Abstract:
A laser welding head capable of producing a plurality of beams that are proficient in providing a unique keyhole. The welding head is movable through a plurality of positions relative to substrates that enables the plurality of beams to engage the substrates in a manner that welds the substrates in a variety of stack-up positions. A welding method using the welding head is also provided.

Description:
RELATED APPLICATION(S) 
       [0001]    This application claims the benefit of Provisional Application Ser. No. 60/821,357, filed Aug. 3, 2006, which is incorporated by reference herein. 
     
     FIELD OF THE INVENTION 
       [0002]    The present invention relates to laser welding systems adapted to provide a plurality of laser beams. 
       BACKGROUND OF THE INVENTION 
       [0003]    The statements in this section merely provide background information related to the present disclosure and may not constitute prior art. 
         [0004]    Laser welding has been known and used in the automotive industry, as well as other industries, for some time. Generally, it is known to use lasers to weld steel plates together. It is common to coat these steel plates with a protective coating that inhibits rust and other materials that reduce the useful life of the plates. The use of coatings, such as a zinc coating, for example, results in the plates being galvanized or galvannealed. The use of a coating on the plates, however, causes problems when welding the plates together. That is, the boiling temperature of materials used to coat the plates is different than a melting temperature of the steel used in the plates. As such, when welding the coated plates together, the coating may boil and infiltrate the molten pool of the weld and cause it to spatter, become porous, or both. The spattering and porousness of the weld results in the weld being mechanically weak and prone to corrosion. 
         [0005]    To overcome the infiltration of the coating into the molten pool it is common to weld the coated plates together when they are separated from each other by a gap. This gap assists the coating in its gaseous form to be expelled through the gap away from the molten pool. Notwithstanding, the use of a gap is impractical due to increases in manufacturing time and cost. 
         [0006]    Accordingly, there is a need for an improved laser welding system that reduces spattering and porous welds that are caused by the coating of the plates infiltrating the molten pool of the weld and causing the weld to spatter or become porous. Moreover, it is desirable to be able to weld a pair of coated plates together without having any gap between them. 
       SUMMARY OF THE INVENTION 
       [0007]    The present teachings have been developed to overcome the drawbacks of conventional welding systems. In this regard, the present teachings provide a laser welding head capable of producing a plurality of beams that are proficient in providing a unique keyhole. The welding head is movable through a plurality of positions relative to substrates that enables the plurality of beams to engage the substrates in a manner that welds the substrates in a variety of stack-up positions. A welding method using the welding head is also provided. 
         [0008]    Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating the preferred embodiment of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention. 
     
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]    The present invention will become more fully understood from the detailed description and the accompanying drawings, wherein: 
           [0010]      FIG. 1  is a perspective view of a laser system according to the present teachings; 
           [0011]      FIG. 2A  is a top perspective view of a laser head of the laser system according to the present teachings; 
           [0012]      FIG. 2B  is a bottom perspective view of the laser head of the laser system according to the present teachings; 
           [0013]      FIG. 3  is a perspective view of the laser head with a housing cover removed from the laser head; 
           [0014]      FIG. 4  is a front perspective view of the laser head engaging a pair of laser beams to a pair of substrates to be welded; 
           [0015]      FIG. 5  is a cross-sectional view of a pair of beams engaging a pair of substrates to be welded; 
           [0016]      FIG. 6  is a cross-sectional view of a pair of beams engaging a substrate to be welded in a deep penetration welding method; 
           [0017]      FIGS. 7A and 7B  are top perspective views of keyholes formed using the welding system according to the present teachings; 
           [0018]      FIGS. 8A and 8B  are top perspective view of keyholes and weld pools formed using the welding system according to the present teachings; and 
           [0019]      FIGS. 9A to 9D  are cross-sectional views of various stack-ups of substrates that may be welded together using the welding system of the present teachings. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0020]    The following description of the preferred embodiment(s) is merely exemplary in nature and is in no way intended to limit the invention, its application, or uses. 
         [0021]    Referring to  FIG. 1 , the laser welding system according to the present teachings will now be described. As shown in  FIG. 1 , the laser system  10  includes a laser head  12  that is movable through a plurality of positions by a machine such as a robot  14 . The laser head  12  is movable by the robot  14  relative to a substrate stage  16 . In this regard, the robot  14  is capable of moving the laser head  12  in the x-, y-, and z-axis directions. Further, the robot  14  is capable of rotating the laser head  12  about the z-axis through 360°, as well as rotating the head  12  through a plurality of points relative to the x- and y-axes. In this manner, the laser head  12  is movable through a locus of points that is substantially hemispherical in nature. It should also be understood, however, that the substrate stage  16  is movable through a plurality of positions (i.e., in the x-, y-, and z-axis directions) relative to the laser head  12 , as well as angularly movable relative to the laser head  12 . Further, other devices capable of moving the head  12  include a gantry or any other motion system known to one skilled in the art. 
