The energy consumption of the process of joining steel sheets with the use of clinching with and without an additional rivet, and analysis of sheet deformation and mechanical strength of joints

In the case of thin-walled structures, the geometric accuracy of the products is particularly important. The implementation of the parts joining process requires various tools and technological equipment. For clinching joints, their geometric quality, energy requirement of the forming process, and joint load capacity are very important issues. Clinching technology has been used for at least two decades, and research is still being conducted to improve it. This paper presents the results of research on the impact of process modification, i.e., the use of different tools and the shape of the additional rivet, on the joint geometrical quality, the energy consumption of the forming process, distortion of the DX51D+Z/275 steel samples, and the load capacity of the joints. The research was carried out using three different sets of tools. The effect of changing the shape of the rivet (the hole and its diameter and depth) on the reduction of the forming force and the energy consumption of the joining process was analysed. For joints made with different tools and with an additional rivet, an analysis of the size of the sheet deviation was performed. Changes in the geometric structure of the joint interlock and changes in the surface flatness of the sheets in the area of the joint axis were observed. The impact of the proposed rivet modification on the change in the effectiveness of the forming process was determined. Identification of the strength of the joints was also carried out in the shear test of the lap joint. The energy consumption up to fracture was calculated. It has been found that it is possible to significantly increase the joint load capacity and reduce the forming force of the clinch-rivet joints.


