Additive manufacturing – A review of hot deformation behavior and constitutive modeling of flow stress

Hot working, as an important group of post-processing routes for additive manufacturing technology (3D printing), is used to reduce the solidification/processing defects and anisotropy of properties, grain refinement, improvement of mechanical properties, processing of pre-formed parts, and increasing the applicability domain. Accordingly, the present state of the art of the elevated temperature deformation behavior and constitutive description of flow stress during thermomechanical processing of additively manufactured parts is summarized in this monograph. Besides the effects of temperature and strain rate (represented by the Zener-Hollomon parameter), the significance of initial phases and the type of additive manufacturing process on the hot deformed microstructure, restoration processes of dynamic recovery (DRV) and dynamic recrystallization (DRX), flow stress, workability, and hot deformation activation energy is critically discussed. In this regard, the α'-martensite in Ti-6Al-4V titanium alloy produced by selective laser melting (SLM), the precipitates in aluminum alloys (such as 2219 Al alloy) produced by wire and arc additive manufacturing (WAAM), and the Laves phase in Inconel 718 superalloy produced by laser metal deposition (LMD) are remarkable examples. The utilization of innovative methods with in situ hot working effects such as additive friction stir deposition (AFSD) is also enlightened. Regarding the constitutive equations for modeling and prediction of hot flow stress, the reports on the strain-compensated Arrhenius model, artificial neural network (ANN) approach, DRX/DRV kinetics models, Johnson-Cook equation, and Fields-Backofen formula are presented, and the potentials of the modified, simplified, and physically-based approaches are discussed. Finally, the future prospects in this research field such as the hybridization of additive manufacturing with hot forming processes, work-hardening analysis for obtaining the onset of DRX, unraveling the effects of as-built microstructure, developing processing maps, proposing some physical-based unified constitutive models, and investigation of novel and/or widely-used alloys such as austenitic stainless steels, high-entropy alloys, and aluminum alloys (e.g. AlSi10Mg alloy) are proposed.