The effect of heat treatment parameters on hardening of AlZn5.5MgCu alloy

V diplomski nalogi smo zasledovali vpliv parametrov toplotne obdelave na utrjevanje zlitine AlZn5.5MgCu (AA 7075). Zlitino smo stalili ter ulili v jekleno kokilo in jo toplotno obdelali. Toplotna obdelava je vključevala homogenizacijsko žarjenje na temperaturi 460 °C, raztopno žarjenje na temperaturi 480 °C, gašenje v vodi ter staranje na sobni temperaturi in temperaturah 100, 200 in 300 °C. Izvedli smo tudi dvostopenjsko staranje pri katerem je prvo staranje potekalo na 100 °C, drugo pa na 200 °C. Zasledovali smo vpliv časa homogenizacijskega žarjenja na mikrostrukturo ter vpliv temperature in časa staranja na trdoto. Mikrostrukturna analiza je bila narejena na vrstičnem mikroskopu, za napovedovanje mikrostrukturnih sestavin in potek strjevanja pa smo uporabili Thermo-Calc. Ugotovili smo, da so v mikrostrukturi v litem stanju poleg dendritov primarne faze α prisotne faze Al12Cu4Mg8Zn, Al7Cu2Fe in Mg2Si, ki so razporejene na mejah kristalnih zrn faze α. Zaradi neravnotežnega strjevanja je opaziti izrazite mikroizceje in neravnotežne evtektike. S Thermo-Calcom smo izračunali, da je likvidus temperatura pri 634,5 °C, temperatura solidus pa pri 461 °C. S homogenizacijskim žarjenjem so bile mikroizceje odpravljene po 4 urah. S toplotnim utrjevanjem smo najvišjo trdoto dosegli pri vzorcu, ki se je naravno staral. Višjo trdoto kot jo ima zlitina v gašenem stanju, smo zasledili še po staranju zlitine na 100 °C do časa staranja 24 ur, staranju na 200 °C, kjer trdota narašča vse do časa staranja 15 minut in po dvostopenjskem staranju do časa staranja 1 ure. Temperatura staranja na 300 °C je bila previsoka, kar se kaže v takojšnem znižanju trdote. V mikrostrukturi zlitine starane na 300 °C so z vrstičnim elektronskim mikroskopom dobro vidni drobni, enakomerno porazdeljeni izločki faze MgZn2.In this thesis the effect of heat treatment parameters on hardening of AlZn5.5MgCu alloy (AA 7075) was followed. The alloy was melted and casted in steel mold and then heat treated. Heat treatment included homogenization annealing on 460 °C, solution heat treatment on 480 °C, quenching in water and aging on room temperature and on temperatures of 100, 200 and 300 °C. Two-stage aging was also performed with the first aging at 100 °C and second at 200 °C. The effect of homogenization annealing time on the microstructure and the effect of the temperature and aging time on hardness were investigated. Microstructure analysis was made by scanning electron microscope (SEM), for predictions of microstructural constituents and solidification process Thermo-Calc was used. It was discovered that in the microstructure of casted alloy, in addition to the dendrites of primary phase α, the phases Al12Cu4Mg8Zn, Al7Cu2Fe and Mg2Si are present, which are distributed on the grain boundaries of phase α. Due to the non-equilibrium solidification the micro segregations and non-equilibrium phases were formed. Thermo-Calc determined that the liquidus temperature of alloy was 643,5 °C and solidus 461 °C. By homogenization annealing the microsegregations were eliminated after 4 hours. With heat treating the highest hardness was reached at specimen that was aging naturally. The higher hardness as hardness of specimen in quenched condition was also found at aging at 100 °C til aging time of 24 hours, at aging at 200 °C, where hardness was increasing till 15 minutes and at two-stage aging till 1 hour. The aging temperature at 300 °C was too high, which was apparent in and immediate decrease in hardness. In the microstructure of the alloy that was aged at 300 °C, fine homogenously distributed precipitates of MgZn2 phase were easily visible by a scanning electron microscope