Ova doktorska disertacija obuhvatila je istraživanja na aluminijumskim legurama iz serije 6000 (Al-Mg-Si legure, komercijalne aluminijumske legure EN AW-6060 i EN AW-6082). Na pomenutim legurama sprovedena je različita termomehanička obrada, nakon čega je ispitivan uticaj režima prerade na mehanička, fizička, toplotna i strukturna svojstva. Ispitivanja su rađena u cilju karakterizacije ispitivanih legura i sagledavanja uticaja prisutnih legirajućih elemenata (hemijskog sastava) na njihove strukturne, mehaničke, fizičke i toplotne karakteristike čime je dat i određeni naučni i stručni doprinos boljem poznavanju ispitivanih Al-Mg-Si legura nakon starenja (prirodnog i veštačkog), kao i nakon termomehaničke obrade koja je sprovedena u cilju proučavanja njenog uticaja na efekat ojačavanja starenjem. Ciljevi istraživanja usmerili su ovu disertaciju tako da doprinese boljem poznavanju uticaja režima primenjene termomehaničke obrade na strukturu i osobine ispitivanih Al-Mg-Si legura, što će uz nesumnjiv teorijski doprinos omogućiti i proširenje njihove primene u najsavremenijim industrijama, gde je trenutno velika potražnja za materijalima sa aluminijumskom osnovom sa poboljšanim osobinama.
Doktorska disertacija je podeljena u osam poglavlja. U uvodnom delu dat je kratak osvrt na veliku potražnju u industriji u poslednjim decenijama za materijalima koji po određenim osobinama mogu zameniti materijale iz oblasti crne metalurgije, s obzirom na to da njihova proizvodnja iziskuje veliku potrošnju energije i dovodi do zagađivanja životne sredine, kao i na ulogu aluminijuma i njegovih legura u ovom domenu.
Osnovne napomene o aluminijumu i legurama na njegovoj osnovi, uz kratak osvrt na primenljivost ovih legura u industriji, kao i kratak opis uticaja postupaka prerada na strukturu i karakteristike aluminijuma i njegovih legura prikazan je u drugom poglavlju.
U trećem poglavlju opisane su osnovne teorijske postavke i uslovi za pojavu precipitacionog ili taložnog ojačavanja (ojačavanja starenjem), kao jedne od najčešće korišćenih metoda za poboljšanje osobina kod aluminijumskih legura, pa i kod Al-Mg-Si legura.
U četvrtom poglavlju predstavljeni su postojeći publikovani rezultati o karakterizaciji legura sistema Al-Mg-Si.
Ciljevi istraživanja prikazani su u petom poglavlju.
U šestom poglavlju opisan je način izvođenja eksperimenata i dat je uvid u metode kojima je moguće izvršiti potrebna merenja i karakterizaciju dobijenih uzoraka. Za karakterizaciju legura korišćeno je više savremenih, standardizovanih metoda i tehnika.
Eksperimetalni rezultati svih izvršenih ispitivanja sistematizovani su i izneti u sedmom poglavlju ovog rada uz odgovarajuću analizu i diskusiju rezultata. Legure EN AW-6060 i EN AW-6082 dobijene su od strane „Al-Cu metali― firme iz Niša u obliku flahova i izrađene su po standardima koji diktiraju parametre izrade. Kompletan materijal je podvrgnut žarenju radi uklanjanja fabričke strukture i dobijanja najmekšeg stanja (temper O). Ovo žarenje je urađeno na temperaturi od 550°C u trajanju od 6 h. Nakon uklanjanja fabričke strukture, izdvojeni su uzorci u setove i svaki od setova je bio podvrgnut različitoj termičkoj ili termomehaničkoj obradi.
U prvom setu cilj je bio dobijanje početnih uzoraka koji su korišćeni za poređenje, stoga je grupa uzoraka podvrgnuta rastvornom žarenju u trajanju od 1 h na temperaturi od 550°C i kaljenju u vodi sa ledom radi dobijanja presićenog čvrstog rastvora (temper W).
Zatim je pristupljeno isptivanju parametara koji definišu precipitaciono ojačavanje (starenje). U ovom setu ispitivan je uticaj temperature rastvornog žarenja, pa su stoga uzorci podvrgavani različitim temperaturama rastvornog žarenja (510°C, 530°C, 550°C, 570°C, 590°C), nakon čega su uzorci ispitivani odmah nakon kaljenja, nepotpunog starenja i potpunog starenja.
