Field of the Invention This invention relates to a method of lap welding a first sheet to a second sheet in overlapping relationship with each other using a high density radiation beam such that the resultant weld possesses high peel strength and more particularly to a method of lap welding two sheets that completely fuses the sheets together across their region of overlap for preventing the sheets from peeling apart or wrinkling or buckling in the region of the weld during forming or shaping of the sheets after welding has been completed.

Background of the Invention

High energy density radiation beam welding, such as is accomplished using a laser beam or electron beam, is becoming more widely used to join one sheet to another sheet during the course of manufacturing a product made up of at least in part by the sheets. Laser welders use a highly focused beam of light energy directed onto one or both sheets to join them together, while electron beam welders direct a stream of electrons onto the sheets to be welded to heat them and fuse the sheets together.

One type of welding joint commonly used to secure one sheet to another sheet is a lap joint where an edge portion of one sheet is placed in overlapping fashion against an edge portion of another sheet before welding the sheets together in the overlap region. Typically, before welding, the sheets are held against each other by clamps and the beam is directed against one or both of the sheets to join them together in the overlap region.

In one type of lap joint weld, a partially or fully-through penetrating weld is used to join the sheets together in what is referred a lap seam weld. In producing

a lap seam weld, the beam is directed against the upper surface of the top sheet with enough energy density and for a sufficient period of time such that the beam melts and fuses through the top sheet penetrating completely through the top sheet and at least partially through the bottom sheet. However, this type of lap joint welding method produces a weld seam which does not cover the entire overlapping surfaces leaving at least some portion of the overlapped surfaces unwelded. This results in a lap joint that is open and, therefore, not completely fused where the sheets overlap each other, making it possible, even highly likely, for the sheets to peel apart from each other as well as wrinkle or buckle in the region of the lap weld during post-welding forming of the sheets or for the lap seam weld to fail during use of a finished product constructed of the sheets. In addition to low peel strength, an open lap joint can have unfused areas where cracks can easily form initiating undesirable failure of the weld joint after repetitive or cyclical loading such as what can occur during use of a finished product constructed of the sheets. These unfused areas of the lap weld joint can also collect moisture and contaminants leading to undesirable corrosion in the weld joint which can later also lead to failure of the open lap weld. Additionally, the shear strength of the weld may be poor if the weld is not wide enough which can contribute to poor fatigue strength that, in turn, can cause premature weld failure.

Unfortunately, the integrity of the sheets can be negatively impacted in other ways if the weld is too wide, such as can be characteristic of the laser-mash seam lap welding method disclosed in Bϋdendbender, U.S. Patent No. 4,945,202. For example, if sheets are coated with a corrosion resistant coating, such as a zinc coating, heat generated during welding can vaporize the coating leaving the sheets unprotected in the area of the weld. If this unprotected area is too large, the "self-healing"

properties of the zinc coating may not be able to prevent corrosion from forming in the weld area as well as in the heat affected zone (HAZ) surrounding the weld. Moreover, even if a lap seam weld of sufficient width could be produced so that it joined the sheets together completely across the overlap of the sheets by weaving the beam across the overlap with the beam impinging on the exposed top surface of the top sheet, it would require a relatively large amount of energy and adversely affect the formability of the sheets because the weld would have a rather large cross-sectional area and surrounding heat affected zone resulting in a more brittle weld region that would not lend itself to be formed successfully, such as by bending, deep drawing, roll-forming, flanging, piercing, or another forming method. Moreover, weaving the beam across the overlap reduces the weld speed and destroys coating on the beam impinging surface, that is, the top surface of the top sheet, and induce coating contamination in the weld nugget. In addition to the lap seam joints just discussed, another type of lap joint is a lap fillet joint. In a lap edge joint, the sheets are overlapped such that their edges are parallel and generally in line with each other. Unfortunately, for all of these types of lap joints, the joint is open with portions of the overlap welded, dramatically reducing peel strength and increasing the likelihood for corrosion as well as making these joints difficult to form or shape after welding without weld failure or forming defects such as wrinkling or buckling occurring.

