Strength of self-piercing riveted Joints with conventional Rivets and Rivets made of High Nitrogen Steel

The use of high-strength steel and aluminium is rising due to the intensified efforts being made in lightweight design, and self-piercing riveting is becoming increasingly important. Conventional rivets for selfpiercing riveting differ in their geometry, the material used, the condition of the material and the coating. To shorten the manufacturing process, the use of stainless steel with high strain hardening as the rivet material represents a promising approach. This allows the coating of the rivets to be omitted due to the corrosion resistance of the material and, since the strength of the stainless steel is achieved by cold forming, heat treatment is no longer required. In addition, it is possible to adjust the local strength within the rivet. Because of that, the authors have elaborated a concept for using high nitrogen steel 1.3815 as the rivet material. The present investigation focusses on the joint strength in order to evaluate the capability of rivets in high nitrogen steel by comparison to conventional rivets made of treatable steel. Due to certain challenges in the forming process of the high nitrogen steel rivets, deviations result from the targeted rivet geometry. Mainly these deviations cause a lower joint strength with these rivets, which is, however, adequate. All in all, the capability of the new rivet is proven by the results of this investigation.


Intr Introduction oduction
A further reduction in fuel consumption and CO2 emissions in traffic can be achieved through lightweight design. This is resulting in an increasing proportion of high-strength steel and aluminium in car bodies. To join these materials, self-piercing riveting (SPR) is a well-established technique. By contrast to resistance spot welding, SPR turns out to be beneficial, due to its capability to join multi-material structures and its better joint properties [1]. The four stages of the SPR process are demonstrated in Fig. 1. Two or more sheets are clamped between a blank holder and a die. A punch presses a semi-tubular rivet into the sheets. The rivet pierces the sheet on the punch side before flaring in the sheet on the die side. This then creates an interlock [2].  ocess stages and charact acteristic joint par eristic joint paramet ameters in the cr ers in the cross-section of a se oss-section of a sev ver ered joint accor ed joint according t ding to [3] o [3] Several methods can be used to inspect the quality of the resulting joint, especially in respect of its strength. The characteristic joint parameters as per [4] can be determined over the cross-section of the joint (see Fig. 1). These are an indicator of the expected joint strength. Standardised experimental tests or, as an alternative, numerical simulation are used for reliable determination of the joint strength [5]. For experimental examinations, specimens are tested destructively under defined load conditions using a testing machine. Parameters like maximum force, maximum displacement and energy adsorption until failure can then be determined on the basis of the measured force and displacement during the test. The failure mode can also be analysed. Typical failure patterns for self-piercing riveted joints, which are illustrated in Fig. 2, are known from [6] and [7]. With conventional rivets, whether the rivet foot is torn from the die-sided material or the rivet head is torn from the punch-sided sheet depends on the combination of the amount of interlock and the strength and thickness of the sheets. In general, a low strength in the punch-sided sheets results in the rivet head being torn from the punch-sided sheet and the emergence of a slot. Rivet fracture is promoted by brittle rivet materials. The rivet is essential for SPR. For this reason, it has been continuously improved in the past.
A promising approach is the use of stainless steel with high strain hardening as the rivet material. This has certain advantages. Due to the corrosion resistance of the material, the coating is no longer required and, since sufficient material strength is achieved during forming, the heat treatment can be omitted. This shortens the manufacturing process. Because the strength of stainless steel is achieved by cold forming, systematic adjustment of the local strength is also possible. This offers the opportunity to improve the rivet's deformation behaviour and hence increase the joint strength. Attempts to use stainless steel as rivet material have been published in [8] and [9]. The strength attained with the rivet materials used is not, however, adequate in respect of the requirements of modern sheet materials. just a single rivet geometry is elaborated in [3]. The aim of the rivet design is to guarantee attainment of the required values for the characteristic joint parameters and a crack-free joint and to avoid any bending and compression of the rivet. In addition, manufacturability by cold forming and feedability to the riveting system using standard equipment are ensured. The rivet geometry of [3] is used as the target for elaborating a manufacturing concept for rivets in high nitrogen steel 1.3815 in [10]. A two-step concept is used for the cold forming process. The high strain hardening of the high nitrogen steel causes very high tool loads. The foot chamfer is thus increased from 60°to 70°compared to the target in [3] in order to reduce the stress prevailing in the tools. Due to the fact that the stress in the tools cannot be sufficiently reduced and that the tool materials used are already the most capable ones, it proved necessary to reduce the forming stroke. The head diameter is much lower than intended because of the reduced forming stroke. An investigation concerning the rivet's surface condition demonstrates that SPR regarding the joining process is possible without rivet coating [11].
Thus, the high nitrogen steel rivets are not coated. As the conventional rivets are heat-treated and thus the hardness is almost homogenous within the rivet, the hardness within the high nitrogen rivet results from the strain hardening during the forming process. Because of the high strain within the rivet head, the hardness is higher than the hardness within the rivets made of treatable steel in the same area. Nevertheless, before joining, the hardness within the rivet foot is lower. The deformation behaviour during the joining process and the change in hardness during the process are examined in [12].

