Evolution of Microstructure in Semi-Solid of Al-2wt%Si-0.5wt%Mg and Al-4wt%Si-05wt% Alloys during Isothermal Holding

Abstract

Thixoforming technology has made considerable advances since its initial discovery in 1973. Although, after almost 40 years, few raw materials are still used as thixoforming materials. The Al-Si-Mg alloys often used for semi-solid state processing are notably casting alloys like A356 and A357. The development of new alloys specially tailored for thixoforming is a new challenge in this area of research. This paper will present some results related to the study of Al-2wt%Si-05wt%Mg and Al-4wt%Si-05wt%Mg alloy obtained via conventional foundry followed by ultra-refining. The results are compared with a commercial Al-7wt%Si-05wt%Mg alloy. Also presents its morphological evolution in the re-heating of the material to obtain the solid fraction of 45% and 60% in four different holding times 0, 30, 90 e 210 seconds. After re-heating, with the determination of grain size and globules, held the study Rheocast Quality Index (RQI) and Shape Factor (SF), parameter that helps to quantify the efficiency of the treatment globularization to obtain semi-solid structure with globular structure. The morphological evolution showed that the increase of holding time resulted in a minor increase in the globule size of the structure. The RQI values show that there is little morphological evolution in the treatment of globularization for alloys with percentage of Si, indicating alloys with percentage below of 4wt% Si is the most appropriate for thixoforming.