Introduction
Automation and computerisation of manufacturing processes (so called "Industry 4.0") is the latest trend in the global industry.In mass production, assembly lines for the thin-walled elements are equipped with industrial robots that are connected to the IT network (Fig. 1a).The prevailing trend of minimising the carbon footprint and the computerisation of the manufacturing processes forces, among others, to modify assembly process and optimise them to reduce environmental impact.The integration of specific joint assembly systems makes it necessary to introduce changes in production processes aimed at increasing production efficiency and introducing better control of the production process (Fig. 1b).The new introduced joining techniques allow for increasing the stiffness and strength of the joints and elements.However, not all solutions allow to reduce the energy consumption of the process.
Until recently, the only way to join car body elements was using based on resistance spot welding, welding, or laser soldering.These processes generate harmful gaseous compounds that affect the purity of the natural environment.For some time, clinching (CL) has been one of the methods used to join thin-walled structures of vehicles [1,2].Clinching is becoming more and more commonly used in the assembly processes of car bodies [3−10].Sheets of car body structures can be welded with additional adhesive between sheets [11].The adhesive layer is not a major obstacle in the welding process.The coherence of the material is maintained during the joining process, and there is no material damage of the joined elements.Clinching is also used to join galvanised sheets without significantly damaging the zinc coating [12].It can replace resistant welded joints in some cases [13].Clinching technology is constantly being developed and new variations of the clinching process are being developed [14].One of the modifications of the process concerns the local heating of the materials before joining [15,16].It can be used to join when a hole has already been made in the lower layer [17].Another widely used method of joining without the need to make holes is self-piercing riveting (SPR) [18,19].However, in the SPR joining process, the top layer of the material is pierced.SPR can be used to join various materials, including increasingly widely used aluminum alloys [20,21] and titanium [22,23].Another modification of riveting technology without drilling a hole is joining with a high-hardness rivet, by means of which a hole is punched [24−29].Another method used to connect the vehicles construction is friction stir welding (FSW) [30].
In the literature, it can be observed that new types of joint structures are constantly being developed, requing the development of new types of joining processes [31−34].
Clinching allows for joining steel sheets [35−37], aluminum alloys [38−41] and even titanium alloys [42−46].The clinching method is constantly developed and modified [47,48].In the case of using an additional rivet to form a clinching joint, a significant increase in joint strength is obtained [49].The authors presented the results of the research on joining sheets of material (H220PDthickness = 0.8 mm, DC06thickness = 0.7 mm, and DX51Dthickness = 0.9 mm) of different thicknesses.For all tested joints, the load capacity of the clinch-rivet joint increased several times compared to the clinching joint.Another paper [50] presents extended research on joining steel sheets with a thickness of 0.8 mm (DX51D, H220PD, DC06, Ra-K 40/70) of the 5754 aluminum alloy in the H111 technological state.The largest interlock was obtained for the combination of aluminum alloy sheet material.In all cases, the plates were not pierced.Connections made with an additional rivet allow to obtain 93%−97% of the load capacity of welded connections for the same materials [51].In the case of DX51D sheets with a thickness of 0.9 mm, a lower load force was observed, but obtained with a smaller displacement value than for the welded joint.Clinchriveting can be successfully used to join different types of aluminum alloys with different technological states, for example: 5754 (H11, H22, H24 states) and 6082 (T6 state) [52].From the tested combinations of the sheet arrangement in the joint, the highest load capacity (2.016 kN) was obtained for the upper sheet-6082-T6 and the lower sheet-5754-H22.
The use of a solid rivet modification, i.e. with a through hole, reduces the forming force.Paper [53] presents the effect of the diameter of the rivet hole on the formation and load capacity of the lap joint of DX51D steel sheets with a thickness of 1.5 mm.The lowest forming force was obtained for a hole with a diameter of 2.5 mm.However, no significant reduction was achieved compared to a joint formed without an additional rivet.
The clinchriveted joints can be formed using a rivet of different hardness.The authors of the paper [54] analysed the influence of different rivet hardnesses (350 HV1, 400 HV1, 420 HV1) on the forming force, interlock parameters, and joint load capacity for DX51D steel sheets with a thickness of 1.5 mm.They carried out a load-displacement analysis of the clinchriveted joints for H-shaped samples in mixed-loading tests.The lowest load capacity was obtained for the joint with a rivet with a hardness of 350 HV1, and for the other two hardnesses the values were very similar.
The use of joining with an additional rivet (without loss of material cohesion) is also possible when using a flat die.This is a way of simplifying the shape of the die.In the paper [55], for a flat die the influence of the sheet holder force on the interlock parameters and joint load capacity for sheets made of 5052 aluminum alloy (thickness = 2.0 mm) was analysed.It has been shown that the greater the holder force, the greater the interlock parameters and the load capacity of the connection.
A rivet with a hole can be used to regenerate a damaged clinching joint.The issue of strengthening a damaged connection made of 2.0 mm thick sheets of 5052 aluminum alloy with a tubular rivet made of 6111 aluminum alloy is presented in [56].After placing the tubular rivet, the joint was pressed with varying force.A flat die was used in the experiment.A similar method for strengthening a damaged clinching joint was presented in [57,58].The authors used a full 6061 aluminium alloy rivet to regenerate the joint of 5052 aluminum alloy.The reinforced connection showed a higher load capacity than the undamaged clinching connection without the rivet.The paper [59] presents research on joining with the use of a solid rivet and a hole rivet in a two-stage forming process.The authors used a rivet made of the 6111 aluminum alloy to join sheets with 2.0 mm thickness made of the 5052 aluminum alloy.They obtained a significant increase in the size of the interlock and the load capacity of the joint.The results of the optimisation related to the location of the special groove on the cylindrical surface of the solid rivet and the impact on the load capacity of the connection are presented in the paper [60].In this case, the full rivet was also made of Cr12MoV steel with a hardness of 62 HRC, and the joined sheets of 2.0 mm thickness were made of 5052 aluminum alloy.A flat die and a slotted holder were used for joint forming.
Each of the methods of joining thin-walled structures causes, to a greater or lesser extent, deviations of the material in the vicinity of the joint axis.During joining, in addition to local changes in the structure of the material, there is a specific deviation of the surface of the sheets [61] (Fig. 2a).Examples of deformations of thin-walled elements are shown in Fig. 2b.The issue of predicting the surface deviation of the joined aluminum alloy elements with the use of SPR joining technology was analysed by Cai et al. [62].During the research, they analysed the effect of the length of the rivet on the joining process parameters and deformation of the aluminum alloy sheets.In addition, for a specific aluminum alloy door panel, an algorithm was applied to predict the deformation of specific surfaces.Tozaki et al. [63] studied the behaviour of joined materials in the structure and deformations around the joint.For various shapes of the tool and the technological parameters of the friction stir welding process, they performed an analysis of the microstructure and load capacity of the lap joint of aluminum alloy sheets.Friction stir welding causes an increase in temperature at the connection point, which affects changes in the material and thermal deformations [64].Hwang et al. [64] investigated the influence of technological parameters of friction stir welding on the range of temperature impact in the joined materials.
Often, in addition to joining thin-walled layers of the structure, it is necessary to form an additional thread in a specific place.This effect can be achieved by mechanical locking (or pressing) or friction pressing of additional fasteners [65,66].The friction pressing of an element with a thread (FSR) allows the use of a different design of the detachable connection [67].FSR causes the material to deform plastically and thermally around the injection place.A flash of material is formed around the pressed in additional element, which may affect the adhesion of the joined elements.Mechanical clamping of additional elements allows you to quickly create a place with a thread.The thread allows for the assembly of a separable connection.In the papers [65,66], the authors present the analysis of the load capacity of a joint with a plastically clamped additional element with a thread.They proposed a variant of the connection technology in which there is already a flange in the material and that is used to tighten the threaded fastener.
This paper presents research, results, and their analysis regarding the possibility of using a modified steel deformable rivet with a hardness of 400 HV1 as an additional element for forming a clinching joint.To form the clinching connection with an additional rivet, a full rivet was used, a rivet with a through hole, and a rivet with hole depth of 3.0 mm were used.To compare the effect of using an additional rivet, the clinching joint was formed without a rivet using a rigid circular die and a die with movable segments.Furthermore, an analysis of the energy consumption of the forming process was made for different variants of tools and the shape of the rivet.The basic geometry of the interlock on the cross-section of the joints and the deformation of the samples were measured.The load capacity of the lap joints was also tested for a DX51D steel sheets of 1.5 mm thickness with a zinc coating.

