Abstract:
Provided is a method and tool for butt welding a first material piece of metal to a second material piece of metal at opposed longitudinal edges. The second material piece has a greater material thickness than the first material piece. The first material piece has a higher tensile strength and/or a higher inching temperature and/or a higher yield strength and/or a higher modulus of elasticity than the second material piece. A thickened longitudinal edge is provided at the first material piece. The thickening of the longitudinal edge is at least partly produced by at least simple folding, beading or winding of the first material piece or at least partly by folding the first material piece at its edge. The two material pieces are friction-stir welded at their front along the opposed longitudinal edges.

Description:
FIELD OF THE INVENTION 
     This invention relates to a method for butt welding a first material piece of metal to a second material piece of metal at opposed longitudinal edges, wherein the second material piece has a greater material thickness than the first material piece. This invention furthermore relates to a friction stir welding tool. 
     BACKGROUND 
     In the industry, in particular the automotive industry, there is an increasing trend towards the utilization of mixed materials for the construction of vehicle bodies for weight and/or cost reasons. Typically, materials of different tensile strength, different melting temperatures and/or different yield strengths and/or different moduli of elasticity are combined with each other. The challenge consists in cohesively connecting or joining the different materials such that large forces and bending moments can be transmitted. For weight reasons, the sheet metal thicknesses furthermore should be optimized corresponding to their strength values, whereby for example steel has a smaller sheet metal thickness than aluminum. 
     In the prior art, such joints previously have been made from different materials by lap welding, as with lap joints good strength characteristics can be achieved. As regards the lap joints, it was found to be disadvantageous, however, that a thin gap is formed at the joint, into which moisture can penetrate. Due to the penetrating moisture and the different electric potentials of the joint partners, a corrosion cell can form, whereby the welded joint or the welded component can corrode. In addition, such joints cannot be used completely in the visible part of a product, in particular of a vehicle, as a welding seam or an overlap is to be seen, which is perceived as disturbing with regard to the appearance. 
     Alternatively, it is known from the prior art that two materials can be butt-welded by applying a friction stir welding method. This welding technique among other things offers the advantage that no welding seam is visible, which is perceived as disturbing. Furthermore, by means of the friction stir welding joint in contrast to the lap welding joint higher fatigue strengths can be achieved, as there is no overlap region in which notch effects occur. With the known method, however, only materials of equal material thickness can be connected with each other for different types of joint, so that the resulting strength values meet the requirements. When different materials of different material thickness would be connected with each other at their butt ends, the forces and bending moments transmittable by the joint would correspond to the strength of the material with lower strength based on the cross-section of the thinner material piece. This is due to the fact that in such joints the cross-section of the thinner material piece at the joining line is combined with the strength properties of the material with lower strength, so that the transmittable forces of the joint always are smaller than the transmittable forces of the respective material pieces. 
     DE 1 901 281 U describes a classical welding method in the case of a lap joint in the region of a boiler bottom, wherein the thin bottom is backed up with a sheet metal at its edge. Both are welded to the thicker jacket wall of the boiler. 
     In MAG welding the problem exists that a thin sheet metal can burn away very easily due to the extreme input of heat. In this case, lap welding can be made safer by bending over the edge of the sheet metal, as is proposed by DE 10 2010 004 283 A1. 
     Friction stir welding methods, in which two sheets of equal materials, but of different thickness, are welded to each other, are described in DE 699 33 978 T1 and JP H 10-193 143 A. These documents propose to build-up weld onto the thinner sheet metal or to glue on a further thin sheet metal of the same material. 
     A friction stir welding method also is shown in JP 2010-036 230 A, in which two sheets of different materials, but of the some thickness, are stir-welded by a tool which has a conically tapered pin. 
     It therefore is the object of the invention to create a cost-optimized method for butt welding, with which two material pieces of different materials are welded to each other, wherein the strength characteristics of the joint are improved. 
