Causes and effects of laser cladding coating toughness reduction
1. Reasons for the decrease of toughness of cladding layer
(1). Stress
Due to the great difference in thermal expansion coefficient between cladding material and matrix, it is easy to produce internal stress during cladding. The cladding process is a process of rapid melting and rapid solidification. In this process, there will be a temperature gradient difference, and various stresses will be generated, such as thermal tensile stress caused by uneven contraction of the cladding layer due to the different temperature gradient and thermal expansion coefficient of the material, and organizational stress caused by the phase transition of the metal in the cooling process. And the constraint stress caused by extruding the uncontracted part when the cladding layer shrinks due to the temperature gradient difference. The internal stress includes thermal stress, structural stress and constraint stress, and thermal stress is the main stress. In the process of laser cladding, the fracture toughness is much less than the generated thermal tensile force, which is the main cause of crack production. Cracks form when the thermal stress exceeds the strength limit of the material.
The results show that the non-uniform distribution of high-volume chromium borides and carbides in the laser cladding nickel-based silicon carbide coating, resulting in excessive thermal stress, is the main reason for the crack. In the process of laser cladding, brittle compounds are easy to form at the grain boundary, and the thermal expansion coefficient of these brittle compounds and the cladding layer is very different, which leads to the stress concentration at the grain boundary, and the cracking is easy to result from the reduction of toughness.
(2). Defects in the cladding process
Small defects such as pores and micro-cracks will also occur during the cladding process. The existence of pores may be caused by the dissociation and gasification of a substance in the material caused by high-energy laser. Another possibility is that inert gas is needed as a protective gas in the laser cladding process, and the protective gas will be involved in the molten pool. The gas is left in the condensate pool too late to be discharged to form pores. In the process of rapid condensation, the existence of pores will cause micro-cracks. If the components such as deoxygenation and slag-making in the cladding powder cannot float up in time, they will be preserved in the cladding coating. These inclusions will also increase the possibility of cracks in the coating and reduce the structural strength and toughness of the cladding layer. At the same time, the existence of pores is also the help of crack initiation and expansion. Pores make the cladding layer loose, and it is easy to produce stress concentration around the pores, which increases the sensitivity of cracks in the cladding layer.
2. Influencing factors of toughness of cladding layer
The fracture toughness of laser cladding layer is affected by many factors, including the choice of cladding material, laser power, powder feeding rate and the temperature of cladding matrix. The main cause of coating cracking is the internal stress caused by the difference of physical properties between cladding layer and substrate. When the cladding powder material and the substrate material with similar physical properties are selected, the melting and solidification of the two materials are almost synchronized, which can effectively reduce the cracking possibility of the cladding layer and improve the toughness of the coating. The process parameters of laser cladding also have a direct influence on the generation of cracks. With the increase of laser power, the cracks of the coating first increase and then decrease, and the laser scanning rate and powder feeding rate also have similar effects. Preheat treatment of the substrate before cladding can significantly reduce the temperature gradient between the molten pool and the substrate and improve the stress distribution of the cladding layer.
Methods for enhancing the toughness of coatings
1. Optimization of cladding powder composition
Laser cladding is to melt the powder and matrix to form a metallurgical coating, the coating performance and powder selection and treatment is very important. The toughness of the Fe-based amorphous alloy coating is relatively poor, and it is very easy to produce cracks in the preparation process or in the actual use because of the stress set in the internal shear zone. The rims continue to expand until brittle fracture occurs, but the special disordered atomic arrangement structure and no crystal boundary make the iron-based amorphous crystals have excellent wear resistance and corrosion resistance. The inner shear zone of amorphous alloy is easy to cause stress concentration, where micro-cracks are easily formed, and the expansion of micro-cracks can produce fracture cracks. The coating with high hardness and toughness was obtained by laser cladding test on the surface of steel plate after mixing nickel-coated carbon nanotubes with iron-based amorphous powder by ball milling. It was found that the amorphous nanocrystalline composite structure with good toughness was formed in the coating after nickel-coated carbon nanotubes were added. Meanwhile, nickel plating and ball milling can effectively avoid the formation of brittle carbides in the laser cladding process, so as to ensure that the coating has good fracture toughness.
The addition of an appropriate amount of rare earth elements can improve the absorption rate of the powder to the high-energy laser beam, make the cladding powder melt evenly and fully, better form a metallurgical bond with the matrix, reduce the probability of porosity, and can be used for homogenizing, solid solution strong and strong toughening of the cemented carbide coating.