An alternative to the aforementioned lap joint constructions is conventional mash seam welding such as is disclosed in Kerby, U.S. Patent No. 3,159,419. As is disclosed, a pressure roll electrode on each side of the overlapped sheets melts, fuses and welds the sheets together across the overlap region while pressure applied by the rolls reduces the thickness of the overlap. After

resistance mash seam welding, the sheets form part of a blank that is formed for later assembly as part of an automobile. However, because of the relatively large cross-sectional size of these welds and its associated HAZ, the weld joints are located away from areas of the blank that are greatly formed or bent. There are other disadvantages to resistance mash seam welding in addition to lacking formability. One further disadvantage is that resistance mash seam welding is not suited for lap joint welding of more complex two- and three-dimensionally contoured sheets. A still further disadvantage is that any low vaporizing temperature coating on the sheets, such as zinc coating, is vaporized over a wide area during welding which can leave the lap joint with little or no corrosion protection. For sheets having such a coating on both sides, the coating can also vaporize in the overlap area and become trapped in the molten metal leading to welds possessing poor integrity which can lead to premature weld failure. Lentz, et. al, U.S. Patent No. 4,769,522, discloses a method of laser welding using a sophisticated apparatus and fixture for joining overlapped ends of a sheet to form a container body. A laser beam is directed into a "mouth" where the sheets are overlapped and acutely angled relative to each other with the beam impinging against portions of the adjacent surfaces of the overlapped sheet ends to heat them above their melting temperature. Before solidifying, the "mouth" is closed by the fixture pressing the sheet ends into overlapping contact with each other to fuse them together in a lap joint arrangement.

The method disclosed in Lentz requires that the sheet ends be accurately spaced apart from each other, accurately located, and fixtured relative to one another during scanning of the sheet by the laser beam before quickly bringing the sheet ends into contact with each other before solidification occurs. Of course, should the

sheet ends not be pressed together quickly enough before the sheet solidifies, the resulting weld joint is of poor integrity and low peel strength, both of which could lead to premature weld failure. Therefore, this welding method is susceptible to leaving portions of the overlap unfused reducing significantly formability and weld peel strength while resulting in decreased corrosion protection in the weld region.

An article in the December 1993 issue of the periodical Welding Journal entitled "Laser Beam Welding Goes into High-Speed Production of Home Hot Water Tanks", discloses a method of laser welding for joining together two generally cylindrical tank halves to form a tank of a hot water heater. The bottom tank halve has a radially outwardly turned lip for guiding the top tank halve into tight-fitting, intimate contact with the lower halve necessary for welding the two halves completely about their peripheries. There must be no fit-up gap anywhere along the weld joint interface where the upper and lower tank halves are overlapped so that the laser beam will not pass through the joint interface during welding. Unfortunately, the unwelded portion of the outwardly turned lip of the upper tank halve used to guide the lower tank halve into tight-fitting contact produces an open lap joint adversely affecting the peel strength of the weld joint. Additionally, because this welded joint is open, it is also susceptible to crack formation and corrosion. Furthermore, if deep drawing of the weld region is attempted after welding is performed, the weld can yield to crack failure because of reduced peel strength in the weld region and due to wrinkling and buckling in the area of the lap joint because of the aforementioned unwelded overlapping outwardly turned lip.

Autogenous laser and electron beam butt-welding methods have been used to form blanks prior to forming them, as is disclosed in the 1992-93 edition of The Industrial Laser Handbook entitled "Tailored Welded

Blanks: A New Alternative in Automobile Body Design" and in an August 1974 article in the periodical Welding Journal entitled: "Production Electron Beam Welding of Automotive Frame Components" . In constructing a tailored welded blank component, a first sheet is cut to the desired shape and butt-welded to another sheet using a laser or electron beam gun. After welding, the component is formed. However, before butt-welding, accurate edge preparation of the sheets where they are to be joined must be done so that the edge of one of the sheets is virtually perfectly parallel to the edge of the other sheet before butt-welding can be performed.

Even when done properly, however, edge preparation is time consuming and costly. Typically, during edge preparation, the edge of each sheet to be joined is machined to make it parallel with the edge of the other sheet so that there is virtually no gap between the sheets when they are butted against each other for welding. If edge preparation is not properly performed, when the edges of the sheets are butted against each other to be welded, any gap that is too large between the sheet edges can result in a poor weld joint between the sheets. This can lead to the sheets not being joined together in that area, possibly causing weld failure during forming or even more undesirably, later, when the sheets are in use.

Conventional welding methods, such as shielded metal-arc, submerged arc, self-shielded flux-core and gas- shielded arc welding processes, are also not suitable because the resultant welded sheets cannot be easily formed, especially deeply drawn, in the region of the weld without negatively impacting weld strength and weld integrity, possibly resulting in weld failure during forming as well as cracking or peeling of the weld joint as well as wrinkling or buckling in the region of the weld. Conventional welding methods are also ill-suited for welding sheets having corrosion resistant or low vaporizing temperature coatings because they produce

relatively wide welds destroying the coating across the region of the weld and its surrounding HAZ. Finally, the production rates that can be achieved using conventional welding processes are relatively slow, further making their use in these types of applications economically undesirable.

