Abstract:
A method and apparatus for friction stir welding that produces a weld of significantly reduced surface roughness at significantly higher welding rates, in materials that are difficult to weld, such as non-extrudable aluminum alloys. The method includes cooling the stir welding tool during the welding process, thereby reducing the tendency of softened metal to adhere to the rotating pin and shoulder of the tool. The apparatus includes a tool with internal spaces or an external jacket, through which coolant can be pumped to remove heat and cool the tool during welding operations. In another embodiment, the apparatus includes a device for spraying a coolant onto exterior surfaces of the distal end of the welding tool to thereby remove heat from the tool, and the surrounding workpiece, during welding.

Description:
FIELD OF THE INVENTION 
     The invention relates to a method and apparatus for friction stir welding. More particularly, in accordance with the invention, excess heat produced in the friction stir welding process is removed so that a smoother weld surface is produced. 
     BACKGROUND OF THE INVENTION 
     Friction stir welding (FSW) is a relatively new welding process for joining together 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 the FSW apparatus and process may be found in Patent Publications WO 93/10935; WO 95/26254; and U.S. Pat. No. 5,460,317, all of which are hereby fully incorporated by reference. One of the useful apparatus for FSW is shown in FIGS. 1A and 1B. As shown, two parts, exemplified by plates  10 A′, and  10 B′ are aligned so that edges of the plates to be welded together are held in direct contact on a backing plate  12 ′. An FSW tool W has a shoulder  14 ′ at its distal end, and a non-consumable welding pin  16 ′ extending downwards centrally from the shoulder. As the rotating tool W′ is brought into contact with the interface between plates  10 B′ and  10 A′, 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 produces 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, causing commingling of material, which upon hardening, forms a weld. The tool is moved longitudinally along the interface between plates  10 A′ and  10 B′, thereby forming an elongate weld all along the interface between the plates. The welding tool&#39;s shoulder  14 ′ prevents softened material from the plates from escaping upwards, and forces the material into the weld joint. When the weld is completed, the welding tool is retracted. 
     Welds produced by the prior art friction stir welding process can produce smooth welds for certain materials, but for non-extrudable aluminum alloys, the maximum spindle speed is severely limited by adherence of the material to the welding tool shoulder and pin. For these alloys, exemplified by aluminum alloys 7075, 2014, 2090, and 2024, as the spindle speed increases, and correspondingly the heat input to the weld increases, the surface texture of the upper surface of the weld degrades by becoming rougher. At higher spindle speeds, and higher heat input, the aluminum material adheres and builds up on the welding tool shoulder, tearing away material from the sides of the weld surface. For long welds, this condition can cause such excessive buildup that continuing the weld becomes impossible. Also, the overheated welding tool can sometimes partially tear away surface material from the center of the weld surface, producing a “fish scale” appearance on the upper surface of the weld which progressively worsens along the length of the weld. For certain applications such a rough weld surface is undesirable, and requires additional machining to produce a smooth surface. Rough surfaces often provide points of initiation of fatigue cracks, and are therefore generally undesirable, especially if the welded part is to be used under conditions that could cause fatigue, such as cyclical conditions of applied load. There exists a need for a FSW process that produces a weld of reduced surface roughness that would not require subsequent machining, for most applications, and that would have a uniform, smooth surface texture. 
     SUMMARY OF THE INVENTION 
     The invention provides a method and apparatus for producing a friction stir weld of difficult to weld materials, such as non-extrudable aluminum alloys, that has a smoother surface than heretofore achieved with conventional friction stir welding equipment. The weld is produced at higher speeds and has a commercially acceptable surface smoothness so that it does not require subsequent machining for most purposes. 
     In accordance with the invention, it has now been found that the rate of welding limitation on non-extrudable materials, imposed by the increasing roughness of the weld surface as welding rate increases, is caused by excessive heat generated during the friction stir welding process at the surfaces of contact between the tool and the workpiece being welded. While a certain amount of heat is necessary to cause softening of the material to form the weld, excessive heat causes the softened material to adhere to the rotating pin and shoulder of the friction stir welding tool. The rotational and lateral movement of the tool against these adhesive-type forces causes the irregular weld surface. Therefore, the invention provides a method of friction stir welding that includes the step of simultaneously cooling the welding tool during the welding process to remove excess heat. This method allows a significant increase in welding rate, preferably at least about a 20% increase, and more preferably at least about a 100% increase, while maintaining an acceptable weld smoothness. Moreover, the invention provides apparatus for friction stir welding that are cooled by a coolant. 
