Correlações entre microestrutura, parâmetros térmicos e propriedades mecânicas de ligas de alumínio da série 5xxx solidificadas unidirecionalmente

Abstract

Processing high-Mg content Aluminum Alloys by continuous casting technics is considered an industry challenge. The present research aims to characterize 5xxx alloy series, considering composition based on the commercial alloy 5052 with different Mg contents (2.4wt.%, 2.6wt.% and 3.2wt.%, the latter intentionally extrapolated from the maximum limit) through transient directional solidification experiment. The results allowed to comprehend the effect of Mg content with respect to thermal parameters (cooling rate and VL- tip growth rate), microstructural parameters (primary and secondary dendritic spacing - Ai and À2) and mechanical properties (Brinell hardness) of different alloys. Microstructural analyzes were performed using polarized light microscopy and scanning electron microscopy (SEM) of samples taken from different ingot positions (between 3mm and 110mm from the water-cooled face). The experiments reached cooling rates between 7 and 30°C/s (considering the closest thermocouple to the bottom). The experimental growth law for microstructural evolution of Ai were obtained, resulting in: À1 = 640 TL 1/4 to the alloy with 2.4wt.%Mg and À1 = 300 TL 1/4 to the sample with 3.2wt.%Mg, in which experimental the exponent -1/4 is in agreement with the expression proposed by Jackson and Hunt modified by the analytical model of Garcia and Clyne. Secondary dendritic growth laws were also obtained concerning their variation with the growth rates, resulting in the following expressions:X2 = 27 VL-11 for the 5052-2.4wt.%Mg and X2 = 15.0 VL-11for the 5052-2.6wt.%Mg and 5052-3.2wt.%Mg* alloys. Thus, 5052 alloys containing higher Mg content may allow a significant decrease on X2 value for a certain VL. The hardness values were directly related to the size and distribution of the intermetallic particles and showed À2 as the main variable to affect mechanical properties for the tested alloys.