Processing and Microstructure Characteristics of As-Cast A356 Alloys Manufactured via Ultrasonic Cavitation during Solidification

The microstructures of the middle and bottom parts

Figure 3 shows the optical microscopy results in the middle part of the cast ingots. As shown in Figure 3(a) and (b), the A356 samples without UST treatment during solidification have coarser microstructures than the ones with UST treatment. Applying UST during solidification can break up the dendritic structure and thus more fine globular grain structure can be obtained. Figure 3(c) and (d) shows the microstructure of A356 with nanoparticles (Al₂O₃ or SiC) with UST treatment during solidification. They have similar microstructures with those shown in Figure 3(b), which means adding nanoparticles will not change the microstructure significantly, and the UST is the main cause of grain refinement. This is confirmed by Figure 3(e), which shows that coarser dendritic grains were obtained in the middle part of the A356 sample reinforced with 1.0 mass% Al₂O₃ processed without UST treatment during solidification.

Figure 3:

Optical micrographs in the middle part of the cast samples (a) A356 without UST treatment [9] and (b) A356 with UST treatment during solidification and (c) A356 sample reinforced with 1.0 mass% Al₂O₃ and UST during solidification and (d) A356 sample reinforced with 1.0 mass% SiC and UST during solidification and (e) A356 sample reinforced with 1.0 mass% Al₂O₃ without UST during solidification.

The mechanisms of how UST during solidification refine the microstructure can be related to the pressure and temperature cycle change in the melt. They are probably enhancing dendrite fragmentation and increase heterogeneous nucleation in the melt during solidification [22]. UST can produce strong convection and shock waves. Strong convection can cause and promotes diffusion of solute, what’s more, convection will cause local temperature and composition variations. Shock waves can induce the breakage of the melting root. Since dendrites usually start melting at the root because of local temperature rise and segregation, UST can promote dendrite fragmentation. On the other hand, when applying UST, melt temperature will change periodically at high frequencies, which means some part of melt is superheated and some part is undercooled. This phenomenon causes increase of nuclei in the melt, which will refine the microstructure [23].

The microstructure of the top part

The height of the top part of the ingots is about 25 mm, which is the part that is adjacent to the ultrasonic probe. As shown in Figure 4(a), the microstructure of A356 obtained via 3 min UST treatment during solidification is dendritic grain, which fits the conclusion of some literature very well [24]. In [24], the authors indicated that the ultrasonic grain refinement occurs almost below the radiating face of the probe, so microstructure of the top part is not as good as the part below the radiating face of the probe. However, all the top part of the samples with the added nanoparticles (Al₂O₃ or SiC) and UST during solidification have fine globular grains, as shown in Figure 4(b) and (c). However, the microstructure of the top part of the A356 sample reinforced with 1.0 mass% Al₂O₃ without UST during solidification consists of dendritic grains, which is similar with the microstructure shown in Figure 4(a). Figure 4 indicates that the microstructure of the ingot section adjacent to the immersed cylindrical face of the probe can be modified only if both the “addition of nanoparticles” and the “use of UST during solidification” approaches are considered. Applying UST during solidification or adding nanoparticles alone will not change the morphology of the top part microstructure from a dendritic grain to a globular grain. The reason for this can be explained as follows: when applying UST during solidification, the ultrasound intensity of the top part is so weak that it cannot increase the nucleation potential or enhance the dendrite fragmentation (as shown in Figure 4(a)). The added nanoparticles are disperse very well into the melt, but nanoparticles have no effect on grain refinement (as shown in Figure 4(d)) until UST is applied during solidification. The nanoparticles (Al₂O₃ or SiC) in the top part will be activated by weak ultrasound, which in turn will increase the nucleation potential during solidification, and consequently to refine the microstructure (as shown in Figure 4(b) and (c)). In addition, since the ultrasound intensity in the middle and bottom parts is strong enough to refine the microstructure (increase nucleation and enhance dendrite fragmentation), the effect of the UST activated nanoparticles on the microstructure refinement cannot be distinguished.

Figure 4:

Optical micrographs for the top part of the cast samples (a) A356 with UST treatment during solidification and (b) A356 sample reinforced with 1.0 mass% Al₂O₃ and UST during solidification and (c) A356 sample reinforced with 1.0 mass% SiC and UST during solidification and (d) A356 sample reinforced with 1.0 mass% Al₂O₃ without UST during solidification.

Globular grain size distribution

Table 1 shows the globular grain size distribution of the measured samples at various locations. The microstructures of the control group and of the top location of the A356 samples obtained via 3 min UST during solidification have not been measured because they are not revealing pure globular grain structures.

