Generally, single material is difficult to meet the increasingly stringent industrial requirements, but multi-material components with gradient properties have broad application prospects. 316L stainless steel with excellent corrosion resistance and mechanical properties is widely used in the nuclear industry, but its high temperature stability is poor. In contrast, nickel base alloy, Inconel 718, has good thermal stability, creep strength and radiation resistance at high temperature. Because the density, specific heat capacity, thermal conductivity and other thermal physical parameters of the materials mentioned above are similar, it is easy to form a multi-material component with good interfacial bonding. Therefore, Inconel 718 can be combined with 316L to form multi-material 316L/Inconel 718, which is resistant to high temperature and large temperature difference. It can meet the strict environmental requirements of nuclear reactor and combustion wall in engine. Compared with the traditional multi-material component forming method, the laser directed energy deposition technology has obvious advantages, which can flexibly control the powder distribution and has high densification degree.
In this paper, a numerical model of laser directed energy deposition (LDED) forming of heterogeneous materials is proposed based on the finite element method, and the temperature field in the LDED forming of 316L/Inconel 718 multi-material process is established. The influences of laser power and scanning speed on the interface thermal behavior, interface defect evolution, and interface bonding performance are studied. The formation mechanism of interface defects driven by thermal action is revealed. Meanwhile, the 316L/Inconel 718 samples are formed by LDED, and the interface bonding properties of heterogeneous materials under different process parameters are characterized to verify the accuracy of the model.
When 316L material is deposited, the longitudinal section of the molten pool is narrow at the front and wide at the back (Fig. 5). The cometary tail at the end of the molten pool indicates that there is a large temperature gradient at the front of the molten pool, which could be attributed to the fact that the laser scanning speed is higher than the solidification rate of the molten pool. The heat accumulation effect and the change of thermal conductivity caused by the melting and solidification behavior of the molten pool make the laser spot center in front of the maximum temperature value. In addition, when the scanning speed of the Inconel 718 layer increases from 7 mm/s to 20 mm/s (laser power is 1100 W), the maximum temperature gradient of the molten pool increases from 6.02×105 ℃/m to 1.19×106 ℃/m [Fig. 7(a)]. The lifetime of liquid phase decreases from 0.52 s to 0.125 s [Fig. 8(b)], and the remelting depth decreases from 0.45 mm to 0.22 mm (Fig. 10). When the laser power of Inconel 718 layer increases from 900 W to 1500 W (scanning speed is 10 mm/s), the maximum temperature gradient decreases from 8.15×105 ℃/m to 6.93×105 ℃/m [Fig. 7(b)], and the lifetime of liquid phase increases from 0.3 s to 0.4 s [Fig. 8(a)]. The remelting depth increases from 0.28 mm to 0.48 mm (Fig. 9).
With the increase of vIN718, the temperature of 316L/Inconel 718 in the positive direction of Z axis decreases gradually, which is accompanied by the decrease in the lifetime of the 316L remelting liquid phase and the molten pool size. The maximum temperature gradient is located on the 316L substrate surface and decreases gradually along the positive direction of the Z axis. With the increase of PIN718, the positive temperature of 316L/Inconel 718 in Z axis gradually increases together with the lifetime of the 316L remelting liquid phase and the size of molten pool. The maximum temperature gradient is located on the 316L substrate surface, and decreases first and then increased gradually along the Z axis. Combined with the microstructural analysis of the LDED 316L/Inconel 718 sample, it is found that the experimental results are in good agreement with the three-dimensional finite element temperature field simulation results, indicating that the model can effectively analyze the thermal behavior of the LDED multi-material forming process.
