Patent Application
20180214983
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 aluminum workpieces.
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) within an intended weld site. A laser beam is then directed at 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 establishes a molten weld pool within the workpiece stack-up. The molten weld pool penetrates through the metal workpiece impinged upon by the laser beam and into the underlying metal workpiece or workpieces to a depth that intersects each of the established faying interfaces. 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—typically miliseconds—once it impinges the top surface of the workpiece stack-up. After the molten weld pool is formed and stable, the laser beam is advanced along the top surface of the workpiece stack-up while tracking a predetermined weld path, which has conventionally involved moving the laser beam in a strict forward direction without any side-to-side variation. Such advancement of the laser beam translates the molten weld pool along a corresponding course relative to top surface of the workpiece stack-up and leaves behind molten workpiece material in the wake of the advancing weld pool. This penetrating molten workpiece material cools and solidifies to form a weld joint comprised of re-solidified workpiece material. The resultant weld joint fusion welds the overlapping workpieces together.
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 welds. 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. At each weld site where laser welding is performed, the laser beam is directed at the stacked panels and conveyed along a predefined laser beam travel path, which may be configured to produce the weld joint in any suitable overall shape including, for example, as a circular spot weld joint, a stitch weld joint, or a staple weld joint. 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, load-bearing structural members, etc.) is typically an automated process that can be carried out quickly and efficiently.
Aluminum workpieces are an intriguing candidate for many automobile component parts and structures due to their high strength-to-weight ratio and their ability to improve the fuel economy of the vehicle. The use of laser welding to join together aluminum workpieces, however, can present challenges. Most notably, aluminum workpieces almost always include a protective coating that covers an underlying bulk aluminum substrate. This protective coating may be a refractory oxide coating that forms passively when fresh aluminum is exposed to atmospheric air or some other oxygen-containing medium. In other instances, however, the protective coating may be a metallic coating comprised of zinc or tin, or it may be a metal oxide conversion coating comprised of oxides of titanium, zirconium, chromium, or silicon, as disclosed in U.S. Patent Application No. US2014/0360986, the entire contents of which are incorporated herein by reference. The protective coating inhibits corrosion of the underlying aluminum substrate through any of a variety of mechanisms depending on the composition of the coating. But the presence of the protective anti-corrosion coating also makes it more challenging to autogenously fusion weld aluminum workpieces together by way of laser welding.
The protective anti-corrosion coating is believed to affect the laser welding process by introducing weld defects in the final laser weld joint. When, for example, the protective coating is a passive refractory oxide coating, the coating is difficult to break apart and disperse due to its high melting point and mechanical toughness. As a result, residual oxides may accumulate in the molten aluminum weld pool and contribute to the formation of weld defects, such as porosity, in the solidified weld joint. As another example, if the protective coating is zinc, the coating may readily vaporize to produce high-pressure zinc vapors (zinc has a boiling point of about 906° C.) at the faying interface of the aluminum workpieces. These zinc vapors may, in turn, diffuse into and through the molten aluminum weld pool created by the laser beam unless provisions are made to vent the zinc vapors away from the weld site, which may involve subjecting the workpiece stack-up to additional and inconvenient manufacturing steps prior to welding. The other materials mentioned above that may constitute the protective anti-corrosion coating can present similar issues that may ultimately affect and degrade the mechanical properties of the weld joint.
The unique challenges that underlie the use of laser welding to fusion join aluminum workpieces together have lead many manufactures to reject laser welding as a suitable metal joining process despite its potential to bestow a wide range of benefits. In lieu of laser welding, these manufacturers have turned to mechanical fasteners, such self piercing rivets or flow-drill screws, to join together two or more aluminum workpieces. Such mechanical fasteners, however, take much longer to put in place and have high consumable costs compared to laser weld joints. They also increase manufacturing complexity and add extra weight to the part being manufactured—weight that is avoided when joining is accomplished by way of autogenous fusion laser welds—that offsets some of the weight savings attained through the use of aluminum workpieces in the first place. A comprehensive laser welding strategy that can make aluminum laser welding a viable option in even the most demanding manufacturing settings would thus be a welcome addition to the art.
