Ultrasonic Welding Of Carbon Fiber Reinforced Nylon 66 Composite Without Energy Director

Carbon fiber reinforced Nylon 66(C_f/Nylon 66)composite is being increasingly applied to body-in-white due to its good combination of low density,thermal stability and good mechanical property.Ultrasonic welding(UW)has been widely used in joining C_f/Nylon 66 composite because it is fast,suitable for mass production,and offers good cosmetic quality.To meet the requirements for polymeric composites in automotive industry,carbon fiber reinforced Nylon 66 composite without energy director was selected and its ultrasonic welding mechanism was investigated in this study.The macrography,microstructure,shear tensile and fatigue properties of the joint were studied systematically.The effect of welding parameters,humid environment and contact conditions among horn,workpiece and anvilon the weld quality of joints were also discussed,the main innovative results are as follows:(1)Investigation of the ultrasonic welding of C_f/Nylon 66 composite without energy director indicated that the C_f/Nylon 66 composite can be double-side and single-side welded.The optimal welding parameters for double-sideultrasonic welding was a weld time of 2.1 s and weld pressure of 0.15MPa while for the single-side welding is a weld time of 1.3s and weld pressure of 0.19 MPa.(2)Thermal degradationof C_f/Nylon 66 compositeduring ultrasonic welding and humid environment had an influence on the weld quality of C_f/Nylon 66 joint.Decomposition index of Nylon 66 and amplitude of the workpieces during ultrasonic welding process can beproposed as indications in monitoring the weld quality of C_f/Nylon 66 joint.The joint strength can meet the strength requirements of the body-in-white when the decomposition index ranged within 12%–30.5%.The moisture had little influence on the joint strength when the amplitude of the workpieces was larger than 90μm during welding.The deleterious effect of moisture on the weldability of C_f/Nylon 66 can be reversed by completely redrying before welding.(3)The influence of contacts among horn,workpiece,anvil on the weld quality of C_f/Nylon composite was studied.Although the large misalignment angles between the horn and workpieces(i.e.,more than 4 degrees)or gaps between workpieces(i.e.,above 1.0 mm)resulted in a significant decrease in weld strength,weld area,and cosmetic quality,the gap between the lower workpiece and anvil had a slight influence on the weld quality of the joint.The weld sizescan be estimated based on the proposed ultrasonic wave propagation model and weld size estimation model,which were close to the measured ones.(4)An online inspection procedure of joint quality and repairing method of the unqualified joint were developed based on the horn displacements and durations of the horn-displacement curves in stage III.By analyzing and comparing the values of the horn displacement and duration of a joint in stage III to that of target range,the quality weld waseffectively determined without interruption of the welding operation.The double-pulse vibration was not only used to repair unqualified joint,but also enhanced the fatigue property of the weld.(5)The fatigue property of the joint can be improved by the preheat process.The suitable preheating temperature for UW C_f/Nylon 66 composite was within the range of 95°C–145°C.The joints welded with preheated workpieces of125°C not only avoidedthe decomposition in the composite but also decreased temperature gradient of the joint during ultrasonic welding,thus,the endurance limit of the joint increased by almost 30%compared with that of the non-preheated workpieces.