Key Engineering Materials Vols. 651-653

Abstract: A method for the virtual process design of combined quasi-static and electromagnetic forming processes based on a thorough process simulation is developed. Its flexibility is demonstrated by means of an identification problem for process parameters yielding a minimum bottom edge radius in round cup forming. Particularly, an optimum double exponential current pulse is identified. This class of pulses is parameterized as an example for pulses with mono-directed current employed to reduce the wear of the tool coil.

Abstract: The main goal of the paper is development of a numerical model for explosive welding involving geometry and properties of major process components, i.e. base plate, flyer plate and explosive material. To properly replicate material behavior under these severe conditions the Coupled Eulerian Lagrangian (CEL) approach is used. Series of numerical simulations are realized based on the developed model in order to relate the process variables to the physical parameters. That will be used in further work to establish how these can be used to predict whether or not bonding will occur.

Abstract: Joining materials by forming is an interesting approach to the manufacture of hybrid (multi material) parts. By establishing a cold pressure weld between metallic surfaces, high quality joints with superior properties can be achieved. Reliable cold welding conditions are difficult to set up, however, since the weld initiation requires extraordinary clean, virtually sheer surfaces. Until today such conditions could only be achieved under a high vacuum conditions. Various studies on cold pressure welding reported that under vacuum welds can be established at significantly lower deformation than in a normal atmosphere. Since adverse deformation is currently needed in industrial cold pressure welding processes like the cold roll cladding of metal bands, a new process with in-line electrochemical surface treatment, is investigated. The ECUF process is intended to supply clean and thereby highly activated surfaces to the cold pressure welding process.This paper presents first results on the weld-ability of copper specimens with regard to the influence of the welding environment: air, argon and KCl solution. Butt welds were made by pressure welding of previously fractured specimens.

Abstract: This paper deals with the investigation of the metallurgy of a dissimilar Ti-6Al-4V-stainless steel joint linear friction welded. In particular two different stainless steel were considered: AISI 304 and AISI 316. These two alloys differs in the Molybdemun content. Metallographic observations, EDS analysis and Vickers Microhardness measurements were carried out, particular attention was focused on the study of the intermetallic compounds and on the microstructures of the different zones produced by the process. As usual for solid state welding processes, three different zones can be identified: the parent material, the heat affected zone (HAZ) and the thermo-mechanical affected zone (TMAZ), furthermore a very thin joining line, rich of intermetallic compounds, was also observed. In this zone diffusive phenomena also occurred resulting in a variation of the alpha phase content on the titanium side.In the TMAZ, the bimodal microstructure of the parent material was deformed and the presence of elongated alpha grains with broken beta-phase particles was established. Moreover it was observed that in the weld region, exposure to supertransus temperatures (995°C) combined with hot-deformation working and rapid cooling after joining induced the recrystallization of a martensitic beta grain structure. Concerning the joint between Ti-6Al-4V and AISI 316 some cracks were observed within the weld line, this due to the presence of brittle intermetallics compounds in this zone. The formation of these intermetallics was promoted by the presence of Molybdenum.

Abstract: The present investigation aims at studying the effect of the tool geometry and of rotational velocity of the tool, at different welding velocities, on the tensile shear strength of the friction stir welded joints realised with blanks of different thicknesses. The proposed trial and error methodology was based on experiments, numerical simulations and microstructure observations.It was observed that, at the lowest rotational velocity, the slender tool determines tensile shear strength values lower than those obtained with the thick tool in particular at the highest welding velocity investigated. The numerical simulation evidenced a wider stirred zone for the thick tool when compared with that realised with the slender tool at the lowest rotational velocity. Microstructure observations evidenced that the increase in the welding velocity determines reduced stirred zones and an homogenisation of material particularly relevant for the slender tool.

Abstract: Upsetting and extruding riveting is a new joining method, which is mainly used to join castings. In order to investigate the effect of geometric dimensions of punch and upper sheet hole diameter on the quality of joints, models with different geometric parameters were simulated via ABAQUS. According to the simulation results, the riveting process could be divided into five stages. Besides, diameter difference on rivet tail and interference value on upper sheet hole wall were selected as indicators to evaluate quality of joints. And a group of parameters is obtained for a better quality of joints. Finally, the simulation results were validated through experiments.

