DESC:TECHNICAL FIELD
The present disclosure relates in general to a field of manufacturing. Particularly, but not exclusively, the present disclosure relates to joining of metallic sheets. Further, embodiments of the present disclosure disclose a method for joining the metallic sheets through a self-piercing rivet which is made of same material as that of the plurality of metallic sheets.

BACKGROUND

Sheets or sheet metals are commonly used in various industrial applications such as, but not limited to, automobile industry, manufacturing industry, and the like. Various components of different shapes, configurations, and dimensions may be obtained by joining two or more sheets together. Hence, joining of the sheets may be considered as one of the important industrial process. Conventionally, different techniques are employed to join the sheets. Some of the conventional techniques may include, but may not be limited to, Resistance Spot Welding, Fusion Welding, Friction Stir Spot Welding, riveting, fastening, clinching and adhesive bonding. Each of the aforementioned techniques or methods may have their own advantages and limitations. For instance, lower static and fatigue strength may be associated with clinching joining process. The joining of sheets through any of the conventional joining techniques requires filler or an electrode, which may have different material composition from the sheet. The different material composition of sheets and electrode or filler makes corrosion unavoidable. Further, recycling of sheets joined through any of the conventional process may be difficult. The adhesive bonding and friction stir spot welding processes are time consuming joining processes.

With the increase in popularity and demand for joining of sheet metal, various efforts have been made to join the sheet metals with ease. One such development is use of self-piercing rivets for joining of the two or more sheets. The development of the self-piercing rivets has taken the centre stage in sheet joining process and application of the self-piercing rivets technology has broadened its base in the automobile industry.

The self-piercing riveting process for joining the sheets is a mechanical joining and a cold forming process. The two or more sheets may be joined by driving a rivet towards an upper sheet and flaring the skirt of the rivet in the lower sheet without fracturing the lower sheet. The self-piercing rivet joining process does not require a pre-drilled hole unlike the conventional riveting process.

In the conventional self-piercing riveting process, the rivet is made of the material which is having higher strength when compared to the sheets. For example, aluminum sheets may be joined using steel rivets and therefore, there is a surface contact between rivet and sheets. However, the differences in material composition may make the joint susceptible to corrosion. The direct surface contact between the sheets and rivet may be avoided to a certain extent by using adhesive between sheets and rivet, which may reduce the surface contact but cannot be eliminated completely.

The present disclosure is directed to overcome one or more limitations stated above or any other limitation associated with the prior art.

SUMMARY

One or more shortcomings of the prior art are overcome by a method as claimed and additional advantages are provided through the provision of a method as claimed in the present disclosure. Additional features and advantages are realized through the techniques of the present disclosure. Other embodiments and aspects of the disclosure are described in detail herein and are considered a part of the claimed disclosure.

In an exemplary embodiment of the present disclosure, a method for joining a plurality of metallic sheets is disclosed. The method includes, heating the plurality of metallic sheets to be joined to a predetermined temperature. The plurality of metallic sheets is then positioned on a die in hot condition. A ram punch that accommodates a self-piercing rivet is operated to pierce the self-piercing rivet onto the plurality of metallic sheets. The self-piercing rivet is made of the same material as that of the plurality of metallic sheets.

In an embodiment of the present disclosure, heating the plurality of metallic sheets that are to be joined, to a predetermined temperature, infuses a reduction in the finite flow strength of the surface of the plurality of metallic sheets

In an embodiment of the present disclosure, a portion of the plurality of metallic sheets is heated for joining.

In an embodiment of the present disclosure, the plurality of metallic sheets are heated in a heating chamber to a predetermined temperature.

In an embodiment of the present disclosure, the predetermined range of temperature defined with respect to melting point of the material of the plurality of metallic sheets and the self-piercing rivet. Particularly, for a plurality of metallic sheets and a self-piercing rivet made of aluminum, the temperature of the plurality of metallic sheets ranges from about 300°C to about 450°C.

In an embodiment of the present disclosure, the self-piercing rivet is punched at an ambient temperature thereby producing a difference in the finite flow strength from the heated plurality of metallic sheets.

In an embodiment of the present disclosure, the self-piercing rivet is configured to plastically deform through at least one sheet of the plurality of metallic sheets. The self-piercing rivet completely pierces through at least one sheet of the plurality of metallic sheets and at least partially pierces through the subsequent sheet of the plurality of metallic sheets to join the plurality of metallic sheets.

