Abstract:
Welding using a laser, which leaves keyhole portions at each pass, that allow gases to vent. That keyhole portion is an area within the interior portion, e.g., an inside of a spiral or a circular arc. The keyhole is not processed by the laser and gases can escape. The laser later circles back to process the area.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application claims priority to U.S. Provisional Application Serial No. 60/720,081, filed on Sep. 23, 2005 and 60/720,404, filed on Sep. 26, 2005. The disclosures of the prior applications are considered part of (and are incorporated by reference in) the disclosure of this application. 
     
    
     BACKGROUND  
       [0002]     It is often difficult to weld metals that are formed of two different materials. For example, coated steel may be one metal that is difficult to weld—especially when the coating has a lower melting point than the base metal. Different welds of this type, including a lap joint, have been attempted. However, this has been a challenge.  
         [0003]     One problem is that during the welding process, the coating may evaporate violently at the weld interface because of the coating&#39;s lower boiling point than the base metal. This may cause undesirable effects within the weld joints. This may also lead to defects, including high levels of porosity, as well as expulsion of the base metal from the weld pool.  
         [0004]     Different approaches have been attempted to deal with this problem. One of the approaches uses a laser welding technique, with plural sheets, each in a vertical position. The weld is carried out from bottom to top in order to allow gravity to lengthen an area through which the vapors can escape. This approach has been difficult to use in production.  
         [0005]     Another technique attempts to remove the coating prior to the weld. However, this requires additional processing, and hence can be relatively expensive.  
         [0006]     Gap welding requires a constant joint gap during the weld process and hence may be difficult to maintain. The joint design may also be changed in order to attempt to allow an escape path for the gases from either one or both sides of the joint.  
       SUMMARY  
       [0007]     The present application teaches a technique of using a laser to make welds, preferably on coated steel where the coating is of a lower boiling point than the base metal. The laser follows a path which defines an interior portion at each pass of the laser, leaves an area within that interior portion which is not processed by the laser at a first time, and through which gases can escape at that first time, and later returns to process said area.  
         [0008]     In an embodiment, the laser follows a substantially spiral path, where elements which form the spiral may be of random geometric shapes. For example, in an embodiment, a broken spiral path is followed.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]      FIG. 1  illustrates a typical lap weld;  
         [0010]      FIG. 2A  shows a picture of a typical laser weld;  
         [0011]      FIG. 2B  shows a picture of a weld using the present techniques;  
         [0012]      FIG. 3  is an illustration of a beam path; and  
         [0013]      FIG. 4  is an enlargement of the illustration of the details of the beam path.  
     
    
     DETAILED DESCRIPTION  
       [0014]     The general structure and techniques, and more specific embodiments which can be used to effect different ways of carrying out the more general goals, are described herein.  
         [0015]     A structure used in an embodiment is shown in  FIG. 1 . A laser  100  is equipped with a scan head  105 , which allows the laser output beam to be moved. The operation is controlled by a controller  120  which produces a signal that controls the output position of the laser beam  110 . The laser can be a 2.5 kW Co 2  laser.  
         [0016]     The laser traverses the path  125  as shown in  FIG. 1 . The path is a swirling path, and has outer edges  126 , defining a “keyhole”  127  within the outer edges. Different paths can be used, as well as different directions. For example, while  FIG. 1  shows a straight-line path overall, the path can be curved or can be in any other general shape. The term “swirling” is intended to denote any shape of any type that returns to itself, which can be formed of curved arcs or straight lines as parts of the swirl.  
         [0017]     The technique does not require special positioning of the pieces, the pieces can be clamped tightly together, and the process can be used to laser weld those pieces anywhere that is required using normal laser weld techniques.  
         [0018]     Conventional laser welding could produce violent outgassing of the coatings, which could cause explosion of the molten metal and messy welds. The present technique may avoid that by using the technique of leaving unprocessed areas or “keyholes”. For example, laser welding with coated steel using normal welding procedures can produce the effect shown in  FIG. 2A . This effect includes holes such as  200  which are caused by the expulsion of the molten material.  
