Influence of AlSi7Mg alloy melt state on microstructure and mechanical properties

Zlitina AlSi7Mg je toplotno obdelovalna zlitina, ki je podvržena umetnemu staranju, da doseže večjo trdnost. Ulivanje obravnavane zlitine poteka v razmeroma ozkem temperaturnem območju, ki je določeno s sestavo taline, debelino stene ulitka ter velikostjo in težo ulitka. Če je temperatura ulivanja nižja od optimalne, se votlina forme ne napolni, tanke stene se prehitro strdijo, kar posledično povzroči motnje v procesu, poroznost ter slabo površino ulitka. Visoka temperatura ulivanja povzroči krčenje med strjevanjem in posledično izgubo dimenzijske točnosti. Visoka temperatura prav tako podaljša čas strjevanja ter tako zmanjša produktivnost. V okviru magistrskega dela smo na laboratorijskem nivoju simulirali tehnološko pot priprave taline zlitine AlSi7Mg. Cilj magistrskega dela je bil ugotoviti optimalne pogoje priprave taline in najprimernejšo območje livnih temperatur. Pri eksperimentalnem delu smo talino pregreli in ulivali pri različnih temperaturah. Med strjevanjem smo zasledovali ohlajevalno krivuljo v dveh različnih merilnih celicah. S programom ThermoCalc smo izvedli termodinamični izračun. Ulitke smo razrezali in pripravili vzorce za kemijsko analizo, analizo DSC, metalografske in mehanske preiskave. Pri kemijski analizi med posameznimi vzorci ni prišlo do bistvenih sprememb, zato je potek strjevanja vzorcev ulitih v iste celice podoben. Tudi karakteristične temperature, reakcijske entalpije ter mikrostruktura so med vzorci primerljive. Razlike se pojavijo pri hitrosti ohlajanja med vzorci, ulitimi v celice, narejene po postopku Croning, in vzorci, ulitimi v jeklene celice, kjer je mikrostruktura bolj fina zaradi večjih ohlajevalnih hitrosti. Iz termodinamičnih izračunov in mikrostruktur, posnetih z elektronsko vrstično mikroskopijo ter energijsko disperzijsko spektroskopijo, lahko predvidevamo, da so v mikrostrukturi primarni zmesni kristali ?-Al in evtektske faze ß-Si, Al18Fe2Mg7Si10, Mg2Si, Al9Fe2Si2 ter Al3Ti. Rezultati meje tečenja, natezne trdnosti, raztezka in trdote se med vzorci bistveno ne razlikujejo, iz rezultatov pa lahko predvidevamo, da je najprimernejša temperatura litja v območju od 730 °C do 750 °C.AlSi7Mg alloy is a heat treatment alloy that is subjected to artificial aging to achieve higher strength. The casting of this alloy takes place in a relatively narrow temperature range, which is determined by the composition of the melt, the wall thickness of the casting, as well as the size and weight of the casting. If the casting temperature is lower than optimal, the cavity of the mould does not fill upthe thin walls solidified too quickly, which in turn leads to disruptions of the process, porosity, and poor casting surface. High casting temperature causes shrinkage during solidification and consequent loss of dimensional accuracy. The high temperature also prolongs the solidification time, thus reducing productivity. As part of the master\u27s thesis, we simulated the technological path of preparation of the melt of the AlSi7Mg alloy at the laboratory level. The aim of the master\u27s thesis was to determine the optimal conditions for melt preparation and the most suitable range of pouring temperatures. In the experimental part, the melt was overheated and poured at different temperatures. During solidification, a cooling curve was observed in two different measuring cells. We performed a thermodynamic calculation using Thermo-Calc software. The castings were cut and samples were prepared for chemical analysis, DSC analysis, metallographic and mechanical testing. In the chemical analysis, there were no significant changes between the individual samples, so the course of solidification of the samples cast into the same cells is similar. The characteristic temperatures, reaction enthalpy and microstructure are also comparable between the samples. Differences occur in the cooling rate between samples cast into cells made in the Croning process and samples cast into steel cells, where the microstructure is finer due to higher cooling rates. From thermodynamic calculations and microstructures recorded by scanning electron microscopy and EDS analysis, it is assumed that there are the primary α-Al crystals and eutectic phases β-Si, Al18Fe2Mg7Si10, Mg2Si, Al9Fe2Si2, and Al3Ti in the microstructure. The results of yield strength, tensile strength, elongation and toughness do not differ significantly between the samples, and the results indicate that the most appropriate casting temperature is in the range from 730 to 750 ⁰C