In an embodiment of the present disclosure, the self-piercing rivet comprises a rivet head and a plurality of legs that extend from the rivet head. The plurality of legs that extend from the rivet head are adapted to bend during punching to join the plurality of metallic sheets.

It is to be understood that the aspects and embodiments of the disclosure described above may be used in any combination with each other. Several of the aspects and embodiments may be combined together to form a further embodiment of the disclosure.

The foregoing summary is illustrative only and is not intended to be in any way limiting. In addition to the illustrative aspects, embodiments, and features described above, further aspects, embodiments, and features will become apparent by reference to the drawings and the following detailed description.

BRIEF DESCRIPTION OF ACCOMPANYING DRAWINGS

The novel features and characteristic of the disclosure are set forth in the appended description. The disclosure itself, however, as well as a preferred mode of use, further objectives, and advantages thereof, will best be understood by reference to the following description of an illustrative embodiment when read in conjunction with the accompanying figures. One or more embodiments are now described, by way of example only, with reference to the accompanying figures wherein like reference numerals represent like elements and in which:

FIG.1 illustrates a cross-sectional view of a punch and die assembly which is employed to join a plurality of metallic sheets using a self-piercing rivet made of same material, in accordance with an exemplary embodiment of the present disclosure.

FIG.2 illustrates a schematic view of a high strain rate machine used for joining the plurality of metallic sheets using the self-piercing rivet at high temperature, in accordance with an exemplary embodiment of the present disclosure.

FIG.3 illustrates a sectional view of the self-piercing rivet, in accordance with an exemplary embodiment of the present disclosure.

FIG.4 illustrates a cross sectional simulation views sheets joined using self-piercing rivet under various parameters, in accordance with an exemplary embodiment of the present disclosure.

FIG.5 illustrates a cross section view of the sheets joined with the self-piercing rivet, in accordance with an exemplary embodiment of the present disclosure.

The figures depict embodiments of the disclosure for purposes of illustration only. One skilled in the art will readily recognize from the following description that alternative embodiments of the system illustrated herein may be employed without departing from the principles of the disclosure described herein.

DETAILED DESCRIPTION

The foregoing has broadly outlined the features and technical advantages of the present disclosure in order that the detailed description of the disclosure that follows may be better understood. Additional features and advantages of the disclosure will be described hereinafter which form the subject of the disclosure. It should be appreciated by those skilled in the art that the conception and embodiments disclosed may be readily utilized as a basis for modifying or designing other method for carrying out the same purposes of the present disclosure. It should also be realized by those skilled in the art that such equivalent constructions do not depart from the scope of the disclosure as set forth in the disclosure. The novel features which are believed to be characteristic of the disclosure, both as to its organization and method of operation, together with further objects and advantages will be better understood from the following description when considered in connection with the accompanying figures. It is to be expressly understood, however, that each of the figures is provided for the purpose of illustration and description only and is not intended as a definition of the limits of the present disclosure.

The terms “comprises”, “comprising”, or any other variations thereof, are intended to cover a non-exclusive inclusion, such that an assembly, apparatus or method that comprises a list of components or steps does not include only those components or steps but may include other components or steps not expressly listed or inherent to such setup or apparatus or method. In other words, one or more elements in a system or apparatus proceeded by “comprises… a” does not, without more constraints, preclude the existence of other elements or additional elements in the system or apparatus.

Embodiments of the present disclosure discloses a method that is employed for joining metallic sheets using a self-piercing rivet. The method involves steps of heating the plurality of metallic sheets to a predetermined temperature. Heating the plurality of the metallic sheets that are to be joined, infuses a reduction in the finite flow strength of the surface of the plurality of metallic sheets. The heated plurality of metallic sheets may be then positioned on a die, a ram punch which accommodates a self-piercing rivet may be operated to punch the self-piercing rivet onto the plurality of metallic sheets. The self-piercing rivet may be maintained at an ambient temperature while the plurality of metallic sheets are heated to a predetermined temperature. The self-piercing rivet comprises a rivet head and a plurality of legs that extend from the rivet head. The plurality of legs of the self-piercing rivet may completely pierces through at least one sheet of the plurality of sheets and at least partially pierces through the subsequent sheet of the plurality of sheets thereby joining the plurality of metallic sheets. The self-piercing rivet may be made of the same material as that of the plurality of metallic sheets. Since, the self-piercing rivet is made of the same material as that of the plurality of metallic sheets, corrosion at the joints of the plurality of the metallic sheets may be avoided. This improves life of the joint.

