Utrjevanje in korozijske lastnosti zlitine EN AW-2011 v različnih stanjih : magistrsko delo

Tršar, Ajda (avtor), Nagode, Aleš (mentor)

Aluminijeve zlitine skupine 2xxx, kamor spada preiskovana zlitina EN AW-2011, za dosego optimalnih mehanskih lastnosti potrebujejo ustrezno obdelavo. Pogost in zelo primeren postopek je toplotno oz. izločevalno utrjevanje, ki zajema raztopno žarjenje, gašenje in staranje, zaradi gnetljivosti pa je pogosta tudi mehanska predelava v vročem in hladnem. Ob obdelavi se zlitini spremenijo mikrostrukturne sestavine in lastnosti, ki neposredno vplivajo na korozijsko obstojnost. Cilj magistrskega dela je bil preučiti zlitino EN AW-2011, opraviti mikrostrukturno karakterizacijo po litju, homogenizaciji, izločevalnem utrjevanju ter po hladni deformaciji, pri tem pa ugotoviti vpliv omenjenih operacij na trdoto in korozijsko odpornost. Zlitino EN AW-2011 smo stalili in ulili v bakreni kokili pravokotne oblike, ploščata ulitka pa nato homogenizacijsko žarili 16 ur pri 520 °C. Enega od ulitkov smo vroče valjali pri temperaturi 500 °C, temu pa je sledilo še hladno valjanje do stopnje deformacije 95 %. Drug ulitek smo toplotno obdelovali po postopku izločevalnega utrjevanja, pri čemer je bila temperatura raztopnega žarjenja 525 °C, temperatura staranja 190 °C in čas staranja od 0,5 do 16 ur. Določitev temperature homogenizacijskega žarjenja, temperature staranja in predvsem karakterizacijo mikrostrukturnih sestavin smo si olajšali s faznim diagramom, Scheilovo krivuljo in diagramom deležev faz, izrisanimi s programom Thermo-Calc. Vzorce smo metalografsko pripravili, jim izmerili trdoto z metodo po Vickersu in analizirali mikrostrukturo na svetlobnem in vrstičnem elektronskem mikroskopu. Za analizo kemijske sestave posameznih mikrostrukturnih sestavin je bila uporabljena energijsko disperzijska spektroskopija (EDS). Na izbranih vzorcih smo izvedli potenciodinamske meritve korozije, iz rezultatov katerih smo določili korozijsko obstojnost vzorcev v različnih stanjih. Mikrostrukturo zlitine po izločevalnem utrjevanju sestavljajo kristalna zrna ?-Al, sferični delci Bi-Pb ter intermetalne faze Al7Cu2Fe. Pri staranju nastali izločki so se najprej opazili na kristalnih mejah (t=2 h), nato pa še znotraj kristalnih zrn (t=16 h). Trdota staranih vzorcev je naraščala od 81 HV do 124 HV pri času staranja 4 h, nato pa je začela padati. Hladno deformiranim vzorcem je trdota naraščala od izhodiščnih 103 HV do 146 HV pri 95 % stopnji deformacije. Staranim vzorcem se je korozijska odpornost z daljšim časom staranja slabšala, valjanim pa z večjo stopnjo deformacije izboljševala.

• Jezik: Slovenski jezik
• Ključne besede: zlitina EN AW-2011, deformacijsko utrjevanje, toplotno utrjevanje, Thermo-Calc, mikrostrukturna analiza, korozija, trdota
• Vrsta gradiva: Magistrsko delo/naloga
• Tipologija: 2.09 - Magistrsko delo
• Organizacija: NTF - Naravoslovnotehniška fakulteta
• Kraj izida: Ljubljana
• Založnik: A. Tršar
• Leto izida: 2023
• Št. strani: XVI, 61 f.
• PID: 20.500.12556/RUL-147525
• UDK: 669
• COBISS.SI-ID: 160989443
• Datum objave v RUL: 07.07.2023

Sekundarni jezik

• Jezik: Angleški jezik
• Naslov: Hardening and corrosion properties of EN AW-2011 alloy in different conditions : master's thesis

Izvleček

In order to achieve required mechanical properties, aluminium 2xxx series alloys (including the examined alloy EN AW-2011), require proper processing. Common and very suitable heat treatment is precipitation hardening, which includes solution annealing, quenching, and aging. Due to alloy’s formability, hot or cold mechanical processing can be used too. These treatments alter the microstructural constituents and properties of the alloy, thereby influencing its corrosion resistance. The aim of the master's thesis was to study the EN AW-2011 alloy and analyse its microstructure at casting, homogenization, precipitation hardening and cold working. Additionally, the objective was to investigate how these processes affect the hardness and corrosion resistance of the alloy. The EN AW-2011 alloy was melted and poured into rectangular shaped casting form made of copper. Flat castings were first homogenized at 520 °C for 16 hours. After that, the first sample was subjected to both hot rolling at 500 °C and cold rolling to a strain of 95 %. The second sample, on the other hand, underwent precipitation hardening. Temperature of solution heat treatment was 525 °C, time and temperature of aging were from 0,5 to 16 hours at 190 °C. Choosing temperature of homogenization annealing, temperature of aging and most importantly, characterization of microstructural constituents were much easier using phase diagram, Scheil curve and diagram of solid fractions, all made with Thermo-Calc software. Samples were metallographically prepared. Then, the hardness of the samples was measured using the Vickers method, and the microstructure was analysed using light and scanning electron microscopy. For the chemical composition analysis of certain constituents, energy dispersive X-ray spectroscopy (EDS) was employed. In addition, the corrosion resistance of selected samples was measured using the potentiodynamic method. The microstructure of the alloy after precipitation hardening consists of α-Al crystal grains, spherical Bi-Pb particles, and Al7Cu2Fe intermetallic phases. During aging, precipitates were first observed at the grain boundaries (t=2 h), and later in the grain interior (t=16 h). The hardness of the aged samples increased from 81 HV to 124 HV at the aging time of 4 h, and then started to decrease. The cold worked samples exhibited an increase in hardness from the initial 103 HV to 146 HV at a strain of 95%. The corrosion resistance of the aged samples deteriorated with longer aging times, while the rolled samples exhibited improved corrosion resistance at a higher degree of deformation.

• Ključne besede: EN AW-2011 alloy, work hardening, precipitation hardening, Thermo-Calc, microstructure analysis, corrosion, hardness