A method of laser welding a workpiece stack-up that includes overlapping aluminum workpieces is disclosed. The workpiece stack-up includes two or more aluminum workpieces, and at least one of those aluminum workpieces (and preferably all of the aluminum workpieces) includes a protective anti-corrosion coating. The term “aluminum workpiece” as used here in the present disclosure refers broadly to a workpiece that includes a base aluminum substrate comprised of at least 85 wt. % aluminum. The aluminum workpiece may thus include a base aluminum substrate comprised of elemental aluminum or any of a wide variety of aluminum alloys. Moreover, the protective anti-corrosion coating that covers the base aluminum substrate is preferably the passive refractory oxide coating that naturally forms when fresh aluminum is exposed to atmospheric air or some other source of oxygen. In alternative embodiments, however, the anti-corrosion coating may be a zinc coating, a tin coating, or a metal oxide conversion coating. The disclosed method minimizes the impact that these and other anti-corrosion coatings may have on the properties of final weld joint.
To begin, the laser welding method involves providing a workpiece stack-up that includes two or more overlapping aluminum workpieces (e.g, two or three overlapping aluminum workpieces). The aluminum workpieces are superimposed on each other such that a faying interface is formed between the faying surfaces of each pair of adjacent overlapping aluminum workpieces. For example, in one embodiment, the workpiece stack-up includes first and second aluminum 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 aluminum workpiece situated between the first and second aluminum workpieces. In this way, the first and second aluminum workpieces have first and second faying surfaces, respectively, that overlap and confront opposed faying surfaces of the third aluminum workpiece to establish two faying interfaces. When a third aluminum workpiece is present, the first and second aluminum 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 back over on itself and hemmed over a free edge of another part.
After the workpiece stack-up is provided, a laser beam is directed at, and impinges, a top surface of the workpiece stack-up to create a molten aluminum weld pool that penetrates into the workpiece stack-up and intersects each faying interface established within the workpiece stack-up. The power density of the laser beam is selected to carry out the laser welding method in either conduction welding mode or keyhole welding mode. In conduction welding mode, the power density of the laser beam is relatively low, and the energy of the laser beam is conducted as heat through the aluminum workpieces to create only the molten aluminum weld pool. Indeed, the molten aluminum weld pool created during conduction welding mode is relatively shallow, typically having a width at the top surface of the workpiece stack-up that is greater than a penetration depth of the molten aluminum weld pool into the workpiece stack-up. In keyhole welding mode, the power density of the laser beam is high enough to vaporize the aluminum workpieces and produce a keyhole directly underneath the laser beam within the molten aluminum 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 molten aluminum weld pool. As such, the molten aluminum weld pool created during keyhole welding mode typically has a width at the top surface of the workpiece stack-up that is less than the penetration depth of the weld pool.
The laser beam is advanced relative to the top surface of the workpiece stack-up along one or more predefined travel paths following creation of the molten aluminum weld pool. In particular, within each laser beam travel path, the laser beam is advanced from a start point to an end point, which may be the same or different points on the top surface, to thereby translate the molten aluminum weld pool along a course that corresponds to the travel path of the laser beam. Such advancement of the laser beam leaves behind molten aluminum workpiece material in the wake of the travel path of the laser beam and the corresponding course of the weld pool. This molten workpiece material quickly cools and solidifies into a weld joint comprised of re-solidified aluminum that autogenously fusion welds the aluminum workpieces together. Here, in the disclosed laser welding method, the weld joint is strong and durable, and its structure and properties are consistently attainable in a manufacturing setting as a result of the peculiar travel path of the laser beam between the start and end points, as will be further explained below. Eventually, upon reaching the end point, the laser beam is removed from the top surface of the workpiece stack-up.
Unlike conventional laser welding practices, in which the laser beam is advanced unidirectionaly in a strict forward direction, the laser beam in the disclosed method experiences movement in two directions as it is advanced relative to the top surface of the workpiece stack-up. Specifically, while being advanced along any of the one or more laser beam travel paths, the laser beam moves in a forward direction away from the start point and towards the end point and further moves back and forth in a lateral direction transverse to the forward direction. The back and forth movement of the laser beam that occurs while the laser beam is also moving in the forward direction is believed to more effectively disturb (e.g., fracture and break down, vaporize, or otherwise) the protective anti-corrosion coating as compared to purely unidirectional movement in the forward direction. The back and forth movement of the laser beam is also believed to clear a wider swath of the protective anti-corrosion coating in an around the course of travel of the molten aluminum weld pool. These two effects attained through the bidirectional movement of the laser beam ultimately minimize the occurrence of source porosity and other related weld defects in the resultant weld joint.
