The aluminium alloys are materials extensively employed in various application fields, for example the transportation one, thanks to their characteristic lightness and to their adequate corrosion resistance in different environments. However, they suffer from a limited mechanical strength. In order to tackle this deficit, a possible solution is the introduction of a reinforcement, usually of ceramic nature, in the matrix; the new materials are called metal matrix composites (MMCs). While the low density is preserved, the corrosion resistances may decrease. To address this issue, a superficial treatment, which increases the superficial hardness and the isolation from aggressive agents, is required. One of the best coatings which allow to obtain these results is the diamond-like carbon (DLC) film, but this layer is too thin to prevent the substrate from being subjected to intensive stresses caused by external loads. For this reason, a second superficial treatment, applied previously to the deposition of the DLC film, must be employed for producing a cover with characteristics lying between those one of the metallic component and the outer layer (the DLC one). The overall structure is known as double layer. The most obvious choice for this role falls on a known and industrially spread process: this is the anodizing treatment. In this work, after a long discussion of the anodizing process both from the theorical and industrial point of view, the attention is focused on the optimization of the oxy-hydroxide cover’s properties obtained through anodizing of two specific aluminium alloys: the latter are the 6082 and the 7075 alloys, both heat treatable alloys. For each alloy, a suitable pretreatment’s sequence has been selected, following this scheme: alkaline bath, rinse, acid bath, rinse. Than, the samples have been anodized in different electrolytical bathes, chosen in function of the information available in literature. In order to compare the results obtained with different anodizing conditions, it has been chosen to fix current density at 20 mA/cm2 for 60 minutes. At the end, the specimens have been heated to remove the residual water. The anodized films have been analysed by Vickers’ microindentation test. For the covers which had given the higher values of hardness, Scanning Electron Microscopy (SEM) analyses have been performed both the surfaces and for the cross-sections. The chemical composition has been acquired by studying the X-rays emitted by the film during the interaction with the electrons’ beam (EDS-SEM). The analyses’ results pointed out two anodizing conditions for each alloy which have allowed to obtained hard, thick and homogeneous covers. For the 6082 alloy, the best process conditions are: (1) sulphuric acid 1.88 M at 2°C and (12) sulphuric acid 1.84 M and citric acid 0.80 M at 15°C. For the 7075 alloy, the best results have been obtained by the following solutions: (3) sulphuric acid 1.03 M at 2°C and (9) a mixture of sulphuric acid 2.06 M, oxalic acid 0.08 M, ethylene glycol 0.36 M, hydrochloric acid 0.036 M and aluminium chloride 3 mM at 15°C. Then, four samples for each alloy (the hardest ones) were cut in two parts, one of which was sealed by immersion in boiling water for 1 hour. The so-obtained 16 specimens have been subsequently coated with a DLC layer grown by Plasma Assisted Chemical Vapor Deposition (PACVD) in an industrial plant. Lastly, these samples plus the two untreated alloys were tested by linear potential scan voltammetry (LPS) and by wear-resistance test. The LPS voltammetry and the wear resistance test have shown that the samples anodized in condition 9 (for the 6082 alloy), 4 and 12 (for the 7075 one) and subsequently sealed offer the most effective protection to corrosion and wear attacks.
Le leghe di alluminio sono dei materiali estensivamente utilizzati in diversi settori applicativi, come per esempio quello relativo ai trasporti, grazie alla loro caratteristica leggerezza e alla resistenza ai fenomeni di corrosione in diversi ambienti di comune impiego. Soffrono però di una limitata resistenza meccanica. Per sopperire a questo deficit, una possibile soluzione è quella di introdurre nella matrice dei rinforzi, solitamente di natura ceramica, dando origine così ai materiali compositi a matrice metallica (MMC). Mentre viene preservata la bassa densità, la resistenza a corrosione può calare. Per affrontare questo problema, è richiesto un trattamento superficiale che implementi la durezza superficiale e l’isolamento agli agenti aggressivi. Uno dei migliori rivestimenti che permettono di ottenere questo risultato è un film in diamond-like carbon (DLC), ma questo rivestimento è troppo sottile per impedire che il substrato sia soggetto a sforzi intensivi dovuti all’applicazione di carichi esterni. Per questo motivo, deve essere applicato un secondo trattamento superficiale, precedente alla deposizione del DLC, che permetta di produrre un film con proprietà intermedie tra quelle del componente metallico e quelle del rivestimento in DLC. Il sistema complessivo viene definito doppio film (Double Layer). La scelta più ovvia per questo trattamento propedeutico ricade su un processo noto e diffuso a livello industriale: si sta parlando del processo di anodizzazione. In questo lavoro, dopo un’estesa trattazione del processo di anodizzazione sia dal punto di vista teorico sia da quello del processo industriale, si focalizza l’attenzione sull’ottimizzazione delle caratteristiche del rivestimento di ossiidrossido ottenuto via anodizzazione per due leghe particolari: esse sono la lega 6082 e la lega 7075, entrambe facente parti della famiglia delle leghe di alluminio trattabili termicamente. Per ciascuna delle leghe è stata selezionata un’opportuna sequenza di pretrattamenti della superficie da anodizzare seguendo il seguente schema: bagno alcalino, risciacquo, bagno acido, risciacquo. Successivamente, i componenti sono stati anodizzati in diversi bagni elettrochimici, selezionati in base alle informazioni reperite in letteratura. Per confrontare i risultati ottenuti con diverse condizioni di anodizzazione, si