Formation Mechanism And Regulation Of Thermal Damage In Cfrp Short-pulsed Laser Cutting

Carbon fiber reinforced polymers(CFRPs)are advanced materials that have high specific strength/specific modulus,exhibit excellent fatigue and corrosion resistance,and are widely used for aerospace components.Due to the multi-layer structure of CFRPs,as well as their high strength and high hardness characteristics,it is difficult to meet the manufacturing requirements of low defects and high consistency when using traditional mechanical drilling and milling techniques.Short-pulse laser processing has been adopted as a new technology for CFRP manufacturing because it is a non-contact processing method that has high pulse energy and a narrow pulse width among other advantages.However,as the heterogeneity,anisotropy,and the two material phases of CFRP give rise to large differences in thermal properties,there are significant differences in thermal efficiency when CFRPs are processed using laser cutting.These different efficiencies make it easy to produce large-scale thermal damage,which directly leads to a weakening of the mechanical properties of the components.As the thermal damage has become an obvious challenge for the short-pulse laser processing of CFRPs,it is necessary to conduct in-depth research on the formation mechanisms of thermal damage,the effect of multiple process parameters on the damage behavior,and the mechanical properties of components that have sustained thermal damage.In this paper,a large damage regulation method based on thermal effect suppression is proposed to achieve low-damage and high-quality manufacturing of CFRPs.The main research contents of the paper are as follows:(1)To study the formation of material damage under the action of a short-pulse laser,a three-dimensional,variable point concentric circle/spiral multi-pass scanning strategy for hole cutting is proposed,and a mesoscopic model for short-pulse laser ablation that considers the material heterogeneity and anisotropy is established.The differences in the ablation of the carbon fiber and the resin matrix under the action of laser pulses were analyzed,and a new ablation mechanism was found whereby the thermal accumulation behavior was found to induce fragmentation of the carbon fiber filaments under the impact of intermittent laser energy.It was revealed that the significant difference in thermal effect during the cutting process was the main reason for the initiation of thermal damage and that the fiber-resin interface effect exacerbates the damage formation.The coupled effect of thermal conduction and heat accumulation during laser cutting was explored,and the damage formation process and expansion law under anisotropy were clarified.The accuracy of the proposed model was verified by experiments.(2)To examine the material damage behavior under the interactive influence of the multi-process parameters for cutting holes,the weight loss during the laser cutting and the pyrolysis process as well as the ablation products of the carbon fiber and resin matrix were explored,and the differences resulting from laser ablation of the two-phase materials were analyzed.The temperature at which weight loss of the carbon fiber during pyrolysis occurs was found to be much higher than that of the resin,and the ablation products of the fibers mainly consisted of fatty alcohols,sugars,and other organic compounds that have larger molecular structures than the ablation products of the resin(such as CO2 and phenol).Based on the three-dimensional variable point concentric circle/spiral multi-pass scanning hole cutting process,the formation process of thermal damage including the heat-affected zone,shape error,and surface microscopic defects induced by the hole cutting under multi-process parameters was analyzed.Combined with the difference in two-phase ablation,the influence law of multi-process methods and parameter interaction on the damage were clarified,a relationship between the damage and the process parameters was formulated,and the optimal process parameter set was obtained.(3)To investigate the influence of the thermal damage induced by short-pulse laser hole-cutting on the mechanical properties of the materials,the dynamic and static mechanical properties of the materials were evaluated,and the effects of laser cutting and mechanical drilling under different loads(such as tensile,bending,and fatigue loads)were compared and analyzed.The mechanical properties of the un-drilled specimens and those subjected to laser cutting and mechanical drilling were found to be comparable,which verifies the applicability of short-pulse laser cutting.The fracture morphology of the mechanical test specimens was characterized,the differences in the fracture modes of the specimens surfaces with different layering angles were analyzed,and the main failure modes of the material for different types of thermal damage were clarified.The overall progression of the damage is that the cracks along the fiber orientation from the orifice,and gradually expand to fiber tearing and delamination,followed by failure.The effects of thermal damage on the dynamic and static mechanical properties of the material were analyzed,and the correlation between the thermal damage caused by laser cutting and the dynamic and static mechanical properties of the materials was obtained.(4)Based on the research results,a method for controlling damage during laser cutting that involves an adjustable pulse width and assisted cutting using transparent and thermally stable dimethicone is proposed to inhibit the oxidation reaction and heat conduction effect on the surface of the CFRPs.An interlaced scanning strategy is proposed to adjust the laser scanning order of the multi-track hole cutting strategy;the use of this strategy effectively prolongs the time interval of the pulse action in adjacent tracks,reduces the pulse overlap rate,and reduces the heat accumulation effect.Moreover,a short-pulse laser cutting hole “interlaced scanning compound dimethicone-assisted”thermal damage regulation method based on thermal conduction and heat accumulation coupled thermal effect suppression was proposed that can significantly reduce damage.The heat-affected zone at the edge of the cutting hole(optimal 37.61 μm to 14.55 μm)is reduced with less loss in the mechanical properties of the material.