A progressive refinement in alloy microstructure was observed with decreasing step thickness for all casting and heat treatment conditions. As demonstrated in Fig. 2, the 6.4 mm steps featured finer dendritic structures and secondary phases than the 25.4 mm steps. This refinement was caused by an increase in solidification rate, which is typically present in the thinner sections of castings. As shown in Fig. 3, the average SDAS of the 25.4, 12.7 and 6.4 mm steps were 40, 27 and 19 µm, respectively. For a given step size, there was no considerable difference in SDAS between each alloy condition, with any minimal changes attributable to casting variation. As displayed by the optical micrographs in Fig. 2, the observed Base microstructure was typical for 319 alloy (Ref 2, 35). In addition to the α-Al matrix, the eutectic Si, α-Al15(Fe,Mn)3Si2, β-Al5FeSi, Al2Cu, and Al5Mg8Cu2Si6 phases were observed. For all alloy conditions, the α-Al15(Fe,Mn)3Si2 phases appeared coarse with Chinese script morphologies, whereas the β-Al5FeSi phases appeared as needles. The Al2Cu and Al5Mg8Cu2Si6 phases were present in complex eutectic structures. The morphologies of the Si and Al-2Cu phases were influenced by the Sr content and heat treatment condition.
In the Base castings (Fig. 2), the eutectic Si particles, appearing darkest in the optical micrographs, were coarse and acicular, as typically observed in the unmodified condition (Ref 36). However, the sharp edges of the particles were significantly rounded in both the HIP and HT conditions, as depicted in Fig. 4(a) and (b). Spheroidization of the Si particles during heat treatment has been reported in the literature for various Al-Si alloys, and its effectiveness was found to be enhanced as both the temperature and treatment time were increased (Ref 14,15,16). Evidently, the 2-h treatment at 500 °C conducted on both conditions was sufficient to achieve a similar result, albeit at an early stage before complete spheroidization. In contrast, Sr additions were highly effective at transforming the Si particles into a fine, fibrous morphology, as shown in Fig. 4(c) and (d). While the Si particles in the 50Sr condition could be characterized as partially modified, fine lamellae, the 150Sr condition featured a well-modified, fibrous eutectic structure. The other secondary phases were not noticeably affected by the Sr additions.
Furthermore, the Al-2Cu particles were considerably influenced by the 500 °C heat treatments in both the HIP and HT conditions. The Base condition featured relatively large, blocky Al2Cu particles and some eutectic Al2Cu lamellar structures (Fig. 5a). However, these phases were largely dissolved during the heat treatments, as indicated by their severely fragmented morphologies in Fig. 5(b). This dissolution was quantified by Al2Cu phase area percentage measurements, as shown in Fig. 6. The Al2Cu area percentage in the microstructure did not vary much with decreasing step thickness, despite the change in solidification rate. As well, Sr content did not have much effect on the amount of Al2Cu present relative to the Base castings. All three of these conditions had an average area percentage of approximately 2%. In contrast, both the HIP and HT conditions equivalently featured significant reductions in Al2Cu area percentage to an average of approximately 0.6%, which is characteristic for the solution heat treatment of this alloy (Ref 2). Given the similarity between these two conditions, it is evident that the Al2Cu dissolution was not affected by the high pressure during the HIP treatment, but rather was caused by the elevated temperature, itself.
Physical and Mechanical Properties
The density measurements of the samples are shown in Fig. 7. The average density of the Base casting was approximately 2.773 g/cm3, without considerable difference between the step thicknesses. This corresponded well to the pore-free density of 319 alloy, typically listed in the literature as 2.79 g/cm3 (Ref 32), which indicated approximate porosity levels of only 0.6%. The 12.7 mm step solidified faster than the 25.4 mm step of the casting, and interdendritic shrinkage pores tend to be smaller and more uniformly distributed in finer microstructures (Ref 35, 37). Nonetheless, the change in SDAS between these two steps (27 to 40 µm) was likely too minimal to achieve an appreciable change in density.
