Hole Damage of Carbon Fiber Reinforced Thermoplastic Composites Using Helical Milling

In this paper, helical milling experiments were carried out to investigated the hole quality of carbon fiber reinforced thermoplastic composites (CFRTP). The effect of machining parameters such as rotation speed, revolution speed and axial feed depth on cutting temperature, cutting force, characteristics and formation of typical hole damages were investigated. The transition between different damage forms induced by changing machining parameters was also analyzed. Additionally, in order to reduce the effect of cutting temperature on the hole damage, additional cooling assistance technology was proposed. The experimental results showed that increasing rotation speed, revolution speed and axial feed depth increased the cutting temperature by 46.43%, 12.06% and 95.97%, respectively. The cutting force decreased with the increase of rotation speed, but increased with the increase of rotational speed and axial feed depth. However, when the axial feed depth reached 0.45 mm, the axial force decreased to a certain extent, which was related to the softening of the material caused by the continuous contact between the bottom material and the milling tool. Burrs were mainly damage occurred at the entrance and exit of the hole. The entrance damage was caused by the material softening at high temperature during machining and then pushing by the cutting force. Owing to the different thermal conductivity of the carbon fiber and the thermoplastic resin, the resin was easily suffering soften during machining. The softened resin was easily to produce interfacial debonding under the push of the cutting force, so the entrance burr tended to appear in the resin area of the material surface. The exit damage was caused by the bottom material softening during machining and then breaking after reaching yield strength. The exit burr was more obvious because the cutting temperature was higher than that at the entrance. Both entrance damage and exit damage were aggravated with the increase of above machining parameters. The hole wall damages were mainly manifested in three forms: coating, deformation and crack. The phenomenon of coating was more serious at the exit of the hole because the cutting temperature was higher than that at the entrance of the hole. The material which had been softened during machining was pushed by the tool and chip friction, leading to the obvious deformation of the hole wall. When the axial force was large, the tool pushed the material at the hole wall strongly, resulting in cracks. With adding cooling assistance, the heat and chips in the machining area were taken out, so that the cutting temperature decreased greatly, and the phenomenon that the matrix being easily to plasticize by heat was obviously improved. The hole quality was significantly improved, and the chip adhesion on milling tool induced by high temperature basically disappeared. However, there were some phenomena of the machining cylinder adhering due to the insufficient cooling. In conclusion, the cutting temperature is the main factor dominating the hole quality of CFRTP. The increase of cutting temperature will lead to the softening of resin matrix and the variation of chip morphology from powder to continuous slice, which has an impact on the cutting force. The softening of resin has a significant impact on hole damage. The cooling assistance can significantly reduce the cutting temperature and thus inhibit the damage. © 2023 Chongqing Wujiu Periodicals Press. All rights reserved.