WO2012175127A1

WO2012175127A1 - Friction stir welding tool with shoulders having different areas methods using such tool; product welded with such tool

Info

Publication number: WO2012175127A1
Application number: PCT/EP2011/060471
Authority: WO
Inventors: Wojciech OSIKOWICZ
Original assignee: Sapa Ab
Priority date: 2011-06-22
Filing date: 2011-06-22
Publication date: 2012-12-27

Abstract

A friction stir welding tool for joining two abutting work pieces is described, wherein said tool has an axis of rotation (R), and comprises a first shoulder (1), a second shoulder (2), and a pin (3) arranged so as to connect the first and second shoulders, wherein said first shoulder (1) has a first contact surface, which is to be in contact with one surface (4) of the work pieces to be joined during welding, said first contact surface having a first area essentially perpendicular to the axis of rotation, and said second shoulder (2) has a second contact surface, which is to be in contact with an opposite surface of the work pieces to be joined, said second contact surface having a second area essentially perpendicular to the axis of rotation, wherein the first contact surface and the second contact surface have different areas; This application describes also a method of joining work pieces, a product comprising joined pieces, and the use of such friction stir welding tool.

Description

FRICTION STIR WELDING TOOL WITH SHOULDERS HAVING DIFFERENT AREAS METHODS USING SUCH TOOL; PRODUCT WELDED WITH SUCH TOOL

Technical Field

The present invention pertains to a friction stir welding tool, a method of joining work pieces, to a product comprising joined pieces, and the use of friction stir welding tool.

Background of the invention

It is known in the art to use a friction stir welding (FSW) for joining of two metal or plastic work pieces. In essence, FSW employs a non-consumable rotating tool, which interacts with the work pieces to generate frictional heating and thus plasticizes the material in weld zone. Local softening of the material in the weld zone facilitates mixing of material from the work pieces and upon translation of the tool along the welding direction the joint is formed. The plasticized material tends to run-off from the weld zone under process loads, which in turn might lead to formation of internal or surface breaking defects in the joint. This is circumvented by confining softened material in the weld zone with aid of the work pieces material surrounding the weld zone, tool shoulder(s) and appropriate support, such as backing bar, if required. For instance, in US 7 686 202 for friction stir welding of the flat metal pieces, the confinement of the softened material is achieved by the use of two shoulders interconnected by a pin and by the work piece material itself. In case of other configurations of the work pieces than flat ones, e.g. with a limited access and space for the support from the bottom of the weld zone, the known conventional tools and methods are not applicable.

For example, this problem appears in manufacturing of double skin panels from multi-void hollow extrusions made of aluminium alloy material by joining extrusions with FSW into the double skin panels, having beneficial strength-to-weight ratio. These panels might be used in building constructions such as bridges or the similar, or in a mass transportation applications such as marine, rail stock and automotive applications requiring strong lightweight constructions being resistant to corrosion. Such multivoid hollow profiles extruded from aluminium alloys usually comprise a first flat portion or plate or skin, a flat second portion or plate or skin, and interconnecting inner perpendicular or inclined walls or trusses arranged between the first and second flat portions forming a generally triangular shape in a cross section perpendicular to the extrusion direction as shown in e.g. US 7 665 651 . US 7,665,651 B2 relates to butt joining of two multivoid hollow extrusions into a double skin panel by FSW. Each deck panel comprises an upper plate and a lower plate. In order to make FSW possible, a vertical plate member is arranged between the upper and lower plates to provide support in location where the friction stir welding is performed.

The inclined inner walls are carrying and distributing the load providing the strength for the entire panel. For uniform load distribution and thus the improved strength of the panel, the load lines of actions shall preferably intersect within the profile flat portions or plates, within the profile body. The profiles inner inclined walls shall form a regular triangular shape hollows in a cross section perpendicular to the extrusion direction. Such constructions reduce risk of excessive bending moments acting on panels during exploitation.

The need of the support to the welding area from the downwards side during FSW as described above might be solved in different ways by a separate support placed under the welded area (backing bar), or alternatively as illustrated in US 7,665,651 it might be formed by work piece extensions underneath (integral backer). This solution has a disadvantage of forming crack like notch in the vicinity of the weld which adversely affects fatigue performance of the construction. Besides that, moisture can accumulate in the crack between the faying surfaces and lead to accelerated corrosion in the welded section, thus weaker construction.

