Laser-directed energy deposition (LDED) is an effective technique for processing Inconel 718. However, because of the overlap of cladding tracks and the stacking effect of the deposition layers with unmelted powders, the processing accuracy and surface quality of the as-deposited parts are poor; thus, subsequent substrate processing must be performed before use. Electrochemical machining (ECM) can effectively improve machining accuracy and surface quality and has a wide range of applications in the precision manufacturing of difficult-to-process metals (such as nickel-based superalloys). Therefore, ECM is used for the subsequent substrate processing of LDED-Inconel 718 components. However, the processing quality of LDED components with inhomogeneous microstructures is unclear, particularly when nonwater-based electrolytes are used. Therefore, the microstructural characteristics and dissolution behavior of the constituent phases of the LDED-Inconel 718 alloy under a pulsed current and ethylene glycol electrolyte are systematically investigated to improve the surface quality of the LDED-Inconel 718 alloy.
In this study, the LDED-Inconel 718 alloy is used as the research object. The parameters of the high-frequency narrow-pulse current with a frequency of 30?100 kHz, duty cycle of 30%?80%, and saturated NaCl glycol electrolyte are employed to perform electrolyte jet machining (EJM) experiments. The dendritic morphologies and constituent phases of the as-deposited Inconel 718 alloy and the micro-morphologies of the machined surface after the EJM experiments are characterized using scanning electron microscope (SEM). Confocal laser microscope is performed to measure the central region of the groove along the X-axis. The surface-machining quality of the groove is characterized based on the surface roughness, and the machining precision is evaluated based on the depth-to-width ratio of the groove profile.
The results show that the microstructure of the as-deposited Inconel 718 alloy is composed of the γ matrix phase, Nb-segregated γ phase, and inter-dendritic phase (mainly the γ/Laves eutectic phase), as shown in Fig.2. At a current density of 10.50 A/cm2, the surface roughness increases with increasing pulse frequency, and the smallest surface roughness (Ra=1.562 μm) and highest machining accuracy (Fig. 5) are obtained when the pulse frequency is 30 kHz. The surface roughness first decreases and then increases with an increase in the duty cycle, whereas the machining precision is optimum when the duty cycle is 60% (Fig.7). In the direct-current mode, the surface roughness decreases with increasing current density. When the current density reaches 10.50 A/cm2, the surface quality is the best (Ra=0.526 μm). This is because a high current density can easily induce the formation of surface-supersaturated salt films and effectively inhibit selective dissolution and reduce the surface roughness. However, in terms of the machining accuracy, the processing localization in the high-frequency narrow pulse mode is better (Fig.9).
Two stages (transpassivation dissolution and salt film leveling stages) exist during the direct-current mode EJM process. During the first stage, the composition and structure of the transpassivation films on the surface of the Inconel 718 alloy result in a dissolution rate difference. During the second stage, the salt films on the Inconel 718 surface eliminate the difference in the dissolution rate of each phase, and a smooth surface is formed. In the pulse current model, the dissolution process is divided into transpassive dissolution, salt film leveling, and salt film vanishing stages. However, the surface quality of the alloy is poor because of the periodic on-off property of the pulse current. The current density around the edge of the groove is very low owing to the pulse current characteristics; thus, the transpassive film is difficult to break, which improves the machining accuracy.