Rezultati iz ovog seta poslužili su za definisanje optimalne temperature rastvornog žarenja koja bi se koristila u ostalim eksperimenatima.
Nakon toga akcenat je stavljen na ispitivanje parametara veštačkog starenja. Ispitivan je uticaj izotermalnog i izohronog starenja na osobine ispitivanih legura (temper T6). Uzorci su najpre rastvorno žareni na tempearturi od 550°C, pa kaljeni u vodi sa ledom i nakon toga su veštački stareni. Izotermalno starenje je definisano sa dve različite temperature 180°C i 200°C i vršeno je u trajanju od 1 h do 8 h, nakon čega su uzorci bili podvrgnuti karakterizaciji. Pri izohronom starenju korišćena su dva vremena, 30 minuta i 60 minuta, sa opsegom temperatura od 160°C-330°C.
Nakon toga ispitivan je uticaj prirodnog starenja na osobine legura, kao i uticaj prirodnog starenja na veštačko starenje (temper T4 i uticaj tempera T4 na temper T6). Uzorci su najpre rastvorno žareni na temperaturi od 550°C, pa kaljeni u vodi sa ledom i nakon toga su prirodno stareni na sobnoj temperaturi različito vreme: 3, 10, 25, 40, 55 i 70 dana. Posle tako definisanih vremena prirodnog starenja, uzorci su podvrgnuti veštačkom starenju na temperaturi od 180°C u trajanju od 5 h za leguru EN AW-6060 i 6 h za leguru EN AW-6082.
Uticaj termomehaničke obrade na ispitivane legure pratio je uticaj stepena unete deformacije na precipitaciono ojačavanje. Naime, svi uzorci su rastvorno žareni na temperaturi od 550°C nakon čega su kaljeni u vodi sa ledom. Jedna grupa kaljenih uzoraka je hladno valjana različitim stepenima deformacije od 20 %, 30 %, 40 % i 50 %, a zatim su veštački stareni prema parametrima određenim za izotermalno starenje, gde su dobijene maksimalne vrednosti tvrdoće, a to je starenje na temperaturi od 180°C u trajanju od 5 h za leguru EN AW-6060 i 6 h za leguru EN AW-6082. Druga grupa uzoraka je odmah posle kaljenja veštački starena, pa je nakon starenja podvrgnuta hladnom valjanju stepenima deformacije od 20 %, 30 %, 40 % i 50 %.
Nakon sprovedenih postupaka termomehaničke obrade vršena su ispitivanja mehaničkih, fizičkih, toplotnih i strukturnih osobina legura. Ispitivanja mehaničkih osobina obuhvatila su merenja tvrdoće i mikrotvrdoće po Vickers-u. Promene fizičkih osobina praćene su merenjem električne provodnosti. Toplotne osobine ispitivane su uz pomoć merenja toplotne difuzivnosti, toplotne provodnosti i specifične toplote, kao i uz pomoć DSC/DTA analize, dok je mikrostrukturna analiza urađena primenom optičke mikroskopije (LOM), skenirajuće elektronske mikroskopije sa energetsko-disperzivnom spektroskopijom (SEM-EDS) i transmisione elektronske mikroskopije (TEM).
Dobijeni rezultati pokazuju da je nakon izvršenog žarenja dominirala ravnotežna β faza. Dobijene su najniže vrednosti tvrdoće i mikrotvrdoće, i visoke vrednosti električne provodnosti i toplotne difuzivnosti, dok je mikrostrukturna analiza pokazala visoku homogenost strukture kod svih uzoraka.
Ispitivanje presićenog čvrstog rastvora pokazalo je da su vrednosti tvrdoće i mikrotvrdoće porasle, a vrednosti električne provodnosti opale, dok je toplotna difuzivnost gotovo ostala indentična u poređenju sa žarenim uzorcima. Na ispitivanim legurama urađena je DSC/DTA analiza koja je pokazala opšte prihvaćeni precipitacioni niz sa pikovima u karakterističnim temperaturnim opsezima. Temperaturni opsezi na DSC/DTA krivama u kojima se javila precipitacija poslužili su za definsanje ostalih eksperimenata, posebno onih koji su uključivali veštačko starenje. Nakon DSC/DTA analize sproveden je postupak merenja toplotnih osobina u temperaturnim opsezima koji odgovaraju temperaturnim pikovima sa DSC/DTA krivih. Toplotna difuzivnost i toplotna provodnost ispitivanih legura dostižu najviše vrednosti u oblastima temperatura gde su se pojavili pikovi pri DSC/DTA analizi.
