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Mechanical and corrosion resistance analysis of laser cladding layer

  • Yinghao Cui , Cong Xie , Jialin Liu EMAIL logo , Shirui Guo and Lujun Cui
Published/Copyright: November 7, 2022

Abstract

To improve the mechanical properties and corrosion resistance of the hydraulic column in the mine, a semiconductor fiber-coupled laser was used to laser-clad iron-based alloy powder on a 27SiMn steel substrate (SUB). The microstructure, mechanical properties, and corrosion resistance of the obtained cladding layer (CL) were studied based on experiments. Results show that equiaxed grains at the top of the CL are fine and tightly arranged. Compared with the SUB, the CL average microhardness is increased by 0.3 times, the average friction coefficient is decreased by 0.12, and the wear amount is significantly reduced. The bonding strength between the CL and the SUB is good, and the tensile strength is increased by 10.82%. Compared with the SUB, the mechanical properties and corrosion resistance of the CL are better. The research has an important guiding significance for the practice of hydraulic column repair and strengthening engineering.

1 Introduction

As an important fossil fuel in production, coal had a high demand for its output. As a key structural component in underground coal mining, hydraulic supports are served in complex underground mining environments [1]. The hydraulic column is the important supporting component between the base and the top beam of the hydraulic support. However, in the coal seam mining process, falling gangue, cinder, coal lumps, and other particles will hit the hydraulic column, causing surface scratches, wear, and depression [2], which will reduce the surface mechanical property. Besides, the underground environment is filled with various corrosive media and relative humidity of more than 75%, which will cause the surface of the hydraulic column to corrode [3], further reducing its surface properties, resulting in a significant reduction in service life. To improve the wear resistance and corrosion resistance of the hydraulic column surface, surface modification technology is usually used to improve its surface properties [4].

Laser has a wide range of applications in product processing [5]. As a mature surface modification technology, laser cladding combines the high-performance metal powder materials with the metal surface that needs to be repaired rapidly by a non-contact high-energy laser beam, to improve the performance of the metal surface [6,7,8]. Wang et al. [9] created an iron-based alloy cladding layer (CL) using a high-speed powder feeding technique, and the results show that the CL microstructure is composed of uniform small particles, which advantages CL hardness and wear resistance. Li et al. [10] prepared pure Fe316L and WC-Fe316L CL with different WC on 27SiMn steel. According to the studies, the CL with various WC possesses higher hardness and a lower wear rate. Wang et al. [11] deposited 304 stainless steel on 27SiMn Steel in multiple layers, which shows that the CL exhibits columnar crystals and the elongation is higher than that of the substrate (SUB). Guo et al. [12] prepared iron based alloy CL on the hydraulic column, and the microstructure in the middle and lower parts exhibits typical directional solidification characteristics. The hardness is distributed in steps, and the wear mechanisms of the CL are mainly adhesive wear, abrasive wear, and spall wear. JG-2 and JG-3 alloys were cladded on hydraulic column 27SiMn by Yang et al. [13]. The microstructure of the CL is a relatively dense dendrite structure. JG-3 cladding has a higher average hardness and lowers wear rate. In terms of corrosion resistance, the JG-2 CL is better. Ouyang et al. [14] used high-speed laser cladding technology to prepare an iron-based alloy CL on 27SiMn steel surface. The CL mainly includes α-Fe, M7C3, M2B, and Cr3Si with equal metallographic composition, and the corrosion resistance is better than the SUB. Li et al. [15] used ultra-high-speed laser cladding to prepare 431 steel CL on 27SiMn SUB. Laser cladding at ultra-high speeds produces a more uniform distribution of dendrites and inter-dendritic components in the CL, as well as improved corrosion resistance. The research on the performance improvement of the hydraulic column is mainly to prepare the CL with an excellent performance by using different laser cladding technologies and processes. However, there are relatively few studies on the overall performance of mechanical properties and corrosion resistance.

In this paper, multi-pass laser cladding of iron-based alloy powder on hydraulic column using coaxial powder feeding laser. Mechanical properties and corrosion resistance of the CL were analyzed from the perspectives of microstructure, microhardness, friction and wear, tensile strength, and electrochemical corrosion, which provided theoretical and experimental support for the improvement of the surface properties of the hydraulic column.

