The technical field of this disclosure relates generally to laser welding and, more particularly, to a method of laser welding together two or more overlapping steel workpieces in which at least one of the steel workpieces includes a zinc-based surface coating.
Laser welding is a metal joining process in which a laser beam is directed at a metal workpiece stack-up to provide a concentrated energy source capable of effectuating a weld joint between the overlapping constituent metal workpieces. In general, two or more metal workpieces are first aligned and stacked relative to one another such that their faying surfaces overlap and confront to establish a faying interface (or faying interfaces) that extends through an intended weld site. A laser beam is then directed towards and impinges a top surface of the workpiece stack-up. The heat generated from the absorption of energy from the laser beam initiates melting of the metal workpieces and creates a molten weld pool within the workpiece stack-up. And, if the power density of the laser beam is high enough, a keyhole is produced directly underneath the laser beam and is surrounded by the molten weld pool. A keyhole is a column of vaporized metal derived from the metal workpieces within the workpiece stack-up that may include plasma.
The laser beam creates the molten weld pool in very short order upon impinging the top surface of the workpiece stack-up. Once created, the molten weld pool grows as the laser beam continues to deliver energy to the workpiece stack-up. The molten weld pool eventually grows to penetrate through the metal workpiece impinged by the laser beam and into the underlying metal workpiece or workpieces to a depth that intersects each of the established faying interfaces. The general shape and penetration depth of the molten weld pool can be managed by controlling various characteristics of the laser beam including its power, travel velocity (if any), and focal position. When the molten weld pool has reached the desired penetration depth in the workpiece stack-up, and optionally been moved within the stack-up by advancing the laser beam along the top surface of the stack-up, the transmission of the laser beam is ceased so that it no longer impinges the stack-up at the weld site. The molten weld pool quickly cools and solidifies (and collapses the keyhole if present) to form a laser weld joint comprised of resolidified composite workpiece material derived from each of the workpieces penetrated by molten weld pool. The resolidified composite workpiece material of the laser weld joint autogenously fusion welds the overlapping workpieces together at the weld site.
The automotive industry is interested in using laser welding to manufacture parts that can be installed on a vehicle. In one example, a vehicle door body may be fabricated from an inner door panel and an outer door panel that are joined together by a plurality of laser weld joints. The inner and outer door panels are first stacked relative to each other and secured in place by clamps. A laser beam is then sequentially directed at multiple weld sites around the stacked panels in accordance with a programmed sequence to form the plurality of laser weld joints as previously described. The process of laser welding inner and outer door panels—as well as other vehicle part components such as those used to fabricate hoods, deck lids, body structures such as body sides and cross-members, load-bearing structural members, engine compartments, etc.—is typically an automated process that can be carried out quickly and efficiently. The aforementioned desire to laser weld metal workpieces together is not unique to the automotive industry; indeed, it extends to other industries that may utilize laser welding including the aviation, maritime, railway, and building construction industries, among others.
The use of laser welding to join together coated metal workpieces that are often used in manufacturing practices can present challenges. For example, steel workpieces often include a zinc-based surface coating for corrosion protection. Zinc has a boiling point of about 906° C., while the melting point of the base steel substrate it coats is typically greater than 1300° C. Thus, when a steel workpiece that includes a zinc-based surface coating is laser welded, high-pressure zinc vapors are readily produced at the surfaces of the steel workpiece and have a tendency to disrupt the laser welding process. In particular, the zinc vapors produced at the faying interface(s) of the steel workpieces are forced to diffuse into and through the molten weld pool created by the laser beam unless an alternative escape outlet is provided through the workpiece stack-up. When an adequate escape outlet is not provided, zinc vapors may remain trapped in the molten weld pool as it cools and solidifies, which may lead to defects in the resulting weld joint—such as entrained porosity—as well as other weld joint discrepancies such as spatter, blowholes, and undercut joints. These weld joint deficiencies, if sever enough, can unsatisfactorily degrade the mechanical properties of the laser weld joint.