The method of the present disclosures may be used in any application including but not limited automobile, manufacturing, or any other industrial process where a joining of sheets through riveting process is involved.

The present disclosure is further explained with the help of one or more figures of exemplary embodiments. However, such exemplary embodiments should not be construed as limitation of the present disclosure. The method for joining sheets using single material self-pierce rivet is as shown in figures. It is to be noted that slight variations in the constructional features or the method of operation is to be considered as part of the present disclosure.

The following paragraphs describe the present disclosure with reference to FIG.1 to 5. In the figures, the same element or elements which have similar functions are indicated by the same reference signs.

FIG. 1 is an exemplary embodiment of the disclosure which illustrates a cross-sectional view a punch and die assembly (100) for joining a plurality of metallic sheets (4) using a self-piercing rivet (101). Each metallic sheet of the plurality of metallic sheets (4) may be made of the same material and may be configured to be joined for further processing. Further, the plurality of metallic sheets (4) may be arranged in a manner including at least one of a stack and a lateral overlap. As shown in FIG. 1, the punch and die assembly (100) comprises of a die (5), where the plurality of metallic sheets (4) are placed on the die (5). The plurality of metallic sheets (4) may either be pre-heated to a predetermined temperature and may then be positioned on the die (5), or the plurality of metallic sheets (4) may be heated upon positioning on the die (5), in a heating chamber (7). The punch and die assembly (100) also comprises of ram punch (1), that accommodates a self-piercing rivet (101). The self-piercing rivet (101) and the plurality of metallic sheets (4) are made of the same material.

In an embodiment, the predetermined temperature for heating the plurality of metallic sheets (4) may be dependent on melting point and recrystallization temperature of material with which the plurality of metallic sheets (4) are made.

Further, finite flow strength of a surface of at least one metallic sheet of the plurality of sheets (4) may be reduced by heating the plurality of metallic sheets (4). In an embodiment, at least one metallic sheet of the plurality of metallic sheets (4), configured to be positioned at either ends of the stack or each of the plurality of metallic sheets (4) of the lateral overlap, may include a surface exposed to the heating chamber (7). This surface of each of the plurality of metallic sheets (4) may be subjected to reduced finite flow strength. Reduction in the finite flow strength of the surface of the plurality of metallic sheets (4) may assist the self-piercing rivet (101) to pierce the plurality of metallic sheets (4) with relatively lower force, when compared with pierce the plurality of metallic sheets (4) at an ambient temperature. While the plurality of metallic sheets (4) are heated, the self-piercing rivet (101) may be maintained at the ambient temperature, such as room temperature ranging from about 15°C to about 30°C. In this way, a difference in the flow strength of the plurality of metallic sheets (4) and that of the self-piercing rivet (101) may be achieved. This difference in the flow strength of the self-piercing rivet (101) and the plurality of metallic sheets (4) may enable the self-piercing rivet (101) to pierce through the plurality of metallic sheets (4). In an embodiment, heating may be a localized heating, and the metallic sheets (4) may be heated only in the regions to be pierced by the self-piercing rivet (101) for joining.

In an embodiment, the ram punch (1) may be dislodged from an initial position, to reside proximal to the die (5), after the plurality of metallic sheets (4) are heated and placed on the die (5). The ram punch (5) accommodating the self-piercing rivet (101) may then be operated to punch and/or pierce the self-piercing rivet (101) onto the plurality of metallic sheets (4). This operation of the ram punch (1) may plastically deform the plurality of the metallic sheets (4) upon impact from the self-piercing rivet (101). Further, the self-piercing rivet (101) may completely pierce through at least one metallic sheet (e.g. sheet that may be positioned at the top, facing the self-piercing rivet (101)) of the plurality of sheets (4). Additionally, the self-piercing rivet (101) may at least partially pierce through at least one subsequent sheet of the plurality of metallic sheets (4), which is positioned downward from the at least one metallic sheet facing the self-piercing rivet (101). Also, during punching and/or piercing, a portion of the plurality of metallic sheets (4) may be deformed (e.g. bent or curled) relative to movement of the self-piercing rivet (101), whereby the portion of the plurality of metallic sheets (4) may be joined due to such deformation. Thus, the plurality of metallic sheets (4) are joined together by the self-piercing rivet (101) that is made of the same material as that of the plurality of metallic sheets (4).