Methodology for measuring sheet deviations and load capacity of joints formed with different sets of tools
A DX51D+Z/275 (according to PN-EN 10346: 2015-09 [68], material number 1.0917) steel sheet with a thickness of 1.5 ± 0.15 mm with a zinc coating was used to test the clinching joints formation process.The true stress-true strain diagram of the sheet material, according to PN -EN ISO 6892-1:2020-05 [69], is shown in Fig. 3a.The microstructure of the steel was identified on the etched specimen (Fig. 3b).The standard and modified rivets used in the experimental tests are made of carbon steel with 205 GPa Young's modulus (E), 0.3 Poisson's ratio (), and 400 HV1 hardness.Microhardness measurement was performed in accordance with the PN-EN ISO 6507-1:2018-05 standard [70].The compression force-displacement diagrams in the rivet upsetting tests are shown in Fig. 4. In the cross section of the solid rivet (Fig. 5a), the macrostructure of the rivet material was observed (Fig. 5b).The identification of the material by analysing the chemical composition in microareas was performed by using the Hitachi S-3400N scanning electron microscope, which uses X-ray energy dispersion spectroscopy (EDS), according to the ASTM E-1508-12 standard [71].An accelerating voltage of 20 kV and a spot size < 10 nm were used.A backscattered electron (BSE) detector was used.The obtained X-ray spectra is shown in Fig. 5c.The tests of forming process of all joints, including three variants of the rivet geometry, were performed in the Pressed Joint Laboratory of the Machine Design Department at Rzeszow University of Technology.The stand enables the formation of various plastically shaped joints up to 100 kN.The "CMB" C-frame press was equipped with an EMPK linear servo drive, force measurement systems with an accuracy of 0.5% of the pressing force, and the displacement measurement with an accuracy of 0.01 mm.The stand has interchangeable sets of forming tools for 3 types of joints (Fig. 6a): 1. clinchinground punch and solid die (CL/CL).2. clinchinground punch and die with movable segments (CL/CR).3. clinchrivetingrivet feeder punch, deformable rivet and "SKB" die with movable segments (CR/CR).The detailed geometry and arrangements of the forming tools are included in Fig. 6b.In addition, three variants of the rivet geometry were used for the tests (Fig. 7).Based on the standard rivet geometry (Fig. 7a), a specific modifications were made: hole with a certain depth (Fig. 7b) and through hole (Fig. 7c).The individual nomenclature of the joints is listed in Table 1.Table 1 The tools and an additional elements arrangements In the case of modification of the rivet geometry, a reduction in the weight of the rivet was obtained.The largest decrease was obtained for the diameter of the 2.5 mm through hole.For individual variants of the rivet shape, individual weight change parameters were measured and calculated, which are listed in Table 2.In the case of forming clinching joints without a rivet, a punch with a diameter of d = 5 mm was used (Fig. 6b).In the case of joints with an additional rivet, a rivet of the same diameter as the forming punch (CL) was used -Fig.6b and Fig. 7.For joints without a rivet, the thickness of the embossment was set at X = 0.75 mm, and for joints formed with a deformable rivet the zero distance between the upper surface of the rivet and the upper surface of the upper sheet was set (Fig. 8).Fig. 8 The final position of forming tools for "CL/CR" method (left side), "CR/CR" method (right side).sfrpunch displacement, hr-rivet height, X -minimal thickness of the embossment, ∑tstotal sheets thicknesses, sfcdisplacement for joints without rivet For all variants of the joints, observations and measurements of the characteristic parameters of the joints interlocks were made (Fig. 9).Observations and measurements of the joint geometry were made using the VHX7000 optical microscope.The microscope was equipped with a VH-Z20R/Z20T zoom lens.The parameters were analysed for a zoom of 100×, which allowed one to obtain a single image with a resolution of 2048 × 1536 px with a working area of 3.05 mm by 2.28 mm.The lens distance was 25.5 mm.The distance between the pixels of the recorded image was 0.0015 mm, in both directions of the XY plane.Identification of the size of the sheets deviation from the initial position was carried out by measuring the deviations of the outer surfaces of the sheets of the sample with dimensions as in Fig. 10a.On the other hand, to test the joint load capacity and the energy consumption, lap joints samples were prepare with various arrangements of the forming tools and the rivet geometry (Fig. 10b).The lap joints for the shear tests were made in accordance with the ISO 12996:2013 standard [72].
In the case of measuring the deviation of the sheets, the origin of the axis system was adopted on the sheet surface -Fig.11.In order to increase the accuracy of the measurement of the sheet edge deviation line, the part deformed as a result of samples preparing (cutting process) was omitted.On the press, during the strip cutting process, the sheets were pressed down with a holder.The size of the sheet surface deviations was measured with the ATOS Capsule 200 MV200 scanner (Fig. 11a).The measurement system was characterised by the maximum measurement error: sphere spacing error 0.008 mm and length measurement error: 0.009 mm.After the scan, points were set along the path shown in Fig. 11b.

Results and Discussion
Changing the type of joining technology for one sheet layout arrangement affects the energy consumption, interlock geometry, and load capacity of pressed joints.It is possible to use different sets of tools for one C-frame stand."CR" technology requires a frame with more clearance than a rivetless joint forming tool kit.The rivet feeder system with a punch and a rivet feeding mechanism has much larger dimensions than the punch holder for clinching joints [53].In the case of the manipulator, the weight of the tools is important."CL" technology allows the use of a rigid tool and a relatively simple solid die, but no significant load capacity of the sheet joint is achieved.The use of a matrix with movable segments allows to reduce the energy consumption of the joining process.The use of an additional deformable element (rivet) significantly changes the forming process and the maximum load capacity of the joint.For selected variants of the forming process, the size of the deviation of the joined sheets was measured.The influence of the die used in the clinching joint formation process on the maximum load capacity of the lap joint was also analysed.

