The cross orthogonal stacking method was used to perform multi-layer and multi-pass laser wire cladding on a 20 mm thick Q345B low carbon steel plate, and the macroscopic morphology, microstructure, phase composition, microhardness and corrosion resistance of the cladding layer were studied. The results show that the cladding layer obtained by the multi-layer and multi-pass laser wire filling process has good macroscopic formation and no obvious defects such as pores and cracks; the cladding layer is mainly composed of cladding zone, overlap zone, phase change affected zone, fusion zone and heat affected zone; the parent material structure is mainly ferrite and pearlite, and the cladding layer microstructure is mainly ferrite, widmanstatten and martensite; due to the influence of microstructure and grain size, the hardness of the cladding layer is overall stepped, and the average hardness of the cladding layer is 320.13 HV, which is higher than that of the parent material; in 3.5% NaCl solution, the polarization curve of the cladding layer shows a passivation region, and its corrosion resistance is better than that of the parent material. The multi-layer and multi-pass laser wire filling cladding process can meet the preparation requirements of cladding layers in actual engineering.
Keywords: Q345B low carbon steel; laser wire cladding; cross orthogonal stacking; microstructure and properties

With the development of the economy and society, my country’s demand for marine oil and gas resources continues to increase. Focusing on the exploration and development of marine resources is a practical need for the development of my country’s petroleum industry [1-2]. Due to the complex service environment of marine engineering structures, they are more susceptible to damage than traditional structures. Therefore, the daily maintenance of marine engineering equipment has become a key issue that needs to be solved urgently [3]. Q345B steel is a low-alloy high-strength steel with good comprehensive properties and excellent weldability. It is widely used in marine engineering and bridge construction [4].

As an advanced protective and repair coating technology, laser cladding provides an efficient near-net-shape forming process for high-precision repair of key parts and preparation of coatings with advanced material properties [5]. During the multi-layer and multi-pass cladding process, the heat-affected zones of adjacent welds overlap, forming areas that have undergone two or more thermal cycles. The microstructure of these areas is particularly complex [6], and the microstructure composition phase, recrystallization rate, precipitate scale and inclusion morphology change continuously throughout the process [7]. Therefore, during the multi-layer and multi-pass cladding process, there are often weak points in the cladding area, which are prone to failure during use. For example, electrolytic corrosion and stress corrosion are often observed near the welded joints of pressure vessels during use [8].

Wu et al. [9] used laser cladding technology to prepare a continuous and dense Mo2NiB2 cladding layer on a steel substrate. The coating has high hardness, good wear resistance and corrosion resistance, improves the performance of the substrate, and ensures the safe and stable service of marine engineering equipment. Li et al. [10] used laser wire cladding to repair the corroded parts of 316L stainless steel surface and obtained a multi-layer multi-pass cladding layer of 308L stainless steel. The coating is mainly composed of austenite and a small amount of ferrite, with a tensile strength and elongation of 548MPa and 40%, respectively, which is about 86% and 74% of the substrate.

In this paper, laser wire cladding technology is used to prepare Q345B laser cladding layer by cross orthogonal stacking. The macroscopic morphology, microstructure, phase composition, microhardness and corrosion resistance of the multi-layer multi-pass cladding layer are studied, which provides a basis for the on-site repair of marine engineering structures.

1 Laser wire cladding experiment

1.1 Experimental materials

The experimental substrate material is Q345B carbon steel, and the wire cladding material is AFEW6-86 alloy steel wire with a diameter of 1.2 mm. The chemical compositions of the two are shown in Table 1.

1.2 Multi-layer and multi-pass laser wire cladding process
In actual engineering applications, the workpiece will be affected by forces in different directions during operation, so the influence of anisotropy needs to be considered. In order to reduce the influence of anisotropy, the path of the cladding layer is planned, the additive direction of the welds in the same layer is consistent, the directions of the welds in adjacent stacking layers are perpendicular to each other, and the layers are orthogonal. Its cross-orthogonal stacking path is shown in Figure 1.