To deter high-pressure zinc vapors from diffusing into the molten weld pool, conventional manufacturing procedures have called for laser scoring or mechanical dimpling at least one of the two steel workpieces at each faying interface where a zinc-based coating is present before laser welding is conducted. The laser scoring or mechanical dimpling processes create spaced apart protruding features that impose a gap of about 0.1-0.2 millimeters between the faying surface on which they have been formed and the confronting faying surface of the adjacent steel workpiece, which provides an escape path to guide zinc vapors along the established faying interface and away from the weld site. But the formation of these protruding features adds an additional step to the overall laser welding process and is believed to contribute to the occurrence of undercut weld joints. It would be a welcome addition to the art if two or more steel workpieces—at least one of which includes a surface coating of a zinc-based material—could be laser welded together without having to necessarily score or mechanically dimple any of the steel workpieces in order to consistently form a durable weld joint with sufficient strength.
A method of laser welding a workpiece stack-up that includes overlapping steel workpieces is disclosed. The workpiece stack-up includes two or more steel workpieces, and at least one of those steel workpieces (and possibly all of the steel workpieces) includes a surface coating of a zinc-based material such as zinc or a zinc-iron alloy. The zinc-based surface coating preferably has a thickness that ranges from 2 μm to 50 μm. And while a zinc-based surface coating protects the underlying steel from corrosion, among other notable benefits, it can evolve high pressure zinc vapors when heated during laser welding. The evolution of such zinc vapors, in turn, can be a source of porosity in the laser weld joint and can also lead to other abnormalities such as spatter and blowholes. The disclosed laser welding method minimizes the impact that zinc-based surface coatings may have on the laser weld joint without requiring—but of course not prohibiting—the practice of certain procedures such as, for example, the intentional imposition of gaps between the steel workpieces at the faying interface where the zinc-based surface coating is present by way of laser scoring or mechanical dimpling.
To begin, the laser welding method involves providing a workpiece stack-up that includes two or more overlapping steel workpieces. The steel workpieces are stacked together such that a faying interface is formed between the faying surfaces of each pair of adjacent overlapping steel workpieces at a weld site. For example, in one embodiment, the workpiece stack-up includes first and second steel workpieces having first and second faying surfaces, respectively, that overlap and confront one another to establish a single faying interface. In another embodiment, the workpiece stack-up includes an additional third steel workpiece situated between the first and second steel workpieces. In this way, the first and second steel workpieces have first and second faying surfaces, respectively, that overlap and confront opposed faying surfaces of the third steel workpiece to establish two faying interfaces. When a third steel workpiece is present, the first and second steel workpieces may be separate and distinct parts or, alternatively, they may be different portions of the same part, such as when an edge of one part is folded over a free edge of another part.
After the workpiece stack-up is provided, a preliminary welding laser beam is directed at, and impinges, a top surface of the workpiece stack-up at an initial spot location to create a preliminary molten steel weld pool that penetrates into the workpiece stack-up from the top surface towards the bottom surface. The power density of the preliminary welding laser beam is selected to carry out this particular portion of the disclosed laser welding method in either conduction welding mode or keyhole welding mode. In conduction welding mode, the power density of the preliminary welding laser beam is relatively low, and the energy of the preliminary welding laser beam is conducted as heat through the steel workpieces to create only the preliminary molten steel weld pool. In keyhole welding mode, on the other hand, the power density of the preliminary welding laser beam is high enough to vaporize the steel workpieces and produce a keyhole directly underneath the preliminary welding laser beam within the preliminary molten steel weld pool. The keyhole provides a conduit for energy absorption deeper into workpiece stack-up which, in turn, facilitates deeper and narrower penetration of the preliminary molten steel weld pool.
The preliminary welding laser beam may be fixedly trained at the initial spot location on the top surface or it may be moved relative to a plane of the top surface at the initial spot location until the preliminary molten steel weld pool grows to the desired size. The preliminary molten steel weld pool may partially or fully penetrate the workpiece stack-up. In a preferred embodiment, for example, the preliminary molten steel weld pool is grown so that it intersects each faying interface (single interface in a two-workpiece stack-up or both interfaces in a three-workpiece stack-up) established within the workpiece stack-up, meaning that the preliminary molten steel weld pool fully traverses a thickness of the first steel workpiece and at least partially traverses a thickness of the second steel workpiece. Once the preliminary molten steel weld pool has reached the desired size, in terms of depth and diameter, the transmission of the preliminary welding laser beam is ceased at the initial spot weld location, causing the preliminary molten steel weld pool to solidify into a preliminary weld deposit. The preliminary weld deposit either partially or fully penetrates the workpiece stack-up depending on the acquired depth of the preliminary molten steel weld pool. Additional preliminary weld deposits may be formed at other initial spot locations in the same way. Anywhere from one to eight preliminary weld deposits are preferably formed depending on the size of the weld deposit(s) as well as the compositions of the steel workpieces.
