Additive Manufacturing of Inconel 718 by Meltpool and Grain Boundary Engineering

Additive Manufacturing (AM) achieves significant cost savings and enables complex geometries that are otherwise impossible to fabricate using conventional manufacturing processes. AM offers a new paradigm in design of additive alloys with complex microstructure by using rapid solidification, meltpool dynamics, and cyclic heat treatment of AM processes. The objective is to minimize the trial-and-error prints and improve quality of alloys by using a building block strategy with verification and test validation. This includes meltpool engineering for each layer, fabrication of coupons with desired microstructure, and novel alloy design for improved components. Integrating materials technology, materials design, and manufacturing innovation is a new frontier of AM development. AM process parameters are characterized in a case study for (1) meltpool engineering (MPE) and prediction of the process thermal map, density map, and temperature time transformation history to establish a roadmap for fabrication; (2) grain boundary engineering (GBE) to perform micro-scale material modeling of alloy composition and predict the grain size, mechanical strength, fracture, fatigue, and creep crack growth properties due to defects and precipitates; and (3) thermal-structural analysis incorporating MPE and GBE to assess part quality, reduce costs, and accelerate qualification of AM components. This includes (i) void prediction at the coupon level, (ii) macro-void print error calculations at element level, (iii) scatter in material strength and establishment of allowable, (iv) prediction of fracture control plan, (v) computing part distortion and inherent strain due to different print strategies and baseplate removal residual stresses, and (vi) net-shape and warpage measurements. Different AM process parameters result in unique alloy composition and microstructure due to different thermal history, precipitation, and property response surfaces. AM process maps hasten new additive alloy development and help characterize new alloy design envelopes. 3D-printed parts produced by Laser Power Bed Fusion (LPBF) need to be qualified and may suffer from: (i) defects (micro, macro), (ii) net-shape warpage, (iii) high residual stresses, (iv) surface roughness, (v) inconsistent density and voids, (vi) anisotropic microstructures due to variable cooling rates, (vii) scatter in mechanical properties, and (vii) poor fracture and fatigue performance. AM defects (e.g., unfused powder, balling, humping, and keyholing) are affected by variations in power and speed as well as hatch spacing that result in pores, thermal cracks, rough surface finish, and warping. Some of these defects are closely related to thermal behaviors during printing, in which materials go through multiple stages of heating, melting, and cooling resulting in the final microstructure of alloy. Prediction of these outcomes leads to optimization and build solutions.