"Growing requirements in the transport industry with regard to safety, lightweight construction, fuel economy and functional integration can usually only be met by a mix of materials. The mantra is: ""the right material in the right place"". However, these hybrid material...\n\n"Growing requirements in the transport industry with regard to safety, lightweight construction, fuel economy and functional integration can usually only be met by a mix of materials. The mantra is: ""the right material in the right place"". However, these hybrid material concepts can only be realized if the different components and materials are combinable respecting their individual properties. FlexHyJoin deals with the joining of metals and thermoplastic fibre reinforced polymer composites (TP-FRPC) by means of implementing two complementary joining technologies (laser and induction joining). The overall goal of the project is the construction of a fully automated production cell for joining metal with TP-FRPC including quality assurance for the automotive industry. The most important requirements to be fulfilled by the production cell are: weight neutrality, cost efficiency, time efficiency and bonding strength. With state of the art joining solutions, these requirements can only be partially met. Most joining methods such as adhesive, screw or bolt connections require additional weight. Adhesives are also expensive, while screw and bolt connections weaken the TP-FRPC structurally and cause an unfavourable load condition.
For this reason the FlexHyJoin project combines laser joining (LJ) for complex, smaller geometries and induction joining (IJ) for larger geometries in a single prototype production cell. The advantage of these joining processes is that components can be joined without need of additional material. This saves weight (weight neutrality) and costs (cost efficiency). With LJ and IJ, the components are heated in a single contactless process that goes easy on the material and joint to each other by use of a pressure device (time efficiency). By means of laser structuring of the metallic surface, high connection strength can be achieved. The key for the automation is an online process control and quality assurance. Non-destructive testing (NDT) shall ensure a 100% inspection of the joints and thus an automated and comprehensive quality assurance.
The aim of the FlexHyJoin project was to set up a fully automated production cell for joining metal with TP-FRPC for the automotive application. The validation of the developed technologies was performed with a roof stiffener for a Fiat Panda. Laser-structured metal brackets are attached to a TP-FRPC roof stiffener located behind the B-pillar to facilitate the assembly to the car body.
Laser joining (LJ) and induction joining (IJ) were selected as suitable joining methods. LJ is more purposeful for smaller and more complex geometries, while IJ has advantages for larger geometries. The most relevant joining parameters affecting bonding strength were investigated in order to obtain the ideal parameters for the joining process and process time. In addition, clamping and pressing devices for both joining methods were developed which were specially designed for the demonstrator?s geometry. The temperature distribution in the materials to be connected and in the joining zone itself was optimized, and the joining path was introduced as a quality assurance feature. FlexHyJoin represents a solution for mass series application for IJ as well as the combination of LJ and IJ.
To improve the bonding strength, the surface of the metallic joining partners is textured using a single mode cw (continuous wave) fibre laser. This increases the joining surface and the mechanical clamping by undercuts.
Finite element (FE) simulations were performed and their results validated in order to analyse the joining processes and the bond strength, as well as the behaviour of the joint. Explicit thermal simulations were created to simulate the IJ and LJ processes.
Lock-in Thermography was selected as the quality assurance method. With the help of this non-destructive testing (NDT) method, the requirements for a complete and fully automated quality assurance of the joint were realized.
Prototype production cell:
All individually developed process components were integrated into a fully automated production cell for the joining of a thermoplastic roof stiffener with 3 metal brackets (left, right, central). An articulated 6-axis robot transports the respective material component to each process step and places them in the clamping devices. Various monitoring systems were implemented in the production cell. The individual processes are monitored either with thermal monitoring systems, such as pyrometers and thermal cameras, or highly precise measuring systems, e.g. displacement sensors.
Dissemination / Exploitation:
The project results were made available to a broad expert audience through numerous publications, presentations at issue-specific conferences and exhibition appearances. One highlight was the participation at the EU Industry Day in Brussels in February 2018, presenting the roof stiffener demonstrator part among various current Horizon 2020 research projects. In March, a re-engineered Fiat Panda body-in-white was exhibited during the world?s largest fair on composites (JEC) in Paris. During a first dissemination event at a medium-size conference on material innovations (MatX) in June in Nuremberg, both of the above mentioned exhibition pieces were shown. This was enhanced by oral presentations, a workshop on the future of hybrid materials and making the production cell come alive by a virtual reality system for all conference participants to try out. Furthermore, the FlexHyJoin technology was presented to industrial companies from the automotive industry at a dissemination event in Italy in October. The functionality of the prototype production cell was successfully demonstrated live during a project closing dissemination event open to the public in December 2018 at one of the consortium members? company site.\n\nFlexHyJoin makes it possible to realize weight-neutral hybrid metal and TP-FRPC joints with high bonding strength in a cost-efficient, time-efficient, and fully automated mass production process for automotive applications. As joining methods, LJ and IJ are combined to join laser-structured metal brackets with a TP-FRPC plastic part. Lock-in Thermography is used to perform a 100% NDT inspection of the joints.
This opens up new possibilities in the design and selection of materials in automobiles to advantageously combine specific material properties in order to save time, costs, waste and minimize CO2 emissions by saving weight.
The technology represents a broad application potential and can be transferred to a wide variety of geometries and almost all metal/thermoplastic material combinations. This favours a technology transfer to other applications and sectors, such as aviation, sports & recreation, energy, etc.
There already have been requests from industry companies to have their materials tested in the production cell, i.e. the cell is kept available to the consortium and their prospective customers for at least 3 years after the end of the project to perform tests. In this way other future applications are conceivable by using the FlexHyJoin technology.
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