In order to improve the corrosion resistance of high-strength steel for engineering and reduce the cost of materials, a Fe-Al alloy cladding layer was prepared on the surface of Q960E high-strength steel by coaxial powder feeding process using fiber laser, and the influence of different scanning speeds on the structure and corrosion resistance was studied. The results show that the grain morphology, element distribution and content of the cladding layer are all affected by the scanning speed. The phase is mainly composed of Fe3Al phase with DO3 structure and FeAl phase with B2 structure in eutectic form. The hardness of the cladding layer gradually increases with the increase of scanning speed due to the influence of grain refinement. The self-corrosion potential of the cladding layer under three scanning speeds first increases and then decreases with the increase of scanning speed, and the self-corrosion current density first decreases and then increases. The pitting pits on the surface of the polarized cladding layer first change from deep to shallow with the increase of scanning speed, and then the pitting pit area expands.

As an important load-bearing structural component, high-strength steel for large-scale engineering machinery faces severe natural environment outdoors. Once the steel fails due to corrosion, the consequences will be very serious. Protecting the steel through surface engineering technology is an efficient and economical method.Fe-Al Alloy is a new type of cheap, low-density material that has been focused on in recent years. It has good oxidation resistance, wear resistance and corrosion resistance. At present, many researchers have used surface technology to study Fe-Al alloy as a coating, hoping to introduce aluminum elements into the coating to replace precious metal elements with Fe-Al alloy coating to participate in the protection of the substrate material, providing the possibility for economic efficiency in industrial production.

Laser cladding, as an efficient and green method, can achieve metallurgical bonding between the coating and the substrate, and can make high-strength steel have excellent mechanical properties and certain corrosion resistance. The control of the molten pool during laser cladding will directly affect the performance of the final coating. Due to the fast cooling rate of laser cladding, defects such as pores, inclusions and cracks may form during the cooling process of the molten pool. In addition, the change of aluminum content in Fe-Al alloy has a great influence on its own brittleness, and the cracks will seriously affect its corrosion resistance. Therefore, it is necessary to accurately control the process parameters during the cladding process. In this paper, a well-formed Q960E high-strength steel surface was prepared by laser cladding. Fe-Al alloy cladding layer, the effect of laser scanning speed on the microstructure and hardness of Fe-Al alloy cladding layer was studied, the corrosion resistance of Fe-Al alloy cladding layer was studied by electrochemical test, and the potential application prospects of Fe-Al alloy coating in corrosion resistance were explored.

1 Experimental materials and methods

The Q960E high-strength steel plate with a size of 200 mm×6 mm×5 mm was selected as the substrate. Its chemical composition is shown in Table 1. Before the experiment, the substrate was polished and wiped with alcohol. Figure 1 shows the 6061 aluminum alloy powder and pure iron powder used as cladding materials. They are spherical powders with a particle size distribution between 50 and 100 μm. The chemical composition is shown in Tables 2 and 3. The two powders were mixed in a ball mill at a speed of 100 r/min for 4 h in a ratio of 1:1 and then dried.

This experiment uses the ILAM series fiber laser laser cladding system of Chengdu Qingshi Laser Technology Co., Ltd for surface laser cladding .Coaxial synchronous powder feeding was used throughout the process, the protective gas flow rate was 23 L/min, and the powder feeding gas flow rate was 7 L/min; the laser power was 2 800 W, the laser scanning speed was 50, 75, 100 mm/s, and the overlap rate was 40%.

The coating was processed into a metallographic specimen with a size of 10 mm×10 mm×6 mm using a wire cutting device, and the cross section of the coating specimen was embedded with epoxy resin. After grinding with sandpaper of No. 80, 240, 400, 600, 800, 1 000, 1 500, and 2 000, diamond polishing paste was used to polish away surface scratches, and 4% (mass fraction) nitric acid alcohol etchant was used for metallographic corrosion. OLYMPUS GX71 metallographic microscope was used for metallographic observation, and JEM-7001F scanning electron microscope and EDS were used to observe the microstructure of the cross section of the specimen and perform component analysis. Smart Lab type X X-ray diffractometer was used to analyze the phase of laser cladding layer, and the scanning speed was 4°/min in the range of 20°~100°. Wilson Wolpert 401MVD microhardness tester was used to test the hardness of the coating cross section from the surface to the substrate at intervals of 30 μm, with a load of 100 g and a loading time of 15 s. Coster CS310 electrochemical workstation was used to test the electrochemical corrosion of the cladding surface. The size of the electrochemical sample was 10 mm×10 mm, and it was encapsulated with epoxy resin, and polished after grinding with 400~2 000 sandpaper. The experimental temperature was 25 ℃, and a three-electrode system was used. The reference electrode used SCE-saturated calomel electrode, the auxiliary electrode used platinum electrode, the corrosion medium was 3.5% (mass fraction) NaCl solution, the polarization curve test potential range was -1~1 V, and the scanning speed was 1 mV/s. The impedance spectrum test frequency range is 10′-2~10’5 Hz, and the amplitude is 5 mV.