     SUMMARY 
     The invention provides a method for butt welding a first material piece of metal to a second material piece of metal at opposed longitudinal edges, wherein the second material piece has a greater material thickness than the first material piece and the first material piece is made of a material having a higher tensile strength and/or a higher melting temperature and/or a higher yield strength and/or a higher modulus of elasticity than the second material piece, wherein the following steps are carried out:
         Providing a thickened longitudinal edge at the first material piece for locally increasing the material thickness of the first material piece at the longitudinal edge, wherein the thickening of the longitudinal edge is at least partly produced by simple folding, beading or winding of the first material piece or is at least partly produced by folding the first material piece at its edge, and   Friction stir welding of the two material pieces along the longitudinal edges opposed at their front.       

     The different material properties set forth above have in common that these are physical material properties which are responsible for the strength of the material pieces. 
     The idea underlying the invention provides that the thickness of the material piece with the smaller material thickness is increased only locally on its contact side, in order to increase the joint cross-section between the two joint partners. The joint cross-section in the joining region thus can correspond to the one which occurs in the case of two material pieces of the same material thickness. It thereby is achieved that in the case of a connection of two materials of different thickness a component with high strength characteristics can yet be produced, which also occur in a friction-stir welded component of material pieces having the same thickness. In addition, in the joint produced in this way the corrosion problem is avoided due to the existing gap, which can occur in a conventional lap welding method. The invention provides for combining materials of different tensile strength values, melting temperatures and/or yield strengths with each other, which in addition have different material thicknesses, wherein there is yet obtained a component with correspondingly high strength characteristics. 
     According to the invention it is provided that the thickening of the longitudinal edge is at least partly produced by simple folding, beading or winding of the first material piece. The free end of the material piece associated to the longitudinal edge is beaded over, for example, whereby the thickening is obtained at that longitudinal edge which forms the contact surface to the second material piece. Thus, the thickening of the longitudinal edge is formed integrally on the first material piece. Surprisingly, it has turned out that contrary to the prevailing doctrine, which states that a material piece cannot or only very insufficiently be welded to another material piece with its shaped region, welded components with high strength characteristics very well can be produced by the invention. Alternatively, the thickening of the longitudinal edge can at least partly be produced by folding the first material piece at its edge, in particular by folding at right angles. The fold forms a contact surface to the second material piece, via which the end-face contact is produced. The welded joint accordingly is made between the contact surface at the fold and the longitudinal edge of the second material piece. Via the fold, the cross-section of the end face of the first material piece is increased correspondingly. 
     Outside the thickening, the second material piece is at least twice as thick as the first one. 
     The invention in particular provides that the thickened longitudinal edge of the first material piece contacts the opposed longitudinal edge of the second material piece at its front, before they are welded. 
     The width of the thickening can lie in the range between 2 and 30 times the material thickness of the first material piece. In this way, a correspondingly strong thickening is formed, which is able to absorb the occurring forces and moments. 
     In particular, the thickening is at least partly formed by folding several times, in which a portion of a free edge of the starting material is located inwards between two adjacent, succeeding portions, or in which a free edge of the starting material is folded in zigzag fashion. By folding several times, the thickening of the longitudinal edge at the first material piece can become correspondingly larger. The number of folds is suggested by the relative differences in material thickness between the first and the second material piece. 
     The fold can have a height of 0.5 to 1.5 times the material thickness of the second material piece, in particular 0.7 to 1.3 times the material thickness. The fold hence need not exactly correspond to the material thickness of the joining partner, wherein correspondingly good strength values are achieved nevertheless. 
     In particular, the first material piece includes a bearing surface for the underside of the second material piece produced by folding again, which preferably has been produced by folding again at right angles. The bearing surface can form a supporting surface for the second material piece. Via the bearing surface, a lap joint between the first material piece and the second material piece additionally can be produced in the region of the bearing surface. Alternatively, the second material piece also can be welded to the bearing surface, in order to increase the stability. 
     According to a further aspect of the invention, the first material piece is made of a steel and the second material piece is made of a light metal or transition metal, preferably of aluminum or copper. This material combination represents the material combination typically used in vehicle construction or light-weight vehicle body construction. 
     The first material piece can have a material thickness smaller by at least 40% than the second material piece. It hence is ensured that the first material piece, which is formed of a material of higher strength, can be formed correspondingly thinner, whereby material is saved. 
     In particular, the second material piece is formed thicker by a multiple than the first material piece, so that an optimum strength-weight relation can be achieved in the produced component. 
