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The aim of this work was to assess the repeated-remelting influence upon the mechanical properties, thermomechanical properties, chemical composition and structure changes of the selected material. An Al-Cu-type aluminum alloy was chosen on the basis of the ever increasing experiments with non-ferrous metals in the industry. The technical nomenclature of the selected alloy is RR.350 according to the German standard ALUFOND 60. The RR.350 alloy is known for its poor foundry properties which deteriorate due to remelting and affect mechanical properties and the cast-material structure. This negative influence upon the structure and usable properties of a re-melted alloy is further confirmed in the submitted paper. The samples for tensile-strength determination were cast into a metal mould. The gating system and the riser, which served as a charge for the second melt, were removed from the casting. In this way we re-melted the material four times. The samples were machined and ruptured within the temperature range between 20 °C and 350 °C. A sample for metallography and hardness determination (HBS) was taken from the cast material. It can be seen in the tensile-strength diagram that the mechanical properties of the first melt are higher, by 11 % at the temperature of 20 °C, than the properties of the third melt. This difference is evident up to 100 °C. At the temperatures of above 100 °C the cast-material strength characteristics are the same. This tendency shows itself on all the materials tested so far. The hardness and microhardness evaluations show that the material reaches the highest values with the fourth melt. This phenomenon is attributed to the repeated reoxidation and exclusion of oxide membranes. Further, the material structure properties and chemical-composition change were evaluated. The results of this study confirmed a negative influence of alloy remelting upon the material properties and structure. Namen tega dela je bil oceniti ponavljanje pretaljevanja na mehanske lastnosti, termomehanske lastnosti, kemijsko sestavo in spremembe mikrostrukture izbranega materiala. Izbrana je bila vrsta aluminijeve zlitine Al-Cu na podlagi naraščanja preizkusov na neželeznih kovinah v industriji. Tehnična oznaka izbrane zlitine, skladno z nemškim standardom ALUFOND 60, je RR.350. Zlitina RR.350 je poznana zaradi slabih livarskih lastnosti, ki se s pretaljevanjem poslabšujejo in vplivajo na spremembe mehanskih lastnosti in mikrostrukture v litem stanju. V predstavljenem članku je potrjen negativni vpliv pretaljevanja na uporabne lastnosti. Vzorci za natezne preizkuse so bili uliti v kovinsko kokilo. Ulivni in napajalni sistem, ki se je uporabljal za sekundarno napajanje, je bil odstranjen iz ulitka. Tako je bil material štirikrat pretaljen. Izdelani vzorci so bili porušeni v temperaturnem intervalu med 20 °C in 350 °C. Iz ulitega materiala so bili odrezani vzorci za metalografijo in za meritve trdote (HBS). Iz nateznih diagramov je razvidno, da ima prva talina za okrog 11 % višjo natezno trdnost pri 20 °C v primerjavi s tretjo talino. Ta razlika se opazi do temperature 100 °C. Pri temperaturah nad 100 °C so trdnostne lastnosti materiala v litem stanju enake. Ta tendenca se je pokazala pri vseh do sedaj preizkušenih materialih. Primerjava trdote in mikrotrdote kažeta, da je najvišjo vrednost dosegel material četrte taline. Ta pojav se pripisuje ponovljeni reoksidaciji in odsotnosti oksidnih kožic. Ocenjene so bile tudi značilnosti mikrostrukture in določene kemijske sestave. Rezultati teh preiskav so potrdili negativen vpliv večkratnega pretaljevanja na mikrostrukturo in lastnosti materiala. |