2 Results and Analysis

2.1 Macromorphology of Cladding Layer

Figure 2 shows the macromorphology of Fe-Al alloy cladding layer prepared at 2 800 W laser power and scanning speeds of 50, 75 and 100 mm/s and its morphology after flaw detection. As the scanning speed increases, no obvious cracks appear in the cladding layer after flaw detection. The surface of the cladding layer gradually changes from flat to slightly undulating. This undulation mostly appears in the overlap area. As the scanning speed increases, the cooling rate accelerates, resulting in discontinuous molten pool formation.

Figure 3 shows the original surface morphology and cross-sectional metallographic structure of Fe-Al alloy cladding layer samples prepared at different scanning speeds. As the scanning speed increases, the thickness of the cladding layer gradually decreases due to the decrease in the laser action time on the powder and the molten pool per unit time. The thickness of the cladding layer prepared at 50 mm/s is about 350 μm, and the thickness of the cladding layer prepared at 75 mm/s is about 250 μm, 100 mm/s, the cladding layer thickness is about 180 μm, and the heat affected zone range is also decreasing. The microstructures under the three scanning speeds are dense, without holes, and the grains are mainly columnar crystals. The grain growth direction grows with the cladding direction. The change in scanning speed also has a certain impact on the formation of the molten pool. The laser input energy in the center of the molten pool is higher than that on both sides. The central area will melt more powder and flow to both sides of the molten pool. As the scanning speed increases, the molten liquid in the central area of ​​the molten pool solidifies before it has time to flow to both sides. The morphology of the molten pool gradually forms a wave crest as the center and a wave trough on both sides, which overlaps with each pass, and the slope of the wave crest gradually increases.

2. 2 Microstructure of the cladding layer

Figure 4 is the SEM cross-section of the Fe-Al alloy cladding layer prepared at different scanning speeds Figure, the cladding layer is composed of eutectic structure, with needle-like and plate-like structures distributed in the grains and at the grain boundaries, and staggered at a certain angle in the grains. As the laser scanning speed increases, the grain structure becomes finer, and the needle-like structure growing in the grains also becomes finer.

The cladding layer is mainly composed of cellular crystals near the substrate interface, columnar crystals in the cladding zone and the middle area, and planar grains on the surface. This phenomenon is mainly due to the different degrees of G/R ratio along the molten pool to the substrate surface, where G is the temperature gradient at the front of the liquid alloy solidification interface, and R is the interface velocity. The G/R at the interface between the molten pool and the substrate is initially large, resulting in a relatively small compositional supercooling. The grains mainly grow as cellular crystals. As G/R decreases, composition supercooling increases, and the grains advance to the surface in the form of columnar crystals. At the interface between the molten pool and the shielding gas, G/R increases again, and the grains solidify in a plane. In the overlap area, the solidified cellular grains grow into columnar crystals again, so there are more columnar crystals in the overlap area. For the cladding layer at different scanning speeds, due to the reduction of input energy, the thinner cladding layer means the faster cooling rate, which makes the G/R value smaller, so more grains grow in the form of columnar crystals. The increase in scanning speed will cause a greater degree of composition supercooling.

Figure 5 is an EDS image of the cross-sectional direction of the Fe-Al alloy cladding layer prepared at different scanning speeds. At a speed of 50 mm/s, the aluminum content of the cladding layer shows a small amount of fluctuations, and the overall content is uniform. With the increase of scanning speed, at a speed of 75 mm/s, the aluminum content in the cladding layer is more uniform, and the scanning speed is 100 mm/s, the aluminum content is higher on the surface and gradually decreases from the surface to the substrate. This is because as the laser scanning speed increases, the energy density of the laser acting on the molten pool decreases, the dilution rate decreases, and the diffusion range of the aluminum element is limited.