Summary of the Invention

The present invention is directed to a method of lap welding using a high energy density radiation beam for joining one sheet that has been overlapped with another sheet to form a lap joint having a weld interface where the sheets are overlapped by acutely angling the beam relative to the plane of the weld interface and training the acutely angled beam on at least one of the sheets for producing a lap weld in the overlap region that fuses the sheets completely together across the overlap region such that the weld possesses high peel strength to enable the sheets to be formed after welding. The lap welding method of this invention utilizes a high energy density radiation beam that preferably is a laser beam or a beam of electrons, for producing a lap weld having no unfused area in the region of the weld interface of the sheets so that the sheets will not peel apart, wrinkle, buckle or split apart in the area of the weld during forming or shaping of the sheets after welding is completed or during use after forming.

The resultant lap weld construction minimizes dust collection and corrosion in the weld region because cracks and crevices in the lap weld are minimized and are preferably prevented. Finally, the resultant lap weld construction also prevents fatigue failure because the lap weld joint is completely fused and welded virtually eliminating any crack initiation sites in the region of the weld. Preferably, the sheet material is a metal such as aluminum, steel, copper or another metal. The sheets can be coated with an organic or a metallic coating, such

as, for example, a zinc coating, if galvanized material is to be used.

In operation, the sheets are overlapped and fixtured for welding with one edge of one of the sheets overlying the other of the sheets and the edge of the other sheet underlying the one sheet forming a lap joint. Preferably, the width of overlap of the sheets is at least 50% of the thickness of the sheet having the thinnest cross sectional thickness and is no greater than the smallest of either of the following: preferably, about twice the thickness of the thinnest sheet or 1.5 times the thickness of the thickest sheet, for producing a welded lap joint of high strength, good integrity and possessing high peel strength. If the edge of either sheet or both sheets have burrs from processing, such as during slitting, cutting, or blanking the sheet edges, the sheets can be overlapped such that any burrs space one sheet apart from the other sheet along at least a portion of the region of sheet overlap, preferably creating a gap between the sheets. Preferably, the gap created between the sheets in the region of overlap is no greater than about 0.10 millimeters, is preferably less than about 10% of sheet thickness and is typically about 0.05 millimeters or less. Of course, the adjacent surfaces of the sheets where they are overlapped can bear against each other with no gap at all between them. If the sheets are fixtured for welding by clamps, preferably, clamp pressure or distance between the clamps can be varied to regulate the gap between the sheets so that the gap is not too large to prevent allowing an excessive amount of beam energy to pass through the weld interface during welding.

After the sheets have been fixtured, the beam is trained on one or both sheets to weld them together. During welding, the beam is preferably acutely angled relative to the plane of the weld interface and is directed toward the plane of the weld interface.

Preferably, the beam is trained on the forward edge of one of the sheets and the adjacent surface of the other sheet or only on the forward edge of one of the sheets. To influence the formation, geometry and flow of the weld nugget during welding, the sheets can be oriented relative to the direction of gravity, F _g , so that the plane of the weld interface is acutely angled relative to the direction of gravity, F _g .

When welding is completed, the resultant lap weld extends from front to back of the overlap leaving no region of the overlap unfused to provide the weld with high peel strength so that the sheets will not peel apart during forming or shaping of the sheets, even in the region of the weld. Preferably, after welding is completed, the sheets form a blank that is shaped or formed using any shaping or forming method.

Objects, features and advantages of this invention are to provide a method of welding using a high energy density radiation beam for joining sheets overlapped in a lap joint configuration which produces a weld having high peel strength and which will not peel apart during forming of the sheet after welding; produces a blank that can be conventionally formed even in the region of the weld such as by shaping, deep drawing, flanging, blanking, hydro-forming, bending, roll forming, die stamping, or any other forming method; utilizes the direction of gravity to influence formation, geometry and flow of the weld nugget during welding; can utilize the direction of gravity to advantageously orient relative to the plane of the weld interface the keyhole in the weld region created during high energy density beam welding; is easily adaptable to continuous laser lap welding of a pair of sheets uncoiled from coil stock and overlapped with each other; is tolerant of joint fit-up gap in the overlap region and thereby minimizes or eliminates edge preparation before welding; can advantageously be used to lap weld sheets of unequal thicknesses; can be used to lap

weld two sheets of equal thickness; can be used with both autogenous and filler metal welding applications; can be used to lap weld sheets of relatively thin cross sectional thickness of 5 millimeters or less; can be used to lap weld sheets spaced slightly apart by metal processing burrs; can be used to join sheets coated with organic or inorganic coatings; is economical because it can be used to unpeelably lap weld two sheets together with each of the sheets having different desired mechanical or other desired properties to form a blank that can be later conventionally formed to produce a product having the desired properties in desired area of the finished product; is a method that is simple, flexible, economical and reliable; and which produces a 'lap weld that has no unfused area across the region of the overlap from front to back of the overlap, is durable, possesses good integrity, high peel strength, high tensile strength, and high fatigue resistance to cyclical and repetitive loading, and which will not peel apart during shaping or forming of the weld region.

