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Michieletto, Francesco (2016) Innovative forming processes of aluminium alloys sheets and tubes at elevated temperature. [Tesi di dottorato]

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Abstract (inglese)

In the last two decades the international community has been looking for solutions to preserve the environment, and in particular the atmosphere, from the CO2 emissions through the car exhausts, considered one of the main responsible of the greenhouse effect and, therefore, of the Earth temperature increase. Rules and limits were fixed in the 1997 with the Kyoto Protocol that entered in force in 2005, by which the international community signed the legal responsibility for producing vehicles with CO2 emission limited to 95g/km to be reached in 2020. The production of cars using lightweight materials can represent an optimal solution because the lower weight means lower energy consumption. Therefore, the automotive companies are now investigating the feasibility of producing parts made of lightweight materials to replace conventional steels for the car chassis and body-in-white components, but without decreasing the passenger safety.
High resistance steels and aluminium alloys have demonstrated to be the best solution thanks to their low density, high corrosion resistance and excellent stiffness-to-weight ratio. In case of use of aluminium alloy sheets and tubes, it is possible to reduce the car weight of about 15–20 % with also a consequent weight reduction of all the connected vehicle parts and therefore a substantial reduction of the pollutant exhausts.
The main limit of light alloys is the poor formability and the high springback exhibited during room temperature deformation. Temperature assisted processes have proven to increase material formability: Superplastic and Quick Plastic Forming, already used for shaping aluminium sheets, have shown a relevant increase in the material formability allowing to form very complex parts but are extremely expensive due to the very long process times, therefore not applicable for mass production. On the other hand, cold and warm hydroforming processes, nowadays at the state-of-the-art for shaping hollow components, exhibit very high initial investment cost due to the high pressure of the fluid used as deformable mean and to the high tons presses needed for keeping the dies closed during the process. Moreover, a strict forming temperature limit is fixed by the fluid boil and burst temperatures, which may limit the material formability.
In this research work, innovative forming processes were investigated to prove the feasibility of shaping aluminium sheets and tubes at high temperature, exceeding the limits of the already available process technologies. In particular, the Hot Stamping (HS) technology was applied to form 5xxx and 6xxx series aluminium alloys proving the capability of stamping an automotive component on a hot stamping industrial plant, and thus validating the laboratory tests results. An experimental apparatus able to work with the innovative technology of the Hot Metal Gas Forming (HMGF) process was designed and developed to form aluminium alloy tubes. In doing so, resistance heating was used as heating system and cold air in pressure was used to bulge-up the tubes during the process. The formability of different 6xxx series aluminium alloys tubes was investigated by means of free bulging tests and, afterwards, shaping component inside a die, evaluating the influence of the most important process parameters. Finally, in collaboration with an industrial company, the shaping of an aesthetic component with also the evaluation of the surface appearance was carried out demonstrating the applicability of the new process to form an industrial part.

Abstract (italiano)

