Advanced Materials Research Vols. 966-967

Abstract: Fusion welding of dissimilar metals is in the most cases difficult or even impossible as a result of different melting points and the development of undesirable brittle intermetallic phases. This often leads to joint strengths considerable below the tensile strength of the base materials. By using Friction Stir Welding (FSW) it is possible to reduce the development of the intermetallic phases of Al/Mg-joints significantly but not to avoid them completely. Hence a hybrid welding system at the WKK of the University of Kaiserslautern was developed called “Ultrasound Supported Friction Stir Welding (US-FSW)” with the aim to shatter the brittle interlayer lines and to scatter fragments in the welding area during the FSW process. Pre-investigations have shown that for Al/Mg-US-FSW-joints the strength can be increased up to 30% in comparison to conventional FSW. Moreover for the reliable detection of nonconformities in the weld during a post-process inspection by suitable non-destructive testing (NDT) methods is necessary. Also there is a strong need for better process monitoring and control by in-process NDT methods. Furthermore the corrosion behavior of the basic materials and hybrid-joints was investigated by electrochemical methods indicating an increased corrosion of the Mg alloy in the area of the Al/Mg-butt weld.

Abstract: Friction Press Joining (FPJ) is a suitable method for producing composites of aluminum and thermoplastics in lap joint configuration, which is based on modified Friction Stir Welding (FSW). During the joining process, a rotating cylindrical tool is pressed onto an aluminum surface. The resulting friction generates heat that is conducted to the bonding zone, leading to localized softening of the thermoplastics. In combination with the tool’s axial force and a suitable pre-treatment of the aluminum surface, a resilient composite compound is created. This paper presents the results of a surface pre-treatment of aluminum using laserradiation. The textures are essential for a strong connection, as they can significantly influence effective joining mechanisms, such as microscopic and macroscopic form fit. The experiments were carried out using different surface treatments by means of a single-mode laser for joints of aluminum (EN AW-6082 T6) and glass fiber reinforced polyamide (PA6 GF15). The aim of the study was an increased understanding of process behavior and joining mechanisms. The shear strength could be increased by 40 % compared to previous studies with the presented laser surface treatment.

Abstract: In transportation sector the reduction of moving masses without the decrease of safety parameters is a key factor for future economic success. One possible approach for this is the use of different metallic materials in composite construction. Therefore, it is essential to establish a reliable component connection by means of suitable and cost-effective joining technologies. Mechanical joining technologies such as self-piercing riveting and mechanical clinching have proven to be effective methods for joining lightweight materials like aluminium and ductile steels. As these technologies require formability or pre-holing of the joining partners, the field of application is limited by the mechanical properties of the joining partners. Great potential for joining hot stamped steels, which have a very low elongation at fracture and therefore a low formability, offers the shear-clinching technology. For a systematic development of the shear-clinching technology, detailed investigations of the process are required. This paper presents an analysis of the material behaviour during the shear-clinching process and the reference process – clinching with pre-hole.

Abstract: Multi-material and hybrid constructions are increasingly used in the automotive industry with the aim of achieving significant weight reductions of conventional car bodies, and thereby lead to effective reductions of fuel consumption. In this respect, the use of aluminum and short fiber reinforced plastics represents an interesting material combination. A full exploitation of such a material combination requires a suitable joining technique. Among different joining techniques, clinching represents one of the most appealing alternatives for automotive applications. This contribution deals with the experimental tests for determination of material behaviour of two representative materials PA6GF30 and EN AW 5754, which are used for parameterization of material models needed for numerical analysis of the clinching process using the FE software LS-DYNA. With regard to the material modeling of the aluminum sheet, an isotropic material model based on the von Mises plasticity implemented in LS-DYNA was chosen. For the description of the strain hardening behaviour of the aluminum sheet at high equivalent plastic strains, the hydraulic bulge test was carried out in addition to the uniaxial tensile test. For modeling of the short fiber reinforced thermoplastic a semi-analytical model for polymers (SAMP-1) available in LS-DYNA was taken. This material model uses an isotropic pressure dependent yield surface for the description of homogeneous materials. Finally, the FE model of clinching process is presented and an outlook of planned activities is given in terms on determination of the yield surface and hardening behaviour of PA6GF30 at high plastic strains.