Following the formation of the preliminary weld deposit(s), a principal welding laser beam is directed at, and impinges, the top surface of the workpiece stack-up radially outside of and away from the initial spot location(s) where the preliminary weld deposit(s) have been formed to create a principal molten steel weld pool that penetrates into the workpiece stack-up from the top surface towards the bottom surface and intersects each faying interface established within the stack-up. The power density of the principal welding laser beam, like before, is selected to carry out this particular portion of the disclosed laser welding method in either conduction welding mode or keyhole welding mode. The designation of the laser beams as “principal” and “preliminary” is not necessarily intended to indicate a difference in laser beam type, although such distinctions are not foreclosed, but rather is meant to specify the sequence in which the laser beams act on the workpiece stack-up and to differentiate where on the top surface of the stack-up the laser beams are directed. In particular, the preliminary welding laser beam is used to form the preliminary weld deposit(s) first, and, afterwards, the principal welding laser beam is advanced relative to the plane of the top surface of the workpiece stack-up around the preliminary weld deposit(s) to form a principal laser weld joint. The preliminary weld deposit(s) are formed basically to promote the strength and integrity of the principal laser weld joint, which is the primary structural joint that fusion joints the steel workpieces together.
The principal welding laser beam is advanced relative to the plane of the top surface of the workpiece stack-up along a beam travel pattern following creation of the principal molten steel weld pool and, optionally, the keyhole. Advancing the principal welding laser beam along the beam travel pattern translates the keyhole and the principal molten steel weld pool along a route that corresponds to the patterned movement of the principal welding laser beam relative to the top surface of the workpiece stack-up. Such advancement of the principal welding laser beam along the beam travel pattern leaves behind a trail of molten steel workpiece material in the wake of the principal welding laser beam and the corresponding route of the principal molten steel weld pool. This trail of molten steel workpiece material quickly cools and solidifies into resolidified composite steel workpiece material that is comprised of steel material from each steel workpiece penetrated by the principal molten steel weld pool. After the principal welding laser beam has completed its advancement along the beam travel pattern, the transmission of the principal welding laser beam within the annular weld area is ceased to terminate energy transfer to the workpiece stack-up. The collective resolidified composite steel workpiece material obtained from advancing the principal welding laser beam along the beam travel pattern provides the principal laser weld joint that autogenously fusion welds the workpieces together.
The beam travel pattern traced by the principal welding laser beam includes one or more weld paths that lie within an annular weld area as projected onto the plane (the x-y plane) of the top surface of the workpiece stack-up. The annular weld area that delimits the beam travel pattern surrounds a center area on the plane of the top surface that spans the at least one preliminary weld deposit. Consequently, as the principal welding laser beam moves along the beam travel pattern within the annular weld area, it does so without impinging on the center area. This type of patterned movement of the principal welding laser beam induces changes to the fluid velocity field within the principal molten steel weld pool, which agitates the weld pool and disturbs entrained zinc vapors, thereby promoting zinc vapor evolution from the weld pool. Additionally, the formation of the preliminary weld deposit(s) can reduce the amount of vaporizable zinc within the regions of the workpiece stack-up beneath the center area and annular weld area by boiling zinc and/or converting zinc to zinc oxide. As such, the composite resolidified steel workpiece material that constitutes the principal laser weld joint is less liable to include a debilitating amount of entrained porosity or be accompanied by other laser welding discrepencies such as spatter and/or blowholes.
In a preferred embodiment, a remote laser welding apparatus is used to form both the at least one preliminary weld deposit and the principal laser weld joint in the workpiece stack-up. The remote laser welding apparatus includes a scanning optic laser head that houses optical components that can move a laser beam relative to the plane at the top surface of the workpiece stack-up and also adjust a focal point of the laser beam up or down along a longitudinal axis of the laser beam. Separately-transmitted laser beams can thus be transmitted from the scanning optic laser head to form, in sequence, the at least one preliminary weld deposit and the principal laser weld joint. In particular, within a predetermined weld site, the scanning optic laser head directs the preliminary welding laser beam at a spot location on the top surface of the workpiece stack-up to form the preliminary weld deposit, and can optionally do so multiple times to form additional preliminary laser deposits, if desired. Then, after formation of the preliminary weld deposit(s), the same scanning optic laser head directs the principal welding laser beam at the top surface of the workpiece stack-up within the annular weld area and advances the laser beam along the beam travel pattern to form the principal laser weld joint.
