Determine Suitable Joining Methods For Multi-material Construction Methods

2023 Author : Hannah Pearcy | [email protected] . Last modified: 2023-06-01 01:49

Developments in the field of vehicle construction are increasingly dealing with the challenges in the transition from the use of fossil fuels to the electrification of the drive train. Driven by increasing dynamics in the political environment and growing regulatory requirements, the focus is increasingly on lightweight solutions as a key technology for fulfilling legal requirements and customer requirements.

Comprehensive lightweight construction concept

The HigHKo joint project funded by the Federal Ministry for Economic Affairs and Energy (BMWi) sought to meet these challenges in the area of lightweight construction. Together with the project partners of Porsche AG, ElringKlinger AG and Fraunhofer IPA, a lightweight construction concept was developed that took the requirements of the main system components such as the battery system, chassis and body into account at an early stage so that significant weight reductions could be achieved. In addition to material innovations, this also involved the integration of new process and joining technologies.

adhesive

Expert discussion on gluing - making electric motors reliable together

In the focus of innovative vehicle concepts, lightweight construction is becoming increasingly important because it offers significant mass reduction potential and can therefore reverse the so-called weight spiral in vehicle construction [1]. The increasing need for increased safety, more comfort and improved driving performance has led to a continuous increase in vehicle weight [1, 2, 3, 4]. Lightweight construction is of particular relevance for electric vehicles, which have a significantly higher weight than comparable vehicles with an internal combustion engine [5], with the battery module accounting for a considerable proportion of the total vehicle weight.

Due to the higher weight of the battery cells, the surrounding body structures have to be reinforced, which in turn starts a weight spiral, which in turn has a negative effect on the range of the electric vehicles [5, 6]. For the economic success of the new drives, it is therefore imperative to achieve an increase in range through significant weight reductions. The body in particular, which accounts for about a third of the weight of the overall vehicle, has great potential [2, 3].

The four pillars of lightweight construction

For the development of appropriate lightweight structures, four pillars of lightweight construction have now been established in automotive engineering [2]. These include, on the one hand, lightweight design, which strives to minimize weight by optimizing shape and topology, whereby the structure remains uniformly stiff with little use of material [2, 4, 7]. In the concept of lightweight construction, weight is reduced by systematically omitting unnecessary components such as the spare wheel or simple load-bearing structures [2, 4]. In addition, lightweight manufacturing is used, in which weight saving potentials are achieved through manufacturing, manufacturing and assembly processes, such as through the use of thickness-optimized sheets and profiles [2, 7]. The fourth approach is lightweight construction,which enables weight reduction through the use of lightweight materials (Fig. 1) such as aluminum, magnesium or fiber composite plastics (FRP) [8].

In the automotive industry, however, a substitution of traditional metallic structures with modern lightweight materials such as carbon fiber reinforced plastics (CFRP) will not be useful in the future due to the high material costs. A lever to reduce the high costs of pure FRP construction methods is the use of multi-material or mixed construction methods, which enable different materials within a component to work together in a manner that is suitable for the load and function [9, 10]. Here, the material is selected which, taking economic and production-technical requirements into account, optimally fulfills the requirements placed on the respective component of the vehicle body with a minimum weight [4, 10]. The mere mass reduction in the implementation of the mixed construction is not the only advantagebut by combining different materials, in addition to reducing weight, a significant improvement in the required mechanical component properties is achieved. For example, loads can be absorbed in highly stressed structural components by the local use of carbon fiber reinforced plastics [1].

Joining technology as a success factor for mixed construction

However, the numerous advantages of the mixed construction face considerable challenges, whereby the joining technology between foreign materials is a central problem. For example, specific material combinations fundamentally exclude certain joining processes [9]. The force introduction points and transition areas where different materials meet are also critical [2, 8]. Here it is important to take into account the problems caused by the divergent material properties such as different thermal expansion, contact and crevice corrosion [9]. It is a priority for the connection technology that the advantages of mixed construction are not due to unfavorable consequences of inadequate joining processes such as increased weight,low rigidity or strength can be leveled in the joint area [2].

