Abstract:
A method of welding metal components together including moving a laser beam in a first direction along an interface between a pair of metal components such that in the vicinity of the focused beam, metal from each component is vaporized to produce a keyhole in a pool of molten metal. The laser beam is oscillated in a direction different from (e.g., transverse to) the first direction such that the keyhole oscillates through the pool of molten metal and molten metal fills into the keyhole as the position of the keyhole changes.

Description:
RELATED APPLICATION 
   This application is a continuation-in-part of U.S. application Ser. No. 10/155,688, filed May 24, 2002 now U.S. Pat. No. 6,740,845 entitled “Laser Welding With Beam Oscillation”. 

   FIELD OF THE INVENTION 
   The present invention relates to laser welding, and more particularly, welding with an oscillating laser beam to create a moving and refilling keyhole. 
   BACKGROUND OF THE INVENTION 
   Common welded joints include butt joints, lap-penetration joints, and lap-fillet joints. Laser welding is a method of joining metal components using a focused beam of coherent light to melt adjoining components and allowing the melt to solidify into a joint. While butt joints may be produced by laser welding, they are not always suitable in the aerospace, automotive, and marine industries. Laser welding of lap-penetration joints and lap-fillet joints is more difficult to accomplish.  FIGS. 1 and 2  depict the weld region during laser beam welding of a lap-penetration joint in which a laser beam is directed at the region of an interface  2  between components  4  and  6 . Relative movement is effected along the interface  2  between the laser beam and the assembly of components. The laser beam may cause a portion of metal in the weld region to volatilize to produce a keyhole  8  bounded by molten metal  10 . The keyhole  8  advances with the movement of the laser beam in the direction of the arrow A. Molten metal  10  solidifies behind the advancing keyhole  8  to create a joint between the components. 
   In practice, production of lap-penetration joints and lap-fillet joints via laser welding is limited. For example, in a lap-penetration joint as shown in  FIGS. 1 and 2 , it is well established that the width W of the weld at the interface  2  should be equal to or exceed the thickness t of the thinnest of the components being joined. The welding process must be controlled to minimize formation of voids in the welds that are caused by instabilities in the keyhole and/or volatilization of low melting constituents with high partial pressures (e.g., Mg). In addition, laser welding is relatively costly. The laser beam is generally operated at or above 10 6  W/cm 2 ; efficiency dictates a need to weld at rates of at least 80 inches per minute (ipm) at this power level. With welding components up to 0.1 inch thick, it is possible to produce the required 0.1 inch weld width W at speeds exceeding 120 ipm. The formation of voids can be adequately controlled by use of defocused beams or bifocal optical systems. However, with thicker materials it is progressively more difficult to achieve the required weld width while still maintaining acceptable weld quality and speeds of travel. 
   With laser welding lap-fillet joints, the welding system must accommodate variations in lateral placement of the laser beam relative to the joint edge and gaps between the components to attain performance comparable to deposits made with gas metal arc welding (GMAW) at rates that justify using costlier laser welding systems. 
   One option for overcoming the challenges in laser welding lap-penetration and lap-fillet joints is to use beam integrators, focusing optics (mirrors or lenses) with longer focal length or defocused beams. However, to ensure reliable and consistent optical coupling between the laser and components to be joined and to achieve localized melting at the joint, the power output of the laser system must be increased to compensate for the reduction in power density. With sufficient power output, widened welds can be produced in the more placid conduction mode rather than the keyhole mode. Unlike the latter mode, which involves translation of a cavity (or keyhole) along the joining area, the conduction mode is achieved by translating a molten pool of metal along the joining area. By minimizing the violent volatilization of metal within the keyhole, the more placid conduction mode can eliminate the instabilities inherent with the keyhole mode. As a result, the conduction mode minimizes the formation of voids in the laser welds. However, implementing this approach necessitates using very powerful lasers (e.g., 18 KW to 25 KW, depending on the application) and costly laser generating systems, which makes the approach impractical for many industrial applications. 
   Another approach to increasing the effectiveness of laser welding is described in U.S. Pat. No. 4,369,348 by oscillating the laser beam at frequencies of over 1000 Hz. This very rapid movement of the laser is intended to distribute and time average the intensity of the laser at a frequency greater than the thermal response time of the metal. In this manner, the time averaged intensity of heat experienced by a particular location at the interface between the metal components being joined is greater than the intensity of heat experienced without oscillation. However, operation of a laser beam at oscillation frequencies of over 1000 Hz is difficult and costly. In addition, the only way to implement this approach is to weld in the conduction mode where a continuous molten pool is maintained. 
   Accordingly, a need remains for a low cost, effective method of laser welding. 