The disclosed method of laser welding a workpiece stack-up comprised of two or more overlapping steel workpieces involves, first, forming at least one preliminary weld deposit in the workpiece stack-up with a preliminary welding laser beam and, second, forming a principal laser weld joint by impinging a top surface of the workpiece stack-up with a principal welding laser beam and advancing the principal welding laser beam relative to a plane of the top surface along a beam travel pattern confined within an annular weld area. The annular weld area and, thus, the beam travel pattern, surrounds a center area that spans the at least one preliminary weld deposit previously formed in the workpiece stack-up. The number of preliminary weld deposits spanned by the center area, which is not impinged by the principal welding laser beam during its advancement along the beam travel pattern, may range from a single preliminary weld deposit to a plurality of preliminary weld deposits, with a typical number of preliminary weld deposits ranging anywhere from one to eight. Each of the preliminary weld deposits may intersect each of the faying interfaces established within the workpiece stack-up.
The principal laser weld joint, which is the primary joint that fusion welds the overlapping steel workpieces together at a weld site, is less liable to include entrained porosity or be accompanied by spatter or blowholes for at least two reasons: (1) the patterned movement of the principal welding laser beam promotes more aggressive zinc vapor evolution from the corresponding principal molten steel weld pool; and (2) the preceding formation of the at least one preliminary weld deposit acts to remove vaporizable zinc from the workpiece stack-up in the regions beneath the center area and the annular weld area. Moreover, if any porosity is present in the resolidified composite steel workpiece material of the principal laser weld joint, the conductive heat transfer that emanates radially inward from the annular weld area during laser welding has the affect of sweeping porosity into a region of the principal laser weld joint beneath the center area defined on the plane of the top surface of the workpiece stack-up. This is noteworthy since centrally located porosity is less likely to affect the mechanical properties of the principal laser weld joint compared to porosity located at the perimeter of the joint.
The at least one preliminary weld deposit and the principal laser weld joint can be formed using the same laser welding apparatus. For example, a remote laser welding apparatus or a conventional laser welding apparatus may be operated to form the at least one preliminary weld deposit and the principal laser weld joint in succession using the preliminary welding laser beam and the principal welding laser beam, respectively, that may or may not differ in their beam characteristics (e.g., power level, focal point location, travel speed, etc.). Each of the preliminary welding laser beam and the principal welding laser beam may be a solid-state laser beam or a gas laser beam depending on the characteristics of the steel workpieces being joined and the laser welding apparatus being used. Some notable solid-state lasers that may be used are a fiber laser, a disk laser, a direct diode laser, and a Nd:YAG laser, and a notable gas laser that may be used is a CO2 laser, although other types of lasers may certainly be employed. In a preferred implementation of the disclosed method, which is described below in more detail, a remote laser welding apparatus is operated to sequentially form both the at least one preliminary weld deposit and the principal laser weld joint through the use of a solid-state state laser that can serve as both the preliminary welding laser beam and the principal welding laser beam.
The disclosed laser welding method may be performed on a variety of workpiece stack-up configurations. For example, the disclosed method may be used in conjunction with a “2T” workpiece stack-up (
Referring now to
The workpiece stack-up 10 may include only the first and second steel workpieces 12, 14, as shown in
The term “faying interface” is used broadly in the present disclosure and is intended to encompass a wide range of overlapping relationships between the confronting first and second faying surfaces 28, 32 that can accommodate the practice of laser welding. For instance, the faying surfaces 28, 32 may establish the faying interface 34 by being in direct or indirect contact. The faying surfaces 28, 32 are in direct contact with each other when they physically abut and are not separated by a discrete intervening material layer or gaps that fall outside of normal assembly tolerance ranges. The faying surfaces 28, 32 are in indirect contact when they are separated by a discrete intervening material layer such as a structural adhesive—and thus do not experience the type of interfacial abutment that typifies direct contact—yet are in close enough proximity that laser welding can be practiced. As another example, the faying surfaces 28, 32 may establish the faying interface 34 by being separated by gaps that are purposefully imposed. Such gaps may be imposed between the faying surfaces 28, 32 by creating protruding features on one or both of the faying surfaces 28, 32 through laser scoring, mechanical dimpling, or otherwise. The protruding features maintain intermittent contact points between the faying surfaces 28, 32 that keep the faying surfaces 28, 32 spaced apart outside of and around the contact points by up to 1.0 mm and, preferably, between 0.2 mm and 0.8 mm.