The laser beam can be advanced along myriad travel paths that incorporate both movement in the forward direction and movement back and forth in the lateral direction. For example, in a preferred embodiment, the laser beam is oscillated from the start point to the end point in a sinusoidal pattern that includes repeating waves. These repeating waves have peak-to-peak amplitudes and wavelengths that gauge the movement of the laser beam in the lateral direction and the frequency of such movement, respectively. And, while those characteristics of the travel path of the laser beam may vary depending on several factors, a preferred implementation involving the sinusoidal pattern includes repetitive waves having peak-to-peak amplitudes ranging from 0.1 mm to 6.0 mm and wavelengths ranging from 0.1 mm to 6.0 mm. Of course, other laser beam travel paths may be implemented besides those that embody the sinusoidal pattern. Some examples of alternative travel paths include those that embody a rectangular wave pattern, a zig-zag wave pattern, and a continuous loop pattern, to name but a few.
The disclosed method of laser welding a workpiece stack-up comprised of two or more overlapping aluminum workpieces calls for advancing a laser beam relative to a top surface of the workpiece stack-up such that the laser beam experiences movement in forward direction as well as back and forth movement in a lateral direction. Any type of laser welding apparatus, including remote and conventional laser welding apparatuses, may be employed to advance the laser beam relative to the top surface of the workpiece stack-up. Moreover, the operational power density of the laser beam may be selected to perform the method in either conduction welding mode or keyhole welding mode. The laser beam may thus be a solid-state laser beam or a gas laser beam depending on the characteristics of the aluminum workpieces being joined and the laser welding mode desired to be practiced. Some notable solid-state lasers that may be used are a fiber laser, a disk 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 used so long as they are able to create the molten aluminum weld pool. In a preferred implementation of the disclosed method, which is described below in more detail, a remote laser welding apparatus directs and advances a solid-state laser beam at and along the workpiece stack-up while practicing laser welding in keyhole welding mode.
Referring now to
While the faying interface 30 is, broadly speaking, established between the portions of the first and second faying surfaces 20, 24 that overlap and confront one another, the particular attributes of the faying interface 30 can take on several different forms. For instance, the overlapping and confronting portions of the faying surfaces 20, 24 may directly or indirectly contact one another. The faying surfaces 20, 24 are in indirect contact when they are separated by an intermediate material—such as a thin layer of weld-through adhesive or sealer—yet remain in close enough proximity that remote laser welding can still be practiced. Additionally, the overlapping and confronting portions of the faying surfaces 20, 24 can make complimentary flush contact (direct or indirect) at the weld site 32, meaning that the faying surfaces 20, 24 are closely mated together and are not purposefully separated by gaps or spaces imposed by intentionally formed protruding features. This type of close complimentary contact, which allows for small indiscriminate breaks or spaces as a result of acceptable tolerances in the size and shape of the workpieces 12, 14 or otherwise, is permitted since the disclosed method provides another mechanism (i.e., bidirectional laser beam movement) to help counteract the possible adverse effects associated with the boiling of zinc at the faying interface(s). And, while not necessarily required, one or both of the faying surfaces 20, 24 may include protruding features formed by laser scoring, mechanical dimpling, or otherwise, to assist in zinc vapor escape, if desired.