   SUMMARY OF THE INVENTION 
   This need is met by the method of the present invention which includes focusing a laser beam onto a joining region between a pair of metal components to form a continuous molten pool and establishing a keyhole therein. The molten pool is a mixture of metals from each of the components. By simultaneously translating the focused beam along the joining region in a first direction and oscillating the beam relative to the molten pool at a direction different from the first direction, the keyhole is continuously moved and immediately refilled by the adjoining molten metal. Welding takes place as the keyhole penetrates through the molten pool and melts the components being welded at the interface therebetween. In the vicinity of the focused laser beam, the molten pool is vaporized to produce a keyhole which is translated with the oscillating beam. As the beam oscillates in a direction different from the first direction (e.g., transverse to the first direction), the keyhole oscillates through the pool of molten metal and molten metal fills into the keyhole as the keyhole oscillates. In this manner, the keyhole continuously is produced and then is refilled with molten metal that solidifies to produce a weld. 
   The weld may be a lap-penetration weld, a butt weld, or a lap-fillet weld. Depending on the type of joint, thickness combination, and welding position, the laser beam oscillates at a frequency of about 5 to about 120 Hz and advances in the first direction at a rate of about 5 to about 400 ipm. When citing a range of frequency or dimension herein, the range includes all intermediate values, such as for the rate of the first direction, the rate would include 5, 6, 7, and on up through 400 ipm. The oscillations may be linear, circular, elliptical, or a combination thereof, or any other shape that accomplishes a moving keyhole. The thinner of the metal components may be over 0.1 inch thick. The present invention allows laser joining various types of joints with customized weld dimensions (e.g., 0.25 inch interfacial weld width between 0.25 inch thick components) up to about 0.50 inch wide. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic cross-sectional view of a pair of metal components laser welded to form a keyhole according to the prior art; 
       FIG. 2  is a top view of the stack of-components shown in  FIG. 1  operated according to the prior art; 
       FIG. 3  is a perspective view of a pair of metal components laser welded according to the method of the present invention; 
       FIG. 4  is a top view of the stack of components shown in  FIG. 3 ; 
       FIG. 5  is a schematic cross-sectional view of the metal components shown in  FIG. 4  taken along line  5 — 5 ; 
       FIG. 6  is a schematic cross-sectional view of the metal components shown in  FIG. 4  taken along line  6 — 6 ; 
       FIG. 7  is a schematic cross-sectional view of metal components hybrid laser welded according to another embodiment of the invention; and 
       FIG. 8  is a schematic cross-sectional view of metal components hybrid laser welded via an alternative of the embodiment of the invention shown in  FIG. 7 . 
   

   DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
   In the method of the present invention, metal components (such as steel, aluminum alloys or titanium alloys) are laser welded together. As shown in  FIG. 3 , radiation  20  (such as a laser or plasma beam) is focused in a round spot over an interface  22  between a pair of metal components  24  and  26 . The metal components  24  and  26  of  FIG. 3  are stacked upon one another to form a lap-penetration weld. This is not meant to be limiting; other weld joints may be produced according to the method of the present invention, such as butt welds and lap-fillet welds. The laser beam  20  travels in the direction of arrow A which may follow a linear path or a path of another configuration. The path of arrow A determines the location of the joint between the components  24  and  26 . 
   While the laser beam  20  travels in the direction of arrow A, the laser beam  20  also is oscillated in the direction of double arrow B. Double arrow B is at an angle to arrow A such as transverse to the direction of arrow A. In  FIG. 3 , the laser beam  20  is shown as oscillating in a linear path perpendicular to arrow A, but this is not meant to be limiting. The laser beam  20  may travel in other paths, such as circular, elliptical, sinusoidal, or the like. In the vicinity of the focused laser beam  20 , as shown in  FIGS. 4–6 , the metal of the components  24  and  26  melts and vaporizes (as shown by the outline of vaporized metal at  27  in  FIG. 6 ) to produce a keyhole  28  surrounded by molten metal  30 . Metal vapors  27  from the molten metal  30 , which is a mixture of components  24  and  26 , escape from the keyhole  28  and produces a plume or plasma above the surface of the upper component  24 . The focal point of the laser beam  20  is shown schematically at  25  in  FIGS. 5 and 6  and is generally well below the upper surface of the upper component  24 . Oscillation of the laser beam  20  causes the keyhole  28  to fill in with molten metal  30  and reform as a new keyhole  28  adjacent thereto. As the keyhole  28  continuously moves from one position to another position across the path of the arrow A and vacates its previous position in the pool of molten metal  30 , the vacated keyhole  28  fills in and reforms as a new keyhole  28 . This process has the appearance of movement of the keyhole transversely through the molten metal  30  with the molten metal  30  acting to continuously “heal” the vacated keyhole  28 . In this manner, a weld having an interfacial width W is produced that is significantly wider than the welds attainable using the prior art welding techniques. For example, when welding lap-penetration joints, the implementation of the invention affords joining with welds having an interfacial width equal to or wider than the thickness of the thinner part being welded. Welds produced using this method are typically two to five times the width of laser beam welds produced using conventional methods. Wider welds are particularly helpful in producing lap-penetration welds in thicker components, i.e., components thicker than 0.1 inch and up to about 0.25 inch thick and for achieving complete fusion at the faying edge when the structure requires the lower component to be perpendicular to the upper component. 
