The effect of ultrasonic field on the microstructure and corrosion behavior of Fe-based amorphous coating applied to selective laser melting

This study was conducted to investigate the effect of the ultrasonic field on the preparation of amorphous/crystalline Fe-based coatings. For this purpose, Fe86.3Si 5.9B3.2Cr4.6 (wt%) powder was deposited on GTD-111 superalloy substrate with and without ultrasonic field by selective laser melting method. After coating, the corrosion behavior, microstructure, and hardness of the amorphous coating were investigated. The results showed that in conditions without ultrasonic vibration, the growth of grains starts in columnar form. At the same time, the use of an ultrasonic field inhibits columnar growth and creates equiaxed grains. In addition, the ultrasonic field increased the amorphous phase by 34.5%. This is attributed to the increased solidification rate caused by the cavitation effect. The experimental results of corrosion show that the self-corrosion current density of 6.83×10−7 A⋅cm2 is obtained due to the refinement of the microstructure and the increase of the amorphous phase. The wear results showed that the increase in the amorphous phase, as well as the decrease in the grain size, reduction in the grain ratio, and the increase in the tendency to equiaxed grains when using the ultrasonic field, reduce the coefficient of friction by 97%.


Introduction
Amorphous alloys have attracted the attention of researchers and industrialists due to their unique properties, including high hardness and good resistance to corrosion and wear [1]. Amorphous alloys, known as metallic glasses, are obtained from the rapid solidification of metal alloys whose structure consists of ordered atoms with a short and uniform range [2]. One of the advantages of amorphous alloys compared to crystalline alloys is the elimination of crystal defects [2]. Amorphous Fe-based alloys are one of the most promising engineering materials in the industry due to abundant natural resources, low material costs, and high potential. One of the challenges of making Fe-based amorphous alloys is to reach the critical cooling rate or even beyond it. But reports show that the critical size of Fe-based metallic glasses is only centimeter-sized [3]. This issue limits the application of Fe-based alloys only as substrates. But good physical, chemical, and mechanical properties have been reported in using these alloys as coatings. Research that has been done in recent years focuses on the process of how to make amorphous Fe-based coatings [4,5]. It is stated in the literature that selective laser welding (SLM), and thermal spraying processes are instrumental methods for preparing Fe-based amorphous coatings due to fast solidification speed, high deposition rate, and low cost [6]. Despite its advantages, the thermal spraying technique is limited in its application in this field due to limitations such as the high porosity of the coating and low bond strength between the coating and the substrate [7]. Laser-based processes is known as one of the best methods of coating materials, especially amorphous Fe-based alloys [8]. This is attributed to low heat input, high solidification rate (10 4 −10 6 K s −1 ), high speed, Marangoni effect, near zero heat affected zone, good metallurgical bonding of the coating to the substrate, and surface alloying capability [9]. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.
Jiang et al [10], by strengthening the high entropy AlCoCrFeNi alloy by 5% Fe, increased the tendency to form the amorphous phase during SLM transition. The formed amorphous phase also strengthened the printed material through the reduction of the size of the crystalline grains and the precipitation of the FCC phase in the grain boundaries of the BCC matrix. Li et al [11] fabricated a high-entropy FeCoCrNiMn alloy composite by SLM with a combination of high strength and toughness. The interesting point of their research was the identification of two different high entropy phases in addition to the amorphous phase in the surface area of the high entropy alloy and metallic glass. The identification of the amorphous phase was attributed to the rapid solidification properties of the SLM surface. Research has also shown that increasing the amorphous phase during SLM enhances the anti-corrosion behavior of the material [12]. Lu [13] et al reported the successful processing of crack-free amorphous FeCrCBMo coating by laser cladding technique, which included amorphous and crystalline phases and was well bonded to the substrate (45 steel) and also had excellent corrosion resistance. In a similar research, Gargarella [14] et al showed the distribution of the δ-ferrite phase in homogeneous dendrites of amorphous coating of FeCoBNbSi alloy as the reason for increasing hardness and corrosion resistance. Yeh et al [15] also synthesized a Fe-based composite using a direct laser deposition technique and obtained high hardness in the coating due to the repetition of amorphous and crystalline phases.
