Interface microstructure of SUS301S plated with Zinc by ultrasonic. Figure 2 shows the SEM and EPMA images of the interface microstructure observation. Figure 2 (a) shows an SEM image of SUS310S plated material with no ultrasonic and a plating time of 60 seconds. As observed in the result showing the interfaces, the Zn-Al alloy plating structure on the SUS310S steel plate bonded with an irregular interfacial microstructure, and the appearance of the bonded interface is more discontinued in segments. Furthermore, plated material is peeled off, or delamination occurs during the cutting process, and even it is possible that the oxide is entrapped in bonding gaps at the microstructure interface. After that, the defective area expands over the period, and very few bonded regions are observed. However, the microstructure of the interface is roughened with incomplete wettability, and impurities are formed like many dross particles with patches of oxide skin. Figure 2 (b), SEM image shows the result of SUS310S plated with zinc under the conditions of ultrasonic, 60 s plating time, and under the horn (5 mm). It was observed that the non-bonding interface between the Zn-Al alloy and SUS310S steel was significantly reduced compared to without ultrasonic. When ultrasonic pressure is applied, the wave does not inhibit the damaged structure but is confirmed to minimize the defective interfacial bonded structure and prevent the formation of impurities at the interface. "It helps the improvement of δ phase growth with increases in the Fe-Al layer generated at the interface between the ζ phase with high binding characteristics as well as this grown δ phase gradually improves Fe-Zn alloy formation by suppressing ζ phase growth and aiding in overcoming the weak bonding force". Figure 2 (c), SEM image shows the result of SUS310S plated material under the conditions of ultrasonic, 3 s plating time and under the horn (5 mm). In this interface, a similar output was generated, but when it is compared with the tested 60 s plating duration, it shows a steadily increased bonded region of the plated steel interface. For example, If the plating exhibits various growth kinetics depending on plating time and temperature, it has an impact on layer growth on the steel surface, especially a short period immersed up to 733 K at this stage up to the δ phase layer when ζ phase growth becomes slow. However, the gamma phase layer is not proven in this situation because it forms only after extended periods. In addition," the short immersion times encountered in galvanizing, only one δ phase morphology has been reported and confirmed"11,12. Figure 2 (d), shows the SEM image of SUS310S plated material under the conditions of ultrasonic, 3 s plating time, and horn side (5 mm). The plated structure was generated in the form of a defective bonding structure on the surface of the SUS310S steel plate, and also observed that the interface zones of steel surfaces were rough bonding with an irregular path of structures. Compared to under the horn (5 mm) with 3 s plated time, The impact of insufficient ultrasonic waves on the region of steel surface generates defective structures of porosities and dull patches, and the peeled skin effect in parallel found more discontinued segments due to less reaction between the Zn-Al alloy and the steel.
Figure 2 (e) shows the area analyzed by EPMA mappings of the interface of the SUS310S, to which ultrasonic waves were applied under the horn (5 mm) for 60 seconds. It was found that the formation of reaction layers are confirmed forms at the interface composed of Zn-Al and SUS310S. Initially, it is confirmed that the Al is concentrated more active at the interface of Fe-Zn. It is considered that Al was continuously supplied to the surface of the steel by the molten metal stirring effect of the ultrasonic waves, and the reactivity improved the bonding structure by decreasing the diffusion boundary layer, which promoted the formation of the Fe-Al alloy layer. Because they generate back diffusion that moves against their self-concentration gradient, Al atoms in the Zn liquid phase are concentrated at the SUS steel plate interface in this manner14. Fe atoms diffuse in the liquid phase of the Zn-Al alloy immediately after immersion, and the Fe concentration varies due to the induced Al diffusion. Then, Al attempts to stabilize the chemical potential at the interface to build a layer. In addition, with the cavitation effect of the ultrasonic pressure waves, Al continually reacted to the surface of the steel plate in the molten melt. This activity increased the joint Al diffusion flow velocity on the SUS steel plate. Furthermore, by generating an inhibition layer of Fe2Al5 between the steel and Zn-Al alloy, Al improves the fluidity of the Zn melts as well as suppresses the brittle Fe-Zn intermetallic layer at the plating steel interface15. From this observation, compared to without ultrasonic conditions, it was confirmed that the absence of a stirring effect causes the formation of a gaseous bubble, which should slowly dissolve due to gas diffusion from the bubble to the melts. As a result of "the influence of the diffusion boundary layer on the reaction time of Al content or it is static Al content".
Figure 3 illustrates the overall improvement of the interfacial bonding structure as a result of modifications ito the ultrasonic application mechanisms, time variations, and horn location of the plating material. According to this graph, when compared to ultrasonication and 60s plating, short time plating with applying waves produces a smooth bonding interface structure of steel with a low non-contact plated surface and a low porosity on the interface. "The cavitation effect on molten Zn-Al alloys with increased Al concentration between the Zn-Fe interface and ultrasonic pressure aids in the prevention of contamination at the interface zone as well as the reduction of defective structures".
