Charakterisierung der Fügezone von laserbasiert gefügten Hybridverbunden aus teilkristallinen thermoplastischen Kunststoffen und Metallen

Das thermische Fügen ermöglicht die direkte Herstellung von Kunststoff-Metall-Verbunden ohne Verwendung von Zusatzwerkstoffen oder Fügehilfselementen und bietet damit ein hohes Potenzial für die industrielle Produktion von Großserienprodukten. Dabei stehen aufgrund ihrer Eigenschaftsprofile einerseits Aluminiumlegierungen und Stähle und andererseits teilkristallinen Kunststoffe im Mittelpunkt des Interesses. Der industrielle Einsatz dieses Fügeprozesses setzt allerdings eine fundierte Kenntnis über die entstehende Fügezone und die vorliegenden Abhängigkeiten von Werkstoff und Prozessgrößen voraus. Die Arbeit beschäftigt sich deshalb mit der grundlegenden Charakterisierung der Fügezone laserbasiert gefügter Hybridverbunde aus teilkristallinen thermoplastischen Kunststoffen mit Metallen am Überlappstoß, die Beschreibung der Interaktion zwischen Fügeprozess und Werkstoffen und die daraus resultierenden Effekte auf die Verbundeigenschaften.Thermal joining of polymers to metals enables a direct connection between both materials without the use of a filler material or joining element. In this work, the joining zone in laser-based joining between semi-crystalline plastics and metals was examined and evaluated regarding mechanical properties of the joint. In order to address a large number of industrial applications, aluminum (EN AW 6082) as well as steel (X5CrNi18-10) were used as metal joining partner and PA 6, PA 6.6 and PP were applied on the side of the plastic materials. At the beginning, the investigations on the joining zone were carried out on spot joints and a half-section setup. The characterization was based on experimental investigations and thermal simulation. A generalization of the joining zone was given by material-dependent isotherms. Further examinations addressed the processes of melting and solidification, bubble formation and melt flow within the joining zone. The results were correlated to the temperature distribution respectively material specific properties. Thereby, the melting layer is decisive for forming a joint between both materials and changed in morphology as well as material properties compared to the base material. The size of the melting layer was highly sensitive to the metal sheet thickness, the laser beam power and the melting interval of the thermoplastic joining partner. Regarding joint formation, the penetration of microscopic surface structures was investigated. Thereby, the temperature distribution plays a dominant role compared to the joining time and the increasing volume during phase transition solid-liquid supports the penetration too. The results were transferred to overlap joints and the joining zone was characterized depending on the energy input per unit length. On this basis, the mechanical properties of a PA 6-EN AW 6082 joint were investigated towards cohesive failure. An increasing energy input per unit length results in a reduced ductility in short-term testing and a decreased fatigue strength. Thereby, the heat treatment of the thermoplastic material led to secondary crystallization which was identified as primary influence for the change in mechanical properties and verified by heat treated control specimens.Das thermische Fügen ermöglicht die direkte Herstellung von Kunststoff-Metall-Verbunden ohne Verwendung von Zusatzstoffen oder Fügehilfselementen. Im Rahmen der Arbeit wurde die Fügezone zwischen teilkristallinen Kunststoffen und Metallen im laserbasierten Fügen beschrieben und der Einfluss auf die mechanischen Verbundeigenschaften ermittelt. Zur Abbildung eines breiten Anwendungsspektrums wurden die Untersuchungen anhand von Aluminium (EN AW 6082) und Stahl (X5CrNi18-10) sowie für PA 6, PA 6.6 und PP durchgeführt. Die Charakterisierung der Fügezone erfolgte an Punktverbindungen sowie unter Einsatz eines Halbschnittversuchsstandes. Auf Basis von experimentellen Untersuchungen sowie der thermischen Simulation konnte die Fügezone anhand charakteristischer Isothermen verallgemeinert werden. Darauf aufbauend wurden wesentliche Vorgänge innerhalb der Fügezone (Schmelzen/Erstarren, Blasenbildung, Strömung) erfasst und mit dem Temperaturfeld bzw. werkstoffspezifischen Eigenschaften verknüpft. Die Schmelzzone innerhalb des Kunststoffes ist dabei maßgeblich für die Verbundherstellung zwischen beiden Werkstoffen und weist gegenüber dem Grundwerkstoff eine veränderte Morphologie sowie modifizierte Materialeigenschaften auf. Die Größe der Schmelzzone zeigt eine hohe Sensitivität gegenüber der Materialstärke des metallischen Fügepartners, der Laserstrahlleistung sowie dem Schmelzintervall des Kunststoffes. Im Hinblick auf die Verbundentstehung wurde die Bedeutung der Temperaturverteilung gegenüber der Fügezeit zur Füllung der Oberflächenstrukturen sowie eine unterstützende Wirkung der Volumenzunahme im Phasenübergang fest-flüssig nachgewiesen. Die gewonnen Erkenntnissen wurden auf Überlappverbindungen übertragen und die Fügezone in Abhängigkeit der Streckenenergie charakterisiert. Darauf aufbauend erfolgte die Untersuchung der mechanischen Eigenschaften eines Verbundes aus PA 6 mit EN AW 6082 hinsichtlich des kohäsiven Versagensverhaltens. Steigende Streckenenergien führten zu einer verringerten Duktilität des Verbundes in der mechanischen Kurzzeitprüfung sowie zu einer nachteiligen Beeinflussung des Ermüdungsverhaltens. Als maßgeblicher Effekt wurde die Sekundärkristallisation des Kunststoffes identifiziert, die auf eine Wärmebehandlung der Schmelzzone im Fügeprozess zurückgeführt und anhand von Auslagerungsversuchen an Vergleichsproben verifiziert werden konnte