2 Experimental material and methods

The SUB selected in this test is 27SiMn steel commonly used in coal mine hydraulic columns, with a size of 100 × 50 × 20 mm, and chemical composition content of C: 0.3%, Si: 1.25%, Mn: 1.3%, S: 0.035%, Ni: 0.3%, Cr: 0.3%, etc. The cladding material is iron-based alloy powder, the powder particle size is 150–240 mesh, and the chemical composition is C: 0.75 %, Mn: 2 %, Si: 2.5 %, Cr: 17 %, Ni: 29%, etc.

The laser cladding experimental device consists of a CNC machining center LDM8060, a high-power semiconductor fiber-coupled laser with a spot diameter of Φ3 mm, and a water-cooling machine system. Removing surface rust and surface oxides are required before experimentation. The iron-based alloy powder was put into the HD-E804-45A drying box, and dried at 120°C for 120 min to remove the moisture in the iron-based alloy powder, to prevent the powder from blocking the powder feeding pipeline in the experiment.

Based on the previous research results, this experiment adopts the optimized process parameters, laser power: 2,000 W, powder feeding rate: 15 g/min, scanning speed: 6 mm/s, and overlap rate: 50%, to prepare an iron-based alloy CL.

Using the above process parameters, laser cladding was performed on the SUB 27SiMn steel of 100 mm × 50 mm × 20 mm to obtain a CL with a surface size of 100 mm × 35 mm. The obtained cladding specimen and base 27SiMn steel were cut into non-standard tensile specimens, the dimensions of which are shown in Figure 1. The gauge length of the tensile specimen is 37.3 mm, the thickness of the CL in the cladding tensile specimen is 2 mm, and the thickness of the bottom SUB portion is 3 mm.

Figure 1 
               Dimensions of non-standard tensile specimens.
Figure 1

Dimensions of non-standard tensile specimens.

After the experiment is completed, according to the standard of GB/T 13298-2015 Inspection methods of microstructure for metals the samples are cut with a wire cutting machine, and the cut samples are inlaid with metallographic inlays. After corroding the cross-section of the CL with aqua regia for 10 s, the microstructure of the CL was observed under a microscope. Hardness, friction coefficient, and tensile properties of specimens in different regions of the cladding section were measured. The microstructure of the friction and wear area was studied using an SEM. Finally, the general electrochemical measurement system CHI600E was used to test the corrosion resistance of the samples.

3 Experimental results and analysis

To explore the mechanical properties and corrosion resistance of the CL, after the experiment was completed, the microstructure, microhardness, friction and wear properties, tensile properties, and electrochemical corrosion properties of the CL were analyzed.

3.1 Microstructure analysis of CL

Figure 2 shows the microstructure of the CL of the prepared cladding sample. According to the solidification principle [16], the growth pattern of dendrites is related to the ratio of the temperature gradient G to the crystallization rate R (G/R). When the laser cladding starts, the matrix material 27SiMn steel and iron-based alloy powder at room temperature form a liquid metal molten pool under the irradiation of a laser beam. At this time, the G/R ratio is the largest, and the grains are not spontaneously nucleated on the solid–liquid surface. Figure 2(a) shows that the dendrites begin to grow upward in a planar crystal form, forming a bright fusion line at the bottom of the CL. Metallurgical bonding between CL and SUB. With the progress of laser cladding, the heat generated by the laser beam accumulates on the SUB and the newly formed solid–liquid surface, the temperature rises rapidly, and the G/R of the newly formed solid–liquid surface gradually decreases. At this time, the dendrites began to grow upward in the form of cellular crystals, and the growth direction was antiparallel to the direction of heat flow.

Figure 2 
                  Microstructure of the CL: (a) bottom, (b) middle, and (c) top.
Figure 2

Microstructure of the CL: (a) bottom, (b) middle, and (c) top.

