| Carbon fiber/epoxy resin composites (abbreviation:C/E composites), because of their excellent strength-to-weight ratios, stiffness-to-weight ratios, designable, fatigue resistance, fatigue resistance and weight saving, have been widely used for manufacturing parts in the fields of high-tech equipment, such as launch vehicle, missile and aircraft/spacecraft. The percentage of C/E composites in aircraft/spacecraft is an important symbol of its advancement. Lots of connecting holes, shaped windows and contour edge need to be machined on large-scale composite structures. However, owing to their laminated structure, anisotropy and high heat-sensitive of matrix, irreparable damages are easily be generated during machining. These damages, such as fuzzing, spalling and delamination, are multi-scale and multi-formation, which become a great hidden danger to the high reliability and long life aircrafts. High-quality and high-efficiency machining of C/E composites becomes a hot issues at home and abroad, and it is regarded as a difficult problem in manufacture field. With the high requirement of machining quality and efficiency and increasing application, manual machining does not satisfy any more. Machining technology of C/E composites becomes one key factor to dominate the development and manufacture of the structures in airspace and national major projects. Hole-making of C/E composites is taken as an example and investigated in this paper to promoting the machining technology of the material, including mechanism of damage generation, cutting tool, process and so on.In order to study the reason for low machining quality of C/E composites, the mechanical behavior of C/E composites is investigated in three factors:anisotropy of material, cutting angle and friction between cutting tool and material. The results show that high machining quality can be obtained when the fiber direction is45°or90°; fuzzing is easily generated when the fiber direction is0°or135°due to the fiber can not be removed completely. Carbon fiber is easily removed when the rake angle is negative or zero, while fuzzing often appears when the rake angle is positive. Bending of carbon fiber leads to the generation of crack. Then, the fiber is broken on its tensile side, which is regarded as the main reason for the material damage. Friction between tool material and C/E composites is different when the tool material changes. The friction coefficient between single crystal diamond or polycrystalline diamond and C/E composites is much lower than that between carbide and C/E composites. Mechanism of machining damage is the theoretical basis and prerequisite for high-quality machining of C/E composite structures. Both the thermo-mechanical properties of the material and the variation of thrust force during hole-making were discussed. Based on this, it is pointed out that the machining quality of C/E composites depends on cutting force as well as cutting heat. However, the influence of cutting heat on machining quality has two stages. When the material temperature is lower than the glass transition temperature of the resin, cutting heat has little influence on the machining quality. When the material temperature is higher than the glass transition temperature of the resin, there is a dramatic decrease of both the resin matrix properties and the critical stress. As a result, small cutting force can lead to serious machining damage such as delamination or spalling. Besides, single-layer C/E composites drilling results show that the material properties have a great contribution on the machining quality. When the interlaminar tensile strength is high enough, high machining quality can be obtained using traditional drilling.During machining of C/E composites, when the tool material is high-speed steel or carbide, machining damage is hard to be avoided. The influence of tool material and tool structure on cutting force and cutting heat during drilling was investigated. It is concluded that tool material has a great influence on cutting temperature, while tool structure has a great influence on cutting force. Comparing with traditional carbide tool, cutting temperature decreases40.3%using diamond coated tool; cutting temperature decreases52.6%using PCD tool. Comparing with twist tool, thrust force generated using core tool decreases70%during hole-making, which is caused by the decrease of pushing force. Furthermore, take an aerospace component as an example, the performace of diamond tool is verified during machining.Due to the material properties and coolant forbidden, high machining quality is difficult to be obtained during traditional drilling process. The influence of orbital drilling and low-temperature air cooling on cutting force, cutting heat and machining quality was studied. It is found that cutting force generated by orbital drilling is the same as that generated by traditional drilling. However, comparing with traditional drilling, the temperature during orbital drilling decreases more than69℃, which is about36%. Air cooling can effectively decrease the temperature combining with core drilling. Compared with using no cooling method, the temperature of cutting area using internal air cooling method decreases by162.3℃, which is about61.3%. And, internal air cooling method avoids the blocking of tool hole. Take an aero-structure as an example, the air cooling method is verified during actual machining. |
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