Temperatura rastvornog žarenja ja imala uticaja na osobine ispitivanih legura. Ovaj uticaj je posebno evidentan kod više legirane legure i pri potpunom starenju. Ova grupa eksperimenata definisala je temperaturu rastvornog žarenja koja je korišćena u svim ostalim eksperimentima.
Izotermalno kao i izohrono starenje je imalo veliki uticaj na osobine ispitivanih legura. Mehaničke osobine starenih uzoraka obe legure postepeno rastu u odnosu na kaljeno stanje, dostižući maksimalne vrednosti usled precipitacionog ojačavanja i pojave metastabilne β’’ faze i nakon toga opadajući usled prestarevanja legura. Električna provodnost, toplotna provodnost i toplotna difuzivnost postepeno rastu sa porastom intenziteta precipitacije. Mikrostrukturna
ispitivanja potvrđuju rezultate dobijene merenjem mehaničkih, fizičkih i toplotnih osobina.
Prirodno starenje je pokazalo veliki uticaj na osobine ispitivanih legura, kao i na proces naknadnog veštačkog starenja. Mehaničke osobine su postepeno rasle zbog ojačavanja starenjem, dok je porast vrednosti fizičkih osobina zavisio od tipa legure. Uticaj prirodnog starenja na veštačko je takođe zavisio od tipa legure i bio je ili pozitivan ili negativan u zavisnosti od količine legirajućih elemenata u leguri.
Termomehanička obrada imala je velikog uticaja na osobine ispitivanih legura. Kod obe legure mehaničke osobine su poboljšane sa porastom stepena unete deformacije, bez obzira na to da li je deformacija uneta pre ili posle veštačkog starenja. Električna provodnost kod obe legure opada sa porastom stepena unete deformacije i niža je od one dobijene samo za veštački staren uzorak, a viša od onih dobijenih za kaljeno stanje. Uneta deformacija imala je veliki uticaj na raspadanje presićenog čvrstog rastvora, na homogenizaciju strukture i raspored legirajućih elemenata u strukturi, što je pokazano uz pomoć SEM-EDS analize i mape distribucije elemenata.
Na kraju u osmom poglavlju, na osnovu razmatranja i analize svih dobijenih rezultata uobličeni su zaključci o uticaju primenjene termomehaničke obrade na poboljšanje osobina ispitivanih komercijalnih aluminijumskih legura EN AW-6060 i EN AW-6082. Naglašena je međusobna zavisnost mikrostrukture i ispitivanih osobina legura od primenjenog termomehaničkog režima prerade. Izvedeni zaključci sadrže koncizno izražene rezultate istraživanja koji odgovaraju postavljenim ciljevima istraživanja.
Svi dobijeni rezultati su potvrđeni i literaturnim podacima, a u velikoj meri su dali i nova saznanja o uticaju primenjene termomehaničke obrade na poboljšanje osobina EN AW-6060 i EN AW-6082 legura počevši od: primene kratkotrajnog visokotemperaturnog starenja u industrijskim uslovima za izradu aluminijumskih legura u obliku tempera T6 pri kojima su dobijene visoke vrednosti mehaničkih, fizičkih i toplotnih osobina; uticaja prirodnog starenja koje gotovo ne može biti izbegnuto čak ni u najsavremenijim uslovima pri izradi aluminijumskih legura; primene deformacije pre i posle veštačkog starenja, što omogućava dobijanje veoma visokih vrednosti mehaničkih osobina i dobro homogenizovanu mikrostrukturu kod ispitivanih legura iz ove serije; mogućnosti primene dobijenih rezultata u najsavremenijim industrijama gde je trenutno velika potražnja za materijalima sa poboljšanim toplotnim osobinama.