With the further decrease in G/R, the shape of the cellular crystal gradually changed from small at the bottom to coarse. The G/R ratio lowers to a specific value at the center of the CL, and the nucleation rate of the crystal is larger than the growth rate at this point. Figure 2(b) shows that the grains are transformed from coarse cellular crystals to dendritic crystals, and at this time dendritic crystals will grow freely in the opposite direction of heat flow in the middle of the CL, forming a dendrite network.

At the top of the CL, the high-temperature heat generated by the molten pool is mainly absorbed by the air medium. The melt gradually becomes slightly supercooled from superheating, and the ratio of G/R will be reduced to the minimum value at this time. The dendrites also transformed from dendritic crystals to fine equiaxed crystals and continued to grow. Figure 2(c) shows that on the top of the CL, the fine equiaxed grains are closely arranged.

3.2 Microhardness analysis

The microhardness distribution of the prepared cladding sample section is shown in Figure 3. The position of the fusion line where the cladding zone (CZ) is combined with the SUB is the zero position in the figure. The maximum hardness of the CL is 408.6 HV0.3, and the average hardness is 383.3 HV0.3. The average hardness of the SUB is about 275.6 HV0.3. The average hardness of the CL is about 0.3 times higher than the SUB. In the heat-affected zone (HAZ), the bonding area of the CL is to the SUB. The hardness increased sharply from 331 HV0.3 of the CL to 741.1 HV0.3. Maintaining high hardness in the range of 0.5 mm, the maximum hardness is 746.5 HV0.3. The hardness gradually decreased to about 283 HV0.3 as it approached the direction of the SUB.

Figure 3 
                  Microhardness distribution of cladding sample.
Figure 3

Microhardness distribution of cladding sample.

During the process of irradiating the iron-based alloy powder and the SUB with a high-energy laser to form a molten pool, the HAZ hardness will change sharply. The high temperature generated by the molten pool will be quickly transferred to the SUB, the CL and the SUB will remain at a high temperature. After the cladding is completed, it will be rapidly cooled to room temperature. The HAZ will be quenched and hardened during this process, and the hardness will increase by a large amount [17]. With the further transition towards the SUB, the temperature generated by the molten pool gradually decreases to a temperature where it cannot be quenched and hardened. As a result, the hardness of HAZ gradually decreased, and finally decreased to the SUB hardness.

3.3 Friction and wear analysis

The friction and wear properties of the CL can be analyzed from three aspects: friction coefficient, wear amount, and wear morphology.

3.3.1 Friction coefficient and wear amount

Figure 4 shows the friction coefficient curve of both the cladding coating and SUB. In the running-in stage of friction initiation, the surfaces of the two samples were rubbed against the protruding parts of the anti-abrasive material GCr15, respectively. The friction coefficient is unstable due to the small contact area. As the friction continues, the protruding parts of the counter-grinding material are gradually smoothed. The contact area between the two increases, and the friction enters a stable friction stage. The friction coefficient of the two samples was gradually stabilized. The average friction coefficient of the CL was 0.46, while that of the SUB was 0.58.

Figure 4 
                     Friction coefficient curve of SUBs and cladding sample.
Figure 4

Friction coefficient curve of SUBs and cladding sample.

Under the same wear test conditions, the wear amounts of the CL and SUB are 1.36 and 14.7 mg, respectively. The wear amount of the CL only accounts for 9.25% of the SUB. Because the wear amount of the sample is negatively correlated with the hardness, and the above test shows that the CL hardness is greater than the base metals, the CL wear amount is eventually smaller than the base metals. From the perspectives of friction coefficient and wear amount, the obtained CL has better wear resistance.

3.3.2 Wear morphology

Figure 5 is the SEM micro-morphology of the SUB and cladding samples after wear. From Figure 5(a), the worn SUB surface has consistent and clear furrows, and there are also many signs of lamella falling off on the surface. In Figure 5(c), there is no obvious furrow shape on the surface of the CL. Although there are traces of lamella falling off on the surface, it is significantly reduced compared to the SUB. Figure 5(b) and (d) are the microscopic topography of the worn SUB and cladding sample after 1,500 times magnification by scanning electron microscope. The surface of the worn SUB is a large piece of flaky layer peeling off. The CL surface is a little portion of a flaky layer that peels away and is made up of a large number of abrasive particles of various sizes after grinding. The wear mechanism of the SUB 27SiMn steel is primarily adhesive wear, whereas the CL wear mechanism is primarily abrasive wear, as shown in the above analysis.