Table 4 shows the changes in the EDS point scanning element content of the Fe-Al alloy cladding layer prepared at different scanning speeds from the surface to the substrate. The changes in aluminum and iron elements are basically the same as the trend shown by the line scan. The cladding layers obtained at the three scanning speeds have reached the highest aluminum content in the area close to the surface, and the aluminum content in the middle and lower parts begins to decrease due to the dilution of the substrate material. At 75 mm/s, the dilution rate decreases, which increases the aluminum content of the cladding layer. The aluminum content of the cladding layer at a scanning speed of 100 mm/s is the lowest.

2. 3 Phase analysis of cladding layer

Figure 6 is the XRD spectrum of the in-situ synthesized Fe-Al alloy cladding layer at scanning speeds of 50, 75, and 100 mm/s. The main diffraction peaks correspond to FeAl The (110)(200)(211)(220) crystal planes of the Fe3Al phase and the (220)(400)(422)(440) crystal planes of the Fe3Al phase, due to the influence of the growth orientation of a large number of columnar grains, the diffraction peak intensity at 64° is increased compared with the standard card intensity. Different scanning speeds will affect the intensity of the diffraction peak, but have no effect on the phase composition of the cladding layer.

In order to study the difference in phase structure between the needle-like structure and the matrix structure in the microstructure, the EDS surface scanning of the cladding layer prepared at 50 mm/s was performed. The results are shown in Figure 7. There is a significant change in the element distribution between the needle-like structure and the matrix. The enrichment of the Fe element in the needle-like structure is reduced, and the Al content is slightly increased compared with the matrix. At the same time, the C content increases significantly. From the changes in the Fe and Al contents, it can be analyzed that these needle-like structures may be FeAl phases with B2 structure. At the same time, there is a larger atomic gap space in the FeAl phase for solid solution of C atoms. FeAl The phase has a wide composition range. The Fe and Al alloy powders initially form the FeAl phase in the molten pool. During the solidification of the molten pool, the FeAl phase begins to transform in a eutectic manner when it reaches the Fe3Al phase composition range of the DO3 structure. Due to the high cooling rate of laser cladding, part of the FeAl phase does not have time to undergo phase transformation, and finally remains in the grains and between the grain boundaries in the form of needle-shaped and lath-shaped structures.

2. 4 Analysis of microhardness of cladding layer

Figure 8 is the microhardness curve of the Fe-Al alloy cladding layer along the cross-sectional direction at different scanning speeds. The hardness curve passes through the cladding layer, the heat-affected zone and the substrate respectively. The hardness of the cladding layer prepared at a scanning speed of 50 mm/s is between 275~295 HV, the hardness of the cladding layer at a scanning speed of 75 mm/s is between 286~300 HV, and the hardness of the cladding layer at a scanning speed of 100 mm/s varies between 308~330 HV. Figure 9 is the grain size distribution diagram of the cladding layer at different scanning speeds. The grain size of the cladding layer prepared at a scanning speed of 50 mm/s is mainly distributed in 30~35 μm, and the columnar grain size of the cladding layer prepared at a scanning speed of 100 mm/s is more concentrated in 10 μm. For the cladding layers prepared at different scanning speeds, the cooling speed of the molten pool becomes faster due to the increase in scanning speed, resulting in a greater undercooling, which makes the cladding layer grains finer and the lath structure inside the grains finer, thereby improving the hardness of the cladding layer. Although the aluminum content also changes with the scanning speed, the hardness of the cladding layer still gradually increases with the increase in scanning speed, so the morphology of the grain structure plays a dominant role in the hardness. The thickness of the molten pool formed at different scanning speeds is different, but the overall hardness change trend of the cladding layer is the same. The hardness of the cladding layer is lower than that of the substrate material and the heat affected zone has a tendency to be partially strengthened.

2. 5 Analysis of electrochemical corrosion performance of cladding layer

Figure 10 is the potential polarization curve of the Fe-Al alloy cladding layer prepared at different scanning speeds. According to The corrosion potential and corrosion current density obtained by Tafel extrapolation method are shown in Table 5. The corrosion potential at 50 mm/s is -0.937 V. When the scanning speed increases to 75 mm/s, the corrosion potential also rises to -0.887 V. Compared with 50 mm/s, the corrosion resistance is improved to a certain extent. When the scanning speed increases to 100 mm/s, the corrosion potential decreases slightly. The corrosion current density represents the speed of corrosion. As the scanning speed increases, the corrosion current density decreases first and then increases. Therefore, appropriately increasing the scanning speed is beneficial to improving the corrosion resistance. The increase in corrosion potential is directly related to the aluminum content. Increasing the scanning speed is beneficial to reducing the dilution rate of the Al content. Therefore, the corrosion potential at 75 mm/s increases by 0.05 V. After the scanning speed is increased to 100 mm/s, the aluminum content decreases due to the limited molten metal powder, and the corrosion potential cannot continue to increase but decreases instead. During the corrosion process of the alloy cladding layer, aluminum atoms will form a passive film on the surface of the cladding layer. As the surface Fe corrodes, more Al atoms are exposed to continue to strengthen the passive film. It can also be seen from the polarization curve that the cladding layers at the three scanning speeds all have a certain range of passive layers. When Cl- destroys the passive film, it begins to destroy the cladding layer to form pitting.

