Vpliv hitrosti ohlajanja na strjevanje zlitine AA 6008 : diplomsko delo

Podobnik, Jan (author), Vončina, Maja (mentor)

Zlitine na osnovi aluminija zaradi njihovih raznovrstnih lastnosti uvrščamo med najpomembnejše materiale sodobnega časa. Posebno mesto med lahkimi materiali v industrijski rabi zavzemajo zlitine skupine 6xxx, ki poleg aluminija vsebujejo tudi večje količine silicija in magnezija. Gre za skupino zlitin srednjih trdnosti, dobrih korozijskih obstojnosti in izvrstne kombinacije mehanskih lastnosti, ki se večinoma koristijo pri iztiskanju. Zlitine skupine 6xxx so proizvedene s kontinuirnim oz. pol-kontinuirnim ulivanjem drogov. Zaradi neravnotežnega poteka strjevanja, ki je posledica visokih hitrosti ulivanja, so zlitine podvržene nehomogeni porazdelitvi legirnih elementov, nastanku neželenih faz in kristalni segregaciji. Nepravilnosti v materialu negativno vplivajo na številne lastnosti. Za doseganje enakomernih porazdelitev dodanih elementov, taljenje nizko taljivih evtektikov in kristalnih segregacij se koristijo postopki homogenizacije. V sklopu naloge smo analizirali vpliv hitrosti ohlajanja na strjevanje in oblikovanje mikrostrukture zlitine EN AW 6008, ki sodi v skupino 6xxx. Vzorce zlitine smo pridobili iz treh različnih delov rezine litega droga, ki smo jih z namenom preiskave homogenosti analizirali z diferenčno termično analizo DSC ter energijsko disperzijsko spektrometrijo, s katero smo analizirali kemijsko sestavo. Na podlagi DSC-krivulj in termodinamskih izračunov smo določili potek taljenja in predvideli, katere ravnotežne faze so prisotne v strjeni mikrostrukturi ter katere faze so v mikrostrukturi prisotne ob upoštevanju pogojev neravnotežnega strjevanja in stopnje difuzivnosti elementov. Vzorec iz litega droga smo tudi stalili in ulili v croning merilno celico ter v jekleni stožec z namenom spremljanja strjevanja preiskovane zlitine pri različnih hitrostih ohlajanja. Vrsto, velikost in porazdelitev mikrostrukturnih sestavin smo analizirali z uporabo optičnega mikroskopa. Iz primerjave DSC-krivulj smo ugotovili znatne razlike v kemijski sestavi med posameznimi mesti lite rezine droga ter naše ugotovitve potrdili z rezultati kemijske analize. Pri analizi mikrostrukture smo opazili manjšanje primarnih zrn z večjimi hitrostmi ohlajanja. Evtektske faze pa so z večanjem hitrosti ohlajanja postale vse bolj zaobljene in razpršene.

• Language: Slovenian
• Keywords: skupina 6xxx, DSC analiza, nehomogenost, hitrost ohlajanja, strjevanje, mikrostruktura.
• Work type: Bachelor thesis/paper
• Typology: 2.11 - Undergraduate Thesis
• Organization: NTF - Faculty of Natural Sciences and Engineering
• Place of publishing: Ljubljana
• Publisher: [J. Podobnik]
• Year: 2021
• Number of pages: XI, 32 f.
• PID: 20.500.12556/RUL-131175
• UDC: 669
• COBISS.SI-ID: 85688067
• Publication date in RUL: 23.09.2021

PODOBNIK, Jan, 2021, Vpliv hitrosti ohlajanja na strjevanje zlitine AA 6008 : diplomsko delo [online]. Bachelor’s thesis. Ljubljana : J. Podobnik. [Accessed 16 March 2025]. Retrieved from: https://repozitorij.uni-lj.si/IzpisGradiva.php?lang=eng&id=131175

Secondary language

• Language: English
• Title: Influence of cooling rate on the solidification of AA 6008 alloy : diploma work

Abstract

Aluminium-based alloys are due to their diverse range of properties classified among the most important materials of modern times. A special place among light materials in industrial use is occupied by alloys of the 6xxx group, which, in addition to aluminium, also contain significant amounts of silicon and magnesium. It is a group of alloys of medium strength, good corrosion resistance and an excellent combination of mechanical properties, which are mostly used in extrusion. Alloys of group 6xxx are produced with continuous or by semi-continuous casting in rods. Due to the unequilibrium solidification process, which is a consequence of high casting speeds, alloys are subjected to inhomogeneous distribution of alloying elements, formation of undesired metastable phases and crystal segregation. Irregularities in the material negatively affect many properties. Homogenization processes are used to achieve an even distribution of added elements, melting of low-melting eutectics and crystal segregations. In the frame of this work, we analyzed the influence of the cooling rate on solidification and the formation of the microstructure of the alloy EN AW 6008, which belongs to the group 6xxx. Alloy samples were obtained from three different parts of a cast rod slice, which were analyzed for the purpose of investigating homogeneity by DSC differential thermal analysis and energy dispersion spectrometry, which was used to analyze the chemical composition. Based on DSC curves and thermodynamic calculations, we predicted the melting / solidification process and predicted which equilibrium phases are present in the solidified microstructure and which phases are present in the microstructure considering the conditions of nonequilibrium solidification and the degree of diffusivity of the elements. The cast rod sample was also melted and poured into a croning measuring cell and into a steel cone mould to monitor the solidification of the investigated alloy at different cooling rates. The type, size, and distribution of microstructural components were analyzed using an optical microscope. From the comparison of DSC curves, we found significant differences in the chemical composition between the individual sites of the cast slice of the rod and confirmed our findings with the results of chemical analysis. In the microstructure analysis, a decrease in primary grains with higher cooling rates was observed. The eutectic phases, however, became more rounded and diffuse as the cooling rate increased.

• Keywords: group 6xxx, DSC analysis, inhomogeneity, cooling rate, solidification microstructure.