Method to repair voids in aluminum alloys

The present invention provides a method to repair a void in an aluminum alloy, particularly a void resulting from an exit hole left from friction stir welding (FSW). The method includes machining the void to provide a tapered bore (34) through a parent metal, i.e., the aluminum alloy (30), providing grooves and ridges (36) on a sidewall of the tapered bore, inserting into the bore a consumable tapered plug (38) having an included angle less than, or equal to, that of the tapered bore, and rotating the plug inside the bore under an applied load to plasticize both its surface and the ridges on the parent metal tapered bore. The tapered plug is preferably attached at its proximal end to a nonconsumable heat sink (40) to remove excessive heat generated during rotation of the plug. Also, a support bracket is temporarily attached to the aluminum alloy adjacent the tapered bore to receive a distal end of the plug (38) and to react to the applied load. The plasticized material (48) at the interface between the plug and the tapered bore, upon hardening, produces a strong bond consisting of refined and recrystallized fine metal. The heat sink and the support bracket can be later trimmed away.

FIELD OF THE INVENTION 
This invention relates to a method for repairing voids in an aluminum alloy 
or a weld, and more particularly to a method for filling exit holes left 
from friction stir welding of aluminum alloys. 
BACKGROUND OF THE INVENTION 
Friction stir welding (FSW) is a relatively new welding process for joining 
parts of materials such as metals, plastics, and other materials that will 
soften and commingle under applied frictional heat to become integrally 
connected. A detailed description of an FSW apparatus and process may be 
found in Patent Publications WO 93/10935 and WO 95/26254; and U.S. Pat. 
No. 5,460,317, all of which are herein fully incorporated by reference. 
One useful apparatus for FSW is shown in FIGS. 1A and 1B and includes a 
shoulder 14' at its distal end, and a nonconsumable welding pin 16' 
extending downward centrally from the shoulder. As shown, two parts to be 
welded together, exemplified by plates 10A' and 10B' on backing plate 12', 
are aligned so that edges of the plates along the weld joint are in direct 
contact. As the rotating tool W' is brought into contact with the weld 
interface between plates 10A' and 10B', the rotating pin 16' is forced 
into contact with the material of both plates, as shown. The rotation of 
the pin in the material and rubbing of the shoulder against the upper 
surface of the material produce a large amount of frictional heating of 
both the welding tool and the plate interface. This heat softens the 
material of the plates in the vicinity of the rotating pin and shoulder, 
and in concert with deformation created by the rotating pin, causes 
commingling of material, which, upon hardening, forms a weld. The tool is 
moved longitudinally along the interface between plates 10A' and 10B', 
thereby forming an elongate weld along the interface between the plates. 
When the weld is completed, the welding tool is retracted. 
FSW has been successfully used for welding aluminum alloys. For welding 
together flat workpieces, suitable run-on and run-off extensions can be 
utilized. These extensions, or end-tabs, provide starting and stopping 
points along a weld seam that may be later trimmed away. The end-tabs 
serve to transition a FSW tool to and from the workpieces without causing 
undue disturbance to the weld. A problem arises when welding workpieces 
that cannot have end-tabs, for example, workpieces with a circumferential 
geometry such as domes and cylinders, or workpieces with any continuous 
curved surface. In these situations, the FSW tool must be retracted from 
the weld at one point. The retraction leaves an exit hole behind on the 
weld, which remains unfilled and creates a discontinuity in the weld path. 
To repair holes in steel, friction hydro pillar processing (FHPP) has been 
developed. FIG. 2 is a schematic diagram of a prior art FHPP for steel. 
The process involves rotating a consumable rod 20 coaxially in a generally 
cylindrical hole 22 to be filled, while under an applied load to generate 
continuously a plasticized layer 24. The rotating rod 20 heats with 
friction to generate plasticized layer 24 in an almost hydrostatic 
condition. Plasticized layer 24 develops at a rate faster than the feed 
rate of rod 20, causing plasticized layer 24 to rise along hole 22 while 
leaving beneath a dynamically recrystallized deposit material 26. FHPP, 
which was developed specifically for steel, however, is not directly 
applicable to aluminum alloys due to aluminum's reduced high-temperature 
strength and high oxidation rate behavior. Specifically, while a certain 
amount of heat is necessary to cause softening of aluminum to fill a void, 
excessive frictional heat generated during FHPP tends to degrade the 
structural and metallurgical properties of aluminum alloys. A need exists 
to provide a method for repairing voids in aluminum alloys, in particular, 
filling exit holes left from FSW process, with material the same as, or 
similar in composition to, the parent material in such a manner that the 
repaired section has structural and metallurgical properties that are the 
same as, or better than, the parent metal. 
SUMMARY OF THE INVENTION 
The present invention provides a method to repair a void in an aluminum 
alloy, particularly a void resulting from an exit hole left from friction 
stir welding (FSW). The method includes machining the void to provide a 
tapered bore through a parent material, i.e., the aluminum alloy. In 
accordance with the present invention, grooves and ridges are provided on 
a sidewall of the tapered bore. A consumable tapered plug having an 
included angle less than, or equal to, that of the tapered bore is fitted 
to the bore, preferably with the tip of the plug extending beyond the bore 
depth. The tapered plug is preferably attached at its proximal end to a 
larger nonconsumable section that serves as a heat sink. The heat sink 
serves to remove excessive heat generated during rotation of the plug. 
