EFFECT OF ADDITIONAL ALLOYING AND HEAT TREATMENT ON THE PHASE COMPOSITION AND MORPHOLOGY IN Al-Mg-Si TYPE CASTING ALLOY

Abstract

The structure of permanent mold and high pressure die castings of the AlMg5Si2Mn alloy after alloying with Li and Sc has been investigated by scanning and transmission electron microscopy, hardness and microhardness measurements, energy dispersive X-ray analysis. Three conditions, as cast, solution treated and aged, were investigated. It was shown that in as-cast state, the structure of an alloy having the nominal composition AlMg5Si2Mn consists of four phases: first-the Al based solid solution, second-the (Al) + (Mg 2 Si) eutectic, third-the primary Mg 2 Si crystals and fourth-the a-Al(Mn, Fe)Si phase. Similar phases were observed in the alloys containing Sc or Li. After two days of storing in an as-cast condition, the solid solution in all tested alloys decomposes and forms zebra-crossing shaped precipitates. TEM examinations revealed that these precipitates nucleate heterogeneously on dislocations. The solution treatment at 575.0°C results in spheroidization of the Mg 2 Si lamellas, dissolution of the precipitates and formation of a-Al(Mn, Fe)Si dispersoids, nucleating on the surfaces of Mg 2 Si lamellas. In the Sc containing alloys, the formation of Al 3 Sc was detected after 120 min soaking. Further heating resulted in the growth of these precipitates. Aging of the Al-Mg-Si alloys leads to an increase of hardness in all studied alloys. This effect is mainly related to precipitation strengthening, via solid solution decomposition and formation of b''-phase. In Li-alloyed specimens, plates of b Mg 2 Si phase were observed together with small cubic-shaped d' Al 3 Li precipitates.