the present invention includes a method of friction plug welding an article , comprising several stages . preferably , the first stage is making a hole that is preferably tapered in the article to be welded , although machining a tapered hole is not necessarily required in friction plug push welding where ( in certain situations generally characterized when the article to be welded is softer ( having lower hardness ) relative to the harder ( having higher hardness ) plug ) the plug will form a hole , self bore or imbed into the material either while rotating or not . a tapered plug is then inserted through the tapered hole , then the plug is attached to a chuck of a motor which can both pull on the tapered plug and rotate it . some connection means , such as threads or locking retention interface , are provided on the tapered plug to facilitate pulling the plug . the second stage , or heating cycle is always required to weld the plug to the article . this stage preferably consists of rotating the plug while pulling ( placing the plug in tension axially ) into intimate contact with the hole &# 39 ; s surface , or region surrounding the hole . other forms of heating may also be utilized , including but not limited to , using electricity to assist in the heating process , or vibrational energy such as oscillatory rotation rather than the preferred method of continuous rotation , or lateral , axial or some combination thereof , rapid displacement ( such as ultrasonic welding ) to impart sufficient energy to assist in the heating the weldment . the plug ( preferably tapered , with a taper the same as or preferably different from the taper of the hole ( if it is tapered ), and rotating the plug relative to the part while moving the plug in the direction such to make contact with the hole &# 39 ; s surface , until contact is made , and forcing the plug into the surface of the hole by pulling on the plug ( imposing a tensile force in the plug in the plug &# 39 ; s axial direction ) all while continuously spinning the plug relative to the article . the weld continues to . the third stage is the braking stage . this rapid deceleration of rotation , if rotation is used , or otherwise defined as rapid decline of energy input to zero or near zero , is necessary to performing a successful weld . preferably , the fourth stage which is also referred to as the forging stage , is a period of cooling in which no further heating energy is intentionally applied to the weldment and energy in the form of heat is dissipated . during this stage , it is preferable to maintain either the same axial tensile load , or a different axial tensile load whether that be greater or lesser , to cause densification and or maintain or create a sound metallurgical bond or weldment . in the current application , although not necessarily required in other applications , excess sections of the plug are cut off and material further removed via grinding and sanding to make it smooth with the initial weldment and / or surrounding materials &# 39 ; surfaces . the present invention also includes the plug . the typical maximum temperature for this process , using typical process parameters , using aluminum alloys is 900 ° f . as measured by an embedded thermocouple located within 0 . 100 ″ from the original interface at an approximate depth of 33 % to 50 % through the substrate &# 39 ; s thickness . the first cycle ends after a pre - programmed time ( typically less than 5 seconds , preferably from 0 . 25 seconds to 2 seconds , more preferably from 0 . 5 seconds to 2 seconds , and most preferably in about 1 second ), displacement during heating ( also referred to as ‘ burn - off ’) ( typically 0 . 010 inches to 0 . 5 inches , preferably 0 . 025 inches to 0 . 250 inches , more preferably 0 . 050 inches to 0 . 150 inches , and most preferably 0 . 075 inches to 0 . 125 inches ), or temperature ( typically 500 ° f . to 1000 ° f ., preferably 700 ° f . to 1000 ° f . as measured with an imbedded thermocouple within 0 . 1 ″ of the original interface buried up to a depth of about 50 % through the substrate &# 39 ; s thickness . the forging phase for our current typical weld geometry exerts an axial tensile load of typically 1000 pounds to 20 , 000 pounds , preferably 6000 pounds to 15 , 000 pounds , more preferably 8000 pounds to 12 , 000 pounds , and most preferably 9000 pounds to 11 , 000 pounds , as the weld cools . after several seconds , ( typically less than one minute , preferably about 5 seconds ), the weld has cooled sufficiently to remove the tensile load and remove the tooling and weld equipment fpw hole geometries are an important factor to creating a successful weld . the current hole is made by mechanically removing the weld bead flush to the top and bottom sides or the surrounding plate or sheet , drilling a pilot hole , and counter sinking with a counter sink cutter . the current included angle is typically 20 ° to 120 °, preferably 40 ° to 90 °, and most preferably 40 ° to 60 °, although it is possible to perform similar welds with no angle ( a straight bored hole ), or opposite bored tapers ( those which the taper of the plug and hole are opposing each other ). see the figures . the hole is drilled to a depth such that the minor diameter is approximately the same as the diameter of the plug &# 39 ; s shaft . the major diameter of the plug is typically equal to or greater than the major diameter of the hole . current shaft diameters have ranged from ⅝ ″ to ¾ ″ in diameter ; however , diameters ranging from between 1 ″ and 1 . 