A number of other solutions have been proposed, i.e. providing the support for plasticized material in the welding zone, as illustrated in JP 2007-253210, by the equal-sized shoulders attached to the pin on the both sides (so called balanced Bobbin FSW tool) or similarly to the tool of US 6,199,745 adjusting the distance (or the pin length) between two opposite shoulders. The tool of JP 2007-253210 does not provide the possibility to weld the other or opposite flat portion of the aforementioned profiles due to lack of access for the tool. The equal-sized shoulders are applying essentially symmetrical loads for plasticizing the material and they hold the plasticized material in the joint area in place from the both sides of the welded pieces. Symmetrical loads in turn eliminate forces acting in the normal direction to the profile plates.

Several methods are available for joining deck panels, such as soldering, gluing, conventional welding and friction stir welding. The joint shall desirably be quickly and cost- effectively formed, strong, fatigue and corrosion resistant, without internal defects which compromises the corrosion and mechanical integrity of the structure. Additionally, the joint shall have no or little influence on the joined objects. Joining by FSW presents extraordinary advantages in production of panels for use in different applications, as it allows manufacturing of large, homogenous panels based on extruded profiles. The FSW tool frictionally heats the work piece material locally to a plasticized state at a temperature substantially below the melting temperature. Additionally, joining of the work pieces is made without any added filling material. Thus the joined panel structure is substantially free of heat distortion. In addition, the absence of melt-related and filler-induced defects known from fusion welding results in excellent mechanical properties and tightness of the welds. Also risk of intermetallic corrosion is eliminated. The rotating tool leaves a relatively smooth weld face, flush with the work piece surface. This results in longer time to crack initiation under cyclic loading. Clearly a universal FSW tool and process allowing welding work pieces independently on their configuration are needed.

Summary of the Invention

An objective of the present invention is to provide FSW tool for efficient joining of work pieces even having a limited access for the tool, for example multivoid hollow extrusions, tube- shaped products, bended panels, etc. Another objective is to provide a method of joining the work pieces by means of such FSW tool. A further objective is to provide a new product with improved strength parameters after welding such as an example, a double skin panel construction comprising two or more multi-void hollow extrusions joined by such method. The panel has essentially the same specific strength-to-weight ratio as the individual extruded profiles. These objectives are met by the present invention. The tool of the invention shall allow for simultaneous or in turn welding from any side of the double skin panels or tubes made of extruded aluminium profiles. The corresponding welding method shall provide joining of the work pieces having different or sophisticated configurations, even these with a limited access for the tool, like between the two inclined inner walls of the multi-void hollow extrusions with a narrow space in between members of construction or a small radius at the location of the FSW joint, such that the preferred load distributions is achieved.

The present invention relates to a friction stir welding (FSW) tool for joining of abutted work pieces. Said friction stir welding tool has an axis of rotation, and comprises a first shoulder, a second shoulder, and the pin arranged so as to connect the first and second shoulders. The first shoulder has a first contact surface, which is to be in contact with one surface of the both abutting work pieces to be joined during welding, said first contact surface having a first area (a-ι) essentially perpendicular to the axis of rotation, and the second shoulder has a second contact surface, which is to be in contact with an opposite surface of the two abutting work pieces to be joined. Said second contact surface has a second area (a₂) essentially perpendicular to the axis of the pin rotation, and the first contact surface and the second contact surface have different areas. Thereby, it allows joining two adjacent work pieces with a limited space on one side of the work pieces.

The invention further relates to a method of joining two adjacent work pieces by said FSW tool. The method comprises the following steps: assembling the tool having a first shoulder, a second shoulder, and a pin connecting the first and second shoulders; rotating the tool with a predetermined speed; aligning the tool adjacent to the two abutting work pieces so that the first tool shoulder is placed in the plane of the first surface of the both work pieces and the second tool shoulder is placed in the plane of the opposite surface of the both work pieces; and linearly moving the pin along the abutment of the two work pieces while having the shoulders in a contact with the corresponding opposite surfaces of the both work pieces so as to perform the welding by heating the material of the work pieces to the plasticized condition in the joint area due to the friction between the shoulders and the work pieces surfaces. A further step may comprise an entry notch at the abutment of the work pieces so as to ensure the correct position of the rotating pin at the beginning of the welding process.

