Study On Heat And Mass Transfer Mechanisms During Ultrasonic Welding Of Carbon Fiber Reinforced Thermoplastic Composites

Carbon fiber reinforced thermoplastic composite(CFRTP),as one of the new lightweight materials,is widely used in manufacturing and other industries.It is an urgent need for the application of this material to develop suitable connection technology.Among them,ultrasonic welding is one of the ideal technologies to realize CFRTP connection.Understanding the heat and mass transfer mechanism of CFRTP under ultrasonic is the key to accurately regulate welding parameters and improve welding quality.However,the multi-scale characteristics of CFRTP geometry and the coupling effect between CFRTP and ultrasonic are very complicated,and the relevant mechanism is still unclear.Based on the servo-driven ultrasonic welding experiment of CF/PA6 composites with no energy director(ED),combined with single factor sensitivity experiment and multi-factor sensitivity experiment,the interaction between welding parameters(welding energy,ultrasonic amplitude,welding head velocity and trigger pressure),and the influence mechanism on the thermal physical phenomena and welding quality of CFRTP welding were studied.By comparing the PA6 and CF/PA6 welding under different welding energies,the influence of the presence of carbon fiber(CF)on welding heat generation,heat transfer and melting phenomenon was explored.The acoustic-thermal coupling theoretical model of ultrasonic welding was derived to analyze the acoustic propagation path and the energy transfer and transformation law of welding process,and the relationship between acoustic-thermal transformation and material properties and input energy was obtained.The microheterogeneity and anisotropy of the thermodynamic parameters of CF were fully considered,and the mini-scale thermodynamic coupling model of PA6 and CF/PA6 was constructed to further reveal the mechanism of the influence of the microstructure effect of CF addition on the viscoelastic heat generation,temperature distribution and melting phenomenon under ultrasonic action.According to the ultrasonic welding model of polymer,the acoustic thermal conversion ability of materials is positively correlated with the loss modulus and density of materials,negatively correlated with Young’s modulus,and the internal heat source generated is linearly correlated with the input power.The servo-driven ultrasonic welding experiments of CF/PA6 composites without energy director show that the characteristics of constant welding rate and variable power can make the plates tightly bonded without ED,especially for direct contact of the composite materials with rough surface.The power output can be adjusted according to the melting state of the materials,and the ideal joint with the highest relative strength of 70 % can be obtained.The joint formation process involves a complex thermodynamic coupling process,and the welding pressure is directly affected by the welding head speed and trigger pressure.The single factor experiment shows that on the one hand,the pressure can increase the normal stress and the strain in the frictional heat generation stage,so as to improve the frictional heat generation and the viscoelastic heat generation stage.On the other hand,excessive pressure(> 400 N)will greatly limit the horizontal relative motion between two plates,thus affecting the friction heat production;In addition,the greater the welding pressure is,the greater the welding power will be in order to rapidly melt a large number of materials,thus affecting the process and quality of joint formation.Design of experiment considering the interaction of welding factors found that welding energy had a significant effect on joint strength,and there was a strong interaction between welding head speed,welding energy and ultrasonic amplitude.By comparing the welding process of PA6 and CF/PA6 ultrasonic welding under different welding energies,it is found that when the surface of the weldment is smooth,the melting phenomenon of the ultrasonic welding without ED first occurs in the corresponding area of the edge of the welding head.The fibers raised on the rough surface of CF/PA6 can be regarded as a small ED,which causes the stress and energy to be concentrated and forms a small melting region nearby.At the same time,CF/PA6 temperature is higher under the same welding energy.From the macroscopic experiment,the addition of CF increases the loss modulus of the material,thus increasing the viscoelastic heat production,and the specific heat capacity near the melting point is only half of that of PA6,increasing the temperature rise rate.From the microscale simulation,the CF inside CF/PA6 causes some inhomogeneity,which makes the isothermal surface distorted along the CF extension direction.Since CF is an elastic body,both high strain and high viscoelastic heat are generated near the interface between CF and PA6.The high thermal conductivity of CF reduces the heat accumulation effect on the underside.In addition,the ultrasonic frequency increases from 15 k Hz to 60 k Hz,and the mean temperature of CF/PA6 cell model increases from 217 ℃ to 379 ℃,while the amplitude increases from 70% to 90%,and the mean temperature only increases from 212 ℃ to 226 ℃.The research results of this paper can provide theoretical basis for the improvement of control methods of ultrasonic welding system and provide important support for promoting the large-scale application of high-performance CFRTP.