5 ″ are planned . current plug major diameters ( the diameter on the larger side of the taper ) typically range from ¾ ″ to 1 ″, but have been made less than ¾ ″ and may be made in excess of 1 . 5 ″. the backing support ( also referred to as a pressure foot or collet ) is another important aspect in forming a successful weld . it reacts the axial load , and also serves as a reservoir to contain the extruded flash . the depth , diameter and profile of this support are optimized for each set of conditions , e . g . weld parameters , plug / hole geometrical design , plate thickness , etc ., and is important in creating a defect - free weld . in the pull method , the backing support ( also referred to as a pressure foot ) consists of a collet of an appropriate diameter and geometry . currently , the diameter of the collet is larger than the shaft of the plug that passes through it by at least 0 . 002 ″, typically from 0 . 002 ″ to 0 . 100 ″, and preferably from 0 . 010 ″ to 0 . 050 ″. the profile of the collet may include a bevel , taper , groove or other type of depression to act as a reservoir for plasticized material to flow . successful welds have been made with backing supports with beveled edges and without beveled edges . examples of several backing plates are shown in fig8 , and 14 a - 17 b . the advantage of pulling instead of pushing is that all equipment can be placed on one side of the object being repaired ( such as the rather large and fragile external tank of the space shuttle ). this makes the logistics of performing a plug weld much easier in some cases than if standard plug push welding were to be performed . in the preferred embodiment of the method of the present invention , a tapered hole is drilled from one side of the article being repaired . a tapered plug is then inserted through the tapered hole , then the plug is attached to a chuck of a motor which can both pull on the tapered plug and spin it . some connection means , such as threads or locking retention interface , are provided on the tapered plug to facilitate pulling the plug . the plug is pulled while spun by the motor . preferably the plug is pulled also after the spinning stops , with a load the same as or different from the load while spinning . after the spinning has taken place and the plug is welded in place , the excess part of the plug is cut off and the weld machined down to make it smooth . pulling a tapered plug during plug welding allows all equipment , including a backing plate , to be on one side of the article being welded . pull welding eliminates the need for large backing structures that must react high loads associated with friction plug push welding , often exceeding 10 , 000 pounds force , while at the same loads deflect an amount often less than 0 . 25 inches . the present inventors use or contemplate using plug welding ( push and / or pull ) with 2195 al — cu — li alloy and 2219 al — cu alloy . development of plug welding at lmmss has included plugs of extruded al — cu — li 2195 in the t3 and t8 condition welded into ( 1 ) al 2195 - t8 sheet , extrusion and plate , ( 2 ) 2195 - t8 / 2195 - t8 fusion welded sheet , extrusion and plate with al 4043 filler wire , ( 3 ) 2195 - t8 / 2219 - t8 fusion welded sheet , extrusion and plate with al 4043 filler wire , ( 4 ) friction stir welded 2195 - t8 / 2195 - t8 sheet and plate , where all sheets and plates were of thicknesses greater than 0 . 12 ″ and no greater than 1 . 00 ″ and all plugs were a diameter between 0 . 500 ″ and 1 . 