An experimental investigation of ultrasonic assisted milling (UAM) of carbon fibre reinforced polymer (CFRP) and the effect of machining on the BMI 5250-4 matrix resin

Milling of Carbon Fibre Reinforced Polymer (CFRP) is necessary for component accuracy prior to assembly of aircraft. Recently, ultrasonic assisted milling (UAM) which combines conventional machining (CM) with ultrasonic vibration on the cutting tool, has shown beneficial outcomes with respect to the machinability of some metals, however, limited UAM of CFRP has been reported. In this thesis, milling (CM and UAM) of a CFRP incorporating Bismaleimide 5250-4 (BMI 5250-4) resin was carried out in a wide range of cutting parameters and environments (dry, conventional cutting fluid (CCF) and CO2 cryogenic). Machinability was examined in terms of tool wear, cutting forces and surface roughness. In terms of machinability with conventional cutting tools, machining in a CO2 had a positive effect on tool life, despite an increase in cutting forces, compared to CCF and dry. UAM was found to reduce cutting forces by up to 10 %, compared with CM, however, this did not yield any benefit in terms of tool wear and/or workpiece surface roughness. When dry machining employing an abrasive diamond tool, CFRP material adhesion was a feature. The application of UAM in this instance yielded, reduced workpiece adhesion on the cutting tool and improved workpiece surface roughness. Machining of CFRP must be performed below the glass transition temperature (Tg) of the resin to avoid the degradation of the properties of the matrix resin. In this research new findings in the temperature initiated during machining and the consequential effects on the polymer utilised Fourier transform infrared spectroscopy (FTIR) and differential scanning calorimetry (DSC) which is well established in polymer characterization. FTIR and DSC was carried out to investigate the effect of machining on the chemical and material properties of BMI 5250-4 such as Tg and changes to matrix resin chemical bonding, which has been closely associated with degradation of the machined part. Further analysis of the machined surface by DSC indicated that the Tg of the matrix resin had been exceeded during the machining process and led to degradation of the BMI 5250-4 in some cases. An observed reduction of the maleimide double bond (C=C) at 825 cm-1 wavelength by FTIR signified that further post-curing of BMI 5240-4 had occurred which suggested that a higher cutting temperature was developed at the machine tool tip than recorded with the infrared camera. CM dry machining, FTIR analysis also confirmed the formation of isocyanate-derived products (C≡N) at 2250 cm-1 wavelength, a bond associated with the point at which BMI 5240-4 is thermally degraded having experienced temperatures in the range 400 to 600 °C. This result suggests that when CM dry machining the actual cutting temperature experienced by the BMI 5240-4 was at least 400 °C. The formation of isocyanate-derived products was not observed for UAM dry machining, suggesting that ultrasonic vibration of the cutting tool may reduce the cutting temperature in the primary shear zone, however this temperature reduction was insufficient to arrest observed post curing effects and shift in the Tg. Other aspects of the FTIR analysis revealed that despite the improvements to workpiece surface roughness when milling with CCF there was an increased presence of moisture (-OH bond) in the BMI 5240-4 resin which may have a detrimental effect on the durability of the material over time. Machining CFRP has been enhanced by the introduction of the chemical analysis. It suggests that DSC and FTIR exploration of the thermal history of the CFRP can provide more information about the temperature than typical thermal measurement during machining such as thermal cameras and thermocouples. The management of the milling process of CFRP can now be related to the management of the temperature at the tool tip and the effect on polymer characteristics. As a consequence, milling of CFRP in CO2 exhibited improvement in tool wear, an observed reduction in cutting temperature, and sustenance of the chemical properties of BMI 5250-4. However, there was no significant benefit in additionally employing UAM in a CO2 environment. The research has provided a new insight in the milling of polymer composites and could be beneficial in avoiding thermal degradation of the machined part, maintaining the quality of machined part and avoiding scrap parts at the end of machining processes