Analysis of the Energy consumption of the forming processes
An important role in the process of forming clinching joints is the pressure on the joined sheets, hence the holder springs are used in the punch system.Sometimes there is a blockage of the joint embossment in the die after forming, which is why die ejector systems are used (additional spring in die system).There are many publications in which the authors present an analysis of the formation of pressed joints, but omit aspects of energy consumption of the joining technology.Figure 12 shows the forming forcedisplacement curves, taking into account the holder spring force, obtained during the formation process.The use of other die than solid, e.g."SKB" (with movable segments), allowed to reduce the forming force and energy consumption (Fig. 13).It is possible to form a joint with an additional rivet and a die with movable segments [59].The use of an additional solid rivet resulted in an increase in the forming force and the energy consumption of the joining process.The stiffness of the C-frame press for forming clinching joints is extremely important [73].Hence, it is extremely important to look for possibilities to increase the load capacity of the clinching joint, but not to increase the forming force [53].
An important role in the process of forming clinching joints is the pressure on the joined sheets, hence the holder springs are used in the punch system.Sometimes there is a blockage of the joint embossment in the die after forming, which is why die ejector systems are used (additional spring in die system).There are many publications in which the authors present an analysis of the formation of pressed joints, but omit aspects of energy consumption of the joining technology.Figure 12 shows the forming forcedisplacement curves, taking into account the holder spring force, obtained during the formation process.The use of other die than solid, e.g."SKB" (with movable segments), allowed to reduce the forming force and energy consumption (Fig. 13).It is possible to form a joint with an additional rivet and a die with movable segments [59].The use of an additional solid rivet resulted in an increase in the forming force and the energy consumption of the joining process.The stiffness of the C-frame press for forming clinching joints is extremely important [73].Hence, it is extremely important to look for possibilities to increase the load capacity of the clinching joint, but not to increase the forming force [53].
Fig. 12 The examples of the forming force-displacement diagrams for different tools arrangement Fig. 13 The comparison of the average values of the forming force and energy consumption for different joining technologies The idea of using an additional rivet is to increase the load capacity of the joint.Therefore, it was decided to investigate how modification of the shape (and thus also weight) of the rivet will influence the maximum forming force and energy consumption of the joining process.Figure 14 shows the forming force-displacement diagrams of joint with an additional rivet.The shape of the modified one (with a hole made in the axis to a depth of 3.0 mm) looks like a rivet for self-piercing riveting (SPR).The difference in the process is significant, in the case of "SPR" the rivet pierces the upper layers of the sheets [18], while in the case of "CR" there is no loss of material cohesion [49].For two different cases of the shape of the rivet with a hole, a comparison of the maximum forming force and energy consumption of the "CR/CR" joint was made (Fig. 15).In the case of the rivet with a hole of 1.0 and 1.5 mm, no significant differences were observed.Tests have shown that the use of a rivet with a 2.0 mm diameter through hole significantly reduces both the forming force and energy consumption.The differences between the forming force and the energy consumption for the two varieties of the hole in the rivet are at a similar level.The mechanical characteristics of the rivet influence the reduction of the forming force-displacement diagram course and the increase of the maximum force [54].
Fig. 14 The forming forcedisplacement diagrams for joint with rivet with and without hole Fig. 15 The influence of the diameter of the rivet hole on the forming force and energy consumption