     In one embodiment the coolant is circulated in the body of the tool to remove excess heat. In this embodiment, the friction stir welding tool of the invention has a tool body with a rotatable, usually non-consumable, pin and shoulder at its distal end that are adapted for stir welding parts together. The tool body has an internal space that is in heat-conducting communication with the pin, and preferably also the shoulder, of the welding tool. The internal space is adapted for flowing a coolant therethrough to remove excess heat from the tool, including heat conducted from the shoulder and pin. 
     In another embodiment, heat is removed from the friction stir welding tool by a jacket that surrounds a distal portion of the tool body. The jacket has an inlet that is in fluid communication with a source of coolant, and an outlet for exit of heated coolant. Thus, when the tool is in use, coolant flows through the jacket removing heat from the tool, so that excess heat is removed from the rotatable pin and shoulder. 
     In another embodiment, the removal of heat is achieved by spraying a coolant (such as cold air, or a liquid coolant, such as water) onto the tool, and surrounding surfaces being welded, during the welding step. Preferably, the tool portion being cooled is equipped with fins to facilitate heat removal. 
     In accordance with the invention, friction stir welds of even non-extrudable aluminum alloys are produced at commercially useful rates and have such a reduced surface roughness texture that they may be used commercially. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The foregoing aspects and many of the attendant advantages of this invention will become more readily appreciated as the same becomes better understood by reference to the following detailed description, when taken in conjunction with the accompanying drawings, wherein: 
     FIG. 1A is a schematic diagram of a prior art friction stir welding tool; 
     FIG. 1B is a schematic end view showing a prior art friction stir welding tool in use; 
     FIG. 2 illustrates a friction stir welding apparatus of the invention, including a nozzle for providing a coolant; and 
     FIG. 3 is a schematic diagram, in cross-section, of an embodiment of an internally cooled welding tool of the invention; 
     FIG. 4 is a schematic side view, in partial cross section, showing an externally jacketed embodiment of the tool of the invention; 
     FIG. 5 is a schematic side view of an air cooled, finned embodiment of a tool of the invention; 
     FIG. 6A is an optical micrograph showing details of the surface of a weld using a prior art friction stir welding tool; and 
     FIG. 6B is an optical micrograph of a surface of a weld made in accordance with the invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     In accordance with the invention, excess heat is removed from a friction stir welding tool (FSW) to reduce the degree of adherence between the tool and softened, difficult to weld material, such as non-extrudable aluminum alloys, so that a weld with a smoother surface is produced at a faster rate. Such smooth surface welds have many potential advantages, not only aesthetic, but also in reducing the risk of the initiation of fatigue cracking and corrosion. Moreover, the production of such welds eliminates, or reduces, the need for costly further machining of the weld to produce a smooth surface. The invention also increases welding rate by allowing higher FSW tool rotational speeds. 
     In accordance with the method of the invention, excess heat is removed from the friction stir welding tool while the friction stir weld is being formed. Thus, the heat removal is simultaneous with the welding operation. While the method and apparatus of the invention are applicable to all kinds of material that are subject to friction stir welding, the invention is particularly useful when applied to materials that are difficult to friction stir weld, such as the non-extrudable aluminum alloys, exemplified by the 2024, 7075, 2014, and 2090 alloys. In the prior art, these alloys are typically welded at a much slower rate than the extrudable aluminum alloys, in order to produce a weld that extends throughout the workpiece, and that has a surface smoothness that is commercially acceptable, without need for subsequent machining. Thus, while a one-quarter inch thick extrudable aluminum alloy, such as alloy 6061, may be welded with a friction stir welding tool rotating at 1600 rpm, at a rate of 15 inches per minute, to produce a smooth weld; a non-extrudable alloy would have to be welded at a lower tool rotation rate and slower rate of welding. Typically, in the prior art, a one-eighth inch thick non-extrudable alloy, such as alloy 2024, may be welded at a tool rotation speed of about 500 rpm for a weld rate of 3.5 inches per minute. This produces a weld throughout the workpiece, that has a surface of acceptable smoothness, without need for subsequent machining. In accordance with the invention, the same one-eighth inch thick workpiece of 2024 alloy can be welded at a tool rotation speed of at least about 800 rpm to produce a weld at a rate of about eight inches per minute, that extends throughout the workpiece and that has a commercially acceptable smooth surface. Similarly, a one-eighth inch thick workpiece of 7075 aluminum alloy (a non-extrudable alloy), can be welded with a tool rotating at 1100 rpm, and a welding rate of 13 inches per minute to produce a weld that extends throughout the workpiece, with a surface of acceptable smoothness without subsequent machining. In the prior art, not using the tools and method of the invention, the same 7075 alloy workpiece would have been welded with a tool rotating at 600 rpm and producing a weld at a rate of only seven inches per minute. 