Local Shielding Gas Supply in Remote Laser Beam Welding

The use of shielding gases in laser beam welding is of particular interest for materials interacting with ambient oxygen, e.g., copper, titanium or high-alloy steels. These materials are often processed by remote laser beam welding where short welds (e.g., up to 40 mm seam length) are commonly used. Such setups prevent gas nozzles from being carried along on the optics due to the scanner application and a small area needs to be served locally with inert gas. The article provides systematic investigations into the interaction of laser beam processes and parameters of inert gas supply based on a modular flat jet nozzle. Based on the characterization of the developed nozzle by means of high-speed Schlieren imaging and constant temperature anemometry, investigations with heat conduction welding and deep penetration welding were performed. Bead-on-plate welds were carried out on stainless steel AISI 304 for this purpose using a disc laser and a remote welding system. Argon was used as shielding gas. The interaction between Reynolds number, geometrical parameters and welding/flow direction was considered. The findings were proved by transferring the results to a complex weld seam geometry (C-shape)

Transient Arc Characteristic of a Commutation Switch Utilizing High Velocity Contact Separation

Commutation circuits are commonly used for low on state resistance and high current interruption capability if this can not be achieved by a single device or single current switching path. In case of fault detection, a very fast commutation of the current from the low-impedance main current path into the parallel high-current interrupting path is necessary. For single usage applications a low-cost approach is the utilization of a pyrotechnical switch in the low-impedance path. Compared to other electromechanical switches, those switches provide very high velocity of contact separation and thus a fast arc voltage rise with short commutation times. Here, measurements of the contact movement of a pyrotechnical switch were carried out using optical high speed imaging and an arc elongation up to 100ms−1 was calculated. From this, transient arc characteristics were measured in a simplified commutation network during the period until current zero in the low-impedance main current path

Effect of local gas flow in full penetration laser beam welding with high welding speeds

Spatter formation is a major issue in deep penetration welding with solid-state lasers at high welding speeds above 8 m/min. In order to limit spatter formation, the use of local gas flows represents a technically feasible solution. By using the gas flow, the pressure balance inside the keyhole, and therefore the keyhole stability, is affected. Existing investigations demonstrate a reduction in spatter and pore formation for partial penetration welding up to a welding speed of 5 m/min. However, the effect of the gas flow is not yet clarified for full penetration welding at welding speeds above 8 m/min. By using a precisely adjustable shielding gas supply, the effect of a local gas flow of argon was characterized by welding stainless steel AISI304 (1.4301/X5CrNi18-10). The influence of the gas flow on the melt pool dynamics and spatter formation was recorded by means of high-speed videography and subsequently analyzed by image processing. Schlieren videography was used to visualize the forming flow flied. By the use of the gas, a change in melt pool dynamics and gas flow conditions was observed, correlating to a reduction in loss of mass up to 70%. Based on the investigations, a model of the acting effect mechanism was given

Feasibility study of using integrated fiber optical sensors to monitor laser-assisted metal-polymer joining

The possibilities and challenges of using fiber optical sensors to monitor the laser-assisted joining of metal-polymer joints have been described in this article. Fundamental investigation proves the basic suitability of the measuring method for this application and studies the effect of essential influencing variables of the joining process - e.g., the clamping force - on the resulting sensor signals. In addition, the strain state (because of the process temperature and shrinkage of the polymer) of the parts to be joined can be traced as a function of the joining partners, the process parameters, and the material thicknesses. It is shown that the fiber optical method is suitable for process monitoring directly in the joining zone of metal-polymer hybrids and providing a tool for detailed strain measurements in the joint zone during subsequent component testing

Gap and force adjustment during laser beam welding by means of a closed-loop control utilizing fixture-integrated sensors and actuators

The development of adaptive and intelligent clamping devices allows for the reduction of rejects and defects based on weld discontinuities in laser-beam welding. The utilization of fixture-integrated sensors and actuators is a new approach, realizing adaptive clamping devices that enable in-process data acquisition and a time-dependent adjustment of process conditions and workpiece position by means of a closed-loop control. The present work focused on sensor and actuator integration for an adaptive clamping device utilized for laser-beam welding in a butt-joint configuration, in which the position and acting forces of the sheets to be welded can be adjusted during the process (studied welding speeds: 1 m/min, 5 m/min). Therefore, a novel clamping system was designed allowing for the integration of inductive probes and force cells for obtaining time-dependent data of the joint gap and resulting forces during welding due to the displacement of the sheets. A novel automation engineering concept allowed the communication between different sensors, actuators and the laser-beam welding setup based on an EtherCAT bus. The subsequent development of a position control and a force control and their combination was operated with a real time PC as master in the bus system and proved the feasibility of the approach based on proportional controllers. Finally, the scalability regarding higher welding speeds was demonstrated