In order to achieve short cycle times, gas temperatures of 500 °C or higher are used for welding plastics by means of serial hot gas welding. These temperatures are often distinctly higher than the decomposition temperature of the plastic to be joined. The thermal stresses can lead to chemical degradation and physical aging. This can negatively affect the lifetime of the weld and lead to premature failure. In this study, the fusion layers of polypropylene and polyamide are analyzed in terms of their thermal and thermal oxidative degradation using viscometry and FTIR spectroscopy. The results show that a detection of chain degradation and degradation products in the individual melt layer depths is possible. Furthermore, correlations between the depth of degradation and the weld strength can be identified. This characterization shows the first results of a holistic approach to the influence of thermal degradation in the weld seam as a function of the process parameters and their effects on weld seam properties. With the right joining strategy, it is possible to achieve high weld strength without the use of nitrogen and the associated material degradation. This could result in a significant cost reduction for the industry.

This work establishes the relationships between several key Friction Stir Welding process parameters and the resulting microstructural and flexural properties of the welded joint. A series of four single parameter experiments were run. The parameters investigated were pin diameter, feedrate, shoe temperature, and pressure time. Butt welds were made in 6 mm thick stress-relieved extruded polypropylene sheet. Three-point bend tests were used to determine the ultimate flexural strength and coincident strain. The maximum bend angle before failure was used to label the welds as "good or bad." An optical microscope capable of cross polarization was used to examine and photograph the weld microstructure. Welds were evaluated according to 1) DVS bend angle, 2) flexural properties, and 3) weld microstructure. All welds made surpassed the DVS requirements for classification as a "good weld" established for hot-gas, extrusion, and laser welding processes. Most welds met the bend angle requirement for hot-plate welds. Welds created for this work maintained 80-92% of base material flexural strength. In the majority of the welds, the strength was between 85 and 90% of base material. The FSW joints showed a flexural strength of 10500 psi, compared to a base material strength of 12400 psi. Four microstructural zones were found to exist in the FSW joints. These were: 1)advancing interface, 2) retreating interface, 3) bottom disturbance, and 4) central zone. Several common microstructure types and defects were found to exist in the welds. These were: 1) spherulites, 2) voids, 3) root defects, 4) flow lines, and 5) onion skin. A distinct correlation was observed between weld microstructure and flexural properties. Those welds whose microstructure most nearly resembled the base material demonstrated the best flexural properties. This can be accomplished by operating with a low feedrate, a high shoe temperature, and a large pin.