     From the foregoing, it is clear that the invention allows a significant increase in the rate of rotation of the friction stir welding tool, with a concomitant dramatic increase in the rate of welding, in inches per minute. Preferably, the invention allows an increase in welding rate of at least about 20%, most preferably at least about 100%, while maintaining a weld surface smoothness that is usually commercially acceptable, without requiring subsequent machining, although such machining may optionally be performed for specific applications. 
     The invention provides a range of apparatus for removing excess heat, and the preferred embodiments of these apparatus are illustrated in the accompanying FIGURES, for ease of explanation. Clearly, other apparatus that perform the same function, of removing excess heat, so that a smoother weld surface is produced at a faster rate, are also within the scope of the invention. 
     Referring to FIG. 2, a schematic side view of a preferred embodiment of an apparatus in accordance with the invention, a substantially cylindrical weld tool body  30  having a proximal end  34 , for operative connection to a drive motor for rotating the tool, and a distal end  36 , that is equipped with a shoulder  38  and a substantially cylindrical pin  40  extending axially downward through a center of the shoulder. As shown, the pin  40  has a tip  42  and an outer surface that is helically grooved. The shoulder  38  is usually slightly peaked upward, from its circular periphery to the pin  40  at its center, at an angle of about 10°. 
     In accordance with the invention, a nozzle  50  extends in proximity to the welding tool body  30 , in particular to the distal end  36 , when a workpiece  20  is being welded on a backing plate  24 . The rotating cylindrical pin has a tip  42  at its distal end that extends through the workpiece  20  to a depth to provide a minimal clearance between it and the backing plate  24 . Thus, the pin extends substantially through the entire thickness of the workpiece  20 , to produce a continuous weld  26  through the entire workpiece. The nozzle is in fluid communication with a source of coolant, such as a liquid or air, that is supplied under pressure to the nozzle, so that coolant exits the nozzle in a mist that impinges directly on the distal end  36  of the tool, and the surrounding workpiece  20 . Thus, the coolant removes excess heat from both the exposed portion of the shoulder  38  that is above the workpiece  20  during welding, the workpiece  20  itself, and the weld  26  that is being formed. Heat travels by conduction from the hot rotating pin  40  and the shoulder  38  to their surroundings, namely, the workpiece  20  and the weld  26 , from which the coolant then removes the heat. As a result, the temperatures of the surface of the rotating shoulder  38  and workpiece are significantly lower than would have been the case, but for the supply of coolant. These reduced surface temperatures caused by removal of excess heat, as explained above, provide a smoother weld surface at a faster weld rate. The amount of coolant should be controlled to avoid removal of so much heat as to interfere with the welding operation. A coolant rate of about 0.01 gpm is usually suitable and the rate may readily be optimized for a specific application. 
     FIG. 3 illustrates, schematically, in cross section, an internally cooled FSW tool in accordance with the invention. As shown, the substantially cylindrical tool body  60  has a proximal end  62  for coupling to a motor for rotating the tool, and a threaded distal end  64 . A cap-shaped shoulder  66 , with a circular shoulder base  65  and an internally threaded collar  67 , is threadingly engaged to the distal end  64  of the FSW to produce an internal cylindrical space  74  between the base  63  of the tool body  60  and the base  65  of the shoulder. A pin, preferably with a helically grooved exterior, extends downward from the center of the base of the shoulder. The tool includes an internal space, preferably a serpentine or tortuous internal space, that is in fluid communication with a source of coolant, and a sink for receiving heated coolant. In the embodiment shown, the internal space is made up of vertical spaced and horizontal bores. Thus, a substantially horizontal inlet bore  70  penetrates to about the center of the distal end  64  of the cylindrical tool body  60 . A central vertical bore  72  extends downward from the farthest extent of the horizontal bore  70  to exit from the base  63  of the distal end  64  of the tool body  60  so that it is in fluid communication with space  74 . An annular space  76 , concentric with the central bore  72 , surrounds the central bore  72  and extends from the base  63  of the tool body  60  to below the inlet bore  70 . Thus, the central bore  72  is in fluid communication with the annular space through internal space  74  at the very distal end  63  of the tool body  60 . An exit bore  78  extends from an upper end of the annular space  76 . Coolant fluid entering the inlet bore  70  flows down the central bore  72 , into the internal cylindrical space  74 , into the annular space  76  and out of the exit bore  78 . 
