Coupling mechanisms of multiple defects and corrosion behaviors in SS 316L/IN718 graded materials under different laser powers

Abstract

This study used laser directed energy deposition (LDED) technology to fabricate stainless steel 316L (SS 316L)/Inconel 718 (IN718) functionally graded material (FGM) under different laser powers. Through a combination of experimental characterization and numerical simulation, the study systematically investigated the effects of laser power on deposition layer defects (pores, cracks, poor surface formability, and element segregation) and their intrinsic correlation mechanisms. It also investigates the effects of laser power on hardness and corrosion resistance. The results showed that pores in the molten pool originate from disturbances caused by high-speed powder injection and bubble generation induced by defects in the previous layer. The efficiency of porosity elimination is closely related to the melt viscosity, flow velocity, and molten pool residence time controlled by heat input. With increasing laser power, the melt viscosity decreases and the melt pool retention time increases, which facilitates bubble escape and reduces lack-of-fusion porosity. However, excessive power can result in excessively deep and wide melt pools, increasing the susceptibility to hot cracking. Under medium to low power, the melt pool flow is relatively gentle, and surface diffusion is limited, resulting in a flatter formed surface. In contrast, high power causes accelerated lateral flow and exacerbated surface unevenness. High roughness regions induce stress concentration, promoting crack initiation at the 50 % SS 316L/50 % IN718 and 25 % SS 316L/75 % IN718 gradient layers. Nb and Mo segregation accumulates under low laser power due to limited diffusion, and concentrates under high power due to intense melt convection—both of which hinder uniform distribution. The hardness first increases and then decreases with increasing laser power, reaching a maximum average hardness of 204.43 HV_0.5 at 1000 W and a minimum of 183.44 HV_0.5 at 1400 W. Corrosion resistance at high laser powers (1000 W–1400 W) is superior to that at low laser powers (600 W–800 W) but exhibits a downward trend at 1400 W. This study reveals the coupling mechanisms by which laser power induces multiple defects in FGMs and their impact on performance. It provides theoretical guidance for optimizing DED process parameters and enhancing the forming quality of graded materials.