Analysis of the interactions between the morphology and the properties of weld seams during laser transmission welding of plastics by a three-dimensional, spatially re-solved determination of the crystallinity of the weld seam by means of Raman microscopy
Analytical Chemistry
Final Report Abstract
The aim of the research project was to determine the cause-effect relationships between the temperature field present in the weld seam, the resulting weld seam morphology and the mechanical properties of laser-welded components. One main focus was on the development and qualification of a new measurement method based on Raman microscopy for the spatially resolved determination of crystallinity. Equally important was the analysis of the influence of the morphology on the short-term and long-term properties of the components and weld seams. The basic objectives of the application were successfully achieved both on the Raman microscopy side and on the mechanical characterisation of polymer welds. For this purpose, an innovative Raman microscope was developed, which allows fast, large-area, locally finely resolved and polarisation-dependent investigations of (polymer) thin sections. Thus, complete weld seam thin sections with dimensions of approx. 1.3 x 2 mm could be measured with a resolution of 12 µm (> 17000 measuring points) polarisation-dependently in 2 hours despite admixed carbon black. For the subsequent evaluation of the recorded spectra, a method based on spectral reconstruction was developed, with the help of which false colour images of the locally resolved crystalline, amorphous and intermediate phase components could be generated. Through the further development of an FE model, simulations of the expected temperature curves and cooling rates in the weld seam could be carried out as a function of selected process parameters. The correlation between local cooling rate and polymer morphology was validated by Raman spectroscopy and confirmed by fine resolution DSC measurements. For this purpose, thin sections of previously welded polymer compounds with varying process parameters were examined. It was shown that the crystallinity of the weld seam is lower than the surrounding crystallinity in the laser-unaffected material due to melting and rapid cooling. Furthermore, it was shown that an increased line energy during the welding process leads to an increase in the crystallinity of the weld seam due to the increasing maximum temperature and the resulting lower cooling speed. Also, by means of Raman microscopy, a ring of high crystallinity could be resolved, which surrounds the melted area of the weld, independent of the process parameters. Analyses of the micro-mechanical properties of the welds showed a clear correlation between the locally prevailing morphology and the tensile strength of the examined specimens. It could be recognised that even a slight decrease in the local crystallinity leads to a significantly reduced tensile strength in this area. The work carried out clearly shows that Raman spectroscopy is an excellent tool for assessing polymer processing. This potential should also be exploited in future work, with the medium-term aim of using the technique in situ for in-process measurements.
Publications
- "Correlation between weld seam morphology and mechanical properties in laser transmission welding of polypropylene", Procedia CIRP 94, 691–696 (2020)
M.-L. Röhricht, T. Stichel, S. Roth, P. Bräuer, M. Schmidt and S. Will
(See online at https://doi.org/10.1016/j.procir.2020.09.119) - "Spatially-resolved crystallinity determination of polymer welding seams by Raman-microscopy", Procedia CIRP 94, 796–801 (2020)
P.A.B. Braeuer, L.A. Bahr, M.-L. Röhricht, M. Schmidt and S. Will
(See online at https://doi.org/10.1016/j.procir.2020.09.128) - "Correlation between the spatial weld seam morphology and the spatial-temporal temperature profile in laser transmission welding of polypropylene", Lasers in Manufacturing Conference (2021)
E. Mayer, T. Stichel, T. Frick, S. Roth and M. Schmidt
