Abstract
Ultrasonic welding of thermoplastics is widely applied in automobile and aerospace industries. Increasing the weld area and avoiding thermal decomposition are contradictory factors in improving strength of ultrasonically welded polymers. In this study, relations among the loss modulus of carbon-fiber-reinforced polyamide 66 composite (CF/PA 66), time for obtaining stable weld area, and time for CF/PA 66 decomposition are investigated systematically. Then, a double-pulse ultrasonic welding process (DPUW) is proposed, and the temperature evolutions, morphologies and structures of fractured surfaces, and tensile and fatigue properties of the DPUWed joints are measured and assessed. Experimental results show the optimal welding parameters for DPUW include a weld time of 2.1 s for the first pulse, a cooling time of 12 s, and a weld time of 1.5 s for the second pulse. The DPUW process enlarged the weld area while avoided decomposition of CF/PA 66 under appropriate welding parameters. Compared to the single-pulse welded joint, the peak load, weld area, and endurance limit of the DPUWed joint increased by about 15%, 23% and 59%, respectively. DPUW also decreases the variance in strengths of the joints.
Highlights
Polymer composites play an increasingly significant role in automobile and aerospace industries due to low density, high specific strength, and corrosion resistance [1]
These results indicate the selection of doublepulse ultrasonic welding process (DPUW)
Overheating does not emerge, and no porous area occurs in the joint due to controlling the temperature distribution. These results indicate the selection of DPUW parameters based on the temperature in upper workpiece is reasonable
Summary
Polymer composites play an increasingly significant role in automobile and aerospace industries due to low density, high specific strength, and corrosion resistance [1]. There is an urgent need to develop methods to enlarge weld area but avoid thermal decomposition in the meantime for ultrasonically welded thermoplastic composites. Wang [19] proposed interfacial preheating treatment in ultrasonically welded polyamide 6 composites and found proper preheating conditions helped to concentrate more weld energy at the joining surface and effectively improved the joint quality. Zhi [18] preheated whole workpieces prior to ultrasonic welding, the optimum weld time shortened from 2.1 s to 1.5 s, and the degree of thermal decomposition decreased, because the lower loss modulus reduced heat generation in the workpiece. A multipulse ultrasonic welding method is proposed to improve the weld quality while avoiding thermal decomposition. Weld quality will be improved by increasing energy dissipation at the faying interface while inhibiting thermal decomposition. The temperature evolution, peak load, and fatigue properties of the DPUWed joint are assessed systematically
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