The columnar to equiaxed transition of horizontal unsteady-state directionally solidified Al-Si alloys

Abstract

Experiments were conducted to investigate the influence of thermal parameters on the columnar to equiaxed transition during the horizontal unsteady-state directional solidification of Al-Si alloys. The parameters analyzed include the heat transfer coefficients, growth rates, cooling rates, temperature gradients and composition. A combined theoretical and experimental approach is developed to determine the solidification thermal variables considered. The increasing solute content in Al-Si alloys was not found to affect significantly the experimental position of the CET which occurred for cooling rates in the range between 0.35 and 0.64 K/s for any of three alloy compositions examined. A comparative analysis between the results of this work and those from the literature proposed to analyze the CET during upward vertical solidification of Al-Si alloys is reported and the results have shown that the end of the columnar region during horizontal directional solidification is abbreviated as a result of about six times higher thermal gradient than that verified during upward unidirectional solidification of alloys investigated.

Influence of upward and horizontal growth direction on microstructure and microhardness of an unsteady-state directionally solidified Al-Cu-Si alloy

“…It is observed in Figures 6a and 6b that power laws equal to -1.1 and -0.55 characterize the experimental variation of primary spacing with growth rate and cooling rate, respectively, i.e, λ1 = 96(VL) -1.1 and λ1 = 396(TR) -0.55 . This is in agreement with observations reported by ROCHA et al [10], PERES et al [5], CARVALHO et al [7], CRUZ et al [15] BARROS et al [13] COSTA et al [12] and GOMES [16] that exponential relationships and λ1 = constant(VL) -1.1 and λ1 = constant(TR) -0.55 best generate the experimental variation of primary dendritic arms with VL and TR along the unsteady-state solidification of Al-Cu, Al-Si, Al-Sn, Al-Cu, Al-Cu-Si and Al-Cu-Si alloys, respectively. With a view to analyzing the effect of Si element in binary Al-3wt.% Cu alloy as well as the influence of growth direction on the length scale of the dendritic microstructure (1) the average, maximum and minimum values of the correlation between 1 and TR of this work are plotted in Figure 7 and compared with the experimental equations obtained by BARROS et al [13] and GOMES [16], whose works have been developed to horizontal and upward directional solidification, respectively.…”

“…In order to determine more accurate values of these parameters, the results of experimental thermal analysis, shown in Figure 4a have been used to generate a plot of position from the metal/ mold interface as a function of time corresponding to the liquidus front passing by each thermocouple. This methodology has been detailed in our recently published articles [7][8][9][10][11][12][13][14]. A curve fitting technique on such experimental points has generated a power function of position as a function of time (see Figure 4b).…”

“…The derivative of this function with respect to time has yielded values for VL, shown in Figure 5a. Figure 5b shows the TR profile, which was calculated by considering the thermal data recorded immediately after the passing of the liquidus front by each thermocouple [7][8][9][10][11][12][13][14]. It can be seen in Figures 5a and 5b that the use of a water cooled mold imposes higher values of VL and TR near the metal/mold interface and a decreasing profile along the casting due to the increasing thermal resistance of the solidified layer with distance from the heat-extracting surface.…”

“…Nowadays these alloys are supplied in a wide range of chemical compositions [1][2][3][4][5][6][7][8][9][10][11][12]. We highlight the Al-Cu-Si ternary system because of particular outstanding properties such as high mechanical strength, low weight and very good fluidity [3][4][5][6][7][8][9][10][11][12].…”

“…This is known that the values of 1, 2 and 3 are strongly influenced by solidification thermal parameters such as growth rate (VL) and cooling rate (TR) [1,2,3,[7][8][9][10][11][12]. Several directional solidification studies have been reported in the literature to characterize and quantify these parameters as a function of solute concentration C0, VL and TR.…”