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- Laser direct joining; metal-polymer connections; surface texturing; transmission joining; (1)
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Remote ablation cutting with continuous wave laser radiation is a process commonly used for the surface pretreatment of large area metallic parts before joining of metal-polymer hybrid structures, due to their cost-effectiveness and high average powers when compared to short-pulsed lasers. The process requires high power-densities and multiple irradiations at high scanning speeds to achieve the desired kerf depth. In this work a fast modulated cw fiber laser is used to investigate the possibilities of increasing the material removal rate by periodically dropping the laser power and therefore modifying the recoil pressure at the vapor-liquid interface in the interaction zone to assist in melt expulsion. It was found that power modulation with frequencies in excess of 10 kHz can lead to an increased material removal rate when compared to continuous wave processing at comparable average and peak power levels, enabling higher kerf aspect ratios in a single irradiation.
2.0 μm fiber lasers provide a high beam quality and a high‐power output, which makes them ideal for welding and cutting a wide variety of commercially used plastics, as well as marking plastics, metals or even food. At 2.0 μm wavelength the intrinsic absorption of most thermoplastics is high enough to weld or cut without applying any absorbent additives or coatings. Compared to commonly used near‐infrared systems, welding with 2.0 μm lasers can improve heat distribution and gap bridging and enable transmission welding for challenging materials.
Fiber lasers are increasingly replacing the commonly used diode lasers in laser transmission welding of plastics for challenging applications due to their superior beam quality and the ability to use long working distances and small spot sizes. At the same time, these smaller spot sizes increase the risk of thermal degradation. In this work, we investigate different scanning strategies that allow controlling the weld seam widths. These strategies are characterized regarding the demands on the performance of the scanner system, the risk of thermal degradation and the mechanical properties of the resulting welds.
Laser direct joining of metals and plastics is a promising process for producing strong and reliable joints for lightweight hybrid structures without the need for adhesives, primers or mechanical fasteners. Commonly this process consists of two separate steps using at least two processing stations: the surface of the metal part is textured to achieve an increased surface roughness and improved mechanical interlock between the components. In another step, the thermoplastic components or the matrix of thermoplastic fiber reinforced composites are selectively heated to allow the melt to flow into the prepared metal structures. In this work, a new method is presented that combines highly efficient laser surface texturing of metallic parts and the selective laser heating of the joining zone in a transmission joining process using a single laser beam source running in different operation modes. This enables a very compact and cost-efficient processing station.
Residual stresses in welded parts can limit the mechanical performance and reliability of the components. In polymers, especially in amorphous thermoplastics, tensile residual stresses in the presence of certain media can lead to environmental stress cracking, which can result in catastrophic failure of the connection. In this work the mechanisms of stress generation in welding of plastics is compared to metallic materials and methods for evaluating residual stresses are discussed in terms of suitability to polymer materials. The application of solvents in liquid and vapor phase is shown to be a useful tool for intentionally inducing stress cracking in laser-welded parts to evaluate and compare residual stress levels and orientations in the weld zones. Appropriate process parameters are examined for reducing welding stresses.
The usage of laser transmission welding of polymers as a joining technique in industrial applications is often limited by the optical properties of the joining partners and the limited gap-bridging capability. By using alternative laser wavelengths or multiple beam sources it is possible to adjust the radiation to the properties of the materials and increase the weld seam quality. Especially for scattering materials, parts without specific laser absorbing additives and in applications where relatively large gaps occur due to manufacturing tolerances of the parts, adapted wavelengths can lead to improved results. In this work, the gap bridging during quasi-simultaneous laser transmission welding is investigated in welding experiments and thermomechanical FE-simulations. Usage of a laser wavelength in the range of 2.0 μm is found to be beneficial in terms of gap-bridging, compared to usually applied diode or solid state lasers emitting at about 1 μm due to the higher intrinsic absorption in unmodified thermoplastic materials. This leads to increased temperatures in the transparent joining partner and therefore greater thermal expansion. Furthermore the radiation shows an increased penetration depth in carbon black filled, laser absorbing materials, also leading to increased thermal expansion and improved gap bridging. The wavelength of 2.0 μm is also found to improve the strength of weld seams when turbid materials or materials with scattering additives are used as laser transparent parts, since scattering at small particles is strongly wavelength dependent, so the longer wavelength features better control of the resulting intensity distribution in the joining zone.
Laser transmission welding is an established joining technology for the creation of strong, hermetic and aesthetic weld seams between thermoplastic parts. However, weld seam properties are strongly dependent on the optical properties of the materials involved. This paper investigates the wavelength-dependent absorption properties of polymeric materials and carbon black, their influence on temperature field generation and the resulting melt pool geometry in laser transmission welding. A FE simulation model is developed to examine the possibilities of influencing the temperature fields during contour and quasi-simultaneous laser transmission welding by adapting the wavelengths under consideration of the absorption and scattering properties. The application of laser wavelengths in the spectral range of 1400 nm to 2000 nm leads to modified temperature fields and melt pool geometries, which are expected to feature a better load-bearing capacity and a much improved gap-bridging capability.
Laser transmission welding is widely used for the creation of strong, hermetic and optically appealing joints between thermoplastic parts. Amorphous polymers like polycarbonate and poly(methyl methacrylate) offer excellent optical properties, making them suitable for this joining technique and especially attractive for lighting and decorative applications, for example in the automotive industry. However, the rapid and non-uniform temperature changes during the process can lead to residual stresses, which can impair weld strength and result in part failure due to environmental stress cracking. In this work it was found, that the application of 2 µm laser radiation can lead to reduced stress-cracking-susceptibility compared to conventional laser transmission welding with wavelengths of ~1 µm.
Laser surface texturing of metals is usually performed using pulsed laser sources. Short and ultrashort pulsed laser systems offer the highest machining quality and processing flexibility. Most of these beam sources however feature relatively low average powers or high system prices. Continuous wave fiber lasers in comparison feature high average powers at moderate costs. In this work the laser power of a continuous-wave fiber laser is rapidly modulated to investigate the possibilities to improve laser surface texturing processes by periodically changing the interaction between the surface tension in the melt pool and the vapor pressure created during high power laser processing. It was found that the intermittent nature of modulated continuous wave laser radiation can improve the melt expulsion from the processing area, leading to high material removal rates and ablation efficiencies while also limiting the heat input into the substrate material.
Das Laser-Direktfügen von Metallen mit Polymeren beruht auf zwei wesentlichen Teilprozessen: Der Oberflächenstrukturierung des metallischen Fügepartners und dem Aufschmelzen des thermoplastischen Fügepartners an der Grenzfläche. Bisher werden dafür meist zwei verschiedene Werkzeuge verwendet. Kurzgepulste Laserstrahlung für die Oberflächenstrukturierung und kontinuierliche Laserstrahlung für den Fügeprozess. In einem Forschungsvorhaben der Technischen Hochschule Nürnberg mit der Evosys Laser GmbH wurde ein neuer Ansatz entwickelt, bei dem beide Teilprozesse mit derselben cw-Laserstrahlquelle durchgeführt werden, wobei ein besonders effizienter Strukturierprozess durch eine schnelle Leistungsmodulation des Lasers realisiert wurde. (cw: continuous wave; deutsch: ununterbrochene Welle, Dauerstrichlaser)