Macrostructure and measurements of the joint interlock parameters
The formation of the joint with a punch and a solid die is characterised by a limited flow of the material.The shape and geometry of the die used in the forming process influence the mechanism of material flow [74].The use of a die with movable segments (instead of a solid die) resulted in a greater displacement of the material in the radial direction, especially in the segment area (Fig. 16).Thus, the interlock parameters increased (Fig. 17).
(a) (b) (c) Fig. 16 The comparison of the macrostructure of the joint: a) formed by using solid die and die with movable segments, b) formed by using punch, an additional rivet, and die with movable segments, c) the outlines of the upper (left view) and lower (right view) sheet Fig. 17 The interlock parameters for different joining technologies The formation of the joint with a punch and a solid die is characterised by a limited flow of the material.The shape and geometry of the die used in the forming process influence the mechanism of material flow [74].The use of a die with movable segments (instead of a solid die) resulted in a greater displacement of the material in the radial direction, especially in the segment area (Fig. 16).Thus, the interlock parameters increased (Fig. 17).
As the diameter of the hole in the type II rivet increased, the sheet material under the rivet was pushed into the hole.Thus, the sheet material is pressing on the side surface of the rivet.In a joint with a type II rivet and a diameter of 2.5 mm, the rivet had a similar radius of the inner and outer walls (Fig. 18 − detail No. 1).The rivet was pressed to a greater extent in the direction of its insertion.In the case of a rivet with a hole with a diameter of 1.0 mm, there was a part of the conical surface of the countersink (Fig. 18 − detail 2).The differences between the height and shape of the type II and III rivets compared to rivet type I are shown in Fig. 19.The greatest difference in the depth ((h1) type II and III rivets in sheets was obtained for the diameter of the hole of 2.5 mm, and the highest radial rivet high in the upper sheet for a rivet with a hole of 1.0 mm.In the case of type II rivet, the difference in radial displacement (d1) on diameter d1 was greater than for the type III rivet.A rivet with a thinner wall pressed the joined sheet materials to a greater extent (up to 2.5 mm).Therefore, a larger volume of material was pushed into the hole in the transverse directions of the rivet (Fig. 20).For all joints, there was no material cohesion loss of the upper sheet.The issue of pressing an additional deformable aluminum alloy rivet, during which the upper sheet is pierced, was presented in [75].Piercing of the upper sheets occurs in the case of selfpiercing riveting [18,76], or in another joining method such as shear clinching [77].When using a rivet with a hole of 1.5 mm, an improvement in material flow into the free space of the rivet hole was observed.The larger the diameter of the hole in the rivet, the greater the material volume of the upper (Vt) and lower (Vb) sheets was -Fig.20b.The principle of measuring the volume of material under the rivet is shown in Fig. 20a.In the case of the type III rivet, the change in the volume of the upper sheet material (Vt) under the rivet was proportional.The material flow outside the rivet and inside the rivet hole is influenced by the forming force (Fig. 14) and the joint macrostructure (Fig. 18).

Fig. 18
The macrostructure of the "CR/CR" joints formed with different diameters of the rivet hole (do in mm) Fig. 19 The comparison of the rivet outline for rivets with through hole and 3 mm depth hole (solid lines)backgraound is a solid rivet (dashed line).do in mm After measurements on the microscope and the recording results, charts of the relationship between the impact of the diameter and shape of the hole on the change in the size of the interlock tu (Fig. 21a) and the neck thickness    were made (Fig. 21b).Compared to the joint with a solid rivet for a diameter of the rivet hole of 1.0 mm (CR/CR1.0), a value of tu decreases about 3.12%.In the case of a rivet with a through hole and diameter from 1.0 to 2.0 mm, no difference in the size of the interlock tu was observed.The use of a rivet with a hole diameter of 2.5 mm (CR/CR2.5)resulted in a decrease in size tu by 9.68% compared to the joint with a rivet with a hole diameter of 2.0 mm (CR/CR2.0).The amount of wall thinning    in the case of using a solid rivet and a rivet with a 1.0 mm hole (CR/CR1.0)did not change.A significant increase in wall thickness was observed when the diameter of the hole in the rivet was increased to 2.0 mm (CR/CR2.0).For the rivet with the hole with a certain depth, instability of changes in the values of the tu and    parameters was observed (Fig. 21).The values were lower than for the solid rivet and the rivet with a through hole.However, the value of wall thinning    in the case of a hole with a diameter of 2.0 mm (CR/CR-2.0)decreased by 3.7% (compared to the joint with a solid rivet).In combination with a hole rivet with depth 3.0 mm and diameter of 1.5 mm, a higher value of    was obtained than for the solid rivet (increase by The use of a rivet with a through hole with different diameter and a rivet with a hole depth of 3.0 mm resulted in changes in the geometrical parameters of the embossment (Fig. 22).Changing these parameters affects the formation of an interlock (Fig. 21).The analysis of the above results shows that the diameter of the hole in the rivet for which significant changes were obtained was 2.0 mm (Fig. 21, 22).The larger the diameter of the hole in the rivet, the greater was the change in the maximum diameter of the upper sheet in the joint interlock (d2) (Fig. 22a).The largest reduction in diameter d2 was obtained when the hole in the rivet was increased from 2 mm to 2.5 mm.However, the use of a rivet with a hole made at a certain depth caused smaller changes in the d2 diameter.The effect of a through hole and a hole made at a certain depth had a similar effect on changes in the minimum distance between the upper and lower sheets (h3).The smaller the hole in the rivet, the greater part of its material is upset, which causes a greater displacement of the sheet material in the radial direction, thus increasing the maximum diameter of the embossment (d3) -Fig.22b.
Changes in the height of the rivet with the hole in the joint interlock were proportional to the values of the diameter of the through hole (Fig. 22b).In the case of a rivet with a hole made to a certain depth, the changes in the height of the rivet in the joint interlock were not linear.Differences in the outline of rivets with a through hole and those made to a certain depth are also visible in Fig. 19.The use of a rivet with a hole reduced the maximum forming force and energy consumption (Fig. 15).A better effect was shown by using a rivet with a through hole than made to a certain depth.
During the formation of the "CR" connection, the rivet was upset, and its material flows in the radial direction, pushing the joined sheets out.In the lower part of the rivet, its diameter (d1) was the largest (Fig. 22c).In experimental tests of the joint forming process with different arrangements of the shape and geometry of the rivet, there was no loss of material cohesion (Fig. 16b, Fig. 18).The highest radial flow of the rivet material was observed for the rivet with a 1.0 mm diameter through hole (Fig. 22c).And for a rivet with a hole at a certain depth, the highest radial flow was observed for a hole diameter of 2.0 mm (do = 2.0 mm).