EIS test was performed on the cladding layer at different scanning speeds. Figure 11a is the Nyquist diagram at different scanning speeds. The fitting results using Ziew software are shown in Table 6. Figure 11b is the Bode diagram of the Fe-Al alloy cladding layer at different scanning speeds. The cladding layer at 50 mm/s has the highest |Z| and the most stable passivation film, but the difference is not much with the other two speeds. The maximum phase angle appears between -60° and -80°. The slope of the cladding layer at 75 mm/s decreases in the low-frequency region. At this time, the influence of point defects in the passivation film becomes smaller. The equivalent circuit is shown in Figure 12. Rs is the solution resistance between the platinum electrode and the passivation film on the surface of the cladding layer, Rc is the resistance of the cladding layer, CCPE1 is the double-layer capacitance of the cladding layer, Rct is the charge transfer resistance between the passivation film and the cladding layer, CCPE2 is the double-layer capacitance of the cladding layer substrate, and n1 and n2 are the dispersion coefficients of CCPE1 and CCPE2, respectively. Impedance arcs appeared in the cladding layers at three scanning speeds, and the radius of the impedance arc decreased with the increase of scanning speed. The corrosion resistance of the cladding layer can be analyzed by polarization resistance (Rc+Rct). Rc and Rct have no order of magnitude difference, and the two jointly affect the corrosion resistance. The cladding layer has the largest Rc value at 50 mm/s, and CCPE1 is the smallest, and the passivation film on the surface of the cladding layer is more dense. The cladding layer at 75 mm/s has the largest Rct and the smallest CCPE2, and the passivation film has fewer point defects, which corresponds to the Bode diagram.

2. 6 Surface morphology of the cladding layer after polarization

Figure 13 is the surface morphology of the Fe-Al alloy cladding layer prepared at different scanning speeds after polarization. Different pitting pits appeared at the three scanning speeds. Wider and deeper pitting pits appeared at 50 mm/s. The deep pitting pits decreased at 75 mm/s, followed by large area and shallow depth corrosion pits. At 100 mm/s, the deep pitting pits decreased, followed by large area and shallow depth corrosion pits. There are fewer deep pitting pits under 100 mm/s scanning speed, and the corrosion is mainly shallow pitting pits with large diameters, and the radius of shallow pitting pits is also larger. For this material that mainly forms a strong and stable passive film, pitting has a huge impact on it. The scanning speed directly affects the density of point defects between grains in the cladding layer. The reduction of grain size increases the density of grain boundaries in the cladding layer, which helps the diffusion of Al elements during the corrosion process, and the passive film is more easily formed near the grain boundary. The increase in scanning speed reduces the grain size of the cladding layer, and the deep pitting pits after polarization are significantly reduced. However, the density of the passive film is reduced due to the reduction of Al content in the cladding layer under 100 mm/s. The increase in grain size reduces the density of point defects and maintains the strength of the passive film.

Conclusion Discussion

1) With the increase of scanning speed, the thickness of the cladding layer decreases, the columnar grains in the structure increase and gradually refine, and the needle-shaped FeAl phase distributed between the grains and grain boundaries also refines.

2) The cladding layer is mainly composed of Fe3Al phase and FeAl phase. The extremely fast cooling rate makes the FeAl phase that does not have time to undergo phase transformation distributed in the grain boundaries and between the grains in the form of laths and needles respectively.

3) With the increase of scanning speed, the hardness of the cladding layer increases due to grain refinement on the one hand and the refinement of the FeAl phase in the grains on the other hand, and the hardness gradually increases from 275 HV to
330 HV.

4) The self-corrosion potential of the Fe-Al alloy cladding layer increases first and then decreases with the increase of scanning speed, and the self-corrosion current density decreases first and then increases. Due to the increase of scanning speed, the grains of the cladding layer are refined, and the pitting pits on the surface change from deep to shallow after polarization, and then the area of ​​the shallow pitting pits expands.