Abstract: The trend towards lightweight construction in automotive engineering causes additional effort and higher expense in vehicle manufacturing, because new materials or, respectively, new material combinations require adapted production and processing methods. Various combinations of metallic and fibre-based structures (GRP-/ CFRP components) presuppose convenient joining methods. In this context, an innovative joining method for combining sheet metals with carbon textiles is going to be developed at the Institute for Metal Forming Technology (IFU, University of Stuttgart / Germany). The goal of this research work is motivated by the prevention of any damage of the used textile fibre structures during the joining process (compared to mechanical joining methods like screwing or riveting). Based on the semi-solid forming technology, the new joining process is going to be developed to create a material integrated interlock between fibres and metallic components.This paper deals with the first fundamental investigations, conducted at IFU, which have already shown the technical feasibility of this new type of joining technique. The research work to be carried out comprises the usage of different sheet alloys: the combinations steel-aluminium, aluminium-aluminium and steel-steel are to be joined with layers of carbon fibre fabrics. By this innovative joining method, a firmly bonded and non-aging connection between textile and metallic materials is to be produced, without the need of any adhesive materials or associated preparative cleaning methods.

Abstract: The ongoing trend of miniaturization makes hybrid joint also for the micro range necessary. Existing solutions often have restrictions due to the principle of joining. Therefore a new joining technology, which is realized by a plastic forming process based on TEA-CO₂-laser induced shock waves, is used at BIAS. This technology enables the joining of different sheet materials with thicknesses between 20 µm and 300 µm. The manufacturing of the joint is an incremental process where several laser induced shock waves are needed to form the undercut, which presents the joint itself. For the analysis of the incremental forming behavior of this process a 50 µm thick forming sheet of aluminum (Al99.5) is joined with a 100 µm thick stainless steel (1.4301) die sheet. The first ten laser pulses are leading to relative high induced strain while for forming of the undercut 200 laser pulses are needed. The incremental induced strain per laser pulse decreases exponentially with the amount of used laser pulses. This behavior is explained by the acting pressure distribution of the induced shock wave and the contact area.

Abstract: Spin-blind-riveting (SBR) is a newly developed joining process, which combines the advantages of conventional blind riveting and flow drilling screws. With this technology, it is possible to join two different materials by one-sided accessibility without the need of pre-drilling holes. This complex process cannot be simulated by 2D finite element method. Therefore, a more realistic 3D finite element model for the SBR process is developed using the commercial software package ABAQUS. The applicability of this work is demonstrated for joining magnesium alloy AZ31B and carbon-fibre reinforced plastics. Dynamic effects, thermomechanical coupling, material damage laws, and contact criterion were taken into account in the model. The Johnson–Cook material constitutive equation was used, considering the effects of strain, strain rate, and temperature on material properties. Finally, through simulation, the joint formation, stress distribution and riveting temperature were obtained. Furthermore, a series of experiments were carried out to validate the simulation results. The numerical results are in a good agreement with the experimental results and confirm the promising properties of SBR joints between metal and fibre-reinforced plastics.

Abstract: In multi-material-design, e.g. in the automotive industry, mechanical joining processes like self-pierce riveting are well established, because of their amount of advantages. However, adhesive bonding with one-component structural adhesives is increasingly being used. The combination of the specific advantages of both joining techniques in the form of hybrid joints leads to synergies of quality and reliability, such as high corrosion resistance and better damping properties. A critical issue is the generation of global deformations of the different parts of the mechanical joints. These global deformations of the sheet metal between two or more mechanical connectors (e.g. rivets) are caused by the formation of adhesive bags during the riveting process, before the adhesive curing takes place. This research focuses on the time-dependent formation process of these bags. The aim is to achieve a reduction of global deformations based on detailed knowledge of the adhesive flow during the manufacturing of the joint by means of experiments and simulations. For this purpose experimental techniques and measurement methods for deformations over time are presented for different setups of hybrid joint types of self-piercing rivets in combination with adhesive bonding. The challenge is to track rapid and small surface deformations very accurately in the ongoing mechanical joining process. High-speed optical measurement technology like Point-Tracking and surface scanning are used to track the resulting deformations experimentally. Numerical investigations, which include the interaction of the solid matter influenced in the mechanical joining process and the fluid adhesive, are presented. On the basis a fully coupled fluid-structure interaction simulation of a single hybrid joint, a surrogate model for a multi-point hybrid joint is developed. The comparison of experimental data with simulations allows deriving the pressure distribution and flow velocities inside the adhesive layer. The influence of various parameters can be interpreted based on the physics of the interacting system, ultimately resulting in optimization helpful to the automotive industry.