A combined optical-thermal model for laser-assisted fibre placement of thermoplastic composite materials

Abstract

Automated fibre placement of thermoplastic composites has potential to produce high performance composite structures in a fast, clean, automated, out-of-autoclave process. Structures are manufactured in an additive fashion by depositing pre-impregnated thermoplastic composite plies layer-by-layer, with a placement head on an articulated robot for example. During placement, the plies are welded together in situ, which is achieved by melting the surfaces of the plies to be bonded with application of heat. Previously investigated heat sources include convective approaches such as hot gas torches and direct flames, conductive approaches in the form of hot shoes and rollers, or radiative approaches in the case of a lasers and infra-red lamps. In order to maximise the placement rate for productivity, the heat input must be increased correspondingly so as to maintain the temperatures necessary for bond development. Recent developments in near-infrared diode laser technology have resulted in new heat sources for the process with unsurpassed output power, efficiency, reliability and uniformity. Remotely mounted sources deliver the laser via light cable to compact optics modules on the placement head. The optics can produce large rectangular spots, ideal for uniform and progressive heating of the composite. This thesis investigates the near-infrared laser-assisted thermoplastic fibre placement process in detail for carbon fibre/PEEK composites. Topics covered include detailed studies of the optical interaction between the laser and the composite and the presence of a shadow, development of a combined optical-thermal model which provides robust predictive capability for the temperature history, as well as investigation of the evolution of inter-laminar bonding, void dynamics and crystallinity throughout the manufacturing process of a laminate. Placement trials were performed where samples were manufactured under conditions that reflect those for a practical manufacturing process. The effects of placement rate and process temperature were investigated. High process temperatures in excess of 500 degrees Celsius were found to produce the highest mechanical strengths, explained by higher rates of intimate contact development and molecular healing at elevated temperature. Void content and crystallinity predictions also suggest the best properties at high process temperatures of 600 degrees Celsius. The short time periods of high temperature exposure avoid significant degradation when processing at such high temperatures. The high cooling rates result in a highly amorphous bond interface during the initial placement pass, and results from this work show that significant bond development can occur for interface temperatures lower than 250 degrees Celsius. Bond strength predictions were performed using experimentally validated models from literature. Predictions were compared and contrasted with experimental results, as well as with currently proposed models. One of the currently proposed models was found to provide robust predictions over a wide range of process conditions. The resulting strength of the samples was also compared with those reported in literature for other heat sources. Results show that the near-infrared laser-assisted thermoplastic fibre placement process is capable of producing completely bonded samples at placement rates as high as 400 mm/s, four times higher than achieved by other heat sources previously reported in the literature. Show more