Yangtze River Delta G60 Laser Alliance Introduction
This article studies the significant impact of magnetic field on the defects and performance of the cladding layer. The morphology evolution, phase composition, element distribution, thermal expansion and thermal stress of the alloy were studied.
Abstract
Co-based laser cladding layer was prepared on 42CrMo substrate. applies a magnetic field during laser cladding. The causes of cracks and pores in the cladding layer were systematically analyzed. The magnetic field has a significant impact on the defects and properties of the cladding layer. The morphology evolution, phase composition, element distribution, thermal expansion and thermal stress of the alloy were studied. The results show that the magnetostrictive effect reduces the thermal expansion and thermal stress during magnetization-induced laser cladding and reduces the crack sensitivity of the cladding layer. The average microhardness and morphology of the cladding layer are improved than those of the cladding layer without magnetic field assistance. The magnetic field can improve the uniformity of element distribution in the cladding layer and reduce element segregation, thereby reducing the crack sensitivity of the cladding layer.
. Introduction
Laser cladding is a process in which a high-energy laser beam interacts with metal powders and substrates to build new parts, repair or improve the mechanical properties of worn parts. As an important laser manufacturing technology, laser cladding plays a vital role in parts remanufacturing.
Different from traditional removal technology, laser cladding is based on the incremental law of materials. metal powder is completely melted during the forming process, forming a metallurgical bond on the base material. After decades of development, laser cladding technology has become a rapidly growing part of advanced manufacturing technology. Laser cladding is a surface modification technology commonly used in remanufacturing. It can prepare coatings with low dilution rate, small thermal deformation, high bonding quality and excellent mechanical properties on the surface of the substrate. Laser cladding is a revolution and breakthrough in manufacturing.
Schematic illustration of various laser beams : (a) negative defocus, (b) focused and (c) positive defocus plane.
However, defects, cracks and pores have been the main problems restricting the development of this technology. Cracks in the cladding layer will reduce the life and mechanical properties of the cladding layer.
The occurrence of cracks in the cladding layer can be summarized as the following reasons:
(1) The mismatch in thermophysical parameters (such as thermal expansion coefficient and elastic modulus ) between the laser cladding material and the substrate will cause cracks during laser cladding Thermal Stress. Thermal stress cracks are the most common type of cracks during laser cladding.
When the thermal expansion coefficient and elastic modulus of the cladding are greater than those of the base material, the cladding is subject to tensile stress and increases crack susceptibility (see Figure 1(a)). Thermal stress will cause the joint between the cladding and the substrate to be stretched, causing the cladding to crack. When the thermal expansion coefficient and elastic modulus of the cladding are smaller than those of the base material, the thermal stress experienced by the cladding is smaller and the crack sensitivity is reduced (see Figure 1(b)).
Figure 1 Schematic diagram of laser cladding layer cracking: (a) Tensile thermal stress causes cracking of the cladding layer; (b) The cladding layer is subjected to compressive thermal stress.
(2) During the solidification process, the brittle zone of the cladding layer is very sensitive to cracks. When there are hard phases such as martensite , carbides and borides in the cladding layer, the brittleness of the cladding layer is more significant, which may lead to a decrease in the fracture toughness of the cladding layer, thereby increasing the strength of the cladding layer. Crack susceptibility.
(3) During the laser cladding process, complex carbides or low-melting intermetallic compounds produced by segregation may melt, resulting in local grain boundary films or local brittle areas in the partially melted zone.
Laser cladding of low-expansion materials can reduce the thermal expansion difference between the substrate and the cladding material to a certain extent, but this often seriously affects the mechanical properties of the cladding layer. laser cladding of Invar alloy (Fe-36Ni) reduces the crack sensitivity of the cladding layer. Therefore, laser cladding layers with low expansion, high hardness, and no cracks are challenges for laser cladding research.
In order to solve the crack problem of laser cladding, many scholars have done a lot of work.At present, research on magnetic-assisted laser cladding mainly focuses on controlling fluid flow, metal solidification, and heat and mass transfer to improve the microstructure, microhardness, corrosion resistance, wear resistance and other properties of the cladding layer. Another way controls cladding cracking is to introduce a buffer layer between the substrate and the cladding. buffer layer is made of a material that is highly resistant to cracks and has good compatibility with the physical properties of the base material, enabling the formation of thick and crack-free cladding.