As shown in
At least one of the first or second steel workpieces 12, 14—and sometimes both includes a surface coating 40 that overlies the base steel substrate 36, 38. Still referring to
Referring back to
Some examples of a suitable solid-state laser beam that may be used in conjunction with the remote laser welding apparatus 18 include a fiber laser beam, a disk laser beam, and a direct diode laser beam. A preferred fiber laser beam is a diode-pumped laser beam in which the laser gain medium is an optical fiber doped with a rare earth element (e.g., erbium, ytterbium, neodymium, dysprosium, praseodymium, thulium, etc.). A preferred disk laser beam is a diode-pumped laser beam in which the gain medium is a thin laser crystal disk doped with a rare earth element (e.g., a ytterbium-doped yttrium-aluminum garnet (Yb:YAG) crystal coated with a reflective surface) and mounted to a heat sink. And a preferred direct diode laser beam is a combined laser beam (e.g., wavelength combined) derived from multiple diodes in which the gain medium is semiconductors such as those based on aluminum gallium arsenide (AlGaAS) or indium gallium arsenide (InGaAS). Other solid-state laser beams not specifically mentioned here may of course be used.
The scanning optic laser head 54 includes an arrangement of mirrors 56 that can maneuver the laser beam 24 relative to a plane oriented along the top surface 20 of the workpiece stack-up 10 within an operating envelope 58 that encompasses the weld site 16. Here, as illustrated in
The arrangement of mirrors 56 and the z-axis focal lens 60 cooperate during operation of the remote laser welding apparatus 18 to dictate the desired movement of the laser beam 24 within the operating envelope 58 at the weld site 16 as well as the position of the focal point 62 along the longitudinal axis 64 of the beam 24. The arrangement of mirrors 56 includes a pair of tiltable scanning mirrors 68. Each of the tiltable scanning mirrors 68 is mounted on a galvanometer 70. The two tiltable scanning mirrors 68 can move the location at which the laser beam 24 impinges the top surface 20 of the workpiece stack-up 10 anywhere in the x-y plane of the operating envelope 58 through precise coordinated tilting movements executed by the galvanometers 70. At the same time, the z-axis focal lens 60 controls the location of the focal point 62 of the laser beam 24 in order to help administer the laser beam 24 at the correct power density. All of these optical components 60, 68 can be rapidly indexed in a matter of milliseconds or less to advance the laser beam 24 relative to the top surface 20 of the workpiece stack-up 10 at a travel velocity that may reach as high as 120 m/min (meters per minute) while positioning the focal point 62 of the laser beam somewhere between 100 mm above (+100 mm) the top surface 20 of the workpiece stack-up 10 and 100 mm below (−100 mm) the top surface 20 along the longitudinal beam axis 64.
A characteristic that differentiates remote laser welding (also sometimes referred to as “welding on the fly”) from other conventional forms of laser welding is the focal length of the laser beam 24. Here, as shown in best in
As part of the disclosed laser welding method, and referring now to
The preliminary molten steel weld pool 80 (and the keyhole 82 if present) may be grown to any of a variety of sizes. As shown in
Once the preliminary molten steel weld pool 80 (and the keyhole 82 if present) has reached the appropriate size, the transmission of the preliminary welding laser beam 76 is ceased at the initial spot location 78. Ceasing transmission of the preliminary welding laser beam 76 at the initial spot location 78 may involve halting the transmission of the laser beam 76 from the scanning optic laser head 54 or simply moving laser beam 76 outside of the initial spot location 78 relative to the top surface 20 of the workpiece stack-up 10. By ceasing transmission of the preliminary welding laser beam, the keyhole 82 (if present) collapses and preliminary molten steel weld pool 80 solidifies into the preliminary weld deposit 74, as illustrated in
The at least one preliminary weld deposit 74 may include a plurality of deposits 74 formed in a similar fashion. In particular, a second preliminary welding laser beam 76 may be directed at a second spot location 78 within the weld site away from the previously-formed preliminary weld deposit 74. The second preliminary welding laser beam 76 is operable to form a second preliminary molten steel weld pool 80 (with an optional keyhole 82) that solidifies into a second preliminary weld deposit 74 following cessation of the laser beam 76 at the second spot location 78. This same process may be repeated to form any number of preliminary weld deposits 74. In fact, in a preferred embodiment, anywhere from one to eight preliminary weld deposits 74 may be formed in close proximity within the workpiece stack-up 10. Moreover, the grouped preliminary weld deposits 74 may be the same or different in terms of their penetration depth and size. To be sure, in one embodiment, all of the plurality of preliminary weld deposits 74 may fully penetrate the workpiece stack-up 10 and have a diameter between 2 mm and 4 mm at the top surface 20. In other embodiments, however, only some of the preliminary weld deposits 74 may fully penetrate the workpiece stack-up 10 while others may only partially penetrate the stack-up 10.