Abstract: Joining using forming processes in the field of bulk metal forming is currently experiencing a renaissance both within research projects in academia and industrial manufacturing processes. One main challenge of such research activities is any improvement of joint strength. In contrast to conventionally joined interference fits, such components can be manufactured without heating or thermal loads. Within framework of the priority program SPP 1640 of the German Research Foundation, a common lateral extrusion process for a solid internal part and an external tube actually is being developed at the Institute for Metal Forming Technology, Stuttgart. Aim of this project is to manufacture a form-closed as well as force-closed joint composed by a high strength external material and a light weight internal material for light-weight components. The present work describes numerical investigations of this process and shows aspects of process development. This includes, among other things, development of a new tool concept, which provides divided punches to determine the influence of different punch motion on the forming and joining process. Spring back behavior, induced residual stress distribution and occurring contact area depending on different material combinations are also investigated. Moreover, the process limits regarding component geometry and force requirement are shown.

Abstract: In the last decades, manufacturing of layered composite materials has become an interesting topic in industrial development. Joining properties of adhesively bonded materials are characterized by a complex interaction of plastic deformation, thermo-mechano-chemical coupling effects, adhesion and diffusion. Additionally, the interactions between the microstructures involved in the process have to be taken into account. The design of new joining technologies requires a fundamental understanding of the mechanisms which is difficult to achieve by working solely experimentally. The present study therefore deals with modeling the essential effects characterizing joining. Additionally, special attention is paid to the experimental characterization of the involved materials at the macro and micro levels. The microstructure of materials (as e.g. AA1050, AA2024 and AA5754), which have a wide range of applications in engineering structures, is numerically and experimentally investigated. Moreover, a general cohesive zone element formulation in the framework of zero-thickness interface elements is developed. This enables the accurate and efficient modeling of the interface based on an interfacial traction-separation law.

Abstract: Due to an ongoing trend of function compaction miniaturization gets more and more important in industrial production. This makes hybrid joints under various conditions also in the micro range necessary. Existing joining solutions often have restrictions due to the principle of joining. Thus in this article a new high speed forming method for the micro range is shown, which is based on plastic deformation by laser induced shockwaves. First of all it is shown how metal sheet-sheet joints can be realized with this method. With the produced joints tensile tests are carried out, where a maximum shearing force of 26.7 N could be achieved. For a detailed process understanding, the near-field of the acting pressure of the TEA-CO₂-laser induced shockwaves is measured. Moreover it is determined that the ignition point of the TEA-CO₂-laser induced plasma out of aluminum is about 8 mm above the surface.

Abstract: A mechanical clinching using counter pressure of a rubber disk was developed to join the ultra-high strength steel sheets having low ductility. In the proposed process, the interlock was increased by the increment of metal flow with the counter pressure of rubber disk in the die cavity. The two kind of ultra-high strength steel sheets having different ductility were used in the mechanical clinching. The effect of the shape of rubber disk on the deforming behaviour of the sheets was investigated. The joinability was improved under the appropriate shape of rubber disk for both sheets, and then the sheets having 56% of reduction area were successfully joined whereas the sheets were not joined without the counter pressure. Although the joinability of the sheets having 43% of reduction area was improved, the cracks occurred in the upper sheet around the punch sidewall. The maximum static load and the fatigue limit of the joined sheets were measured in the tension-shearing and cross-tension tests. It was effective for the improvement of joinability in the mechanical clinching of ultra-high strength steel sheets to use the counter pressure of the rubber disk.

Abstract: The conventional clinching of steels is currently limited to tensile strength less than 800 N/mm2 and to elongation at fracture more than 14 %. To realise the clinching of high-strength steels, the sheet can be heated locally at the joint, to improve ductility. Thereby the material characteristics outside the joint should be maintained. This could be achieved by means of short-time laser heating. The short-time tempering behaviour of press hardened steel 22MnB5 has been analysed. The mechanical properties during a short-time heat treatment were investigated by thermo-mechanical analysis in a deformation dilatometer. Thereby laser-assisted clinching shall be established and an efficient form-closed and force-closed connection shall be produced. As a result, the press hardened steel 22MnB5 could be clinched by laser assistance for the very first time.

Abstract: Round point clinching with rotational tool movement is a novel technique to join endless sheets of same or different metals in a quick and economic way. The main challenges are the asymmetry of the resulting clinch points as well as the non-perpendicular impact and retraction of the tools. To address these challenges, the material flow during the joint formation is closely examined. For this purpose an experimentally validated simulation strategy was developed. The influence of the process parameters on the material flow was then analyzed using this simulation model. Based on these crucial insights, an optimization approach is presented.