Brief Description of the Drawings

These and other objects, features and advantages of this invention will become apparent from the following detailed description of the best mode, appended claims, and accompanying drawings in which:

FIG. 1 is a perspective view of a pair of sheets overlapped and fixtured with its plane of overlap acutely oriented to the direction of gravity and being lap welded by a high energy density radiation beam using a method of welding of this invention;

FIG. 2 is an enlarged fragmentary sectional view of the overlapped sheets oriented generally horizontally relative to the direction of gravity; FIG. 3 is an enlarged fragmentary sectional view of the sheets of FIG. 2 taken along line illustrating the

sheets being welded;

FIG. 4 is an enlarged fragmentary sectional view of the sheets of FIG. 2 illustrating the resultant lap weld after welding has been completed; FIG. 5A is a trace depicting the beam of FIG. 1 following a straight line path of travel longitudinally along the weld interface of the overlapped sheets;

FIG. 5B is a trace depicting the beam of FIG. 1 following a discontinuous straight line path; FIG. 5C is a trace depicting weaving the beam of

FIG. 1 as it travels along the weld interface;

FIG. 5D are traces depicting generally circular spinning of the beam of FIG. 1 as it travels along the weld interface; FIG. 5E are traces depicting oblong spinning of the beam of the FIG. 1;

FIG. 6A illustrates circular and oblong focused beam spot shapes;

FIG. 6B illustrates generally rectangular focused spot shapes;

FIG. 7 is a perspective view of a pair of overlapped sheets being welded using the welding method of this invention by a laser beam having a generally rectangular focused spot; FIG. 8A is an enlarged fragmentary sectional view of a pair of sheets overlapped, fixtured and oriented relative to gravity for being welded by a high energy density radiation beam using the method of welding of this invention; FIG 8B is an enlarged fragmentary sectional view of the sheets of FIG. 8 illustrating the sheets being welded while oriented relative to gravity;

FIG. 8C is an enlarged fragmentary sectional view of the overlapped sheets oriented relative to gravity

illustrating the resultant lap weld after welding has been completed;

FIG. 9A is a perspective view of an apparatus for continuously lap welding a pair of of sheets from coil stock using the welding method of this invention;

FIG. 9B is a side view of the continuous welding apparatus in the direction of 9B--9B of FIG. 9A;

FIG. 9C is an enlarged fragmentary view of the overlapped sheets being welded by the continuous welding apparatus of FIG. 9A;

FIG. 10 is a perspective view of a pair of three-dimensionally contoured sheets being located and fixtured for welding;

FIG. 11 is an enlarged fragmentary sectional view of the clamped sheets during welding;

FIG. 12 is a perspective view of the three- dimensionally contoured sheets being welded after being located and fixtured for welding;

FIG. 13 is a top view of a flat vehicle door panel and a flat hinge reinforcing panel;

FIG. 14 is a top view of the hinge reinforcing panel placed in overlapping contact with the door panel blank and being welded to the door panel to form a blank;

FIG. 15 is an enlarged fragmentary sectional view of the panels taken along line 15--15 of FIG. 14 illustrating in more detail welding of the hinge reinforcing panel to the door panel blank;

FIG. 16 is an enlarged fragmentary sectional view of the panels after welding has been completed; FIG. 17 is a top view of the panels after welding illustrating more clearly that they form a blank; and

FIG. 18 is a perspective view of the blank after being formed by a conventional forming process .

Detailed Description of the Invention

Referring now to the drawings, FIGS. 1-4 illustrate a method of lap welding a first or top sheet 30 overlapped with a second or bottom sheet 32 using a high energy density radiation beam 34 for joining the sheets 30 & 32 together producing a lap weld 110 having high peel strength such that the sheets 30 & 32 preferably create a blank 38 that can be formed after welding using conventional forming methods such as bending, deep drawing, roll forming, hydro-forming, shaping, flanging or another shaping or forming process without the sheets 30 & 32 peeling apart during forming. The sheets 30 & 32 are preferably constructed of weldably similar material and are preferably constructed of metal such as a steel, aluminum, copper, or another metal capable of being welded. Alternatively, there may be applications of this invention where non-weldable, dissimilar materials, such as nickel and silver, can be diffusion bonded together if sufficient pressure is applied simultaneously with the heat input of the laser. Preferably, the high energy density radiation beam 34 is a laser beam. Alternatively, an electron beam can be used.

