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. 

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

The study of heterostructured materials (HSMs) answered one of the most pressing questions in the metallurgical field: “is it possible to greatly increase both the strength and the strain hardening, to avoid the “inevitable” loss of ductility?”. From the synergy between the deformation modes of zones with greatly different flow stress, low stacking fault energy (SFE) alloys can reduce their typical trade-off between strength and ductility. Stainless steel (SS) is a low-SFE material, which is widely applied for structural, biomedical, biosafety, food-processing, and daily applications. The possibility to combine its corrosion resistance and biocompatibility with the outstanding mechanical behaviour of HSMs can convert SS into a promising option for low-cost and high-effective advanced material. This paper reviews all the microstructural aspects of HS SS obtained by different processing methods and their correlation with crystallographic texture and properties such as mechanical, corrosion, biological, and magnetic characteristics. The critical comparison between experimental and modelling findings is also presented in terms of the deformation mechanisms, microstructural and texture features. Thus, the processing-microstructure-properties relationship in HS SS is the focus of this publication. The multi-disciplinary perspectives of HS SS are also discussed. This review paper will serve as a reference for understanding and designing new multi-functional HS SSs. 

Heterostructured stainless steel: Properties, current trends, and future perspectives

Additive manufactured magnesium (Mg) alloys have widespread applications in the medical industry as orthopedic implants and biomedical stents, and in the transportation/automotive industry due to their status as the lightest structural metallic alloys. The printability of Mg alloys is challenging due to the high oxidization rate, rapid evaporation, and susceptibility to gas trapping. Accordingly, there might be many defects in the as-built parts such as cracking and delamination, porosity and lack of fusion, residual/thermal stresses and distortion, inhomogeneous/columnar microstructure, anisotropy in mechanical/physical properties, inclusions, and non-equilibrium phases formed during solidification. The removal of these defects has a great practical significance, where the post-processing heat treatments without altering the geometry of the parts are often sought in this regard. Accordingly, the present overview article focuses on and critically discusses the recent progress/advances in the application of hot isostatic pressing (HIP), solution annealing (solutionizing), and aging/precipitation heat treatment for the modification/homogenization of the microstructure and improvement of mechanical/functional properties of Mg alloys for the first time. The main fabrication methods are selective laser melting (SLM) from the category of powder bed fusion (PBF) processes, as well as wire and arc additive manufacturing (WAAM) from the category of directed energy deposition (DED) processes. Moreover, the adaptation of the friction stir processing (FSP) technology into additive manufacturing for grain refinement via dynamic recrystallization (DRX) and defect/pore closure (due to the elevated-temperature thermomechanical processing effects), combined severe plastic deformation (SPD) and thermal post-processing, hybridization of additive manufacturing for Mg alloys, and future prospects have been summarized. 

Post-processing heat treatment of lightweight magnesium alloys fabricated by additive manufacturing: A review

Constitutive modeling of hot deformation behaviour of AlCrFeNi multi-component alloy

High entropy alloys (HEAs) is a new generation of alloys with multi-component systems and emerged as a potential candidate for the industrial applications due to their excellent mechanical properties at cryogenic, ambient and elevated temperatures. The chemistry changes through microstructural heterogeneities leads to a significant strength increment in HEAs as compare to the conventional solid solutions. However, the strength increment is often related to the ductility loss and vice-versa, which is known as strength-ductility trade-off. Therefore, the modifications in the conventional strengthening mechanisms are required to overcome this dilemma. The conventional strengthening mechanisms have been revisited and revised by the materials scientists to break these trade-offs through adjusting the stacking fault energy, twins formation and phase transformation in multicomponent systems. Herein, the different strengthening mechanisms including, solid solution strengthening, grain boundary strengthening, precipitation strengthening and phase transformation strengthening are systematically reviewed with regard to overcome the strength-ductility trade-off in HEAs. 

Strengthening mechanisms in high entropy alloys: A review

A review of hot deformation behavior and constitutive models to predict flow stress of high-entropy alloys

Superplasticity of fine-grained magnesium alloys for biomedical applications: A comprehensive review

Under specific deformation conditions, thermoforming in the supercooled liquid region (SLR) might lead to Newtonian flow characterized by high strain-rate-sensitivity-index (m) of ∼1 and achieving superplastic ductilities. Both incipient deformation-induced crystallization and rapid increase in the stress-assisted free volume lead to decreased m-values and transition to non-Newtonian flow. The maximum elongations can be achieved near the transition strain rate from Newtonian to non-Newtonian flow. The effects of heating rate to the thermoforming temperature, total processing time, and the extent of the SLR are significant for structural stability against the in-situ crystallization during elevated-temperature hot deformation. Increasing the deformation temperature normally accentuates the superplastic behavior, but high temperatures might promote crystallization and loss of superplastic ductility . Future prospects for research have been recognized as optimization of the volume fraction of the crystalline (reinforcement) phase in BMG composites, superplasticity of high-entropy BMGs, thermoplastic micro-formability, ultrasonic-assisted forming, improved thermoplastic formability by alloying method, and superplasticity of BMG parts fabricated by additive manufacturing processes. 

Reza Motallebi

Papers

A review of hot deformation behavior and constitutive models to predict flow stress of high-entropy alloys

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

Post-processing heat treatment of lightweight magnesium alloys fabricated by additive manufacturing: A review

Superplasticity of fine-grained magnesium alloys for biomedical applications: A comprehensive review

Superplasticity of bulk metallic glasses (BMGs): A review