Fabrication of Tailor-Made Metallic Structures for Lightweight Applications and Mechanical Behaviour

Tailor-made metallic structures are fabricated by welding, adhesive bonding, and mechanical joining methods. Here the aim is not only to fabricate lightweight structures, but also to develop novel methods of joining. Lightweight structures are advantageous in several ways including reduction of fuel consumption and vehicle emissions. Developing novel methods of joining is advantageous due to the possibility of joining of dissimilar materials, improved mechanical performance, and microstructures. In the chapter, initially, tailor-welded blanks (TWB) are introduced, and after that, fabrication of TWBs by laser welding, friction stir welding, and friction stir additive manufacturing are elaborately discussed. Some critical issues in modeling the deformation during fabrication of TWBs is also discussed. A brief account of mechanical behavior of adhesive bonded sheets and mechanical joining are presented in the later part.

A contribution on versatile process chains: joining with adaptive joining elements, formed by friction spinning

Nowadays, manufacturing of multi-material structures requires a variety of mechanical joining techniques. Mechanical joining processes and joining elements are used to meet a wide range of requirements, especially on versatile process chains. Most of these are explicitly adapted to only one, specific application. This leads to a less flexibility process chain due to many different variants and high costs. Changes in the boundary conditions like sheet thickness, or layers, lead to a need of re-design over the process and thus to a loss of time. To overcome this drawback, an innovative approach can be the use of individually manufactured and application-adapted joining elements (JE), the so-called Friction Spun Joint Connectors (FSJC). This new approach is based on defined, friction-induced heat input during the manufacturing and joining of the FSJC. This effect increases the formability of the initial material locally and permits them to be explicitly adapted to its application area. To gain a more detailed insight into the new process design, this paper presents a detailed characterization of the new joining technique with adaptive joining elements. The effects and interactions of relevant process variables onto the course and joining result is presented and described. The joining process comprises two stages: the manufacturing of FSJC from uniform initial material and the adaptive joining process itself. The following contribution presents the results of ongoing research work and includes the process concept, process properties and the results of experimental investigations. New promising concepts are presented and further specified. These approaches utilize the current knowledge and expand it systematically to open new fields of application.

Automatic generation of cross sections from computed tomography data of mechanical joining elements for quality analysis

In this work, we present a methodology for shifting from a conventionally destructive, manual quality analysis for repetitive processes towards a non-destructive and largely automated process. The objects subjected to the quality analysis are mechanical joining elements like rivets or flow-drilling screws. We propose an algorithm that can automatically find and extract such joining elements from a computed tomography (CT) scan, rotate these elements to an upright orientation and eventually generate radial cross sections parallel to the elements’ longitudinal axis. The proposed algorithm was tested on five grayscale-based computed tomography volumes, with one synthetically generated volume. We will discuss both, cases in which the duo of CT and our proposed algorithm produces satisfying results, as well as cases in which it fails. Limitations of both the scan acquisition process and the proposed algorithm will be elaborated on and potential improvements will be mentioned.

Low Temperature Soldering of Laser Structured and Metal Coated Fiber Reinforced Plastics

Assemblies containing fiber-reinforced plastic (FRP) and metal parts are typically fastened together via mechanical joining or adhesive bonding. Mechanical joining processes tend to weaken FRP parts by cutting fibers, while adhesives require long cures and often lead to inseparable material compounds. This paper evaluates a new joining method in which plastic parts are laser treated, then metallized via wire-arc spraying, and finally soldered to mating metal parts using a low-temperature process. Due to the effective increase in interface area resulting from laser structuring, bond strengths of up to 15.5 MPa can be achieved.

Influence of various procedures for the determination of flow curves on the predictive accuracy of numerical simulations for mechanical joining processes

The predictive quality of numerical simulations for mechanical joining processes depends on the implemented material model, especially regarding the plasticity of the joining parts. Therefore, experimental material characterization processes are conducted to determine the material properties of sheet metal and generate flow curves. In this regard, there are a number of procedures which are accompanied by varying experimental efforts. This paper presents various methods of determining flow curves for HCT590X as well as EN AW-6014, including varying specimen geometries and diverse hardening laws for extrapolation procedures. The flow curves thus generated are compared considering the variety of plastic strains occurring in mechanical joining processes. The material data generated are implemented in simulation models for the joining technologies, clinching and self-piercing riveting. The influence of the varied methods on the predictive accuracy of the simulation model is analysed. The evaluation of the differing flow curves is achieved by comparing the geometric formation of the joints and the required joining forces of the processes with experimentally investigated joints.

Development of General data-based Process Models for Self-Pierce Riveting

The determination of ideal process parameters for mechanical joining processes such as self-pierce riveting currently requires a comprehensive understanding of the process, the availability of the materials to be joined and the corresponding system technology. General process models can simplify the use of these joining technologies, accelerate development cycles and thereby reduce the effort for implementation into production. In this paper, the development of general data-based process models for the mechanical joining method self-pierce riveting with semi-tubular rivet is described. Extensive experimental and numerical investigations with more than 2300 joint combinations for steel and aluminum sheets with tensile strengths between 240 - 1020 MPa were generated for the building of the models. Based on these results, different meta-models are fused into general data-based process models for the self-pierce riveting process in order to show the general relationships between material properties, process parameters and joining results. The paper discusses the acquisition of the experimental and numerical data, the statistical methods for evaluation and the application of the data-based process models.

New Approach for Versatile Self Piercing Riveting: Joining System and Auxiliary Part

The increasing use of multi-material constructions lead to a continuous increase in the use of mechanical joining techniques due to the wide range of joining possibilities as well as the high load-bearing capacities of the joints. Nevertheless, the currently rigid tool systems are not able to react to changing boundary conditions, like changing the material-geometry-combination. Therefore research work is crucial with regard to versatile joining systems. In this paper, a new approach for a versatile self-piercing riveting process considering the joining system as well as the auxiliary joining part is presented.

Influence of the Rivet Coating on the Friction during Self-Piercing Riveting

The number of multi-material joints is increasing as a result of lightweight design. Self-piercing riveting (SPR) is an important mechanical joining technique for multi-material structures. Rivets for SPR are coated to prevent corrosion, but this coating also influences the friction that prevails during the joining process. The aim of the present investigation is to evaluate this influence. The investigation focuses on the common rivet coatings Almac® and zinc-nickel with topcoat as well as on uncoated rivet surfaces. First of all, the coating thickness and the uniformity of the coating distribution are analysed. Friction tests facilitate the classification of the surface properties. The influence of the friction on the characteristic joint parameters and the force-stroke curves is analysed by means of experimental joining tests. More in-depth knowledge of the effects that occur is achieved through the use of numerical simulation. Overall, it is shown that the surface condition of the rivet has an impact on the friction during the joining process and on the resulting joint. However, the detected deviations between different surface conditions do not restrict the operational capability of SPR and the properties of uncoated rivet surfaces, in particular, are similar to those of Almac®-coated rivets. It can thus be assumed that SPR with respect to the joining process is also possible without rivet coating in principle.

Approach for the Automated Analysis of Geometrical Clinch Joint Characteristics

Due to their cost-efficiency and environmental friendliness, the demand of mechanical joining processes is constantly rising. However, the dimensioning and design of joints and suitable processes are mainly based on expert knowledge and few experimental data. Therefore, the performance of numerical and experimental studies enables the generation of optimized joining geometries. However, the manual evaluation of the results of such studies is often highly time-consuming. As a novel solution, image segmentation and machine learning algorithm provide methods to automate the analysis process. Motivated by this, the paper presents an approach for the automated analysis of geometrical characteristics using clinching as an example.