Furthermore, density did not vary much between the Base and the HT condition, but high-temperature treatments at 500 °C were not expected to influence porosity (Ref 2). However, the HIP treatment increased the density of the 25.4 mm step to 2.777 g/cm3, which corresponded to 0.1-0.2% less porosity than the Base condition. It was unusual that the high pressure during the HIP treatment did not promote an increase in density in the 12.7 mm step, and that the improvement in density for the larger step thickness was relatively small. This may have been caused by several factors. The HIP treatment was designed to simultaneously apply high temperature and pressure to collapse internal pores by small-scale plastic flow (Ref 6). This works effectively for removing hydrogen gas porosity, as the hydrogen is soluble in the Al matrix and can diffuse out of the casting. Yet, pores filled with nitrogen, oxygen, or other relatively insoluble gases are generally more difficult to collapse using a HIP treatment. Additionally, surface pores and those interconnected to the surface can be infiltrated by the inert gas at high pressure in the HIP environment, which can prevent porosity reductions (Ref 11). Moreover, given that the porosity levels in the Base castings were already relatively low, at less than 1%, it is possible that HIP treatment was unable to promote considerable further improvements in density.
In contrast, density was noticeably decreased by increasing the Sr content, and the effects were more pronounced for the slower solidifying samples. For example, when increasing Sr content from 50 to 150 ppm, the density of the 25.4 mm step was reduced to approximately 2.766 and 2.746 g/cm3, respectively. Yet, the 12.7 mm step only experienced a significant decrease in density in the 150Sr condition, which featured 2.754 g/cm3. Eutectic Si modification has been reported to be associated with increases in porosity levels or changes in its dispersion (Ref 23, 38,39,40,41). This was attributed to increases in hydrogen solubility in molten Al, reductions in the surface tension of the melt, decreases in the required hydrogen concentration for the nucleation of pores, or modification of the characteristics of the solidification growth mode. Since the 50Sr condition was only partially modified, it follows that an intermediate density level was observed. Nonetheless, the reduction in porosity associated with increasing solidification rate for microstructural refinement was evident for both of the Sr-containing alloys.
The mechanical properties of the samples are shown in Fig. 8. For all conditions, there was a general increase in hardness, ultimate tensile strength (UTS), and ductility with decreasing step thickness. Yet, yield strength (YS) was relatively unaffected. For example, the UTS and ductility of the Base casting increased considerably from approximately 144 MPa and 1.5% to 210 MPa and 4.5% between the 25.4 mm and 6.4 mm steps, respectively. This improvement in mechanical properties can be attributed to the refinement in microstructure which is associated with increasing solidification rate in thinner steps, as discussed in Section 3.1. The finer SDAS values and more uniform distribution of finer secondary phases effectively impeded dislocation motion through the material and caused strengthening (Ref 20, 35, 42).
Hardness and UTS were generally increased in the HT condition. For example, the hardness of the 12.7 mm step was improved from approximately 75 to 81 HB, relative to the Base condition. At high temperature, significant Al2Cu dissolution was observed, and the Si particles were found to spheroidize, as discussed in the previous section. Both of these effects influenced mechanical properties. The former promoted solid solution strengthening, but it may have been balanced by weakening due to the loss of stable Al2Cu secondary phase particles (Ref 20, 43). As well, the samples were brought to room temperature after heat treatment with forced air, which may have enabled the formation of some nano-sized Al-Cu precipitates during cooling. Further strengthening likely occurred during precipitation at ambient temperatures following the heat treatment, via natural aging. Moreover, spheroidization of the Si particles can mitigate the stress concentrations promoted by their coarse and acicular morphology. Nonetheless, the heat treatment at 500 °C was only 2 h long, compared to the recommended solution heat treatment time at this temperature of approximately 12 h (Ref 18, 32). Thus, the full effect of a solution heat treatment was likely unrealized. Accordingly, since heat treatments are more effective on finer phases with larger surface area-to-volume ratios, the 25.4 mm steps were not appreciably influenced by the HT condition.
The HIP treatment produced similar results to the HT condition, featuring generally higher hardness, YS and UTS values than the Base condition. Given the equivalent heating temperature and time as the HT condition, the dissolution of Al2Cu and spheroidization of Si occurred in the same way, as presented in the preceding section. Yet, the HIP produced some additional benefits due to the applied high pressure. These were predominantly observed for the 25.4 mm step. As presented in Fig. 7, the densities of the HIP samples were found to be higher than the Base or HT samples for the 25.4 mm step, which was associated with reduced porosity. It is well known that porosity is detrimental to the mechanical properties of a material. Hence, the improvement in the HIP condition relative to the HT condition may be attributed to the decrease in porosity. Furthermore, after high-temperature treatment, the HIP samples were cooled via Ar, whereas the HT samples were cooled with forced air. Consequently, a difference in cooling rates may have promoted a greater supersaturation of Cu following the former treatment, thereby enabling strengthening via natural aging to occur more efficiently.