This doctoral dissertation included the investigation of aluminium alloys from 6000 series (Al-Mg-Si alloys, commercial aluminium alloys EN AW-6060 and EN AW-6082). Various thermomechanical treatments (TMT) were applied to these alloys, after which the influence of these regimes on the mechanical, physical, thermal and structural properties was examined. The investigations were performed in order to characterize the alloys and to investigate the influence of alloying elements (chemical composition) on structural, mechanical, physical and thermal properties. These investigations also gave a certain scientific and professional contribution to better understand the investigated Al-Mg-Si alloys after ageing (natural or artificial), as well as after the thermomechanical treatment that was applied in order to study its effect on the precipitation hardening process. The research aim in this doctoral dissertation was to contribute to a better understanding of the influence of applied thermomechanical treatment on the structure and properties of the investigated Al-Mg-Si alloys. This will undoubtedly give the theoretical contribution and the expansion of the use of these alloys in modern industry, where currently there is a high demand for aluminium-based materials with improved properties.
The doctoral dissertation is divided into eight chapters. The introductory section gives a brief overview of the great demand in the industry for materials that can replace materials in the field of ferrous metallurgy, since the production of ferrous metals requires high energy consumption and leads to environmental pollution, as well as to the role of aluminium and its alloys in this domain.
Basic notes on aluminium and its alloys, with a brief overview of the applicability of these alloys to the industry, as well as a brief description of the impact of processing operations on the structure and characteristics of aluminium and its alloys, are presented in Chapter Two.
The third chapter describes the basic theoretical assumptions and conditions for the appearance of precipitation hardening or ageing as one of the most commonly used methods for improving the properties of aluminium alloys including the Al-Mg-Si alloys.
Chapter 4 presents the published results on the characterization of Al-Mg-Si alloys.
The research goals are presented in Chapter Five.
The sixth chapter describes the method of performing the experiments and gives an insight into the methods that can be used to perform the necessary measurements and characterization of the obtained samples. Several modern, standardized methods and techniques have been used to characterize the alloys.
The experimental results of all the tests performed are systematized and presented in the seventh chapter of this paper with appropriate analysis and discussion of the results. The alloys EN AW-6060 and EN AW-6082 were produced and delivered by the company ―Al-Cu metali‖ in the form of rectangular bars. All necessary standards that dictate the manufacturing parameters were followed during the production. The received material was subjected to annealing to remove the fabricated state and to obtain the lowest strength and highest ductility (temper O). This annealing was done at a temperature of 550°C for a period of six hours. After obtaining adequate temper, the bars were cut to the appropriate dimensions and the samples of both alloys in this state were separated to be subjected to different HT or TMT.
Firstly, the goal was to obtain the initial samples which would later be used for comparison in every other experiment. The samples of both alloys were subjected to solution heat treatment at 550°C and then quenched in ice water in order to obtain a super saturated solid solution (αssss) (temper W).
After that, investigation of the parameters that define the ageing treatment was started. In this set of samples, the influence of the solutionizing temperature was examined, and in this set
samples were subjected to different solutionizing temperatures (510°C, 530°C, 550°C, 570°C, 590°C), where the samples were examined immediately after quenching, after partial ageing and after complete ageing. Results of these experiments were later used as a guide in defining the optimal solution heat treatment temperature.
Focus was then put on the investigation of the parameters of artificial ageing. The influence of isothermal and isochronal ageing on the properties of the investigated alloys (temper T6) was examined. The samples were solution heat treated at 550°C for 1 hour, quenched in ice water and then artificially aged. Isothermal ageing was defined at two different temperatures, 180°C and 200°C and was carried out for 1 to 8 hours after which the samples were subjected to characterization. In isochronal ageing, two 30-minute and 60-minute intervals were used with a wide range of temperatures from 160°C to 330°C.
The effect of natural ageing (NA) on various properties of the investigated alloys, as well as the effect of natural ageing on artificial ageing (AA) (temper T4; and the influence of temper T4 on the temper T6) was then investigated. The samples were solution heat treated at 550°C for 1 hour, quenched in an ice water and then naturally aged for 3, 10, 25, 40, 55 and 70 days. After natural ageing, samples were subjected to artificial ageing at 180°C for 5 h (EN AW-6060) and at 180°C for 6 h (EN AW-6082).
Lastly, in the fourth set of experiments, the influence of TMT on the properties of the investigated alloys was studied. The influence of deformation on precipitation hardening was studied by introducing a different degree of deformation. The samples were solution heat treated at 550°C for 1 hour, quenched in ice water. One group of samples was cold rolled with different deformation degrees (20 %, 30 %, 40 %, 50 %) and then artificially aged according to the parameters obtained after the isothermal ageing for maximal hardness. The second group was aged according to the parameters obtained after the isothermal ageing for maximal hardness and then cold rolled with different degrees of deformation (20 %, 30 %, 40 %, 50 %).
