Claims:WE CLAIM:
1. A method for mechanical joining for two dissimilar steels over Self Piercing Riveting (SPR), the method comprising:
maintaining dimension of rivet with hardness 510±30 Hv, length 5.5 mm, and shaft diameter is 5.5 mm;
maintaining stack thickness of 2.2-2.6 mm;
maintaining diameter of a pip die at 10 mm;
maintaining depth of the pip die at 2 mm;
maintaining pip at zero height;
maintaining interlock 0.291-0.379 mm; and
maintaining Tmin at 0.277-0.258 mm.
2. The method as claimed in claim 1, wherein the rivet is made of boron steel.
3. The method as claimed in claim 1, wherein the two dissimilar steels have top steel and bottom steel.
4. The method as claimed in claim 3, wherein the top steel is hard dual phase steel.
5. The method as claimed in claim 3, wherein the bottom steel is sheet of softer interstitial free galvanealed steel.
6. The method as claimed in claim 1, wherein range of actual rivet penetration is -0.020 mm to -0.326 mm.
Dated on 02nd March, 2019
, Description:OVER SELF PIERCING RIVETING (SPR)

TECHNICAL FIELD
[0001] The present disclosure, in general, relates to the field of metal joinng and, in particular, relates to a method for mechanical joining of two dissimilar steels over self-piercing riveting (SPR).

BACKGROUND
[0002] Background description includes information that may be useful in understanding the present invention. It is not an admission that any of the information provided herein is prior art or relevant to the presently claimed invention, or that any publication specifically or implicitly referenced is prior art.
[0003] Self-piercing riveting (SPR) is an environment friendly mechanical joining process, commonly used for joining aluminium/softer metal sheets in the automotive industry with improved fatigue properties. Their application to steels sheets particularly for specific strength combination of steel thickness is not clearly available.
OBJECTS OF THE DISCLOSURE
[0004] In view of the foregoing limitations inherent in the state of the art, some of the objects of the present disclosure, which at least one embodiment herein satisfy, are listed hereinbelow.
[0005] It is a general object of the present disclosure to propose a method for joining of dissimilar strength steel combination viz. dual phase steel of 600 MPa tensile strength with interstitial free galvanealed steel of 300 MPa using self-piercing riveting (SPR) process.
[0006] It is an object of the present disclosure to propose a range for total stack thickness and individual sheet thickness for specific combination.
[0007] It is an object of the present disclosure to propose suitable rivet (strength and size) and die (size) for specific steel combination.
[0008] It is an object of the present disclosure to propose penetration values and corresponding interlock and Tmin values to achieve improved fatigue properties for specific stack combination
[0009] These and other objects and advantages of the present invention will be apparent to those skilled in the art after a consideration of the following detailed description taken in conjunction with the accompanying drawings in which a preferred form of the present invention is illustrated.

