Research On Heat Transfer Characteristics During Ultrasonic Welding Of Carbon Fiber Reinforced Thermoplastic Composites

In the context of " carbon neutrality and emission peak",it is imperative to reduce the weight of automobiles.Carbon fiber reinforced thermoplastic composites(CFRTP)have the advantages of high specific strength and modulus,relatively easy fabrication and handling,selflubrication and good corrosion resistance.It is an ideal substitute material for automobile body manufacturing.Ultrasonic welding(USW)can form high-quality joints at lower temperatures.It has the advantages of short welding time,high cleanliness and reliability,small joint deformation and high degree of automation of industrialized processes.It is ideal as a connection technology for CFRTP.The USW technology mainly heats the material and forms a nugget through the micro frictional heat generation between the CFRTP material interface and the viscoelastic heat generation in the material,and forms a joint after cooling.In this paper,a three-dimensional thermal-mechanical coupled cubic element finite element numerical model is established for the typical position of CF/PA6 in the USW process.This model can fully reflect the interaction between carbon fibers and PA6 matrix during the USW process.In this paper,the effect of carbon fibers on the ultrasonic welding of composites during the USW process was investigated.The temperature variation in the model,the evolution of the melting front,the thermal conductivity of the bottom surface and the viscoelastic heat generation of the model are analyzed.By changing the thermal conductivity and mass content of carbon fibers,their effects on the USW process were analyzed.The results show that USW is a highly coupled thermal-mechanical process.Compared with the pure PA6 model,the temperature of the CF/PA6 model increases faster,and the melting front increases faster.Under the same ultrasonic vibration conditions,the addition of carbon fibers can accelerate the temperature rise and melting of CF/PA6.The melt fronts of the different carbon fiber-arranged models grow differently at different times.The CF/PA6 with inclined fiber alignment model had faster temperature and melting front increases in the early stage(t < 50 ms)of the USW process,while the CF/PA6 with vertical fiber alignment model had a faster increase in the later stage(t > 50 ms).For models with carbon fibers,the presence of carbon fibers significantly changes the strain field in the model,with regions of greater strain consistent with regions of greater viscoelastic heat generation power.During the USW process,the power density of the PA6 substrate changed significantly with the welding time and the position of the typical point,and the peak power density of the typical point had an obvious correlation with the temperature of the typical point.The power density peaks at 80 ℃ ～ 100 ℃,which is the result of the joint effect of PA6 matrix strain and loss modulus.In the early stage of the USW process,the bottom surface heat conduction is the main heat source,and in the later stage of the USW process,the viscoelastic heat generation is the main heat source.Due to the presence of carbon fibers,the PA6 matrix in the model has a larger strain,and the viscoelastic heat generation power is larger than that of the pure PA6 model.The CF/PA6 with inclined fiber alignment model has higher viscoelastic heat generation,while the CF/PA6 with vertical fiber alignment model has higher bottom surface thermal conductivity.The higher the thermal conductivity of carbon fiber,the faster the temperature rises in the CF/PA6 model,and the faster the melting front rises.The thermal conductivity of carbon fibers has little effect on the viscoelastic heat generation power,and the thermal conductivity of carbon fibers has a limited change in the strain field in the model.However,by changing the temperature field of the model,the loss modulus of the PA6 matrix is changed,thereby changing the viscoelastic heat generation power.The higher the carbon fiber content,the faster the temperature rises in the CF/PA6 model,and the faster the melting front rises.In the model with high carbon fiber content,the PA6 matrix strain is larger,the viscoelastic heat generation power is higher,the initial bottom surface thermal conductivity is larger and its decrease is faster.This paper is of great significance to the promotion of USW technology in the industrial field of CFRTP.