2. Additional transition layer
Using powder with good toughness and good matching with the physical properties of the matrix as the bottom layer, that is, setting such a transition layer or gradient layer in the middle of the matrix and the cladding layer can reduce the internal stress between the cladding layer with high hardness and the matrix, and reduce the cracks caused by excessive stress. The intermediate transition zone can alleviate the residual stress caused by the different thermal expansion coefficient between the substrate and the cladding layer, thereby enhancing the toughness of the coating and preventing the cracking of the coating. A layer of Ni20Cr coating is deposited on the surface of die steel as the base layer, and then two layers of Ni60A coating are continuously deposited on the Ni20Cr coating. The results show that adopting Ni20Cr coating as the base layer can effectively improve the metallurgical bonding between the coating and the substrate. Greatly reduce cracks, pores and other defects in the coating.
3. Optimization of process parameters
In the process of laser cladding, the laser beam power P, scanning rate V, spot diameter D, etc. have important effects on the quality of the cladding layer. The dilute release rate is the embodiment of the coating mass, and the dilute release rate is affected by the specific energy E.
The results show that too large or too small specific energy E is not conducive to obtaining the coating with excellent performance. If E is too low, the dilution rate of the coating will be correspondingly low, the matrix and the cladding layer can not obtain good metallurgical bonding, and the surface of the cladding layer is also prone to porosity cracks and other defects. If the specific energy E is too high, the dilution rate will increase accordingly, and the metals in the molten pool will be fully mixed, and the excellent properties of the cladding powder cannot be exerted. It is found that the laser scanning speed is too fast, resulting in uneven formation of the cladding layer, the scanning speed is too slow, will form a larger crystal structure, the appropriate scanning speed can obtain a relatively dense cladding layer and better toughness.
3. Heat treatment
(1)Preheat and post heat treatment
The heat treatment of laser cladding coating is basically to order the phase and homogenize the elements in the coating. Proper preheating of the substrate can effectively reduce the temperature gradient in the coating, reduce the thermal stress and improve the toughness of the coating. The results show that proper preheating of the matrix can significantly reduce the cooling rate of the cladding layer, reduce the residual stress and inhibit the crack generation. As shown in the figure below, cracks can be observed when the matrix is not preheated (arrow indicates), but no obvious cracks can be seen when the matrix is preheated at 200 ℃.
In order to improve the toughness of the cladding layer and reduce the cracking tendency of the coating, the preheated matrix method is relatively simple, and sometimes can not achieve the expected effect. It is found that preheating and heat preservation can significantly reduce the cooling rate of the molten pool, thereby improving the temperature distribution of the molten pool, reducing the intercrystalline eutectic hard phase, increasing the ductile phase γ in the melting process, which makes the toughness of the cladding layer increased and the residual thermal stress decreased, and the cracks decreased or even disappeared. Heat treatment of powder can improve the bonding ability of powder and reinforced phase, and also improve the hardness and toughness of cladding coating. Heat treatment of the obtained coating can homogenize the coating, significantly reduce the defect density of the coating, release the residual stress of the coating, and improve the strength and toughness of the coating. The post-heat treatment can improve the tensile strength and cracking resistance of the coating and the elongation ratio, yield strength and ultimate tensile strength of the cladding layer.
Preheating and post-heat treatment can significantly reduce the temperature gradient of the molten pool, increase toughness and inhibit the generation of cracks, but the preheating and post-heat treatment process will produce too high temperature, which is easy to affect the internal stress distribution of thin-walled parts, and may lead to workpiece deformation. Therefore, preheating and post-heat treatment are limited in strengthening or repairing precision thin-walled parts.
(2)Laser remelting
One or more laser remelting immediately after laser cladding can eliminate surface defects of the coating and improve the toughness of the coating. Ni60/50%WC composite coating without crack was obtained by laser cladding + remelting treatment on 45 steel surface. It is found that laser remelting has the functions of secondary slagging exhaust, healing cracks, improving surface roughness, and improving the non-uniformity and compactness of coating structure. The Fe-based amorphous coating is prepared by three laser scanning methods, that is, the first laser scanning heats the matrix, the second laser cladding forms the amorphous coating, and then the third laser remelting of the amorphous coating gets the non-crack coating. Laser remelting basically eliminates the coating defects, and greatly improves the elastic modulus and toughness of the coating.
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