         [0022]    Now referring to  FIG. 2A ,  FIG. 2B , and  FIG. 3 , the laser head  12  of the laser welding system  10  will now be described. As shown in  FIGS. 2A and 2B , the head  12  is formed of housing cover  13  attached by fasteners such as a plurality of screws  11  to a housing plate  15 . Attached by housing plate  15  is a mounting flange  17  that connects the head  12  to robot  14 . The head  12  houses a laser device  16 . The laser device  16  is comprised of a fiber laser receptor that is connected to an outer surface  18  at a first end  20  of the housing  13 . In other words, the laser device  16  is attached to the housing  13  at an inlet portion  20 . The laser device  16  is capable of emitting a laser or some other type of energy from the inlet portion  20  to a second end  22 , or output portion  22 . Although the present embodiment is described using a fiber laser receptor as the laser device  16  for emitting a laser beam, it should be understood that any device known in the art sufficient for emitting a concentrated form of energy that may be focused and reflected may be used with the present teachings without departing from the spirit and scope thereof. In this regard, by non-limiting examples, devices that may be used in the laser welding system  10  according to the present teachings include a Nd:YAG laser, a diode laser, an electron beam emitter, and the like. As for a power of the laser device, it is preferable to use a device adapted to emit energy in the range between 4 kW to 10 kW. 
         [0023]    The head  12  includes a plurality of adjustment devices  24 ,  26 , and  28  for adjusting various parameters of the laser head  12  including an adjustment of the angle between a first and second beam (knob  24 ), an adjustment of the inter-spot distance between the first and second beams (knob  26 ), and an adjustment of a power ratio between the first and second beams (knob  28 ). These adjustment devices extend from housing cover  13 , but are connected to various instruments inside the head  12  that are mounted to housing plate  15 . 
         [0024]    Referring now to  FIG. 3 , it can be seen that housing cover  13  has been removed from housing plate  15  to expose the devices within the head  12 . Inward (i.e., within the housing cover  13  from the inlet  20 ) from the laser device  16  is a beam splitter  30 . The beam splitter  30  is a device adapted to reflect and transmit the energy  32  emitted by the laser device  16 . That is, the beam splitter  30  is adapted to split the laser emitted energy  32  into a leading beam  34  and a trailing beam  36 . 
         [0025]    To split the energy beam  32 , the beam splitter  30  is a device formed to have aperture (not shown) which deflects the radiation (i.e., energy)  32  emitted from the laser device  16  with a higher divergence, and lets radiation  32  with a lower divergence pass through downstream (i.e., toward the outlet  22 ). More specifically, as the energy beam  32  enters the housing  13  from laser device  16 , it diverges or spreads outward. As the beam  32  contacts the beam splitter  30 , radiation that has spread too far (i.e. the higher divergence radiation) contacts the beam splitter at positions outside of the aperture. This higher divergence radiation is reflected by the beam splitter  30  downward to form the trailing beam  36 . Radiation  32  that has not spread too far (i.e., the lower divergence radiation) arrives at the beam splitter  30  and is able to pass through the aperture downstream toward a second end (outlet  22 ) of the head  12  to form the leading beam  34 . In this manner, as stated above, the beam splitter  30  is adapted to reflect and transmit the energy  32  emitted by the laser device  16 . 
         [0026]    The radiation  32  with a low divergence passes through the aperture towards a collimator  38 . This lower divergence radiation that passes through the aperture of the beam splitter  30  toward the collimator  38  will, as stated above, subsequently form a first beam  34  or “leading beam”  34 . Because the beam splitter  30  reflects the radiation  32  with a higher divergence, the quality of the leading beam  34  is increased, and reduced spot sizes of the leading beam  34  can be achieved which allows larger depth of focus. Further, clipping the outer parts of the radiation  32  also enables an angle between the leading and trailing beams  34  and  36  that is smaller than a full divergence of a single beam. By the phrase “leading beam,” it is meant a first laser beam to engage substrates to be welded. The so-called leading beam  34  is generally used to weld at least a pair of substrates together, as well as open a keyhole that will be described later. Controlling parameters of the leading beam  34  such as its spot size, power, and the like also enable increasing the welding speed with the leading beam  34 . For example, a smaller spot size is advantageous in achieving increased welding speeds, which in turn increases productivity. 
         [0027]    The radiation  32  with the higher divergence is deflected downward toward a reflecting mirror  40 . The higher divergence radiation deflected by the beam splitter  30  will, as also stated above, subsequently form a second beam  36  or “trailing beam”  36 . By the phrase “trailing beam,” it is meant a second laser beam to engage substrates to be welded. The trailing beam  36  is generally for controlling a shape of the keyhole that is formed during welding. In this regard, controlling parameters of the trailing beam  36  such as its spot size, power, as well as angle relative to the leading beam  34  increase the ability of the leading beam  36  to control the formation of the keyhole. Controlling formation of the keyhole is advantageous in controlling a quality of the weld between substrates. 
