Influence Of Pores/Interface Bonding Characteristics On The Joint Performance Of Laser Direct Joining Of CFRTP And Al Alloy

Carbon fiber reinforced thermoplastic(CFRTP)has become the preferred material for high-end equipment to reduce weight and increase efficiency due to its excellent mechanical properties such as light weight,high strength,and impact resistance.In order to meet the requirements of high-end equipment components to withstand huge,complex,and variable loads,CFRTP still need to be used together with metal materials such as aluminum alloy(Al)in the joining part.Realizing the reliable joining of CFRTP/Al is the key to ensure the service performance of this type of structure.The laser joining technology uses the weldability of CFRTP to achieve fast and non-destructive joining of CFRTP and Al,which is suitable for reliable joining of this type of laminated structure.However,CFRTP is very sensitive to temperature changes,and the temperature field gradient has strong time-varying and large gradient,which often results in internal pore defects in CFRTP.It also seriously influences the performance of CFRTP/Al laser joining joints.In this paper,the influence of pores/interface bonding characteristics on the joint performance of laser direct joining of CFRTP and Al were studied by quantitatively extracting the pores and interface bonding characteristics.The main research contents are as follows:(1)In order to achieve the quantitative extraction of the features of pores and interface bonding characteristics,a pore extraction method based on grayscale transformation and image derivation and an effective interface extraction method based on grayscale normalization and filtering have been proposed by taking digital images of joint section and cross section,and using the gray difference between the inside and outside of the pore edge and the bound/unbound part of the interface.In combination with the extraction method based on gray normalization and filtering,the quantitative indicators of porosity,pore distribution and interface combined area have been proposed.(2)Based on the above-mentioned pore and interface bonding characteristics extraction algorithm,the CFRTP/Al laser joining experiment of variable laser power has been carried out to analyze the influence of laser power on the pores and interface bonding characteristics.The results show that the effective bonding area of the interface increases with the increase of laser power,but a large number of pores are formed at the interface under high power,which makes the interface performance decrease after reaching the peak value.The maximum failure load of the joint under this process is only 3840 N.(3)In order to analyze the influence of pores and interface bonding characteristics on the joint performance,a script for generating joint geometric models considering the true distribution characteristics of pores has been written.Based on the cohesive zone model and Hashin failure criterion,a tensile shear simulation model of laser joining joints considering interface and CFRTP failures has been established.The model analyzes the influence of different pores and interface bonding characteristics on the joint performance,and based on this,gives the optimal interval of the pore and interface bonding characteristics: on the basis of ensuring the interface bonding performance,the formation of pores in areas where shear stress is concentrated should be avoided as much as possible.(4)In order to improve the joint performance of laser joining CFRTP to Al alloy,according to the proposed pores and interface bonding characteristics optimization conditions,the interface filling resin layer process has been used as the carrier to ensure the full integration of the interface and inhibit the formation of pores in the joint.The maximum failure load is increased to 6660 N,which further improves the performance of laser joining joints.The research results of this paper clarify the influence of the characteristics of pores and interfaces on the joint performance of laser joining CFRTP and Al alloy,which could provide a certain theoretical basis for reasonable control of defects and promotion of interface bonding characteristics.It also provides a certain theoretical basis for improving the performance of laser joining joints,and have certain guiding significance for the further improvement of subsequent laser joining technology.