SUMMARY
[0010] This summary is provided to introduce concepts related to a method for mechanical joining of two dissimilar steels over self-piercing riveting (SPR). The concepts are further described below in the detailed description. This summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used to limit the scope of the claimed subject matter.
[0011] In an embodiment, the present disclosure relates to a method for mechanical joining of two dissimilar steels over self-piercing riveting (SPR). The method includes maintaining dimension of rivet with hardness 510±30 Hv, length 5.5 mm., shaft diameter is 5.5 mm; maintaining stack thickness of 2.2-2.6 mm; maintaining diameter of a pip die at 10 mm; maintaining depth of the pip die at 2 mm; maintaining pip at zero height; maintaining an interlock at 0.291-0.379 mm; and maintaining Tmin at 0.277-0.258 mm.
[0012] In an aspect, the rivet is made of boron steel.
[0013] In an aspect, the two dissimilar steels have top steel and bottom steel. In said aspect, the top steel is hard dual phase steel and the the bottom steel is sheet of softer interstitial free galvanealed steel.
[0014] In an aspect, range of actual rivet penetration is -0.020 mm to -0.326 mm.
[0015] Various objects, features, aspects, and advantages of the inventive subject matter will become more apparent from the following detailed description of preferred embodiments, along with the accompanying drawing figures in which like numerals represent like components.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWING
[0016] The illustrated embodiments of the subject matter will be best understood by reference to the drawings, wherein like parts are designated by like numerals throughout. The following description is intended only by way of example, and simply illustrates certain selected embodiments of devices, systems, and methods that are consistent with the subject matter as claimed herein, wherein:
[0017] FIG. 1A illustrates a microstructure of parent material being Dual Phase (DP) 600 steel, in accordance with an embodiment of the present disclosure;
[0018] FIG. 1B illustrates a microstructure of parent material being Interstitial Free (IF) steel, in accordance with an embodiment of the present disclosure;
[0019] FIG. 2 illustrates a lap shear joint geometry, in accordance with an embodiment of the present disclosure;
[0020] FIG. 3A illustrates a cross section of self-piercing riveting (SPR) joints at actual penetration of -0.020 mm, in accordance with an embodiment of the present disclosure;
[0021] FIG. 3B illustrates a cross section of SPR joints at actual penetration of -0.326 mm, in accordance with an embodiment of the present disclosure;
[0022] FIG. 4 illustrates a cross section of resistance spot weld (RSW) joint, in accordance with the state of the art;
[0023] FIG. 5A illustrates a rivet geometry, in accordance with an embodiment of the present disclosure;
[0024] FIG. 5B illustrates a die geometry, in accordance with an embodiment of the present disclosure
[0025] FIG. 6 illustrates a fatigue life comparison of SPR and Spot welds, in accordance with an embodiment of the present disclosure; and
[0026] FIG. 7 illustrates a method a method for mechanical joining of two dissimilar steels over self-piercing riveting (SPR), in accordance with an embodiment of the present disclosure.

DETAILED DESCRIPTION
[0027] The detailed description of various exemplary embodiments of the disclosure is described herein with reference to the accompanying drawings. It should be noted that the embodiments are described herein in such details as to clearly communicate the disclosure. However, the amount of details provided herein is not intended to limit the anticipated variations of embodiments; on the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the present disclosure as defined by the appended claims.
[0028] It is also to be understood that various arrangements may be devised that, although not explicitly described or shown herein, embody the principles of the present disclosure. Moreover, all statements herein reciting principles, aspects, and embodiments of the present disclosure, as well as specific examples, are intended to encompass equivalents thereof.
[0029] The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of example embodiments. As used herein, the singular forms “a”, “an”, and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises”, “comprising”, “includes”, “consisting” and/or “including” when used herein, specify the presence of stated features, integers, steps, operations, elements and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components and/or groups thereof.
[0030] It should also be noted that in some alternative implementations, the functions/acts noted may occur out of the order noted in the figures. For example, two figures shown in succession may, in fact, be executed concurrently or may sometimes be executed in the reverse order, depending upon the functionality/acts involved.
[0031] Unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art to which example embodiments belong. It will be further understood that terms, e.g., those defined in commonly used dictionaries, should be interpreted as having a meaning that is consistent with their meaning in the context of the relevant art and will not be interpreted in an idealized or overly formal sense unless expressly so defined herein.
[0032] Embodiments explained herein pertain to a joining method of dual phase steel of 600 MPa tensile strength with interstitial free galvanealed steel of 300 MPa tensile strength having stack thickness range of 2.2-2.6 mm by using mechanical joining technique named Self Piercing Riveting (SPR) process with improved fatigue properties over conventional resistance spot welding (RSW) process. This embodiments explained herein further relates to (i) a stack thickness having dissimilar steel combination (ii) rivet strength & size and die selection, (iii) optimization of rivet penetration values, and (iv) optimum values of interlock and Tmin values.
[0033] The present disclosure further relates to the joining of dissimilar steel sheets with improved fatigue properties over conventional resistance spot weld which is only possible with proper selection of rivet and die combination, optimized rivet penetration values and optimum values of interlock and Tmin values. Chemical composition and mechanical properties of parent materials have been listed in Table 1 and Table 2 respectively:
Table 1: Chemical composition of the chosen steel grades (wt-%)
Materials C Mn Si P S Al Nb Ti Cr N
IF 0.003 0.08 0.007 0.009 0.007 0.039 0.011 0.035 0.014 --------
DP 600 0.09 0.86 0.358 0.015 0.008 0.038 ------- ------- 0.021 0.0045