The method may alternatively comprise the steps of: drilling a hole in the abutment of the two work pieces for receipt of the pin of the tool; inserting the pin (3) of the tool through the hole; assembling the tool having a first shoulder, a second shoulder, and a pin arranged to connect the first and second shoulders, while the first shoulder is placed in the plane of the first surface of the work pieces and the second shoulder is placed in the plane of the opposite surface of the work pieces; starting rotation of the tool with a predetermined speed; and linearly moving the pin along the abutment so as to perform the welding.

The invention also relates to a product such as a panel made of the extruded profiles obtained by the method. In addition, the present invention relates to the use of said friction stir welding tool for joining two abutting work pieces. The invention is applicable to the friction stir welding of different materials such as metals and plastics.

Another aspect of the invention is the use of the friction stir welding tool as described above for friction stir welding of two abutting work pieces, where the work pieces are two metal bended or flat panels, or sheets, which may be hot and/or cold rolled from casted ingot or billet, preferably are metal double skin deck panels. Brief Description of Drawings

Further features and advantages of the invention will now be described with reference to the enclosed schematic figures, where

Figure 1 is a perspective view of a friction stir welding tool according to the invention , and multi-void hollow extrusions to be joined illustrating one method of friction stir welding;

Figure 2 is a cross-sectional view of two multi-void hollow extrusions which upon welding form double-skin panel according to the invention;

Figure 3a is a schematic view of a friction stir welding tool of the invention;

Figure 3b is an exploded view of the friction stir welding tool of Figure 3;

Figure 4 is a schematic view of another embodiment of a friction stir welding tool of the invention which allows adjustment of the shoulders positions relative each other and a control of the applied forces;

Figure 5 illustrates a three-dimensional view of one possible embodiment of a friction stir welding tool of the invention;

Figure 6 is a schematic view of a cross section of weld joint obtained by the friction stir welding tool of the invention;

Figures 7-8 illustrate flow schemes for the method of joining two abutting work pieces Figures 9-10 illustrate the prior art friction stir welding tool and methods of joining two abutting work pieces by conventional friction stir welding tool.

Figure 1 1 illustrates the different kinds of products manufactured by use of the FSW tool according to the invention

Detailed description of the invention

The invention will now be described in details by referring to the appended drawings. Corresponding elements of the different embodiments have the same reference numerals. It is to be understood that the scope of the invention is not limited to the embodiments as shown in the appended Figures, which are used only for clarification purpose.

Figure 1 shows a cross-sectional view of a two adjacent and abutting hollow extrusion profiles 10, which are being joined by a friction stir welding tool 20 according to a method of the invention. Figure 2 shows a product in form of a double skin panel, which has been obtained by welding with friction stir welding tool 20 of two multivoid extruded profiles. It is to be noted that even though the invention is illustrated by means of multivoid extrusions, having flat upper and lower skins, the FSW tool of the invention can as well be used to join profiles, panels or other details of any design, having flat or curved shape. In this description the invention is described in relation to joining two work pieces of identical shape. However, it should be noted that work pieces of different shape can be joined equally well. For the sake of simplicity the same reference numerals have been used for both details to be joined, when illustrated in the appended drawings.

The tool 20 is schematically illustrated in Figures 3a and 3b. It is to be noted that the tool illustrated in Figures 3a and 3b are merely examples of an FSW tool of the invention, and that various other designs of the tool's parts with different conventionally known surface features may be conceivable.

The friction stir welding tool 20 of the present invention comprises a pin 3 rotated about its axis (R) which is also a rotation axis (R) of the tool and a first and a second radial extensions at both ends of the pin, so called shoulders 1 , 2. The first shoulder 1 has a first contact surface A1 , which is to be in contact with one surfaces 4 of the work pieces 10 formed in this embodiments by the extruded profiles 10 to be joined during welding, and the first contact surface A1 has a first area ai , which is essentially perpendicular to the axis of rotation R. The second shoulder 2 has a second contact surface A2, which is to be in contact with opposite surfaces 5 of the two work pieces to be joined, said second contact surface A2 having a second area a₂ essentially perpendicular to the axis of rotation R. These contact surfaces A1 and A2 have different contact areas ai and a₂. In the embodiment shown in Fig 1 and Fig 3a-3b, the second contact surface area a₂ is smaller than the first contact surface area ai . Accordingly, the radial dimension r2 is smaller than the radial dimension r1 . It is to be understood that the tool 20 can be turned upside down during operation, and the contact surface area ai maybe smaller then the contact surface area a₂.

