Abstract:
A method is disclosed for welding a first part and second part together. A spacer bead is first formed on the first part by directing a laser beam on one side of the first part. The second part is then assembled to the one side of the first part. The second part is then welded to the first part by directing a second laser beam in a partially circular pattern adjacent the spacer bead. An end portion of the weld terminates radially inside the partially circular pattern.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit of U.S. provision application Ser. No. 62/295,312 filed Feb. 15, 2016, the disclosure of which is hereby incorporated in its entirety by reference herein. 
     
    
     TECHNICAL FIELD 
       [0002]    This disclosure relates to laser welding processes for welding parts having an anti-corrosion layer. 
       BACKGROUND 
       [0003]    A wide variety of welding processes are used to join sheet metal panels or other types of parts together. Laser welding is one type of welding process that offers advantages such as the ability to weld from one side without requiring access to the back side of the parts to be welded as is required for spot or resistance welding. Laser welding also may be performed without a filler wire like manual inert gas (MIG) welding or tungsten inert gas (TIG) welding. Laser welding eliminates maintenance of weld tips, electrodes and torches. 
         [0004]    Welding parts that are coated with an anticorrosion layer of zinc or a zinc based coating composition creates problems for laser welding processes because the coating has a lower melting point than a steel substrate panel. When the coating is heated during a laser welding operation, the coating vaporizes creating smoke plumes and gaseous emissions. Plumes of smoke are removed from the path of the laser by blowing air across the welding area. Gaseous emissions from vaporizing the coating applied to an area of two parts that are assembled together in a face-to-face relationship must be permitted to escape from between the panels or the gaseous emissions may bubble up through the molten laser weld and create porosity in the weld. 
         [0005]    It has been proposed to provide a series of protuberances, or bumps, between panels through the use of a “humping effect” wherein a laser is used to heat the inner surface of one of two panels that are to be joined together. After the panels are placed face-to-face with the protuberances between the panels, a laser weld is formed through one side of the assembly while off-gassing from the coating is emitted through a space between the panels created by the bump shaped protuberances. One problem with bump shaped protuberances is that it is difficult to consistently control the height of the bumps. 
         [0006]    Another problem with laser welding panels arranged in a face-to-face relationship is that the weld may have increased porosity at the tail end of the weld. Porosity in the weld is unacceptable if the porosity affects more than a specific length of the weld. Laser welds that replace spot welds are generally configured in a C-shape that is less than 9 mm in diameter and have a curved length of 25 mm. If excessive porosity is encountered, there is insufficient space for extending the length of the C-shaped weld. 
         [0007]    This disclosure is directed to solving the problems of facilitating off-gassing coatings and assuring weld quality by reducing weld porosity, and other problems relating to laser welding. 
       SUMMARY 
       [0008]    According to one aspect of this disclosure, a method is disclosed for welding a first and second part together. A spacer bead is first formed on the first part by directing a laser beam on one side of the first part. The second part is then assembled to the one side of the first part. The second part is then welded to the first part by directing a second laser beam in a partial circular pattern adjacent the spacer bead and forming an end portion of a weld terminating radially inside the partially circular pattern. 
         [0009]    According to another aspect of this disclosure, the method may further comprise providing a coating on at least one of the first and second parts that is between the first and second parts when assembled together in the assembling step and off-gassing the coating from between the first and second parts that are separated by the spacer bead. 
         [0010]    The step of forming a spacer bead may be performed by directing the first laser beam toward spaced locations on the one side to form a plurality of spaced raised areas. The step of welding the second part to the first part may be performed by directing the second laser beam in a partial circular path outside the spaced raised areas with the end portion being formed inside the partial circular path. 
         [0011]    The step of forming a spacer bead may be performed by directing the first laser beam in a C-shaped path to form a C-shaped bead. The step of welding the second part to the first part may be performed by directing the second laser beam in a partial circular path outside the C-shaped bead with the end portion being formed inside the C-shaped bead. 
         [0012]    The step of forming a spacer bead may be performed by directing the first laser beam in a first C-shaped path to form a first C-shaped bead and by directing the first laser in a second C-shaped path radially outside the first C-shaped path to form a second C-shaped bead. The step of welding the second part to the first part may be performed by directing the second laser beam in a partial circular path between the first and second the C-shaped beads with the end portion being formed inside the first C-shaped bead. 
         [0013]    According to another aspect of this disclosure, a weld is disclosed that joins together first and second parts. The weld includes a spacer bead integrally formed on one side of the first part and a weld bead connecting the second part to the first part. The weld bead includes a partially circular shaped weld portion partially encircling the spacer bead and an end portion of the weld that terminates radially inside the partially circular shaped weld portion. 
         [0014]    The spacer bead may include a plurality of spaced raised areas and the partially circular shaped weld portion may extend around the spaced raised areas with the end portion extending radially inside the partially circular weld portion. 
         [0015]    The spacer bead may be a C-shaped bead and the partially circular shaped weld portion may extend around the C-shaped bead with the end portion extending radially inside the C-shaped bead. 
