Gerson Meschut

Innovative and Highly Productive Joining Technologies for Multi-Material Lightweight Car Body Structures

Driven by increasing costs for energy and raw material and especially by the European CO2-emission laws, automotive industry faces the challenge to develop more lightweight and at the same time still rigid and crash-stable car bodies, that are affordable for large-scale production. The implementation of weight-reduced constructions depends not only on the availability of lightweight materials and related forming technologies, but also on cost-efficient and reliable joining technologies suitable for multi-material design. This article discusses the challenges and requirements for these technologies, based on the example of joining aluminium with press-hardened boron steels, what is considered as a very important material combination for affordable future lightweight mobility. Besides a presentation of recent developments for extending the process limits of conventional mechanical joining methods, new promising technologies such as resistance element welding are introduced. In addition, the performance, advantages, and disadvantages of the presented technologies are compared and discussed.

Innovative joining technologies for multi-material structures

The affordable implementation of lightweight constructions in automotive engineering depends not only on the availability of suitable processing technologies for new lightweight materials but also on suitable, cost-efficient joining methods for multi-material combinations with high process reliability. Therefore, joining technology plays a key role in realizing energy-efficient vehicles. The systematic development of joining methods is necessary to overcome the metallurgical and thermal incompatibility of steel/aluminium or steel/fibre-reinforced plastic combinations. This paper presents two innovative and highly productive joining technologies and characterizes these processes based on their technological properties for one specific steel/aluminium material combination.

Mechanical properties of an innovative shear-clinching technology for ultra-high-strength steel and aluminium in lightweight car body structures

Comfort- and safety-related requirements lead to weight increase of automotive structures. Modern constructions in automotive body in white (BIW) production confront this upward weight spiral with multi-material design. This restricts conventional thermal joining technologies. The basic idea of mechanical joining technologies is a force- and form-fitting joint. Clinching technologies create a joint only by plastic deformation. The formability of the joining partners limits this technology, especially when joining ultra-high-strength steels (UHSS) in multi-material design. State of the art to realise this is multi-stage clinching with pre-hole. Industrial applications in BIW are, e.g. the brackets for mouldings in the upper windscreen frame of Daimler’s S-class and the battery holder of Daimler’s M-class. Innovative shear-clinching enables joining by forming hot formed UHSS and ductile aluminium in a single-stage process. The spherical punch-sided tool set prevents any harm of the punch-sided ductile material. The cutting die initialises a crack in the die-sided material with limited formability. This paper presents detailed studies on the mechanical behaviour of state of the art multi-stage clinching with cylindrical and tapered pre-hole under quasistatic and fatigue testing methods to evaluate the influence of the interlock. These results were compared to the single-stage shear-clinching technology to derive further recommendations.

Influences of interface and surface pretreatment on the mechanical properties of metal-CFRP hybrid structures manufactured by resin transfer moulding

The combination of sheet metal and carbon-fibre-reinforced plastic (CFRP) is a promising approach in the sector of automotive lightweight construction. The hybrid structures allow a symbiotical usage of the specific advantages of each material. First of all, this article specifies the process chain by manufacturing hybrid materials with an intrinsic resin transfer moulding (RTM) process. Subsequently, research results regarding the interface between metal and CFRP component as well as the surface pretreatment of metallic component with laser structuring are illustrated and discussed. By means of four-point-bending tests, it is found that the mechanical properties of metal-CFRP hybrid structures are improved by using a glass fleece or an epoxy-based adhesive film as intermediate layer or due to surface pretreatment of metallic component with laser structuring. Additionally, a finite-element simulation for a four-point-bending test of a hybrid part is compared to an experiment for the linear elastic region, where strain and stress distributions are focused.

Relativverschiebungen elastisch ausgleichen

Die Verkleidung von Stahlrahmen mit Schubfeldern aus Faserverbundkunststoffen ist im Automobil-, im Nutzfahrzeugbau und im Schienenfahrzeugbau eine kosteneffiziente Möglichkeit, leichte und zugleich hoch feste Strukturen zu konstruieren. Eine Herausforderung dabei ist aber, die Materialien mit unterschiedlichen Wärmeausdehnungskoeffizienten so zu verbinden, dass Relativverschiebungen ausgeglichen werden können.

On the Design, Characterization and Simulation of Hybrid Metal-Composite Interfaces

Multi-material lightweight designs are a key feature for the development of innovative and resource-efficient products. In the development of a hybrid composite, the interface between the joined components has to be considered in detail as it represents a typical location of the initialization of failure. This contribution gives an overview of the simulative engineering of metal-composite interfaces. To this end, several design aspects on the microscale and macroscale are explained and methods to model the mechanical behavior of the interface within finite element simulations. This comprises the utilization of cohesive elements with a continuum description of the interface. Likewise, traction-separation based cohesive elements, i.e. a zero-thickness idealization of the interface, are outlined and applied to a demonstration example. Within these finite element simulations, the constitutive behavior of the connected components has to be described by suitable material models. Therefore, inelastic material models at large strains are formulated based on rheological models.