         [0028]    The beam splitter  30  is slidably movable along an optical axis (i.e., along an axis that travels from the inlet portion  20  to the outlet portion  22  of the head  12 ) of the leading beam  34  to adjust a power ratio of the leading beam  34 . More particularly, the beam splitter  30  is positioned on a stage  42  that is slidably movable in a direction from the inlet  20  of the housing cover  13  toward the outlet  22  of the housing cover  13  along a track  44 . The stage  42  is threadingly connected to the track  44 , and to move the beam splitter  30 , the adjustment device  28  (see  FIG. 2B ) that extends from the inlet  20  side of the housing cover  13  may be rotated to rotate the track  44  in a manner that the stage  42  translates along the threading (not shown) of the track  44 . 
         [0029]    To increase the power ratio of the leading beam  34  compared to the trailing beam  36 , the beam splitter  30  is moved along track  44  to a position closer to inlet portion  20 . In this manner, more radiation  32  with a lower divergence is allowed to pass through the aperture of the beam splitter  30 . To decrease the power ratio of the leading beam  34  compared to the trailing beam  36 , the beam splitter  30  is moved along track  44  to a position farther away from inlet portion  20  (i.e., toward outlet  22 ). In this manner, the radiation  32  emitted from laser device  16  is allowed to diverge or spread further, which in turn lessens the amount of lower divergence radiation that passes through the aperture of the beam splitter  30 . Regardless, it should be understood that adjustment of the beam splitter  30  relative to the laser device  16  enables the power of each beam to be adjusted relative to the other. Preferably, however, the ratio of power of the leading beam  34  compared to the trailing beam  36  is 75:25. More preferably the ratio is between 80:20 or 45:55. Although the above ratios are preferred, it should be understood that any power ratio between the two beams may be used without departing from the spirit and scope of the present teachings. 
         [0030]    The leading beam collimating lens  38  and a leading beam focusing lens  46  are located downstream from the beam splitter  30 . The collimating lens  38  is mounted to a support bracket  48 . Support bracket  48  is a substantially L-shaped bracket that is fixedly mounted to housing plate  15 . Due to the beam splitter  30  being slidably movable, a distance between the beam splitter  30  and the leading beam collimating lens  38  is variable depending on the desired power ratio of the beams  32  and  34 . This is beneficial to maintain proper collimation of the low divergence radiation that is allowed to pass through the beam splitter  30 . A collimating lens  38  is a device that reduces the divergence of the radiation. 
         [0031]    After the leading beam  34  passes through the leading beam collimating lens  38 , the beam  34  travels downstream toward the leading beam focusing lens  46 . The focusing lens  46  is mounted to a slide bracket  49  that is slidably coupled to the support bracket  48 . Slide bracket  49  is connected to support bracket  48  via a track (not shown), which in turn is coupled to an adjustment device  50  that may be rotated to move the slide bracket  48  and lens  46  along a surface of support bracket  48  in a manner that can focus and un-focus the leading beam  34 . Adjustment device  50  can be reached by removing removable plate  51  from housing cover  13  (see  FIG. 2A ). 
         [0032]    The leading beam focusing lens  46  focuses the radiation to the desired focal plane. After passing through the leading beam focusing lens  46 , the leading beam  34  passes through a cover glass  52  and cross jet device  54  located at an outlet  22  of the head  12 . Cover glass assembly  52  is a removable piece mounted to housing plate  15 . Cover glass assembly  52  may be removed as needed for cleaning or when the assembly  52  needs to be replaced. Cover glass assembly  52  prevents spattering from entering the head  12  to protect the optical components of the head  12  from becoming damaged. Just downstream from cover glass  52 , cross jet device  54  is fixedly mounted to outlet portion  22  of housing plate  15 . Cross jet device  54  deflects any spattering that occurs during the welding process from contacting or landing on cover glass assembly  52 . 
         [0033]    The radiation  32  with a higher divergence is deflected by the beam splitter  30  toward a mirror  40 . After being deflected by the beam splitter  30 , the radiation passes through a trailing beam collimating lens  58  that reduces the divergence of the radiation. The trailing beam collimating Tens  58  is mounted to the stage  42  that supports the beam splitter  30  and, accordingly, is slidably moveable therewith. The radiation passes through the trailing beam collimating lens  58  toward the trailing beam reflective mirror  40 . 
         [0034]    The trailing beam reflective mirror  40  is mounted to a mounting plate  60  that is slidably moveable in a direction from the inlet side  22  of the head  12  to the outlet side  22  of the head  12 . In this manner, the reflective mirror  40  can maintain a proper optical alignment with beam splitter  30  and all other optical components in all six axes (i.e., the x-, y-, and z-axes, and three angles of rotation). That is, the reflective mirror  40  is movable to stay dynamically aligned with beam splitter  30 . The mounting plate  60  is in turn mounted to a trailing beam adjustment plate  62 . Preferably, the reflective mirror  40  is formed of copper and may be water cooled to enable the reflective mirror  40  to reflect the radiation  32  emitted by the high power laser device  16 . 