R Results and Discussion esults and Discussion
At the beginning of the investigation, experimental joining tests are carried out. The joining parameters for P-rivet, HD2-rivet and improved rivet are taken from [3]. With the high nitrogen steel rivet, the joining stroke is adjusted due to the deviation in the rivet length. The cross-sections of the joints riveted with high nitrogen steel rivets by comparison to the joining results from [3] are shown in Fig. 4. Additionally, the joining stroke and the measured joining force of the individual joints are given. All joints meet the required quality in the respect of the characteristic joint parameters.  Fig. 4. Cr Fig. 4. Cross-sections of se oss-sections of sev ver ered joints and joining par ed joints and joining paramet ameters with diff ers with differ erent ri ent riv vets ets The joint strength is determined by experimental tests as explained above. The determined strengths of the joints under quasistatic load are shown in Fig. 5. As already mentioned in [14], it must be noted that the shear strength is higher than the tensile load in general. The dependency of the joint strength on the material strength of the parts to be joined, which is also explained in [14], is evident in the present study too. The strength of the aluminium limits the joint strength regardless of the rivet properties. However, the joint strength is significantly influenced by the selected rivet and the main differences caused by the rivets are not influenced by the load direction. The strength achieved with the improved rivet compared to the P-rivet is lower with material combination 1, while the difference compared with the HD2-rivet with material combination 2 is small. With material combination 1, the strength achieved by the high nitrogen steel rivet is similar to the strength achieved by the improved rivet. With material combination 2, however, the strength achieved by the high nitrogen steel rivet is even lower than that for the improved rivet. The reasons for these insights can be determined by means of an analysis of the joint failure mode (see from the punch-sided sheet. Because of that, the strength achieved with the improved rivet is similar to the strength achieved by the HD2-rivet. The drop in strength that results when using the high nitrogen steel rivet is caused by the even smaller diameter of the rivet head, which is a consequence of the deviations caused by the forming process. Under shear load, the joints with HD2-rivet and improved rivet fail through the tearing of the rivet foot from the die-sided sheet. With the high nitrogen steel rivet, by contrast, the joint fails through a combination of the tearing of the rivet head from the punch-sided sheet and the tearing of the rivet foot from the die-sided sheet, which is also a consequence of the smaller rivet head diameter. The failure modes tally with the insights of [15]. With material combination 1, the determined joint strength is higher than the determined strengths with a similar material combination in [15] even for the high nitrogen steel rivet, while the joint strength with material combination 2 when using the high nitrogen steel rivet is on the same level as the results of [15]. on the findings of [16], it can be assumed that the fatigue strength under shear load is even higher than under tensile load, as is also the case with the tests under quasistatic load. On the contrary, for material combination 2 with punch-sided EN AW-5083 and die-sided HCT780X, the joint strength is lower compared to joints with conventional rivets. The main failure mode with high nitrogen steel rivets for material combination 1 is the tearing of the rivet foot from the die sided sheet. In this case, the lower strength of the joints with high nitrogen steel rivets can be attributed to the smaller interlock formation compared to P-rivets. As concerns the failure behaviour for joints of material combination 2, the failure mode differs depending on the load. Under tensile load the tearing of the rivet head from the punch-sided sheet is the reason for failure, while under shear load an additional tearing of the rivet foot from the die-sided sheet occurs. The observed results can be correlated with deviations between the target geometry and the actually achieved geometry of the manufactured rivets, which are a consequence of the reduced forming stroke. Thus, the lower joint strength results from the smaller head diameter of the high nitrogen steel rivets. Nevertheless, a competitive joint strength is achieved using the new rivets. Overall, based on the results of the present investigation, it can be stated that joints with an adequate strength can be realised with rivets made of high nitrogen steel. Thus, the high potential offered by high nitrogen steel as rivet material is confirmed.
Future studies will involve correcting the deviations due to the forming process and improving the joint strength through the local adjustment of the material strength within the rivet. In addition, the corrosion resistance of the rivets and riveted joint will be examined.