Objective Currently, traditional engineering materials are facing the contradiction between strength and toughness. Several organisms in nature have comprehensive high strength and toughness properties owing to the long-term evolution of heterogeneous characteristics; thus, they have become a model for people to imitate. However, achieving heterogeneous forming using the traditional manufacturing process is difficult; the selective laser melting (SLM) with high degree of freedom and precision provides a new opportunity for forming parts with this feature. Currently, the heterogeneous forming of SLM is mainly achieved through multimaterial and different-layer forming. However, the studies regarding the same-layer forming are inadequate, and the same-layer formed parts’ performance is poor compared with different-layer formed parts. The reason is that the laser penetration is reduced in the same-layer forming, and the overlapping efficiency of the melt pool at the boundary is poor, leading to poor bonding quality between heterogeneous materials, making it difficult to achieve metallurgical bonding. Therefore, this study improved the interface bonding quality of SLM316 L/IN718 heterogeneous parts with the same layer based on optimizing the laser remelting process. In addition, the mechanical properties of the remelted parts were significantly improved compared with those without remelting, making further innovation and expansion to form SLM bionic structure materials.
Methods The metal materials used in this study are atomized 316L and IN718 powders. After laser remelting, the top surface morphology and roughness of 316L were observed and measured using a laser confocal microscope. For heterogeneous forming of different layers 316L/IN718, the sequence of 316L-IN718-316L was used, and the upper surface of each material was remelted after forming. For heterogeneous forming of the same-layer 316L/IN718, the intermediate IN718 was formed after forming both sides of 316L, and laser remelting was performed at the interface joint. Scanning electron microscope (SEM) and energy dispersive spectrometer (EDS) were used to observe the microstructure and element distribution of the interface. The tensile test was performed at room temperature using DDL100 electronic universal testing machine at a strain rate of 3 mm/min, and the tensile strength and elongation were measured. Finally, SEM was used to observe and analyze the fracture morphology of tensile specimens.
Results and Discussions The effect of different laser remelting parameters (laser power, scanning speed, and remelting times) on the surface roughness of 316L is different (Fig. 5). When the laser power is 300 W, scanning speed is 250 mm/s, and remelting times is 5, the surface roughness exhibits the lowest values of 2.4, 2.4, and 2.7 μm. After laser remelting, the surface quality of 316L parts is effectively improved, indicating that this optimization process is expected to improve the interface bonding quality of heterogeneous parts, and the interface transition zone of 316L/IN718 heterogeneous layer becomes smoother with a width of 450 μm (Fig. 7). However, the interface bonding quality of the 316L/IN718 heterogeneous layer is poor without remelting and the pore diameter is approximately 0.18--0.35 mm. Moreover, after remelting, the spheroidization and pores in the transition zone almost disappear, and the interface bonding quality is improved (Fig. 8). EDS analysis results show a gradient trend in the element diffusion of same-layer 316L/IN718 heterogeneous parts after laser remelting. Consequently, the distribution of Fe, Ni, and other elements is more uniform (Figs. 9 and 10). The tensile test results in Fig. 11 show that the tensile strength of remelted sample increases from (104.77±45.26) to (507.33±58.3)MPa and the elongation is approximately 11%. This proves that the laser remelting optimization process improves the performance of SLM 316L/IN718 heterogeneous components. The fracture morphologies of same-layer 316L/IN718 tensile parts before and after remelting were observed (Fig. 12). The crack was found to deflect at the poor joint, leading to rapid fracture failure of the sample without remelting; however, no obvious transition was observed at the fracture after remelting optimization, indicating that 316L and IN718 exhibit good metallurgical bonding. Both samples exhibit brittle fracture in macroscopic view. However, numerous dimples and tearing edges are observed at the fracture of remelted specimens, indicating that the fracture mode is brittle and ductile fractures.
Conclusions After remelting, the surface roughness of 316L decreases from 7.1 to 2.7 μm by 62%; the flat micromorphology and good element diffusion of the 316L/IN718 transition zone exhibit the effect of remelting in improving the bonding quality of the 316L/IN718 heterogeneous interface. The increase in tensile strength of the sample from (104.77±45.26 )MPa before optimization to (507.33±58.3) MPa after optimization also verifies the feasibility of the optimized process. Both the unremelted and remelted same-layer 316L/IN718 heterogeneous tensile parts exhibit brittle fracture characteristics in the macroscopic view. Simultaneously, the interface joints of the remelted samples exhibit obvious dimple fracture characteristics and no secondary cracks and unmelted powders were observed, indicating that laser remelting can considerably improve the quality of interface bonding.