         [0035]    To slidably move the reflective mirror  40  to stay in proper optical alignment with the beam splitter  30 , the mounting plate  60  is connected via an attachment bracket  64  to a panel  66  including a slot  68 . Panel  66  is in turn fixed to trailing beam collimator  58 . The slot  68  is angularly oriented relative to mounting plate  60  such that when the adjustment device  28  that extends from the housing cover  13  is rotated to move stage  42  and collimator  58 , a pin  70  located on the collimator  58  that is engaged with the slot  68  is moved through positions in the slot  68  that slides the mounting plate  60  along tracks  61  formed on a surface of the trailing beam adjustment plate  62 . As the reflective mirror  40  is slidably movable upon actuation of adjustment device  28 , the reflective mirror  40  can be actuated to stay optically in-line with beam splitter  30  and collimator  58 . 
         [0036]    The reflective mirror  40  is also movable to adjust an angle between the leading beam  34  and the trailing beam  36 . To adjust the angle between the beams  34  and  36 , the reflective mirror  40  may be moved either toward or away from the collimator  58  and beam splitter  30 . To actuate the mirror  40  in this manner, the mirror  40  is pivotably mounted to mounting plate  60 . By rotation of adjustment knob  24 , mirror  40  is pivoted such that the mirror  40  is moved toward or away from collimator  58  and beam splitter  30 . As the mirror  40  pivots in this manner, the mirror  40 , and angle of the mirror  40 , is independently adjustable such that an angle of the trailing beam  36  relative to the leading beam  34  is adjustable. 
         [0037]    Preferably, the angle of the trailing beam  36  relative to the leading beam  34  is adjustable between the angles of 10 to 20 degrees. More preferably, the angle between the trailing beam  36  relative to the leading beam  34  is adjustable between the angles of 10 to 15 degrees. Notwithstanding, one skilled in the art will acknowledge and appreciate that design of the optical system in laser head  12  may be modified to accommodate any angle between the leading and trailing beams as may be desired. Regardless, the minimum angle between the two beams  32  and  34  is less than a divergence of the entering radiation  32 . 
         [0038]    After reflection of the trailing beam  36  by the trailing beam reflecting mirror  40 , the trailing beam  36  travels toward the outlet  22  of the head  12  to a trailing beam focusing lens  72 . The trailing beam focusing lens  72  focuses the trailing beam  36  to the desired focal plane. The trailing beam focusing lens  72 , similarly to the leading beam focusing lens  46 , is mounted to another support bracket  74  that is slidably moveable along an inner surface of the housing cover  13 . To adjust a focus of the lens  72 , an adjustment device  76  that is coupled to the support bracket  74  is engaged to slide the support bracket  74  and focusing lens  72  in a direction between the trailing beam reflecting mirror  40  and the outlet  22  of the head  12 . Adjustment device  76  may be reached by removing cover plate  53  from housing cover  13  (see  FIG. 2A ). 
         [0039]    It is preferable that the focal plane of the trailing beam  36  matches that of the leading beam  34 , but the present teachings should not be limited to such a configuration. More particularly, it should be understood that the leading and trailing beams  34  and  36  may be focused at differing focal planes in accordance with the welding task at hand. After passing through the trailing beam focusing lens  72 , the trailing beam  36  passes through the cover glass  52  and cross jet device  54  located at an outlet  22  of the head  12 . 
         [0040]    Changing the inter-spot distance requires moving the mirror  40  and focusing lens  72  of the trailing beam  36  only. That is, the mirror  40  and focusing lens  72  are mounted to adjustment plate  62  which in turn is mounted to inter spot distance adjustment plate  63 . By rotating adjustment knob  26  (see  FIG. 2B ), adjustment plate  62 , and mirror  40  and focusing lens  72 , may be actuated toward and away from an edge  65  of housing plate  15 . In this manner, the inter-spot distance between the leading beam  34  and trailing beam  36  may be adjusted. It should also be understood that the focusing lenses  46  and  72  for the leading and trailing beams, respectively, can be adjusted individually. In this manner, the spot size of the leading and trailing beams  32  and  34  may be adjusted. Further, if a different shaped beam is being used (e.g., elongated or a customized shape) for the trailing beam  36 , the focusing lens  72  can be adjusted to change a size of the shape. 
