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, Volume 11, Issue 3, pp 36–41 | Cite as

Laser structuring for out of autoclave repair

  • Widyanto Surjoseputro
  • Sven Blümel
  • Peter Jäschke
  • Dieter Meiners
Materials CFRP Repair
  • 170 Downloads

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 repair of composite parts can be considered being already optimally implemented industrially, but at the same time it is still part of an ongoing research. The main goal is the implementation of the highly automated repair process by trained personnel with regard to the repair preparation, involving methods of defect removal and also the implementation of the repair process itself.
Figure 1

Laser-scarfed CFRP-structure with the dimensions 180 mm × 350 mm × 2.4 mm, in four steps of 0.4 mm depth each (© LZH)

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

Only in very few cases, the parts in need of repair are limited to a solely two-dimensional geometry. Therefore, it is necessary to ensure the adaptability of the developed laser-based scarfing and actual repair method for three-dimensional free-form surfaces. A demonstrator, Figure 2, which was also set up for the development of the cutting process with the same equipment, was used for the development of the required process adjustments and evaluation of the method. For the adaptation of the laser process, it is necessary to know the surface to be processed and to adjust the scanning program, so that the focus of the laser always stays on the surface of the material. This requires an exact positioning of the scanner relative to the location of the part within the scanner program. A so-called 3-D scanner allows for the adjustment of the focus position without repositioning the scanner itself, which simplifies the scarfing process enormously and accelerates it particularly in the processing of complex structures.
Figure 2

Laser-trimmed demonstrator part (left) and laser-scarfed free-form surface at the front of the demonstrator (right) (© LZH)

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

In its initial state, a prepreg material, despite the correct resin-fiber ratio, can show a non-homogenous resin distribution on the surface or a non-homogenous impregnation of the woven fibers, respectively, due to manufacturing reasons. The irregularities in an autoclave prepreg will be eliminated at the latest after the application of pressure and heat. However, the pressure applied in the out of autoclave method is not sufficient to optimally compress the autoclave prepreg, Figure 3 (left). Therefore, the autoclave prepreg needs to be precompressed before it can be applied in the repair process using the out of autoclave method. Through precompression, the prepreg will be conditioned to reach the required single layer thickness and resin amount for out of autoclave process conditions.
Figure 3

Micrographs of CFRP laminates, manufactured using the out of autoclave method with a non-precompressed (left) and a precompressed autoclave prepreg (right) (© PUK | TU Clausthal)

The application of pressure and heat will be limited only to the area to be repaired.

The precompression process of the autoclave prepreg and the state of the prepreg at each step of the precompression can be seen in Figure 4. The first precompression step is the warming of the prepreg in an oven or a heat press. In warming the prepreg, the viscosity of the resin, which is otherwise highly viscous at room temperature, will be decreased, leading to a slight expansion of the woven fabric. Promoted by the capillary effect and the low-viscous resin, a re-impregnation of the fiber bundles takes place during the expansion of the woven fabric and therefore the optimum impregnation of the fiber bundles can be ensured, Figure 3 (right).
Figure 4

Precompression process of the autoclave prepreg for the application in the out of autoclave process (© PUK | TU Clausthal)

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.

A forming tool made of CFRP is preferably used as a flexible repair method. The CFRP forming tool offers freedom of design and good thermal properties. With relatively little effort, the prepreg material adapts to nearly any contour of the part to be repaired, provided that a sample of the original contour is available. An original part or another area with the same contour as the the damaged part to be repaired can serve as a sample mold. The resin of the prepreg will optimally adapt to the surface structure or roughness of the original part or sample mold. The heat expansion in the forming tool and the original part is almost identical, since the original prepreg is being used. The CFRP forming tool is made using the out of autoclave method and the precompressed prepreg, Figure 5. The design of the prepreg lay-up for the forming tool must take into consideration that the thickness of the forming tool is sufficient to withstand the compression pressure of 1 bar maximum during the out of autoclave process.
Figure 5

Repair process of 3-D fiber composite parts using the out of autoclave process (© PUK | TU Clausthal)

The surface structure of the original and repaired parts are nearly indistinguishable.

Component Repair

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 result of the repair work on the generic demonstrator can be seen in Figure 6 (right). Due to the specific layout of the repair patch on the top layer, the appearance and the surface structure of the original and repaired parts are nearly indistinguishable.
Figure 6

Comparison between the original (left) and demonstrator repaired using the out of autoclave process (right) (© PUK | TU Clausthal)

Outlook

The preparation of repair work using lasers is distinguished by a high degree of automation at ablation rates comparable to the manual method. A further advantage of the laser process is the force-free and wear-free manufacturing especially of thin-walled and flexible parts. The application of adjusted optics and robots as a handling technology allows for industrial-scale laser-scarfing of large and complex contoured 3-D parts, Figure 7.
Figure 7

Process setup for the laser processing of 3-D CFRP parts (© LZH)

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

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.

Notes

Thanks

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.

Copyright information

© Springer Fachmedien Wiesbaden 2018

Authors and Affiliations

  • Widyanto Surjoseputro
    • 1
  • Sven Blümel
    • 2
  • Peter Jäschke
    • 2
  • Dieter Meiners
    • 3
  1. 1.Mahr Metering SystemsGöttingenGermany
  2. 2.Laser Zentrum Hannover e. V.Germany
  3. 3.TU ClausthalGermany

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