Alternatively, external cooling, such as a cooling fluid, can be employed 
to remove excess heat. A support bracket is temporarily attached to the 
aluminum alloy adjacent the tapered bore. The support bracket includes a 
cavity that is machined so that the tip of the plug is fitted to the 
cavity. As the consumable tapered plug is rotated inside the tapered bore 
under an applied load, the plug and the parent material in proximity to 
their interface become plasticized and commingled to form a bonding 
interface, due to the normal component of the applied load, frictional 
heat, and hydrodynamic force. The ridges on the surface of the tapered 
bore are crushed and plasticized by the plug material to further enhance 
local deformation and commingling of the plug material and the parent 
material. Upon hardening of the plasticized material at the interface, the 
heat sink and the support bracket can be trimmed away, leaving the tapered 
bore thoroughly bonded with the plug material at the interface consisting 
of refined and recrystallized fine metal.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
The invention provides a method for repairing a void in an aluminum alloy, 
particularly a void resulting from an exit hole left when a friction stir 
welding (FSW) tool is removed from the workpieces at the conclusion of an 
FSW process. The method achieves this goal by inserting a consumable plug 
into a bore that has been machined into the aluminum alloy to subsume the 
void, and rotating the plug inside the bore under an applied load to 
plasticize and commingle the edge of the plug material and the adjacent 
parent material. 
The following description describes the present invention in the context of 
repairing a void that results when a FSW tool is removed from workpieces 
that are to be welded together. It should be understood that the methods 
of the present invention are equally applicable to voids that may be 
present in aluminum alloy workpieces that do not necessarily result from 
an exit hole in a FSW weld. 
FIGS. 3A through 3E illustrate preferred setup procedures for repairing a 
void in an aluminum alloy FSW weld according to the present invention. 
FIG. 3A illustrates an aluminum workpiece 30 with an anomaly void 32. 
Anomaly void 32 can be an exit hole left from the friction stir welding 
process or it can be a void resulting from the production or machining of 
the aluminum alloy workpiece. Void 32 is machined to provide a tapered 
bore 34 as illustrated in FIG. 3B. Tapered bore 34 has an included angle 
.alpha., which is dependent on the flow properties of both the parent 
material and the consumable plug material. For example, a low flow-stress 
material such as pure aluminum will require a dimension different from 
what is desirable for a high-strength alloy such as 7075 T6 aluminum. In 
the illustrated embodiment, .alpha. is about 60.degree.. Referring to FIG. 
3C, grooves and ridges 36 are provided on the surface of tapered bore 34. 
As before, the dimensions of small ridges 36 are dependent on the flow 
stress of the parent material and the consumable plug material. In one 
embodiment, 72 ridges per inch was satisfactory. Into thus grooved tapered 
bore 34, a consumable tapered plug 38 having an included angle less than 
or equal to that of tapered bore 34 is inserted, as in FIG. 3D. 
Preferably, the tip of consumable tapered plug 38 extends beyond a rear 
surface of aluminum workpiece 30. Further, consumable tapered plug 38 is 
preferably attached at its proximal end opposite its tip to a 
nonconsumable larger section 40. Larger section 40 has a proximal end 42 
that is operatively connected to a motor, not shown in the FIGURE, for 
rotating larger section 40 and, thus, consumable tapered plug 38. 
Referring to FIG. 3E, preferably, a support bracket 44 is temporarily 
attached to aluminum workpiece 30 adjoining consumable tapered plug 38. 
Support bracket 44 includes a cavity 46 machined to receive the tip of 
consumable tapered plug 38. Support bracket 44 is provided to react to a 
load applied to tapered plug 38 at its proximal end and to prevent 
plasticized plug and parent material from deforming, i.e., other than 
locally at the faying interface. 
FIGS. 4A through 4D illustrate procedures which follow the setup procedures 
hereinabove described with respect to FIGS. 3A-3E according to a preferred 
embodiment of the present invention. FIG. 4A schematically illustrates 
consumable tapered plug 38 attached at its proximal end to nonconsumable 
larger section 40, inside the tapered and grooved bore in aluminum 
workpiece 30, with its tip received by cavity 46 of support bracket 44. As 
larger section 40 is rotated by a motor with which it is operatively 
connected, consumable tapered plug 38 rotates while contained inside the 
tapered and grooved bore and cavity 46. Referring now to FIG. 4B, as 
tapered plug 38 rotates under an applied load, the plug material and the 
parent material of the aluminum workpiece in proximity to their interface 
become plasticized and commingled with each other, becoming generally 
plasticized material 48. The plasticization of these materials is due to 
the normal component of the applied load, frictional heat generated by the 
plug rotation, and hydrodynamic force. 