     According to one aspect of the invention it is provided that the thickening is formed such that the material thickness at the thickening corresponds to the material thickness of the second material piece at the opposed longitudinal edge. This ensures that by butt welding the two material pieces, a welded joint extending over the complete and face can be produced between the two material pieces, whereby the strength characteristics of the joint are improved correspondingly. The joint cross-section in the joining region hence is optimized. 
     In particular, the first and/or second material piece each is a sheet metal. Sheet metals typically are used in lightweight construction, as they are particularly weight-saving. 
     According to a further aspect of the invention it is provided that a friction stir welding tool is used, which includes a pin which in the region of the opposed longitudinal edges penetrates into at least one material piece and has a shoulder resting on the upper side and/or on the underside of the material pieces. It hence is ensured that at least one of the two materials can be plasticized over its entire end face due to the pin, which can be formed as rotary body. In addition, mechanical pressure can be exerted on the first and/or the second material piece via the shoulder. 
     In particular, the length of the thickening at least corresponds to the shoulder radius of the tool used for friction stir welding, whereby it is guaranteed that the tool shoulder can rest completely on the thickening. 
     A further aspect of the invention provides that the pin used in the method has a contoured enveloping shell surface formed during its rotation, which substantially corresponds to the cross-sectional geometry of the end face of the thickening, in particular is concavely curved. The enveloping shell surface is formed by the points on the circumferential surface of the pin located farthest on the outside radially to the axis of rotation, so that the pin e.g. like a milling cutter has a concavely axially extending portion, which defines the enveloping shell surface, and portions radially located further on the inside. 
     The rotating pin, which generates the plastification of the material pieces, can rest directly against the end face of the thickening or at least has a constant distance over the entire height. This ensures that between the two material pieces a cohesive connection of uniform quality can form over the entire end face height. 
     According to a further aspect of the invention it is provided that the pin is moved completely or chiefly in the second material piece and/or that the pin first penetrates into the second material piece. The second material piece typically is formed as softer material, whereby the pin is easier to move in this material piece. Furthermore, the pin does not wear out so quickly in the second material piece due to the lower hardness. 
     A further aspect of the invention provides that the pin and the at least one supported shoulder rotate with different rotational speeds. The input of energy into the joining region thereby can be optimized, as corresponding to the existing conditions the pin rotates relative to the shoulder. The shoulder also can be formed to be standing still, so that only the pin rotates. 
     In particular, the in can have a distance to the first material piece which corresponds to one fifth of the diameter of the pin and/or amounts to 0.2 mm. This distance is enough to sufficiently plasticize the first material piece and nevertheless have a safety distance, so that the pin does not penetrate into the firmer first material piece and wears out. 
     The shoulder of the tool can have a diameter which corresponds to between 1 and 25 times the material thickness of one of the two material pieces. 
     The pin can have a diameter between one quarter and three quarters of the shoulder diameter. 
     A further aspect of the invention provides that during welding the material pieces are placed on a counter-bracket against which the tool is pressed. The counter-bracket has a step or a receiving groove which holds the first material piece at the edge of the thickening opposite to the second material piece and/or which has a depth which largely compensates the difference in the material thicknesses. Thus, the upper sides or undersides of the material pieces lie in one common plane or have a height difference which is smaller than the difference in material thickness. The step and/or the receiving groove ensure that the shoulder of the tool lies on a planar surface, whereby it is ensured that the pin can move exactly along the end face. An angular offset of the tool with respect to the material pieces thus is prevented. The height differences of the butt-weld joint between the two material pieces can be adapted to the situation by the step or the receiving groove, so that a good welded joint can be achieved. 
     According to a further aspect, the thickening is at least partly formed by folded portions which form layers, wherein at least two of the layers forming the thickening are spaced from each other before welding, wherein during welding the tool exerts an axial force on the thickening, which presses the layers onto each other and reduces, in particular eliminates the spacing. As a result, there can be used starting materials which have a partly beaded longitudinal edge which is closed by the tool during friction stir welding to the second material piece such that the desired material thickness of the first material piece is achieved. Furthermore, due to the final formation of the thickening during friction stir welding the cohesive connection of the layers of the thickening can be produced at the same time, for example by the energy input of the friction stir welding tool itself. 