Compendium lightweight

Collect ideas for lightweight construction

These obstacles make it clear that experience in the field of classic joining processes cannot be transferred to multi-material systems [2]. The selection and use of material-appropriate joining processes is therefore of particular importance so that the specific material properties can be optimally used [8, 9].

Various joining technologies are currently used to produce firm, permanent connections between body components (see Fig. 2). Within the framework of the lightweight construction concepts and the associated mixed construction method, thermal joining processes such as welding reach their limits or cannot be used. In this context, the different melting temperatures, temperature resistances and diverging heat conduction coefficients of the non-species materials have a disadvantageous effect on their thermal joining ability [11]. In addition, due to the different thermal expansion coefficients of the joining partners, tension and component distortion can occur, which are associated with a loss of dimensional accuracy and functionality of the component.

Alternative mechanical joining processes

Alternatives are offered by mechanical joining processes such as punch rivets or clinching, which are suitable as low-distortion techniques for the positive and non-positive connection of dimensionally critical assemblies [12]. One of the main advantages of these processes is that, due to the lack of heat input, different materials can be joined together in a cost-effective and energy-efficient manner while maintaining their respective material properties [11].

Put

Versatile joining technologies

Another low-heat joining technique is gluing, with which practically all technically usable materials can be joined together in a material-fit manner. The connection created by adhesion is built up very gently, since the gluing process requires neither excessive heat, such as during welding, nor structurally weakening holes, such as riveting. The generally large-area adhesive also ensures an even distribution of stress in the component and can show advantages in terms of structural damping [13].

In addition to the processes mentioned above, hybrid joining technologies that combine the use of adhesives with mechanical joining processes have become established within mixed-body car bodies [1, 4, 7, 12]. Such hybrid connections particularly meet the requirement profiles for the joining technology in body-in-white with regard to rigidity, strength and energy absorption, even in the event of a crash [12, 14].

Methodology for the selection of joining processes

In mixed construction, the selection of the most efficient joining methods, taking into account the design and material requirements, poses major challenges for the designer in the development process. Currently, the selection process within the company is mainly based on the personal knowledge and experience of the employee with the help of text-based tools such as specialist literature or construction catalogs. This procedure is not only time-consuming and inefficient, but also non-transparent and not reproducible. To systematize this process, Fraunhofer IPA developed the database-based application JoinIT, which provides a methodology for the selection of suitable joining technologies as a knowledge database, taking into account the different levels of a morphological production system.

The application is based on a database that includes the joining processes according to DIN 8593 as a basis. In an iterative selection process, by defining the influencing factors that constitute the joining task, joining partners, the geometry of the joining point and other requirements for the joining task, suitable solution paths can be derived by comparing the respective characteristics of the joining technology (see Fig. 3).

Classification of materials according to DIN standards

The basis of the developed selection methodology is the determination of the joining partners involved from a material perspective, whereby in addition to the classic construction materials such as metals and plastics, the material types of ceramics as well as composite and wood materials are taken into account. The classification of the materials corresponds to the systematics according to DIN standards according to material classes, groups and numbers. The selection process can thus be based on a defined material hierarchy path, both at the level of superordinate material classes and groups up to the respective material numbers.

In addition to the materials to be joined, the surface properties of the parts to be joined are decisive for the joining process. This makes it necessary to also integrate methods of surface treatment (e.g. blasted, polished), surface hardening (e.g. flame-hardened, laser-beam hardened) and surface coating (e.g. galvanized, powder-coated) of the parts to be joined into the methodology. The procedures considered in relation to the three sub-areas were determined using specialist literature.

Multi-material construction

Join plastics with magnesium sheets by injection molding

Geometrical requirements taken into account

Geometric requirements of the joint are also included in the methodology to further specify the selection. These include the joint types commonly used for joining (eg butt joint, overlap joint) and requirements regarding accessibility to the joint (one-sided, two-sided).