Existing research shows how to reduce cracks in laser cladding layers. It is difficult to eliminate all cracks by adjusting the laser cladding process parameters ; pre- or post-processing steps and buffer layers also incur high time and money costs during the cladding process.
The foundation for eliminating cracks is to systematically understand the generation mechanism of cracks during laser cladding. Research on crack control of magnetic-assisted laser coatings mainly focuses on the stirring magnetic effect. Stirring magnetic effect can make the temperature distribution of the molten pool uniform, refine the grains, improve coating segregation, and release stress concentration. Currently, there are few studies on controlling the thermal expansion coefficient difference between the substrate and cladding materials to reduce thermal stress and crack susceptibility. Research on the magnetostrictive effect in reducing thermal stress during laser cladding has not yet been reported.
(a) Laser cladding operation window for bead spacing and higher defocus distance, (b) Effect of bead spacing on the lateral shape of double beads, (c) Optical image showing (b) and (d) two-layer fusion Close-up view of crack defects in the surface topography and transverse cross-sectional shape of the coating.
This study takes Co-Fe-Cr-B-C composite powder as the research object. Combining CALPHAD calculations and EDS technical methods, the generation mechanism of cracks in laser cladding layers was studied. The influence of magnetic field on the morphology evolution, microhardness and crack sensitivity of the cladding layer is discussed.
Experiments and methods
.1. Materials and sample preparation The substrate material used in the
experiments was 42CrMo with a size of 85×12×12mm. Sand the substrate with sandpaper to remove rust and impurities. The average roughness of the substrate is approximately 0.54μm.
.2. Experimental equipment
laser cladding system is shown in Figure 2(a). The laser cladding process was performed in a 2 kW FL020 fiber laser system coupled with a KUKA robotic arm (KUKA KR30, Germany) and a magnetic field generator device . A 0.3 mm high silicon steel core is placed outside the magnetic field generation module to reduce magnetic losses (see Figure 2). The magnetic induction intensity adjustment range is 0-80mT, measured with a magnetic field meter (SJ700, Guilin Senjie Technology Co., Ltd.). The magnetic induction intensity test area is the central part of the workpiece (see Figure 2(b)).
Figure 2 Schematic diagram of the laser cladding experimental system.
During the laser cladding process, argon gas with a purity of 99.99% is used as a protective gas to prevent the mixed powder from being oxidized in the molten pool.
3. Results and discussion
3.1. Characterization of cladding layer defects
The solidification process of laser cladding is non-equilibrium, and defects such as cracks are easily formed in high-hardness cladding layers. Figure 3 shows the microstructure of the S1 laser cladding layer. It can be seen from the micrograph that there are multiple cracks and pores in the cladding layer.
Figure 3 Microstructure and EDS of S1: (a) Microstructure of section S1; (b) Average element content of the material near pore 1; (c) Average element content of the material close to crack 2; (d) Near pore 2 The average element content of the material; (e) The average element content of the material near pore 3; (f) EDS mapping of the defect area. The EDS mapping of the defect area of
is shown in Figure 3(f). The element content of crack 2 is measured along the length of the crack edge through EDS point picking analysis, and a set of data is collected every 0.05 mm. Since crack 1 originates from pore 1, in order to better understand the crack behavior, we use the EDS point picking method to analyze the average element content around the edge of pore 1, and the number of point picking is set to 10. The element content analysis method of pores 2 and 3 is the same as that of pore 1.
The mechanism of cracks in the cladding layer is different. Crack 2 originated at the interface between the cladding and the substrate. Crack 1 and crack 3 originate from the cladding layer, which is different from crack 2. This means that the crack generation mechanism is different.The thermal expansion coefficient of
material close to the selected crack area and the 42CrMo substrate is shown in Figure 4. It is worth noting that the thermal expansion coefficient of the material close to crack 1 is not consistent with the thermal expansion coefficient of crack 2. This is mainly due to the uneven diffusion of elements in the molten pool caused by rapid heating and cooling during laser cladding (see Figure 3(b) and (c)).
Figure 4 The thermal expansion coefficient of the defect area and the substrate.
It is worth noting that the thermal expansion coefficient of the material close to crack 1 is lower than that of the base material, but the crack will still appear. This may be due to the presence of a large amount of C element in the material close to crack 1, and carbides significantly reduce the fracture toughness (see Figure 3(b)). In addition, pores can easily cause stress concentration and cracks.