000 ″. it has been discovered that a groove in the enlarged diameter portion of the plug can help improve the performance of the present method . tests have shown that a plug containing a groove whose size , profile and locations have varied widely , but have the general shape , profile and location of ⅛ ″ to ¼ ″ deep groove , with a ‘ vee ’, ‘ square ’, or ‘ u ’ bottom , ⅛ ″ to ¼ ″ beyond the end of the taper , increases the strength and reliability of the weld and is also believed to contribute to the elimination of all weld defects , surface or internal . it is believed that this is so because the groove interrupts heat transfer away from the weld interface and / or allows deformation of the welding surface portion of the plug . the core of the plug is unaffected by the groove , but is beneficial to the weld process in that it is believed to contribute to the dissipation of heat from the plug &# 39 ; s core , where low temperatures are preferred to help maintain structural integrity , both tensile and torsional . a hydraulically powered direct drive weld system has been used for all current development at lockheed martin ; however , an electrically powered direct drive , or inertia drive weld system may also be used . the inventors have discovered that satisfactory welds occur most frequently when the plug diameter is large enough to maintain a mechanically stable cool core . for this reason , plug diameters have continued to increase , and more powerful weld equipment has been acquired . techniques have been developed to weld larger diameter plugs while minimizing the required motor power . one such discovery entails varying the axial stroke rate during the weld process to decrease the initial contact friction . in this process , it is preferable for the plug and article to contact slowly , thereby reducing the rotational friction at contact . after the boundary between the plug and article plasticizes , then it is preferable , although not required , to increase the stroke rate , thereby increasing the rate of heating at the interface to achieve weld temperatures . this discovery significantly reduces the required power to perform welds , and is advantageous in performing large welds whose power requirement exceeds that which the system is designed to deliver . it has been found that for our current equipment and process , for example , a plug having an initial contact diameter of 0 . 5 ″ to 0 . 75 ″ can be pulled at an initial stroke rate of 15 inches per second and perform well , but larger plugs whose contact diameter is 0 . 75 ″ to 1 . 0 ″ best perform when pulled at an initial stroke rate of 5 to 10 inches per second . lockheed martin has found that with its current equipment and process , the preferable operating range at which to rotate the plug is 4000 - 6000 rpm prior to contact between the plug and hole &# 39 ; s surface , and it is also preferable to maintain a minimum of 3000 rpm during the duration of the heating cycle . successful welds have been created at much slower speeds , as low as but not limited to 1000 rpm prior to contact and as high as , but limited only by the equipment capability , of about 7000 rpm prior to contact . the plug of the present invention preferably has a connection means comprising a standard external thread . the thread can be , for example , right - hand ⅝ ″ with 18 threads per inch . other methods for holding the plug in the chuck may also include internal threads and key grooves or flats . the first embodiment 10 of the plug apparatus of the present invention is shown in fig1 a and 1b . plug apparatus 10 includes a plug portion 11 , a shaft 12 , and a threaded portion 13 . the second embodiment 20 of the plug apparatus of the present invention is shown in fig2 a and 2b . plug apparatus 20 includes a plug portion 21 , a shaft 22 , and a threaded portion 23 . the third embodiment 30 of the plug apparatus of the present invention is shown in fig3 and 4 . plug apparatus 30 includes a plug portion 31 , a shaft 32 , and a threaded portion 33 . plug apparatus 30 is shown welded to a plate 40 at the termination of a plasma weld including a plasma weld crown pass 41 and a plasma weld root pass 42 . similarly , a friction stir weld ( or any weld ) could be used in place of the plasma weld . a fourth embodiment of the apparatus of the present invention , plug apparatus 50 , is shown in fig5 a - 5d . plug apparatus 50 includes a plug portion 51 , a shaft 52 , a threaded portion 53 , a bore 54 , a v - shaped groove 55 , and a tapered portion 56 . bore 54 extends downward to about the top of the taper 56 ( preferably no deeper than dimension h in fig5 b . bore 54 could be used to inject cooling fluid during the weld cycle . a fifth embodiment of the apparatus of the present invention , plug apparatus 60 , is shown in fig6 a - 6d . plug apparatus 60 includes a plug portion 61 , a shaft 62 , a threaded portion 63 , an annular , straight - walled groove 65 , and a tapered portion 66 . a sixth embodiment of the apparatus of the present invention , plug apparatus 70 , is shown in fig7 a - 7d . plug apparatus 70 includes a plug portion 71 , a shaft 72 , a notched motor - gripping portion 73 , a bore 74 , a tapered portion 76 , a shear portion 77 , and an internally threaded bore 79 for pulling . a seventh embodiment of the apparatus of the present invention , plug apparatus 80 , is shown in fig8 . plug apparatus 80 includes a plug portion 81 , a shaft 82 , a notched motor - gripping portion 83 , a v - shaped groove 85 , a