Роботизирани процеси за автоматско полагање на композитни влакна/ленти (AFP/ATL) при производство на делови од композитни материјали

Abstract

Automated fiber placement and automated tape laying (AFP/ATL) with laser assisted heating (LAFP/LATL) is an attractive manufacturing technology for the development of lightweight and high performance components, primarily for the aerospace, automotive, military and many other dominant industries worldwide. The products are commonly manufactured from a material called prepreg, which consists of carbon fibers impregnated with polymer resins. The process comprises the automated laying of fiber reinforced thermoplastic tapes to formation of a layer which merges with the previous one until the build up a final laminate structure. For the automated laying of the prepreg tape or fibers, a robotic installation is used for precise laying of the tape along predefined paths. The investigation in the frame of this doctoral thesis focuses on the in situ obtaining high performance composite parts based on thermoplastic prereg, without additional processing. For this purpose, the technological parameters of the process, the interlaminar bonding of the layers in the laminate structure as well as the properties of the prepreg materials and the obtained composite structure have been analyzed. This was done through a combination of experimental work and design of the experiments and with optical and scanning electronic microscopy (SEM) to study the microstructure of prepreg materials and laminates and their relationship to process parameters. For the investigation, three types of unidirectional prepreg materials based on carbon fibers and thermoplastic polymer matrices PPS and PEEK have been used. The laminate plates from the three types prepreg have been produced by ATL process and for the determining of the influence of some technological parameters, the design of experiments have been applied. The full factorial experimental design has been applied and base on that eight combinations of laminated plates were produced separately from the three types prepreg, with variation of the three most influenced factors: processing temperature, laser placement angle and compacion pressure of the roller. For all produced specimens, the flexural strength have been tested and on the basis of the received experimental data it was created the regression equitations which the best describes the processes. For all three types of laminate plates, it has been found that the processing temperature and the compaction pressure of the roller significantly influence on the flexural strength of the laminate plates. The influence of the lasser placement angle affects insignificantly. There is an interaction between all factors that also affects the process but with smaller negligible effect on the flexural strength. Based on the calculation made for the adequacy of the researched process, it has been concluded that the regression equations are adequate and describe the process quite well. The change of the processing conditions makes a change in the mechanical characteristics of the laminate plates, as a result of a well achieved interlaminar bonding between the layers in the laminate. Namely, the samples obtained at the highest investigated processing temperatures and compact roller pressure have the highest values for flexural strength. Also, the samples obtained at the highest compaction roller pressure and the lowest processing temperature and the laser placement angle of 25° have good flexural strength. The samples obtained at the lowest compaction roller pressure and processing temperature have shown the lowest value of flexural strength. Microscopic analysis of the samples with the highest values of flexural strength have shown a good interlaminar bonding between the layers and a small percentage of pores compared to the samples with the lowest values of flexural strength where the pore content is higher. In the frame of this doctoral dissertation, laminate plates have been produced with the AFP process on the best processing conditions obtained from the ATL process. From the flexural strength testing it has been found that they have lower values of about 25% compared to those obtained under the same conditions with the ATL process. Microscopic analyses have shown the presence of higher pore content and weaker interlaminar bonding between the layers. It has been concluded that the AFP procedure can be successfully applied to obtain small and asymmetrical parts, where the ATL prosesse is inappropriate due to the complex geometry of the parts. Finally, in order to show the advantage of automated processes in terms of accuracy, productivity and obtaining high performance products, laminated plates with manual laying and compression molding have been produced, which are with lower quality and with defects in their structure.