The effect of the refractory material on the phase transformation parameteres during forming of the Al-8wt%Si-3wt%Cu structure

Abstract

Očvršćavanje legura aluminijuma se najčešće odvija u termootpornim oblogama sačinjenim ili od metala ili od oksida koji su stabilni na visokim temperaturama. Značajno različita toplotna provodljivost između metala i termootpornih oksida prouzrokuje očvršćavanje sa različitim brzinama hlađenja. U ovom radu smo formirali sekundarnu leguru Al-8wt%Si-3wt%Cu u kalupu od nerđajućeg čelika, tankozidnoj čaši od nerđajućeg čelika i debelozidnoj čaši od cirkonijum oksida. Tok formiranja očvrsle strukture je praćen zaronjenim termoparovima koji su omogućili kompjutersku analizu krive hlađenja. Parametri na koje je brzina hlađenja imala najznačajniji uticaj su vrednosti pothlađenja formiranja primarnih kristala aluminijuma, vreme rasta primarnih kristala aluminijuma i ukupno vreme očvršćavanja od pojave prvih čvrstih kristala do formiranja potpuno očvrsle strukture. Al-8wt%Si-3wt%Cu legura aluminijuma formirana u metalnim kalupima ima manju veličinu zrna i posledočno veću zateznu čvrstoću, manju makroporoznost i manje hrapavu površinu od legure formirane u oblogama od termootpornih oksida. Ispitivane termootporne obloge su uticale na formiranje tri osnovna mikrokonstituenta u Al-8wt%Si-3wt%Cu leguri u različitim vremenskim intervalima, što je dovelo do formiranja različite mikrostrukture, gde odluka o izboru materijala termootporne obloge zavisi od isplativosti celokupnog procesa, zahtevanog kvaliteta spoljnih površina i zahtevane minimalne čvrstoće konačnog proizvoda.Solidification of the aluminum alloys takes place in heat-resistant refractory materials made of either metal or oxides that are stable at high temperatures. The significantly different thermal conductivities between metals and heat-resistant oxides cause solidification with significantly different cooling rates. In this work, we formed a secondary Al-8wt%Si-3wt%Cu alloy in a stainless steel mould, a thin-walled stainless steel cup, and a thick-walled zirconium oxide cup. The course of the formation of the solidified structure was monitored by immersed thermocouples, which enabled the computer analysis of the cooling curves. The parameters on which the cooling rate had the most significant influence are the undercooling values of the formation of primary aluminum crystals, the time of the growth of primary aluminum crystals and the total solidification time from the formation of the first solid crystals to the formation of a fully solidified structure. The Al-8wt%Si-3wt%Cu aluminum alloy formed in metal mould has a smaller grain size and consequently higher tensile strength, lower macro-porosity and less rough surface than the alloy formed in refractory oxide coatings. The examined heat-resistant refractory material influenced the formation of three basic micro-constituents in the Al-8wt%Si-3wt%Cu alloy in different time intervals, which led to the formation of a different microstructure, where the decision on the choice of material for the heat-resistant refractory material depends on the profitability of the entire process, the required quality of the external surfaces and required minimum strength of the final product