In addition to the characterization of the joining partner and joining point, the selection process can be further systematized by specifying additional requirements for the joining task. This includes technical, economic and ecological aspects and their respective characteristics, the selection of which is based on DIN standards, construction catalogs and specialist literature. Technical requirements relate, for example, to the types of stress on the joint (e.g. tensile load, shear load), media resistance requirements (e.g. oil resistance, splash water resistance) or media tightness requirements (e.g. airtight, watertight). Economic requirements include, for example, the automatability of the joining technology (e.g. partially automated,fully automated) or the scalability of the number of parts to be joined per year. The ecological aspects, for example, summarize requirements with regard to emissions (e.g. low-noise, radiation-free) or energy consumption (e.g. low, high).

Program implementation of the JoinIT application

The selection methodology developed was mapped in a database architecture and filled with initial data for exemplary joining projects. The JoinIT database-based application was then implemented in the form of a dynamic web application. The graphical user interface of the web application (see Fig. 4) is described in more detail below.

As part of the selection process, the operator first defines the joining partners (1). By selecting the part to be described in each case, a dropdown menu (2) opens in which the required information can be made in accordance with the selection method. The material selection takes place in a structured process along the stored material hierarchy path to the desired level of detail (3). The parts to be joined can then be characterized in more detail by specifying further requirements with regard to material thickness and their surface treatment. Numerical information, such as the material thickness, is set using slide controls (4), while information with Boolean values, such as a specific surface coating of the joining partner to be defined, is activated via checkboxes (5). If a request cannot be restricted at the time of the search, no entry is made in this field.

Friction stir welding

Joining using friction and pressure

Diverse narrowing of the requirements

After a detailed definition of the joining partners, the selection can be limited according to the methodology by providing further information on the geometric type of the joining connection as well as on technical, economic and ecological requirements for the joining task (6). In the sense of a clear appearance and a high level of user friendliness, the individual requirements are assigned to clusters, the possible characteristics of which are minimized (7). Only when a specific cluster is selected does a dropdown menu (8) open with the stored characteristics, whereby the entries can be made analogously using slide controls (e.g. temperature resistance) and checkboxes (e.g. oil resistance).

The search for results in the selection process is based on the exclusion principle. A search algorithm compares the entries in the database for compliance with the selected requirements in real time. Every defined requirement that is not met by a joining process leads to the immediate exclusion of the respective joining process in the result display.

User-friendly and clear

The solution quantity is presented to the developer by specifying the reference joining projects stored in the database in the result field (9). Via a link, the reference projects of the solution set with their details regarding the process description and possible responsible employees in the company can be viewed in detail.

The results of the database-based application are presented in accordance with the web shop method, which is characterized by a high degree of user-friendliness and clarity due to the easy and successive selection and deselection of requirements as well as a live update of the solution quantities in the result field. The procedure thus offers significant advantages in the areas of time requirements, intuitiveness, multiple selection and input correction.

Methodical and reproducible procedure

The joining technology between the different materials represents a special challenge for the body-in-white construction in mixed construction and all hybrid component structures, since the respective material mix requires or excludes the use of different joining technologies. A decisive factor for success is the most efficient selection in the development process, taking into account the constructive and material requirements with regard to the function, quality and economy. In order to simplify this selection process in the conception and design phase, the computer-assisted application JoinIT was developed, which enables a methodical and reproducible procedure when selecting suitable joining processes in multi-material systems. JoinIT is to be used beyond the underlying research project and, as a generally accessible knowledge database, to make it easier for third parties to select suitable joining methods.

Creep molding process

IWS engineers form modern lightweight parts for aircraft

thanksgiving

The joint project HigHKo - highly integrative rear car concept was funded as part of the funding program "Efficiency increase vehicle drives" with funds from the Federal Ministry for Economic Affairs and Energy (BMWi) and supervised by the project executing agency Mobility and Traffic Technologies (TÜV Rheinland). The authors thank the BMWi for the funding granted and the project management agency and all participating consortium partners for their support.

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* Thomas Götz and Dr.-Ing. Marco Schneider, Fraunhofer Institute for Manufacturing Engineering and Automation IPA