         [0016]    The spacer bead may be a first C-shaped bead. A second C-shaped bead may be provided that is disposed radially outside and concentric with the first C-shaped bead. The partially circular shaped weld portion may be disposed between the inner and outer C-shaped beads and the weld end portion may extend radially inside the first C-shaped bead. 
         [0017]    The above aspects of this disclosure and other aspects will be described below with reference to the attached drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0018]      FIG. 1  is a side elevation view of a vehicle door frame showing weld locations on the assembly. 
           [0019]      FIG. 2  is an exploded perspective view of the vehicle door frame assembly. 
           [0020]      FIG. 3  is a diagrammatic partial cross-section view of a remote laser welding tool. 
           [0021]      FIG. 4  is a programming diagram for forming three raised dimple spacers on a panel. 
           [0022]      FIG. 5  is a plan view of the three raised dimple spacers formed using the programming diagram of  FIG. 4 . 
           [0023]      FIG. 6  is a chart showing the power in Watts of the laser output as the laser forms a 2 mm dimple on a panel. 
           [0024]      FIG. 7  is a programming diagram for forming an inner and an outer C-shaped spacer on a panel. 
           [0025]      FIG. 8  is a plan view of the inner and the outer C-shaped spacer on a panel formed with the programming diagram of  FIG. 7 . 
           [0026]      FIG. 9  is a programming diagram for a G-shaped laser weld for joining panels together to form an assembly. 
           [0027]      FIG. 10  is a photograph of the G-shaped laser weld on a panel formed with the programming diagram of  FIG. 9 . 
           [0028]      FIG. 11  is a programming diagram for a G-shaped laser weld on an outer panel overlaid on a programming diagram for forming three raised dimple spacers on an inner panel. 
       
    
    
     DETAILED DESCRIPTION 
       [0029]    The illustrated embodiments are disclosed with reference to the drawings. However, it is to be understood that the disclosed embodiments are intended to be merely examples that may be embodied in various and alternative forms. The figures are not necessarily to scale and some features may be exaggerated or minimized to show details of particular components. The specific structural and functional details disclosed are not to be interpreted as limiting, but as a representative basis for teaching one skilled in the art how to practice the disclosed concepts. 
         [0030]    Referring to  FIG. 1 , a vehicle  10  is shown in part to illustrate a door frame  12  that defines a door opening  14  of the vehicle. A plurality of weld locations  16  used to assemble the door frame  12  is shown as an example. Part of the door frame is commonly referred to as the “A-pillar”  18 . 
         [0031]    Referring to  FIG. 2 , parts of the door frame  12  are shown in an exploded view to better illustrate the component parts of the door frame  12 . A hydro-formed tube front rail  20  reinforces the A-pillar (shown in  FIG. 1 ). Other parts of the A-pillar  18  include a roof rail header panel  22  and a windshield header panel  24 . The hydro-formed tubular front rail  20  extends to the “B-pillar”  26 . 
         [0032]    Referring to  FIG. 3 , a remote laser head  30  is shown that may be used to form laser welds on the vehicle  10 . Laser light is received through a laser light cable receiver  32  from a remote location. A plurality of pointer diodes  36  is provided to direct the laser light beam. A cover slide  38  is provided for collimation of the laser light beam. Adjustments of the laser beam in the Z direction (vertical) are made by a Z variation linear motor  40 . X and Y scanner mirrors  42  are provided to control movement of the laser beam in the X and Y directions. A focusing lens  44  focuses the laser on the object to be welded. A focusing optic cover slide  46  is provided to cover the focusing lens  44 . A cross-jet  48  is used to direct a flow of air across the laser light beam to eliminate any plume of smoke or gas from the path of the laser beam. 
         [0033]    Referring to  FIG. 4 , laser path plots  50 A,  50 B and  50 C are provided to show how the laser is controlled to traverse an inner surface of a panel  52 . The laser path plots  50 A,  50 B and  50 C have a length of approximately 2 mm and are arrayed as spaced partial sides of a triangular array. 
         [0034]    Referring to  FIG. 5 , a plurality of raised dimples  54 A,  54 B and  54 C are shown on a panel  52 . The height of the raised dimple is controlled in  FIG. 6 . 
         [0035]    Referring to  FIG. 6 , a series of power offset plot  56  is shown to illustrate how the power of the laser is controlled to form the three raised dimples  54 A- 54 C. Each power offset plot  56  shows that the laser is initially provided with a spike of power of, for example, 6000 watts after which the power is reduced after travelling 1 mm to approximately, for example, 1000 watts. In the second millimeter, the power of the laser output increases again to, for example, 6000 watts until it reaches a peak whereupon the power output of the laser is reduced to 0 watts. This process is repeated three times to provide the three raised dimples  54 A- 54 C that have a controlled height. 