FIG.2 illustrates a high strain rate machine (6) used for joining the plurality of metallic sheets (4) using the self-piercing rivet (101) at high temperature. As shown in FIG. 2, the machine (6) comprises the heating chamber (7), including, but not limited to, a furnace, Ultrasonic heating compartment, induction heating source, a laser emitting chamber, and the like. In an exemplary embodiment, the plurality of metallic sheets (4) are placed over the die (5) and are heated inside the heating chamber (7) to a temperature ranging from about 300°C to about 450°C, whereby reducing the flow strength of the plurality of metallic sheets (4). The ram punch (1) may be operated to punch the self-piercing rivet (101) onto the plurality of metallic sheets (4), to join the plurality of metallic sheets (4) with at least one subsequent metallic sheet.

In one embodiment, one or more localized portions of the plurality of metallic sheets (4) may be heated for joining. The one or more localized portion of heating may include a periphery of each of the plurality of metallic sheets (4), center of the plurality of metallic sheets (4), and the like, to suitably reduce the flow strength of the plurality of metallic sheets (4), without affecting the flow strength of remaining portions.

In an embodiment, the portion of the plurality of metallic sheet (4) that are heated for joining, may be referred to as localized region of joining.

In one embodiment, the plurality of metallic sheets (4) may be heated at multiple regions, individually or simultaneously, to pierce and join the plurality of metallic sheets (4) at multiple regions. Further, all the regions for joining the plurality of metallic sheets (4) may also be heated simultaneously.

In one embodiment, a chamber may be provided inside the die (5) or the ram punch (1) for the supply of a coolant to prevent the plurality of metallic sheets (4)

FIG.3 illustrates a sectional view of the self-piercing rivet (101). As seen in FIG.3, the self-piercing rivet (101) comprises of a rivet head (2) and a plurality of legs (3), that are extended from the rivet head (2). The rivet head (2) may be detachably fixed to the ram punch (1) and the plurality of legs (3) may be adapted to pierce through the plurality of metallic sheets (4), upon operation of the ram punch (1). Further, the plurality of legs (3) of the self-piercing rivet (101) may be adapted to fractionally bend during punching and/or piercing such that, each of the plurality of metallic sheets (4) may be deformed to join with subsequent metallic sheet of the plurality of metallic sheets (4).

In one embodiment, the plurality of legs (3) of the self-piercing rivet (101) may be joined to form a single circular shaped self-piercing rivet (101).

In one embodiment, the plurality of legs (3) may alternatively be joined to the rivet head (2) to form a perimetral profile such as, but not limited to, circular, square, hexagon and the like, to suitably pierce and join the plurality of metallic sheets (4).

In an exemplary embodiment, experiments are conducted with the plurality of metallic sheets (4) and the self-piercing rivet (101) being made of aluminum as a material, and FIG.4 illustrates cross sectional simulation views of the plurality of metallic sheets (4) joined using the self-piercing rivet (101) under various parameters. Further, as the plurality of metallic sheets (4) and the self-piercing rivet (101) are made of aluminum, the predetermined temperature for heating the plurality of metallic sheets (4) may be set in the range of about 300°C to about 450°C. Additionally, the various combinations of parameters that are considered for experimental analysis is indicated in the Table 1 below.

Parameter P1(Temperature in °C) P2 (ram rate in mm/sec) P3 Rivet diameter (inside to outside) ratio in mm/mm P4 die (5) diameter (inside to outside) ratio in mm/mm
M1 300 100 3.5/7.0 2.0/10.5
M2 450 25 3.5/5.5 2.0/9.0
M3 350 75 3.5/5.5 2.0/10.0
M4 400 50 3.5/6.0 2.0/9.6
M5 300 75 3.5/6.5 2.0/10.0
M6 450 50 3.5/6.0 2.0/9.6
M7 350 50 3.5/6.0 2.0/9.6
M8 400 75 3.5/6.5 2.0/10.0
M9 300 50 3.5/6.0 2.0/9.6
M10 450 75 3.5/6.5 2.0/10.0
M11 350 25 3.5/5.5 2.0/9.0
M12 400 100 3.5/7.0 2.0/10.5
M13 300 25 3.5/5.5 2.0/9.0
M14 450 100 3.5/7.0 2.0/10.5
M15 350 100 3.5/7.0 2.0/10.5
M16 400 25 3.5/5.5 2.0/9.0