Results of degradation and deviation of the sheet surface
In the area of joint embossment, the bottom surface of the sheet was observed.Observation on the microscope allowed for the analysis of changes in the surface of the galvanised sheet.For individual characteristic areas of the embossment, photographs were taken (Fig. 23).The state of the sheet surface with the zinc coating before joining is shown in Fig. 24.Areas with characteristic changes in the embossment surface were subjected to a more detailed analysis.The analysis made it possible to observe the effect of the arrangement of the joint forming tools on surface degradation (Fig. 25).In the case of forming the "CR/CR" joint, the pressure of the rivet on the lower plate blocked the radial movement of the sheet material.The zinc coating accumulated at the bottom of the die groove.Figure 26 shows the limit of accumulation of the zinc coating.
Measurement with an optical scanner allowed for 3D visualisation of the surface deformation of the material and observation of changes in the embossment, especially in areas of contact with the edges of the tools used (Fig. 27).For the appropriate cross-sectional planes, measurement paths were prepared for the value of sheet surface deviation from the initial position (as shown in Fig. 11).

Fig. 23
The zoomed fragments of the outer surface of the embossment of the joint made with different tools Fig. 24 The macrostructure of the sheet surface of the zinc coating before joint formation Fig. 25 The macrostructure of the sheet surface in the characteristic places of embossment The joining of sheets with an anticorrosion coating slightly changes the course of the forming forcedisplacement diagram and changes the condition and thickness of the zinc coating [78].During the plastic forming of the material of the joined sheets, the external surface of the sheets is degraded.When sheets are joined using clinchriveting technology, the rivet fills the joint space from the side of the punch system.The deformed material of the upper sheet is filled with a rivet at the place where an additional rivet is pressed.However, the material of the bottom sheet (outer side of the embossment) is only protected by a zinc coating.When a clinchriveted joint is formed, the material of the bottom sheet is pressed into the die cavity.The zinc coating is deformed together with the material of the lower sheet (Fig. 25).As a result, the zinc coating is degraded in characteristic areas of the outer surface of the lower sheet.The greatest degradation of the surface of the bottom sheet during the forming tests occurred in the case of formation of the "CL/CL" joint (Fig. 25f).On the surface of the bulge of the embossment, macrogrooves are visible in a specific direction.In the case of the "CL/CR" joints, the bulge of the embossment is larger, however, such large discontinuities of the material did not occur (Fig. 25g).The fixed segments of the "SKB" die block circumferential deformation of the material.The use of a deformable rivet to form the clinchriveted joints resulted in a reduction in material discontinuities (Fig. 25h).The rivet used was deformed during sheet metal pressing.In place of the smallest plastic flow of the bottom sheet material (in the vicinity of the joint axis), the surface pressures were the highest.Therefore, the possibility of plastic flow was the smallest (Fig. 25d).The use of an additional rivet instead of a rigid punch resulted in the fact that the material of the bottom sheet could be moved to a greater extent in the radial direction.Such a state of impact of the rivet and tools on the lower plate resulted in the formation of characteristic macroflow areas (Fig. 25a).Changing the "CL" rigid die to an "SKB" die with sliding segments resulted in greater flow of the material, i.e. with reduced resistance of the lower sheet (Fig. 25b).There, the bottom sheet material came into contact with the die, and plastic flow was accompanied by lower flow resistance.Hence, smaller stops of adhesive retention of the flowing material were formed on the surface.
The smallest degradation of the outer surface of the lower sheet occurred in the case of forming a joint with the punch (Fig. 25c).Both in the case of using the "CL/CR" and "CR/CR" tool systems, there was a directed material flow on the edge of the movable segments.The friction occurring between the movable segments and the lower sheet caused the formation of grooves (Fig. 25i).In the case of the "CL/CL" connection, the area close to the outer surface of the embossment was above the die groove and was not limited by friction (Fig. 25e).