   Suitable frequencies of oscillation of the laser beam  20  are about 5 to about 120 Hz and may be about 10 Hz. The laser beam may advance along the interface at a rate of about 5 to about 400 ipm, or about 40 to about 200 ipm, or about 80 ipm. 
   In certain instances, it may be helpful to include a source of filler material, such as a filler wire. Filler material may be added during welding and may be in the form of a wire, having a diameter of about between 0.030 and 0.062 inch, or a powder. The filler material may be an alloy selected based on the desired attributes of the weld using established engineering principles. The filler material may be added to the front or rear of the molten pool, typically, at an angle of between 15 and 60 degrees off of horizontal, i.e., the plane of the upper component. Processing gas may also be utilized to shield the molten pool and to redirect the vaporized metal away from the beam and material interaction zone, which is commonly referred to as plasma suppression. The gas typically is provided at the front or side of the weld pool through a nozzle directed to the rear or side, respectively, of the pool at an angle of between 30 and 60 degrees off of the horizontal. 
   EXAMPLE 
   A lap-penetration laser weld was produced between a pair of 0.196 inch thick Alclad alloy 6013-T6 plates with 0.045 inch diameter alloy 4047 filler wire at 35° feed angle, 90 ipm wire feed rate using 10 KW power CO 2  laser (110 cfh flow rate of helium as plasma suppressing gas applied from the moving front) traveling at 80 ipm focused 0.25 inch below the top surface of the plate stack up. The laser was linearly oscillated in a direction transverse to the welding direction at 400 ipm, 0.25 inch total oscillation width (i.e., 0.125 inch center to center). The resultant interfacial weld width was slightly greater than 0.22 inch. 
   Another embodiment of the invention involves laser-based hybrid welding. The phrase “laser-based hybrid welding” is meant to include welding processes that include a second welding process (e.g., GMAW or plasma welding) in addition to laser welding. In this embodiment, a second welding process (arc welding) is combined with the above-described laser welding with a self-healing keyhole. 
   Referring to  FIG. 7  and by way of example, a GMA (gas metal arc) or MIG (metal inert gas) welding process is employed in combination with the above-described laser welding with self-healing keyhole. Other suitable arc welding processes may be used such as plasma welding. An MIG welder  40  is positioned behind the laser beam  20  in the direction of travel A. Alternatively, as shown in  FIG. 8 , the arc welder  40  may be positioned in advance of the laser beam  20 . 
   The MIG welder  40  generally includes a torch  42  with a continuously fed consumable welding wire  44 . An electric arc  45  between the tip of the wire  44  and the molten pool  30  continuously melts the wire  44 . Inert processing gas passing through the torch  42  (depicted by arrows  46 ) supports the arc  45  and shields the molten metal  30  from oxidation. The longitudinal axis of the welding wire  44  forms an angle α with vertical axis of the laser beam  20 . Angle α may be about 10–50 degrees, preferably about 30 degrees. The size and shape of the pool of molten metal  30  differs slightly between the arrangements of  FIGS. 7 and 8 . In the arrangement of  FIG. 7  where the MIG welder  40  follows the laser beam  20 , a deeper pool of molten metal  30  forms than when the MIG welder  40  advances in front of the laser beam as shown in  FIG. 8 . 
   There are several advantages of combining MIG welding with laser beam welding with beam oscillation to induce a self-healing keyhole. The deposition rate of the welding wire from the MIG welder is higher when combined with laser welding. A greater volume of molten metal is produced in which the self-healing keyhole can be established and oscillated. With more base material and more welding wire melted per unit length of the weld than in either of the laser welding with a self-healing keyhole or MIG welding alone, welding speed can be increased. The larger volume of molten metal provides a larger volume of metal to fill the self-healing keyhole, which in turn enables the keyhole to heal. In addition, the keyhole heals more readily and uniformly. In addition, due to the inherent characteristics described above, the combination of beam oscillation with MIG welding provides a wider width of penetration at the weld root which is advantageous for enabling adequate penetration of the side walls when producing butt welds. 
   Having described the presently preferred embodiments, it is to be understood that the invention may be otherwise embodied within the scope of the appended claims.