Titanium alloys are widely adopted for aerospace, biomedical and automotive applications: they show very good strength compared to their reduced weight, high bio-compatibility and still good mechanical properties at high temperatures. However they typically represent a consistent manufacturing challenge because of poor machinability compared to steels and alluminium alloys. Traditional oil-based cutting fluids are not so efficient in cooling the cutting zone, so, when machining Titanium and other difficult-to-machine alloys such as Nickel-based alloys or Co alloys, very high temperature are reached in the tool-workpiece interface, leading to poor tool life and possible workpiece material alterations. In recent years the increase for environmental and social sustainability concern is rising the interest for new “green” and socially responsible approaches in machining, trying to substitute traditional cutting oil, which also represent a risk for operators health, with more environmental-friendly alternatives. This works aims to understand the possible modifications that may occur when machining Ti-6Al-4V alloy comparing cryogenic cut with oil-based and dry machining. Scientific literature study gives a global idea of the problem, however the large majority of scientific papers focused on turning processes, where the positioning of the cutting nozzle is easier and more precise. Few studies have been found on milling processes of Ti-6Al-4V and none of them used a through-tool delivery approach for the cutting fluid which is the state of the art for titanium alloys machining in industrial applications. Experimental milling tests, both for tool life and for surface integrity analysis, were executed at MUSP laboratory in Piacenza. To have a global overview of the cutting process influence on the material, cross sections of the machined specimens were analysed with SEM and optical microscope. Moreover also X-Ray analysis for residual stresses was executed. Globally it was found that cryogenic machining can improve workpieces surface quality with respect to traditional oil-based cutting fluid.
Le leghe di titanio sono molto diffuse in ambito aerospaziale, biomedico e automobilistico: sono caratterizzate da una buona resistenza, se comparata al ridotto peso, elevata biocompatibilità e buone caratteristiche meccaniche ad alta temperatura. Purtroppo rappresentano anche una sfida dal punto di vista manufatturiero a causa della loro scarsa lavorabilità alle macchine utensili, soprattutto se comparata ad acciai e leghe di alluminio. I tradizionali fluidi da taglio non sono abbastanza efficienti per raffreddare correttamente la zona di taglio, perciò, le lavorazioni di Titanio e altre leghe a scarsa lavorabilità come leghe di Nickel o Cobalto, sono caratterizzate da alte temperature della zona di taglio e quindi scarsa durata degli utensili e possibili alterazioni del materiale lavorato. Negli ultimi anni anche il settore manufatturiero ha mostrato interesse verso la sostenibilità ecologica e sociale, anche cercando di sostituire i tradizionali metodi di lubro-refrigerazione basati su olii minerali o sintetici, i quali, oltre ad essere altamente inquinanti, rappresentano anche un rischio per la salute degli operatori, con alternative più sostenibili ecologicamente e socialmente. Questo studio mira a comprendere le modifiche che il materiale scelto (Ti-6Al-4V) subisce dopo una lavorazione di fresatura confrontando diversi sistemi di lubro-refrigerazione: azoto liquido con un emulsione al 12% di acqua e olio a 40 Bar e con il taglio senza alcun lubro-refrigerante. La letteratura scientifica è maggiormente incentrata sui processi di tornitura, dove il posizionamento degli ugelli è più agevole e preciso. Pochi studi sono incentrati sul processo di fresatura e nessuno di questi utilizza frese con adduzione interna del fluido da taglio, che rappresentano lo stato dell’arte per la lavorazione del titanio a livello industriale. I test sperimentali sono stati eseguiti presso il laboratorio MUSP (Macchine Utensili e Sistemi di Produzione) di Piacenza. Le sezioni dei provini lavorati sono state osservate al microscopio (sia ottico che elettronico). Inoltre è stata condotta un’analisi delle tensioni residue al variare della distanza dalla superficie lavorata. I risultati di questo studio mostrano che la lavorazione criogenica può migliorare la qualità dei manufatti.
Tool wear and surface integrity in cryogenic milling of Ti-6Al-4V titanium alloy
Guerra, Riccardo
2019/2020
Abstract
Titanium alloys are widely adopted for aerospace, biomedical and automotive applications: they show very good strength compared to their reduced weight, high bio-compatibility and still good mechanical properties at high temperatures. However they typically represent a consistent manufacturing challenge because of poor machinability compared to steels and alluminium alloys. Traditional oil-based cutting fluids are not so efficient in cooling the cutting zone, so, when machining Titanium and other difficult-to-machine alloys such as Nickel-based alloys or Co alloys, very high temperature are reached in the tool-workpiece interface, leading to poor tool life and possible workpiece material alterations. In recent years the increase for environmental and social sustainability concern is rising the interest for new “green” and socially responsible approaches in machining, trying to substitute traditional cutting oil, which also represent a risk for operators health, with more environmental-friendly alternatives. This works aims to understand the possible modifications that may occur when machining Ti-6Al-4V alloy comparing cryogenic cut with oil-based and dry machining. Scientific literature study gives a global idea of the problem, however the large majority of scientific papers focused on turning processes, where the positioning of the cutting nozzle is easier and more precise. Few studies have been found on milling processes of Ti-6Al-4V and none of them used a through-tool delivery approach for the cutting fluid which is the state of the art for titanium alloys machining in industrial applications. Experimental milling tests, both for tool life and for surface integrity analysis, were executed at MUSP laboratory in Piacenza. To have a global overview of the cutting process influence on the material, cross sections of the machined specimens were analysed with SEM and optical microscope. Moreover also X-Ray analysis for residual stresses was executed. Globally it was found that cryogenic machining can improve workpieces surface quality with respect to traditional oil-based cutting fluid.| File | Dimensione | Formato | |
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https://hdl.handle.net/10589/169812