The student Iñigo Rodriguez Bogajo obtained an EXCELLENT CUM LAUDE grade in his Joint doctoral thesis with the University of Ljubljana
The student Iñigo Rodriguez Bogajo obtained an EXCELLENT CUM LAUDE grade in his Joint doctoral thesis with the University of Ljubljana
The student Iñigo Rodriguez Bogajo obtained an EXCELLENT CUM LAUDE grade in his Joint doctoral thesis with the University of Ljubljana
- Thesis title: Sustainable machining of carbon fibre – Ti6AI4V stacks with assistance of liquefied CO2
Court:
- Presidency: Mihael Sekavcnik (University of Ljubljana)
- Vocal: Joaquín Barreiro García (Universidad de León)
- Vocal: Miguel Arizmendi Jaca (Universidad de Navarra)
- Vocal: Roman Sturm (University of Ljubljana)
- Secretary:Patxi Aristimuño Osoro (Mondragon Unibertsitatea)
Abstract:
Carbon Fibre Reinforced Polymers (CFRPs) and Ti6Al4V titanium alloys are widely used for structural applications in aeronautics due to their superior weight-to-strength ratio and corrosion properties. These materials are often combined to create hybrid CFRP/Ti6Al4V stacks. Thus, drilling is one of the most critical operations in aircraft manufacturing due to the large amount of holes needed to assemble components made of different materials. Good hole quality is crucial to ensure the durability of such assembled structures.
A wide range of machinability and hole quality defects arise when drilling CFRP/Ti6Al4V stacks. On the one hand, the abrasive nature of the CFRP rounds the cutting edge, and delamination can occur when being machined due to the brittleness of the material. Ti6Al4V on the other hand, can accelerate tool wear due to adhesion. The stacks are usually machined in a single operation from the CFRP to the Ti6Al4V, which creates additional problems such as the deterioration of the CFRP hole walls by the titanium chips.
Coolants and lubricants are employed in many sectors and applications to supress machinability and hole quality problems when drilling difficult to cut materials. However, for the case of CFRP/Ti6Al4V stack drilling, conventional water and oil based emulsion coolants and lubricants cannot be used, as they can degrade the composite phase due to moisture absorption. Moreover, emulsion coolants are known for being hazardous for the environment and the health of the operators. Techniques such as the combination of liquid carbon dioxide (LCO2) and minimum quantity lubrication (MQL) have emerged as sustainable cooling and lubrication alternatives to conventional emulsions. However, their feasibility to drill CFRP/Ti6Al4V stacks and their effect on health and environmental aspects when machining aeronautical components is yet to be proven.
In this context, the main objective of the thesis is to define a process window for LCO2+MQL assisted drilling of CFRP/Ti6Al4V stacks and to perform a sustainability evaluation of this technique, considering environmental, economic and social aspects. The optimal machining parameters, drill geometry and cooling lubrication flow rates were experimentally determined. Concerning the sustainability analysis, the effect of employing LCO2+MQL coolants on the airborne emissions at the workplace was characterised, and the environmental advantages of using such coolants in comparison to dry drilling were evaluated through life cycle assessment.