However, to increase the percentage of amorphous phase in the alloy and increase the hardness and other properties, it is necessary to increase the freezing speed of the laser-processed coating. Wang et al [16] also reported the high corrosion resistance and hardness of Fe 47 Cr 15 Mo 14 Co 3 C 15 B Fe-based amorphous coating on AISI 1020 steel, which they applied with a high-speed laser cladding technique. The advantage of the high-speed laser cladding technique compared to the laser cladding technique was the lower heat input and, as a result, the higher solidification rate. Using ultrasonic waves simultaneously as laser cladding is an exciting technique for refining the microstructure by increasing the solidification speed. Zhuang [17] et al applied 316 stainless steel coating as a hybrid laser-ultrasonic coating and found that ultrasonic waves with an amplitude of 17.5 μm have the best effect on increasing the mechanical properties. In a report by Li et al [15], it was confirmed that ultrasonic waves enhance crystal-to-amorphous transformation because it quickly causes amorphization of the coating at low temperature and stress.
Various researches have reported the use of ultrasonic field simultaneously with SLM as the factor of grain refinement, increasing its tendency to form an amorphous structure [18,19]. However, few studies have studied the conversion behavior of crystalline to amorphous coatings using ultrasonic waves. In this research, Fe-based alloy was simultaneously applied by SLM-ultrasonic on GTD-111 nickel-based superalloy substrate. Its corrosion, hardness, and wear properties will be discussed through phase and microstructural analysis.

Experimental procedure
The substrate used in this research was GTD-111 nickel-based superalloy with dimensions of 50×50×2 mm 3 with chemical composition (wt%): 13.9 Cr, 4.8 Ti, 3.1 Al, 9.6 Co, 2.9 Ta, 3.7 W, 1.5 Mo, 0.014 B, 0.04 Zr, 0.1 C, balance Ni. The powder used as a coating to make it amorphous was the Fe 86.3 Si 5.9 B 3.2 Cr 4.6 (wt%) alloy with a particle size of 10 to 40 μm and an almost spherical morphology, which was prepared by vacuum melting gas atomization. Figures 1(a)-(c) shows the morphology, distribution, and XRD analysis of the above particles. The coating powders were dried in an oven at 120°C for 2 h before coating. The coating was done by the SLM method. To process the above powders, the specimens were printed using an EOS M290 system (EOS GmbH, Germany). The SLM machine mainly comprises a fiber laser, a computer system for process control, an automatic powder distribution system, and an inert Ar gas protection system. The Fe-based powder was spread on the substrate by an automatic powder distribution system until a layer thickness of 0.5 mm was reached. An alternating X-Y direction laser scanning strategy was applied between successive layers, and the aperture distance between neighboring scan paths was 40 μm. The laser scanning speeds (m s −1 ), laser beam focus (μm), laser power (W), and hatch distance (μm) were 0.4, 100, 100, and 80, respectively. The SLM machine was equipped with an external ultrasonic field with a maximum ultrasound output range of 20 kHz, an ultrasonic generator of 25 μm, an ultrasonic frequency of 20 kHz, and a vibration amplitude of 0.5 μm. The schematic diagram of the ultrasonic vibration-assisted SLM system is shown in figure 1(d). SLM and ultrasonic parameters were used based on detailed studies and optimal determined parameters in our previous works and other studies [19,20]. To investigate the microstructure and phase, the specimens printed by Electron Discharge Machining (EDM) machine were cut in the direction of the building and after going through the metallographic steps including cold epoxy mount, ground to 3000 grit with SiC sandpaper, and mechanical polishing with aqua regia were etched. To investigate the microstructure of the specimens processed by SLM, a field emission scanning electron microscope (FE-SEM) equipped with energy-dispersive x-ray spectroscopy (EDS) and electron backscattering diffraction (EBSD) was used. Electrochemical corrosion tests were performed using a threeelectrode system (Shanghai Chenhua CHI660E). The test solution was 3.5 wt% NaCl solution which was tested for electrochemical polarization curves. Corrosion current density (Icorr) was measured by the TOEFL extrapolation method. To identify the phases, an x-ray diffraction analysis device (XRD, D8 ADVANCE, BRUKER, GER) was used. In particular, the XRD has a scanning range of 30 to 90°with a scanning rate of 0.02°s −1 . To investigate the hardness, the specimens were subjected to the Vickers hardness tester (HVS-1000A) with a load and time of 500 g and 10 s, respectively. MS-T3001 sliding friction tester was used to investigation the wear behavior of SLM and SLM-ultrasonic specimens. The silicon nitride ball was also chosen as the friction pair. The values of wear radius, load, rotation speed, and test time were selected as 4.5 mm, 1000 g, 400 rpm, and 30 min, respectively. shows the microscopic images of the specimens processed by SLM. As can be seen, the appearance of the coatings is healthy and without any cracks. Figure 2(c) also shows the results of the XRD analysis. As can be seen, in the SLM specimen, several strong peaks are observed at 44°, 65°, and 83°angles, which are related to α-Fe, FeCr, Fe 3 Si, and FeSi 2 phases. In addition, two weak peaks are observed at 42°and 46°a ngles, which indicates the presence of an amorphous phase. In order to identify weak peaks, square root calculation was used, the results of which are shown in figure 2(d). Weak peaks between 43°and 58 of SLM specimen are due to the low heat input during SLM, which is one of the advantages of laser material processing [21]. The XRD results of the SLM-ultrasonic coating show that in addition to the weakening of prominent peaks, the peaks related to FeCr and FeSi 2 phases have completely disappeared. XRD results calculated by square root showed that FeCr, and FeSi 2 phases have become amorphous due to the decrease in intensity and broadening. This indicates that the ultrasonic field plays a significant role in creating the amorphous phase of the coating. To determine the volume fraction of the amorphous phase using the parameters obtained from the XRD results, from equation (1) [22] where V f : the volume fraction of the amorphous phase, A a : the integral region of the broad diffraction peak intensity concentration region, A c : the integral region of the same interval of the crystal diffraction peak was used.

