Structure and mechanical properties of optimized extrusion welds

Abstract

AbstractA knowledge of how welding parameters affect the mechanical properties of welds is important. However, the mechanical properties of welds cannot be characterized by nondestructive testing methods. Because of its sensitivity to process conditions, extrusion welding of polypropylene‐homopolymer (PP‐H) was used to investigate the effects of welding parameters on the resulting mechanical properties of welds. Overall optimization of the welding process to obtain stable conditions during welding, which required a redesign of the welding shoe and the welding geometry, resulted in improved weld properties through better build‐up of critical weld areas and suppression of void formation. Investigation of material heating characteristics led to a new air nozzle design. The effect of air temperature and welding velocity on the temperature and thickness of the molten layer was determined. The effects of individual process parameters on the structure and mechanical behavior of welds were established, thereby making it possible to specify narrow limits on the values of the weld parameters for producing high‐quality welds. The quality of these joints cannot be determined by short‐time tests because, even with severe testing conditions, cracks occur in the bulk material. Polarized optical microscopy was used to correlate crack behavior with the build‐up of a specific multilayer structure in the weld area. Long‐term tests demonstrated that, in both the time‐to‐crack and crack behavior, the joining area is not the weakest link in an extrusion weld when the welding parameters are chosen correctly.