Negli ultimi decenni, la comunità internazionale è alla continua ricerca di provvedimenti per salvaguardare l’atmosfera e l’ambiente terrestre. In campo automobilistico e dei trasporti la produzione di biossido di carbonio dai gas di scarico delle autovetture, meglio conosciuto come CO2, è ritenuto tra i maggiori responsabili del rafforzamento dell’effetto serra e dunque dell’innalzamento del clima terrestre. Per porre un concreto rimedio e regolamentare l’efficienza sul consumo medio di un autoveicolo, con il protocollo di Kyoto stipulato nel 1997 ed entrato in vigore nel 2005, la comunità internazionale si è impegnata legalmente alla produzioni di veicoli in grado di rispettare il limite di emissione di 95 g di CO2 per kilometro entro l’anno 2020. L’alleggerimento complessivo di un automobile è sicuramente tra le soluzioni più immediate per la riduzione delle particelle inquinanti, in quanto veicoli più leggeri richiedono minore forza motrice e di conseguenza minore consumo di energia. Per questo motivo le compagnie automobilistiche negli ultimi anni sono alla ricerca di materiali innovativi per sostituire l’acciaio che comunemente è impiegato per la realizzazione di telai e parti di carrozzeria, senza pregiudicare la sicurezza dei passeggeri.
Gli acciai alto resistenziali ma soprattutto le leghe leggere, hanno dimostrato essere delle ottime alternative grazie alle loro proprietà di bassa densità, resistenza alla corrosione, ed ottimo rapporto rigidezza-peso. Con l’utilizzo di parti stampate ma anche di elementi tubolari in lega di alluminio il peso medio della sola scocca di una vettura può essere ridotto del 15 – 20 %, portando ad un conseguente ridimensionamento di tutte gli organi connessi ed ad una sostanziale riduzione delle emissioni dannose.
La principale limitazione nella lavorazione delle leghe di alluminio è la loro scarsa attitudine a subire deformazione plastica a temperatura ambiente collegata oltretutto ad un elevato ritorno elastico. Per far fronte a questa problematica, numerosi processi innovativi utilizzanti alta temperatura sono stati o sono tuttora in fase di studio con l’obiettivo principale di incrementare la formabilità del materiale. I confermati processi di deformazione di lamiera di alluminio quali Superplastic Forming e Quick Plastic Forming, hanno dimostrato sicuramente un vantaggio in termini di formabilità riuscendo oltretutto a generare parti complesse, ma sono d’altro canto estremamente costosi e soggetti a tempi molto lunghi di processo, per cui non applicabili per produzioni in larga scala. L’idroformatura a freddo e a tiepido, invece, che rappresenta l’attuale tecnologia all’avanguardia per la sagomatura di parti cave, oltre a necessitare di elevati costi iniziali connessi alle elevate pressioni del fluido necessarie per la deformazione e alle presse ad alto tonnellaggio richieste per la chiusura degli stampi durante l’iniezione del liquido stesso, presenta severi limiti nella temperatura massima di processo. Infatti le emulsioni acqua olio generalmente impiegate come mezzo deformante risultano infiammabili al di sopra del campo tiepido per l’alluminio, limitando dunque il range termico utilizzabile per il processo e di conseguenza la formabilità del materiale.
In questo lavoro di ricerca sono stati studiati processi innovativi per la produzione di componenti di alluminio in lamiera e tubolari che superassero i limiti di processo delle attuali tecnologie produttive. In particolare la tecnologia dello stampaggio a caldo (Hot Stamping), oggigiorno applicata agli acciai alto resistenziali, è stata applicata con successo su lamiere di alluminio serie 5xxx e 6xxx, e validata con test industriali eseguiti su una vera linea di stampaggio producendo un componente automobilistico. Inoltre è stato realizzato e sviluppato un prototipo in grado di operare con la tecnologia innovativa del Hot Metal Gas Forming, che utilizza gas in pressione invece di fluidi per deformare componenti tubolari al alta temperatura. Prove di formabilità su tubi di alluminio serie 6xxx, ma anche la realizzazione di componenti in stampo, hanno permesso inoltre lo studio di numerosi aspetti critici per il processo. In fine, la sagomatura di un componente industriale in collaborazione con una azienda, curando oltretutto la qualità estetica del formato, ha permesso di verificare l’applicabilità e l’efficacia di questo processo anche a livello industriale.

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Tipo di EPrint:Tesi di dottorato
Relatore:Bruschi, Stefania
Correlatore:Ghiotti, Andrea
Dottorato (corsi e scuole):Ciclo 28 > Scuole 28 > INGEGNERIA INDUSTRIALE > INGEGNERIA CHIMICA, DEI MATERIALI E MECCANICA
Data di deposito della tesi:31 Gennaio 2016
Anno di Pubblicazione:31 Gennaio 2016
Parole chiave (italiano / inglese):Sheet metal, Hot stamping, Aluminium alloy, Hot deformation, Metal forming, Hot Metal Gas Forming, Tube forming
Settori scientifico-disciplinari MIUR:Area 09 - Ingegneria industriale e dell'informazione > ING-IND/16 Tecnologie e sistemi di lavorazione
Struttura di riferimento:Dipartimenti > Dipartimento di Ingegneria Industriale
Codice ID:9474
Depositato il:20 Ott 2016 09:55
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