Table. 1

As shown in FIG. 4, the process M1, M5, M9 and M13, of the exemplary experimental results illustrate that the temperature of 300°C does not sufficiently reduce the flow strength of the plurality of metallic sheets (4). Thus, the self-piercing rivet (101), travelling at different speed range for different process, is buckled under the strength of the plurality of metallic sheets (4).

The process M2, M6, M10 and M14, of the exemplary experimental results illustrate that at a temperature of 450°C, the flow strength of the plurality of metallic sheets (4) is drastically reduced. Therefore, the self-piercing rivet (101), travelling at different speed for different process, can pierce the plurality of metallic sheets (4) easily. However, the second sheet surface flares and comes in contact with the self-piercing rivet (101).

The process M3, M7, M11 and M15, of the exemplary experimental results illustrate that at a temperature of 350°C, the flow strength of the plurality of metallic sheets (4) is reduced. Therefore, the self-piercing rivet (101), travelling at different speed for different process, easily pierces the plurality of metallic sheets (4) without flaring the second sheet surface.

FIG.5 illustrates the fundamental joining parameters in a self -piercing rivet (101). The optimum speed of ram punch (1) to form a high load bearing lap joint may be identified by adjusting the three fundamental joining parameters as shown in FIG.5. The first fundamental joining parameter is radius (R). The radius (R) indicates the change in thickness between the two corresponding loci, that lie along the thickness of the subsequent sheet.

In one embodiment, the radius (R) indicates the reduction in the thickness of the plurality of the subsequent sheet after the plurality of subsequent sheet undergoes deformation by the self-piercing rivet (101).

The second fundamental joining parameter is curvature (C) of the self-piercing rivet (101) as seen in the FIG.5, and the third fundamental joining parameter is length (L). The length (L) indicates the depth of piercing of the self-piercing sheet (101) into the plurality of the subsequent sheet as shown in FIG.5.

The table 2 illustrates the joining parameters at various combinations of ram punch (1) speed and the fundamental joining parameter (R, L and C). Further, based on the experimental results, the high load bearing joint is formed at 350°C temperature and at a ram punch (1) speed of 75 mm/sec.

Ram speed/Parameter (mm) 25 mm/s (M11) 50 mm/s (M7) 75 mm/s
(M3)
C 0.74 0.636 0.67
R 1.36 1.32 1.35
L 0.38 0.54 0.73

Table 2

Further, the aforesaid experimental analysis and the various parameters considered therein should not be construed as a limitation as other parameters such as, but not limited to, humidity, grade of material, and the like may also be considered for further analysis. Also, it should be understood from the present disclosure that aforementioned method of joining may not be limited to the plurality of metallic sheets (4) and the self-piercing rivet (101) being made of aluminum, but may also be employed to join the plurality of metallic sheets being made of materials, including, but not limited to, iron, stainless steel, copper, nickel, polymers, composites and the like.

Advantages

In an embodiment, recycling the sheets becomes easier since plurality of sheets and the self-piercing rivet are of same material.

In an embodiment, using a material for the self-piercing rivet that is same as that of the plurality of metallic sheets, prevents corrosion at the joints of the plurality of metallic sheets, and thereby improves life of the joint.

In an embodiment, using a material for the self-piercing rivet that is same as that of the plurality of metallic sheets, reduces the overall weight of the structure/component.

Equivalents

With respect to the use of substantially any plural and/or singular terms herein, those having skill in the art can translate from the plural to the singular and/or from the singular to the plural as is appropriate to the context and/or application. The various singular/plural permutations may be expressly set forth herein for sake of clarity.

