Klaus Dilger

Wirtschaftliches Fügen in Serie und Reparatur

Um beim strukturellen Kleben die Vorteile der elektromagnetischen Direkterwarmung im industriellen Prozess nutzen zu konnen, bedarf es einer Abstimmung der jeweils eingesetzten Klebstoffe auf den Ausharteprozess. Nachdem der erste Teil dieser Folge (s. adhasion 12/08, S. 39) einen Uberblick uber den derzeitigen Stand der Technik und das experimentelle Vorgehen vermittelte, wird im Folgenden beantwortet, auf welche Weise sich verschiedene strukturelle Klebstoffe in ihrer Schnellhartbarkeit unterscheiden.

Duroplastische CFK mit dem Laser vorbehandeln

In bisherigen Forschungsarbeiten zur Laserbearbeitung von unverstärkten und faserverstärkten Polymeren wurde der Einfluss verschiedener Wellenlängen vom UV-Bereich (248 nm /1, 2/ und 355 nm /3/) über den sichtbaren grünen Bereich (532 nm) /4/, den nahen IR-Bereich (1064 nm) /5/ bis zum mittleren IR (10600 nm) /6/ untersucht. Entscheidend für das Bearbeitungsund damit auch für das Klebergebnis nach der Vorbehandlung von Faserverbundwerkstoffen mittels Laserstrahlung ist die Art der Wechselwirkung zwischen Laser und Verbundwerkstoff. Diese hängt neben der Pulslänge, welche die Dauer der Wechselwirkung zwischen Laserstrahlung und Material bestimmt, im Wesentlichen von der Wellenlänge der verwendeten Strahlung ab. Letztere hat in zweierlei Hinsicht einen entscheidenden Einfluss. Zum einen bestimmt sie – bedingt durch die wellenlängenabhängige Absorption insbesondere des Matrixmaterials – die Eindringtiefe der Strahlung, bzw. ob die Wechselwirkung mit der Oberfläche der Matrix oder den darunterliegenden Fasern stattfindet. Zum anderen wird in der Literatur zwischen photochemischem und photothermischem Abtrag unterschieden /7, 8, 9/, die Anteile dieser Mechanismen sind wiederum wellenlängenabhängig. Photochemischer Abtrag findet dann statt, wenn die Energie eines oder weniger Photonen groß genug ist, um chemische Bindungen aufzubrechen. In einer Polymermatrix existieren hauptsächlich kovalente Bindungen zwischen den einzelnen Kohlenstoffatomen, deren Bindungsenergie in der Größenordnung von 5,0 eV liegt. Anhand der Kopplung zwischen Wellenlänge und Photonenenergie über das Planck śche Wirkungsquantum und die Lichtgeschwindigkeit (E=h ⋅ c/λ) ergibt sich die Wellenlänge eines Photons mit dieser Energie zu etwa 250 nm. Um direkt Bindungen innerhalb der Polymermatrix aufzubrechen, ist also Laserstrahlung im ultravioletten Wellenlängenbereich nötig /10, 11/.

Defect Detection in Friction Stir Welding by Online Infrared Thermography

Friction Stir Welding (FSW) is a complex process with several mutually interdependent parameters. A slight difference from known settings may lead to imperfections in the stirred zone. These inhomogeneities affect on the mechanical properties of the FSWed joints. In order to prevent the failure of the welded joint it is necessary to detect the most critical defects non-destructive. Especially critical defects are wormhole and lack of penetration (LOP), because of the difficulty of detection. Online thermography is used process-accompanying for defect detecting. A thermographic camera with a fixed position relating to the welding tool measures the heating-up and the cool down of the welding process. Lap joints with sound weld seam surfaces are manufactured and monitored. Different methods of evaluation of heat distribution and intensity profiles are introduced. It can be demonstrated, that it is possible to detect wormhole and lack of penetration as well as surface defects by analyzing the welding and the cooling process of friction stir welding by passive online thermography measurement. Effects of these defects on mechanical properties are shown by tensile testing.

High quality laser welding by reducing the ambient pressure

Modern solid-state lasers present a high potential for application in many fields. In addition to the high electrical efficiency, the beams can be transported via fiber-optic cables at a wavelength around 1 µm, thus enabling a simple integration of the laser beams into a processing machine. In practice, however, the potential cannot be fully exploited for applications requiring seams of the highest quality. This is due to the spatter formation increasing with rising welding speed and the tendency of dropping formation underneath the workpiece, getting stronger in the case of penetration welding with increasing sheet thickness. These seam failures influence both, the surface quality with regard to its technical functionality and the strength of the weld seam. However, weld seams with the highest quality requirements are in demand in a wide range of applications, for instance in power-train applications in automobile manufacture.This paper deals with laser beam weldings with solid-state lasers at reduced ambient pressure. By means of diagnostic investigations, the influence of the ambient pressure on the welding process is demonstrated with the aim of qualifying the procedure for industrial implementation. While in electron beam welding a fine vacuum of p < 10-3 hPa is normally used, in laser beam welding processes a very much lower reduction of the ambient pressure to approx. p = 101 hPa already effects a considerable quality improvement as regards spatter formation and seam-geometry properties, thus significantly minimizing the expenditure for pumps and plants compared to the electron beam welding.Lateral camera views show that in the vacuum, the metal vapor plume typical for welding with solid-state lasers and the formation of spatters can be significantly reduced. In the vacuum, the welding process also presents a higher stability against variations of penetration depths and is more robust to modifications of parameters. Because of these advantages, especially for applications with high requirements for seam quality and low spatter formation, the procedures will be introduced into the market in the next few years.Modern solid-state lasers present a high potential for application in many fields. In addition to the high electrical efficiency, the beams can be transported via fiber-optic cables at a wavelength around 1 µm, thus enabling a simple integration of the laser beams into a processing machine. In practice, however, the potential cannot be fully exploited for applications requiring seams of the highest quality. This is due to the spatter formation increasing with rising welding speed and the tendency of dropping formation underneath the workpiece, getting stronger in the case of penetration welding with increasing sheet thickness. These seam failures influence both, the surface quality with regard to its technical functionality and the strength of the weld seam. However, weld seams with the highest quality requirements are in demand in a wide range of applications, for instance in power-train applications in automobile manufacture.This paper deals with laser beam weldings with solid-state lasers at reduced am...