Laser cladding is a technology that uses high energy laser beam as heat source to melt and solidify the filler material coated on the surface of the substrate, forming a metallurgical bond between the two, and then improving its surface properties. Compared with other surface strengthening technologies, laser cladding has a series of advantages such as fast cooling rate, easy metallurgical bonding between coating and substrate, small heat-affected zone, low dilution rate, small deformation of substrate, easy automation, and no pollution. Therefore, the technology has broad application prospects in aerospace, mining machinery, petrochemical, automobile, ship, electric power, railway and other industries.
However, laser cladding is a fast heating and cooling process. The temperature gradient of the substrate and cladding layer, the uneven distribution of hard phase in the cladding layer and the difference in physical properties between the cladding layer and the matrix material will have a certain impact on the dimensional stability and mechanical properties of the cladding layer, which will lead to the initiation and propagation of cracks. The formation of cracks in cladding layer has a great influence on the service life of parts, which is an urgent problem to be solved in the industrial application of laser cladding technology.
Laser cladding is a process of rapid heating and cooling and complex metallurgical reaction. At present, the research on the crack of cladding layer mainly focuses on a single control method, and there is a lack of systematic research. In this study, Ni60 alloy cladding layer was prepared on 42CrMo steel surface by pre-laid powder laser cladding technology. Firstly, the crack formation mechanism and crack sensitivity were analyzed, and then the influence of different laser power and preheating temperature on crack was studied, in order to provide reference for the crack control of laser cladding Ni-based alloy.
Test materials and methods
1. Test material
In this test, 42CrMo alloy steel is selected as the matrix material in the laser cladding test, and the round plate size is Φ150 mm×10 mm. Sand the 42CrMo steel surface with sandpaper before laser cladding, and clean it with alcohol and acetone to ensure that there are no other impurities on the substrate. The cladding powder was selected with Ni60 alloy, and the particle size was 53~ 150 μm. The chemical composition of Ni60 alloy was shown in Table 1.
Table 1 Chemical composition of Ni60 alloy %
|
m(C) |
m(Si) |
m(Cr) |
m(Ni) |
m(Mo) |
m(Fe) |
m(B) |
|
=0.70 |
=4.50 |
= 17.0 |
=60.0 |
= 3.0 |
=5.0 |
=2.70 |
2. Test methods
The LWS-1000 Nd: YAG laser was selected for laser cladding by prelaying powder and multi-lap process. Sample preparation parameters are as follows: laser power 270 ~ 300 W, scanning speed 300 mm/min, preheating temperature 170 ~ 270 ℃, lap rate 50%. After laser cladding test, Zeiss Stemi305 stereoscope was used to observe the surface morphology of cladding layer. The prepared cladding layer is cut into a sample size of 5 mm×10 mm×10 mm, and then HCl+HNO3 solution with a volume ratio of 3 ∶ 1 is used to corrode the polished cladding layer cross-section. Jiangnan MR5000 metallographic microscope and Regulus8230 scanning electron microscope were used to observe the microstructure of Ni60 cladding layer, and EDS was used to analyze the distribution of elements near and without cracks in the cladding layer qualitatively and quantitatively. VTD401 digital microVickers hardness tester was used to measure the microhardness of the cross-section of cladding layer. The loading load was 50 g and the holding time was 10 s. The phase was analyzed by D/MAX2500VL/PC rotating target X-ray diffractometer.
Conclusion
1. The microstructure of the cladding layer is mainly composed of γ - (Fe, Ni), Fe0.64Ni0.36 and M23C6. The cracks in this test are basically through-penetration cracks, which generally originate from the surface of the cladding layer and extend to the junction of the cladding layer and the matrix, and most of the cracks directly extend through the entire cladding layer. The difference of thermal properties between the matrix and the cladding layer, the temperature gradient and the segregation of the hard phase in the cladding layer have some effects on the crack sensitivity.
2. With the increase of laser power, the crack failure in the cladding layer is obviously improved. When the power is 290 W, there are only a few cracks in the cladding layer and good mechanical properties are maintained. When the power is further increased, the dilution rate of the cladding layer is too large, resulting in the reduction of its performance.
3. With the increase of preheating temperature, the crack failure in the cladding layer decreases gradually. When the preheating temperature is 270 ℃, only a small number of cracks remain in the cladding layer, but too high preheating temperature will destroy the performance of the substrate and the cladding layer, so no higher temperature preheating is carried out.
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