Advantageously, this method of welding enables two or more sheets to be lap welded together to produce a blank 38 that can be formed after welding with the sheets being selected to impart to the finished product formed from the blank 38 certain desired mechanical properties, at least in the region of that sheet having the desired properties. By imparting to the blank 38 desired mechanical properties only where needed, thinner gauge and/or less expensive sheet material can be used elsewhere where these properties are not needed making the blank 38 less costly to manufacture while still being suitable and optimized for its intended use after forming is completed.

As is shown more clearly in FIG. 1 the top sheet

30 is placed in overlapping relationship with the bottom

sheet 32 such that its forward edge 40 overlies the top surface 42 of the bottom sheet 32 and the forward edge 44 of the bottom sheet 32 underlies the bottom surface 46 of the top sheet 30 creating a weld interface 48 in the region of overlap 50 of the sheets 30 & 32. To maintain the sheets 30 & 32 in overlapping relationship with each other during welding, they are held in place by a pair of staggered and spaced apart clamps or fixtures 52 & 54. Preferably, the clamps 52 & 54 orient the overlapped sheets 30 _ 32 such that the plane 56 of the weld interface 48 is acutely angled relative to the direction of gravity, F _g , as is indicated by the angle θ of the angular indicator 58 shown in FIG. 1. Preferably, forward edge 60 of clamp 52 is spaced slightly from forward edge 40 of sheet 30 and a forward edge 62 of clamp 54 is spaced from forward edge 44 of sheet 32. To remove heat from each sheet during welding, each clamp or fixture 52 & 54 can have a coolant 66 flowing through channel 64. Suitable coolants water, an alcohol-water mixture, or another conventional refrigerant can be used. To provide good heat transfer from each sheet 30 & 32 to its associated fixture 52 & 54, each fixture is preferably constructed of copper or another good heat conducting substance or metal. However, this clamp substance or metal must possess a high melting temperature and is preferably not weldable to the material of sheet 30 or 32. Alternatively, rather than removing heat by conduction, heat can be removed from the sheets 30 & 32 during welding by blowing air onto the sheets, thereby removing heat by forced convection.

After the sheets 30 & 32 have been fixtured, a high energy density radiation beam welding apparatus 68, such as the laser 70, emits a beam 34 from a gun assembly 72 that impinges against one or both sheets 30 & 32 at the region 50 where the sheets 30 & 32 overlap each other. As is shown in FIG. 1, the laser welding apparatus 70 has a laser source 74, such as a CO ₂ laser, CO laser, excimer

laser, or a solid state laser, for producing an unfocused laser beam 76. The unfocused beam 76 is focused and trained onto at least one of the sheets 30 & 32 during welding to heat the sheet material above its melting point or melting points so that it will fuse the two sheets together. The unfocused beam 76 can be directed and focused by a series of optics 78, such as lenses or mirrors, to control the shape of the focused beam 34 or to further control the movement of the beam 34 relative to the sheets 30 & 32.

During welding, the laser gun assembly 72 moves relative to the clamped sheets 30 & 32 so that the focused beam 34 travels along a weld line 80 to weld the sheets 30 & 32 preferably across the entire width of the weld interface 50 so that there is no unfused area across the region of overlap 50 where the beam 34 has impinged. As is shown in FIG. 1, during welding, the beam 34 travels relative to the weld interface 48 in the Y direction as is shown by a three-dimensional coordinate reference axis 82 illustrated. So that the sheets 30 & 32 move relative to the focused laser beam 34, the fixtured sheets can remain stationary while the beam moves. Alternatively, the beam 34 can remain stationary while the clamps 52 & 54 carry the sheets 30 & 32. To move beam 34 along the sheets, the laser gun assembly 72 is preferably carried by a robot or a one-dimensional gantry if the sheets have straight edges overlapped with each other, a two-dimensional gantry if, for example, the overlapped sheet edges are curved, or a three-dimensional gantry is preferably used if the overlapped sheet edges are three-dimensionally contoured. FIG. 2 illustrates sheets 30 & 32 overlapped, fixtured and oriented relative to the direction of gravity, F _g , such that the plane 56 of the weld interface