During the cladding experiment, the shielding gas is pure argon gas with a gas purity of 99.99%. First, an orthogonal experiment was carried out using a single-layer single-pass cladding method to explore the optimal process parameters for single-pass cladding; then, a multi-layer single-pass stacking method was used to study the influence of the lifting height between layers on the weld forming quality, and a multi-layer single-pass weld with a straight cladding layer and good forming effect was obtained. On the basis of the above, the influence of different overlap rates on the forming quality of the cladding layer was studied, and it was found that when the overlap rate was 40%, the height between each pass of the cladding layer was relatively uniform, the surface formation was relatively flat, and the metallurgical bonding between each pass was the strongest. The lifting height between the experimental layers is 0.8 mm for each of the first two layers and 0.7 mm for each of the subsequent layers. The specific experimental parameters are shown in Table 2.

1.3 Analysis and testing method of cladding layer
Wire cutting was used to cut metallographic samples from the prepared multi-layer and multi-pass cladding layer. The sample surface was ground after being embedded with epoxy resin at room temperature. Sandpaper of different roughness was used to polish until no scratches were left. Then, the sample was polished with a polishing machine to obtain a metallographic sample cross section with a mirror effect. The sample was corroded with 4% nitric acid alcohol solution to etch out the visible cladding layer interface, rinsed with alcohol and blown dry, and the microstructure of the sample was observed with a metallographic microscope; the phase composition and evolution of the cladding layer were scanned and analyzed in the range of 30°~100° using X-ray diffraction technology; the chemical element analysis of the cladding layer was performed using an energy spectrometer; the microhardness of different areas of the cladding layer cross section was tested using an HVS-1000Z Vickers hardness tester; the polarization curves and impedance spectra of the cladding layer and the parent material were tested in a 3.5% NaCl solution using a VersaSTAT 3F electrochemical workstation with a saturated calomel electrode as the reference electrode and a platinum electrode as the auxiliary electrode, and their corrosion resistance was compared and analyzed.

2 Experimental results and analysis
2.1 Macromorphology analysis of cladding layer
The laser wire-filled cladding layer was prepared by a cross-orthogonal stacking experiment of 29 (length) × 15 (width) × 12 layers (height). The cladding layer has a good forming effect, a smooth surface, no macro defects such as cracks and unfused, and obvious vertical height. The macroscopic morphology of the cladding layer is shown in Figure 2. During the multi-layer multi-pass laser wire cladding experiment, the cladding process of the latter layer will produce a remelting reaction on the previous cladding layer, resulting in a downward flow at the edge of the cladding layer. At the same time, during the cladding process, due to a certain delay in the start and end instructions of the laser light output, the height of the edge of the cladding layer will be slightly lower than the middle part.

Figure 3 shows the cross-sectional morphology of the multi-layer multi-pass laser cladding layer. No defects such as pores, cracks and inclusions were found. A dense metallurgical bond was formed between the cladding metal and the base material. There was obvious vertical height, and the thickness of the cladding layer was 11.5 mm.

2.2 Microstructure analysis of cladding layer
The cooling of the welding pool is a phase change process, and the microstructure of the phase change depends on the chemical composition and cooling conditions of the weld metal [11]. The microstructure of each area of ​​the cladding layer was observed using a metallographic microscope, as shown in Figure 4. The cladding layer includes the cladding zone (cladded zone, CZ), the overlay zone (ovelapped zone, OZ), the phase transition affected zone (phase transition affected zone, PAZ), the fusion zone (fusion zone, FZ), the heat affected zone (heat affected zone, HAZ) and the base metal (base metal, BM) [12]. The base metal microstructure is mainly composed of ferrite and a small amount of pearlite. The main element Mn added to Q345B steel not only has a significant strengthening effect on ferrite, but also reduces the toughness-brittleness transition temperature, increases the amount of pearlite, and improves the strength of pearlite.