     According to one aspect of the invention it is provided that the welding process is path-controlled or force-controlled. The pin thus extends corresponding to a predefined path or is adjusted to a certain maximum lateral force, so that the pin moves along the longitudinal edge of the first material piece and laterally presses against the longitudinal edge of the first material piece with the predefined force. The pin of the tool moved in a force-controlled way preferably cannot penetrate into the harder material piece. 
     It can possibly be provided that the pin slightly “scratches” the first material piece. When the first material piece is not liquefied, changes in the crystal structure can occur in the first material piece, wherein according to the preferred embodiment of the invention, however, the first material piece is not even brought into the pasty phase. 
     The method according to the invention in particular is carried out on a machine tool or, preferably, on a freely programmable industrial robot. 
     The invention furthermore relates to a friction stir welding tool, in particular for carrying out the method as described above, wherein the friction stir welding tool includes a pin which in the region of the opposed longitudinal edges penetrates into at least one material piece and has a shoulder resting on the upper side and/or on the underside of the material pieces, wherein the pin has a contoured, in particular concave enveloping shell surface during the rotation about its axis of rotation. The pin is formed as rotary body which rotates, in order to penetrate into the material piece. After penetrating into one of the two material pieces, the pin continues to rotate such that the immediate environment of the pin is plasticized in at least one material piece, whereby a cohesive connection between the two material pieces can be produced. 
     In particular, it is provided that the shoulder and the pin are rotatable relative to each other, wherein they can be driven with different rotational speeds. It thereby is achieved that the largest input of energy is achieved by the pin. For example, the shoulder also can be formed non-rotatable, whereby only the pin rotates, in order to bring about the corresponding plastification of the material pieces. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a schematic representation of a first variant of the method according to the invention for butt welding and producing a component, 
         FIG. 2  shows a second variant of the method according to the invention for butt welding and producing a component, 
         FIG. 3  shows a third variant of the method according to the invention for butt welding and producing a component, 
         FIG. 4  shows a fourth variant of the method according to the invention for butt welding and producing a component, 
         FIG. 5  shows a fifth variant of the method according to the invention for butt welding and producing a component, 
         FIG. 6  shows a sixth variant of the method according to the invention for butt welding and producing a component, 
         FIG. 7  shows a seventh variant of the method according to the invention for butt welding and producing a component, 
         FIG. 8  shows an eighth variant of the method according to the invention for butt welding and producing a component, 
         FIG. 9  shows a schematic representation of the friction stir welding tool according to the invention, and 
         FIG. 10  shows a variant of the method not falling under the invention for butt welding and producing a component. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  schematically shows how a first material piece  10  is welded to a second material piece  12  by means of a friction stir welding tool  14 , in order to form a component  15 . 
     The second material piece  12  has a distinctly greater material thickness than the first material piece  10 . Both material pieces  10 ,  12  are sheet metals. 
     In all embodiments set forth below both material pieces  10 ,  12  are manufactured from sheet metals, namely from sheet metals of different material. The first material piece  10  usually always is a steel sheet, whereas the second material piece  12  is a light metal or a light metal alloy or a transition metal. In particular, the second material piece  12  is made of aluminum or an aluminum alloy. 
     When reference subsequently is made to material thickness of the material pieces  10 ,  12 , this is the preferably constant material thickness of the second material piece  12  and the material thickness of the first material piece  10  outside the thickening  22 . 
     All components produced preferably are body components of a vehicle. 
     The first material piece  10  has a free end  16  which in the shown variant of the method is shaped to obtain a thickening  22  comprising a first portion  18  and a second portion  20 , so that the two portions  18 ,  20  form a first layer  28  and a second layer  30  of the thickening  22 . The thickening  22  thus is obtained by simple folding of the free end  16 . The thickening  22 , this also applies for the remaining embodiments, can be produced for example by beading or by other shaping methods. 
     The thickening  22  forms a longitudinal edge  24  of the first material piece  10 , which at its front opposes a longitudinal edge  26  of the second material piece  12 , wherein before welding the two material pieces  10 ,  12  can contact each other at their front with their two longitudinal edges  24 ,  26 , which is not absolutely necessary, however, as a slight gap can also be present between the material pieces  10 ,  12 . 
