Investigation of the influence of formed, non-rotationally symmetrical pin geometries and their effect on the joint quality of steel and aluminium sheets by direct pin pressing

Abstract

Resource-saving and sustainable production is becoming increasingly important regarding social, political and economic aspects, thus making the use of lightweight-construction technologies a current trend. For this reason, multi-material-systems made of high-strength steel and aluminium as well as metal and fibre-reinforced plastics gain in importance. However, different material properties, e.g. stiffness, thermal expansion coefficients or chemical incompatibilities, are challenging for conventional joining technologies. Joining by cold formed pin structures has shown to have high potential for joining multi-material-systems. These pins can be joined either by direct pin pressing into an unperforated joining partner or by caulking, where the pins are inserted through a pre-punched joining partner and the pin head is upset, resulting in a form-fit joint. Usually, cylindrical pins are used for joining. However, non-rotationally symmetrical pin geometries offer the possibility of introducing a predetermined breaking point or reinforcing a connection in the principal force direction. In this work, cylindrical pins as well as non-rotationally symmetrical pin geometries, such as polygonal and oval pin structures, are cold extruded from the sheet metal plane of an HCT590X+Z dual phase steel and joined in the next step with an EN AW-6014 aluminium using direct pin pressing. Since the formation of an undercut has an crucial influence on the joint strength, the investigations will be focused on the resulting joint geometry. In addition, the effect of different pin heights will be examined to analyse the joint formation at different levels of compression of the pin structures. Finally, the joints are evaluated regarding their joint strength in tensile shear tests and cross tension tests. Here the flow resistance of the geometry used as well as the pin height and thus the strain hardening of the pin base during the extrusion of the pins play a decisive role for the shear strength.

Joining of CFRT/Steel Hybrid Parts via Direct Pressing of Cold Formed Non-Rotational Symmetric Pin Structures

“…When considering the pin heights and the required punch penetration depths, it can be seen that, compared to polygonal and cylindrical pins, the elliptical pin structures require a greater penetration depth. This effect has already been demonstrated in [15] and it could be shown that with an increasing pin cross-sectional area compared to the punch cross-sectional area, it leads to a greater material utilization of the material displaced by the punch, since the reduction in the flow resistance into the die reduces the radial material flow into the sheet plane and increases the axial material flow into the die. However, due to the larger cross-sectional area, a larger volume of material is required to achieve the same pin height as in the other pin geometries with smaller cross-sectional areas.…”

“…In the field of CFRT/metal joining, the direct pin pressing process into a locally heated CFRT component has been shown to be an uncomplicated joining process with promising mechanical properties [13]. In the current state of the art, rotational-symmetric pins have been primarily investigated [14], although in Römisch et al [15], the uses of non-rotational-symmetric pins have shown potential to increase the maximum load of the joint in the field of metal to aluminum hybrid joining. Due to the good performance in metal/metal joining, the use of non-rotational-symmetric pin structures also appears promising in the use of CFRT/metal joining.…”

“…Due to the good performance in metal/metal joining, the use of non-rotational-symmetric pin structures also appears promising in the use of CFRT/metal joining. However, the underlying mechanisms in the direct pin pressing of CFRT/metal parts are fundamentally diverse for steel/aluminum joining and consequently, a direct transfer of the knowledge from Römisch et al [15] is not possible. In particular, the joints' dependency on the pin-and fiber orientation of the anisotropic CFRT material in relation to the load direction needs to be investigated.…”

Pin Extrusion for Mechanical Joining from Orbital Formed Tailored Blanks with Local Material Pre-Distribution

“…The subsequent pin extrusion process increases this hardening as well, resulting in overall higher hardness values, especially in the pin foot area. Consequently, an increased load-bearing capacity of the later joint is expected, as suggested in [27]. -During orbital forming, the complete material inside the thickened area is opposed to strain hardening.…”

“…Conventional pin extrusion, particularly for use in subsequent joining, has already been investigated in other works [27], but serves as a comparison for this work to investigate the influence of local material pre-distribution by orbital forming on the pin extrusion process and the properties of the formed pin. Figure 3 shows the surface contour of the conventional sample made of DC04 with a sheet thickness t0 = 2 mm with an extruded pin.…”