Effects of Ultrasonic wave on oxide layer removal. To confirm the oxide removal effect by ultrasonic application, an oxide SiO2 was deposited on the surface of the SUS310S by PVD (Physical Vapor Deposition). Figure 4 shows the result of analyzed the cross-section of the SUS310S surface with the oxide layer by FE-SEM and EDX. Si and O were observed on the surface of the SUS310S plate with a thickness of 2µm.
Figure 5 shows the BSE images of the SiO2 oxide layer covered SUS310S steel plate treated without and with the ultrasonic application to hot-dip galvanized method. Figure 5 (a), the BSE image shows that the SiO2 oxide layer covering the SUS310S steel plate was treated without ultrasonic, 60-second planting time. The result indicates that the plated material of the SUS310S plate was peeled off occurred in between Zn and SiO2 because they are difficult to react with each other. It was also discovered that silicon persists on the steel plate side and that peeling occurs between Zn and SiO2. Perhaps an aluminothermic reaction did not occur because SiO2 is a vitreous, chemically stable condition. Figure 5 (b), the BSE image shows that the SiO2 oxide layer covering the SUS310S steel plate was treated ultrasonically with zero horn distance and 60 s plating time. In this observation, initially, no peeling or delamination occurred between the interface of Zn-SiO2 before the destruction of the oxide film due to the impact of the ultrasonic pressure wave. It also demonstrated improved bondability when compared to the absence of ultrasonic. Subsequently, the result shows that the break on the SiO2 deposition side and the destruction of the oxide layer by ultrasonic waves were observed. Because of the high intensity of the ultrasonic pressure wave, a particular portion of the SiO2 oxide layer was completely broken between the interfaces. But ultrasonic waves have a certain destructive effect, so the whole 2 µm oxide layer could not be completely destroyed after 60 s of application. Predictably, it may be that an alloyed reaction will occur at the SiO2 broken zone. Figure 5 (c), the BSE image shows that the SiO2 oxide layer covering the SUS310S steel plate was treated ultrasonically with zero horn distance and 3 s planting time. The result shows that the Si component remains entirely at the interface observed. Ultrasonic waves cannot remove the oxide layer due to insufficient time for active pressure waves to act on it. Besides, the shorter immersed plating time eliminated the tiny SiO2 oxide layer, but it has not entirely vanished from the interface of the SiO2 covered SUS310S steel plate. Finally, no fracture points could be confirmed on the steel plate.
Figure 6, confirms that the Si content was eliminated at a broken portion of the interface due to the cavitation effect and that the remaining zone was entirely covered by the SiO2 layer. Fe, Zn, and Al were detected simultaneously in the oxide film breakdown area, and Al concertation reactivity occurred at the Zn-Fe interface. EPMA data showed that when the oxide layer was disrupted, the Zn-Fe alloy layer grew because the Al content was more concentrated at the interface, confirming that the oxide composition is not present at the broken SiO2 oxide layer interface.
Hardness on the interface between Zn-Al alloy and SUS 310S. Figure 7 shows the results of evaluating the interfacial hardness properties of the SUS310S steel plated with ultrasonic and the interfacial hardness properties of the SiO2 covered SUS310S steel plated with ultrasonic. Figure 7 (a) shows the results of ultrasonic application on SUS310S steel that was plated with Zn-Al alloy. The measurements were taken from Zn, the interface between Zn and SUS310S, and SUS310S. In the Zn material to be plated, the displacement due to the load was large, and the indentation hardness and Young's modulus were low. However, a load displacement was obtained from the Zn-SUS310S interface containing the Zn, with indentation hardness and Young's modulus significantly near to SUS310S. The hardness of the Zn-SUS310S interface is higher than that of the Zn zone of a plated material. It is confirmed that the interface region obtained good bondability. Figure 7 (b) shows the results of the ultrasonic, covered SiO2 oxide layer-plated material measured values from the Zn-SiO2 interface, SiO2, and SUS310S-SiO2 interfaces, as well as the obtained indentation hardness and Young's modulus. In this comparison of interface characteristics between Zn-SiO2 with an oxide layer and SUS310S-SiO2, the results show that indentation hardness and Young's modulus are increased in SUS310S-SiO2. However, strong interfacial bondability was observed even if there was an oxide layer present. Because the Zn-SiO2 interaction is weak, a peeling effect may occur at the Zn-SiO2 interface, and there is a possibility of greater displacement due to the presence of cracks. It was also discovered that SiO2 has a very high indentation hardness but a comparatively low Young's modulus. Figure 7 (c), shows the comparison of the interface characteristics of Zn-SUS after the ultrasonic applied zone and the interface characteristics of the Zn-SiO2 broken oxide layer zone. In this zone, maximum displacement due to load, indentation hardness, and Young's modulus were all significantly the same in these two locations. The Zn-Fe alloying process occurs at the interface after the oxide layer has been fractured due to phase development. Lastly, they proved that the ultrasonic cavitation effect destroyed the oxide layer physically and made the plated SUS310S stronger in bonding.