Effect of powder bed fusion laser melting process parameters, build orientation and strut thickness on porosity, accuracy and tensile properties of an auxetic structure in IN718 alloy

In this paper the geometry of an auxetic, re-entrant honeycomb structure made from Inconel 718 (IN718), was optimised with respect to the process parameters of laser melting process using a Renishaw AM250 after which the quasi static behaviour was analysed under tensile loading. Two different PBF process parameters were used with 2 different laser energy densities to manufacture the auxetic structures with 3 different strut thicknesses (0.3, 0.6 and 0.9 mm) in 2 building orientations (XY and ZX plane). A strut thickness of 0.6 mm was found to have the least porosity and the best dimensional accuracy. The latter geometry was then manufactured as part of a tensile test sample which were then tested at a strain rate of 0.001 s' after which the stress strain curve, yield stress, structural stiffness, plastic work and Poisson's Ratio were compared. The building directions XY, ZX and XZ of the auxetic structure were investigated, as well as the orientation of the individual cells with respect to the loading direction. XY was found to provide the best mechanical properties and the kinematics of deformation was found to be dependent on the loading direction with respect to the cells direction which resulted in a significant change in Poisson's ratio. Finite Element Analysis was also done in order to compare the stress strain curves and the deformation mode obtained from numerical modelling and experiment, and a good agreement was observed.