Each of said first and second shoulders 1 , 2 has a function to heat the material in the joint area due to friction between the each shoulder contact surface and the both work pieces opposing surfaces and at the same time to hold a plasticized material in the area of the joint. The contact surface A1 , A2 of each shoulder is essentially perpendicular to the axis of rotation (R) of the tool and adapted to be in tight contact with two adjacent surfaces 4, of two flat portions of the two work pieces 10 as multi-void hollow extrusions and the opposing surfaces 5 of the same flat portions to be joined during welding. These contact surfaces have different contact areas, i.e. one is smaller than the other, but the contact surfaces are still sufficient for holding the plasticized flowable material in the place of joining; such design with different contact areas allows the tool use also within a limited space in the area of the joint between the two work pieces to be joined like between the two inclined inner walls in the mentioned above double skin panels of the multi-void hollow extrusions or joint of a bended panel to a tube shape detail. As the contact areas ai and a₂ are different the loads applied by the tool to the opposite sides of the jointed portion of the welded work pieces are different. The loads are preferably balanced through the extrusions and by the application of an external support in accordance with a definite algorithm. In particular, the second area a₂ of the second contact surface of the second shoulder may be smaller than the first area ai of the first contact surface of the first shoulder, said second area a₂ being 75 % or less of said first area a<i. Preferably, the relation of the first ai and second a₂ contact areas is obtained by providing a second shoulder 2 having a smaller diameter than the first shoulder 1. Since the second area a₂ is substantially smaller that the first area ai, it is made possible to join two abutting work pieces where there is very limited space on one side (surfaces 5) of the work pieces 10. For example this is the case when joining the multi-void extrusions into double skin panels, where the perfect triangulation of the inner walls 8, 9, 1 1 or trusses is desired for the improved load distribution, allowing the load vectors intersect within the double deck panel body for the better strength of the construction. A radial dimension, which is the distance from the pin outer surface to an outer edge of the second contact surface of the second shoulder, may be at least 1 .5 mm, preferably 2-50 mm, more preferably 10-30 mm, most preferably 15-25 mm. These dimensions are advantageous for a weld thickness of 15 mm to 20 mm.. The length of the pin 3 can vary in a wide range and depend on the thickness of the work pieces 10, 10' to be welded. The pin 3 is illustrated in Figs 3a and 3b with a conical shape in direction of the rotation, but may instead have a cylindrical or have eanother sshape in direction of the rotation and the pin's surface might be a smooth or machined provided with cuts, grooves, threads and other known in the art advantageous patterns.

The tool illustrated in Figs 3a and 3b has a rectangular, circular or polygonalcross section perpendicular to the axis R of the tool rotation. However, each of the contact surfaces A1 , A2 of the shoulders 1 , 2 may have a flat, tapered, concave or a convex shape in a cross section perpendicular to the axis R of the tool rotation. Convex shoulders can give improved results when welding work pieces with poor tolerances. The shoulders' contact surfaces may be smooth or featureless or may be machined and comprise features like scrolls, ridges, threads, grooves or scopes. Such features improve work piece 10 material deformation, heat input and material mixing. Scrolls facilitate material capture and general improves material flow around the tool and material mixing. The shoulder shape in a cross section perpendicular to the axis R of the tool rotation as well as any contact surface of the shoulders can be combined with any shape of the pin, as described above. The distance between the first contact surface and the second contact surface is adapted to the thickness of the work pieces to be welded (e.i. as illustrated in Fig 4), and is preferably at least 3 mm or more. The pin 3 cross section may reduce in a direction from one shoulder towards the other shoulder. In other words, the pin may taper in a direction from one shoulder towards the other. Thereby, forces and torques needed for welding may be decreased. The pin 3 may further have a cross section perpendicular to the axis of rotation, which is one of a polygonal, circular and oval cross section. The tool shown in Fig 3b has the pin 3 and the shoulders 1 , 2 made all as separate elements, which are assembled to form the tool 20, i.e. the pin 3 is releasable attached to the first and second shoulder. However, alternatively, the pin 3 and one of the shoulders may be made in one piece. Then the position on the pin 3 with the second shoulder can be adjusted relative to the first shoulder such that the pin 3 length will be equal to the thickness of the material to be welded. Alternatively, the pin 3 fixed to the first shoulder can be threaded through the second shoulder which then will be fixed to the pin 3, see Fig 4. Furthermore, at least one of the shoulders 1 , 2 can be made as a separate component that is non-releasable attached to the pin by the means of welding or in the other suitable way, or as a further alternative, the pin 3 and the both shoulders 1 , 2 are made in one piece. These alternative ways of assembling the tool can be applied on any of the above described tool designs.