48 is generally horizontal, with θ being about 90°. During welding, an assist gas 84, such as argon, helium, nitrogen, or a suitable combination of assist gases is

directed from a gas supply 86 through a nozzle 88 in the vicinity where the focused laser beam 34 is impinging against the sheet or sheets. Preferably, as is shown more clearly in FIGS. 2 & 3, a second assist gas injector 90 is provided for directing a supply 92 of assist gas toward the back or "root" side of the weld interface 48 to provide a better weld when finished. The top sheet 30 has a thickness of ti and the bottom sheet 32 has a thickness of t ₂ and the sheets 30 & 32 have a width of overlap, w, which also constitutes the cross section width from front to back of the weld interface 48. Preferably, if ti differs from t ₂ , then the top sheet 30 where the beam 34 is impinging upon is the thickest of both sheets 30 & 32. When overlapped, the front edge 40 of the top sheet 30 overlies the upper surface 42 of the bottom sheet 32 and the front edge 44 of the bottom sheet 32 underlies the bottom surface 46 of the top sheet 30 such that the width of overlap, w, is the distance from the forward edge 40 of the top sheet 30 to the forward edge 44 of the bottom sheet 32 creating a lap joint 94 ready to be welded. The width of overlap, w, is preferably equal to or greater than 50% of the thickness of the thinnest sheet whether it be the top sheet 30 or the bottom sheet 32 and is, preferably, no greater than the smaller of either twice the thinner sheet thickness or about 1.5 times the thicker sheet thickness. Hence, if the top sheet 30 has a thickness, ti, of 3 millimeters and the bottom sheet 32 has a thickness, t ₂ , of 4 millimeters, the width of overlap, w, of the sheets 30 _ 32 is at least 1.5 millimeters and can be up to 6 millimeters, for example, depending upon the strength required of the lap weld.

Preferably, no edge preparation of either sheet 30 or 32 is required before fixturing and welding. The sheets 30 & 32 can be overlapped with one or both sheets

having an outwardly projecting burr 96 or 98 creating a gap, e, between the sheets 30 & 32. These burrs 96 & 98 shown in FIGS. 2 & 3, are typically caused by cutting, slitting, or blanking that has been performed before welding to cut the sheet to the desired size or shape. Alternatively, however, the sheets 30 & 32 can be overlapped and placed against each other without being spaced apart by any gap. Preferably, e, is typically about 0.05 millimeters and is preferably no greater than 0.1 millimeters or 10% of sheet thickness, so that only a negligible amount of beam 34 passes through joint 94. To regulate the gap distance, e, each clamp 52 & 54 preferably can apply a clamping pressure or force, F _c , to the sheet it bears against. If the sheets 30 & 32 are coated, pressure can be controllably applied to reduce the space between the sheets 30 & 32 while enabling coating that has vaporized into gas to escape from the weld interface 48 to prevent the coating vapor from becoming trapped in the weld or otherwise contaminating the weld. If desired, rather than regulating clamping force, F _c , the spacing between the clamps 52 _ 54 can be controlled to regulate gap, e. As is further illustrated in FIG. 2 and

FIG. 3, if the sheets 30 & 32 are coated with a low vaporizing temperature coating 100, such as zinc (galvanized sheet) , the spacing, e provides a path between the sheets 30 & 32 for enabling coating vapor to travel away from the interface 48.

FIG. 2 depicts an angular indicator 58 for indicating the angle, θ, of the plane 56 of the weld interface 48 relative to the direction of the force of gravity, F _g , of 90°. Although the sheets 30 & 32 are shown in FIGS. 2 through 4 with the plane 56 horizontal, it can be angled relative to the direction of gravity, F _g , so that the direction of gravity will influence the

formation, flow and/or geometry of weld nugget 102 that forms lap weld 36 after welding is completed..

The laser beam 34 is acutely angled relative to the plane 56 of the weld interface 48 and directed against the forward edge 40 of the top sheet 30 such as is depicted by beam 34a in FIG. 2, or at both sheets 30 & 32, such as is depicted by beam 34b in FIG. 2. Preferably, the central axis, shown in phantom in FIG. 2, of the beam 34a directed against the forward edge 40 of the top sheet 30 forms an acute angle, CXi, with a plane 106 parallel to the plane 56 of the weld interface 48 as is indicated in FIG. 2. Preferably, the central axis, also shown in phantom in FIG. 2, of the beam 34b directed at both sheets 30 & 32 and generally toward the plane 56 of the weld interface 48 forms an acute angle, 0. ₂ , with a plane 108 parallel to the plane of the weld interface 48 as is also indicated in FIG. 2. Although a pair of beams 34a & 34b are illustrated in FIG. 2, they are shown for depicting what portion of the top sheet 30 or both sheets 30 & 32 a beam 34 can be trained and therefore, preferably, only a single beam 34a or 34b is used to lap weld the sheets 30 & 32 together.

