Dodatek litija aluminijevim livarskim zlitinam : diplomsko delo

Tršar, Domen (avtor), Petrič, Mitja (mentor)

Aluminijeve zlitine z dodatkom litija so znane že skoraj 100 let. Uporabljajo jih v letalski in vesoljski industriji. Glavna prednost dodatka litija je zmanjšanje gostote in izboljšanje mehanskih lastnosti aluminijevih zlitin, še posebej modula elastičnosti in trdnosti le teh. Vsak dodani odstotek litija zmanjša gostoto zlitine za 3 %. Mehanske lastnosti izboljša izločanje faze Al3Li. Glavna pomanjkljivost dotičnih zlitin pa je visoka reaktivnost litija. Ta reagira s kisikom in dušikom ter tvori pline, ki povzročajo plinsko poroznost. Obstajajo kompoziti, ki bi v teoriji lahko zamenjali zlitine aluminija in litija, vendar so ti cenovno manj dostopni. Cilj diplomske naloge je bil raziskati vpliv ohlajevalne hitrosti na kvaliteto ulitkov zlitine AlSi7MgLi in opisati potek strjevanja zlitine. Zlitino smo ulivali v tri različne forme: jekleno formo, Croning formo in grafitno formo. Z opravljeno termično analizo smo spremljali proces strjevanja vzorcev, litih v merilne celice. Ugotovili smo, da hitrejša ohlajevalna hitrost podaja bolj kakovostne ulitke z manj poroznosti. S programom Thermo-Calc smo izračunali termodinamično ravnotežje zlitin. Proučili smo, katere faze naj bi se pojavile v mikrostrukturi vzorcev. Vzorce smo nato analizirali s pomočjo vrstičnega elektronskega in svetlobnega mikroskopa. Tako smo dokazali tudi obstoj določenih faz ter obrazložili potek strjevanja zlitine. Ta se prične z strjevanjem nove faze AlLiSi. Nadaljuje se s strjevanjem faze ?-Al in evtektika (?-Al + AlLiSi). Nato se tvori železova faza Al9Fe2Si2. Sledi strjevanje ternarnega evtektika (?-Al + ß-Si + AlLiSi). Nato pride do transformacije železove faze Al9Fe2Si2 v ? fazo (Al18Fe2Mg7Si10) Na koncu se strjuje še faza Mg2Si. Mehanske lastnosti zlitine smo izmerili s pomočjo naprave za merjenje trdote po Vickersu. Razvidno je, da je trdota višja pri vzorcih z višjo ohlajevalno hitrostjo.

• Jezik: Slovenski jezik
• Ključne besede: Al-Li zlitine, mikrostruktura, mehanske lastnosti, potek strjevanja, izločanje
• Vrsta gradiva: Diplomsko delo/naloga
• Tipologija: 2.11 - Diplomsko delo
• Organizacija: NTF - Naravoslovnotehniška fakulteta
• Kraj izida: Ljubljana
• Založnik: [D. Tršar]
• Leto izida: 2021
• Št. strani: XIII, 40 f.
• PID: 20.500.12556/RUL-131173
• UDK: 669
• COBISS.SI-ID: 85667843
• Datum objave v RUL: 23.09.2021

Sekundarni jezik

• Jezik: Angleški jezik
• Naslov: Lithium addition to aluminium cast alloys : diploma work

Izvleček

Aluminium alloys with lithium addition have been known for nearly 100 years in the aerospace industry. The main advantage of lithium addition is the reduction of the alloy density and the improvement of the mechanical properties, especially the modulus of elasticity and strength. Each added percent of lithium reduces the density of the alloy by 3 %. Improved mechanical properties are caused by the precipitation of the Al3Li precipitate. The main disadvantage of these alloys is the high reactivity of lithium. Lithium reacts with oxygen and nitrogen and forms gases which cause gas porosity. There are composites that could in theory replace aluminium and lithium alloys, however these are less affordable. The aim of the diploma work was to investigate the influence of cooling rate on the quality of AlSi7MgLi alloy and to describe the sequence of solidification of the alloy. The alloy was cast in three different moulds: steel mould, Croning mould and graphite mould. The method of simple thermal analysis allowed us to monitor the solidification process of the alloy. We found that a faster cooling rate delivers higher quality castings with less porosity. Using the Thermo-Calc program, we calculated the thermodynamic equilibrium of the alloys. We examined which phases are expected to occur in the microstructure of the samples. The samples were then analysed using a scanning electron microscope and a light microscope. This also proved the existence of certain phases and explained the course of solidification of the alloy. This begins with the solidification of the new AlLiSi phase. The solidification sequence continues with the solidification of the Al dendrites and eutectic (α-Al + AlLiSi). The iron phase Al9Fe2Si2 is then formed. Then follows the solidification of the ternary eutectic (α-Al + β-si + AlLiSi). After that comes the transformation of the iron phase Al9Fe2Si2 into the π phase (Al18Fe2Mg7Si10). At the end forms the Mg2Si phase. The mechanical properties of the alloy were measured using a Vickers hardness measuring device. It is seen that the hardness is higher in samples with a higher cooling rate.

• Ključne besede: Al-Li alloys, microstructure, mechanical properties, solidification sequence, precipitation