         [0041]    Now referring to  FIG. 4 , a welding method using the welding system according to the present teachings will now be described.  FIG. 4  is a cross-sectional view showing a pair of substrates  78  and  80  held together by a pair of clamps  82  and  84  so that there is no gap, or at least no intended gap, between them. The use of clamps  82  and  84  ensures that the substrates  78  and  80  remain stationary during the welding process, as well as ensure that there is no gap, or at least no intended gap, between them. It should be understood, however, that the present teachings do not require the use of clamps  82  and  84 . That is, it is contemplated that merely resting the substrates  78  and  80  on top of one another is sufficient such that there is no gap, or at least no intended gap, between them. As also shown in  FIG. 4 , it can be seen that beams  34  and  36  are directed toward the substrates  78  and  80  from the head  12  to connect the substrates  78  and  80  via a weld. 
         [0042]    Now referring to  FIG. 5 , the pair of substrates  78  and  80  are disposed on top of each other such that there is no gap, or at least no intended gap, between them. To weld each of these substrates  78  and  80  together, the first beam  34  (leading beam) and the second beam  36  (trailing beam) move across the substrates  78  and  80  in a welding direction. As the beams  34  and  36  move across the substrates  78  and  80 , the substrates  78  and  80  are sufficiently heated and melted to form a molten pool  82 . The molten pool, or weld pool  82 , subsequently forms the weld between the substrates  78  and  80  upon cooling. 
         [0043]    The leading beam  34  is aligned with the substrates  78  and  80  to be substantially orthogonal to the substrates  78  and  80 . With respect to the trailing beam  36 , this beam is angled relative to an orientation of the leading beam  34  by adjusting an angular position of the trailing beam adjustment plate  62 . As shown in  FIG. 5 , the trailing beam  36  is angled from the leading beam  34  by an angle α. In this regard, it is preferred that the angle by which the trailing beam  36  is angled relative to the leading beam  34  is preferably between 10° and 20°, and more preferably between 10° and 15°. Further, the center of each beam (i.e., a center of each spot) is preferably aligned to be parallel along the weld direction. 
         [0044]    It should be understood, however, that the leading beam  34  is not required to be normal to the pair of substrates  78  and  80 . In contrast, the leading beam  34  can be angled relative to the substrates  78  and  80  as well. In this regard, it should be understood that the leading beam  34  can be angled in a forward direction, (i.e., in the weld direction) or in a rearward direction (i.e., against the weld direction) by adjusting an orientation of the head  12  with robot  14 . It is preferred, however, that if the leading beam  34  is to be tilted, then it should be tilted in the rearward direction. 
         [0045]    When engaging the beams  34  and  36  to the substrates  78  and  80 , the beams  34  and  36  should be focused at a depth that ranges from an upper surface  84  of the upper substrate  78  to a lower surface  86  of the lower substrate  80 . This depth is dependent on a number of parameters including a thickness of the substrates  78  and  80 , a thickness of the coatings  88 , a power of the laser device  16 , etc. By selecting a focus depth according to the predetermined parameters, each of the beams  34  and  36  can sufficiently engage the substrates  78  and  80  at an intensity that sufficiently melts the substrates  78  and  80  to provide a uniform weld pool  82  in the welding direction. Further, it should be understood that each beam may be focused at different depths by adjusting the focusing lenses  46  and  72 . 
         [0046]    The laser device  16  used to weld the substrates  78  and  80  together may be any type of laser device known to one skilled in the art. In this regard, however, a laser device such as a Nd:YAG laser, a CO 2  laser, and a fiber ytterbium laser (Yb) are preferred. With respect to a power of these lasers, it should be understood that these lasers are currently commercially available at wattages that range from 1 kW to 30 kW. It should be understood, however, that any power suitable for use of these lasers may be used in accordance with the present teachings. Preferably, the laser device  16  has a power in the range of 4 kW to 10 kW. 
         [0047]    Moreover, although not shown in  FIG. 5 , it should be understood that a shape of the beams  34  and  36  can be any shape known to those skilled in the art. In this regard, the shape or imprint of the beams  34  and  36  may be circular, elliptical, square, rectangular, horseshoe, crescent-shaped, or the like without departing from the spirit and scope of the present teachings. The shape of the beams  34  and  36  is adjustable in that focusing lenses  46  and  72  may be comprised of a plurality of differently-shaped lenses that modify the beams  34  and  36  to the desired shape. 
         [0048]    A distance between the beams  34  and  36 , a so-called inter-beam distance (i.e., inter-spot), may be determined by the various parameters associated with the welding conditions. That is, the inter-beam distance between the leading beam  34  and the trailing beam  36  will be calculated on a case-by-case basis. Various parameters that should be considered include a thickness of the substrates  78  and  80 , the power (wattage) of the laser  16 , the type of laser  16  being used, welding speed, and a thickness of any coating  88  that may be disposed on the substrates  78  and  80 . Additional factors include a length of the weld and the type of material being used as the substrates  78  and  80 . 
