# Recent Progress in Hybrid Additive Manufacturing of Metallic Materials
## 4 Microstructural Characteristics, Mechanical Behavior, and Properties of Hybrid Managing Steels
The material's yield strength increased to around 1860 MPa, but with a lower fracture strain of 5% compared to 23% in the as-built state. Interestingly, the fracture occurred near the interface, which was scarcely distinguishable from the rest of the hybrid material. Direct aging of the hybrid material produced mechanical properties almost similar to the as-built alloy, with fracture occurring in the H13 steel section [46]. These results differed from those reported by [PERSON] et al. [90], in which fracture occurred at the interface. In their study, the hybrid material was produced using LPBF and subjected to ST at 815 \({}^{\circ}\)C, followed by a 982 \({}^{\circ}\)C preheating for 1 h before aging at 490 \({}^{\circ}\)C for 6 h, based on the manufacturer's specifications. This heat treatment procedure resulted in a peak strength of approximately 1700 GPa, owing to Orowan loop strengthening with the help of Co and Mo precipitates and the grain refining effect of Ti [91; 92]. Non-conventional heat treatment protocols have also been adopted [47; 55], whereby direct aging is used to avoid the loss of strength in a peak-aged hybrid base. An intact interface was obtained in both the as-printed and aged state. In the \begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline
**Process-P (W)** & **V (mm/s)** & **H** & **Orientation** & **E (GPa)** & \(\sigma_{y}\) (MPa) & \(\sigma_{\text{abs}}\) (MPa/Pa)** & **Elongation (\%)** & **HVN** \\ \hline \hline DED—350 & 8.3 & 36.1 & Absent & 40 \(\pm\) 10 & 400 \(\pm\) 100 & 900 \(\pm\) 200 & 5 \(\pm\) 3 & 441 \\ PBF—190 & 787 & 0.24 & Longitudinal & Absent & 661 \(\pm\) 24 & 1255 \(\pm\) 3 & 16.2 \(\pm\) 2.5 & 333 \(\pm\) 2 \\ PBF—195 & 800 & 0.24 & Absent & Transverse & 570 \(\pm\) 13 & 944 \(\pm\) 35 & 50 \(\pm\) 1 & Absent \\ PBF—95 & 350 & 0.27 & Longitudinal & Absent & 610 \(\pm\) 10 & 1050 \(\pm\) 20 & 11 \(\pm\) 0 & Absent \\
[87] & 350 & 0.27 & Transverse & Absent & 610 \(\pm\) 10 & 910 \(\pm\) 10 & 3.5 \(\pm\) 1.5 & Absent \\ Conventional processing—N/A &
\begin{tabular}{c} \(\text{\small solution}\) \\ annealed \\ and aged \\ \end{tabular} & N/A & N/A & 199 & 992 & 1018 & 13.4 & 430 \\ \hline \hline \end{tabular} Note: P = laser power, v = scanning speed, H = linear heat input, E = Young’s modulus, \(\sigma_{y}\) = yield strength, and \(\sigma_{\text{abs}}\) = tensile strength; HVN is the Vickers Hardness Number.  
\end{table} 
Table 3: Mechanical properties of additively manufactured 17-4 PH. Figure 9: Microhardness changes across the MS1–H13 interface. Reprinted with permission from [89]. Copyright 2019 Elsevier B.V.  
as-printed state, failure occurred in the printed material M789 at about 1100 MPa, and after aging, failure occurred in the N709 at approximately a 1600 MPa base plate. The effects of direct aging improved the strength of M789 by precipitation of the \(\eta\)-phase, at the same time minimizing the loss of strength in the hybrid base. Based on the literature, developing an optimized heat-treatment schedule is crucial for achieving the desired mechanical behavior in a hybrid material system, regardless of the additive manufacturing technique used. It has been noted that individual heat treatment methods for each material may not provide the best combination of strength and ductility, as observed by [PERSON] et al. [90]. This is particularly relevant for AM managing steels and wrought medium carbon steels, which can benefit from a well-designed heat treatment. The ideal heat treatment procedure should balance the strength and ductility of the hybrid material while eliminating microstructural inhomogeneities, which may vary depending on the specific hybrid material system. A systematic approach is required to arrive at an optimized heat treatment schedule based on the intended application. [PERSON] et al.