On the consolidation quality in laser assisted fiber placement: The role of the heating phase

Laser assisted fiber placement (LAFP) is a promising additive manufacturing technique for large aerospace structures. A thermoplastic composite prepreg tape is heated using a laser and subsequently bonded to previously placed plies under pressure applied by a roller, thereby building a part ply by ply. LAFP has the potential for in-situ consolidation, which allows omission of an expensive post-consolidation step. However, the achieved consolidation quality, especially the interlaminar void content, does not meet the standards of the aerospace industry yet. This thesis aims to investigate the effect of the heating phase on the consolidation state of the incoming tape and its consequences on the governing mechanisms during consolidation. For this purpose, an experimental research has been performed to analyze the state of the tape during the process, the development of intimate contact and the deformation of the tape during LAFP.

The experimental research showed that the incoming tape can deconsolidate during the heating phase, resulting in a rough and fiber-rich surface. This was found to affect the subsequent interlaminar bond development as the matrix-poor surface of the tape hinders intimate contact development. Matrix flow is required to wet the surface of the tape in order to allow bonding. A novel intimate contact development model was developed based on percolation flow of matrix.

Intimate contact development is also found to be hindered by the occurrence of laps and gaps in the lay-up, which are the result of spreading of the tape during placement. Therefore, an experimental analysis was performed to investigate tape spreading. The results show that the deformation is, for the range of settings investigated, not dependent on the applied pressure, but only on the tape temperature. A quasi static tape spreading model has been developed to explain these observations.
The experimental and modeling results in this thesis help to better understand the consolidation process during LAFP. This knowledge was used to develop a modeling framework which forms a valuable tool to further improvement of the tape material and optimization of the LAFP process.