         [0049]    Each of these factors should be taken into consideration because if the beams  34  and  36  are separated by too great a distance, the trailing beam  36  may generate a concavity in the rear keyhole wall, or an indentation may form in a sidewall of the keyhole. The generation of the concavity or indentation results in a non-stable structure for the molten metal in the weld pool  82  at the keyhole walls that further results in spattering and porosity in the weld. As a result, it is preferred that the inter-beam distance is between −2 and 2 mm, more preferably between 0 and 1.2 mm, and most preferably between 0.6 to 0.9 mm. With these inter-beam distances, it should be understood and appreciated that beams  34  and  36  may overlap. By overlapping the beams  34  and  36 , an increase in power density may be achieved which may result in the laser beams  34  and  36  more fully engaging the substrates  78  and  80 . 
         [0050]    The beam strengths are dependent on the type of laser being used, the laser wattage associated with the laser, and whether the beams  34  and  36  are overlapped. It should be understood, however, that the leading beam  34  opens the keyhole in the substrates  78  and  80  and the second beam  36  (trailing beam) expands and controls the shape of the keyhole. The shape and orientation of the keyhole is important to controlling the robustness of the welding process and whether or not the weld results in a satisfactory joining of the substrates. 
         [0051]    With respect to the types of substrates  78  and  80  that may be used, it should be understood that in an automotive application the substrates  78  and  80  are generally steel that are coated with zinc (Zn). Although Zn is preferably used to galvanize or galvanneal the steel substrates  78  and  80 , it should be understood that the present teachings are also applicable to steel substrates that have been coated with other materials. In this regard, it should be understood that in addition to a Zn coating, a magnesium-based (Mg) coating, an aluminum-based (Al) coating, an adhesive coating, or a plastic coating such as polypropylene may be coated on the steel substrates  78  and  80  without affecting the use of the present teachings. Moreover, the present teachings are also applicable to substrates  78  and  80  formed of a material other than steel. In this regard, materials such as aluminum, magnesium, iron, other metals, or alloys thereof may be used. 
         [0052]    While the above description of a leading beam  34  and a trailing beam  36  has been described relative to welding a pair of substrates  78  and  80  that overlap each other, it should be understood that the present teachings should not be limited thereto. More particularly, referring to  FIG. 6 , a dual beam welding method is shown that is beneficial during a deep penetration welding process. The deep penetration welding process may be used when welding a pair of substrates  78  and  80  that must be connected at a location deeper (i.e., at least 6 mm below a surface of the substrate) than a location that is normally used to connect a pair of substrates  78  and  80 . For example, welding an engine block or transmission component at an edge of the block or component, respectively, to another substrate. 
         [0053]    Referring to  FIG. 6 , it can be seen that the leading beam  34  substantially penetrates a distance into the substrate  78 . A problem that arises during these deep penetrations, however, is that the deeply penetrating leading beam  34  may reflect off a surface of the substrate  78  at a root (i.e., bottom) of the keyhole and become a reflected beam  90  that penetrates into the molten pool  82 . The reflected beam  90  may form a bump or surface concavity  92  in the molten pool  82  that, as the leading beam  34  moves in the welding direction, will be swallowed by the molten pool  82 . Once the bump  92  is swallowed, a pore  94  forms in the weld pool  82  as well as the solidified weld  96 . This is undesirable in that a porous weld  96  may be relatively weak. 
         [0054]    To combat the formation of the bumps  92  and pores  94 , the trailing beam  36  (shown in phantom in  FIG. 6 ) angled relative to the leading beam  34  smoothes the molten pool  82  such that the reflected beam  90  does not impinge the molten pool  82  at an angle sufficient to form a bump  92 . The smoothing of the molten pool  82  may be adjusted according to an angle of the trailing beam  36 . That is, the trailing beam  36  may be angled relative to the leading beam  34  to eliminate, or at least substantially minimize, the formation of bumps  92  that lead to the formation of pores  94 . 
         [0055]    Now referring to  FIGS. 7A and 7B , the keyhole geometry described above will be more fully described. In  FIG. 7A , the keyhole  98  when viewed in plan is essentially a conical shape  100  with a rounded first end  102  and second end  104 . The first end  102  is formed by the leading beam  34  and the expanding conical shape  100 , as well as the rounded second end  104 , are formed by the trailing beam  36  which controls and shapes the keyhole  98 . 
         [0056]    The keyhole  98  is an important aspect of the present teachings because the formation and configuration of the keyhole  98  assists in the expulsion of the gas of the coating  88  that is formed during the welding process. When the substrates  78  and  80  are coated with a coating  88  such as Zn, Mg, Al, or the like, these materials have a boiling temperature that is less than a melting temperature of the steel substrates  78  and  80 . As such, during the welding process, these elements are emitted as a gas. Controlling the evacuation of these gases is important to forming a satisfactory weld that securely fastens each of the substrates  78  and  80  together. This is because the emission of these gasses without controlled evacuation during the welding process may result in spattering and/or a porous weld which can lead to a weak connection between the substrates  78  and  80 . 
