The aviation market is rebounding post-COVID, driving the demand for lightweight materials to reduce fuel consumption and CO2 emissions. However, machining Carbon Fiber-Reinforced Plastic (CFRP) is challenging and costly. Microdrilling (⪅1 mm diameter) for acoustic linings, consisting of CFRP skins in a sandwich structure, is widely requested. Laser drilling offers advantages such as smaller hole diameters and wear-free machining. To scale up laser microdrilling, process efficiency and heat control are crucial. This study conducted a thermal evaluation using a short pulse laser and thermal camera. The temperature curves were evaluated taking into account results obtained from studies based on a layout using design of experiments.
Laser Transmission Welding (LTW) is a well-known technique for joining conventionally manufactured high-volume thermoplastic parts, such as automotive injection molded parts. When LTW is used for additively manufactured parts (typically prototypes, low-volume production, or one-offs), the technology must be developed to overcome the difficulties in welding the parts, that result from the additive manufacturing process itself. Compared to injection molding, additive manufacturing results in an inhomogeneous structure with entrapped air within the volume. Therefore, there is a change in the transmissivity behavior in the weld area due to the additive manufacturing process. In order to make LTW available for additively manufactured thermoplastic components, a process chain was developed to support manufacturing. This process chain ranges from the optimization of the additive manufacturing process to the welding process and is supported by an expert system. For the evaluation of the manufacturing process chain, welding experiments with additively manufactured samples were performed. The transparent samples were welded to black samples with varying process parameters in overlap configuration and tensile shear tests were performed. The additive manufacturing process parameters were used to predict the transmittance of the transparent sample and the weld seam strength of welded parts using the expert system.
New lightweight construction concepts should make it possible to reduce the weight of aircrafts and vehicles. This has an effect on energy consumption, in order to reduce CO2 emissions. New material combinations are used to implement these concepts, or in some cases, plastics substitute metals. The production of single-variety plastic components is particularly advantageous, as these can be returned into a recycling cycle. Thermoplastics are suitable for the production of such composites. If such parts are joined by means of adhesive bonding, the adhesive would remain on the components and prevent sorted recycling. Welding does not have this disadvantage. Wide weld seams are required for the transmission of high forces. Applying laser welding, wide weld seams could be generated using conventional beam shaping techniques using a homogeneous intensity distribution. However, such an intensity distribution is critical if the component geometry has small radii. To solve this problem, a welding system was developed as part of the MultiSpot project, which makes it possible to adapt the intensity distribution to the weld path. For the evaluation of the new welding system, different intensity distribution profiles were developed based on bead on plate welds and then transferred to the demonstrator components.
In the aviation industry, carbon fiber reinforced plastics (CFRP) are used as a standard material. Due to the high strength-to-weight ratio, weight can be saved and the fuel consumption, for example of airplanes or cars, can be reduced. More and more thermoplastics are used as matrix materials, because they enable new production and repair processes. In order to reduce repair costs, an automated, reliable and fast process is needed. For the repair of CFRP with polyamide 6 (PA6) matrix material, a laser-based ablation process for the removal of the damaged material and a laser welding process for the refill of the scarf with a patch are being developed. The ablation of the scarf and the cutting of the patch are conducted with a high power nanosecond pulsed laser, which has a maximum power of PL = 1500 W and emits at a wavelength of λ = 1030 nm. For the joining process, an automatically controlled heat conduction welding process is developed. Therefore, a diode laser with a maximum average power of PL = 300 W and a wavelength of λ = 940 nm is utilized. For the analysis of the ablation and cutting process, samples were analyzed in order to determine the heat affected zone. For the evaluation of the welding process, overlap samples were welded and tested to determine the weld seam strength. In addition, cross sections were prepared and analyzed for defects. Finally, the results were correlated in order to determine a high process quality.
Today, parts based on fiber reinforced thermoplastics are used in many different applications in the aerospace and automotive industry. Furthermore, the number of parts made of semicrystalline polyphenylene sulfide (PPS) or polymers belonging to the polyaryletherketone (PAEK) family are increasing due to their excellent chemical and mechanical resistance. For some applications, these parts have to be joined to more complex structures. Besides mechanical fastening or adhesive bonding, laser transmission welding can be used when one of the joining members consists of a natural or glass fiber reinforced thermoplastic. The transmissivity of the joining members for the wavelength of the laser has an influence on the welding parameters. Often, diode lasers are used emitting in a wavelength range from 808nm to 980nm. At this wavelength range, PPS and PAEK have a lower transmissivity than for example at 1530nm. Therefore, a change of the used wavelength should affect the welding process. In order to determine this influence, a study was conducted comparing the welding process with diode lasers emitting at 940nm and 1530nm focusing on welding times. In these investigations, the joining members were made of glass fiber reinforced PPS and carbon fiber reinforced PPS.