After the at least one preliminary weld deposit 74 is formed, the remote laser welding apparatus 18 forms a principal laser weld joint 88 that fusion welds the steel workpieces 12, 14 together at the weld site 16, as shown in
The heat generated from absorption of the focused energy of the principal welding laser beam 90 initiates melting of the first and second metal workpieces 12, 14 to create a principal molten steel weld pool 100 that penetrates into the workpiece stack-up 10 from the top surface 20 towards the bottom surface 22. The principal molten steel weld pool 100 penetrates far enough into the workpiece stack-up 10 that it intersects the faying interface 34 established within the workpiece stack-up 10 between the first and second steel workpieces 12, 14. The principal welding laser beam 90, moreover, preferably has a power density sufficient to vaporize the workpiece stack-up 10 directly beneath where it impinges the top surface 20 of the stack-up 10. This vaporizing action produces a keyhole 102, which is a column of vaporized workpiece steel that may contain plasma. The keyhole 102 is formed within the principal molten steel weld pool 100 and also penetrates into the workpiece stack-up 10 from the top surface 20 towards the bottom surface 22 and intersects the faying interface 34 within the workpiece stack-up 10. The keyhole 102 and the surrounding principal molten steel weld pool 100 may fully (as shown) or partially penetrate the workpiece stack-up 10.
After the principal molten steel weld pool 100 and the keyhole 102 are created, the principal welding laser beam 90 is advanced relative to the plane of the top surface 20 of the workpiece stack-up along a beam travel pattern 104 (
As noted above, the beam travel pattern 104 is traced by the principal welding laser beam 90 with respect to the plane oriented along the top surface 20 of the workpiece stack-up 10 inside the annular weld area 92 and around the center area 98 that spans the at least one preliminary weld deposit 74. As such, the illustrations presented in
Referring now to
Other embodiments of the beam travel pattern 104 are indeed contemplated in addition to those shown in
The principal welding laser beam 90 may be advanced along the beam travel pattern 104 within the annular weld area 92 in a variety of ways. For example, with respect to the spiral beam travel pattern 800 shown in
As the principal welding laser beam 90 is being advanced along the beam travel pattern 104, which is depicted best in
The depth of penetration of the keyhole 102 and the surrounding principal molten steel weld pool 100 is controlled during advancement of the principal welding laser beam 90 along the beam travel pattern 104 to ensure the steel workpieces 12, 14 are fusion welded together by the principal laser weld joint 88 at the weld site 16. In particular, as mentioned above, the keyhole 102 and the principal molten steel weld pool 100 intersect the faying interface 34 established between the first and second steel workpieces 12, 14 within the workpiece stack-up 10. In fact, in a preferred embodiment, as shown best in
The depth of penetration of the keyhole 102 and the surrounding principal molten steel weld pool 100 can be attained by controlling various characteristics of the principal welding laser beam 90 including the power level of the laser beam 90, the position of a focal point 110 of the laser beam 90 along a longitudinal axis 112 of the beam 90, and the travel velocity of the laser beam 90 when being advanced along the beam travel pattern 104. These beam characteristics can be programmed into a weld controller capable of executing instructions that dictate the penetration depth of the keyhole 102 and the surrounding principal molten steel weld pool 100 with precision. While the various characteristics of the principal welding laser beam 90 can be instantaneously varied in conjunction with one another to attain the penetration depth of the keyhole 102 and the principal molten steel weld pool 100 at any particular portion of the beam travel pattern 104, in many instances, regardless of the profile of the beam travel pattern 104, the power level of the principal welding laser beam 90 may be set to between 0.2 kW and 50 kW, or more narrowly between 1 kW and 10 kW, the travel velocity of the principal welding laser beam 90 may be set to between 2 m/min and 120 m/min or, more narrowly, between 8 m/min and 50 m/min, and the focal point 108 of the principal welding laser beam 90 may be located fixedly or variably somewhere between 30 mm above the top surface 20 (+30 mm) of the workpiece stack-up 10 and 30 mm below (−30 mm) the top surface 20.