It will be understood by those within the art that, in general, terms used herein, are generally intended as "open" terms (e.g., the term "including" should be interpreted as "including but not limited to," the term "having" should be interpreted as "having at least," the term "includes" should be interpreted as "includes but is not limited to," etc.). It will be further understood by those within the art that if a specific number of an introduced claim recitation is intended, such an intent will be explicitly recited in the claim, and in the absence of such recitation no such intent is present. For example, as an aid to understanding the description may contain usage of the introductory phrases "at least one" and "one or more" to introduce claim recitations. However, the use of such phrases should not be construed to imply that the introduction of a claim recitation by the indefinite articles "a" or "an" limits any particular claim containing such introduced claim recitation to inventions containing only one such recitation, even when the same claim includes the introductory phrases "one or more" or "at least one" and indefinite articles such as "a" or "an" (e.g., "a" and/or "an" should typically be interpreted to mean "at least one" or "one or more"); the same holds true for the use of definite articles used to introduce claim recitations. In addition, even if a specific number of an introduced claim recitation is explicitly recited, those skilled in the art will recognize that such recitation should typically be interpreted to mean at least the recited number (e.g., the bare recitation of "two recitations," without other modifiers, typically means at least two recitations, or two or more recitations). Furthermore, in those instances where a convention analogous to "at least one of A, B, and C, etc." is used, in general such a construction is intended in the sense one having skill in the art would understand the convention (e.g., "a system having at least one of A, B, and C" would include but not be limited to systems that have A alone, B alone, C alone, A and B together, A and C together, B and C together, and/or A, B, and C together, etc.). In those instances where a convention analogous to "at least one of A, B, or C, etc." is used, in general such a construction is intended in the sense one having skill in the art would understand the convention (e.g., "a system having at least one of A, B, or C" would include but not be limited to systems that have A alone, B alone, C alone, A and B together, A and C together, B and C together, and/or A, B, and C together, etc.). It will be further understood by those within the art that virtually any disjunctive word and/or phrase presenting two or more alternative terms, whether in the description, or drawings, should be understood to contemplate the possibilities of including one of the terms, either of the terms, or both terms. For example, the phrase "A or B" will be understood to include the possibilities of "A" or "B" or "A and B."

While various aspects and embodiments have been disclosed herein, other aspects and embodiments will be apparent to those skilled in the art. The various aspects and embodiments disclosed herein are for purposes of illustration and are not intended to be limiting, with the true scope and spirit being indicated in the description.

Referral Numerals:

Particulars Reference Number
Ram punch 1
Rivet head 2
Plurality of legs 3
Plurality of metallic sheets 4
Die 5
High strain rate machine 6
Heating chamber 7
Self-piercing rivet apparatus 100
Self-piercing rivet 101

,CLAIMS:1. A method for joining a plurality of metallic sheets (4), the method comprising:
heating the plurality of metallic sheets (4) to be joined to a predetermined temperature;
positioning the plurality of metallic sheets (4) on a die (5) in hot condition;
punching a self-piercing rivet (101) onto the plurality of metallic sheets (4) by operating a ram punch (1), to join the plurality of metallic sheets (4), wherein, the self-piercing rivet (101) is made of material same to the material of the plurality of metallic sheets (4).

2. The method as claimed in claim 1, wherein heating of the plurality of metallic sheets (4) be joined to a predetermined temperature infuses a reduction in finite flow strength of a surface of the plurality of metallic sheets (4).

3. The method as claimed in claim 2, wherein a portion of the plurality of metallic sheets (4) is heated for joining.

4. The method as claimed in claim 3, wherein the plurality of metallic sheets (4) are heated in a heating chamber (7) to a predetermined temperature.

5. The method as claimed in claim 4, wherein the predetermined temperature is defined with respect to melting point of the material of the plurality of metallic sheets (4) and the self-piercing rivet (101).

6. The method as claimed in claim 1, wherein the self-piercing rivet (101) is punched at an ambient temperature to produce a difference in the finite flow strength from the heated plurality of metallic sheets (4).

7. The method as claimed in claim 1, wherein the self-piercing rivet (101) is configured to plastically deform through at least one sheet of the plurality of the metallic sheets (4).

8. The method as claimed in claim 7, wherein the self-piercing rivet (101) completely pierces through at least one sheet of the plurality of metallic sheets (4) and at least partially pierces through at least one subsequent sheet of the plurality of metallic sheets (4)

9. The method as claimed in claim 1, wherein the self-piercing rivet (101) comprises a rivet head (2) and a plurality of legs (3) extending from the rivet head (2).

10. The method as claimed in claim 9, wherein the plurality of legs (3) extending from the rivet head (2) are adapted to bend during punching to join the plurality of metallic sheets (4).