In order to explore the influence of TiC content on the cladding morphology and forming quality of iron-based coatings and improve the surface mechanical properties of 65Mn steel. Iron-based TiC composite coatings were prepared on the surface of 65Mn steel by laser cladding technology, and the effects of different TiC addition amounts on the forming quality (porosity), aspect ratio, and microhardness of the iron-based cladding layer were compared and analyzed. The results show that with the increase of TiC addition, the aspect ratio increases first and then decreases. When the mass fraction of TiC is 30%, the aspect ratio reaches a maximum of 5.31, and the porosity decreases at this time. The appropriate addition of TiC can effectively improve the hardness of the composite coating, but too high TiC content will cause excessive gas in the molten pool, increase the porosity, and fail to obtain good coating performance. The research results have a certain guiding role in the field of agricultural machinery tools.
In recent years, laser cladding technology has been widely studied by many scholars in the field of agricultural machinery and has been applied to the repair and strengthening of agricultural machinery parts. Agricultural machinery is an indispensable equipment in agricultural production. Agricultural machinery parts are required to have high strength, hardness, wear resistance and toughness. 65Mn is commonly used in China to manufacture various agricultural machinery parts, such as disc harrows, deep loosening shovels, plowshares, rotary tillage knives, etc. During use, they will inevitably be subject to friction and wear from soil, stones, etc. Their hardness and wear resistance must meet certain use conditions. If they cannot meet the use requirements, they are prone to wear and failure. Frequent replacement of agricultural machinery parts will lead to an increase in the labor cost of agricultural production. Laser cladding technology, as a surface modification technology, has the characteristics of small light spot, concentrated energy, low dilution rate, and strong metallurgical bonding ability between coating and substrate. Therefore, it can significantly improve the surface mechanical properties of the substrate. Because the composition of iron-based alloys is similar to that of the substrate, its forming effect and price are more suitable for the repair of agricultural machinery equipment. TiC, as a hard reinforcing phase, is often used as the second important material to improve the performance of 65Mn steel surface coating. The addition of TiC to the iron-based coating on the surface of 65Mn steel can effectively reduce defects such as pores and cracks in the coating, significantly improve the forming quality and comprehensive mechanical properties of the coating, and is of great significance to broadening its application range.
Many domestic and foreign scholars have studied the mechanical properties of laser-clad iron-based TiC composite coatings. Wang Zhi et al. laser-clad iron-based TiC composite coatings on the surface of 45 steel. The results showed that the addition of TiC greatly improved the hardness and wear resistance of the cladding layer. Cao Jinlong et al. clad Ni60-TiC composite coatings on the surface of 45 steel and studied the effects of different TiC contents (mass fractions) on the mechanical properties of the coatings. The results showed that the addition of TiC can effectively improve the wear resistance of the coating and increase the hardness of the coating. Pi
Ziqiang et al. prepared Fe-WC composite cladding layers with different WC contents on the surface of high manganese steel and concluded that as the amount of WC particles added increased, the hardness and wear resistance of the Fe-WC cladding layer increased. The WC particles played a skeleton role in the cladding layer, and the alloy matrix played a supporting role. The two synergistically increased the wear resistance of the cladding layer. Wang Qianting et al. used laser cladding technology to prepare Fe50-TiC composite cladding layers on the surface of Cr12 die steel. The results showed that the Fe50-TiC composite coating can significantly improve the surface properties of Cr12 die steel, and the average hardness of the cladding layer is about 2.4 times that of the matrix.
From the above studies, it can be seen that laser cladding iron-based TiC composite coatings have a significant effect on improving the mechanical properties of the substrate. There are few reports on the research of laser cladding iron-based TiC composite coatings on the surface of 65Mn steel. In summary, this paper uses laser cladding technology to prepare an iron-based TiC composite coating on the surface of 65Mn steel, studies its morphology, aspect ratio, porosity and surface hardness, and uses the optimized process parameters to verify multiple overlap tests. The research results have a certain guiding role in improving the performance of agricultural machinery tools.
2 Experimental materials and methods
2.1 Experimental materials
The material used in the test is 90mmX50mmX2mm 65Mn steel, and the cladding material is iron-based alloy powder Fe60 and ceramic powder TiC (purity is 99.98%). The main chemical compositions are shown in Tables 1 and 2. TiC powder with mass fractions of 15%, 30%, and 45% are added as hard reinforcement phases, respectively. The morphologies of the Fe60 powder and TiC powder used are shown in Figure 1.
2.2 Experimental methods
Before the test, 800# and 1200# metallographic sandpapers were used to polish the surface of 65Mn steel in turn, and anhydrous ethanol was used to clean it to remove impurities and excess oil, and dried it for use. The XL-F2000W fiber laser processing system was used for laser cladding test. The powder pre-setting method was used. The powder pre-setting thickness was about 1mm. The laser cladding process parameters were set as follows: laser power was 800W, scanning speed was 300mm/min, and defocus was +5mm. After laser cladding, the sample was cut perpendicular to the laser scanning direction using a wire cutting machine. The sample was made using a metallographic inlay machine (XQ-2B). The sample was polished with 400#, 600#, 800#, 1500#, and 2000# metallographic sandpaper in turn. The sample was polished with 1μm diamond paste on a metallographic polishing machine. The sample surface was corroded with 5% nitrate by volume. The microhardness was measured using a digital microhardness tester (MHVD-1000AT) produced by Shanghai Jujing Precision Instruments with a loading force of 500gf (1gf=0.0098N) and pressure maintained for 10s. The hardness values of the cladding layer and the substrate were measured at equal intervals (0.15 mm) from the top of the coating to the substrate, and the average value was taken for three points at each interval.