Table 2: Mechanical properties of the steel grades
Materials Yield strength
(MPa) Ultimate strength
(MPa) Elongation (%)
IF 142 298 46
DP 600 354.95 597.67 27.80

[0034] FIG. 1A illustrates a microstructure of parent material being Dual Phase (DP) 600 steel, while FIG. 1B illustrates a microstructure of parent material being Interstitial Free (IF) steel, in accordance with an embodiment of the present disclosure.
SELECTION OF RIVET AND DIE COMBINATION:
[0035] High strength rivet was used for preparation of lap shear joints which were required for tensile and fatigue testing. Rivet has shaft diameter and length measured 5.5 mm and Vickers hardness value of 510±30 Hv. Pip die was used for joining the DP-IF combination. Pip height was zero which means it was extended till the surface of the die. Pip die has depth of 2 mm and diameter measuring 10 mm. FIG. 2 represents lap shear tensile specimen geometry and FIGS. 5A and 5B represent geometry of rivet and die, in accordance with an embodiment of the present disclosure.
OPTIMIZED RIVET PENETRATION VALUES:
[0036] Selecting proper rivet penetration value is important as it affects interlock and Tmin values and subsequently mechanical properties (static as well as fatigue). Samples were prepared for three desired penetration values of -0.4 mm, -0.8 mm and -1.2 mm and based on change in static properties two penetration values of -0.4 mm and -1.2 mm were selected for fatigue performance evaluation. Actual penetration values corresponding to desired penetration values of -0.4 mm and -1.2 mm are -0.020 mm and -0.326 mm respectively. Table 3 contains details of rivet penetration values and corresponding lap shear tensile strength values:
Table 3: Lap shear tensile strength of DP 600 1.0 mm + IF 1.4 mm stack at different penetration values (average value of 3 samples)
S. No. Stack combinations Desired head height(mm) Actual head height(mm) Maximum Load(kN)
1 DP 600 1.0 mm (top) + IF 1.4 mm (bottom) -0.4 -0.020 6.60
2 DP 600 1.0 mm (top) + IF 1.4 mm (bottom) -0.8 -0.233 7.15
3 DP 6001.0 mm (top) + IF 1.4 mm (bottom) -1.2 -0.326 7.47

[0037] FIG. 3A illustrates a cross section of self-piercing riveting (SPR) joints at actual penetration of -0.020 mm, while FIG. 3B illustrates a cross section of SPR joints at actual penetration of -0.326 mm, in accordance with an embodiment of the present disclosure.
OPTIMUM VALUES OF INTERLOCK AND TMIN VALUES:
[0038] Corresponding to selected rivet penetration values, interlock and Tmin values were measured from cross section of the riveted joints. Range for interlock and Tmin values are 0.291-0.379 mm and 0.277-0.258 mm to achieve defect free joint having improved fatigue properties. FIG. 6 represents fatigue life of SPR joints vis-à-vis RSW joints.
[0039] Table 4 and Table 5 contains details on RSW parameters and tensile properties respectively:
Table 4: Parameters for resistance spot welding of DP 600 1.0 mm and IF 1.4 mm
Current(kA) Electrode force
(kN) Weld time
(ms) Nugget dia.
(mm)
7.5 3 250 5.5

Table 5: Lap shear tensile strength of spot welded joint DP 600 1.0 mm and IF 1.4 mm
Stack combination Nugget dia.
(mm) Maximum load
(kN)
DP 600 1.0 mm (top) + IF 1.4 mm (bottom) 5.5 10.18

