The microstructure evolution and tensile properties of Inconel 718 fabricated by high-deposition-rate laser directed energy deposition

In order to meet the requirements for rapid manufacturing of large-scale high-performance metal components, the unique advantages of high-deposition-rate laser directed energy deposition (HDR-LDED, deposition rate >= 1 kg/h) technology have been attracted great attention. HDR-LDED technology significantly improves the efficiency by simultaneously increasing the mass and energy input on basis of conventional laser directed energy deposition (C-LDED, deposition rate <= 0.3 kg/h), which dramatically changes the solidification condition and thermal cycling effect compared to C-LDED processes. Based on this, Inconel 718 bulk samples were fabricated with a deposition rate of 2.2 kg/h and a height of 75 mm. Through experimental observation combined with finite element simulation, the precipitation morphology, thermal cycling effect and tensile properties at room temperature of the block samples at heights of 6 mm (bottom region), 37 mm (middle region) and 69 mm (top region) from the substrate were investigated. The results show that both temperature interval and incubation time satisfy the precipitation conditions of the second phases because of the intense thermal cycling effect so that delta, gamma '' and gamma' phase are precipitated in the bottom and middle region of the as-deposited sample during the HDR-LDED process. As a result, the micro-hardness and the yield strength of the bottom region (385 HV; 745.1 +/- 5.2 MPa) are similar to those of the middle region (381 HV; 752.2 +/- 12.1 MPa), respectively. And they are both higher than those of the top region (298 HV; 464.7 +/- 44.2 MPa). The tensile fracture mechanism is shown in both fracture and debonding of the Laves phase. The inhomogeneous microstructures and corresponding mechanical property differences of Inconel 718 fabricated by HDR-LDED along the deposition direction suggest the necessity to conduct further research of the post heat treatment in the future.