Laser transmission welding (LTW) is a known technique to join conventionally produced high volume thermoplastic parts, e.g. injected molded parts for the automotive sector. For using LTW for additively manufactured parts (usually prototypes, small series, or one-off products), this technique has to be evolved to overcome the difficulties in the part composition resulted in the additive manufacturing process itself. In comparison to the injection molding process, the additive manufacturing process leads to an inhomogeneous structure with trapped air inside the volume. Therefore, a change in the transmissivity results due to the additive manufacturing process. In this paper, a method is presented to enhance the weld seam quality of laser welded additively manufactured parts assisted by a neural network-based expert system. The designed expert system supports the user setting up the additive manufacturing process. With the results of a preliminary work, a neural network is trained to predict the transmissivity values of the transparent samples. To validate the expert system, specimen of transparent polylactide are additively manufactured with various manufacturing parameters in order to change the transmissivity. The transmissivity of the parts are measured with a spectroscope. The parameters of the additive manufacturing process are used to predict the transmissivity with the neural network and are compared to the measurements. The transparent samples are welded to black polylactide samples with different laser power in overlap configuration and shear tensile tests are performed. With these experiments, the prediction of additive manufacturing parameters with the expert system in order to use the parts for a LTW process is demonstrated.
The global demand for air travel and air transport is expected to increase again in the next couple of years and so the environmental protection will also increasingly come into focus again. In the aviation sector, this means not only saving fuel and reducing emissions but also reducing the noise pollution caused by aircrafts. A typical method for noise reduction is the use of acoustic liners for sound insulation. Among different designs, acoustic liners can consist of sandwich panels, with one perforated, micro-drilled skin layer, a honeycomb structure and a closed rear layer. Wherever the operating conditions allow, the skin layers are made of carbon fiber reinforced plastics (CFRP), due to weight reasons. Compared to conventional drilling methods for CFRP, laser drilling offers unique benefits such as significantly smaller achievable bore diameters, wear free cutting and flexibility in bore diameter. However, for a large-scale application of laser micro drilling, the process efficiency must be increased and a process control is necessary to avoid damage due to excessive heat input. In this investigation, a process control method based on thermography is presented and evaluated. The control mechanism uses the temperature course in the drilling area to decide whether the process can be terminated in order to avoid time losses and unnecessary heat input. This method was found to be very reliable, however, the synchronization between temperature recording and laser irradiation and the data interpretation need further improvement.
Carbon fibre reinforced plastics (CFRP), particularly with a thermoplastic matrix material, have increasingly been used in the last decades. This is especially true in industrial sectors with a strong focus on lightweight applications such as the aviation industry. During the production of CFRP parts imperfections can occur resulting in the need of rework. Furthermore, a damage can occur in service time. In both cases, a large amount of carbon fibres and matrix material has to be mechanically removed, which comes along with high tool wear. Afterwards, the area has to be refilled. This is done by adhesive bonding of CFRP patches. Normal adhesives have long curing times of several hours. To enhance the repair process of thermoplastic CFRP, a two step laser based process was developed. In the first step, CFRP is removed by laser ablation, which allows a high reproducibility and accuracy. Goal is to generate a flat surface with a defined matrix amount. In the second step, laser heat conduction welding is used to refill the removed area with thermoplastic patches. This study was conducted with a carbon fiber fabric within a polyphenylene sulfide matrix. In order to develop a high quality heat conduction process, the ablation process was optimized to generate a defined surface. For the evaluation of the welding process, lap shear samples were welded with different setups. These samples were tested and fraction pattern evaluated.
An adequate use of finite resources is one of the greatest challenges of our times. To address this, lightweight concepts based on continuously fiber reinforced composites (FRC) are already being adapted for the transportation industry, especially within the automotive and the aerospace sectors. In order to broaden the use of lightweight composite structures and components, suitable processing, monitoring and control techniques are required for a variety of materials, constituting a prerequisite for economic, flexible and automated high volume production. In this regard, photonic technologies can provide valuable solutions. In this presentation, the latest developments within the field of FRC laser machining are summarized. For the processing of large structures such as resin transfer molding parts, combinations of galvo scanners with robots or axis systems are of particular interest. For this purpose, both high brightness cw fiber lasers and pulsed systems are used. Within the repair chain for valuable FRC parts, pulsed UV and NIR lasers are used for the precise removal of fiber layers in order to generate a defined scarfing. For both applications, disintegration of the fiber matrix interconnection due to thermal impact has to be avoided. Thermoplastic composites are becoming increasingly important for many industrial applications. In contrast to thermoset systems, welding techniques are particularly applicable. In this context, laser welding is not limited to the joining of transparent-absorbing-combinations, as it is required for conventional laser transmission welding processes but can be extended to the welding of structural parts consisting of high-performance carbon fiber reinforcements.
Today, fiber reinforced materials are present in a wide field of industrial applications. Short glass fiber reinforced composites are mainly used in automotive, aerospace and medical sectors. In recent years, endless fiber reinforced thermoplastics have gained importance as construction material, especially for lightweight assembly. There are different methods of joining thermoplastic materials such as vibration, resistance and induction welding. Another process is laser transmission welding, which can be characterized by its excellent reproducibility, high flexibility and potential for automation. Typically, laser transmission welding can be applied for joining unreinforced or glass fiber reinforced thermoplastic parts. This welding process was now adapted to heat conduction welding for joining thermoplastic CFRP to itself. The goal of these investigations was to determine the influence of the matrix material on the weld seam quality. The experiments were conducted with a carbon fiber fabric reinforced polyphenylene sulfide with natural matrix material as well as containing carbon black. In the first step, the temperature distribution at the upper joining member, where the heat generation occurred, was evaluated. The heat affected width was determined and correlated to the process temperatures in order to develop a process model. In a next step, lap shear samples were welded and tested. These results were then correlated with previous results.
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