         [0057]    Controlling the evacuation of coating gases has a profound effect on the formation of the weld. The expulsion of the gases can be controlled according to Bernoulli&#39;s Law which states that P 1 V 1 =P 2 V 2 . 
         [0058]    According to Bernoulli&#39;s law, in a tube having varying diameters, the product of gas pressure and gas velocity is constant, independent of a change in tube size. In areas having a small diameter cross-section, the gas has a high velocity and low pressure. In contrast, in areas with a large diameter cross-section, the gas has a low velocity and high pressure. Using this principle, the configuration of the keyhole  98  can be used to control gas evacuation which in turn improves the properties of the weld. 
         [0059]    The leading beam  34  creates the narrow portion of the conical structure  100  of the keyhole  98 . In this narrow portion of the keyhole  98 , the gas will have high velocity and low pressure. The momentum of the gas is opposite the welding direction and believed to be proportional with the welding speed, as well as affected by coating thickness and type. 
         [0060]    The trailing beam  36  controls the formation of the remaining portions  104  of the keyhole  98  to have a wider area  104 . In the wider area, the gas will have a low velocity and higher pressure. Due to this increase in the size of the keyhole  98 , the velocity of the gas in the keyhole  98  sufficiently slows so that the gas does not impinge on a rear surface  104  of the keyhole  98 . In other words, the gas velocity is slowed enough to allow the gas to expel in various directions without contacting a rear surface  104  of the keyhole  98 . This prevents, or at least substantially minimizes the spattering caused by the emission of gas from the zinc coating or any other coating. Further, the emission of the coating gas before it impinges on a rear surface  104  of the keyhole  98  also reduces the effect of the weld being porous. Again, this results in a more satisfactory weld bead which increases strength of the weld joining the two substrates. 
         [0061]    In contrast, a keyhole  98  as shown in  FIG. 7B  will not have the desired effect of expelling gas that arises from the coated substrates. Instead, the opposite effect will occur in that the velocity of the gas will increase as it moves toward the rear keyhole wall  104 . Again, this is undesirable because gas impinging on the rear keyhole wall  104  increases the likelihood of spattering in the weld. 
         [0062]    Although the keyhole  98  shown in  FIG. 7A  is shown to have smooth sides, the shape of the keyhole  98  can be varied depending on the type of substrates  78  and  80  being used, a thickness of the substrates  78  and  80 , a coating thickness, a power of the beams  34  and  36 , a diameter of the beams  34  and  36  (or imprint of the beams on the substrates), etc. The primary method of increasing the second or trailing end  104  of the keyhole  98 , however, is to use a trailing beam  36  that has a larger beam diameter or imprint compared to the leading beam  34 . Preferably, the trailing beam  36  has a beam diameter in the range of 0.03 to 4.00 mm, while the leading beam  34  has a beam diameter in the range of 0.02 to 2.00 mm. More preferably, the trailing beam  36  has a beam diameter in the range of 0.4 to 0.8 mm, while the leading beam has a beam diameter in the range of 0.1 to 0.4 mm. Notwithstanding, one skilled in the art will readily acknowledge and appreciate that various parameters of the welding process can be adjusted according to the specific task where the welding of the present teachings is being used to make the present teachings adaptable to a variety of applications, including automotive applications. In addition to having a smaller spot size, the leading beam  34  can be independently adjusted to have a larger depth of focus to assist in forming the opening of the keyhole  98 . 
         [0063]    Although a smoothly formed keyhole  98  similar to that shown in  FIG. 7A  is preferred, other keyhole shapes can be formed without departing from the scope of the present teachings. To form different keyhole shapes, the inter-beam distance may be altered, or the beams  34  and  36  may be off-set from one another. More specifically, referring to  FIGS. 8A and 8B , it can be seen that the planes of the trailing and leading beams  34  and  36  can be varied. 
         [0064]    As shown in  FIG. 8A , each of the beams follows a specific line or axis  106 . That is, a center of each of the beams intersects with this axis  106  such that a keyhole  98  similar to that shown in  FIG. 7A  is formed. In contrast, as shown in  FIG. 8B , the leading beam  34  may follow an axis that is offset from the axis  106  of the trailing beam  36  to make a shape of the keyhole  98  irregular. This is done by rotating the head  12  about the Z axis relative to the substrates  78  and  80 . This irregular or non-symmetric keyhole  108  may be desirable in various applications such as when welding substrates at joints or edges. Moreover, the non-symmetric keyhole  108  will have a non-symmetric flow of the molten material  82  as indicated by the arrows. The non-symmetric flow is advantageous in that it eliminates, or at least substantially minimizes, a center-axis porosity of the weld by separating the location of where the two side streams of molten material  82  flowing from the first end  110  toward the second end  112  of the keyhole  108  meet. Further, the non-symmetric flow changes a location of the last solidification point  114  of the molten alloy  82  which also assists in reducing porosity of the weld. Yet another benefit of this non-symmetric flow configuration is the intentional separation of the final solidification surface from the axis where a porosity may form. In this way, the weld&#39;s mechanical properties are improved (i.e., strengthened). 