Crack 3 originates from inside the cladding and extends to the cladding surface . Figure 5 shows the EDS mapping of the crack zone; this clearly shows that there is strong segregation of C and Fe elements along the crack boundary distribution of crack 3. Significant microsegregation of elements may lead to crack susceptibility. C and B elements are unevenly distributed in the cladding. During the laser cladding process, these two elements float on the surface of the molten pool and play the role of solid solution hardening and slagging (see Figure 5(b) and (c)).
Figure 5 EDS mapping of the crack area: (a) Microstructure of the crack area; (b) Distribution of b element; (c) Distribution of c element; (d) Distribution of Cr; (e) Distribution of iron element; (f ) Distribution of Co element.
Figure 3 also shows that there are several holes at the bottom of the cladding. This is mainly due to the short existence time of the molten pool during laser cladding. Especially at the bottom of the molten pool, the solidification speed is very fast, and the pores formed in the molten pool do not have enough time to float, so the pore sensitivity is very high.
3.2. Evolution of morphology and microstructure
Figure 6(a)-(f) represents the cross-sectional microstructure of S1. Figure 7 represents the width/height evolution of the cladding geometry . The main structure of the cladding layer is dendritic. It should be noted that the microstructure of S1 shows typical columnar dendrites at the bottom of the cladding layer. The grains are coarse and penetrate into the middle of the cladding layer.
Furthermore, the size of the microstructure gradually decreases from bottom to top. The magnetic field has a significant impact on the morphology of the cladding layer. The average heights of
S1, S2, S3 and S4 are approximately 0.70 mm, 0.59 mm, 0.61 mm and 0.68 mm respectively. The average widths of cladding S1, S2, S3, and S4 are approximately 3.47 mm, 3.55 mm, 3.72 mm, and 4.22 mm respectively. Figure 7 shows the effect of magnetic induction on the width/height evolution of the cladding geometry .
As the magnetic field intensity of increases, the cladding W/H gradually increases. When the magnetic field intensity is 0 mT, the average W/H value of the cladding is approximately 4.96. When the magnetic induction intensity is 20 mT, 40 mT and 60 mT, the average width/height of the cladding is approximately 6.02, 6.10 and 6.21 respectively. W.M.Steen believes that the aspect ratio (width/height) should be greater than 5 to avoid cracks and pores in the laser cladding layer. As the magnetic induction intensity increases, the mass transfer and heat transfer in the molten pool are enhanced, promoting the uniform cooling and solidification of the cladding layer. The magnetic field contributes to uniform molten pool morphology. Therefore, the increase in magnetic field strength is beneficial to reducing cracks and pores in the cladding layer.
Figure 6 Microstructure of the S1 laser cladding layer.
Figure 7 Cladding geometry.
Figure 8 shows that the magnetic induction intensity greatly affects the microstructural evolution of the cladding layer. The structure of at the top of the S2, S3 and S4 cladding layers is equiaxed dendrites, and the middle and bottom regions are columnar dendrites. As the magnetic induction intensity increases, the size of the dendrites on the top of the cladding layer significantly decreases, and the distribution is diffuse. When the magnetic induction intensity increases to 60 mT, the microstructure size of the cladding layer further decreases. The fine structure is beneficial to improving the mechanical properties of the cladding layer. The increase in the magnetic field leads to an increase in the intensity of liquid convection near the surface of the molten pool, which strengthens the scouring effect of liquid metal on crystallization, leading to an increase in mechanical damage to columnar dendrites, thereby causing the rise of equiaxed crystal nuclei .
Figure 8 Microstructure of laser cladding layers of S2, S3 and S4.
The G/R ratio determines the morphology of the coagulated tissue. The cooling rate ratio G×R determines the microstructure size of the cladding layer.At higher G×R values, finer microstructures can be obtained. Figure 8 shows that as the magnetic field intensity increases from 0 mT to 60 mT, the microstructure size of the cladding layer becomes finer. The reduced microstructure size helps optimize the microstructure and mechanical properties of the cladding layer. Figure 8 shows that the microstructure changes from columnar dendrites to equiaxed grains from the bottom to the top of the cladding layer. According to solidification theory, this is the result of reduced G/R. The equiaxed microstructure helps to improve the mechanical properties of the cladding layer due to the finer microstructure size. When
is laser cladded in a magnetic field, the microstructure is refined and evenly distributed, eliminating defects such as cracks and pores. When the magnetic field values are 40 mT and 60 mT, there are no obvious cracks in the laser cladding layer. It should be pointed out that cracks still exist in the S2 cladding layer. EDS mapping of the S2 crack zone shows that Cr and C elements are enriched along the crack boundary (see Figure 9). Cr and C easily form brittle phases, and the segregation zone is sensitive to cracks.
