The gripping mechanism may be applied by any method, but hydraulic or 
pneumatic clamping devices are preferred. The extrusion may be held 
stationary at one end and the other end may be pulled to provide the 
required axial tension as shown in FIG. 6. Alternatively, both 
longitudinal ends of the gripped extrusion may be simultaneously pulled to 
provide the required axial tension. Axial tension is typically applied 
using a hydraulic cylinder or mechanical drive as the tensioning source 
for the force F. 
What is required in the method of the present invention is that the gripped 
end portions subject the portions of the extrusion therebetween to axial 
tension, or longitudinal tension, by applying sufficient force, typically 
opposing force in longitudinally opposite directions. It should be 
understood that applying force in one direction while retaining one end of 
an extrusion in stationary position could also be employed to provide 
axial tension greater than or equal to the yield point of the extrusion. 
The axial tension must be sufficient to equal or exceed the yield 
strength, or elastic limit, of the material. It will be appreciated by 
those skilled in the art that yield strength is a function of the 
metallurgy of the material, i.e., alloy deformation history and temper. 
The amount of force required to equal or exceed the yield strength will 
further be a function of the cross-sectional area of the extrusion. 
Exemplary yield strengths for extruded aluminum products are as follows: 
______________________________________ 
Aluminum Yield 
Alloy Temper Strength (ksi) 
______________________________________ 
6009 T4 24 
6009 T6 41 
6061 T4 21 
6061 T6 40 
6063 T4 13 
6063 T6 31 
7075 T6 73 
2024 T4 47 
______________________________________ 
As is explained in detail below, the shaping of the extrusion of the 
present invention removes shape irregularities including bows, twists and 
bends in the extrusion cross-section and length. As such surface 
deformations are removed, the longitudinal length of the extrusion 
typically increases. Also, in the application of the axial tension, the 
length of the extrusion increases at least about 0.25 percent, due to 
permanent longitudinal stretch. Permanent longitudinal stretch experienced 
in applying sufficient tension to all elements of the products for 6XXX 
alloys is typically less than about 3% permanent stretch, and for certain 
alloys may be on the order of 0.5%. For harder alloys, permanent 
longitudinal stretch may exceed about 2-3%, and, for certain 
aluminum-lithium alloys, permanent longitudinal strength could exceed 
about 6-7%. As the length of the extrusion increases, the axial tension is 
held constant or varied to maintain a stress condition at or above the 
yield strength of the extrusion. This may be accomplished by setting the 
axial tension and providing suitable measuring and controlling 
instrumentation to cause the gripping mechanism to move in response, such 
as with a hydraulic cylinder control, as required to maintain the 
sufficient axial tension throughout the stretch shaping operation. The 
combination of applying a specific percentage of axial stretch in 
combination with the shaping operation further improves tolerances by 
taking advantage of the Poisson's ratio effect on the cross-section. 
Before or after the axial tension is applied to the extrusion, an exterior 
shaping die 60 is applied to an outer peripheral portion of the extrusion. 
The exterior shaping die is provided with working faces which conform to 
the final desired cross-sectional shape of the extrusion. In certain 
instances, the shaping die may be provided with working faces which 
overcompensate for anticipated minor springback which may be experienced 
in the shaping process. In a preferred embodiment, the exterior shaping 
die is formed of two or more portions which are applied over the 
extrusion, at a location at or near one longitudinal end portion of the 
clamped extrusion, and are clamped together with a suitable clamping 
device such as jack screws, or pneumatic or hydraulic clamps, to lock the 
die together. It will be understood by those skilled in the art that 
certain complex extrusions will require multiple die sections to 
accommodate complex cross-sectional configurations. 
Once the exterior shaping die is applied to the extrusion and the extrusion 
is in axial tension, the exterior shaping die is advanced in either 
direction, or sequentially in both directions, along the longitudinal 
length of the extrusion. It will be appreciated that the concept of die 
advance includes the use of a stationary die through which an extrusion 
which is maintained in sufficient axial tension is passed. The die may be 
mounted on a traveling mechanism, such as a rail guided car, which insures 
that the die travels in a path which is coincident with the longitudinal 
axis of tension of the extrusion. In a preferred embodiment, the shaping 
die is mounted to a traveling mechanism, or car, which travels along rails 
which run synchronously with the longitudinal axis along which the axial 
tension is being applied. Alternatively, a cable or cables may be applied 
to the die to pull the die along the longitudinal axis of tension to shape 
the extrusion. It has been found that die guides in the traveling 
mechanism insure minimum deviation from the die travel direction during 
shaping. The rate of travel of the die may vary, and it has been found 
that speeds up to 200 feet per minute are adequate to stay ahead of the 
speed of the extruder in an in-line extrusion process. Die speeds up to 
400 feet per minute has no adverse effects on the shaping process based on 
theoretical evaluations. The working faces of the die act to work the 
exterior walls of the extrusion to plastically deform aluminum extrusions 
within or better than the standard tolerance currently established by 
various American and European associations including the Aluminum 
Association. In a preferred embodiment, the process of the present 
invention typically brings the finished extrusion to within less than half 
of the current standard tolerance established by the Aluminum Association 
for aluminum extrusions. 
The exterior shaping die such as two piece die 60 shown in FIG. 5 of the 
present invention must be of sufficient strength, and there must be 
sufficient lubricity to permit plastic deformation in working and 
reorientation of the material of the extrusion. Exemplary die materials 
for the working faces of the die include, but are not limited to, steel 
alloys, zinc alloys, graphite impregnated nylon and certain epoxy die 
materials. A preferred die material is a cast zinc alloy sold under the 
trade name Kirksite. 