     A joining region  27  is formed thereby, in which the friction stir welding tool  14  plasticizes the two material pieces  10 ,  12 , so that the friction-stir welded component  15  is obtained. In the shown variant, the second material piece  12  has a material thickness twice as large as compared to the first material piece  10 , so that in the region of its longitudinal edge  24  the first material piece  10  has the same material thickness as the second material piece  12  due to the thickening  22 . 
     This means that on the thickening the first material piece  10  forms an upper side  32   a  which lies in one plane with an upper side  34   a  of the second material piece  12 , when the two material pieces  10 ,  12  rest on a planar surface with their undersides  32   b ,  34   b.    
     Subsequent to the contacting of the two longitudinal edges  24 ,  26  at their front, the two material pieces  10 ,  12  are friction-stir welded by means of the friction stir welding tool  14 , in order to form the component  15 . 
     For this purpose, the friction stir welding tool  14  includes a pin  36  which in the illustrated representation already has rotatingly pressed into the softer second material piece  12 . The pin  36  is pressed into the material piece  12  to such an extent that a shoulder  38  of the friction stir welding tool  14  rests on the upper sides  32   a  and  34   a.    
     For welding the two material pieces  10 ,  12 , the friction stir welding tool  14  moves along a predefined path in a path-controlled way or moves in a force-controlled way, in that it presses against the longitudinal edge of the first material piece with a predetermined force and moves along its longitudinal edge, so that the two material pieces  10 ,  12  are welded to each other. 
     The pin  36  usually only moves within the second material piece  12 , wherein it even (an option) can have a small distance to the first material piece  10 . In this case, the pin  36  cannot contact the harder first material piece  10  or only can contact it at its front such that the same remains cold enough to not even reach the pasty phase. 
     The pin  36  is formed as rotary body which can rotate relative to the shoulder  38  of the friction stir welding tool  14 . Due to the high rotational speed of the pin  36 , the regions of at least one of the two material pieces  10 ,  12 , which are in the vicinity of the pin  36 , are plasticized, so that a cohesive connection between the first material piece  10  and the second material piece  12  is obtained. A component  15  formed by cohesive connection is produced thereby. 
     In the shown variant, the in  36  of the friction stir welding tool  14  has an enveloping shell surface  40  formed during rotation about its axis, which is formed linear. 
     Due to folding of the free end  16 , a convex cross-section of the first end face at the first longitudinal edge  24  is obtained at the longitudinal edge  24  of the first material piece  10 . The second end face at the second longitudinal edge  26  is designed e.g. planar or convex or has a correspondingly concave cross-section which can have another curvature as the longitudinal edge  24  or the same curvature (i.e. a complementary shape). In the case of the concave design of the longitudinal edge  26 , the two material pieces  10 ,  12  thus can fully rest against each other at their end faces before welding, which however need not necessarily be the case. There can also be a slight gap between the material pieces or a point or line contact. 
     Independent of the embodiment according to  FIG. 1  the following applies: The space between the material pieces  10 ,  12 , which is not filled with material, is filled on welding in that heated material of the second material piece is transported into the space. This is effected by inclining the axis of rotation A of the friction stir welding tool  14 . By such tilting of the axis of rotation A, the shoulder  38  unilaterally penetrates into the second material piece  12  on the upper side  34   a , in order to thereby displace material. By such displacement of the material, material of the second material piece  12  as a whole is pressed into the possibly present gap between the longitudinal edges  24 ,  26 , in order to fill the gap (see  FIG. 1 ). The tool can be inclined both along and transversely to the welding direction or be inclined in transverse and longitudinal direction according to a combination. The axis of rotation A can of course also extend vertically to the surfaces of the material pieces  10 ,  12 , as likewise shown in  FIG. 1 , when e.g. no substantial transport of material is necessary for filling a gap. 
       FIG. 2  shows a second variant of the method, wherein this variant differs from the one of  FIG. 1  to the effect that the end face of the first longitudinal edge  24  has been adapted to the contour of the pin  36  or its shell surface  40  obtained during rotation and each extends flatly. 
     The first longitudinal edge  24  is upset before contacting at its front and before welding to the second longitudinal edge  26 , whereby a first contact surface  42  is obtained at the front end of the first material piece  10  or at its longitudinal edge  24 . The first contact surface  42  formed in this way extends flatly. 