tapered portion 86 , a shear portion 87 , and an internally threaded bore 89 for pulling . an eighth embodiment of the apparatus of the present invention , plug apparatus 90 , is shown in fig9 . plug apparatus 90 includes a plug portion 91 , a shaft 92 , a notched motor - gripping portion 93 , a hole 94 , an annular groove 95 , a tapered portion 96 , a shear portion 97 , and an internally threaded bore 99 for pulling . plug apparatus 90 is shown with repair plate 201 and backloging plate 200 . a ninth embodiment of the apparatus of the present invention , plug apparatus 100 , is shown in fig1 a - 10d . plug apparatus 100 includes a plug portion 101 , a shaft 102 , a notched motor - gripping portion 103 , a hole 104 , an annular groove 105 , a tapered portion 106 , a shear portion 107 , and an internally threaded bore 109 for pulling . fig1 is an isometric view of an embodiment the pulling and spinning apparatus of the present invention . friction pull plug welding apparatus 110 includes steel bars 111 for clamping workpieces , a hydraulic motor 112 , a mounting plate 113 , support frame 114 , hydraulic cylinder 115 , stand off housing 116 , bolts 117 , bolts 118 , and pressure foot 119 . the chuck ( not shown ) is contained within housing 116 . a tenth embodiment of the apparatus of the present invention , plug apparatus 120 , is shown in fig1 a - 12d . plug apparatus 120 includes a plug portion 121 , a shaft 122 , a threaded motor - gripping portion 123 , a bore 124 , and a tapered portion 126 . the hole 124 extends through the plug portion 121 so it will extend through the workpiece when the welding is completed . plug apparatus 120 can be bolted together , and the various parts can be made of the same or dissimilar materials . an eleventh embodiment of the apparatus of the present invention , plug apparatus 130 , is shown in fig1 a - 13d . plug apparatus 130 includes a plug portion 131 , a shaft 132 , a threaded motor - gripping portion 133 , a hole 134 , and a tapered portion 136 . the hole 134 extends through the plug portion 131 so it will extend through the workpiece when the welding is completed . fig1 a - 17b are views of various combinations of plug taper and hole taper ( though externally threaded plug apparatus is shown , any of the plug apparatus shown in any of the figures could be used in these various combinations ). fig1 a and 14b show a ⅝ - 40 plug , 40 ° reversed plug , full chamfered backing plate . fig1 a and 15b show a ⅝ - 40 plug , 40 ° weld repair , chamfered backing plate . fig1 a and 16b show a ⅝ - 40 plug , straight weld repair , chamfered backing plate . fig1 a and 17b show a { fraction ( 5 / 8 )}- 40 plug , 40 ° weld repair , non - chamfered backing plate . the various features of the various plugs can be mixed and matched . the location of the groove is variable , but the groove is preferably above the finished weld . this process is intended to be used on production tools to repair weld defects - and repair or fill the close - out hole left by the removal of a stationary pin friction stir weld ( fsw ) tool as used in performing a friction stir weld . this process has the potential to deter cracks from forming or repair pre - existing cracks or other defects located in welded material as it is stressed in handling , proof testing , or service . this technique has proven to be successful . this process could also be used as a technique for the incorporation of inserts into a substrate , welded or otherwise . see fig1 , 12 , and 13 . friction plug welds have been found to be virtually defect - free using various non - destructive inspection ( ndi ) techniques such as visual inspection , fluorescent dye penetrant , x - ray radiography , ultrasonic and electromagnetic acoustic transducer ( emat ). mechanical testing has been accomplished using plug pull , and tensile testing . plug pulls were abandoned due to excessive sample deformation during testing . tensile tests whose gauge widths have included 0 . 25 ″, 0 . 50 ″, 1 . 0 ″, 1 . 5 ″, 2 ″, 3 ″, 4 ″, 5 ″, 10 ″ and 20 ″ have been performed at room temperature , − 320 ° f . and − 423 ° f . strengths of up to 69 . 8 ksi ( 70 % weld efficiency ) at 77 ° f . and up to 87 . 6 ksi at − 423 ° f . have been recorded . the ductility of these samples is increased over the initial weld from 4 % to 14 % elongation as measured over a 1 ″ gauge length ( a 250 % improvement ) and 4 . 5 % to 7 % elongation as measured over 2 ″ a gauge length ( a 55 % improvement ). it should be noted that several of the tensile test failures occurred outside the friction weld interface , failing in the original fusion weld &# 39 ; s heat affected zone , indicating that the actual strength of the friction weld was probably above that of which these tests show . plug welds have been made in initial welds of extremely poor quality with the same success as those listed above . this demonstrates that defects such as porosity and cracks do not affect the properties of fpws . furthermore , the parameters of these welds can vary over a broad range and do not greatly affect the fpw quality . burn off is defined as the amount of displacement during the heating cycle , past the point when the rotating plug makes intimate contact with the hole . typical values of burn off can range from 0 . 