Results and discussion
The results showed that in the SLM and SLM-ultrasonic specimens, the volume fraction of the amorphous phase is 10.7% and 14.4%, respectively, which shows a 34.5% increase in the ultrasonically processed specimen compared to the non-ultrasonically processed specimen. To investigate the causes of the increase in the amorphous phase under the conditions of using ultrasonics, it is necessary to investigate the way of solidification and microstructure formation. It is stated in the literature that the use of external fields such as ultrasonic and magnetism increases the solidification rate and temperature gradient [23]. In fact, the sound pressure caused by the ultrasonic field in the molten pool alternates between negative and positive in a short period, which results in the intensification of the melt flow in the pool, especially during solidification. Under such conditions, the solidification rate increases even more. In amorphous alloys, this factor is significant, because it creates refined and probably equiaxed grains. Zhao et al [24] applied ultrasonic field during laser cladding of IN625 powder to reduce porosity and increase the area of equiaxed grains. In amorphous materials, before granulation, the formation of the amorphous phase is important, which is under the control of the solidification rate. The production rate of free volume, which is proportional to the cooling rate, is the most important factor in forming an amorphous alloy. So that by increasing the speed of cooling, and finishing solidification, some free volume is immediately solidified, which helps to produce the amorphous phase. High cooling speed is one of the characteristics of laser processing processes, however, the application of an ultrasonic field increases the cooling rate of the molten pool. Applying the ultrasonic field to the amorphous coating showed that by increasing the cooling rate of the molten pool, the FeCr and FeSi 2 phases are solidification before their nucleation in the melt, and the formation of a large amount of Fe 3 Si and α-Fe phases is prevented to a large extent. The XRD results in figure 2(c) confirm this. Figure 3 shows the microstructure of SLM and SLM-ultrasonic specimens. As can be seen, the initiation of solidification in the SLM specimen (figures 3(a), (b)) has started in the form of a columnar. After sufficient growth and increasing the ratio of the temperature gradient to the solidification rate, they are formed into equiaxed grains. As the solidification process continues at the top of the coating, the value of G is the lowest, and the value of R is the highest. This state indicates that G/R is at its lowest possible state. Since the Marangoni convection current is less effective at the top of the coating [25], only the melt closest to the coating powder becomes amorphous, and the rest of the melt becomes crystalline.
Meanwhile, in the SLM-ultrasonic specimen (figures 3(c), (d)), the solidification starts with equiaxed grains and grows to the top of the coating in the same way and ends at the top of the coating with a larger amount of amorphous material. It is mentioned in the literature that the faster the solidification rate, the grains tend to become equiaxed [26]. The high solidification rate at the substrate-coating interface can be attributed to the cavitation effect [27]. In fact, ultrasonic pressure waves cause flow in the molten, and under the right conditions, it causes the rapid formation of micro-bubbles, which generate a strong shock wave when they grow and burst. The energy from bursting bubbles suppresses the columnar growth of grains and causes the formation of equiaxed grains. The cavitation effect is more intense at the top of the coating. As shown in figure 1(b), the reason for this is the application of the ultrasonic field from the top of the coating. Therefore, under such conditions, the cooling speed will exceed the critical speed, and as a result, not only the melt around the coating powder, but also the rest of the coating will be formed in an amorphous form. Xiao et al [28] reported that the critical rate of conversion of crystalline to amorphous material in iron-base alloy Fe 41 Co 7 Cr 15 Mo 14 C 15 B 6 Y 2 was obtained under the conditions of temperature gradient and cooling rate of 10 6 K m −1 and 10 4 K s −1 , respectively.
