It is known that the thermal parameters have an effect on the length of the microstructural scale. It has been evaluated by the experimental results achieved considering the solidification conditions assumed in the present work, such as the growth and cooling rates (VL and TR, respectively). They were obtained from experimental data resulting from horizontal solidification, as presented in Fig. 4a, which in turn were used to determine the values, varying with the position in the as-cast ingot, as shown in Fig. 4b. It is clearly observed that the cooled mold imposes high VL and TR values close to the heat transfer interface, which decrease with the increasing formation of the solid layer. Mathematical power equations given by the general expression A.P-a characterize the VL and TR variation with the position in the as-cast ingot. This significantly influenced the dendritic network, inheriting finer dendritic microstructures for higher VL and TR values, as can be seen in Fig. 4c. As observed, it has been represented by experimental power expressions in the forms of Constant. VL-2/3 and Constant.TR-1/3, which characterize the λ2 coarsening laws with the solidification thermal parameters.
In our works [6, 19, 21, 23] the λ2 values were measured from the cooled interface, in both as-cast and heat-treated samples, processed under the same conditions assumed in the present work. The expressions resulting from the investigations, which characterize the growth laws of λ2 of the aforementioned samples are presented in Fig. 5. It was observed that the heat treatment affected the length of the dendritic microstructure scale, since the λ2 values came be greater in the treated samples by T6, as seen in Fig. 5a. It was also observed that finer microstructures contributed to the dissolution of the Mg2Si-intermetallic compound within the Al-rich matrix [6, 19, 24], as well as potentiated the spheroidization process of the eutectic Si crystals during the solution treatment, as depicted by the microstructures above the graph in Fig. 5. Finer microstructures have also allowed better dissolution of Al2Cu and obtaining spherical-like Si particles during Treatment by T6 in AlSiCu alloys [17, 18, 27, 29].
Figures 6 and 7 present, for both investigated specimens, the results obtained from the tensile tests for two as-cast and heat-treated samples from the heat transfer surface, and studied mechanical properties, respectively. Higher mechanical strengths were achieved for heat-treated samples, as can be seen in Figs. 6a and 6b. It is observed that the analyzed properties performance along the position in the ingot has not allowed to obtain a mathematical relationship as a function of the solidification parameters (VL, TR e λ2), especially for the σUTS values, as seen in Fig. 6a, although finer solidification microstructures have inherited a slight improvement in deformation (E%), as noted in Fig. 6b. This has induced to calculate for both studied samples a resulting average value from all the averages of each position, as shown in Fig. 6c. It can be observed high σUTS and E% values for heat-treated samples, that is, increases from 132 to 194 MPa (≅47%) and 5.8 to 6% (≅5.5%) have been achieved with the heat treatment, confirming the precipitation hardening without deleterious effects on the deformation. In general, the increase in mechanical resistance achieved with heat treatment does not cause damage to deformation, as can be seen in Fig. 6c. Precipitation hardening also have been observed in the results of HV from the work of Reference [20] for the horizontally solidified and heat-treated Al7Si0.3Mg alloy in the same conditions of this work.
However, Souza et al. [29] have established an inverse mathematical relationship of σUTS and E% with λ2 for the horizontally solidified Al7Si3Cu alloy, that is, they have observed higher σUTS and E% values for smaller secondary dendrite arm spacing, but HV and yield strength (σYS) did not vary with λ2. In turn, Sousa et al. [18] have submitted samples of the horizontally solidified Al7Si3Cu alloy to the T6 heat treatment and observed the hardening of the alloy as well as higher HV values with increasing aging time. Chen et al. [24] have reported higher σUTS, σYS and E% values for finer microstructures in as-cast samples and heat treated by T6 of Al7SixMg alloys, also observing higher values of the aforementioned mechanical properties after heat treatment, as well as proposing an inverse mathematical relationship of σUTS, σYS and E% with a function of λ2
Also in a recent study [19], a reversal wear characteristics was observed in as-cast samples of the Al7Si0.3Mg alloy (wt.%) treated by T6, since lower wear volume and rate values were found for finer as-cast microstructures, while the heat-treated microstructures showed the best wear conditions. It has been attributed mainly to the higher λ2 values reported in samples subjected to heat-treatment, as seen in Fig. 5a, as well as the greater amount of spherical-like Si particles for refined microstructures, as noted in Fig. 5b. In addition, the collapse of tertiary dendritic branches during solution treatment as well as the higher global hardness value for greater λ2 also seems to have contributed [19]. Thus, in verification to the results of E%, shown in Fig. 6b, in which the finer heat-treated microstructures have the lowest E% value, combined with the analysis from the author's work [19] on reverse wear parameters performance, it can be deduced that there is an agreement between the results. Ache et al. [28] have depicted better conditions of wear and mechanical properties for finer as-cast and T6-heat treated microstructures of upward solidified Al2MgZn alloys, but they have highlighted that depending on the microstructure features and the contact aspects of the tribological system the wear resistance can have an opposite behaviour, inducing inadequate selection of material condition.
Figures 8 and 9 show the SEM images and element scans mapping by EDS of the fractured areas for two as-cast and heat-treated samples, revealed in two samples solidified under different solidification thermal parameters conditions. In general, it is observed that the features of all fracture surfaces of the studied samples are indicate a mix between brittle and ductile fractures, constituted by cleavage facets, secondary cracks, facets covered with micro-voids, tear ridges and dimples. In addition, the Mg2Si/IMCs-like Mg element, initially segregated within the interdendritic regions of the as-cast samples, as noted by scan mappings in Figs. 8a and 9a, has been completely and uniformly dissolved in the Al-rich matrix during the solution treatment, as can be verified in Figs. 8b and 9b as well as has been reported in our works [6, 19]. It also can be observed by the scan mappings of both investigated samples fibrous-like Si crystals and finer and distributed better Fe-ICMs in position of the ingot near the cooled base, that is, for solidification conditions of high cooling rates and lower λ2 value, as noted in Figs. 7a and 8a.
Regarding morphology and fracture modes analysis from the SEM fractographies of as-cast regions, have allowed to evidence the predominance and the better distribution of small dimples against cleavage facet regions in position close to the heat-transfer surface, deducing the occurrence of a transition from ductile to fragile fracture with decreasing cooling rate as well as with increasing secondary dendritic spacing, as seen in Figs. 8a and 9a, respectively. This fracture feature can be associated by the elongation performance observed along the as-cast ingots, since higher and lower E% values have been obtained for positions near and far from the refrigerated base, respectively, as reported in Fig. 6b. Souza et al. [29] also reported for the horizontally solidified Al7Si3Cu alloy (wt.%) a transition from ductile to brittle fracture along the position of the as-cast ingot, attributing the authors to the decrease in TR as well as the increase in λ2. Chen et al. [24] have depicted that the increase in the cooling rate and, consequently, the refinement of the dendritic microstructure, cause transitions in both fracture path from transgranular to intergranular as well as in the fracture mode from quasi-cleavage for dimple.
In turn, the presence of dominant cleavage regions is observed in the heat-treated fractured samples, as seen in Figs. 8b and 9b, allowing to admit the predominance of brittle fracture in both analyzed positions. It has been observed more extensive cleavage areas in the furthest position from the cooled base, as noted in Fig. 8b. This has inherited a deformation behavior similar to that observed in as-cast samples, i.e., lower E% values have been achieved in heat-treated positions far from the heat-transfer surface.