As shown best in
At least one of the first or second aluminum workpieces 12, 14—and preferably both—includes a protective anti-corrosion coating 38 that overlies the base aluminum substrate 34, 36. Indeed, as shown in
As a result of stacking the first, second, and third aluminum workpieces 12, 14, 40 in overlapping fashion to provide the workpiece stack-up 10, the third aluminum workpiece 40 has two faying surfaces 46, 48. One of the faying surfaces 46 overlaps and confronts the faying surface 20 of the first aluminum workpiece 12 and the other faying surface 48 overlaps and confronts the faying surface 24 of the second aluminum workpiece 14, thus establishing two faying interfaces 50, 52 within the workpiece stack-up 10 at the weld site 32. These faying interfaces 50, 52 are the same type and encompass the same attributes as the faying interface 30 already described with respect to
Referring back to
The scanning optic laser head 54 includes an arrangement of mirrors 58 that maneuver the laser beam 56 within a two-dimensional process envelope 60 that encompasses the weld site 32. The arrangement of mirrors 58 includes a pair of tiltable scanning mirrors 62. Each of the tiltable scanning mirrors 62 is mounted on a galvanometer. The two tiltable scanning mirrors 62 can move the laser beam 56 anywhere in the x-y plane of the top surface 26 encompassed by the operating envelope 60 through precise coordinated tilting movements executed by the galvanometers. In addition to the tiltable scanning mirrors 62, the laser head 54 also includes a z-axis focal lens 64, which can move a focal point 66 (
A characteristic that differentiates remote laser welding (also sometimes referred to as “welding on the fly”) from other more-conventional forms of laser welding is the focal length of the laser beam. Here, as shown in best in
The weld joint 68 is formed between the first and second aluminum workpieces 12, 14 by advancing the laser beam 56 along a predefined travel path relative to the top surface 26 of the workpiece stack-up 10 according to a programmed laser weld schedule. As shown best in
Like the molten aluminum weld pool 74, the keyhole 76 also penetrates into the workpiece stack-up 10 from the top surface 26 toward the bottom surface 28 and intersects the faying interface 30 of the two aluminum workpieces 12, 14. In fact, the keyhole 76 provides a conduit for the laser beam 56 to deliver energy down into the workpiece stack-up 10, thus facilitating relatively deep and narrow penetration of the molten aluminum weld pool 74 into the workpiece stack-up 10 and a relatively small surrounding heat-affected zone. The keyhole 76 may fully penetrate the workpiece stack-up 10, in which case it extends from the top surface 26 of the workpiece stack-up 10 (also outer surface 18) through the bottom surface 28 of the workpiece stack-up 10 (also outer surface 22), as shown here in
After creation of the molten aluminum weld pool 74 (and preferably the keyhole 76), the laser beam 56 is advanced from a start point to an end point relative to the top surface 26 of the workpiece stack-up 10 within the weld site 32. Such advancement of the laser beam 56 occurs along a programmed travel path by coordinating the movement of the tiltable scanning mirrors 62 in the scanning optic laser head 54. The molten aluminum weld pool 74 is consequently translated along a corresponding course since it tracks the movement of the laser beam 56. Accordingly, as the laser beam 56 is advanced along its travel path, the molten aluminum weld pool 74 follows and leaves behind molten aluminum workpiece material in the wake of the progressing weld pool 74. This molten aluminum workpiece material quickly cools and solidifies into the weld joint 68—the weld joint 68 being comprised of re-solidified coalesced aluminum derived from each of the aluminum workpieces 12, 14—that autogenously fusion welds the workpieces 12, 14 together. Once the laser beam 56 reaches the end point of its travel path, the transmission of the laser beam 56 is ceased so that the laser beam 56 no longer impinges the top surface 26 of the workpiece stack-up 10. At this time, the keyhole 76 collapses (if present) and the molten aluminum weld pool 74 solidifies to complete the formation of the weld joint 68. More than one weld joint 68 may be formed within the weld site 32 in a similar manner if desired, as will be explained in more detail below.
Turning now to
The exact shape and profile of the travel path 78 of the laser beam 56 can assume any of a wide variety of profiles while still experiencing movement in the forward direction 80 and movement back and forth in the lateral direction 82. One particularly effective type of bidirectional movement involves periodic oscillation of the laser beam 56 in the lateral direction 82 while moving the laser beam 56 in the forward direction 80. For example, in one embodiment, as shown in
A few alternative examples of a travel path in which the laser beam 56 can experience movement in the forward direction 80 and movement back and forth in the lateral direction 82 are depicted in
Another suitable travel path, which is denoted by reference numeral 78′, embodies a continuous loop pattern that includes a series of interconnected and overlapping loops 114, as shown in
The programmed laser welding schedule that controls the overall laser welding method can execute instructions that dictate the profile of the travel path 78 of the laser beam 56, which can be any of the specific travel paths 78, 78′, 78″, 78′″ shown here as well as variations not shown. It can also execute instructions detailing other parameters of the laser beam 56 including (1) the power level, (2) the travel velocity, and (3) the focal point location relative to the top surface 26 of the workpiece stack-up 10 in the z-direction. Each of these three laser welding parameters may be varied to ensure the laser beam 56 creates the molten aluminum weld pool 74 (and preferably produces the keyhole 76 to the desired penetration depth) and acceptably forms the weld joint 68 while being advanced along its predetermined travel path 78. In many instances, and regardless of the profile of the travel path 78, the power level of the laser beam 56 is typically between 0.2 kW and 50 kW, and more narrowly between 1.0 kW and 10.0 kW, the travel velocity of the laser beam 56 is typically between 1.0 meters per minute and 50 meters per minute, and the focal point location of the laser beam 56 is typically set at the bottom surface 28 of the workpiece stack-up 10.