EDX analysis investigation at the designated points in figures 3(c) and (d) shows that at the beginning of solidification from the substrate-coating interface, the amount of Fe is above 94.3% (table 1). As solidification continues, columnar and equiaxed crystals grow toward solutes and heat [25]. During growth, the Marangoni convection current reduces the amount of Fe by affecting the diffusion of solutes. At the same time, the amount of the other three elements, Si, B, and Cr, increases. However, microstructural studies and EDX analysis results show that the chemical composition has still deviated from the amorphous formation. In the continuation of solidification, in addition to the Marangoni flow, the mechanical vibrations caused by the ultrasonic flow cause rapid solidification of the remaining melt in an amorphous state. The chemical composition of this point of the coating also shows chemical composition close to the original Fe-based powder. Figures 3(e)-(h) also shows the analysis of the grains formed in the coating by EBSD and pole figures in the xy plane (extension of the building z). As can be seen in the SLM specimen (figure 3(e)), columnar grains with a strong texture in the plane (100) and a maximum uniform density of 3.26, which is caused by a significant thermal gradient and a high cooling rate along the direction of the building. In the SLM-ultrasonic specimen ( figure 3(f)), it can be seen that the morphology of the grains has completely changed and has become equiaxed. Also, the pole figures result shows that a stronger texture has been created in the SLM-ultrasonic specimen with a maximum uniform density of 2.59. As mentioned, these cases are attributed to the cavitation effect caused by the ultrasonic field in the molten pool during solidification. The effects of cavitation and acoustics in the SLMultrasonic specimen as shown in figure 4 cause the crushing and distribution of particles in the molten pool. Considering that the maximum amount of amorphous phase in the SLM-ultrasonic specimen was calculated to be 14.4%, it is natural that the amorphous phase is spread throughout the specimen and cannot be seen by the SEM microscope. The XRD results in figures 2(c) and (d) were the most reliable method to identify the amorphous phase.
The hardness measurement results in figure 5(a) also confirm the results of microstructural investigations, EBSD, and IPF. As expected, SLM-ultrasonic specimen with equiaxed grains resulted in a 52% increase in hardness compared to SLM specimen with columnar grains. It should be noted that in addition to being equiaxed, the grains of the SLM-ultrasonic specimen have also become finer. So that the grain size in SLM and SLM-ultrasonic specimens is 12.5 μm and 9.4 μm, respectively, and the grain aspect in SLM and SLM-ultrasonic specimens is 8.6 and 2.5, respectively, by image j software. Therefore, the disordered structure of the amorphous phase (increasing the volume fraction of the amorphous phase), the fineness, and the equiaxed grain in the SLM-  ultrasonic specimen have led to a significant increase in hardness. The decrease in grain size and grain aspect can be attributed to cavitation and acoustic effects ( figure 4). Jiang et al [29] improved the hardness of 3540 Fe/CeO2 coating from 708 to 1148 HV when using ultrasonic in laser cladding. The crushing of the dendrites during solidification and as a result the reduction of the grain size caused by the ultrasonic vibration in the molten pool was the most important factor of this issue. In the research of Wang et al [30], Ti6Al4V alloy specimens prepared by SLM showed a 40% increase in hardness under assistance-ultrasonic conditions. Figures 5(b) and (c) shows the Bode and Nyquist diagrams of electrochemical impedance spectrum of SLM and SLM-ultrasonic specimens in 3.5w% NaCl solution. Nyquist diagrams of SLM and SLM-ultrasonic specimens ( figure 5(b)) show distinct capacitive-resistive arc characteristics in low-frequency and highfrequency regions. After applying the ultrasonic field, the arc of the capacitive resistance of the coating increased, which indicates that the mechanical vibration resulting from the ultrasonic field has a more significant effect on the corrosion behavior of the coating in the chlorine ion corrosive environment. Figure 5(b) shows that the frequency phase angle curves in the Bode diagram are related to a time constant in the high and low-frequency regions and to the arc of the capacitive reactance in the corresponding region in the Nyquist diagram, which indicates that the corrosion condition of the coating surface during corrosion is consistent [31].
The potentiodynamic polarization curves of SLM and SLM-ultrasonic coatings in figure 5(d) show that the self-corrosion potential of the coating increased, and the self-corrosion current density decreased after applying the ultrasonic field. Since the corrosion current density dominates the corrosion resistance, when the corrosion current density is lower, the corrosion resistance is higher. The self-corrosion potential of SLM-ultrasonic coating is the largest, and its self-corrosion current density is the lowest (it shows a low passive current density), which indicates the best corrosion resistance. Comparing the self-corrosion current density of the SLM specimen (Icorr = 17.39×10 −7 A⋅cm 2 ), the self-corrosion current density of the SLM-ultrasonic specimen (Icorr = 6.83×10 −7 A⋅cm 2 ) has decreased by 155%.