025 ″ to 0 . 150 ″. optimum welding conditions occur in the range of 0 . 075 ″ to 0 . 125 ″ burn off for the inventors &# 39 ; current welding equipment ; however , changing this amount by as much as 0 . 050 ″ yields a typical loss in tensile strength of 10 % or less . similarly , reducing the amount of axial load during heating from the optimum 10 , 000 pounds to as low as 4000 pounds , also results in a loss of strength of less than 10 %. varying parameters such as x and y alignment of the plug to the hole does not affect weld strength to a certain point , at which degradation does occur . elimination of the forging cycle seems to have a large effect on fpw strength . work on curved surfaces and tapered thicknesses have also been successful . equipment similar to that shown and described in u . s . pat . no . 3 , 973 , 715 can be used to make the welds of the present invention , except that thrust bearing 18 would be unnecessary and frame member 12 would be replaced with a backing plate of an appropriate geometry to provide support for the workpiece as the pull plug welding occurs . a pull plug weld was performed in a 0 . 140 ″ thick workpiece of 2195 t8a3 al — cu — li sheet welded to another plate of the same properties using a vppa weld process using a 1 4043 filler wire with a plug made of 2195 t8 extruded rod . the welded workpiece was prepared by removing weld bead protruding from the both surfaces of the plate with a pneumatic bead shaver and pneumatic sanding wheel until approximately flush with the surface . the plug had a shaft ( ⅝ ″ diameter ), a tapered section ( 60 degree ) that served as the bonding surface to the workpiece , a head portion of enlarged diameter ( 0 . 950 ″ diameter ) with a groove ( 0 . 140 ″ wide and 0 . 200 ″ deep ), and a threaded portion ( ⅝ - 18rh ) acting as a connection means for removably attaching the plug to the chuck , which acts as the interface to transmit a means of turning and pulling . the hole in the workpiece was 0 . 140 ″ deep and had a 40 degree taper ( the minor diameter of the hole was 0 . 700 ″). a backing plate that was approximately 0 . 150 ″ thick stainless steel and having an opening of 0 . 710 ″ in diameter was placed adjacent the workpiece and on top of the standard pressure foot 119 ( see fig1 ). the workpiece and backing plate ( which served as the collet ) were concentrically aligned and clamped to the pull weld test stand with c - clamps . the tool for producing such a weld is shown in fig1 . the plug was secured to a chuck which was screwed on to the spindle of the plug weld head . the motor initially spun the plug at a rate of about 5000 rpm , a hydraulic ram pulled on the plug at a rate of about 10 inches per minute until physical contact of the plug and the workpiece was made . the spin rate then decreased by approximately 1000 rpm due to the frictional resistance at the weld interface . the hydraulic ram continued to pull on the plug with a force of about 11 , 000 pounds until a displacement of 0 . 100 ″ during the heating cycle was measured . the heating cycle was ceased about 1 to 2 seconds after heating began . at this point , the motor was stopped in approximately 0 . 2 seconds . at the instant that the motor began to brake ( rapidly decrease in speed ) and the hydraulic ram pressure was increased to about 13 , 000 pounds tensile force . this load was maintained on the plug for about another 5 seconds . the weld was allowed to cool for about 10 seconds . the workpiece and the backing plate were then loosened by removing the c - clamps and the ram was extended to its outmost position , pushing the welded workpiece and plug outward . the plate and plug were manually spun in the opposite direction of rotation to loosen the plug from the chuck . visual inspection of the weld was performed . excess plug material was removed using a saw , beadshaver and sanding disc . nondestructive testing indicated that the strength of the weld was defect free . mechanical testing of a 1 ″ gauge width sample at 77 ° f . yielded an acceptable strength of 52 ksi . all measurements disclosed herein are at standard temperature and pressure , at sea level on earth , unless indicated otherwise . all materials used or intended to be used in a human being are biocompatible , unless indicated otherwise . the foregoing embodiments are presented by way of example only ; the scope of the present invention is to be limited only by the following claims .