A multi-void hollow extrusion or work piece 10 as illustrated in Figs 1 -2 comprises a first flat portion 6 and a second opposite flat portion 7, and the inner walls 8, 9, 1 1 interconnecting the flat portions 6, 7, which the inner walls 8, 9, 1 1 arranged in a triangular configuration seen in the cross section to the extrusion direction. The central inner wall 1 1 is perpendicular to the flat portions 6, 7, and the inclined inner walls 8, 9 are arranged on each side of the central inner wall 1 1 , to form an equally sided triangle with one of the flat surfaces 6, 7. As indicated above, any other profile design can be joined by the tool of the invention. A method for joining two abutting work pieces 10 by the above described tool 20 according to the invention may comprise the following steps. The tool is first assembled from the pin 3 and the shoulders 1 , 2. The work pieces are placed with the surfaces to be joined in abutting relationship. Then rotation of the tool 20 is started with a predetermined rotation speed. The tool 20 is aligned adjacent to the two abutting work pieces 10 so that the first shoulder 1 is placed in the plane of the first surfaces 4 of the work pieces 10 and the second shoulder 2 is placed in the plane of the opposite surfaces 5 of the work pieces 10, and the tool is then linearly moved along the abutment of the work pieces 10, while having the tool shoulders 1 , 2 in tight contact with the corresponding surfaces 4, 4' and 5 of the both work pieces 10 for forming a weld joint 15.

A further step of the method of friction stir welding by this tool 20 may comprise making an entry notch at the edge of the abutment of the two work pieces 10 so as to ease the entry of the rotating the tool at the beginning of the welding process.

If a welding tool 20 with at least one releasable shoulder 1 or 2 is used, the method may alternatively comprise the steps of: drilling a hole in the abutment of the two work pieces 10, 10' for receipt of the pin 3 of the tool 20, inserting the pin 3 of the tool through the hole, adjusting the length of the pin 3 to be equal to the flat portions 4 material thickness by moving the releasable shoulder relative the other one, and assembling the tool 20 comprising the first shoulder 1 , and the second shoulder 2 by fixing the releasable shoulder to the pin 3, while the first shoulder 1 is placed in the plane of the first surfaces 4 of the flat portions 6 of the two work pieces 10 and the second shoulder 2 is to be placed in the plane of the opposite surfaces 5 of the same flat portions 6 of the work pieces 10. Then starting rotation of the tool 20 at a predetermined speed, and moving the tool 20 linearly along the adjacent edges of the work pieces 10 so as to perform welding and forming a joint 15. Similarly or simultaneously the other opposite flat portions 7 of the same work pieces (extruded profiles) 10may be welded forming a welded joint 16, and thus a double skin panel of the extruded profiles is obtained.

The tool 20 is advantageously rotated at 100-3000 revolutions/min, and moves linearly along the abutting edges of the two work pieces at a speed of 50-2000 mm/min. The tool 20 typically applies a finite vertical force to the welded work pieces for balancing the forces applied by the different contact areas ai and a₂ of the tool to the work pieces 10, and the vertical force is typically at least 100 N. The tool 20 is preferably manufactured from a material with very good strength, fracture toughness, dimensional stability and wear resistance within the full range of process temperatures. The tool material needs to be reasonably machineable and show good corrosion resistance and chemical stability including stability against elements present in the work piece material within the temperature range involved.

The method of the present invention is particularly useful for joining work pieces or multi-void hollow extrusions made of metal, more particularly of aluminium and its alloys. The tool and the method of the invention can be applicable for the other materials such as plastics and compounds.