As is shown more clearly in FIG. 3, to form a lap weld joint, such as the welded joint 110 shown in FIG. 4 having no area unfused across the width of the interface 48, the beam 34 has a sufficiently high energy density and moves longitudinally along the interface 48 at a desired speed. Preferably, the beam 34 has an energy density of at least 10 ⁵ watts/cm ² where it impinges against one or both sheets. If desired, such as for the reduction of weld defect frequency, filler metal can be supplied at the weld interface 48 during welding, particularly, if the gap, e, between the sheets is larger than 0.1 millimeter. During welding, the heat generate, can vaporize the coating 100, particularly, in the region of the weld interface 48, and preferably the coating vapor 112 flows away from the weld

interface 48. Preferably, assist gas 84 also helps force coating vapor out of the weld interface 48.

As is shown in FIG. 4, when completed, the resultant welded joint 110 has a weld fusion zone 114 that extends from the front of the weld interface 48 to the back or root side of the weld interface 48 such that the sheets 30 & 32, are fused completely in the region of the overlap 50 to produce a weld 110 having high peel strength. Preferably, the aspect ratio of weld 110, h/S, is relatively large. Preferably, after welding, the sheets 30 & 32 can be formed such as by a die stamping process, bending, deep drawing, flanging, roll-forming, hydro- forming, or by any other forming method.

The path of travel of the laser gun assembly 72 can also be discontinuous longitudinally along the weld interface 48 while achieving complete fusion of the sheets 30 & 32 across the weld interface 48 from the front of the interface 48 to the back of the interface 48, producing a "stitch" lap weld having high peel strength. FIGS. 6A & 6B illustrated beam spot shapes at the point where the beam impinges upon the sheet or sheets. The size and shape of these beam spot shapes are chosen to provide the desired energy density where welding is taking place. As is shown in FIG. 6A, focused beam spot 130 is circular and corresponds to, for example, beam 34a illustrated in FIG. 2. Beam spot 132 is circular in shape and is directed onto both sheets 30 & 32 and corresponds to beam 34b shown in FIG. 2. Alternatively, beam 34 can also be oblong in shape as is shown in FIGs. 6A and 6B by beam spot 134 with the major direction of this oblong shape being generally tangent to the weld line 80, such that its longitudinal axis is generally tangent to the plane of the weld interface 48 as is shown by beam spot

136 or it can be angularly offset, such as by an angle, δ, as is depicted by beam spot 138. The angle, δ, is between about ± 45° and preferably, δ, is smaller than or equal to

± 10°. Alternatively, the beam spot can be oscillated back and forth as is depicted by beam spot 140 in FIG. 6B, such as for initiating the melting-solidification, remelting- resolidification cycles. FIGS. 10 through 12 illustrate utilizing the welding method of this invention used to lap weld a pair of three-dimensionally formed or contoured sheets 190 & 192 to produce a three-dimensionally contoured component 194, such as a center pillar for an automotive vehicle body. As is shown in FIG. 10, before welding, the sheets 190 & 192 are overlapped, placed against each other and fixtured by clamps 52 & 54 in preparation for welding. Preferably, the sheets 190 _ 192 are oriented relative .to the direction of gravity, F _g , such that the plane 56 of the weld interface 48 is acutely angled relative to the direction of gravity, F _g , such as is shown more clearly in

FIG. 11. After the sheets 190 & 192 are overlapped and fixtured, the laser gun assembly 72 moves relative to the sheets 190 & 192 to lap weld them together as is shown more clearly in FIG. 9 forming a weld 110.

FIGS. 13 through 18 illustrate construction of a blank 38 (FIG. 17) made of a flat vehicle door panel 200 and a hinge reinforcing panel 202 using the welding method of this invention that is formed after welding to produce a three-dimensionally contoured door panel 204 (FIG. 18) . So that the hinge reinforcing panel 202 can be welded to the flat door panel 200, the door panel 200 undergoes a stamping operation to remove a portion 206 of the door panel 200 in the area where the hinge reinforcing panel 202 will be attached. Preferably, the hinge reinforcing panel 202 is constructed of sheet material having different properties or different thickness from the material of the flat door panel 200. For example, the hinge reinforcing panel 202 can be constructed of stronger material, thicker material or sound dampening material to impart to the finished three-dimensionally contoured door panel assembly 204 the desired properties at least in the

region of where the door hinge attaches the assembly 204 to a vehicle. For example, as is more clearly shown in FIG. 15, the thickness, tj., of the hinge reinforcing panel

202 can be 1.8 millimeters while the thickness, t ₂ , of the flat door panel 200 can be about 0.8 millimeters. Preferably, the panels 200 & 202 are constructed of a formable aluminum, an automotive deep drawing steel or a combination of metals.