Without being bound by theory, the formation of the at least one preliminary weld deposit 74 in the workpiece stack-up followed by the advancement of the principal welding laser beam 90 along the beam travel pattern 104 within the annular weld area 92 is believed to promote good strength—in particular good peel and cross-tension strength in the principal laser weld joint 88. Specifically, the formation of the at least one preliminary weld deposit 74 reduces the amount of vaporizable zinc within the workpiece stack-up 10 beneath the center area 98 and the annular weld area 92 by boiling off zinc or by converting zinc to high-boiling point zinc oxide. This reduction in the amount of vaporizable zinc during formation of the preliminary weld deposit(s) 74 means that less high-pressure zinc vapors will be generated and possibly become trapped in the principal molten steel weld pool 100 during advancement of the principal welding laser beam 90 along the beam travel pattern 104. As a result, the presence of entrained porosity within the resolidified composite steel workpiece material 108 of the principal laser weld joint 88 is kept to manageable levels or altogether eliminated, and the potential for of spatter and blowholes is significantly minimized.
Moreover, the advancement of the principal welding laser beam 90 along the beam travel pattern 104 within the annular weld area 92 has the effect of driving any zinc vapors that may be generated in a radially inward direction towards the interior of the principal laser weld joint 88. The consolidation and induced guidance of zinc vapors towards the interior of the principal laser weld joint 88 occurs either along the faying interface 34 if the portion of the workpiece stack-up 10 beneath the center area 98 does not melt and/or through molten steel if some or all of the portion of the stack-up 10 beneath the center area 98 does melt as a result of conductive heat transfer. By guiding zinc vapors towards the interior of the principal laser weld joint 88, the patterned movement of the principal welding laser beam 90 within the annular weld area 92 effectively sweeps a significant portion of any porosity that may be present into a region of the principal laser weld joint 88 beneath the center area 98 on the plane of the top surface 20 of the workpiece stack-up 10. The concentration of porosity beneath the center area 98 is tolerable since centrally-located porosity is less likely to affect the mechanical properties of the principal laser weld joint 88 compared to porosity located at the perimeter of the weld joint 88.
Referring now to
As a result of stacking the first, second, and third steel workpieces 12, 14, 200 in overlapping fashion to provide the workpiece stack-up 10, the third steel workpiece 200 has two faying surfaces 204, 206. One of the faying surfaces 204 overlaps and confronts the first faying surface 28 of the first steel workpiece 12 and the other faying surface 206 overlaps and confronts the second faying surface 32 of the second steel workpiece 14, thus establishing two faying interfaces 208, 210 within the workpiece stack-up 10 that extend through the weld site 16. These faying interfaces 208, 210 are the same type and encompass the same attributes as the faying interface 34 already described above with respect to
The formation of the at least one preliminary weld deposit 74 and, subsequently, the principal laser weld joint 88 in the “3T” workpiece stack-up 10 are achieved in the same manner as previously described. The formation of each preliminary weld deposit 74, for example, is carried out by directing the preliminary welding laser beam 76 at a spot location 78 on the top surface 20 of the workpiece stack-up 10 within the weld site 16 to create the preliminary molten steel weld pool 80 and optional keyhole 82, as illustrated in
The formation of the principal laser weld joint 88 is carried out by advancing the principal welding laser beam 90 along the beam travel pattern 104 within the annular weld area 92 as discussed above. Such advancement of the principal welding laser beam 90 translates the optional keyhole 102 and the surrounding principal molten steel weld pool 100 along a corresponding route to ultimately yield the resolidified composite steel workpiece material 108 that collectively constitutes the principal laser weld joint 88 and fusion welds the three steel workpieces 12, 14, 200 together. And, like before, in a preferred embodiment, the keyhole 102 and the surrounding principal molten steel weld pool 100 fully penetrate the workpiece stack-up 10, as shown in
The above description of preferred exemplary embodiments and specific examples are merely descriptive in nature; they are not intended to limit the scope of the claims that follow. Each of the terms used in the appended claims should be given its ordinary and customary meaning unless specifically and unambiguously stated otherwise in the specification.
Filing Document | Filing Date | Country | Kind |
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PCT/CN2016/083112 | 5/24/2016 | WO | 00 |