The axial length L of the die 60, as shown in FIG. 5, must be sufficient to 
work the extrusion material. It has been found that the die length should 
exceed about 0.5 inch and may exceed 12 inches. It will be appreciated 
that multiple dies may be utilized in the present invention to shape 
extrusions in stages. 
In one embodiment the axial tension applied to the extrusion may be 
slightly less than the yield strength of the extrusion, such as at 90% of 
the yield point. However, the action of the advancing shaping die may be 
adequate to cause the total axial tension to which the extrusion is 
exposed to exceed the yield point of the extrusion and thereby cause the 
extrusion to be shaped into dimensional conformity as the die is advanced. 
In a preferred embodiment of stretch shaping, there should be adequate 
lubricity to permit the die to travel freely along the extrusion and 
perform localized deformation of the extrusion. Such lubrication may be 
provided in the die material such as through the use of certain epoxy 
material or through impregnation with materials such as graphite. 
Alternatively, a thin film of medium weight lubricant may be applied 
separately or automatically ahead of the die such as from an applicator 
that may be integrally attached to the travelling mechanism, to the 
extrusion to enhance the process with a minimum surface residue. 
Lubrication reduces variations in axial force of the die on the axial 
tension control system, and also improves surface appearance. 
In a preferred embodiment an interior shaping die may be employed with 
hollow portions of extrusions. Such interior dies could be employed 
independent of, or simultaneously with, the exterior shaping die to 
improve and enhance the final dimensional tolerance of the extrusion. In 
some cases an external and an internal die combination can be used in 
axial alignment to one another during shaping to enhance deformation. An 
interior shaping die conforms substantially to the finished inside 
cross-sectional shape or dimension of a closed or substantially closed 
extrusion. A cable mechanism is typically employed to pull the interior 
shaping die and thereby advance the die through the extrusion along a path 
coincident with the axis of axial tension. 
In another embodiment hollow chambers of elongated extrusions may be filled 
with a fluid to provide uniform pressure against the inside walls of the 
extrusion along the length thereof as an exterior shaping die is advanced 
along the length of the extrusion. Such internal fluid and pressure may be 
provided such as by the method disclosed in U.S. Pat. No. 4,704,886, the 
contents of which are incorporated herein by reference. However, the 
internal pressure of this embodiment may be utilized intentionally prior 
to advancing the shaping die to outwardly bulge surfaces of a hollow 
extrusion, which outwardly bulged surfaces may be subsequently worked into 
dimensional tolerance with the use of an external shaping die. In 
instances where such pressure is utilized to deform the extrusion, such 
pressure is typically released prior to the subsequent working with the 
external shaping die by the process of this invention. The process of the 
present invention is typically performed at ambient temperatures but may 
be performed in certain cases and with certain alloys at elevated or at 
lower temperatures, such as to maintain or alter temper during 
deformation. 
After the extrusion is shaped under axial tension, the exterior shaping die 
is opened, the tension is relaxed, or vice versa, and the extrusion is 
removed from the apparatus. The extrusion should be able to be stretch 
shaped and removed within the time it takes to extrude product to such 
length. Therefore, the method of the present invention could be utilized 
if desired as an in-line process for typical extrusion operations. 
The stretch shaping process of this invention may also be employed on 
multiple extrusions simultaneously. In such embodiment, multiple 
extrusions may be placed and maintained in axial tension with one, or 
more, gripping devices. With the multiple extrusions in axial tension, 
above the yield strength of the material, an external shaping die, which 
may be constructed as a unitary die assembly with multiple shaping ports, 
is advanced along the length of the multiple extrusions. The die ports 
have working faces which conform approximately to the finished 
cross-sectional configuration of the respective extrusion which fits in 
such port during stretch shaping. 
In another embodiment the stretch shaping invention may be employed to 
partially reshape extrusions. For example, certain extrusions cannot 
readily be made to final desired configuration due, for example, to 
limitations in extrusion tooling. Yet, by the process of this invention, 
such desired configurations may be obtained by a stretch shaping 
operation. For example, FIG. 7 illustrates a partial perspective view of a 
somewhat complex shaped extrusion 80. FIG. 8 illustrates a final desired 
configuration for the extrusion 80 shown in FIG. 7. Such final desired 
configuration may be accomplished by advancing a die having working faces 
conforming substantially to the final desired cross-sectional 
configuration of the extrusion, along the longitudinal axis of the 
extrusion while the extrusion is in axial tension above the yield point of 
the extruded material. Such shaping brings end portions 82 and 84 in close 
proximity to one another along the length of the extrusion, which final 
configuration may not be readily obtainable in an extrusion process. In 
addition to closing portions 82 and 84, the stretch shaping operation 
accurately corrects other dimensional deviations that may need correcting 
along the length of the extrusion, such as bows, twists or bends. 
Likewise, the stretch shaping method may be employed, for example, to 
shape extruded flanges where it may be desirable to create shaped pockets 
or envelopes to house wire, cable or the like. 
This invention provides a method of performing consistent spatial 
dimensional corrections to an elongated extruded product with an in-line 
electro-mechanical apparatus without inducing significant residual 
stresses in the extrusion. In any event, the residual stresses created by 
the stretch shaping method are less than the stresses normally created by 
alternative local deformation operations and shape reorientation methods. 
What is believed to be the best mode of the invention has been described 
above. It will be apparent to those skilled in the art that numerous 
variations of the illustrated and described details may be made without 
departing from the scope of this invention.