Figure 9 EDS mapping of the S2 crack zone.
When laser cladding is carried out in a magnetic field, due to the stirring effect, Lorentz force will be generated in the molten pool, thus intensifying the movement of the melt. The pores in the molten pool float upward to the surface of the cladding layer, and finally escape from the molten pool.
3.3. Effect of magnetic field on cracks
3.3.1. XRD results and element diffusion
The phase type of the cladding layer was analyzed by X-ray diffraction. Figure 10 shows the XRD results of Co-Fe-Cr-B-C coatings with and without magnetic field. The cladding is mainly composed of CoFe, Fe, Co3Fe7 and Fe-Cr. Changes in magnetic induction do not affect the phase type. Figures 10 and 11 show that the magnetic induction intensity can effectively accelerate the diffusion of elements in the molten pool. However, the phase transition in the cladding is not significant. This may be because the magnetic field strength is too low, resulting in limited changes in the thermal conditions of the molten pool and therefore insignificant changes in phase types.
Figure 10 XRD pattern of laser cladding layer.
Figure 11 Energy spectrum analysis of the cross-section of the laser cladding layer with different magnetic field strengths: (a) Energy spectrum analysis of the S1 cross-section; (b) EDS of the S2 cross-section; (c) EDS of the S3 cross-section; (d) S4 Cross-section EDS.
Figure 11 shows the influence of magnetic field on the element distribution of laser cladding layer.
Figure 11(a) shows that 1.5 mm from the surface of the cladding layer is the mutation point of the element content, which is the fusion line between the cladding layer and the substrate. Compared with the substrate, the Fe, Co and Cr elements in the cladding layer fluctuate significantly, indicating that the element distribution in the cladding layer is not completely uniform, which reveals local microsegregation in the cladding layer. During the laser cladding process, when a magnetic field is applied to the molten pool, the convection of the melt intensifies, making the diffusion of iron, cobalt, and chromium atoms more uniform. The magnetic field significantly reduces the segregation of elements and then minimizes the susceptibility to cracking caused by elemental segregation. As the magnetic induction intensity increases, the diffusion of elements becomes more evenly distributed in the cladding layer. When the magnetic induction intensity increases to 40 mT and 60 mT, the silicon element is diluted from the substrate to the cladding. This phenomenon may be due to the increase in magnetic induction intensity, which strengthens the magnetic stirring effect of the magnetic field on the molten pool. Enhanced flow and convection contribute to the even distribution of elements in the cladding layer.
3.3.2. Delta-E effect
XRD results show that all samples of the coating contain CoFe phase. As a magnetostrictive material, the CoFe phase can trigger the magnetostrictive effect and generate magnetostrictive strain in the contact area between the substrate and the cladding. The magnetostrictive effect counteracts the thermal expansion of the cladding material during magnetic induction laser cladding.
Figure 12 shows the elastic modulus of all samples. The magnetic field has a significant impact on the elastic modulus of the cladding. The elastic moduli of S1, S2, S3 and S4 are 249.6 GPa, 171.1 GPa, 190.2 GPa and 208.7 GPa respectively. The sample without magnetization has the highest elastic modulus. The elastic modulus of all magnetized samples increases with increasing magnetic field strength.
Figure 12 Elastic modulus of cladding.
Figure 13 shows that no magnetically induced laser cladding layer has a higher thermal expansion than the magnetically induced layer. Due to the higher ΔE value, S2 has a lower thermal expansion coefficient in all samples.In other words, in this study, the stronger the magnetostrictive effect, the greater its ability to offset the normal thermal expansion of the cladding material itself. The decrease in the thermal expansion coefficient of S2, S3 and S4 is due to the magnetostrictive effect, which counteracts the thermal expansion of the cladding material.
Figure 13 Thermal expansion coefficient of cladding.