Table 1 shows that for each ingot, the grain size increases from top to bottom and top location has the smallest grain size. The bottom-location globular grain size of A356 samples obtained via 3 min UST during solidification is 151 µm, which is 19.8 % larger than that in the middle location. The addition of the Al₂O₃ nanoparticles can refine the top-location microstructure, grain size of the ingot in the middle location is 19.3 % larger than that in the top location; the bottom-location globular grain size is 45.3 % larger than that in the top location and 21.8 % larger than that in the middle location. The addition of the SiC nanoparticles can refine the microstructure in the top location, grain size of the ingot in the middle location is 10.0 % larger than that in the top location. The globular grain size in the bottom region is 49.0 % larger than that in the top location and 35.6 % larger than that in the middle location. Higher the UST intensity, smaller the grain size; basically the grain size differences indicate the UST intensity differences [24]. The results in Table 1 indicate that UST intensity in the current experiment is high enough to refine the entire ingot microstructure and UST intensity decreases with increasing distance from the radiating face. The globular grain size in top part is smaller than that in the middle and bottom parts of the samples with the added nanoparticles (Al₂O₃ or SiC) and UST during solidification because of the faster cooling rates from the top of the furnace, which was exposed to air cooling, and because of its contact with the ultrasonic probe, which was water cooled.

When compare the same location of different samples, A356 samples without nanoparticles have the smallest grain size. Regarding the globular grain size in the middle location, the addition of the Al₂O₃ nanoparticles increases the grain size by 12.7 % when comparing with that without nanoparticles; the addition of the SiC nanoparticles increases the grain size by 4.7 % when comparing with that without nanoparticles. Regarding the globular grain size in the bottom location, the increase is 14.6 % and 18.5 %, respectively. The comparison results indicate that by adding the nanoparticles the globular grain size will further increase.

Nanoparticle distribution

Figure 5 shows the SEM microstructures at high magnification of the A356 sample reinforced with 1.0 mass % Al₂O₃ and UST during solidification. Many nanoparticles can be found in the matrix as highlighted by the red circle in Figure 5(b) and (c). Those nanoparticles have spherical shape and average size of about 20 nm, which are similar in size with the nanoparticles added during the processed ingots. The SEM results indicate that the Al₂O₃ nanoparticles are dispersed reasonably well into the A356 matrix.

Figure 5:

SEM results of the sample processed with UST + 1.0 mass% Al₂O₃, the red circles highlight the location of the nanoparticles. (a) at X20000 and (b) at X50000 and (c) at X50000.

Figure 6(a) and (b) shows the SEM microstructures at high magnification of the A356 sample reinforced with 1.0 mass % SiC and UST during solidification. Unlike the results in the A356 sample obtained by adding Al₂O₃, a significant number of nanoparticles in Figure 6(b) has been found in the grain boundary area. Those nanoparticles have spherical shape and average size is less than 50 nm, which is similar in size with the nanoparticles added during the manufacturing process. Since the particles are in nanometer size, the EDS accuracy is not enough to identify the particle chemical composition, therefore Figure 6(c) only shows the EDS mapping results at relatively low magnification. In the mapping photo, the red color represents C (carbon), the green color represents Al (aluminum). The EDS results indicated that some of C tends to stay in the grain boundary area. By combining the EDS results with the SEM photo, it can be estimated that the SiC nanoparticles are mostly gather around the grain boundary area.

Figure 6:

SEM results and EDS mapping photo of the sample processed via UST + 1.0 mass% SiC. (a) SEM picture at X500 and (b) SEM picture of red rectangular area in (a) at X50000 and (c) EDS mapping results at X40.

Conclusions

This study determined that the UST treatment during solidification plays a key role in refining the microstructure of the A356 cast ingots. Without UST processing, dendritic structure was obtained; by using UST during solidification, fine globular grain structure was obtained. The addition of the nanoparticles (Al₂O₃ or SiC) is not the main cause for microstructure refinement, however, when UST is applied during solidification, the nanoparticles can modify the microstructure of the ingot part adjacent to the immersed cylindrical face of the probe.

It was determined that the globular grain size increases with increasing the distance from the radiating face. The addition of the nanoparticles (Al₂O₃ or SiC) will slightly increase the grain size.

The SEM and EDS results indicated that the added Al₂O₃ nanoparticles are dispersed reasonably well into the A356 matrix. Some added SiC nanoparticles into the A356 matrix are gathered around the grain boundary area.