è scelto di imporre una densità di corrente di 20 mA/cm2 per 60 minuti in tutte le condizioni. Infine, i prodotti sono stati riscaldati per rimuovere l’acqua residua. I film anodizzati sono stati analizzati con test di microindentazione Vickers. Per i rivestimenti che hanno mostrato i più alti valori di durezza, si è proceduto all’analisi via Scanning Electron Microscopy (SEM) sia della superficie che della sezione. La composizione chimica è stata rilevata grazie allo stesso strumento analizzando i raggi X emessi dalla superficie durante l’interazione con il fascio elettronico (EDS-SEM). I risultati delle analisi hanno individuato due condizioni di anodizzazione che hanno permesso di produrre un rivestimento duro, spesso e omogeneo. Per la lega 6082, le due condizioni migliori sono: (1) acido solforico 1.88 M a 2°C e (12) acido solforico 1.84 M e acido citrico 0.80 M a 15°C. Per la lega 7075, i risultati migliori si ottengono con: (3) acido solforico 1.03 M a 2°C e (9) una soluzione di acido solforico 2.06 M, acido ossalico 0.08 M, glicole etilenico 0.36 M, acido cloridrico 0.036 M e cloruro di alluminio 3 mM a 15°C. I quattro provini per ciascuna lega che hanno mostrato i più alti valori di durezza sono stati tagliati in due parti, una delle quali è stata sigillata attraverso un’immersione del componente in acqua bollente per un’ora. I sedici campioni così ottenuti sono poi stati ricoperti con un film di DLC mediante deposizione da Plasma-Assisted Chemical Vapour Deposition (PACVD) in un impianto industriale. Infine, i campioni più le due leghe non anodizzate sono stati testati attraverso una prova di resistenza ad usura e una prova potentiodinamica a scansione di potenziale lineare. Le prove potenziodinamiche e quelle di usura hanno mostrato che i provini anodizzati nelle condizioni 9 (per la lega 6982), 4 e 12 (per la lega 7075) e successivamente sigillati sono quelli più resistenti ad un attacco corrosivo ed ad usura.
Anodizing of 6082 and 7075 aluminium alloys as a hardening treatment for duplex covering
Palamini, Alberto
2019/2020
Abstract
The aluminium alloys are materials extensively employed in various application fields, for example the transportation one, thanks to their characteristic lightness and to their adequate corrosion resistance in different environments. However, they suffer from a limited mechanical strength. In order to tackle this deficit, a possible solution is the introduction of a reinforcement, usually of ceramic nature, in the matrix; the new materials are called metal matrix composites (MMCs). While the low density is preserved, the corrosion resistances may decrease. To address this issue, a superficial treatment, which increases the superficial hardness and the isolation from aggressive agents, is required. One of the best coatings which allow to obtain these results is the diamond-like carbon (DLC) film, but this layer is too thin to prevent the substrate from being subjected to intensive stresses caused by external loads. For this reason, a second superficial treatment, applied previously to the deposition of the DLC film, must be employed for producing a cover with characteristics lying between those one of the metallic component and the outer layer (the DLC one). The overall structure is known as double layer. The most obvious choice for this role falls on a known and industrially spread process: this is the anodizing treatment. In this work, after a long discussion of the anodizing process both from the theorical and industrial point of view, the attention is focused on the optimization of the oxy-hydroxide cover’s properties obtained through anodizing of two specific aluminium alloys: the latter are the 6082 and the 7075 alloys, both heat treatable alloys. For each alloy, a suitable pretreatment’s sequence has been selected, following this scheme: alkaline bath, rinse, acid bath, rinse. Than, the samples have been anodized in different electrolytical bathes, chosen in function of the information available in literature. In order to compare the results obtained with different anodizing conditions, it has been chosen to fix current density at 20 mA/cm2 for 60 minutes. At the end, the specimens have been heated to remove the residual water. The anodized films have been analysed by Vickers’ microindentation test. For the covers which had given the higher values of hardness, Scanning Electron Microscopy (SEM) analyses have been performed both the surfaces and for the cross-sections. The chemical composition has been acquired by studying the X-rays emitted by the film during the interaction with the electrons’ beam (EDS-SEM). The analyses’ results pointed out two anodizing conditions for each alloy which have allowed to obtained hard, thick and homogeneous covers. For the 6082 alloy, the best process conditions are: (1) sulphuric acid 1.88 M at 2°C and (12) sulphuric acid 1.84 M and citric acid 0.80 M at 15°C. For the 7075 alloy, the best results have been obtained by the following solutions: (3) sulphuric acid 1.03 M at 2°C and (9) a mixture of sulphuric acid 2.06 M, oxalic acid 0.08 M, ethylene glycol 0.36 M, hydrochloric acid 0.036 M and aluminium chloride 3 mM at 15°C. Then, four samples for each alloy (the hardest ones) were cut in two parts, one of which was sealed by immersion in boiling water for 1 hour. The so-obtained 16 specimens have been subsequently coated with a DLC layer grown by Plasma Assisted Chemical Vapor Deposition (PACVD) in an industrial plant. Lastly, these samples plus the two untreated alloys were tested by linear potential scan voltammetry (LPS) and by wear-resistance test. The LPS voltammetry and the wear resistance test have shown that the samples anodized in condition 9 (for the 6082 alloy), 4 and 12 (for the 7075 one) and subsequently sealed offer the most effective protection to corrosion and wear attacks.| File | Dimensione | Formato | |
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Anodizing Of 6082 And 7075 Aluminium Alloys As A Hardening Treatment For Duplex Covering.pdf
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https://hdl.handle.net/10589/175018