As is shown in FIGS. 17 & 18, the hinge reinforcing panel 202 is at least slightly larger in size than the complementary stamped opening 206 in the door panel 200 so the hinge reinforcing panel 202 will overlap the door panel 200 preferably completely about the periphery of the hinge reinforcing panel 202. FIGS. 14 & 15 illustrate the hinge reinforcing panel 202 being lap welded about its periphery to join the reinforcing panel 202 to the door panel 200.

As is shown in FIG. 17, the door panel blank 38 is formed into the three-dimensionally contoured vehicle door panel 204 illustrated in FIG. 18 for assembly as part of a door that will be assembled to a vehicle body. As is more clearly illustrated in FIG. 18, a portion of the hinge reinforcing panel 202 is bent during forming, thereby also bending the lap weld 110.

Example

As an example of the construction of the automobile inner door panel 204, the hinge reinforcing panel 202 has a thickness, tj., of 2 millimeters and the flat door main panel 200 has a thickness, t ₂ , of 1 millimeter. Preferably, both panels 200 & 202 are constructed of an automotive deep drawing steel and which is 70G/70G coated for corrosion protection. When overlapped, the width of overlap, w, of the panels 200 & 202 is about 1.1 millimeter.

A six kilowatt CO ₂ laser can be used with an output beam diameter of about 50 millimeters. The beam delivery system can be composed of flat reflective polished copper optics and one focusing optic with the focusing optic having, for example, a focal length of approximately / = 150 millimeters to produce a focused beam spot where the beam impinges at the weld interface against the panels 200 and/or 202 of about 0.3 millimeter diameter. In this illustration, the focused laser beam 34 is directed at the forward edge 40 of the hinge reinforcing panel 202 at an angle α = 20° relative to the plane 56 of the weld interface 48 and the panels 200 & 202 are fixtured so they are oriented relative to the direction of gravity, Fg, such that the plane 56 of the weld interface 48 is at an angle of approximately θ = 20° so that the direction of gravity, Fg, influences the formation, geometry and flow of the weld nugget. During welding, the laser beam 34 travels along the weld interface 48 in a direction generally tangent to the interface 48 at a welding speed of V = 6 meters per minute.

During welding, assist gas, such as argon, is directed where the beam 34 impinges upon the forward edge 40 of the hinge reinforcing panel 202 at a flow rate of about twenty cubic feet per hour through an injector nozzle 88 having a tube diameter of approximately 4 millimeters. Preferably, assist gas is also injected toward the root side of the weld interface 48 where welding is taking place.

Use and Operation

In use, the method of welding of this invention is used to join one sheet that has been overlapped with another sheet for producing a lap weld that joins the

sheets completely together in the overlap region and which possesses high peel strength to enable the sheets to be formed or shaped after welding, even in the region of the lap weld 110 and a joint with high tensile strength and high resistance to fatigue. The lap welding method of this invention utilizes a high energy density radiation beam 34, that preferably is a laser beam, for producing a lap weld 110 having virtually no unfused area across the region of the weld interface 48 between the sheets so that the sheets will not peel apart during forming or shaping of the sheets or due to fatigue cracking in the region of the weld 110.

In operation, referring once again to FIGS. 1 through 4, the sheets 30 & 32 are overlapped and fixtured for welding. After the sheets 30 & 32 have been fixtured, the laser beam 34 is brought to bear against one or both sheets to weld them together. To weld them together producing a joint having high peel strength, the beam 34 is preferably acutely angled relative to the plane 56 of the weld interface 48 and is directed toward the plane 56 of the weld interface 48.

Preferably, after welding is completed, the sheets 30 & 32 form a blank 38 that is shaped or formed using conventional shaping or forming methods. It is also to be understood that, although the foregoing description and drawings describe and illustrate in detail embodiments of the present invention, to those skilled in the art to which the present invention relates, the present disclosure will suggest many modifications and constructions as well as widely differing embodiments and applications without thereby departing from the spirit and scope of the invention. The present invention, therefore, is intended to be limited only by the scope of the appended claims and the applicable prior art.