As shown in Figs 1 and 2, the two adjacent work pieces 10, 10' may be multi-void hollow extrusions, e.g. for forming double skin panels, where each of extruded profile comprises a first and a second flat portion which are interconnected by confined inner walls as seen in a cross section perpendicular to the extrusion direction. The method of manufacturing such panels include the steps of placing two extruded profiles or multi-void extrusions adjacent to each other, such that the extrusion directions of each profile or extrusions are parallel to each other, and such that an intersection of one flat portion and the inclined inner walls of one profile 10 are facing a corresponding intersection of one flat portion 6 and the inclined inner walls of the other profile 10, such that a narrowing space is formed between the inner inclined confined walls of the profiles or extrusions 10; and joining at least the two flat portions 6, 6' of the two profiles 10 by moving the tool 20 longitudinally along the extrusion direction through the narrowing space, thus forming a double-skin panel (wherein the skin means the outer flat portion 6). Advantageously, both skins 6 of the hollow extruded profiles are connected by the tool 20, by moving the tool's pin 3 longitudinally along the extrusion direction. The welding of the first two flat portions (skins) 6 and the second two flat portions (skins) 7 is preferably performed simultaneously by two identical tools 20 according to the invention. Alternatively, the flat portions or skins 6 and 7 could be welded one after the other by the same tool 20.

In particular, the multi-void hollow extrusions may have inner walls 8, 9 connecting the flat portions 6 7, which inner walls 8, 9 in cross-section to the extrusion direction are arranged in a triangular configuration. Thus, each extrusion 10 can include first and second flat portions 6, 7, which are parallel to each other, a central inner wall 1 1 perpendicular to said flat portions, and inclined inner walls 8, 9 on each side of the central inner wall 1 1 , whereby an equally sided triangle is formed by the two inclined inner walls 8, 9 and one of the flat portions 6 or 7. Such extrusions can be joined according to the present invention to a panel made of multi-void hollow extrusion or a "double-skin panel". Alternatively, the perpendicular central wall 1 1 may be eliminated and the double skin profile 10 might comprise only inclined inner walls 8, 9. Such panels can be used for example in construction of bridges or ship-deck constructions. In order to obtain high strength in such double skin panels, it is often desirable to provide a configuration where the inclined inner wall 8 of one profile 10 form an equally sided triangle also with the inclined inner wall 9 of an adjacent hollow extrusion profile 10'. This has not been possible to achieve with previously available conventional friction stir welding tools. Instead, it has been necessary to include various supporting members as vertical inner walls into the hollow extrusion profile, as illustrated in Figures 9 and 10, or alternatively to use removable supporting members under the welding joint 15 which generally impair weld quality and productivity. The invention also relates to a new product obtained by the method as described above as the tool 20 of the invention allows manufacturing of panels from extruded work pieces of simplified shape and lighter weight, thereby decreasing the amount of required metal at the same strength demand. Alternatively the other products examples of which are illustrated in Fig. 1 1 such as tube-shaped details manufactured from the bended plates or sheets, or the other products such as air or water coolers might be manufactured by these methods. As indicated above, the product is preferably a double-skin panel construction comprising two or more multi-void hollow extrusions joined by the above method. Each extruded profile includes two flat portions and interconnecting inner walls, wherein the weld face width on one surface of the one flat portion of the panel is wider than the weld face width on the opposite surface of the same flat portion of the profile. The product may be the panel made of multi- void hollow extrusions, e.g. for use as the deck panel member in ship or bridge constructions, comprising two or more hollow extrusion profiles joined with the method described above. The rotating shoulders 1 , 2 of the tool 20 leave marks on the surfaces 4, and 5, of the profile portions joined by the tool, which marks are referred to as weld faces 15', 15" of the joint 15. The product comprises at least two portions 6, welded together so as to form a joint 15 having a weld face, where the weld face has different widths on opposite surfaces of the welded product seen in cross section. Since the second contact area a₂ of the tool as described above is smaller than the first contact area a^_\, the weld face widths will have different sizes. The joint 15 between the two portions of the panels thus obtained has a specific appearance as illustrated in Fig. 6, due to the design of the tool 20, and is wider in the region of the larger shoulder contact area ai than in the region of the opposing smaller shoulder contact area a₂, and has a narrowing portion in between. The joint 15 between the panels is wider towards surface 4 in the region of the larger shoulder contact area than towards surface 5 in the region of the opposing smaller shoulder contact area, and has a narrowing portion 17 in between.

