Experimental investigation on machining process performance for carbon fiber reinforced polymer composites: part quality, tool wear and dust emission

The use of carbon fibre reinforced polymer (CFRP) in industries is continuously increasing because of its exceptional properties such as high strength, low weight, and corrosion resistance. Although CFRP parts can be moulded to a near-net-form geometry, machining processes (milling, drilling) are frequently required to achieve the final shape. However, CFRP does not behave like pure plastic or metals during machining. The chip formation for CFRP is largely brittle factures caused by shearing of the fibres and matrix. Also, CFRP produces powdery chips or dust during dry machining, which are currently categorised as a nuisance pollutant and cause of health hazard. Moreover, its inhomogeneity and the abrasive nature of the fibres bring additional challenges during machining: delamination, cutting temperature, dust emission, and limited tool life. Accordingly, this work aims to investigate the milling of CFRP and key factors that affect its machinability. The application of conventional cutting fluid is a common solution for increasing the tool life, reducing cutting forces or lowering the dust emission and dispersion. However, this solution cannot be applied to the machining of CFRP, because moisture may damage the structural integrity of the composite workpiece. In this manuscript-based thesis, an experimental study was first conducted to examine the feasibility and effectiveness of applying atomized cutting fluid in milling of CFRP (article 1, chapter 3). The cutting fluid is broken down into micrometer size droplets that are sprayed directly into the cutting zone to lubricate the chip and tool interface, and also to dissipate the heat generated by fast evaporation. Because of the significantly higher evaporation rate of the atomized cutting fluid, the applied coolant not get absorbed into the workpiece material. In the study, CFRP milling operations were conducted in a range of high cutting speed, up to 40,000 rpm, and feed rate values up to 6 um under different machining conditions: dry condition, and using two types of cutting fluids, a general purpose semisynthetic coolant and a vegetable-oil-based coolant. The result showed that using atomized vegetable oil helps significantly in reducing the cutting force, tool wear, and fibre delamination as compared to the dry milling condition. The machining performance of CFRP could also be influenced by fibre orientation and its interactions with the machining parameters and conditions. In article 2 (chapter 4) the effects fibres orientation (0◦, 30◦ 45◦, 60◦ and 90◦) on chip formation, delamination, tool wear and cutting forces when milling dry and with vegetable oil-based fluid was investigated. It is found that the cutting direction, in relation to the fiber orientation, strongly influences chip formation, the cutting force, fiber breakage, tool wear, surface quality of the part machined and the delamination, according to the machining parameters and conditions used. The delamination percentage when cutting at tool-fiber orientation angles of 0◦, 30◦ 45◦, 60◦ and 90◦ were improved by 65%, 91%, 54%, 66%, and 75%, respectively, under ACF (vegetable oil) conditions at 3um feed rate. The magnitude of the resultant cutting force was found to be greater in the dry condition relative to the ACF condition by more than 23% depending on fibre orientation angles and feed rate used. Some manufacturers might be very hesitant in using any fluid when machining CRFP because moisture may damage the structural integrity of the composite. Machining dry could bring additional challenges to tool performance, part quality and occupational safety. In article 3, (chapter 5), the dry milling of CFRP machining is investigated with special focus on cutting force, specific cutting energy, cutting temperature, tool wear, and fine dust emission. The cutting temperature was examined using analytical and empirical models, and cutting experiments (full factorial design) were conducted to assess the reliability of the theoretical predictions. Finally, results of the experimental optimization are presented, and the model is validated. It was found in, among other conclusions, that during milling of the tested CRPF, fine particles were emitted (diameters ranging from 0.5–10 um) and that their concentration can reach 2776 particles per cubic centimeters. All the obtained results help to better understand specific phenomena associated with milling of CFRPs and provide the means to select effective milling parameters to improve the technology and economics of the process.