         [0019]      FIG. 2B  illustrates a picture of the resultant weld using the present technique, with a swirl diameter of 2 mm, and a beam path speed of 60 mm per second. The weld is continuous, and no outgassing has been caused.  
         [0020]     The applicants believe that the beam manipulation in this way allows the keyhole to stay open for an extended period of time as compared with straight-line welding. When manipulating the beam around the keyhole, the keyhole remains open for a relatively long period of time. This allows the gases from the coating to escape less violently than in the prior art. In addition, by overlapping the beam path, the liquid metal is disturbed in a way that allows air bubbles to escape before being trapped in the metal, but after solidification.  
         [0021]     Different embodiments may use different overlapping patterns. For example, in an embodiment, a spiral pattern may be used.  FIGS. 3 and 4  illustrate a particularly preferred pattern which is a modified form of spiral. The path  400  shown in  FIG. 4  shows the modified spiral form, formed of generally arc shaped sections such as  402  and  404 , which are joined at an area  406 . Rather than a true spiral, this technique is formed by connected arcs, each forming a section of a circle, for example over 300°. Each arc then joins with another arc at the area  406 . However, other different shapes are also contemplated, including concentric square shapes, or concentric curve type shapes of various forms including concentric curve type shapes formed from straight lines which are held together to form a curve.  
         [0022]     In an embodiment, the amount of overlap needs to be great enough to allow for a large enough keyhole to allow for outgassing of the coating. However, the keyhole should not be so large so as to prevent an effective weld. Different embodiments describe different characteristics of that keyhole, using specific numbers.  
         [0023]     In the first embodiment, described above, a swirl type shape is used with a diameter of 2 mm and a path speed of 60 mm per second, using laser power of about 1800 W, and a field of view of about 500 mm.  
         [0024]     The material used may be 0.033/0.039 thick AISI CBDQSK to ASTM A620 Long Term coating to ASTM A308 carbon 10% maximum. The inventor believes that this material has a tin coating, but it may be lead. Either way, this material has an analogous effect to a zinc coating. The coating specifically has a lower melting point than the base material.  
         [0025]     The general structure and techniques, and more specific embodiments which can be used to effect different ways of carrying out the more general goals are described herein.  
         [0026]     Although only a few embodiments have been disclosed in detail above, other embodiments are possible and the inventor (s) intend these to be encompassed within this specification. The specification describes specific examples to accomplish a more general goal that may be accomplished in another way. This disclosure is intended to be exemplary, and the claims are intended to cover any modification or alternative which might be predictable to a person having ordinary skill in the art. For example, other shapes beyond spirals can be used. In addition, other speeds and powers can be used, determined by computer or by trial and error.  
         [0027]     Also, the inventors intend that only those claims which use the words “means for” are intended to be interpreted under 35 USC 112, sixth paragraph. Moreover, no limitations from the specification are intended to be read into any claims, unless those limitations are expressly included in the claims. The computers described herein may be any kind of computer, either general purpose, or some specific purpose computer such as a workstation. The computer may be a Pentium class computer, running Windows XP or Linux, or may be a Macintosh computer. The computer may also be a handheld computer, such as a PDA, cellphone, or laptop.  
         [0028]     Where a specific numerical value is mentioned herein, it should be considered that the value may be increased or decreased by 20%, while still staying within the teachings of the present application, unless some different range is specifically mentioned.  
         [0029]     The programs may be written in C, or Java, Brew or any other programming language. The programs may be resident on a storage medium, e.g., magnetic or optical, e.g. the computer hard drive, a removable disk or media such as a memory stick or SD media, or other removable medium. The programs may also be run over a network, for example, with a server or other machine sending signals to the local machine, which allows the local machine to carry out the operations described herein.