Figure 5 shows an alternative design of a friction stir welding tool of the invention, where the pin has one set of flutes up, one set of flutes down and one neutral set. Alternatively, grooves or other machined patterns can be provided on the tool's shoulders and/or the pin. Figures 7 illustrates a flow scheme for the method of joining two abutting work pieces, including the steps of: assembling 100 the tool; starting rotation 1 10 of the tool; aligning 120 the tool adjacent to the two abutting work pieces, possibly adjusting the distance between the shoulders; and linearly moving 130 the tool along the abutment of the work pieces. A further step may comprise an entry notch at the abutment of the work pieces so as to ensure the correct position of the rotating pin at the beginning of the welding process.

Figure 8 illustrates a flow scheme for the method of joining two abutting work pieces, including the steps of: drilling 200 a hole in the abutment of the two work pieces, for receipt 210 of the pin of the tool; inserting the pin of the tool through the hole, assembling 220 the tool (first and second shoulder 1 , 2 and connecting pin 3) while adjusting the distance between the first and second shoulders, starting rotation 230 of the tool, and linearly moving 240 the pin along the abutment so as to perform the welding. Figures 9 and 10 illustrate prior art methods of joining two abutting work pieces as mentioned above. As can be seen additional perpendicular inner walls are used adjacent the joint (Fig 9) and space in between them is widened in order to accommodate for the conventional FSW tool or a supporting structure as an extension is incorporated in one of the work pieces (Fig 10).

Figure 1 1 shows examples of the alternative profile designs that can be joined by means of the tool of the present invention. Such profilies are e.g. multi-void extrusion, tube-shaped details, different profiles having a limited access for the tool at one side, hollow elements and different products such as panels, constructions, coolers etc used in a various technical fields and for various applications.

It is to be noted that the invention should not be limited by the above described embodiments, but should encompass any alternative that would fall within the scope of the claims.

Claims

1. A friction stir welding too) (20) for joining (wo abutting work pieces (10), which work pieces comprise portions (6) to be joined, said tool (20) having an axis of rotation (R), and comprising

- a first shoulder (1 ),

- a second shoulder (2), and

- a pin (3) arranged so as to connect the first ( 1 ) and second (2) shoulders, wherein said first shoulder (1 ) has a first contact surface (A1 ), which is to be in contact with one surface (4) of the portions (6) of the work pieces ( 10) to be joined during welding, said first contact surface (A1) having a first area (ai) essentially perpendicular to the axis of rotation (R), and

said second shoulder (2) has a second contact surface (A2) , which is to be in contact with an apposite surface (5) of the same portions (6) of (he work pieces (10) to be joined, said second contact surface (A2) having a second area (a^) essentially perpendicular to the axis of rotation (R),

characterized in that the first contact surface (A1 ) and the second contact surface (A2) have different areas (a-i) and (a₂).

2. The toot (20) of claim 1, wherein the second area (a₂) of the second contact surface (A2) of the second shoulder (2) is smaller than the first area (a1 ) of the first contact surface (A1 ) of the first shoulder (1 )_> said second area (a_?) being 75 % or less of said first

3. The tool (20) of claim 1 or 2, wherein the each of the contact surfaces (At , A2) of the shoulders (1 , 2) has one of a fiat, concave or convex shape in a cross section perpendicular to the axis of rotation (R).

4 The too! (20) of claims 1 -3, wherein the pin (3) cross section reduces in a direction from one shoulder towards ihe other shoulder. , The tool (20) of any one of the preceding claims, wherein the pin (3) has a cross section perpendicular to the axis of rotation, which is one of a polygonal, circular and oval cross section.

RECTIFIED SHEET (RULE 91

ISA/ EP 14

6. The tool (20) of any one of the preceding claims, wherein the pin (3) and the shoulders (1 , 2) are made as separate elements.

7. The tool (20) of any one of the preceding claims, wherein the pin (3) and one of the 5 shoulders ( 1 , 2) are made in one piece.

8. The tool (20) of any one of the preceding claims, wherein the pin (3) and the shoulders (1 , 2) are made in one piece.