     The pin  36 , which likewise forms a linear shell surface  40  upon rotation, thus is exactly adapted to the contour of the first contact surface  42 , and vice versa. The cohesive connection between the first material piece  10  and the second material piece  12  thereby can be formed particularly well, as the rotating pin  36  has the same distance to the first material piece  10  and to its longitudinal edge  24  over the entire height of the joining region  27 . 
       FIG. 3  shows a third variant of the method, wherein the same differs in particular in the material thickness of the material pieces  10 ,  12  used. 
     The first material piece  10  shown here has a thickening with triple material thickness, wherein the first material piece  10  has a free end  16  which has been shaped such that it comprises a total of three portions, the first portion  18 , the second portion  20  and the third portion  44 . 
     The three portions  18 ,  20 ,  44  are formed by zigzag folding to a thickening  22 , which comprises a total of three layers. The second layer  30 , which is formed by the second portion  20 , is arranged centrally and on both sides is surrounded by one further layer each, namely the first layer  28  as well as a third layer  46 . 
     The third layer  46  corresponds to the third portion  44  of the folded free end  16 , wherein in the shown variant the third layer  46  is formed such that it has the underside  32   b.    
     By zigzag folding of the free end  16 , a thickening  22  thus can be produced, which can compensate a difference in material thickness, no that the cross-sections of the two end faces of the material pieces  10 ,  12  are adjusted to each other in the joining region  27 , whereby an optimum joint cross-section is obtained between the two material pieces  10 ,  12 . 
       FIG. 4  shows an alternative to the third variant of  FIG. 3 , wherein the two material pieces  10 ,  12  likewise have a material thickness ratio of 1:3. 
     In the variant shown in  FIG. 4 , the free end  16  shown after its shaping again includes three portions  18 ,  20 ,  44 , which form the three-layer thickening  22 . In contrast to the third variant, however, the portions  18 ,  20 ,  44  are arranged differently, so that the thickening  22  is constructed differently. 
     According to this variant, the second layer  30  of the thickening  22  is formed by the third portion  44 , i.e. the end portion of the free end  16 . The third and last portion  44  of the free end  16  has been folded inwards between the two succeeding portions  18 ,  20  as seen from the free end  16 , so that on the whole a snail-shaped thickening  22  is obtained. 
     With the illustrated fourth variant of the method it in turn is possible to increase the material thickness of the first material piece  10  in the region of its longitudinal edge  24  such that it corresponds to the material thickness of the second material piece  12 . The cross-sections in the joining region  27  are adapted to each other such that an optimum joint cross-section is obtained. In this variant, too, the longitudinal edge of the material piece  10  simply is convexly curved in cross-section. The second material piece  12  is shaped complementarily concave at its front. 
       FIG. 5  shows a fifth variant of the method, wherein the shaped free end  16  has two portions  18 ,  20  analogous to the first and the second variant. The material thickness of the second material piece  12 , however, is three times as large as that of the first material piece  10 , analogous to the third and fourth variants. 
     It nevertheless is possible that the thickening  22  of the first material piece  10  at the first longitudinal edge  24  has the same material thickness as the second material piece at the opposed second longitudinal edge  26 . 
     This is achieved in that between the two portions  18 ,  20  of the shaped free end  16  an intermediate piece  48  is inserted, around which the second portion  20  is folded. The intermediate piece  48  typically is formed as separate material strip, which is frictionally or cohesively connected with at least one of the two portions  18 ,  20 . 
     The intermediate piece  48  also can have another material thickness than the first material piece  10 . 
     The thickening  22  thus again is formed with three layers analogous to the third and fourth variants. In contrast to the third and fourth variants, the first layer  28  is realized by the first portion  18  and the second layer  30  is realized by the intermediate element  48 . The third layer  46  of the thickening  22  is provided by the second portion  20 , which then comprises the upper side  32   a.    
       FIG. 6  shows a sixth variant of the method, wherein in the sixth variant the free end  16  of the first material piece  10  is formed folded by 90°, so that the thickening  22  of the first material piece  10  at its first longitudinal edge  24  is obtained by folding the free end  16  at its edge. 