Without being bound by theory, it is currently believed that the bidirectional movement of the laser beam 56, as exemplified by the travel paths 78, 78′, 78″, 78′″ discussed above, helps minimize the occurrence of weld defects derivable from the protective anti-corrosion coatings 38 in the weld joint 68, thereby helping ensure adequate and consistently-attainable strength in the joint 68. The back and forth movement of the laser beam 56 in the lateral direction 82, in particular, are believed to induce constant changes in the molten metal fluid velocity field within the molten aluminum weld pool 74, causing more disturbance (fracture and break down of a refractory oxide coating, boiling and zinc oxide formation of a zinc coating, etc.) of the protective anti-corrosion coating(s) 38. Moreover, due to the movement of the laser beam 56 in the lateral direction 82, such enhanced disturbance of the protective anti-corrosion coating(s) 38 occurs over a wider swath in an around the course of the molten aluminum weld pool 74 in comparison to strict unidirectional movement of the laser beam 56 in the forward direction 80 according to conventional practices of laser welding.
The weld joint 68 formed by the laser beam 56 may assume any desirable overall shape as projected onto the x-y plane of the top surface 26 of the workpiece stack-up 10. For example, the weld joint 68 may be a spiral or circular spot weld joint, a stitch weld joint, a staple weld joint, or any other shape. A plan view of a laser beam travel path possessing a sinusoidal pattern that would result in a circular weld joint, a stitch weld joint, and a staple weld joint is shown in
While being advanced along at least one of the circle paths 120, 122, 124—and preferably all three of the circle paths 120, 122, 124 as shown here in
The formation of multiple weld joints 68 in close proximity to one another may increase the chances that burn through will occur at the weld site 32. To lessen the potential for burn through to occur, the power level and/or the travel velocity of the laser beam 56 may be adjusted to reduce the heat input into the workpiece stack-up 10 after formation of the first of three weld joints 68. For example, and referring still to
The two circle paths 128, 130 may be tracked by the laser beam 56 in any temporal order; that is, the laser beam 56 may be advanced first in time along the inner circle path 128 and second in time along the outer circle path 130, or it may be advanced first in time along the outer circle path 130 and second in time along the inner circle path 128. The power level and/or travel velocity of the laser beam 56 may also be adjusted to reduce the heat input after formation of the first of the two weld joints 68. In fact, in a specific implementation of
The laser beam travel paths depicted in
Additionally, as before, the power level and/or travel velocity of the laser beam 56 may be adjusted to reduce the heat input after formation of the first of three weld joints 68 so as to help reduce the potential for burn through at the weld site 32. Here, for example, when being advanced along the circle path 128, 134, 130 tracked first in time (which may be any of the circle paths 128, 134, 130) with the focal point 66 being located at the bottom surface 28 of the workpiece stack-up 10, the travel velocity of the laser beam 56 is set to between 3.0 meters per minute and 5.0 meters per minute and the power level of the laser beam 56 is set to between 2.8 kW and 3.8 kW. Next, when being advanced along the circle path 128, 134, 130 tracked second in time (the focal point position remaining the same), the travel velocity of the laser beam 56 is set to between 4.0 meters per minute and 6.0 meters per minute and the power level of the laser beam 56 is set to between 2.6 kW and 3.6 kW. Finally, when being advanced along the circle path 128, 134, 130 tracked third in time (the focal point position remaining the same), the travel velocity of the laser beam 56 is set to between 4.0 meters per minute and 6.0 meters per minute and the power level of the laser beam 56 is set to between 2.4 kW and 3.4 kW.
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 |
|---|---|---|---|
| PCT/CN2015/088569 | 8/31/2015 | WO | 00 |