     In order to direct the coolant, a cylindrical coolant collar  80 , concentric with and spaced from the tool body  60 , surrounds the inlet  70  and outlet  78  bores. The collar  80  is sealed against the body  60  of the tool with an upper O-ring seal  82  above the inlet bore  70 , and is also sealed against the tool body with a lower O-ring seal  84  below the exit bore  78 . In addition, the collar  80  is sealed to the tool body  60  by a third O-ring  86  located between the inlet  70  and outlet  78  bores. Thus, the collar  80  forms a separate inlet compartment  90  that is in fluid communication with the inlet bore  70 , and an outlet compartment  96  that is in fluid communication with the outlet bore  78 . A coolant inlet hose  92  is coupled to the inlet compartment  90  of the collar, and a coolant outlet hose  98  is coupled to the outlet compartment  96  of the collar  80 . 
     Control of coolant flow is important to avoid overcooling of the tool thereby interfering with the welding process. A coolant rate of about 0.1 gallons per minute is suitable. 
     An alternative embodiment, using an external cooling jacket, is illustrated schematically in FIG.  4 . In this embodiment, the cylindrical welding tool body  100  has a proximal end  102  adapted for coupling to a means for rotating the tool, and a distal end  104  equipped with a central downwardly extending pin  106 , surrounded by a shoulder  108 . The substantially cylindrical distal end  104  of the tool body  100  is equipped with an external structure designed to dissipate heat, in this instance a series of circumferentially extending fins  120 . The structure increases the surface area of the distal end, thereby permitting removal of larger amounts of heat for more effective cooling. A substantially cylindrical jacket  110  surrounds the finned distal end  104  of the tool  100 , and is sealed against the tool body  100  by an upper O-ring  112 , and a lower O-ring  114 . Thus, the jacket  110  surrounds the fins  120 , and is spaced from the fins to provide an annular region  122  that is in fluid communication with an inlet port  116  of the jacket, and an outlet port  118 . In use, coolant fluid enters the inlet port  116 , flows into the annular space and around the fins  120 , and exits from the outlet port  118 , removing heat from the surface of the tool  100 . This removal of excess heat, that can be controlled by controlling the temperature of incoming coolant and its flow rate, allows the production of a weld of substantially uniform smoothness at a much faster rate, even when a high-strength aluminum alloy, such as aluminum 2024 or 7075, is being welded. As before, a coolant rate of about 0.1 gpm is usually suitable, and the rate may be readily optimized by experimentation for any specific application. 
     In a yet further embodiment, illustrated schematically in FIG. 5, the welding tool is cooled by using cold air as a coolant. In this instance, as above, the welding tool  100  is equipped with a series of circumferentially extending cooling fins  120  on its distal end  104 . However, instead of a surrounding jacket  110 , at least one nozzle  125  is oriented to continuously blast cold air, or another cold gas, onto the fins  120  of the welding tool to provide cooling. As above, this removal of excess heat results in a cooler welding tool so that a weld with a uniform, smooth upper surface is produced, at a faster rate. 
     In accordance with the invention, a weld that is significantly smoother than achievable with prior art friction stir welding techniques and tools is produced. As can be seen from FIG. 6A, an optical micrograph of an aluminum alloy 2024 stir weld at magnification of eight times, a weld produced according to the prior art is rough, having open tears on its upper surface. The weld was produced at a FSW tool rotation speed of 640 rpm, and a weld rate of 6.3 inches per minute. Welds produced in accordance with the invention, exemplified by FIG. 6B, an optical micrograph at the same magnification for the same material, has a uniform, smooth surface, without surface tears. This weld was produced by a FSW tool rotating at 640 rpm and welding at a rate of 6.3 inches per minute. The weld surface shown in FIG. 6B was produced with an air/water mist applied at the junction between the tool shoulder and the weld surface, on the side opposite the direction of welding. It may be expected that this reduction in roughness will reduce the likelihood of fatigue crack initiation and surface corrosion and would therefore prolong the life (and safety) of welded parts. Also, it is expected that long welds could be performed without material buildup on the shoulder. 
     Although only a few exemplary embodiments of this invention have been described in detail above, those skilled in the art will readily appreciate that many modifications are possible in the exemplary embodiments without materially departing from the novel teachings and advantages of this invention. Accordingly, all such modifications are intended to be included within the scope of this invention as defined in the following claims. In the claims, any means-plus-function clauses are intended to cover the structures described herein as performing the recited function, and not only structural equivalents, but also equivalent structures. Thus, although a nail and a screw may not be structural equivalents in that a nail employs a cylindrical surface to secure wooden parts together, whereas a screw employs a helical surface, in the environment of fastening wooden parts, a nail and a screw may nevertheless be equivalent structures.