Influence of the dosing and mixing technology on the property profile of two-component adhesives

Adhesive bonding has become a more and more important joining technique in most branches of industries because of the increasing interest in protecting natural resources and the related trend towards lightweight design. The adhesive bonding technology enables the joining of various, sometimes temperature-sensitive materials, so that multi-material design and thereby, lightweight constructions can be realized. At the same time, the adhesive bonding helps to increase the rigidity of the component (Russo 2011). In industrial practice, not only single-component adhesives are used, but also two-component adhesives whose curing process starts once the components are mixed. In general, the automated conveying, dosing and mixing of the two adhesive components are performed by batchers. The quality of the adhesive bonding can be influenced by the dosing process (Fricke et al. 2009). Within the research project, the boundaries of automated processing of cold curing two-component adhesives were investigated. Therefore, industrially relevant adhesive processing technologies and process parameters were selected. These technologies and parameters were varied and evaluated with regard to their significance for the joining process and adhesive joint as reported by Gräter and Storz (2005) and Stipp (2007).

FE-Based Study of the Cutting Operation within Joining by Forming of Dissimilar Materials Using Shear-Clinching Technology

Lightweight design and modern production technologies are key factors for the success of today’s car manufacturing industry. Resulting challenges, like the usage of new materials in the production chain and the joining of dissimilar material combinations for composite constructions, require the constant improvement and innovative development of production and joining processes. One promising joining technology which allows single stage joining of modern hot formed steels is shear-clinching. For ensuring process reliability and improving strength of shear-clinching joints fundamental studies are required. A possible approach is the numerical analysis of the material flow. To guarantee high quality simulation results it is important to develop a possibility to simulate the material separation during the shear-clinching process. This paper presents an evaluation of possible methods to simulate the indirectly induced cutting of the die-sided material. The numerically gained results are validated by experimental data.

Basic Investigations of Non-Pre-Punched Joining by Forming of Aluminium Alloy and High Strength Steel with Shear-Clinching Technology

Facing a decreasing amount of resources on the one hand and an increasing demand for comfort on the other, more and more attention is being paid to sustainability and care for the environment. Particularly in the automotive sector, lightweight design principles continue to prosper rapidly. As a result, adjusted materials for different applications were developed. Due to the formation of intermetallic phases, most multi-material mixes cannot be welded and require adapted joining technologies. Mechanical joining technologies such as self-piercing riveting and mechanical clinching have proven effective methods of joining lightweight materials like aluminium and ductile steels. New high-strength steels are increasingly used in crash-sections, where limited deformation under impact load is required. These hot stamped steels have a very low elongation at break and therefore a low formability. Currently there is no joining by forming technology without pre-punching available using these grades of steels on die-side. The newly developed shear-clinching process is one possible method of joining this kind of material without additional elements. The fundamental idea of shear-clinching is a single-stage process in which pre-punching of the die-side material is performed by indirect shear-cutting and subsequent forming of the upper layer into this hole. This would immensely enlarge the application segment of mechanical clinching even if hot stamped steels are positioned on die-side. Fundamental studies are required to ensure process reliability and it is necessary to break down the joining process into fragments, like pre-punching and clinching with pre-punched sheet, and superpose them to form the combined procedure shear-clinching. This paper presents a detailed investigation of the sub-process clinching with pre-hole.

Flow drill screwing of fibre-reinforced plastic-metal composites without a pilot hole

Modern lightweight constructions are increasingly based on the use of multi-material design such as fibre-reinforced plastics (FRP) combined with metallic material. The technology of screwing is rated, especially for applications where high-strength and permanent joints are required, among the established joining methods. To achieve a high-quality lightweight design, often, profile-intensive constructions are used, with the effect of one-sided accessibility to the joining area. Against this background, flow drill screwing offers an innovative solution for car body construction as well as the assembly for joining this combination of material. Currently, flow drill screwing with a pilot hole in FRP materials is used in the assembly process. To use this technology without a pilot hole and use it economically in the body framework, the auxiliary joining elements have to be optimised. This publication includes the latest research findings in the context of the development and qualification of joining by using flow drilling screws. It focuses on the specific adaptation of the auxiliary joining element with regard to the geometry, the material and the coating systems. With macroscopic and microscopic sections, as well as studies of strength, ultrasound and corrosion, it is possible to assess the quality of new and original FDS joints.