Laser structuring for out of autoclave repair
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The application of robot-based laser process opens the possibility of true-to-contour scarfing for the repair preparation on free-form surfaces of large 3-D CFRP parts. The subsequent out of autoclave repair process enables the restoration of initial contour and surface conditions of CFRP parts, as demonstrated by TU Clausthal and Laser Zentrum Hannover.
Laser Scarfing of Two-dimensional Parts
The use of lasers is a promising method of manufacturing scarfs as an alternative to the commonly used hand-guided grinding or milling machines. Laser scarfing offers the advantages of an automatable and non-contact method, especially for the application on thin-walled and fragile parts. In order to extend the working area and to reach the full 3-D abilities of the applied optics, for the presented experimental studies, a 3-D scanner was mounted on an industrial robot, which allows scarfing of any size. For the implementation of the scarfing, the required geometry will be divided into several consecutive segments. Here it should be noted that inappropriate processing strategies in the transition area can cause grooves and bumps in the ablated surface structure. Due to the new developed process strategies for robot-based processing, it is possible to carry out scarfing of large surfaces with no apparent transitions between the individual segments. Ablation rates up to 40 mm3/s could be implemented for the investigated CFRP plate.
For the quality control of the scarfed structure, optical methods are particularly suitable. Using laser line scanners, it is possible to generate a three-dimensional profile of the processed surface and thus measure the achieved ablation depths. Other methods, in particular the optical coherence tomography as well as the optical detection of the fiber layer orientation, are suitable for automated control of the scarfing process. Both methods can be integrated into the CFRP laser machining processes.
Laser Scarfing of 3-D Parts
Out of Autoclave Method
The ablation or scarfing of the damage section is the beginning of the repair process chain. In the next step, the material-free section must be filled with new material. The challenge, especially in the automotive industries, is to repair the CFRP parts by using the original material and taking the layer structure into consideration, in order to reach the initial mechanical strength of the parts, and to restore the initial surface structure without any rework required.
CFRP parts made of prepreg material are commonly manufactured using autoclaves. However, the autoclave method is hardly suitable for repair, due to relatively low process flexibility, e.g. the need for an autoclave and, high pressure-resistant forming tools and high energy consumption. In the case of repair, the whole part, regardless of the size of the area to be repaired, will be placed in the autoclave and thus the undamaged area of the part will be unnecessarily exposed to heat.
The better alternative for repairs with prepreg material is the so-called out of autoclave method. As the name suggests, an autoclave is not required in this method. The application of the required pressure and heat for the consolidation and the curing of the prepreg material on the part will be limited only to the area in need of repair, resulting in lower repair-related costs and energy consumption in the out of autoclave process, compared to the autoclave process. However, the out of autoclave method works in a vacuum environment, which under certain circumstances encourages blistering. For this reason, the preparation and handling of the prepreg material are the decisive factors in the out of autoclave process for the success of the repair works.
Precompression of the Prepreg
The application of pressure and heat will be limited only to the area to be repaired.
In the next step, the re-impregnated prepreg is compressed, using a heat press to reach the required single layer thickness. In the process, the excess resin is pressed out of the fiber bundles of the woven fabric and fills in the gaps in the woven fabric. It should be noted that the failure to achieve the minimum required layer thickness leads to a resin deficiency, which can eventually cause an only partially filled section in the repair process. Furthermore, it must be ensured that the resin stays low-viscous during the compression process in order to avoid air pockets. After the compression, the compressed prepreg must be rapidly cooled off while it is still in the press, in order to maintain the reconditioned state of the prepreg, which closely resembles the end state of the out of autoclave process. After completion, the precompressed prepreg can be used in the repair process.
The range of process parameters for an optimum precompression depends mainly on the resin system used in the prepreg. The knowledge regarding the curing behavior of the resin system is decisive for the determination of the optimum warming temperature and time. Reaction kinetic analysis with Differential Scanning Calorimetry (DSC) and rheological analysis can deliver the needed information on the curing behavior of the resin system, such as temperature and time dependency of the curing degree, residual reactivity and gel point of the resin system, respectively.
The selected warming time and temperature can influence the precompression process. At a short warming duration and low temperature, the resin will not be low-viscous enough for re-impregnation and the subsequent compression of the prepreg. On the other side, high warming temperatures could increase the risk of premature curing of the resin, rendering compression impossible. It is also necessary that the prepreg is rapidly cooled off after compression and stored in a cool place until further processing in order to inhibit the curing reaction. It should be noted that the thermal load during precompression narrows the time frame in which the precompressed prepreg can be processed.
An area with the same contour as the damaged part can serve as a sample mold.
Manufacturing of the Forming Tool
For the repair of 3-D parts, such as the generic demonstrator, Figure 2, a forming tool or a pressure element is required to restore the contour and the surface structure of the repaired area to its original state.
The surface structure of the original and repaired parts are nearly indistinguishable.
The actual repair work begins with the preparation of the repair patches cut from the precompressed prepreg. The shape and fiber direction of each repair patch is oriented according to the position of the corresponding patch in the section to be repaired or filled. The patches will be placed in the section to be repaired as a single layer or a stack, depending on the complexity of the contour, followed by the forming tool in the top layer. Subsequently, the section to be repaired, along with the repair patches and the forming tool, is sealed together in a vacuum setup. For a through repair section, the section must be vacuum-sealed on both sides. The vacuum setup and the forming tool will be removed after curing under vacuum and heat.
The out of autoclave method is preferred for repairs due to its process flexibility, and the application of an original autoclave prepreg in this method is possible after a specific precompression process. The flexibility of the repair method is distinguished by the application of a CFRP forming tool made from a precompressed prepreg. It allows for the recoconstruction of nearly any contour and surface structure of the original part in a single process step and without additional surface treatment.
Laser scarfing requires a so-called galvanometer scanner. These scanners project the laser beam onto the workpiece by means of two mirrors. By filling the surface to be processed with lines arranged at a defined distance, the so-called hatch distance, the geometry to be processed is moved over its entire surface with a laser beam and the material is ablated. A defined ablation depth can be achieved by traversing the geometry several times. The size of the work field depends on the applied focusing optics. By adjusting the process parameters, it is possible, depending on the material used, to create defined structural geometries and depths as preparation for the following repair steps.
Prepreg Method and Material
A prepreg, or pre-impregnated fiber material, is commonly used in the autoclave process. In the manufacturing of the autoclave prepreg, the resin-fiber ratio will be set in order to reach the required thickness and the fiber volume content, respectively, under autoclave pressure (commonly 6 to 7 bar) and in a specific curing temperature range. Another variation of prepreg processing is the so called out of autoclave method. As the name suggests, no autoclave is required for the compression of the prepreg material in this method. The required laminate thickness and fiber volume content respectively must be achieved at a compression pressure of 1 bar vacuum maximum.
The sub-project “Development of repair concepts for laser-scarfed CFRP parts” (project number: 13N12758) in the joint research project HolQueSt3D (3-D high performance laser processing for quality and throughput increase and for reliable, automated manufacturing of CFRP lightweight structures) was funded by the German Federal Ministry of Education and Research (BMBF) and supervised by VDI Technologiezentrum GmbH.
The project partners were TU Clausthal, Laser Zentrum Hannover, Volkswagen, Trumpf Laser, Jenoptik Katasorb, KMS Automation und Invent.