Based on the above results, it is clear that the growth pattern of Si primary dendrites has been changed as increasing the Si content in the Al-Si alloy. Generally, on solidification of the Al-Si hypereutectic alloy, the growth of plate-like Si primary dendrites lies on (111) planes and in [211] directions, where it occurs by the twin plane re-entrance edge (TPRE) mechanism [24]. Figure 8(a) illustrates schematically the growth model of the plate-like Si primary dendrites. In the case of Al-30wt.% Si alloy, the Si primary dendrites grew following this model, forming a thin plate-like morphology (shown in Fig. 2(a)). Due to the fragility of pure silicon or by over-energetic sieving, some thin plates were believed to be broken into fine particles (< 0.5 mm) after washing and sieving, which were classified as eutectic Si. This may be the reason why the experimental recovery rate is only 67.2% of the theoretical value.
Increasing the Si content, more Si atoms are supplied. As solidification proceeds, Si atoms diffuse to the growth front and are trapped by grain surfaces. One possibility is that the Si atoms are located at twin grain edges associated to the TPRE growth mechanism, resulting in a fast growth along < 211 > direction (length direction). The other possibility is that they are located at flat surfaces associated to the lateral growth mechanism [22], resulting in a growth tendency along < 111 > direction (width direction). For the Al-Si alloy with Si content below 50 ~ 55wt.%, both the length and width of the Si primary dendrites increased with increasing Si content, as shown in Fig. 3. This could be named coarsening process for the primary Si dendrites. The coarsened Si plates were not easily broken during sieving and washing processes, thereby improved the experimental recovery rate up to 94.9% of the theoretical recovery rate for the Al-55wt.% Si alloy.
When the Si content further increasing to more than 55wt.%, the average length of the Si primary dendrites decreased (shown in Fig. 3), indicating the advantage of the preferred growth along < 211 > direction was hindered. From the result of Fig. 2(e)-(f), more Si primary dendrites appeared, i.e. higher grain density in alloys with Si content of 60 ~ 70wt.%. During solidification, grain growth competes with each other. As the growing front with < 211 > growth direction meets neighboring grains, the growth will be hindered. This hindering effect would be enhanced with increasing the Si content, resulting in the decrease in length of the dendrites as increasing the Si content. However, the growth along < 111 > direction was not hindered since the typical two-dimension structure of the dendrites. Thereby, a multi-layered structure formed for the Al-60wt.% Si alloy (illustrated in Fig. 6), leading to the further increase in thickness.
Generally, the plate-like Si primary dendrites have a sharp tip morphology along < 211 > fast growth direction [24], which would be easily broken due to its fragile property, especially surrounded by plenty of neighboring grains. The broken tip usually has small size (< 0.5 mm, as shown in Fig. 4(b) marked as A), which could not be collected after leaching, resulting in a consumption of the refined Si. On the other hand, some crystals were peeled off from the coarsened or multi-layered dendrites due to extremely high thermal stress, as like the broken part marked B in Fig. 4(c), leading to the other type of consumption. These two kinds of broken crystal contributed to the reduction of the experimental recovery rate for alloys with Si content range of 55 ~ 70wt.%. For the further analysis, Si particles with size range of 0.2 ~ 0.5 mm was employed to characterize the broken crystals, and the statistic results are shown in Fig. 7, where the weight fractions of broken crystals to the total defined eutectic Si (< 0.5 mm) for Al-55wt.% Si, Al-60wt.% Si, and Al-70wt.% Si alloys were calculated, respectively. It can be seen from Fig. 7 that the weight fraction of broken crystals increases with increasing the Si content, indicating the enhanced broken effect at higher Si content. Thus, it is evident that the broken process has a detrimental effect on the recovery of the refined Si.
Consequently, we could summarize the growth pattern of the primary Si dendrites in the Al-Si hypereutectic alloy with various Si contents, as schematic shown in Fig. 8. In the investigated content range, i.e., 30 ~ 70wt.%, the growth of plate-like Si primary dendrites is dominated by the TPRE mechanism. For the Al-30 ~ 50 wt.% Si alloys, the Si primary dendrites grew largely along < 211 > and < 111 > directions with increasing the Si content, i.e., the primary dendrites underwent a coarsening process, resulting in an increase in experimental recovery rate of the refined Si. The highest recovery rate could be achieved for Al-50 ~ 55 wt.% Si alloys. In the case of Al-55 ~ 70 wt.% Si alloys, however, the growth along < 211 > direction was inhibited. Meanwhile, the broken effect originated from grain collision and thermal stress will be enhanced as further increasing the Si content, thereby leading to a decrease in the experimental recovery rate.
From the view of the production cost reduction, the optimum composition of the Al-Si alloy for solvent refining has determined as Al-50 ~ 55 wt.% Si, for which a desirable practical recovery rate, as well an attractive impurity removal efficiency of refined Si, could be obtained. The purity analysis will be detailed discussed elsewhere.