Microstructural Characterization and Mechanical Properties of Inconel 718 Produced by LPBF Using Optimized Processing Parameters

Abstract

Inconel 718, largely used in many fields of activity, is one of the superalloys which can be manufactured by additive methods such as Laser powder bed fusion. This method involves many parameters which impact the material properties. This work focuses on the variation in the laser power, scan speed, scanning strategy and the impact of a 200°C preheating temperature of the base plate. Twenty 1 cm³ cubic samples were printed with varying laser power in the range of 100-200 W and scan velocities in the range of 700-1000 mm/s while keeping the hatch spacing constant at 80 μm and the layer thickness at 30 μm using a 90° scanning strategy. The cube densities were measured using a gas pycnometer and an internal porosity characterisation was conducted using an optical microscope. These cubes were used to study the melt pool shape, also using an optical microscope and the cellular microstructure, the chemical composition and the grain geometry using scanning electron microscopy. Macro-, micro- and nano-hardness tests were performed using HV10, HV0.3 and an indentation depth of 500 nm, respectively. In parallel, four 20 x 40 x 30 mm blocks using 900 mm/s scan speed, 200 W laser power, the same hatch spacing and layer thickness as the cubes, but using 90° and 67° scanning strategies were printed. These blocks were used for grain characterisation and macro- and micro-hardness tests were performed in the same way as for the cubes. Tensile specimens were machined perpendicular to the building direction for uniaxial tensile tests followed by a fractographic analyses. The results of these tests highlighted significant differences in the mechanical properties between the scanning strategies. The 67° strategy gives a higher Young’s modulus, yield strength and ultimate tensile strength but all specimens presented a ductile failure mode. The 200°C preheating temperature allows the production of near fully dense material with a relative density higher than 99.7% while mitigating the formation of hot cracks and oxide inclusions. A higher VED tends to monotonically increase the melt pool (MP) depth, while the grain size first decreases, reaches a minimum, and then increases. The grains maintain a fan-shaped morphology since the VED* range is restricted due to the already optimised processing parameter window studied here.