3 Experimental results and analysis
3.1 Cross-sectional macroscopic morphology analysis
The cross-sectional morphology of single-pass cladding layers with different TiC mass fractions is shown in Figure 2. It can be seen that different TiC mass fractions have a great influence on the macroscopic morphology of the cladding layer, and the interface between the cladding layer and the substrate is well bonded. When TiC is not added (only Fe60 powder), there are a small number of pores and cracks in the cross-sectional morphology of the cladding layer, which is mainly due to the poor self-solubility, poor oxidation resistance and poor fluidity of the iron-based alloy. With the increase of TiC content, the cross-sectional morphology of the cladding layer deteriorates. This is because during the laser cladding process, when the high-energy-density laser heat source acts on the iron-based TiC composite coating, part of the TiC melts and precipitates C, which reacts with oxygen in the air to generate CO. In addition, laser cladding has the characteristics of rapid heating and cooling, which makes it difficult for CO gas to be discharged, and the cross-sectional morphology of the coating deteriorates further. It can be seen that with the increase of TiC content, the probability of cracks and pores in the cladding layer will increase, and the cross-sectional morphology of the cladding layer will gradually deteriorate. The cross-sectional morphology of the multi-pass cladding layer is shown in Figure 3. As the TiC content increases, cracks and pores also gradually increase, further verifying the single-pass cladding effect.
3.2 Porosity analysis
The cross-sectional image of the cladding layer shown in Figure 2 is analyzed to obtain the porosity distribution of the cladding layer, as shown in Figure 4. As the TiC content increases, the porosity shows a wave-like growth trend, with the highest porosity of 0.29%, when the TiC mass fraction is 45%. When the TiC mass fraction is 30%, the porosity decreases to 0.17%. This is because during the laser cladding process, the TiC content increases, the melting rate of TiC is greater than the decomposition rate, the C content in the molten pool decreases, and the C content in the molten pool decreases, and the generated CO gas also decreases, so the porosity decreases. It can be seen that when the TiC mass fraction in the composite coating is 30%, the cladding quality of the cladding layer is optimal.
3.3 Aspect ratio analysis
The ratio of the width to the height of the cladding layer is a key indicator for evaluating the quality of the cladding layer. Figure 4 shows the aspect ratio. It can be seen from the figure that with the increase of TiC content, the aspect ratio shows a trend of increasing first and then decreasing: it reaches the maximum when the TiC mass fraction is 30%, and the aspect ratio is 5.31; when the TiC mass fraction is 45%, the aspect ratio drops to 4.04. At this time, the undissolved TiC powder increases, resulting in more TiC powder not being able to melt in time, thus being retained in the molten pool, causing the molten pool size to decrease, and the aspect ratio becomes smaller. Therefore, the maximum aspect ratio and lower porosity can be obtained when the TiC mass fraction is 30%.
3.4 Microhardness analysis
A digital microhardness tester (MHVD-1000AT) was used with a loading force of 500gf and a pressure holding time of 10S. The hardness values of the cladding layer and the substrate were measured at equal intervals (0.15mm) from the top of the coating to the substrate downward, and the average value was taken for each interval of three points. As shown in Figure 5, the single-pass and multi-pass microhardness of the samples with different TiC mass fractions are improved. This is because TiC, as a hard phase, improves the overall hardness of the coating. Among them: when the TiC mass fraction is 30%, the microhardness of the cladding layer is about 3.6 times that of the substrate, and gradually increases with the increase of TiC addition; when the TiC mass fraction is 45%, the measured microhardness is the highest. This shows that the appropriate addition of TiC has a great influence on the improvement of the coating hardness. The reasons for this are: 1) The melting of the Ti element in the molten pool plays a role in solid solution strengthening; 2) The increase in the C content in the cladding layer leads to an increase in carbides, thereby increasing the hardness; 3) With the addition of TiC, the undissolved TiC is dispersed in the molten pool and plays a dispersion strengthening effect. The 65Mn matrix has not been subjected to work hardening treatment, and its hardness is around 250HVo.5, which is relatively low.
4 Conclusion
In this paper, iron-based TiC composite coatings were prepared on the surface of 65Mn steel by laser cladding technology. The effect of TiC content on the cross-sectional morphology, aspect ratio, porosity and microhardness of the composite coating was studied, and multiple tests were conducted to verify and analyze the results. The following conclusions were obtained:
1) After adding different contents of TiC to Fe60 powder, the hardness of the obtained coating was greatly improved compared with the substrate, and with the increase of TiC content, the hardness of the cladding layer gradually increased. When the mass fraction of TiC was 45%, the hardness was the highest.
2) The amount of TiC added has a great influence on the performance of the coating, but too much TiC content will cause the cladding quality to deteriorate, the gas in the molten pool to increase, and the porosity to increase. Therefore, the amount of TiC added should be within a reasonable range. From the results of this experiment, when the mass fraction of TiC is 30%, the performance of the composite coating prepared by laser cladding is optimal.
3) After verification by multiple overlap tests, the hardness of the coating with TiC added is significantly better than that of the coating without TiC added.
