Research On Thermal Damage Control And Composite Machining Strategy For Laser Cutting Of Carbon Fiber Composites

Carbon fiber composites(CFRP)have been widely used as lightweight materials in automotive,aircraft and other industrial fields due to their excellent properties such as high specific strength and stiffness.Although CFRP parts are mostly manufactured with near-net shape,milling and drilling are inevitable to meet assembly requirements.As a virtually force-free ‘hot processing' method,fiber laser processing presents advantages of high efficiency,no tool wear easy automation,becoming the main processing method of composite materials.Due to the great difference in thermophysical properties of each component,CFRP samples,as heterogeneous materials,tend to cause serious Heat damage(HAZ)during laser processing.Terribly,thermal damage affects the assembly performance and service performance of the corresponding parts,becoming a big challenge for the mature application of laser machining technology.In this paper,experimental study on high power fiber laser cutting CFRP material was carried out to investigate the thermal damage,and the quality of laser-machined kerf was also characterized.Furthermore,based on the heat conduction theory,the temperature field attenuation model was established to simulate the temperature attenuation process in the cooling stage.In addition,the 10mm-thick CFRP laminates were successfully cut by different machining parameters and laserCNC milling hybrid machining process,and the effects of thermal damage degree,machining parameters on the cutting force were investigated.The specific research contents and results are as follows:(1)Continuous-wave fiber laser(CW)cutting 2.0 mm-thick CFRP laminates tests were carried out in this paper.After processing,optical microscope(OM)and scanning electron microscope(SEM)were adopted to observe the morphology of kerf,and the width of HAZ was characterized.Based on the theory of heat conduction,a unidirectional heat conduction model perpendicular to the direction of the kerf was established.According to the model,the temperature attenuation process in the material during the cooling stage was analyzed.Besides,the study found that the fracture end morphology of carbon fiber was different with different lay-up configuration.In the lower part,the fracture ends of the unidirectional CFRP carbon fibers were in a neat,flat,cylindrical section,while the [45°/-45°] 2S CFRP in a conical end.Based on the analysis model,it was shown that the thermal damage was mainly caused by heat conduction and formed in the cooling stage.Moreover,the higher laser power and the lower processing speed aggravated the heat accumulation,resulting in more serious thermal damage.(2)High power fiber laser with a single channel cutting strategy was applied in this work to cut 10.0-mm-thick CFRP laminates,kerf indexes including kerf width,Kerf taper,Kerf depth and the HAZ width was characterized.The relationship between the characterization results and laser power,cutting speed and linear energy was analyzed,and also the summarized cutting performance of 10mm-thick CFRP with different machining parameters.Interestingly,it showed an inversely proportional relationship between the tapering Angle and line energy.The tapering Angle firstly decreased with the increase of the line energy and then stabilized around a small value.On the contrary,the cutting depth and HAZ width were inversely proportional to the line energy.The kerf depth increased rapidly first,then increases slowly until successfully cut through.But the width of HAZ increased before cut through,then decreases sharply,and finally increases very slowly.The objective of this study is to describe the removal mechanism of CFRP thick plate material in laser-CNC milling and adopt the dynamometer to monitor the variation of cutting force during the removal process of HAZ region.The influence of thermal damage and milling parameters on the cutting force was studied.As thermal processing,the thermal damage defects formed by laser machining were mainly affected by thermal accumulation,while CNC milling is cold processing,the main defects and the size of the cutting force.In addition,the result showed that the degraded matrix weakened the curing effect on the fiber,and it significantly reduced the cutting force and tool wear in the process of removing the thermal damage.Luckily,hybrid machining can effectively produce materials without thermal damage.The above research results on thermal damage of laser machining CFRP materials provide the experimental and theoretical basis for process optimization and cutting of thick plates and can promote the further application of laser machining technology.