Fig. 26
The embossment area near the material outflow for the "CR/CR" joint The clinch-riveted joint formation process has been well characterised in [53].Figure 26 shows a characteristic fragment of the embossment of the joint with the use of the rivet I type.In area 1, directional surface deformations of the sheet material with a zinc coating are visible (Fig. 26).The sheet material pressed into the free space of the die groove gradually fills it until the moment when its further filling requires pushing it out towards the outside of the die.A boundary (area 2) of the accumulated zinc coating material was formed in the circle located at the bottom of the die groove.In area 3, the sheet and the coating material were pushed outside the die groove.The plastically flowing material contact the outer edge of the die groove.The material moving in the radial direction was subjected to strong interaction, hence the directional cracks (area 4).The cracks in the lower sheet were created in the final phase of joint formation.In the last phase, there is a strong movement of the rivet material (in its lower part) and the sheets in the direction of the movement of the movable segments of the "SKB" die.The zinc coating at this place is subject to significant degradation.Degradation of the surface of sheets in the place of embossment reduces resistance to corrosion [79,80].The joining of steel sheets and aluminium alloy is a common case in thin-walled structures.In the case of joints with rivet II and III type, the characteristic areas of embossment were very similar to the joint with rivet I type (Fig. 23c, Fig. 27).For the joint with the largest diameter (do = 2.5 mm), a material bulge was formed with the smallest diameter d3 was formed (Fig. 22b, Fig. 27).In the case of forming the "CL/CL" joint, a large rounding (2) of the bottom sheet bend was formed, and the ring of the bulging material (1) does not have a clearly rounded shape (Fig. 28a).The use of the "SKB" die resulted in the formation of characteristic areas on the outer surface of the embossment resulting from the impact of the fixed (4) and movable (5) segments of the die (Fig. 28b).The ring was not formed in this case either (Fig. 23b).The material was slightly pressed out, but most of the material flowed plastically in a direction radial to the die axis.The use of an additional rivet caused that the material of the lower sheet completely filled the die groove (6) − Fig. 28c.In the last phase of the forming process, the rivet flows intensively in its lower part and pushes the sheet material in the radial direction.
In each of the three types of tools arrangements, different maximum sheet deviations were observed.The deviations of the sheets are different from those shown in Fig. 2a.In all analysed cases, both sheets were bent in the same direction according to the diagram shown in Fig. 29.The outline of the upper and lower surfaces of the sheet for joints formed with different tools is shown in Fig. 30.The largest deviation of the sheets was measured on the "CL/CL" joint sample, the smallest for the variant of the tools used to form the "CL/CR" joint.In the case of using a rivet II type, the highest strip deviation was observed for a hole in the rivet with a diameter of 1.5 mm, both for the upper (Fig. 31a) and bottom (Fig. 31b) sheets.A similar situation occurred for joints formed with rivet III type (Fig. 32).However, the difference in the deviation of the sheets between the connection with the rivet III type (do = 1.5 mm) and the rivet I type was smaller than in the case of the rivet II type (Fig. 31, Fig. 32).The smallest deviation of the upper and lower sheets was observed for the joints with a rivet with a hole diameter of 2.5 mm.Figures 30−32 also include the values of the circles measured on the base surface of the upper and lower plates at which the deviations of the plates were equal to zero.From the point of view of the tightness of the connection, it is important that the sheets between them do not bend and that the corrosive agent does not penetrate the joint between the joined sheets.The conducted research [79,80] showed that both electrochemical and galvanic corrosion reduce the load capacity of the clinching joint.

Maximum load capacity and energy dissipation in the strength test of the joints
Figure 33 shows the shear force-displacement diagrams obtained in the strength test for the lap joint.The area under the curve is the basis for determining the energy consumption of the joint failure according to the guidelines provided in the ISO 12996:2013 standard [72].For joints formed with different tools arrangement, the joints were characterised by maximum load capacity for various values of displacement (s).Different values of the maximum forming force were obtained during the joint formation process (Fig. 34).The highest forming forces were obtained for the case of using the rivet and the "SKB" die.For this combination, the highest joint load capacity was also obtained in the shear test.
The shear force-displacement diagrams were very similar for the joint with and additional rivet.Examples of the shear test diagrams for joints with the smallest and largest rivet holes are shown in Fig. 35.The maximum joint shear force was obtained for rivet I type (Fig. 36).The highest energy consumption of joint failure was obtained in the case of connection with a rivet III type (do = 1.5 mm).Compared to solid rivet, the use of a 2.5 mm hole (rivet II type) significantly reduced the required joint forming force by 22.5% (Fig. 15), and the maximum load capacity decreased by 3.8% (Fig. 36).The forming force of the joint with the rivet II type was 2.8% lower compared to that of the "CL/CL" joint (Fig. 34 and Fig. 36), and the maximum shear force was 81.5% higher.Therefore, it was reasonable to modify the full rivet to the version with a hole.For the rivet III type, apart from the case with a 1.5 mm hole, slightly higher load values were obtained than for the joint with the rivet II type.The shear force-displacement diagrams were very similar for the joint with and additional rivet.Examples of the shear test diagrams for joints with the smallest and largest rivet holes are shown in Fig. 35.The maximum joint shear force was obtained for rivet I type (Fig. 36).The highest energy consumption of joint failure was obtained in the case of connection with a rivet III type (do = 1.5 mm).Compared to solid rivet, the use of a 2.5 mm hole (rivet II type) significantly reduced the required joint forming force by 22.5% (Fig. 15), and the maximum load capacity decreased by 3.8% (Fig. 36).The forming force of the joint with the rivet II type was 2.8% lower compared to that of the "CL/CL" joint (Fig. 34 and Fig. 36), and the maximum shear force was 81.5% higher.Therefore, it was reasonable to modify the full rivet to the version with a hole.For the rivet III type, apart from the case with a 1.5 mm hole, slightly higher load values were obtained than for the joint with the rivet II type.