3.3.3. Thermal stress
According to the results obtained in Figure 13, combined with the geometry of the cladding layer, we can conclude the influence of the magnetostrictive effect on thermal stress.
Figure 14 shows the thermal stress of the cladding. Compared with the thermal expansion coefficient variation pattern, the variation curves of all samples show the same trend. In other words, the thermal expansion of the cladding has the most significant effect on thermal stress.
Figure 14 Thermal stress of laser cladding layer.
The slopes of the thermal stress curves of S2, S3 and S4 are significantly smaller than that of S1. This is due to the magnetostrictive effect of the cladding material in the magnetic field, which reduces the thermal expansion coefficient and elastic modulus, thereby reducing thermal stress.
It is worth noting that when the magnetic induction intensity increases from 20 mT to 60 mT, the magnetostrictive effect decreases, resulting in an increase in the thermal expansion coefficient and elastic modulus. The reduction of the magnetostrictive effect of will increase the thermal stress, so the thermal stress value of S2 is lower than that of S3 and S4. In other words, during the laser cladding process, the elastic modulus and thermal expansion coefficient of the cladding layer can be effectively controlled with the help of the auxiliary magnetic field and the magnetostrictive effect of the cladding material. The thermal expansion coefficient is the main factor controlling the thermal stress of the cladding layer. Reducing the thermal expansion coefficient and elastic modulus of Co-Fe-Cr-B-C alloy can effectively reduce the thermal stress caused by the mismatch in thermal expansion and elastic modulus between the substrate and the cladding, thereby reducing crack susceptibility.
3.4. Microhardness
Figure 15 shows the microhardness of the laser cladding layer. The microhardness values of all samples in the cladding layer are higher than those in the heat-affected zone and matrix. The average microhardness of S1, S2, S3, S4 and the substrate are 861 HV, 940 HV, 980 HV, 1056 HV and 302 HV respectively. Figure 15 shows that the magnetic field has a positive effect on the microhardness of the cladding layer. Among all samples, S4 has the highest microhardness value. As the magnetic field intensity increases, the microhardness of the cladding layer increases. The increase in microhardness is closely related to the size of grains in the cladding layer.
Figure 15 Microhardness of laser cladding layer.
In addition, as the magnetic induction intensity increases, the reduction of pores and cracks helps to increase the microhardness of the cladding layer.
4. Conclusion
Co-Fe-Cr-B-C alloy was prepared on 42CrMo substrate by magnetic field laser cladding method. The influence of magnetic field on cladding defects is multifaceted. A smaller magnetic field can produce a larger magnetostrictive effect. Among the samples involved in this experiment, the magnetostrictive effect produced during the 20 mT magnetic induction intensity-assisted laser cladding process is the largest, thereby reducing the thermal stress and solving the thermal expansion and elastic modulus differences between the cladding material and the substrate. causing crack problems. However, the effect of a small magnetic field on the homogenization of elements is not as strong as that of a large magnetic field. At the same time, magnetic field-assisted laser cladding also has a great influence on the geometry of the cladding layer. This study found that when the magnetic field intensity is 60 mT, the aspect ratio of the cladding layer is the largest and the microhardness is the highest. The influence of magnetic field on the crack sensitivity of Co-Fe-Cr-B-C alloy cladding layer is significant, and the following conclusions can be drawn:
• In the magnetic-assisted laser cladding process, magnetic induction intensity is the key factor affecting magnetostrictive efficiency .
•As the magnetic field intensity increases, from 20 mT to 60 mT, the magnetostrictive effect becomes weaker. The high magnetostrictive effect can improve the mechanical properties of the cladding layer by reducing the segregation of brittle phases and pores.
•The magnetostrictive effect can effectively reduce the thermal expansion coefficient and elastic modulus of the cladding, thereby reducing thermal stress and crack susceptibility.
•Magnetic-assisted laser cladding can improve the microhardness of the cladding layer through fine-grained structure without affecting the phase type of the cladding layer.
Source: Effect of magnetic field on crack control of Co-based alloy laser cladding, Optics & Laser Technology, doi.org/10.1016/j.optlastec.2021.107129
Reference: Research and development status of laser cladding on magnesium alloys: A review, Opt. Lasers Eng., 93 (2017), pp. 195-210, 10.1016/j.optlaseng.2017.02.007