Figure 4 (a) shows the microstructure of the cladding area inside the cladding layer, which is composed of lath and needle-shaped ferrite, widmanstatten and a small amount of lath martensite. Due to the different layers, each cladding layer will produce a tempering effect on the previous layer, resulting in uniform grain refinement and clear grain boundaries; Figures 4 (b) and (b-1) show the microstructure of the fusion area, which is composed of ferrite and widmanstatten with uneven grain distribution; Figure 4 (d) shows the microstructure of the overlap area of ​​two welds inside the cladding layer. The bright area in the figure is the fusion line between the two welds. During the cooling process, the molten pool will form columnar ferrite along the heat dissipation direction. Therefore, this area is mainly composed of columnar ferrite and a small amount of pearlite, as shown in Figure 4 (d-1). Due to the double thermal action, the overlap area has uniform grain refinement; Figure Figure 4 (d-2) is the phase transformation affected area, which is mainly composed of ferrite and Widmanstatten. Due to the influence of phase transformation heat, the grain size of this area is slightly larger than that of the overlap area; Figure 4 (e-1) is the microstructure of the heat affected zone. During the welding process, the lower cladding area undergoes tempering, which makes the structure of this area refined and the grain distribution uniform. It is mainly composed of fine-grained ferrite and a small amount of pearlite. Fine-grained ferrite is a transformation product between ferrite and bainite. It is a beneficial microstructure in the welding metallurgical process [11].

Figure 5 is the microstructure of the last cladding layer. This layer is not subjected to laser secondary heating. Compared with other layers, it can maintain the original structure morphology. Its grain size is uniform and the structure is dense. It is mainly composed of ferrite, Widmanstatten and lath martensite.

2.3 XRD and EDS analysis of cladding layer
In order to analyze the phase composition of laser cladding layer, a sample with a size of 10 mm×10 mm×8 mm was cut by wire cutting, and X-ray diffraction test analysis was performed after grinding and polishing. Figure 6 shows the XRD spectrum of multi-layer multi-pass laser cladding layer and parent material. Combining the microstructure and XRD spectrum results, it can be seen that the cladding layer is mainly composed of a large amount of ferrite, part of martensite and widmanstattenite, and no other harmful phases appear. Since columnar ferrite will be formed in the cooling process of laser cladding molten pool, the cladding layer contains a large amount of ferrite. When the heat input of the laser is large during the welding process, the microstructure of the cladding layer will coarsen to a certain extent, and the grain size will increase. At this time, the structure will appear overheated widmanstattenite and lath martensite, and the two structures are staggered.

The chemical composition was analyzed by point scanning at different positions of the sample cross section. The point scanning positions are shown in Figure 7, and the EDS analysis results of different areas are shown in Table 3. Due to the high content of Cr and Ni elements in the welding wire, the Cr and Ni content of the cladding layer is significantly higher than that of the parent material, making the corrosion resistance of the cladding layer better than that of the parent material.

2.4 Microhardness analysis of cladding layer
The microhardness of the sample was measured. During the test, the load was 1000 g, the holding time was 10 s, the measurement path was along the direction from the parent material to the cladding area, and the interval between two adjacent sampling points was 1 mm. The microhardness distribution from the parent material to the cladding area is shown in Figure 8. The average microhardness of the parent material is 172.02 HV, and the average microhardness of the cladding layer is 320.13 HV. The microstructure of the last cladding layer contains a large amount of ferrite, widmanstattenite and a small amount of lath martensite and pearlite. The hardness value of this microstructure area is the highest, which is 325.92HV. The average hardness of the cladding layer is much higher than that of the parent material, meeting the requirements of repair strength. As shown in Figure 8, the hardness of the cladding area is generally distributed in a step-like manner. This is because in the process of multi-layer and multi-pass laser wire filling, each cladding layer will have a post-heating tempering effect on the previous layer during the formation process, and a preheating effect on the next layer. The last cladding layer has a preheating effect without post-heating tempering, which promotes uniform grain refinement and significantly improves hardness.

2.5 Analysis of corrosion resistance of cladding layer
Most metal corrosion is carried out in the form of electrochemical corrosion, and the corrosion process is accompanied by the generation of current, just like a primary battery [13-14]. In order to test the electrochemical corrosion performance of the multi-layer and multi-pass cladding layer, the specimen was placed in a 3.5% NaCl solution to test its Tafel polarization curve and impedance spectrum.