Figure 5 
                     SEM micro-morphology of (a) and (b) SUB 27SiMn steel, CL (c) and (d) after-wear test.
Figure 5

SEM micro-morphology of (a) and (b) SUB 27SiMn steel, CL (c) and (d) after-wear test.

3.4 Analysis of tensile test results

The true stress–strain curves of the SUB and the cladding specimen after the test are presented in Figure 6. The tensile strength of the SUB 27SiMn steel and the cladding sample is 822 and 911 MPa, respectively. The strength of CL bonding to the cladding sample’s bottom SUB is good. Compared with the SUB, the tensile strength of the cladding specimen is increased by 10.82%. The elongation after fracture of the SUB is 16.7%, while the elongation after fracture of the cladding sample is only 1.7%. The elongation after fracture of the cladding sample is 15% lower than that of the SUB. This is because the properties of the specimens after laser cladding are similar to those of castings, with some increase in tensile strength and a decrease in post-break elongation.

Figure 6 
                  True stress–strain curves of SUB and cladding sample.
Figure 6

True stress–strain curves of SUB and cladding sample.

3.5 Electrochemical corrosion analysis

The polarization curves of the SUB 27SiMn steel and the cladding samples are shown in Figure 7. The potentiodynamic (−1 to 0.6 V) polarization curve can well reflect the corrosion resistance of the sample [18]. The corresponding polarization parameters are obtained after fitting the polarization curve using the Tafel extrapolation method. The self-corrosion potential (E corr) and current density (I corr) of the SUB 27SiMn steel are −0.843 V and 1.50 × 10−5 A/cm2, respectively. The E corr and I corr of the cladding samples are −0.553 V and 8.95 × 10−6 A/cm2, respectively. Compared with the SUB, the E corr is increased by 0.29 V, and the I corr is decreased by 6.05 × 10−6 A/cm2.

Figure 7 
                  Polarization curves of SUB 27SiMn steel and cladding sample.
Figure 7

Polarization curves of SUB 27SiMn steel and cladding sample.

The thermodynamic corrosion tendency of the material can be reflected by the E corr. The smaller the algebraic value of corrosion potential, the greater the corrosion tendency of materials. The uniform corrosion rate of the material can be indirectly reflected by the I corr. The smaller the value of the current density, the lower the corrosion rate of the material. The above data shows that the CL has a smaller corrosion tendency and corrosion rate compared to the SUB. The cladding is more resistant to corrosion. Because the Cr concentration of iron-based alloy powder is 17% greater than that of the SUB, the CL possesses stronger corrosion resistance. There is enough Cr on the CL surface and O in the air to generate a thick oxide coating that prevents sodium chloride electrolyte solution corrosion on the CL surface.

4 Conclusion

Through the above experimental research and analysis, it can be seen that the dendrites on the top of the iron-based alloy CL prepared according to the optimal process parameters are closely arranged and small. The microhardness, friction and wear properties, and tensile properties of the CL are better than those of the base material. The CL’s main wear mechanism, abrasive wear, is different from that of the base material 27SiMn Steel. The corrosion tendency and corrosion rate of the CL are lower than that of the base material. In conclusion, the mechanical properties and corrosion resistance of the CL are better than that of the base material 27SiMn Steel.

  1. Funding information: This work was financially supported by the Key Scientific and Technological Projects in Henan Province (202102210068), Young Backbone Teachers of Zhongyuan University of Technology (2021XQG07), Research team development project of Zhongyuan University of Technology (K2021TD002), and China Textile Industry Federation science and technology guiding project plan (20200784).

  2. Conflict of interest: Authors state no conflict of interest.

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Received: 2022-06-06
Revised: 2022-09-01
Accepted: 2022-09-22
Published Online: 2022-11-07

© 2022 Yinghao Cui et al., published by De Gruyter

This work is licensed under the Creative Commons Attribution 4.0 International License.

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