Following the HT or TMT procedures described in this way, measurements of mechanical, physical, thermal and structural properties were made, as appropriate. Mechanical properties were conducted by measuring hardness and microhardness by Vickers method. Changes in physical properties were monitored by measuring electrical conductivity. Thermal properties were investigated by obtaining the values for thermal diffusivity, conductivity and specific heat. Microstructural changes were detected by optical microscopy (LOM) and scanning electron microscopy coupled with energy-dispersive spectroscopy (SEM-EDS).
After annealing, equilibrium β phase dominated the structure. The lowest values of hardness and microhardness were obtained while electrical conductivity and thermal diffusivity values were high. Microscopic investigation confirmed this information and showed high level of homogenity for all samples.
The investigation of quenched state showed that the values of hardness and microhardness increased, electrical conductivity decreased and thermal diffusivity stayed the same in comparison to the heterogeneous annealed samples. DSC/DTA analysis was performed on quenched samples, showing the precipitation sequence and temperature ranges for the formation of characteristic phases that are generally accepted in scientific literature. The DSC/DTA analysis was used for definition of other experiments, especially the ones involving the artificial ageing. Immediately after the DSC/DTA analysis, investigation of the thermal properties in the temperature ranges corresponding to the temperature peaks from the DSC/DTA analysis was performed. Thermal diffusivity and thermal conductivity have the highest values in the temperature ranges where peaks occurred during DSC/DTA analysis.
The temperature of the solution heat treatment influenced the properties of investigated alloys. This influence is particularly evident in the alloy with the higher amount of alloying elements and after complete ageing. This group of experiments defined the solution heat treatment temperature that was used in all other experiments.
Isothermal as well as isochronal ageing had a great influence on the properties of the investigated alloys. The mechanical properties of the aged samples of both alloys gradually increase
with ageing relative to the quenched state, reaching a maximum due to the precipitation hardening and the presence of the metastable β'' phase. Subsequently, values decline due to the overageing of the alloys. Electrical conductivity, thermal conductivity and thermal diffusivity gradually increase with increasing precipitation and relaxation of the matrix. Microstructural investigation confirms the results obtained by measuring mechanical, physical and thermal properties.
Natural ageing has shown a great influence on the properties of the investigated alloys, as well as on the subsequent artificial ageing. The mechanical properties gradually increased, while the increase in physical properties depended on the type of alloy. The effect of natural ageing on artificial was also dependent on the alloy and was either positive or negative depending on the amount of alloying elements.
TMT had a great influence on the properties of investigated alloys. In both alloys, mechanical properties increased with the degree of deformation, regardless of whether it was introduced before or after artificial ageing. The electrical conductivity in both alloys decreases with increasing amount of deformation and is lower than that obtained only for the artificially aged specimen and higher than those obtained for the quenched state. The applied deformation had a great influence on the decomposition of the supersaturated solid solution, on the homogenization of the structure and the arrangement of the alloying elements in the structure, which was demonstrated by means of SEM-EDS analysis and its mapping feature.
Finally, in Chapter 8, based on the consideration and analysis of all the obtained results, conclusions were drawn on the effect of applied thermomechanical treatment on the improvement of the properties of the tested commercial aluminium alloys EN AW-6060 and EN AW-6082. The dependence of the microstructure and the investigated properties of the alloys on the applied thermomechanical processing regime is emphasized. The drawn conclusions contain concisely expressed results of the research that meet the set goals of the research.
All obtained results additionally confirmed the literature data, and to a large extent gave new insights about the influence of HT or TMT on different properties EN AW-6060 and EN AW-6082 alloys. Starting with the application of short-term high-temperature ageing in industry conditions for the production of aluminium alloys in T6 temper. This ageing treatment gave high values of mechanical, physical and thermal properties. Natural ageing effects that are almost impossible to avoid even in the most advanced aluminium alloy plants. Application of deformation before and after artificial ageing, which gave high values of mechanical properties and excellent homogenized microstructure for alloys in this series. Ability to apply the information obtained from the experiments in the most advanced industries where there is currently a high demand for materials with improved thermal properties.
