*3.4. Influence of the Center Segregation Type on the Mechanical Properties of the TRC Strips*

In a previous study, it was shown that the center segregation pattern changed with variation in the casting speed [3]. For the horizontal-type TRC of Al-5Mg alloys, typical channel segregation took place under a relatively low casting speed (3 m/min) and a high RSF (Figure 5a). In contrast, the segregation pattern changed to a segregation band consisting of a mixture of an Mg-rich phase and α-Al as casting speed was increased to the high-speed range, i.e., 5 m/min, which is similar to the microstructure shown in Figure 4c (Figure 5b).

**Figure 5.** 3D images of center segregation in the strips fabricated at (**a**)3m/min, and (**b**)5m/min.

To investigate how the center segregation type affects the mechanical properties of the strips, tensile tests were conducted in both the CD and TD. Figure 6 shows the engineering stress-strain curves

and SEM images of the resulting fracture surfaces. The SEM images on the right-hand side show the magnified microstructure around the center segregation zones of the left-hand images. The obtained results show that the majority of the outer shell region consisted of dimples, which indicate ductile fracture, whereas the Mg-rich segregation zone revealed cleavages, which suggest brittle fracture. For the 3 m/min conditions, the TD sample exhibited a continuous cleavage along the channel segregation (Figure 4c), causing a premature fracture with a rapid stress drop during the tensile test (Figure 4a). In contrast, the 5 m/min samples showed a region of connected dimples and discrete cleavage in the segregation band, and no drastic premature fracture was observed in both the CD and TD, thereby indicating that formation of this band structure should be beneficial in controlling center segregation in the HSTRC process.

**Figure 6.** (**a**) Engineering stress-strain curves and SEM images of the fractured surface of the (**b**)3m/min, CD, (**c**)3m/min, TD, (**d**)5m/min, CD, and (**e**)5m/min, TD samples.

### **4. Conclusions**

In the traditional twin-roll casting (TRC) process of aluminum alloys, the caster was designed to combine metal casting and hot rolling into a single operation. However, the traditional TRC process is generally operated at low casting speeds (<2 m/min). Thus, to increase the casting speed, the TRC concept was amended from hot rolling to rapid solidification through the application of a minimum roll separating force (RSF). Upon increasing the casting speed with a reduced RSF, it was found that the strip texture changed from a rolling texture to a random texture. In addition, in the case of high-speed TRC (HSTRC), direct temperature measurements indicated that the RSF played an important role in maintaining a good contact between the strip and the roll surface during the casting process. This in turn aided in the fabrication of a sound continuous strip with a fine microstructure. Moreover, the present RSF conditions allowed the formation of a band structure consisting of fine globular grains in the mid-thickness region of the strip, which aids in producing a well-dispersed center segregation pattern. The formation of a segregation band structure was beneficial for the fabrication of a sound strip with no scattering of the mechanical properties.

**Author Contributions:** Conceptualization, M.-S.K. and S.K.; methodology, M.-S.K. and S.-H.K.; validation, M.-S.K. and S.K.; formal analysis, M.-S.K. and S.-H.K.; investigation, M.-S.K.; resources, M.-S.K.; data curation, M.-S.K.; writing—original draft preparation, M.-S.K.; writing—review and editing, H.-W.K. and S.K.; visualization, M.-S.K.; supervision, H.-W.K. and S.K.; project administration, H.-W.K. and S.K.; funding acquisition, H.-W.K. and S.K.

**Funding:** This work was supported by the National Strategic Program funded by Ministry of Trade, Industry and Energy (No. 10081329), Republic of Korea.

**Conflicts of Interest:** The authors declare no conflict of interest.