Porosity also significantly affected the mechanical properties of the Sr-containing alloys. As demonstrated in Fig. 8, increasing Sr content progressively decreased the hardness of the samples, down to approximately 71 HB for the 150Sr, 25.3 mm step. Despite the expected benefits to mechanical properties from transforming the Si particles to a fine, fibrous structure, the associated increase in porosity shown in Fig. 7 was likely sufficient to counteract the advantage. Otherwise, the unaffected secondary phases like α-Al15(Fe,Mn)3Si2 may have still acted as fracture initiation sites, promoting the insensitivity of mechanical properties to Si modification observed in this alloy. Nevertheless, compared to the Base condition, the increased ductility of the larger two steps of both Sr-containing alloys to approximately 3% elongation was similar to the two heat treatment conditions. This indicated that the modification of the Si particles, whether thermally or chemically, can effectively enhance alloy ductility. Yet, for optimal results, Sr additions should be combined with porosity removal techniques, such as HIP treatments.
Thermal and Electrical Conductivities
The thermal and electrical conductivities of the 25.4 mm steps are displayed in Fig. 9. The two conductivities corresponded very well to each other for every condition, in accordance with the Wiedemann–Franz law (Ref 20). The thermal and electrical conductivities of the Base casting were found to be approximately 120 W/m K and 28.7% IACS, respectively. These values were close to the standard quantities listed for 319 alloy in reference textbooks, 109 W/m K and 27% IACS (Ref 32). However, standard 319 alloy specifies 5.5-6.5 wt.% Si and 3.0-4.0 wt.% Cu, whereas the composition of the present alloy was on average approximately 5.6 wt.% Si and 2.8 wt.% Cu (Table 2). The higher conductivity can therefore be attributed to the smaller fraction of secondary phases in the microstructure and the reduced concentration of solute in the Al matrix, both of which impede the flow of free electrons.
The conductivity of both the HIP and HT samples increased to approximately 135 W/m K and 32.3% IACS, which was a 12.5% improvement compared to the Base condition. However, the conductivity was nearly identical between these two heat treatment conditions. This indicates that the higher conductivity resulted from the high-temperature treatment itself, with no significant influence from the high pressure applied during the HIP treatment. Even though heat treatment at 500 °C dissolved Cu atoms into the Al solid solution (Fig. 6), which increased the concentration of electron scattering centers, this effect was evidently outweighed by the benefits of Si spheroidization to conductivity. As a result of the rounder Si particles and the coarsening that likely occurred at the expense of smaller particles, the mean free path of the electrons was increased, thereby enhancing both heat and electron transport (Ref 15). Also, some of the dissolved Cu likely precipitated during cooling from 500 °C as well as during natural aging at ambient temperatures prior to the conductivity measurements. Additionally, porosity is known to be detrimental to conductivity (Ref 37, 44), and porosity was somewhat decreased during the HIP treatment (Fig. 7). Nonetheless, the approximate 0.1-0.2% reduction in porosity was likely too minimal to cause a significant change in conductivity.
Conductivity was also progressively improved by increasing the Sr content in the alloy, up to 128 W/m K and 30.7% IACS for the 150Sr condition. This corresponded to almost a 7% improvement in conductivity compared to the Base condition. The addition of Sr was found to modify the Si particles to a fibrous morphology, as discussed in Section 3.1. Similar to high-temperature spheroidization, this modification improved thermal and electrical conductivities by increasing the free electron mobility (Ref 27, 45, 46). The benefits to conductivity increase progressively with the Si modification level, associated with the Sr content. However, at excessive Sr contents, over-modification can occur, causing particle coarsening or reversion to a platelike morphology. For 319 alloy, the optimal Sr addition level was reported to be on the order of 100-200 ppm (Ref 47, 48), which is consistent with the 150Sr condition.
Due to geometric constraints, the effects of casting step thickness on conductivity could not be evaluated in the present study. However, recent work in the literature has shown that conductivity is not affected by solidification rate in the as-cast, unmodified condition (Ref 35, 37), nor in the thermal sand reclamation (TSR) or overaged (T7) heat-treated conditions for a Sr-containing 319 alloy (Ref 2). However, solidification rate has been found to work synergistically with Sr content in modifying the eutectic Si phase (Ref 36). Therefore, it is expected that the 50Sr and 150Sr castings featured slightly higher thermal and electrical conductivities in the smaller step sizes. Similar conductivity results combining solidification rate and Sr content have been reported in the literature (Ref 27, 49).