Sustainable machining of carbon fiber and Ti6Al4V stacks with assistence of liquified CO2

Rodriguez Bogajo, Iñigo (author), Arazola, Pedro José (mentor), Pušavec, Franci (mentor), Cuesta, Mikel (comentor)

Carbon Fibre Reinforced Polymers (CFRPs) and Ti6Al4V titanium alloys are widely used for structural applications in aeronautics due to their superior strength-to-weight ratio and corrosion properties. These materials are often combined to create hybrid CFRP/Ti6Al4V stacks. Composite/metal stacks are usually joined with rivets or fasteners regardless of having an adhesive layer between the composite and metal. 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 issues and hole quality defects arise when drilling CFRP/Ti6Al4V stacks. On the one hand, the CFRP creates abrasive wear, rounding the cutting edge, and delamination can occur when the drill fails to shear the plies of the composite. Ti6Al4V on the other hand, can accelerate tool wear due to adhesion. The stacks are usually machined in a single operation (one-shot) from the CFRP to the Ti6Al4V, which creates additional problems like the scratching of the CFRP hole walls by the titanium chips. Coolants and lubricants are employed in many applications to supress machinability and hole quality problems when drilling difficult to cut materials. However, for CFRP/Ti6Al4V stack drilling, conventional water and oil based emulsion coolants cannot be used, as they can degrade the composite phase due to moisture absorption. Moreover, emulsions can create environmental and health hazards. 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 machining parameters, drill geometry and cooling lubrication flow rates to achieve superior hole quality and minimal tool wear are experimentally determined. Concerning the sustainability analysis, the effect of employing LCO2+MQL coolants on the airborne emissions at the workplace are characterised, and the environmental advantages of using such coolants in comparison to dry drilling were evaluated through life cycle assessment.

• Language: English
• Keywords: sustainable machining, cryogenic machining, drilling, industrial hygiene, aeronautic structures
• Work type: Doctoral dissertation
• Typology: 2.08 - Doctoral Dissertation
• Organization: FS - Faculty of Mechanical Engineering
• Place of publishing: Ljubljana
• Publisher: [I. Rodriguez Bogajo]
• Year: 2024
• Number of pages: XXVI, 184 str.
• PID: 20.500.12556/RUL-160014
• UDC: 621.9:678:629.7(043.3)
• COBISS.SI-ID: 204076035
• Publication date in RUL: 08.08.2024

Secondary language

• Language: Slovenian
• Title: Trajnostna obdelava struktur iz ogljikovih vlaken in Ti6Al4V z asistenco kapljevitega CO2

Abstract

Z ogljikovimi vlakni ojačani polimeri (CFRP) in titanove zlitine Ti6Al4V se zaradi boljšega razmerja med težo in trdnostjo ter korozijskih lastnosti pogosto uporabljajo za konstrukcijske aplikacije v letalstvu. Ti materiali se velikokrat kombinirajo v hibridne sklope CFRP/Ti6Al4V. Tako je vrtanje, zaradi velike količine lukenj potrebnih za sestavljanje sestavnih delov iz različnih materialov, ena najbolj kritičnih operacij pri proizvodnji letal. Zagotavljanje dobre kakovosti izvrtin pa je ključnega pomena za doseganje zadostnih dob trajanja tako sestavljenih struktur. Pri vrtanju struktur CFRP/Ti6Al4V se pojavlja veliko število napak z vidika obdelovalnosti in kakovosti lukenj. Po eni strani krhka in abrazivna narava materiala CFPR zaokroža rezalni rob in je dovzetna za poškodbe zaradi razslojenosti. Ti6Al4V, po drugi strani, lahko zaradi adhezije pospeši obrabo orodja. Strukture CFRP/Ti6Al4V se običajno obdelajo v eni sami operaciji, v smeri od CFRP proti Ti6Al4V. To povzroča dodatno težavo, kot je poškodovanje sten lukenj CFRP s titanovimi odrezki. Pri vrtanju težko obdelovalnih materialov se hladilno mazalna sredstva uporablja v številnih sektorjih in aplikacijah za preprečevanje težav z obdelovalnostjo in kakovostjo lukenj. Vendar pri materialih sklopih CFRP/Ti6Al4V uporaba običajnih emulzij na osnovi vode in olja ni mogoča. Te lahko, zaradi absorpcije vlage, poslabšajo kompozitno fazo. Poleg tega je znano, da so emulzijska hladilna sredstva nevarna za okolje in zdravje operaterjev. Tehnike, kot sta kombinacija kapljevitega ogljikovega dioksida (LCO2) in minimalno mazanja (MQL), so se pojavile kot trajnostna alternativa običajnim emulzijam za hlajenje in mazanje. Njihovo izvedljivost za vrtanje sklopov CFRP/Ti6Al4V ter njihov vpliv na zdravje in okoljske vidike, pri obdelavi letalskih komponent, pa je potrebno še dokazati. V tem kontekstu je glavni cilj disertacije opredelitev procesnih oken za vrtanje CFRP/Ti6Al4V sklopov s pomočjo LCO2+MQL in ocena trajnosti te tehnike ob upoštevanju okoljskih, gospodarskih in družbenih vidikov. S cenilkami kakovosti izvrtine in obstojnostjo orodja so bili eksperimentalno določeni optimalni parametri obdelave, geometrija svedra in pretok hladilnega maziva. V zvezi z analizo trajnosti je bil opisan vpliv uporabe hladilnih sredstev LCO2+MQL na emisije v zrak na delovnem mestu, okoljske prednosti uporabe takšnih hladilnih sredstev v primerjavi s suhim vrtanjem pa so bile določene z oceno življenjskega cikla.

• Keywords: trajnostna obdelava, kriogeno odrezovanje, vrtanje, industrijska higiena, letalske strukture