[0040] FIG. 4 illustrates a cross section of resistance spot weld (RSW) joint, in accordance with the state of the art.
[0041] Further, as mentioned above, the main objective of the present is to propose a joining method of dissimilar strength steel stacks by SPR. While SPR is commonly used for joining soft metals, possibility of SPR with specific rivets (size and hardness combination) for steel stacks with dissimilar grades (tensile strength of steel, thickness combination) is not known. The proposed methodology addresses this requirement.
[0042] FIG. 7 illustrates example a method 700 for mechanical joining of two dissimilar steels over self-piercing riveting (SPR), in accordance with an embodiment of the present disclosure. The order in which the method 700 is described is not intended to be construed as a limitation, and any number of the described method blocks may be combined in any order to implement the method 700, or an alternative method.
[0043] At block 702, the method 700 includes maintaining dimension of rivet with hardness 510±30 Hv, length 5.5 mm., shaft diameter is 5.5 mm.
[0044] At block 704, the method 700 includes maintaining stack thickness of 2.2-2.6 mm.
[0045] At block 706, the method 700 includes maintaining diameter of a pip die at 10 mm.
[0046] At block 708, the method 700 includes maintaining depth of the pip die at 2 mm.
[0047] At block 710, the method 700 includes maintaining the pip at zero height.
[0048] At block 712, the method 700 includes maintaining an interlock at 0.291-0.379 mm.
[0049] At block 714, the method 700 includes maintaining Tmin at 0.277-0.258 mm.
[0050] In an aspect, the rivet is made of boron steel.
[0051] In an aspect, the two dissimilar steels have top steel and bottom steel. In said aspect, the top steel is hard dual phase steel and the the bottom steel is sheet of softer interstitial free galvanealed steel.
[0052] Thus, the present disclosure discloses joining of dissimilar dual phase steel (DP 600) with Interstitial free galvanealed steel (IF). The parent materials used in this disclosure are a cold rolled 1 mm thick DP600 steel sheet exhibiting a ferrite-martensite dual phase structure and a 1.4 mm Interstitial Free (IF) steel sheet exhibiting ferrite microstructure. DP600 steel was galvanealed with an average weight of 80 g m-2 and IF steel was galvanized with an average weight of 100 gm-2. Dual phase steel presents a unique combination of high formability and strength owing to their dual phase microstructure wherein ferrite present good ductility and martensite provide strength. Hardness values of DP and IF steels are found to be 189±6 Hv and 97±3 Hv respectively. Selection of top and bottom sheet is important in SPR process. DP 600 and IF steels are chosen as top and bottom sheet respectively. In an aspect, sample geometry can be chosen as per BS1140-spot welding standard. Further, the rivet penetration can be varied to achieve a range where improved fatigue properties can be obtained. Also, as shown in FIG. 6, fatigue tests are carried out for obtained rivet penetration range for stress ratio of 0.1. Interlock and Tmin values are obtained from the cross sectional images obtained through stereoscope.
[0053] Each of the appended claims defines a separate invention, which for infringement purposes is recognized as including equivalents to the various elements or limitations specified in the claims. Depending on the context, all references below to the "invention" may in some cases refer to certain specific embodiments only. In other cases, it will be recognized that references to the "invention" will refer to subject matter recited in one or more, but not necessarily all, of the claims.
[0054] Groupings of alternative elements or embodiments of the invention disclosed herein are not to be construed as limitations. Each group member can be referred to and claimed individually or in any combination with other members of the group or other elements found herein. One or more members of a group can be included in, or deleted from, a group for reasons of convenience and/or patentability. When any such inclusion or deletion occurs, the specification is herein deemed to contain the group as modified thus fulfilling the written description of all groups used in the appended claims.
[0055] Furthermore, those skilled in the art can appreciate that the terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the present disclosure. It will be appreciated that several of the above-disclosed and other features and functions, or alternatives thereof, may be combined into other systems or applications. Various presently unforeseen or unanticipated alternatives, modifications, variations, or improvements therein may subsequently be made by those skilled in the art without departing from the scope of the present disclosure as encompassed by the following claims.
[0056] The claims, as originally presented and as they may be amended, encompass variations, alternatives, modifications, improvements, equivalents, and substantial equivalents of the embodiments and teachings disclosed herein, including those that are presently unforeseen or unappreciated, and that, for example, may arise from applicants/patentees and others.
[0057] While the foregoing describes various embodiments of the present disclosure, other and further embodiments of the present disclosure may be devised without departing from the basic scope thereof. The scope of the present disclosure is determined by the claims that follow. The present disclosure is not limited to the described embodiments, versions or examples, which are included to enable a person having ordinary skill in the art to make and use the invention when combined with information and knowledge available to the person having ordinary skill in the art.