The polarization curves of the cladding layer and the base material are shown in Figure 9. It can be seen that the polarization curve of the cladding layer has a passivation region, indicating that a dense oxide film is formed on the surface of the cladding layer during the corrosion process. The elements such as Cr, Ni, and Si in the oxide film improve the passivation stability, hinder the diffusion of ions, and improve the corrosion resistance. The self-corrosion potential Ecorr and self-corrosion current density Icorr of the cladding layer and the base material are obtained by data fitting, as shown in Table 4. The self-corrosion potential Ecorr of a metal in an electrolyte solution reflects its sensitivity to corrosion and is an indicator of the material’s resistance to electrochemical corrosion. The smaller the self-corrosion potential, the easier it is for the metal to lose electrons and the weaker its corrosion resistance; the larger the self-corrosion potential, the harder it is for the metal to lose electrons and the stronger its corrosion resistance[14]. As can be seen from Table 4, the self-corrosion potential of the cladding layer is higher than that of the base material, indicating that the cladding layer has strong corrosion resistance. The self-corrosion current density Icorr is proportional to the corrosion rate. The larger the corrosion current, the faster the corrosion rate of the material and the worse the corrosion resistance. As can be seen from the data in Table 4, the self-corrosion current of the base material is higher than that of the cladding layer, indicating that the corrosion resistance of the base material is poor. Therefore, by comparing the size of the self-corrosion potential and the self-corrosion current, it can be concluded that the corrosion resistance of the cladding layer is better than that of the base material.

The cladding layer and the base material were tested by impedance spectroscopy (EIS), and the impedance spectrum Nyquist plots of the two samples are shown in Figure 10. Z’ and Z” are the real and imaginary parts of the measured impedance Z, respectively. Both the cladding layer and the base material present a single capacitive arc characteristic. The larger the capacitive arc radius, the greater the total impedance of the sample and the stronger the corrosion resistance. As shown in Figure 10, the capacitive arc radius of the cladding layer is significantly larger than that of the base material. Therefore, the polarization resistance of the cladding layer is larger, indicating that the corrosion rate of the cladding layer is lower and the corrosion resistance is stronger, which is consistent with the results of the dynamic potential polarization curve.

In summary, the corrosion resistance of the cladding layer is better than that of the base material. First, the cladding material uses AFEW6-86 welding wire, which has higher Cr and Ni content than the base material, so that the cladding layer has higher oxidation resistance and corrosion resistance. In a corrosive environment, when Cr reacts with O elements, a layer of corrosion-resistant oxide film will be formed on the surface, which will separate the metal surface from the corrosive medium, reduce the dissolution process of the anode, and reduce the dissolution rate of the cladding metal, thus improving the corrosion resistance of the cladding layer. The corrosion resistance is improved[15-16]. The second reason is that the grain size distribution in the cladding layer is more uniform due to the increase in heat input.

3 Conclusion
(1) The cladding layer obtained by the multi-layer and multi-pass laser wire welding process has good macroscopic formation, no obvious defects such as pores and cracks, and a good metallurgical bond is formed between the cladding layer and the parent material. There is a significant vertical pile-up, and the thickness of the cladding layer is 11.5mm.
(2) The cladding layer is mainly composed of ferrite, widmanstatten and lath martensite. The Cr and Ni content in the cladding layer is higher than that in the parent material. Cr and Ni elements improve the stability of the passivation film, hinder the diffusion of ions, and improve the oxidation resistance and corrosion resistance of the cladding layer. In addition, due to the increase in heat input, the grain size distribution in the cladding layer is more uniform, so the corrosion resistance of the cladding layer is better than that of the parent material.
(3) The average hardness of the parent material is 172.02HV, and the average hardness of the cladding layer is 320.13HV, the hardness of the cladding layer is much higher than that of the parent material. Due to the influence of microstructure and grain size, the hardness of the cladding area shows a step-like distribution trend as a whole.