The excellent corrosion resistance of the amorphous coating processed by SLM-ultrasonic field can be explained from two aspects: alloying elements and homogeneous structure. The Fe-based alloy coating contains B and Cr elements. Cr is one of the most effective alloying elements for creating a passive film on amorphous alloys [32]. Zhou et al [24] observed reactive surface films on amorphous specimens after extended immersion in an acid solution. Panahi et al [33] also reported that element B stabilizes passive surface films by preventing their solution, including Cr. In addition, grain size and grain aspect are closely related to the corrosion rate of materials. Comparing the grain size and aspect of SLM and SLM-ultrasonic specimens, the SLM-ultrasonic specimen has a smaller grain size and grain aspect than the SLM specimen, which significantly improves the corrosion resistance of the coating. However, increasing grain size and grain aspect of the SLM specimen leads to non-uniform microstructure and poor coating density, which results in poor coating corrosion resistance. It is expected that the Fe-based amorphous coating processed by the SLM-ultrasonic process can be effective as surface protective layers in corrosive environments. Figure 6 shows the wear test results of the SLM and SLM-ultrasonic specimens, which include the coefficient of friction (COF), wear rate, volume loss, and SEM images of the worn surface. In the results of COF ( figure 6(a)), it can be seen that two types of initial and stable wear occur with the passage of time of the sliding process. In both specimens, approximately after 100 m, the stage of transition from initial wear to stable wear occurs. In the stage of stable wear until the end of the 3000 m track, the average value of COF for SLM and SLMultrasonic specimens was 0.77 and 0.39, respectively. Since COF is the state of contact between the specimen surface and the friction pair, the SLM specimen with a COF of 0.39 has a higher wear resistance, and in the SLMultrasonic specimens with a COF of 0.77, harder wear occurs between the ball and the specimens surface and the possibility of sticky wear increases. In confirmation of the COF results, the wear rate and volume loss results in figure 6(b) show that the wear rate of SLM and SLM-ultrasonic specimens is 0.58×10 −6 mm 3 N −1 .m and 0.3×10 −6 mm 3 N −1 .m, respectively, and the wear volume loss is 0.084×10 5 μm 3 and 0.035×10 5 μm 3 , respectively. All the above results indicate that the wear resistance of the SLM-ultrasonic specimen is higher than that of the SLM specimen. The improvement of wear results is attributed to the combined effect of amorphous phase, α-Fe and Fe 3 Si. In addition to this, SLM-ultrasonic coating grain refinement including reduction of grain size and grain aspect, reduction of columnar grains and increase in tendency to equiaxed and amorphous grains, as well as appropriate distribution of the mentioned phases have had a positive effect on increasing wear resistance. The ultrasonic field was the most important factor in these changes. Figures 6(c) and (d) shows that the higher percentage of amorphous phase and the refined microstructure have a higher resistance to the formation of grooves and then the deepening of the grooves. While in the specimen processed without ultrasonic field, the depth of the formed groove is %67 higher than the SLM + ultrasonic specimen.
SEM images of the worn surface show that except for some shallow grooves, the SLM-ultrasonic specimen has a smooth surface. Meanwhile, in the SLM specimen, the created grooves are deeper, which occurred during the sliding path of wear and due to less wear resistance. The EDS results of some parts of the SLM specimen show the presence of Fe and O elements, which shows that the SLM specimen suffered a little oxidation and oxidative wear during wear due to higher COF.

Conclusion
In this work, the study of selective laser melting technique with the help of ultrasonic field was used for the successful fabrication of Fe 86.3 Si 5.9 B 3.2 Cr 4.6 amorphous/crystalline coating and the following results were obtained: 1. Applying the coating using the ultrasonic field increased the growth tendency of grains with equiaxed morphology compared to the columnar state.
2. The cavitation effect caused by the application of the ultrasonic field increased the amorphous phase of the coating by 34.5% compared to the case of without using the ultrasonic field.
3. The simultaneous presence of equiaxed grains and amorphous phase in the coating increased the hardness of the coating by 52%.
4. The increase of amorphous phase, reduction of grain size, and reduction of grain aspect due to the application of ultrasonic field increased the corrosion resistance of the coating by the decrease of self-corrosion current density.

5.
The wear results showed that with the increase of the amorphous phase, the coefficient of friction, wear rate, and the volume loss, are improved by 97%, 93%, and 58%, respectively.

Data availability statement
All data that support the findings of this study are included within the article (and any supplementary files).