The included angle .alpha. of tapered bore 34 previously discussed with 
respect to FIG. 3B is chosen to produce a normal load component sufficient 
to generate plasticized material 48. In addition, the included angle of 
tapered plug 38 is preferably slightly less than that of tapered bore 34 
in order to ease rotation of tapered plug 38, and also not to trap air 
between tapered plug 38 and tapered bore 34 or between ridges 36. When air 
is trapped inside plasticized material 48, it tends to degrade the 
metallurgical and structural properties of plasticized material 48 as it 
becomes recrystallized. Ridges 36 provided on the surface of tapered bore 
34 are crushed by the plug material to further enhance local deformation 
of the plug material and the surrounding parent material. The size of 
ridges 36 is chosen so that the ridges are coarse enough to be crushed by 
the plug material and at the same time fine enough to be completely 
consumed in plasticized material 48 during the repairing operation 
according to the present invention. 
Still referring to FIG. 4B, rotation of tapered plug 38 is continued until 
the entire interface between tapered plug 38 and tapered grooved bore 34 
in aluminum workpiece 30 becomes plasticized and commingled material 48 
and until ridges 36 are completely consumed in plasticized material 48. 
During the repairing operation, larger section 40 serves as a heat sink to 
remove excessive heat generated by constant rotation and rubbing of 
tapered plug 38 against the tapered and grooved bore in aluminum workpiece 
30. In the illustrated embodiment, heat sink 40 is illustrated as a flat, 
cylindrical disk. It is to be understood that heat sink 40 can take other 
forms and still serve effectively as a heat sink. Furthermore, it should 
be understood that, while the provision of a heat sink is preferred, the 
advantages of the present invention as they relate to the provision of the 
ridges on the tapered surface of the bore in aluminum workpiece 30 can be 
achieved by controlling the temperature of the consumable plug and the 
plasticized material by other cooling means. For example, cooling fluids 
could be employed to remove excessive heat from the tapered plug. 
Referring to FIG. 4C, upon completion of bonding the interface between the 
tapered bore in workpiece 30 and tapered plug 38 and hardening of 
plasticized material 48 at the interface, heat sink 40 and support bracket 
44 are trimmed away. The hardened plasticized material 48 has a tip 
extending beyond the rear surface of aluminum workpiece 30. As illustrated 
in FIG. 4D, the tip may be trimmed away to provide a smooth rear surface 
on aluminum workpiece 30. 
In accordance with the invention, a void in an aluminum alloy is filled, 
producing a filled section having structural and metallurgical properties 
that are the same as or better than those of the parent metal. FIG. 5A, an 
optical micrograph of a repaired exit hole in an aluminum alloy 2219 at 
magnification of two hundred times, illustrates how a void filled 
according to a prior art procedure produces a poor bond at the interface 
between a plug material (finer grain) shown in the right and a parent 
material (larger grain) shown in the left. The interface shows no 
integration of consumable plug and parent metal. In the repaired exit hole 
of FIG. 5A, a consumable plug was fitted to a tapered void having no 
ridges, and was rotated without using a heat sink to control excessive 
heat. In contrast, referring to FIG. 5B, which is an optical micrograph at 
the same magnification and of the same material as FIG. 5A, a void filled 
in accordance with the invention has a refined and recrystallized region 
50 (finer grain) forming a strong bond at the interface between plug 
material shown in the left and parent material shown in the right. The 
solid bond shown in FIG. 5B was produced using a tapered and grooved void 
and a consumable tapered plug attached at its proximal end to a heat sink, 
according to a preferred embodiment of the present invention. 
FIG. 6 illustrates a typical product of a method to repair an aluminum void 
according to the present invention. The FIGURE shows a tapered bore in 
aluminum workpiece 30 filled with consumable tapered plug 38 and hardened 
plasticized material 48. The aluminum workpiece 30 is supported at its 
rear surface 52 by support bracket 44 having cavity 46, which contains a 
tip portion of plasticized material 48. 
Though not apparent from FIG. 6, relatively poor bonding is formed at an 
interface between the tip portion of plasticized material 48 and the 
cavity surface. This region at a plug tip was subject to less rubbing 
against the parent metal as compared to the tapered and grooved interface 
region, and thus was not as much plasticized and commingled with the 
parent metal. Thus, to form optimal bonding, it is preferable to form a 
throughbore in an aluminum workpiece and insert a consumable tapered plug 
extending beyond the workpiece depth. In this way, a relatively poor 
bonding portion formed at the tip of a plug can be later easily trimmed 
away. In situations where an optimal quality bonding is not required, 
however, the repair method of the present invention can be used with a 
non-throughbore and without a support bracket. 
Accordingly, the present invention provides a novel method for filling an 
exit hole left from friction stir welding an aluminum alloy. Clearly, the 
same method can be easily applied to repairing other anomalies found in an 
aluminum alloy, such as fatigue cracks and rivet cracks. 
While the preferred embodiments of the invention have been illustrated and 
described, it will be appreciated that various changes can be made therein 
without departing from the spirit and scope of the invention.