         [0065]    As described above, the present teachings are advantageous when welding substrates  78  and  80  that are disposed over each other with no gap, or at least no intended gap, between them. It should be understood, however, that the present teachings are also advantageously applicable to additional substrate configurations.  FIGS. 9A to 9D  depict additional substrate configurations that can be welded using the present teachings. In these various configurations, it should also be understood that the dual beams of the present teachings are advantageous because “fit-up” problems between the substrates  78  and  80  may occur during production. That is, various tolerances of the substrates  78  and  80  or other factors may cause the substrates  78  and  80  to not “fit-up” prior to welding. Notwithstanding, with the welding system  10  of the present teachings, a more robust process is achievable in that the beams may be oriented in a plurality of positions that assist in overcoming the fit-up problems. In addition, a higher quality weld is achievable, as well as increased productivity. 
         [0066]      FIG. 9A  depicts a configuration where edges  116  of the substrates  78  and  80  abut each other. The welding of plates  78  and  80  in this manner is a butt welding. As shown in  FIG. 9A , the welding head  12  may be rotated 90° about the Z axis relative to the substrates  78  and  80  so that the first and second beams  34  and  36  are disposed laterally to each other when moving in a weld direction instead of “following” each other when the beams  34  and  36  are in an axis that matches the weld direction. Because the head  12  is movable through a plurality of points, the beams  34  and  36  may be angled so that the beams  34  and  36  engage the substrates  78  and  80  at an angle sufficient to melt the substrates  78  and  80  and weld them together. In this regard, it should be understood that the head  12 , may be angled at any position that sufficiently enables the substrates  78  and  80  to be welded. 
         [0067]    This configuration is also advantageous when, as shown in  FIG. 9A , the substrates  78  and  80  to be welded include different thicknesses. Moreover, because an intensity of each beam  34  and  36  may be adjusted, a thicker substrate may be engaged by a beam with a higher power relative to a beam that engages a thinner plate. Although substrates  78  and  80  including different thicknesses are shown in  FIG. 9A , it should be noted that the present teachings are adaptable to butt welding substrates  78  and  80  with substantially equal thicknesses as well. 
         [0068]      FIG. 9B  shows a configuration where bent edges  118  of the substrates  78  and  80  meet each other. This configuration is an edge welding configuration, and the welding head  12  may again be rotated 90° about the z axis so that the first and second beams  34  and  36  are disposed laterally relative to each other. Although substrates  78  and  80  including the same thickness are used in  FIG. 9B , it should be understood that substrates  78  and  80  with different thicknesses may be welded together without departing from the scope of the present teachings. 
         [0069]      FIG. 9C  is a configuration where edges  120  of the substrates  78  and  80  overlap each other to form a lap joint  122 . In this configuration, the welding head  12  may be rotated 90° about the z-axis so that the first and second beams  34  and  36  are disposed laterally to each other. Alternatively, the beams  34  and  36  may be aligned in a plane that is in line with a weld direction. Regardless, it should be understood that the beams  34  and  36  may moved in various positions relative to each other to ensure a proper weld between the substrates  78  and  80 . 
         [0070]    Now referring to  FIG. 9D , the substrates  78  and  80  are in a T-joint configuration  124 . When the substrates  78  and  80  are disposed in a T-joint configuration  124 , the welding head  12  may be rotated 90° about the z-axis as well as angled relative (i.e., rotated about the x- or y-axis) to the T-joint  124  so that the first and second beams  34  and  36  are disposed laterally while engaging the substrates  78  and  80  in an angled manner. The maneuverability of the head  12  enables the beams  34  and  36  to precisely engage the substrates  78  and  80  in the T-joint  124  at the necessary angle to ensure a sufficient weld. Again, it should also be understood that the intensity of the beams  34  and  36  may also be properly adapted to properly connect the substrates  78  and  80 . 
         [0071]    It should be understood that although the head  12  has been described above as being rotatable 90°, the head  12  in actuality may be rotated through 360° such that the beams  34  and  36  can be positioned relative to the substrates  78  and  80  in a variety of positions. For example, the head  12  may be rotated through an angle that enables the beams  34  and  36  to form a weld pool  82  similar to that shown in  FIG. 8B , even when welding substrates  78  and  80  in the configurations shown in  FIGS. 9A to 9D . In this manner, the head  12  may be moved such that the beams  34  and  36  are capable to engage the substrates  78  and  80  in way that joins them with a weld bead that is mechanically strong and non-porous. 
         [0072]    The description of the invention is merely exemplary in nature and, thus, variations that do not depart from the gist of the invention are intended to be within the scope of the invention. Such variations are not to be regarded as a departure from the spirit and scope of the invention.