         [0036]    Referring to  FIG. 7 , a laser path plot for forming an inner and outer C-shaped spacer  60  is shown. The concentric dual C-shaped spacer path  60  is formed by initially following the outer C-shape path  60 A with the laser emitting a plurality of controlled pulses. The laser path plot  60 A is initiated at the top end of the outer C and moves in a counter-clockwise direction until it reaches the lower end of the outer C. At this point, the laser beam is redirected to begin following the inner C-shape path  60 B at the top end of the inner C and rotates in a counter-clockwise direction until it reaches the lower end of the inner C. When panels having the dual C-shaped spacer  62  (shown in  FIG. 8 ) are welded together, the joining weld is formed between the outer and inner C-shaped spacers  62 A and  62 B. 
         [0037]    Referring to  FIG. 8 , a dual C-shaped spacer  62  is shown as it is formed on the inner surface of one of the panels to be assembled. The outer C-shaped spacer  62 A and inner C-shaped spacer  62 B form the dual C-shaped spacer  62 . 
         [0038]    Referring to  FIG. 9 , a laser path plot  64  for forming a G-shaped laser weld  66  is illustrated. The G-shaped laser weld  66  is shown in  FIG. 10  to be formed within the C-shaped spacers  62 A and  62 B. A tail  68  is formed on the G-shaped laser weld  66 . With prior art C-shaped laser welds, the tail  68  is the last portion of the weld formed and tends to have increased porosity and weld imperfections. With the G-shaped weld  66 , the end of the weld undercut is driven into the tail  68  of the G-shaped laser weld  66 . 
         [0039]    One of the problems faced by the disclosed welding process is that different materials are used in material stack-ups. For example, the outer panel is welded to the body side inner may be 0.7 mm mild hot dipped galvanized iron (HDGI). The body side inner assembly may include DP800 uncoated steel; and DP800 HDGI (galvanized high strength steel); DP1000 (uncoated high strength steel); high-strength low alloy 340 (HSLA); boron M1A37; Mart 1100 and mild hot dipped galvanized (HDG) steel. 
         [0040]    The equipment used to provide the remote laser  30  includes a Highyag remote scanning head, a Highyag EPS that controls the laser head, a 6000 watt laser and a programmable controller. 
         [0041]    The parameters used to control the remote laser  30  include the laser power, travel speed of the laser beam and the power ramp in/out as the raised dimple is formed and also as the G-shaped laser weld is formed. Critical factors affecting the process include plume suppression air flow to keep the path of the laser clear, controlling the dimple height and also the location of the dimples. Other factors include the part fit-up when two panels are assembled together for welding, and the focal point of the remote laser  30 . 
         [0042]    The plume is a plasma cloud emitted from the weld pool above the keyhole formed by the laser during welding. The plasma cloud absorbs laser energy and decreases the power delivered to the workpiece resulting in less penetration. The cross-jet  48  directs air or another gas to flow across the weld and shifts the plume out of the path of the laser. Plume suppression is not always necessary, however, plume suppression reduces inconsistent energy absorption and results in more consistent weld quality. Successful plume suppression results in more robust quality and penetration. A key factor to plume suppression is providing a consistent air flow from cross-jet  48 . 
         [0043]    Referring to the G-shaped laser weld  66  shown in  FIG. 10 , the welds  66  in one example are 9 mm in diameter and 25 mm in length. If 80% of the 25 mm weld is non-porous, the weld is determined to be acceptable. By forming a G-shaped weld, the tail of the weld is disposed within the partially circular periphery of the weld and can be lengthened to assure that 80% of the length of the weld is not porous resulting in an acceptable weld. 
         [0044]    To provide a raised dimple spacer, an inner side of two panels to be joined is provided with one or more dimples of controlled height to allow for out-gassing of zinc gases from the galvanized coatings. To form the laser raised dimples, the laser is quickly fired to raise the surface approximately 0.1 mm to 0.15 mm. The location of the dimples relative to the subsequently formed weld that joins the two panels is critical for successful out-gassing and prevention of porosity. 
         [0045]    Referring to forming the inner and outer C-shaped spacers  62 A and  62 B, this approach may be especially advantageous for materials such as galvanized HSLA340. HSLA340 is a boron material that is more volatile than other types of steel. 
         [0046]    By following the disclosed method, a remote laser welder may be used to form welds in a fast-paced manufacturing environment. Clamping and stamping quality must be controlled to maintain a part fit-up of between 0.1 mm and 0.3 mm gap across the entire welding surface to achieve acceptable weld quality. Coated materials, such as those coated with galvanized coatings or other zinc-based coatings, requires the inclusion of raised dimples or other spacers to provide an out-gas escape route and prevent out-gassing through the molten weld. It has been found that remote laser welding can perform at least three times as many welds in a single station cycle compared to traditional resistance spot welding. Remote laser welding also allows for product design with single sided access to weld locations and eliminates structural weakness caused by back-side access holes. 
         [0047]    The embodiments described above are specific examples that do not describe all possible forms of the disclosure. The features of the illustrated embodiments may be combined to form further embodiments of the disclosed concepts. The words used in the specification are words of description rather than limitation. The scope of the following claims is broader than the specifically disclosed embodiments and also includes modifications of the illustrated embodiments.