Summary and conclusions
The presented paper explains the influence of the use of different construction of tools and an additional steel rivet with a hardness of 400 HV1 and rivet modification on the energy consumption of the joint and on the macrostructure of the joint.The most important conclusions obtained from the research are as follows: • It is possible to significantly strengthen the clinching joint by using an additional deformable steel rivet without loss of cohesion of the sheet material.However, the use of a solid rivet resulted in a significant increase in the forming force and energy consumption of the joining process.
• The use of a rivet with a hole allowed one to reduce the maximum forming force compared to the case of forming with a solid rivet.In the case of using a rivet with a through hole (rivet II type) with a diameter of 1.0, 1.5 mm, the forming force was slightly higher than that obtained for the rivet with a hole made to a certain depth (rivet III type).For larger hole diameters, up to (2.0, 2.5 mm), the lowest value of the forming force was obtained for the II rivet type.
• The modification of the geometry of the rivet allowed for a significant reduction in the forming force, the course of the forming force-displacement diagrams was also changed.In all the cases of modification of the rivet geometry, the energy consumption of the forming process was higher compared to forming with a punch and a solid round die (CL/CL).
• The highest value of the interlock (tu) was obtained for the case of using a die with movable segments and a solid rivet.The use of a modified rivet with a through hole (rivet II type) and a rivet made to a certain depth (rivet III type) did not increase the value of the interlock (tu).The measured values of this parameter were lower than those of joints with a solid rivet (rivet I type).The amount of sheet thinning (   ) in the joint was lower only for the rivet with a hole of 2.0 mm diameter (rivet III type).
• The largest macro discontinuities on the surface of the bulge on the bottom sheet were obtained in the case of a joint formed with a solid die (CL/CL).The use of a die with movable segments for the "CL/CR" and "CR/CR" process variants resulted in an increase in the maximum diameter of embossment (d3).The side surface of the embossment was not of the same geometry.The outline of the side surface of the embossment was dependent on the type of forming process.The use of an additional rivet to form a joint using a die with movable segments resulted in the formation of an additional bulge of the sheet material on the outer surface of the embossment.Only in the case of use of a rivet in the forming process (CR/CR) the contact surface of the bottom sheet with the movable segments was significantly bent.The movable elements of the "SKB" die caused scratches on the surface of the bottom sheet just before the last phase of the joint forming process.
• The deviation of the sheet surface in the case of a joint formed using a special "SKB" die (CL/CR) with movable elements was lower than in the case of a solid round die (CL/CL).The use of an additional rivet to strengthen the joint load capacity (CR/CR) formed with the use of the "SKB" die resulted in an increase in the deviation of the sheets compared to the joints formed without the rivet (CL/CR).
• The deviation of the lower and upper sheet for all analysed cases of arrangement of the forming tools was in the same direction, and the deviation values at the greatest distance tested from the joint axis had similar values.The use of a rivet with a 2.5 mm diameter through hole resulted in the least bending of the sheets.
In conclusion, this paper concerned several issues related to the formation of an interlock in a clinching technology, changes in the joint geometry, deformation of sheets as a result of local impact with tools, and an additional rivet.While the information on clinch-rivet joint forming has been expanded, there is still great potential for future research to further refine the joining technologies and the results of these studies.On the basis of the presented examples of forming, new observations were formulated on the behaviour of the material, changes on the surface of the embossment depending on the method of forming, in particular, the energy consumption of the process, which have so far been poorly researched.There is still a wide range of technological conditions related to the formation of a connection with a deformable rivet, which could not be considered in this work.Determining the type of rivet material for other sheet materials for different arrangements of thickness and number of sheet layers will be necessary for future scientists studying the formation of the clinch-rivet joint.

Fig. 1
Fig. 1 The industrial robot for joining thin-walled elements with forming force control (a) and the examples of tools used in a C-frame (b)

Fig. 2
Fig. 2 The examples of the parts deformations after joining process: a) basis schemes, b) real thin-walled structures

Fig. 4 Fig. 5
Fig. 4 Examples of the force-displacement diagrams obtained in the upsetting test of the rivets

Fig. 7
Fig. 7 The variants of the shape and geometry of the rivets used in test: a) full rivet (rivet I), b) rivet with through hole (rivet II), c) rivet with a hole depth of 3 mm (rivet III); the dimensions in mm

Fig. 9 Fig. 10
Fig. 9 The interlock parameters of the clinch-riveted joint

Fig. 11
Fig. 11 The measuring apparatus -(a), characteristics of the method of the sheet deviations measurements -(b)

Fig. 20 Fig. 21
Fig. 20 The upper and lower sheet material in the rivet hole -(a) and the influence of the rivet type on the changes in the volume of the sheet material in the rivet hole -(b)

Fig. 22
Fig. 22 The influence of the type and diameter of the rivet hole, used to form "CR/CR" joints, on the geometrical parameters of the interlock -(a), (b), (c)

Fig. 27 Fig. 28 Fig. 29
Fig.27 The view of the embossment for rivet II and III type with different hole diameter (do in mm)

Fig. 30 Fig. 31 Fig. 32
Fig. 30 The deviations of the sheet edge outline: a) upper, b) lower, for different arrangement of joining tools

Fig. 34 Fig. 35
Fig.34 The comparison of the forming force and the maximum shear force for joints formed with different tool arrangements

Fig. 36
Fig.36 The average values of maximum shear force and energy up to maximum shear force for different rivet geometry