     By folding the free end  16 , the first contact surface  42  is provided for contacting with the second material piece  12  at its front. 
     In the variant shown, the folding height of the free end  16  exactly corresponds to the material thickness of the second material piece  12 , so that the first contact surface  42  has the same surface size as the contact surface of the second material piece  12 , which however is not to be understood in a limiting sense. 
     In the seventh variant, the free end  16  accordingly in turn includes only one portion  18  which, however, has been folded. 
       FIG. 7  shows a seventh variant of the method, wherein in the shown seventh variant the free end  16  is formed by two portions  18 ,  20 . 
     Analogous to the sixth variant, the first portion  18  is folded by 90° such that it forms the first contact surface  42  for the second longitudinal edge  26  of the second material piece  12 . 
     The second portion  20  in turn is angled at about right angles to the first portion  18 , wherein it extends below the second material piece  12 , in order to form the bearing surface  54  for the second material piece  12 , on which the underside  34   b  rests. 
     Via the bearing surface  54 , a lap joint can be produced between the first and the second material piece  10 ,  12  or the second material piece  12  is welded to the bearing surface  54 . Both serves for stabilizing the connection of the two material pieces  10 ,  12  and thus of the component  15 . 
     With the variants of the method as shown in  FIGS. 6 and 7  thickenings  22  can be produced, which are able to compensate any material thickness ratio between the first material piece  10  and the second material piece  12 . 
       FIG. 8  shows an eighth variant of the method, wherein during welding the two material pieces  10  and  12  are placed on a counter-bracket  56  which during the welding process forms a stop for the friction stir welding tool  14 . 
     In the variant shown, the counter-bracket  56  includes a stop surface  58  on which the underside  34   b  rests. In the stop surface  58  a receiving groove  60  is formed, which sectionally can accommodate the thickening  22  such that the first underside  32   b  lies in the receiving groove  60  and is guided laterally and secured in its position. 
     In case the thickening  22  is formed thicker than the second material piece  12 , the tool shoulder  38  nevertheless can rest flat on the upper sides  32   a ,  34   a  of the two material pieces  10 ,  12 , so that the pin  36  is aligned exactly vertically. 
     Alternatively it can also be provided that the counter-bracket  56  includes a step instead of the receiving groove  60 , in case the thickening  22  is formed less thick than the second material piece  12 . 
     Furthermore, the counter-bracket  56  includes an axial stop  62  which axially positions the first material piece  10  such that at its front it rests against the opposed longitudinal edge  26  of the second material piece  12  with its first longitudinal edge  24 . The two material pieces  10 ,  12  thus are fixingly aligned to each other in axial direction. 
       FIG. 9  shows the friction stir welding tool  14 , which includes the in  36  which has a contoured enveloping shell surface  40  obtained during its rotation. 
     The shell surface  40  is adapted in cross-section to the cross-sectional geometry of the first contact surface  42  of the first material piece  10 , which is formed of a harder material. The friction stir welding tool  14  or the pin  36  thus have an almost constant distance to the first material piece  10  over the entire height of the welded joint. 
     With its shoulder  38 , the friction stir welding tool  14  can rest on the upper sides  32   a ,  34   a  and on the undersides  32   b ,  34   b.    
     Furthermore, it is also possible to provide a tool with two shoulders, which between themselves accommodate the material pieces  10 ,  12  and rest against the upper side and the underside, so that a counter-bracket becomes unnecessary. 
     Furthermore, the two material pieces  10 ,  12  can be friction-stir welded from both sides at the same time or one after the other, in order to form the component  15 . 
     With the different variants of the method it is possible to connect two material pieces  10 ,  12  of different material thickness and different material properties with each other such that a component  15  with high strength values is obtained, which nevertheless is lightweight. 
     In the automotive industry, steel sheet metals with a thickness of 0.75 mm and aluminum sheets with a material thickness of 1.5 mm typically are used. 
     At such a material thickness ratio of 1:2 an increase of the tensile strength by a factor of 2 is obtained, based on the aluminum sheet, in contrast to material pieces butt-welded directly. 
       FIG. 10  shows a variant not falling under the scope of protection of the present patent, in which the thickening  22  is produced by a further material strip  50  which is connected with the first material piece  10  via at least one connecting point  52 .