10 9. A method of joining two abutting work pieces (10.) by the tool (20) of any one of the preceding claims, the method comprising steps of:

- assembling the tool (20) having a first shoulder (1 ), a second shoulder (2), and a pin (3) arranged so as to connect the first and second shoulders,

- starting rotation of the tool (20) with a predetermined speed,

15 - aligning the tool (20) adjacent to the two abutting work pieces (10) so that the first shoulder (1 ) is placed in the plane of the first surface (4) of the portions (6) of the work pieces (10) to be welded and the second shoulder (2) is placed in the plane of the opposite surface (5) of the portions (6) of the work pieces (10), and

- linearly moving the pin (3) of the tool (20) along the abutment of the work pieces (10,) :0 while having the shoulders (1 , 2) in a contact with the corresponding surfaces (4, 5) of the portions (6) of the work pieces so as to perform the welding.

10. A method of joining two abutting work pieces (10) by the tool (20) of any one of claims 1- 10, the method comprising steps of:

5 - drilling a hole in the abutment of the two work pieces for receipt of the pin (3) of the tool,

inserting the pin (3) of the tool (20) through the hole,

assembling the tool (20) having a first shoulder (1 ), a second shoulder (2), and a pin (3) arranged to connect the first and second shoulders, while the first shoulder (1 ) is 3 placed in the plane of the first surface (4) of the work pieces and the second shoulder

(2) is placed in the plane of the opposite surface (5) of the work pieces,

- starting rotation of the tool (20) with a predetermined speed, and

- linearly moving the pin (3) along the abutment so as to perform the welding. i 1 1. A method according to claim 10 or 1 1 , the two abutting work pieces (10) being multi- vpoid hollow extrusions for forming a double-skin panels, where each hollow extrusion profile (10) comprises a first and a second flat portion which are interconnected by

RECTIFIED SHEET (RULE 91)

ISA / BP confined inner walls (8, 9, 11 ) as seen in a cross section perpendicular to the extrusion direction, further comprising the steps of

- piacing two extruded profiles (10) adjacent to each other, such that the extrusion directions of each profile are parallel to each other, and such that an intersection of one flat portion (6) and the inner walls (B) of one profile (10) are facing a corresponding intersection of one flat portion (6) and the inner wails (S) of the other profile (10), such thai a narrowing space is formed between the inner confined wails (8, 9) of the profiles ( 10);

- joining at least the two first fiat portions (6) of the two profiles (10) by moving the tool (20) of claims 1-9, longitudinally along the extrusion direction through the narrowing space forming a double-skin panei. 2. The method of claim 12, wherein the work pieces (10) are multi-void hollow extrusions comprising inner walls (8, 9, 1 1) connecting the flat portions ( 6, 7), which inner wails (8, 9, 1 1) are arranged in a triangular configuration, where each hoifow extrusion includes a first and a second flat portion, parallel to each other, a central inner wail (1 1) perpendicular to said flat portions (6,7), and inclined inner walls (8, 9) on each side of the central inner wail (1 1 ), wher&by an equaify sided triangie is formed by the two inclined inner waits (8, 9) and one of the fiat surfaces (6,7).

13. The method of Claim 13, wherein the welding of the first two flat portions (6) and the second two fiat portions (7) is performed simultaneously.

14. The method of any one of claims 10-14, wherein the tool (20) applies a finite vertical force to the work pieces (10, 10'), wherein the vertical force is at least 100 N.

15. A product obtained by the method of any one of claims 10-16, comprising at least two portions (6) welded together so as to form a joint (15) having a we!d face (15', 15"), characterized in that the weld faces (15', 15") have different widths on opposite surfaces (4, 5) of the we!ded product.

16. The product of claim 6, wherein the product is a deck panel construction comprising two or more extruded profiles joined by the method of claims 9-16, each extruded profile having two flat portions (6,7) and interconnecting inner walls (8, 9, ), wherein the weld face (15') width on one surface of the flat portion of the panei is wider than the weld face (1 5") width on the opposite surface of the same flat porition of the profile.

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!GA / GP

17. Use of the tool (20) according to any one of claims 1 -9, for friction stir welding of two abutting work pieces (10).

18. Use of the tool (20) according to any one of claims 1-9, wherein the work pieces (10) are metal double skin deck panels.

RECTIFIED SHEET (RULE 91 )

ISA / EP